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Terahertz technology and its applications
Abstract
New advances in different technologies have made the previously unused terahertz frequency band accessible for imaging systems. The ‘terahertz gap’ has a frequency ranges from ∼0.3 THz to ∼10 THz in the electromagnetic spectrum which in between microwave and infrared. The terahertz radiations are invisible to naked eye & in comparison with X-ray they are intrinsically safe, non-destructive and non-invasive. This is such a new field that researchers around the world race to build the first practical system. It resolves many of the questions left unanswered by complementary techniques, such as optical imaging, Raman and infrared. Terahertz spectroscopy has number of applications run from detecting defects in tablet coating, product inspection (industry), spectroscopy (chemistry, astronomy), material characterization (physics), weapons concealed under clothing (airports), detection of cancer and caries. In the pharmaceutical industries it enables nondestructive, internal, chemical analysis of tablets, capsules and other dosage forms. This paper tries therefore not only to provide a brief overview over the imaging technology, but also over the whole range of current systems and research in terahertz technology.
... In this study, the terahertz field is excited theoretically using Eq. (44), which was derived for the self-focused Hollow Gaussian Laser Beam order (n = 2) radiation in Eq. (26) interacting with the magnetized plasma by relativistic nonlinearity. The parameters affecting the excitation of terahertz radiation and power spectrum of terahertz radiation were studied and it was found that terahertz radiation is enhanced by increasing both the intensity of the laser beam, the cyclotron frequency (resulting from the increase in the magnetic field), and the plasma frequency (resulting from the plasma density) The generated terahertz radiation can be used in many applications such as spectroscopy, medical imaging and airport security instead of X-rays [44]. ...
... (44), which was derived for the self-focused Hollow Gaussian Laser Beam order (n = 2) radiation in Eq. (26) interacting with the magnetized plasma by relativistic nonlinearity. The parameters affecting the excitation of terahertz radiation and power spectrum of terahertz radiation were studied and it was found that terahertz radiation is enhanced by increasing both the intensity of the laser beam, the cyclotron frequency (resulting from the increase in the magnetic field), and the plasma frequency (resulting from the plasma density) The generated terahertz radiation can be used in many applications such as spectroscopy, medical imaging and airport security instead of X-rays [44]. ...
This article presents a mathematical model describing the excitation process of terahertz radiation resulting from the relativistic nonlinear interaction of Hollow Gaussian Laser Beam (HGLB) with magnetized collisionless plasma. The effects of laser beam intensity and plasma density on terahertz efficiency conversion have been investigated in the presence of a transverse magnetic field. High terahertz power (reaching GW level) with high stability of terahertz wave has been recorded.
... The Terahertz (THz) frequency range, spanning 0.3-10 THz, presents a promising solution for the demanding conditions on Mars. The superior transmission capabilities of the THz band, coupled with the compactness of associated hardware, align well with the operational requirements of Martian expeditions [8]. ...
... A abs (f, d) = e k(f )·d (8) where k(f ) is the frequency-dependent absorption coefficient. ...
This paper presents the Mars Dust Storm Detector (MDSD), a system that leverages the THz Opportunistic Integrated Sensing and Communications (OISAC) signals between Mars surface assets (rovers and landers) to extract environmental information, particularly dust storm properties. The MDSD system utilizes the multi-parameter sensitivity of THz signal attenuation between Martian communication devices to provide rich, real-time data on storm intensity, particle characteristics, and potentially even electrification state. This approach, incorporating HITRAN spectroscopic data and Martian-specific atmospheric parameters, allows for accurate modeling and analysis. The system's ability to repurpose THz ISAC signals for environmental sensing demonstrates an efficient use of resources in the challenging Martian environment, utilizing communication infrastructure to enhance our understanding of Mars' atmospheric dynamics. The system's performance is evaluated through extensive simulations under various Node Density Factors (NDFs), comparing different interpolation algorithms for dust storm intensity mapping. Results demonstrate that linear interpolation achieves superior accuracy (correlation >0.90) at high NDFs, while nearest-neighbor and IDW algorithms maintain complete spatial coverage in sparse networks. Error analysis identifies dust particle size uncertainty as the primary contributor to estimation errors, though the system shows resilience to Martian atmospheric variations. This work extends the opportunistic use of ISAC technology to planetary exploration, contributing to both Mars atmospheric monitoring capabilities and ISAC applications in the Internet of Space (IoS).
... To address the limitations of current detection methods, terahertz (THz) spectroscopy has emerged as a promising analytical tool. Spanning frequencies between 0.1 and 10 THz [11], it is a non-ionizing, non-destructive technique capable of penetrating nonconductive materials without damaging samples [12]. Due to these unique properties, THz spectroscopy excels in molecular fingerprinting of chemical and biological substances [11][12][13][14][15]. ...
... Spanning frequencies between 0.1 and 10 THz [11], it is a non-ionizing, non-destructive technique capable of penetrating nonconductive materials without damaging samples [12]. Due to these unique properties, THz spectroscopy excels in molecular fingerprinting of chemical and biological substances [11][12][13][14][15]. Its sensitivity to the absorption features of hydrogen bonds, molecular By tuning the incident angle of the light source or applying strain to the substrate, the resonance frequency in the transmission spectrum can be shifted. ...
Terahertz (THz) spectroscopy, an advanced label-free sensing method, offers significant potential for biomolecular detection and quantitative analysis in biological samples. Although broadband fingerprint enhancement compensates for limitations in detection capability and sensitivity, the complex optical path design in operation restricts its broader adoption. This paper proposes a multi-degree-of-freedom stretchable metasurface that supports magnetic dipole resonance to enhance the broadband THz fingerprint detection of trace analytes. The metasurface substrate and unit cell structures are constructed using polydimethylsiloxane. By adjusting the sensor’s geometric dimensions or varying the incident angle within a narrow range, the practical optical path is significantly simplified. Simultaneously, the resonance frequency of the transmission curve is tuned, achieving high sensitivity for effectively detecting cinnamoylglycine. The results demonstrate that the metasurface achieves a high-quality factor of 770.6 and an excellent figure of merit of 777.2, significantly enhancing the THz sensing capability. Consequently, the detection sensitivity for cinnamoylglycine can reach 24.6 µg·cm−2. This study offers critical foundations for applying THz technology to biomedical fields, particularly detecting urinary biomarkers for diseases like gestational diabetes.
... The problem complicates if such waves are in the "Terahertz gap", laying in the range between microwave and infrared, where the realms of electronics and optics overlap. Implementing technologies for generating and detecting Terahertz radiation is particularly challenging [1]. However, the pace of progress in THz technology has accelerated significantly in recent years, 1 Department of Imaging Physics, Delft University of Technology, Delft, the Netherlands; 2 SRON Netherlands Institute for Space Research, witnessing innovative laser sources and highly sensitive detectors. ...
... Implementing technologies for generating and detecting Terahertz radiation is particularly challenging [1]. However, the pace of progress in THz technology has accelerated significantly in recent years, 1 Department of Imaging Physics, Delft University of Technology, Delft, the Netherlands; 2 SRON Netherlands Institute for Space Research, witnessing innovative laser sources and highly sensitive detectors. The initial aim of this project was the development of a reflective Fourier grating to generate a THz local oscillator with seven uniform beams. ...
-We propose a novel approach to multiplex a single Gaussian beam from a THz source into seven beams arranged in a hexagonal configuration, matching an array of Hot Electron Bolometer (HEB) mixers. This configuration is necessary for the High-Resolution Receiver (HiRX) instrument on the proposed NASA Single Aperture Large Telescope for Universe Studies (SALTUS) space mission. The beam splitter relies on a reflector that introduces a phase shift to the incident Gaussian beam; after propagation, the desired amplitude distribution is achieved at the mixer array plane. Unlike existing THz multiplexers, our method does not use a phase grating based on the repetition of a unit cell. Instead, we employ an iterative phase reconstruction (Gerchberg-Saxton) algorithm to retrieve the required phase shift. This paper discusses the scientific motivation, current state of the art, design methodology, simulation outcomes, and experimental validation of the reflector.
... Terahertz radiation, often referred to as the bridge between electronics and photonics, has gained significant attention in recent years because of its unique properties and the vast potential for applications in sensing, wireless communication, healthcare, spectroscopy, security, quality control, and imaging, as described in Figure 1 [22][23][24][25]. Exploring and harnessing this frequency range necessitates a multidisciplinary strategy that encourages close collaboration between high-frequency semiconductor technology for radio frequencies (RF) electronics and alternative photonic-based solutions. ...
The terahertz (THz) region of the electromagnetic spectrum, spanning from 0.1 to 30 THz, represents a prospering area in photonics, with transformative applications in imaging, communications, and material analysis. However, the development of efficient and compact THz sources has long been hampered by intrinsic material limitations, inefficient conversion processes, and complex phase-matching requirements. Recent breakthroughs in nonlinear optical mechanisms, resonant metasurface engineering, and advances in the fabrication processes for materials such as lithium niobate (LN) and aluminum gallium arsenide (AlGaAs) have paved the way for innovative THz generation techniques. This review article explores the latest theoretical advances, together with key experimental results and outlines perspectives for future developments.
... The THz band (0.1 to 10 THz or wavelengths 3 to 0.03 mm) situated between the infrared and millimeter wave regions of the electromagnetic spectrum, is also referred to as the THz gap since this was unexplored until a few decades ago [1][2][3][4][5][6]. Terahertz waves can penetrate a variety of dielectric materials, such as paper, plastic, and many organic materials hence perfect for scanning and imaging applications [7]. These waves have low photon energy, in contrast to X-rays. ...
In this paper, an ultra-wideband (UWB) terahertz Metamaterial Perfect Absorber (MPA) based on the phase change feature of vanadium dioxide (VO2) is proposed. The proposed absorber consists of two square loops of vanadium dioxide (VO2), silicon dioxide (SiO2) as the dielectric substrate and a layer of gold (Au) acting as the ground. Phase transition material VO2 changes the conductivity (conductivity is changed from 200 to 2 × 10⁵ S m⁻¹, concerning temperature 300 K to 341 K) by several orders of magnitude at temperatures above 340 K by shifting from an insulating to a metallic state. The absorption of this suggested absorber can be adjusted from 4% to 100% within the conductivity range of 200 to 2 × 10⁵ S m⁻¹, which corresponds to temperatures between 300 K and 341 K. In the 2.2–6.8 THz range, with a bandwidth of up to 4.6 THz, more than 80% absorption is observed. The absorber design we proposed is polarization insensitive, simple and easily integrable. There are numerous uses for the suggested absorber in communication-related futuristic devices.
... Terahertz (THz) waves are electromagnetic (EM) waves with a wavelength range of 0.03-3 mm, situated between microwaves and infrared. 1 As an important segment of the EM spectrum, THz waves exhibit strong penetration, 2 provide abundant bandwidth resources, and possess distinctive spectral characteristics. 3 Furthermore, they offer a high signal-to-noise ratio in time-domain spectrum measurements. ...
A dual-wideband linear-to-circular (LC) polarization converter with a one-octave band separation (OBS) working mode based on liquid crystal regulation is proposed in this paper. OBS polarization conversion refers to dual-band polarization conversion where the second operating band either completely covers twice the range of the first band or corresponds exactly to twice the frequency range of the first band. The performance of the design, both before and after optimization, is analyzed in detail. The optimization process achieves the desired outcome by introducing cavities into the liquid crystal layer of the initial structure. At zero bias, the optimized structure achieves LC conversion at 0.779–0.976 and 1.429–2.085 THz, with relative bandwidths of 22.45% and 37.34%, respectively. The generated circularly polarized (CP) waves are all right-handed. When the liquid crystal is fully biased, the operating frequency bands shift to 0.758–0.978 and 1.504–2.154 THz, with relative bandwidths of 25.35% and 35.54%, respectively, and the CP waves are all left-handed. The OBS frequency bands for the two typical bias states are 1.558–1.952 and 1.516–1.956 THz, respectively. These bands are fully covered by their respective second operating ranges, enabling the OBS polarization conversion working mode. LC conversion with the same CP wave chirality in dual-wideband operation is achieved through an innovative and advanced design in this paper. Furthermore, it addresses the common issue of high-frequency bandwidth attenuation in dual-band polarization converter designs. Liquid crystals also offer flexible, frequency-reconfigurable operation, a feature not found in conventional designs. The OBS working mode, with the same CP wave chirality, creates a synergistic relationship between operating bands, opening up broad application prospects in advanced high-speed communications, encryption, integration with nonlinear devices, and molecular detection.
... 3 Extending Mueller matrix ellipsometry into the terahertz region, however, has been particularly challenging due to the "terahertz gap"-a frequency range where the development of optical components has lagged behind other spectral regions. 4 Nevertheless, recent advancements are helping to close this gap. A wide range of optical components necessary for constructing functional terahertz full Mueller matrix ellipsometers are now available, including wire grid polarizers made from carbon nanotubes, 5 aluminum, 6 and thin films. ...
We report a Mueller matrix ellipsometer design using dual continuously rotating anisotropic meta wave plates which determines the full set of Mueller matrix elements in the terahertz spectral range. The instrument operates in the frequency domain and employs a frequency tunable, solid state synthesizer based, continuous wave terahertz source with sub-MHz bandwidth. The implemented source permits operation within 82-125 GHz and 170-250 GHz, without and with an additional frequency doubler, respectively. The terahertz transparent meta wave plates consist of 3D-printed polymer based columnar thin film structures with subwavelength dimensions. The rotating wave plates produce sufficient modulation of the Stokes vector components of transmitted terahertz light to use the wave plates in polarization state generator and polarization state detector devices. Fast terahertz light detection rate of a quasi optical solid state detector permits acquisition with few microsecond temporal resolution and electronic sweeping control of the source frequency. We develop a fast frequency sweeping scheme while continuously rotating the terahertz wave plates. Subsequent sorting of measured data permits measurements of Mueller matrix elements at hundreds of different wavelengths. The Mueller matrix elements are obtained by forward numerical reduction of the measured data using an algorithm developed by Ruder et al.[Opt. Lett. 45, 3541 (2020)]. The instrument is combined with a magnetocryostat capable of reaching magnetic field strengths at the sample position of -8 T...8 T and sample temperatures from 4 K...400 K. Hence, the instrument is suitable to measure the full Mueller matrix of samples with magnetic resonances as demonstrated recently by Rindert et al. [Phys. Rev. B 110, 054413 (2024)]. We discuss design, calibration, and example applications.
... Terahertz (THz) radiation band is a tiny region in the spectrum of electromagnetic fields between 0.1 and 10 THz that is currently attracting a lot of attention because of its multiple possible uses [8,9]. This spectrum of radiation, that lies within Conversely, because hollow-core PCF offers the largest area of communication between the radiation and the analyser, it is the preferred method for utilisation. ...
This research presents a novel square hollow‐core photonic crystal fibre (PCF) sensor designed for the detection of food‐grade oils in the terahertz (THz) frequency range. The sensor’s effectiveness is quantitatively evaluated using COMSOL Multiphysics, a sophisticated simulation tool that employs finite element methodology (FEM) to model complex interactions within the fibre structure. Simulation outcomes reveal that, under optimal geometric parameters, the proposed sensor achieves an exceptional relative sensitivity of 98.27% for various edible oils at an ideal frequency of 2.2 THz, significantly outperforming existing technologies. Additionally, the sensor exhibits minimal confinement loss of 1.428 × 10⁻⁸ dB/m and a low effective material loss of 0.004246 cm⁻¹, facilitating accurate detection of slight refractive index variations related to the chemical compositions of different oils. This high sensitivity enables non‐destructive testing, allowing for the analysis of oils without compromising their composition or quality, thereby maintaining the integrity of food products. Ultimately, the proposed PCF sensor enhances food safety monitoring and paves the way for advanced applications in the food industry, ensuring consumers receive high‐quality products.
... Terahertz (THz) wave has wide applications in spectroscopy, imaging, non-destructive testing, biomedicine, security checking, etc [1][2][3][4][5]. THz wave detection plays a key role in the development of THz technologies. ...
Terahertz (THz) frequency upconversion detection is a promising method in the field of THz measurement. This paper focuses on the dependence of the THz frequency upconversion detection characteristics based on the stimulated polariton scattering (SPS) effect in lithium niobite (LN) crystal on the pumping pulse energy. It was found that optimal pumping pulse energy exists, which leads to the largest ratio of the upconverted Stokes signal to the background clutter. In the theoretical part, the background clutter is divided into two parts: the amplified spontaneous Stokes fluorescence and other background clutter. The expressions for the upconverted Stokes signal and amplified spontaneous Stokes fluorescence are derived, and the terahertz wave frequency upconversion detection characteristics varying with the pumping pulse energy are successfully explained.
... Typical applications are found in the fields of space exploration, wireless communication, security screening, material science, biomedical engineering and so on. However, the development and application of THz science and technology are largely limited due to lack of high-performance terahertz sources and detectors [2]. ...
In this paper, we propose a novel design of GaN-based planar Gunn oscillator as terahertz signal source. The oscillator has multiple gates and each gate can be individually biased. By controlling gate bias voltage and distance to the barrier layer, the output oscillating current can be formed in such a way that the higher harmonics are more powerful than the fundamental one. This is because multiple Gunn domains created under the gates in the channel are synchronized. Compared to the conventional single-domain diode, this multi-gate configuration not only increases the frequency and output power but also provides superior harmonic control, leading to higher efficiency and more stable operation at terahertz frequencies. We will review the design details and analysis on domain forming conditions using a physics-based numerical model. The optimal parameters for high power, high DC-RF conversion efficiency and high frequency will be given. Specifically, in dual-gate device, when it works in the dual-domain mode, the second harmonic is enhanced, reaching a frequency of 310.5 GHz with 7.6 mW of power and 11.2% efficiency. The tri-gate device operating in tri-domain mode further enhances the third harmonic to 417.0 GHz, with 9.57 mW of power and 9.23% efficiency. The multi-gate structure allows for more efficient harmonic generation, greater frequency tunability, and better power management, all of which are crucial for advanced terahertz application such as mixing, frequency multiplexing, and signal amplification.
... Terahertz (THz) waves, occupying the frequency range of 0.1 to 10THz, exhibit distinctive modulation properties [1]. The utilization of external physical fields via optical, electrical [2], magnetic [3], and other stimulation methods enables precise control over their electromagnetic characteristics [4]. ...
In this study, a series of temperature-controlled terahertz (THz) wave modulators based on a combination of magnetron-sputtered strontium titanate (STO) thin films and metasurfaces have been proposed. As the sputtering temperature increased from 25 to 600°C, the STO thin films exhibited an evolution from amorphous to polycrystalline states and thus different dielectric responses were observed in the THz band. Differences in the performance of the THz wave modulators such as differences in the variation trends of the amplitude and frequency of the resonance peaks as a function of the heating temperature were observed in the THz transmission spectra. The variation in the permittivity as a function of heating also exhibited different trends for the as-deposited STO films and fabricated modulators when the sputtering temperature was increased. Although a maximum amplitude modulation depth of 29.8% was obtained for the modulator with STO sputtered at 25°C, the frequency-shift exhibited nearly no regular variation on heating. On the contrary, amplitude as well as frequency modulations were achieved for the THz wave when the sputtering temperature of STO for the modulator exceeded 200°C. This study presents a high-temperature sputtering method for fabricating STO temperature-controlled modulators that can provide simultaneous modulation of amplitude and frequency of THz waves.
... terahertz and a wave band of 0.03-3 mm [4]) detection involves the use of their unique optical characteristics to manipulate and guide THz waves. These fibres feature a periodically arranged microstructure that creates an optical frequency gap, enabling precise supervision of light propagation. ...
Fuel adulteration involving the illicit mixing of substances such as kerosene and diesel with petrol poses significant risks to engine performance, environmental safety and consumer health. This paper presents a novel HC-PCF sensor designed to accurately detect and identify adulterants in petroleum-based fuels with unprecedented sensitivity and selectivity. The proposed HC-PCF sensor features a unique circular core structure surrounded by a carefully engineered square cladding region, enabling highly sensitive detection of refractive index changes caused by the presence of adulterants. Through rigorous numerical simulations and optimisation, our design achieves remarkable maximum relative sensitivities of 98.56%, 98.95%, and 99.32% for petrol, kerosene, and diesel, respectively, outperforming many previously reported techniques. A comprehensive analysis of the sensor's performance reveals an ultra-low confinement loss of 4.08 � 10 −10 dB/m, 1.08 � 10 −13 dB/m, and 2.95 � 10 −12 dB/m and effective material loss of 0.0040 cm −1 , 0.0036 cm −1 , and 0.0034 cm −1 , highlighting its exceptional light-guiding capabilities and sensitivity. The sensor's high responsiveness facilitates the detection of even trace levels of adulterants by capturing minute refractive index variations as low as possible, enabling real-time monitoring and timely intervention in adulteration incidents. The proposed HC-PCF sensor exhibits high selectivity, precisely targeting the refractive index signatures of fuels, ensuring accurate detection even in complex chemical environments. Its compact size and robust design make it suitable for deployment in various fuel quality control applications, from transportation to industrial settings. Overall, this work introduces cutting-edge HC-PCF sensor technology that addresses the critical need for reliable fuel adulteration detection with unparalleled sensitivity and selectivity, contributing to enhanced product quality, consumer protection, and environmental sustainability in the energy sector. K E Y W O R D S nanofabrication, nanofibres, nanotechnology, optical microscopy
... terahertz and a wave band of 0.03-3 mm [4]) detection involves the use of their unique optical characteristics to manipulate and guide THz waves. These fibres feature a periodically arranged microstructure that creates an optical frequency gap, enabling precise supervision of light propagation. ...
Fuel adulteration involving the illicit mixing of substances such as kerosene and diesel with petrol poses significant risks to engine performance, environmental safety and consumer health. This paper presents a novel HC‐PCF sensor designed to accurately detect and identify adulterants in petroleum‐based fuels with unprecedented sensitivity and selectivity. The proposed HC‐PCF sensor features a unique circular core structure surrounded by a carefully engineered square cladding region, enabling highly sensitive detection of refractive index changes caused by the presence of adulterants. Through rigorous numerical simulations and optimisation, our design achieves remarkable maximum relative sensitivities of 98.56%, 98.95%, and 99.32% for petrol, kerosene, and diesel, respectively, outperforming many previously reported techniques. A comprehensive analysis of the sensor's performance reveals an ultra‐low confinement loss of 4.08 × 10⁻¹⁰ dB/m, 1.08 × 10⁻¹³ dB/m, and 2.95 × 10⁻¹² dB/m and effective material loss of 0.0040 cm⁻¹, 0.0036 cm⁻¹, and 0.0034 cm⁻¹, highlighting its exceptional light‐guiding capabilities and sensitivity. The sensor's high responsiveness facilitates the detection of even trace levels of adulterants by capturing minute refractive index variations as low as possible, enabling real‐time monitoring and timely intervention in adulteration incidents. The proposed HC‐PCF sensor exhibits high selectivity, precisely targeting the refractive index signatures of fuels, ensuring accurate detection even in complex chemical environments. Its compact size and robust design make it suitable for deployment in various fuel quality control applications, from transportation to industrial settings. Overall, this work introduces cutting‐edge HC‐PCF sensor technology that addresses the critical need for reliable fuel adulteration detection with unparalleled sensitivity and selectivity, contributing to enhanced product quality, consumer protection, and environmental sustainability in the energy sector.
... The terahertz band possesses the benefits of low radiation energy, strong penetration, high resolution, and rich spectral information on molecular structures. It has promising application potential in medical imaging, non-destructive security inspection, in vivo detection, broadband communication, electromagnetic weapons, and other fields [10,11]. ...
Dual-wavelength output devices have a wide range of applications in mid-infrared band difference frequency generation, anti-interference lidar, dual-wavelength holographic interferometry, and other applications. Vertical external cavity surface-emitting lasers (VECSELs) are a type of semiconductor laser that can achieve single-chip dual-wavelength output by designing the chip structure. In this paper, we present a single-chip VECSEL that can switch between dual-wavelength and single-wavelength output modes. The VECSEL can simultaneously emit coaxial laser beams at 967 nm and 1013 nm, with a wavelength spacing of about 45 nm. The degree of mismatch between the gain peaks of the two quantum wells in the gain chip and the corresponding cavity modes is different. By adjusting the pump power, the temperature of the active region can be changed, which alters the matching relationship between the gain peaks and the cavity modes and controls the output mode of the VECSEL. The dual-wavelength output mode maintains a stable wavelength spacing at different operating temperatures. The laser output mode can be switched between single-wavelength and dual-wavelength, and the beam divergence angle is less than 8°. The dual-wavelength output power can exceed 400 mW, and the long-wavelength output power can reach up to 700 mW.
... THz waves experience greater free-space path loss than lower frequency bands, as path loss increases with the square of the frequency, as illustrated in Equation (2). As a consequence, due to its high-resolution sensing capabilities on a small scale, THz radiation is applied in imaging [88], food quality inspection [89], and various pharmaceutical industry processes [90]. Finally, Naftaly et al. [91] reviewed both current and emerging industrial applications, highlighting the significant market potential of THz technology. ...
Recently, with advancements in Deep Learning (DL) technology, Radio Frequency (RF) sensing has seen substantial improvements, particularly in outdoor applications. Motivated by these developments, this survey presents a comprehensive review of state-of-the-art RF sensing techniques in challenging outdoor scenarios with practical issues such as fading, interference, and environmental dynamics. We first investigate the characteristics of outdoor environments and explore potential wireless technologies. Then, we study the current trends in applying DL to RF-based systems and highlight its advantages in dealing with large-scale and dynamic outdoor environments. Furthermore, this paper provides a detailed comparison between discriminative and generative DL models in support of RF sensing, offering insights into both the theoretical underpinnings and practical applications of these technologies. Finally, we discuss the research challenges and present future directions of leveraging DL in outdoor RF sensing.
... The high sensitivity and precision of the graphenebased THz antenna make it an ideal candidate for detecting minute changes in tissue properties, enabling early diagnosis of diseases [12]. In environmental monitoring, the antenna's ability to operate efficiently at THz frequencies makes it suitable for detecting trace gases and pollutants in the atmosphere, offering a powerful tool for real-time environmental analysis [13]. The high directivity and efficiency of the antenna also make it useful for security scanning, where precise detection of concealed objects is critical [14] Further, some other paper has been explained for advanced sensing applications such as [15,16]. ...
This paper introduces the design of a dual-substrate graphene-based antenna for advanced sensing applications in the terahertz (THz) frequency range. The antenna utilizes two distinct substrates, one rectangular substrate made of silicon dioxide (SiO2) and a circular substrate positioned above it. This dual-substrate configuration is designed to enhance the overall performance of the antenna at high frequencies. The radiating patch constructed from graphene is chosen for its unique electrical properties making it highly suitable for THz operations. To ensure proper isolation between the graphene patch and the underlying substrate, a polysilicon ring is employed, which prevents undesired interactions and improves the antenna's efficiency. The antenna is resonant at 1.934 THz, a critical frequency for many sensing and imaging applications. At this frequency, the antenna demonstrates a maximum directivity of 8.2 dBi which indicating focused radiation in the desired direction, and achieves an overall efficiency of 70.19%. The design also shows improved surface current distribution, which helps to reduce losses and optimize radiation performance. These characteristics make this antenna a promising candidate for applications requiring precise THz wave manipulation, such as environmental monitoring, biomedical sensing, and high-resolution imaging systems.
... It demonstrated notable promise for a diverse array of applications in diferent industries, for example, in the next generation of wireless communication [2,3], nondestructive scanning and detection [4], spectroscopy [5], and even biomedical sensing [6]. Tis is due to the unique characteristics of the THz band, which has intrinsically safe, nondestructive, and noninvasive radiation compared with higher frequency electromagnetic radiation such as X-rays and gamma rays [7]. ...
Terahertz (THz)-integrated technology experienced explosive growth and shows excellent potential across many industries. Hence, developing shielding materials is crucial to safeguard waveguides and sensitive devices from unwanted external electromagnetic sources. To date, there is a gap in current research regarding the conclusive use of maghemite nanoparticle (γ−Fe2O3 NP) as an effective nanofiller for THz-shielding material while utilizing the inherent properties of graphene nanosheets (GNS) to create a hybrid polymer nanocomposite. Therefore, for the first time, this study developed a novel hybrid γ−Fe2O3/GNS nanocomposite while assessing the potential role of γ−Fe2O3 nanoparticles for THz-shielding applications. A one-step direct chemical exfoliation method synthesized the GNS. On the other hand, a straightforward thermal decomposition method synthesized the γ−Fe2O3 NP. The nanomaterials were then loaded to poly (methyl methacrylate) (PMMA), with different ratios of γ−Fe2O3 NP (w% = 0, 5, 10, and 15) to fabricate thin films by the evaporative casting technique. The scanning electron microscopy results with elemental dispersive X-ray (SEM-EDX) display the morphological traits of GNS as loosely stacked flat-shaped sheets. The γ−Fe2O3 has predominantly homogeneous spherical-shaped morphology measuring 30–60 nm. At the same time, the films retained some of the distinct features of the polymer matrix and GNS with localized clusters of the magnetic nanoparticles. Results from the X-ray diffraction (XRD) analysis complemented the results of SEM and ATR-FTIR analysis, supporting the existence of pure γ−Fe2O3 NP. Furthermore, the time-domain spectroscopy (TDS) analyzes the THz electromagnetic interference-shielding effectiveness (EMI-SE) at 0.6–1.6 THz of the nanocomposite with 15% of γ−Fe2O3, revealing an SE of ≤20 dB (99.00% wave attenuation) and calculated specific shielding effectiveness (SSE) of ∼30 dB∗cm3/g. These demonstrated improved SE by introducing γ−Fe2O3 NP as an additive magnetic filler. Conclusively, the fabricated hybrid γ−Fe2O3/GNS nanocomposites were considered facile, effective, and comparable THz EMI-shielding materials.
... The electromagnetic spectrum of terahertz (THz) waves spans the frequency range of from 0.1 to 10 THz [1]. It not only demonstrates the properties of microwave and infrared waves but also possesses numerous exceptional advantages. ...
THz waves have garnered significant attention across multiple domains, particularly in high-sensitivity sensing applications. Metamaterials are applicable in THz sensors owing to their exceptional sensitivity, particularly in refractive index measurement and pesticide identification. This paper proposes a THz metamaterial sensor for detecting ether kresoxim-methyl. The sensor comprises a periodic array of silicon cylindrical trimers organized on a silicon substrate. Resonances in the guided mode, dictated by bound states in the continuum spectrum, can be stimulated by meticulously configuring the geometric arrangement of the silicon column trimer. The sensor demonstrates a Q-value of up to 143 at the resonant frequency. The detection of pesticide residues achieved high sensitivity and specificity, with a detection limit of 37.74 μg/cm². This study presents a novel alternative for THz metamaterial sensors characterized by high sensitivity and a broad spectrum of pesticide concentration detection. The sensor platform developed in this paper, utilizing conventional CMOS technology, is posited as a potential detection instrument for herbicide and pesticide residues in agriculture and food.
Grating metasurfaces supporting guided‐mode resonance (GMR) can provide ultra‐sharp resonances for light‐matter solid interactions, facilitating their applications in various fields such as sensing, lasers, and imaging. Bound states in the continuum (BICs) are unique resonant modes located in the radiative continuum domain but fully bound without energy leakage. BIC can be transformed into quasi‐bound states in the continuum (QBICs) and coupled to external radiation to meet the demand for high‐Q resonances. Here, a broadband dielectric terahertz (THz) sensor is presented for GMR, which facilitates the generation of BIC whose QBIC states can reach a quality factor of 10 ³ . The sensor has a signal enhancement factor of more than 10 dB over the entire frequency range of 1.25–2.20 THz and can accurately analyze three absorption peaks of explosive materials. The design breaks through the single‐peak scanning of THz detection by cross‐fertilizing BIC with GMR, generating an angular manipulation ultra‐broadband in the THz spectrum. It provides a powerful tool for ultrasensitive trace analysis and will facilitate many new THz sensing applications. Meanwhile, the structure will support the combination of metasurfaces with flexible materials, improving the performance of flexible devices and broadening their applications in optoelectronic devices.
An ultra-wideband absorption and tripling octave frequency linear to circular polarization conversion tunable metastructure (MS) is proposed, utilizing the phase transition property of vanadium dioxide (VO2). In its metallic state,...
In this study, the design of an adaptable, high-efficiency, and broadband polarization converter in the terahetz (THz) region was carried out in a computer environment with the electromagnetic simulation program CST Microwave Studio. Current density at the resonance frequencies of the designed model, reflection coefficients for the polarization converter, polarization conversion ratios, orthogonal components, and phase differences were examined in Excel graphics. The metamaterial designed as substrate Gold (Au), middle layer Rogers RT5880LZ, and top layer vanadium dioxide (VO2)-Au composition, with the Bruggeman effect model, and the dielectric constant of VO2 being taken as ε=9, has a Polarization Conversion Ratio (PCR), of over 90% in the range of 5.29-13.68 THz the relative bandwidth (RBW) was calculated as 88.40%. When the dielectric constant of VO2 is taken as ε=12 with the Drude model, the RBW is calculated as 89.39% with a PCR of over 90% in the range of 5.20-13.61 THz. The designed model is 5.4 μm thick, adaptable, and highly efficient. In addition, a dynamic metamaterial was designed because the conductivity level of VO2 changes according to temperature.
A multifunctional terahertz metamaterial device assisted by vanadium dioxide (VO2) is proposed. Leveraging the property that VO2 can switch from an insulation state to metal state, the device can achieve linear-to-circular (LTC), linear-to-linear (LTL) polarization conversion, and wideband absorption. The investigation findings indicate that when VO2 is in the insulating state, the structure can accomplish the corresponding LTL and LTC polarization conversions in the band of 1.0 -3.1 THz and 3.1–6.1 THz, respectively. When VO2 is in the metallic phase, the device can achieve over 90% absorption from 3.10 THz to 4.95 THz, with the relative bandwidth of 46%. Furthermore, the impacts of structural dimensions, terahertz wave incident angles and polarization angles on the operating characteristics of multifunctional metamaterial structures have also been explored. The reported multifunctional metamaterial device demonstrates promising application in the area of terahertz technology research and intelligent application.
This work presents a design of a solid-core photonic crystal fiber (SC-PCF). The core is made from Topas material, surrounded by three rings of equally sized hexagonal air holes. The finite element technique is employed for numerical analysis with COMSOL Multiphysics software. It is found that with porosity of 28%, the proposed SC-PCF achieved ultra-flatted dispersion through a wide bandwidth of frequency from 0.8 THz to 1.2 THz and negative dispersion “of -43.2958"ps/THz.cm" at 1 THz. Further, the lowest EML for presented PCF at frequency of 1 THz is very low and it is found to be 0.1853 cm−1 with porosity 28% compared to the PCF without porosity. Additionally, the effective area of PCF is significantly improved and the high value of 1.317 × 10−7 m2 is achieved with porosity 28% at 1 THz compared to that of the PCF without porosity which is found to be of 1.615 × 10−5 m2. The above results indicate that the suggested fiber can be utilized in communication systems with minimal losses and it may be used for achieving a coherent supercontinuum generating spectrum.
The rapid advancement of terahertz (THz) technology has driven an increasing demand for efficient THz sources and detectors, particularly in applications such as spectroscopy, imaging, and wireless communications. Organic (NLO) crystals, renowned for their high nonlinear coefficients, tunability, and flexible molecular design, have emerged as highly promising materials for THz generation. This article highlights the latest progress in the design, synthesis, and investigation of novel organic NLO crystals demonstrating exceptional THz activity, with a focus on recent breakthroughs in ionic and molecular crystals. The discussion delves into the pivotal roles of crystal packing, molecular engineering, and functional group modification in optimizing nonlinear optical properties. Furthermore, the article explores strategies for performance enhancement through molecular engineering and functional group modification, offering insights into the mechanisms driving these advancements. Based on cutting‐edge research on advanced NLO crystals, this study examines future research directions and potential applications, emphasizing the critical need for improved crystal growth techniques, refined theoretical modeling, and enhanced material stability. By providing a comprehensive review of the current state of organic THz optical crystals, this article aims to illuminate the challenges and opportunities within this rapidly evolving field, paving the way for future innovations.
High-resolution three-dimensional (3D) tissue analysis has emerged as a transformative innovation in the life sciences, providing detailed insights into the spatial organization and molecular composition of biological tissues. This review begins by tracing the historical milestones that have shaped the development of high-resolution 3D histology, highlighting key breakthroughs that have facilitated the advancement of current technologies. We then systematically categorize the various families of high-resolution 3D histology techniques, discussing their core principles, capabilities, and inherent limitations. These 3D histology techniques include microscopy imaging, tomographic approaches, single-cell and spatial omics, computational methods and 3D tissue reconstruction (e.g. 3D cultures and spheroids). Additionally, we explore a wide range of applications for single-cell 3D histology, demonstrating how single-cell and spatial technologies are being utilized in the fields such as oncology, cardiology, neuroscience, immunology, developmental biology and regenerative medicine. Despite the remarkable progress made in recent years, the field still faces significant challenges, including high barriers to entry, issues with data robustness, ambiguous best practices for experimental design, and a lack of standardization across methodologies. This review offers a thorough analysis of these challenges and presents recommendations to surmount them, with the overarching goal of nurturing ongoing innovation and broader integration of cellular 3D tissue analysis in both biology research and clinical practice.
Achieving active tunability in metasurfaces remains a critical challenge, with conventional local metasurfaces limited by dispersive wavefront deflection and broad resonances that lack spectral selectivity. In contrast, nonlocal metasurfaces exhibit high selectivity, offering a promising platform for dynamic functionality. Here, an active nonlocal metasurface with exceptional spectral and spatial selectivity is experimentally demonstrated, leveraging the physics of bound states in the continuum and coupling phase. The metasurface achieves a deflected beam with a quality factor of 22 and a narrow beamwidth of 5°, focusing energy more precisely than local metasurfaces across both spectral and spatial domains. By integrating a liquid crystal elastomer substrate, tunable azimuthal deflection of 3° with 4.5% in‐plane deformation is realized. Furthermore, the coupling phase introduces polarization‐dependent in‐plane wavevectors, enabling the spatial separation of orthogonal polarization components while maintaining high selectivity and tunability. This active nonlocal metasurface architecture shows strong potential for polarization‐division multiplexing and demultiplexing with low cost and high environmental adaptation, paving the way for advanced terahertz devices, such as signal relays, processors, modulators, and transmitters, for next‐generation wireless communications.
CMOS terahertz detector had a problem of manufacturing variations. In this paper, we proposed a method for simultaneous correction of all pixels that does not interfere with the global shutter operation by mounting a threshold correction mechanism in the pixel circuit. With this collection technique, the σ of the operation points of the pixels was suppressed by 89.4%. We also improved the linearity of the VCO-ADC as the readout circuit, achieving a linearity of +0.43%/-0.37%.
This research presents the design and analysis of a square core photonic crystal fiber (PCF) sensor for the
detection of hazardous chemical compounds, specifically nerve agents such as Soman and Tabun. The proposed
sensor leverages a unique square core geometry and hybrid cladding structure to achieve superior light
confinement and enhanced wave-matter interaction in the terahertz frequency range. Zeonex is utilized as the
fiber material, offering low absorption loss and thermal stability for reliable performance under varying envi-
ronmental conditions. Numerical simulations demonstrate exceptional sensitivity, reaching 99.78 % for Tabun
and 98.75 % for Soman, with ultra-low confinement losses of 6.170 × 10 13 dB/m and 4.467 × 10 13 dB/m,
respectively. Unlike previous PCF-based sensors, this work pioneers the detection of Soman and Tabun by
exploiting their unique spectral signatures in the terahertz domain. The sensor design is compatible with existing
fabrication technologies, ensuring practical implementation for environmental monitoring and chemical safety
applications. This study marks a significant advancement in non-invasive chemical detection, offering a novel
solution for the rapid and precise identification of hazardous substances.
Flexible electromagnetic metamaterials are a potential candidate for the ideal material for electromagnetic control due to their unique physical properties and structure. Flexible electromagnetic metamaterials can be designed to exhibit specific responses to electromagnetic waves within a particular frequency range. Research shows that flexible electromagnetic metamaterials exhibit significant electromagnetic control characteristics in microwave, terahertz, infrared and other frequency bands. It has a wide range of applications in the fields of electromagnetic wave absorption and stealth, antennas and microwave devices, communication information and other fields. In this review, the currently popular fabrication methods of flexible electromagnetic metamaterials are first summarized, highlighting the electromagnetic modulation capability in different frequency bands. Then, the applications of flexible electromagnetic metamaterials in four aspects, namely electromagnetic stealth, temperature modulation, electromagnetic shielding, and wearable sensors, are elaborated and summarized in detail. In addition, this review also discusses the shortcomings and limitations of flexible electromagnetic metamaterials for electromagnetic control. Finally, the conclusion and perspective of the electromagnetic properties of flexible electromagnetic metamaterials are presented.
This paper introduces compact size graphene holographic antenna array (HAA) with polarization diversity for 360° scanning THz imaging applications. The proposed HAA is characterized by high gain and easily reconfigurable beam direction, number and polarization based on graphene. The HAA is composed of two hologram surfaces, each has 3721 elements and is excited by feeder. The unit-cell consists of fractal cross-shaped graphene patch on silicon dioxide dielectric substrate. The surfaces impedance of each element can be varied from 450 Ω to 267 Ω when biased by chemical potential from 0.3 eV to 1.4 eV. 360° electronic beam scanning is achieved via controlling the impedance distribution of the holographic surfaces with peak gain changed from 17 dBi to 10.2 dBi. The upper surface radiates the beam for coverage space from θ = 0° ± 90° around the +z-axis, while the lower surface coverage the space θ = 180° ± 90° around the -z-axis. Two techniques for polarization diversity are investigated. Technique (1) depends on the phase modulation via holographic surface division into sectors. Phase modulation Ns = 4, 8, 12, and 16 sectors are studied with constant gain of 17 dBi. For Ns = 12 sectors, the AR-bandwidth from 0.8 THz to 0.91THz with co-/cross-polar level of −28 dB. In Technique (2), the polarization is controlled by the feeder types single monopole and 2 × 2 sequential monopoles with peak gain of 17 dBi with AR-BW from 0.65 THz to 1.15 THz. The radiation characteristics of the HAA are investigated for single/dual beam with different polarization state and direction on the coverage space. Dual-beam with different polarization LHCP at (θ = 15°, ϕ = 45°), and RHCP (θ = − 25°, ϕ = 90°) with peak gain 14 dBi. Directive CP-beam is radiated in different directions from the upper and lower hologram array surfaces to achieve 360° space coverage.
This finishing chapter contributes an outlook on indispensable industrial, commercial, and related future scenarios of graphene structured quantum dots nanocomposites. In actual fact, the research on the conversion of graphene quantum dots nanocomposites from lab scale models to large scale production designs is still in rudimentary stages. Nevertheless, number of top down and bottom up practices have been found scientifically valuable to form the graphene
quantum dots derived nanocomposites, however implication of these methods for large scale production have certain processing confines. Furthermore, essential ecological needs and economic aspects must be fulfilled to attain future
high tech graphene nanodots resultant nanomaterials. The final design, processing, and performance challenges must be overcome to attain the efficacious commercialization or industrialization of the graphene quantum dots
nanocomposites.
Methanol (CH₃OH) is a volatile, transparent, and toxic substance widely used in chemical substrates, antifreeze, and industrial applications. Ethanol (C₂H₅OH), in contrast, is commonly used in alcoholic beverages, as a fuel additive, and as an antiseptic. Differentiating between methanol and ethanol is critical due to the severe health risks associated with methanol ingestion, while ethanol is safe for consumption in moderation. To tackle this challenge, we present a highly sensitive and accurate Photonic Crystal Fiber (PCF) sensor designed for the detection of both methanol and ethanol. The proposed sensor demonstrates impressive performance, with maximum relative sensitivities (RS) of 95.72 % for methanol and 97.55 % for ethanol, while operating within the 2.2 to 3.2 THz range. Additionally, it achieves low Effective Material Loss (EML) values of 0.0066 cm⁻¹ for ethanol and 0.0044 cm⁻¹ for methanol, with Numerical Aperture (NA) values of 0.257 and 0.270, respectively. The key advantages of this sensor include its exceptional precision, high sensitivity, and low material loss, making it a reliable solution for accurately distinguishing between methanol and ethanol in various industrial and commercial applications. By providing enhanced detection capability in the THz range, this sensor improves safety monitoring and quality control processes in sectors where these substances are frequently used.
An investigation of five different patch-shaped novel graphene-based terahertz (THz) antennas has been conducted to propose a solution to the inherent narrow-bandwidth problem of microstrip patch antennas. The investigated shapes are hetero-hexagonal, homo-hexagonal, pentagonal, square, and circular. The achieved −10 dB bandwidth for the hetero-hexagonal antenna is 20.35 THz, 19.77 THz for the pentagonal and square antenna, 19.70 THz for the homo-hexagonal antenna, and 19.37 THz for the circular antenna. The antenna demonstrates a remarkable operational frequency range of 9.37–30 THz with an excellent S11 of -41.44 dB. The overall fractional bandwidth of the antenna is 104.76% with a bandwidth ratio of 3.2:1. All the proposed antennas possess high radiation efficiency with a value of more than 84% and a very high gain of more than 15.40 dBi. In addition, these antennas are very compact, with a dimension of 55 × 55 μm2. The antenna performance has been further evaluated for Teflon (εr = 2.1), Roger RO 3003 (εr = 3), and Silicon Nitride (εr = 1) substrates, and it has found that the silicon–nitride (Si3N4) substrate works well with the proposed design. Using the CST Microwave Studio, the antenna’s overall performance is assessed. The proposed antenna’s exceptional gain, wide bandwidth, and efficiency make them perfect for high-speed wireless communication and future networks, including 5G/beyond 5G and 6G. These antennas are designed to operate within the THz range, endowing them with significant value in applications including medical imaging, material inspection, and security screening.
Terahertz (THz) radiation (0.3 to 30 THz) fills the crucial gap between the microwave and infrared spectral range. THz technology is important for applications ranging from imaging to telecommunication to biosensing, but these applications often require precise control and manipulation of the THz frequency and polarization state, which typically requires modulators external to the THz source. A hybrid THz emitter that overcomes this limitation by integrating two THz emitters into a single device to enable pulse shaping and chirality control of the emitted radiation without any external components is demonstrated. The two sources are a spintronic emitter (SE) and a semiconductor photoconductive antenna (PCA). The two emitters respond independently to external parameters: the PCA is controlled by the applied bias voltage, while the SE is controlled by the applied magnetic field. Moreover, a dual‐wavelength excitation scheme allows for control of the relative time delay between the THz emission from each constituent. These properties of the hybrid emitter enable precise control of the mixing of the two signals to control the frequency, polarization, and chirality of the overall THz radiation. This on‐chip hybrid emitter thus provides a powerful platform for engineered THz radiation with wide‐ranging potential applications.
The biological effects of electromagnetic field (EMF) irradiation in the terahertz (THz) range remain ambiguous, despite numerous studies that have been conducted. In this paper, the metabolic response of Escherichia coli K 12 to EMF irradiation was examined using a 1.0 W m–2 incident synchrotron source (SS) in the range of 0.5–18.0 THz for over 90 min of continuous exposure at 25 °C. This continuous SS THz exposure induced periodic decreases in the cell growth after 10, 20, and 40 min of exposure compared to a time-matched control; however, the number of viable cells thereafter grew. The physiological status of treated cells immediately after exposure was assessed by using the direct plate counting technique and electron microscopy. Analysis of scanning electron microscopy (SEM) and high-resolution cryogenic transmission electron (cryo-TEM) micrographs showed that approximately 20% of the SS THz-exposed E. coli cells exhibited a deformed outer membrane, membrane perturbations, and leakage of cytosol. The proteome changes in E. coli cells after 18 h postexposure were associated with cellular response to plasma membrane regulation including phospholipid biosynthetic process and osmotic stress. The results of this study highlighted that E. coli cells can promptly activate the fundamental mechanisms in response to prolonged exposure to THz radiation that are evolutionarily developed to withstand other environmental stressors.
Terahertz frequency radiation possesses a unique combination of desirable properties for noninvasive imaging and spectroscopy of materials. This includes the ability to obtain chemical and structural information about substances concealed within dry packaging, such as paper, plastics, and cardboard. As a result, the application of terahertz frequency spectroscopy for the sensing and identification of materials of security interest, such as explosives and, to a lesser extent, drugs-of-abuse, has caught the attention of a number of researchers and security agencies. We describe terahertz time-domain spectroscopy and examine the terahertz spectra of a wide range of drugs-of-abuse, pure explosives, and plastic explosives.
A new temperature performance record of 199.5 K for terahertz quantum cascade lasers is achieved by optimizing the lasing transition oscillator strength of the resonant phonon based three-well design. The optimum oscillator strength of 0.58 was found to be larger than that of the previous record (0.41) by Kumar et al. [Appl. Phys. Lett. 94, 131105 (2009)]. The choice of tunneling barrier thicknesses was determined with a simplified density matrix model, which converged towards higher tunneling coupling strengths than previously explored and nearly perfect alignment of the states across the injection and extraction barriers at the design electric field. At 8 K, the device showed a threshold current density of 1 kA/cm², with a peak output power of ∼ 38 mW, and lasing frequency blue-shifting from 2.6 THz to 2.85 THz with increasing bias. The wavelength blue-shifted to 3.22 THz closer to the maximum operating temperature of 199.5 K, which corresponds to ∼ 1.28ħω/κ B . The voltage dependence of laser frequency is related to the Stark effect of two intersubband transitions and is compared with the simulated gain spectra obtained by a Monte Carlo approach.
Semiconductor devices have become indispensable for generating electromagnetic radiation in everyday applications. Visible and infrared diode lasers are at the core of information technology, and at the other end of the spectrum, microwave and radio-frequency emitters enable wireless communications. But the terahertz region (1-10 THz; 1 THz = 10(12) Hz) between these ranges has remained largely underdeveloped, despite the identification of various possible applications--for example, chemical detection, astronomy and medical imaging. Progress in this area has been hampered by the lack of compact, low-consumption, solid-state terahertz sources. Here we report a monolithic terahertz injection laser that is based on interminiband transitions in the conduction band of a semiconductor (GaAs/AlGaAs) heterostructure. The prototype demonstrated emits a single mode at 4.4 THz, and already shows high output powers of more than 2 mW with low threshold current densities of about a few hundred A cm(-2) up to 50 K. These results are very promising for extending the present laser concept to continuous-wave and high-temperature operation, which would lead to implementation in practical photonic systems.
Terahertz pulse imaging has been used for the first time to study basal cell carcinoma ex vivo, the most common form of skin cancer. This noninvasive technique uses part of the electromagnetic spectrum in the frequency range 0.1-2.7 THz. A total of 21 samples were imaged; the study was performed blind and results were compared to histology. Each image consisted of possible diseased tissue and normal tissue from the same patient. The diseased tissue showed an increase in absorption compared to normal tissue, which is attributed to either an increase in the interstitial water within the diseased tissue or a change in the vibrational modes of water molecules with other functional groups. Seventeen of the images showed a significant difference between the normal and the diseased tissue. These were confirmed by histology to be basal cell carcinomas. Of the remaining four cases, three showed no contrast and were confirmed as blind controls of normal tissue; the fourth case was a suspected basal cell carcinoma but showed no contrast, and histology showed no tumor. Cross-sections of the terahertz images, showing the terahertz absorption, were compared to histology. Regions of increased terahertz absorption agreed well with the location of the tumor sites. Resolutions at 1 THz of 350 microm laterally and 40 microm axially in skin were attainable with our system. These results demonstrate the ability of terahertz pulse imaging to distinguish basal cell carcinoma from normal tissue, and this macroscopic technique may, in the future, help plan surgery.
We report new results suggesting the feasibility of Raman spectrometry as a tool by which to examine the variability of tablet coatings. Our experiments feature a probe that can operate with a revolving laser focus to average content and coating non-uniformity. Raman spectral changes are correlated with tablet exposure times in a pan coater by means of partial least squares (PLS) multivariate analysis. Statistical models are found to be improved by pre-processing schemes that emphasize spectral changes while minimizing the effects of background light scattering and fluorescence. These pre-processing techniques include multiplicative scatter correction (MSC) and standard normal variate (SNV) transformation, used in concert with Savitzky-Golay second derivative smoothing (SGSD). The two approaches give comparable results yielding R2 values for PLS calibration and cross-calibrated prediction variance regression of 0.999 and 0.997, respectively. Correlation results and model residual values demonstrate that Raman spectroscopy serves sensitively to reflect the coating thickness of the tablets studied.
We report the first 10-450 GHz single pixel reflection spectra of
several energetic materials (explosives). These are measured with an
all-electronic reflection spectrometer, and we compare them to those of
common objects and human skin to show differences in dielectric
contrast. This instrument uses microwave sources to drive picosecond
GaAs nonlinear transmission lines and sampling detectors, so it can form
the basis of a fully integrated circuit spectroscopic imaging system for
screening in security applications
Crystallization of active pharmaceutical ingredient (API) or pharmaceutical drug substances is a crucial unit operation. in this paper challenges and scientific progress over the last two decades in the areas of pharmaceutical crystallization thermodynamics, kinetics, polymorphism, process modeling, and process control were attempted. Effective utilization of such scientific knowledge for optimized control of API crystallization process was discussed. A conceptual PAT framework for API crystallization process control was proposed. Three case studies from the literatures were discussed as hypothetical examples to illustrate the application of PAT framework to pharmaceutical crystallization process.
Crystallization of active pharmaceutical ingredient (API) or pharmaceutical drug substances is a crucial unit operation. In this paper challenges and scientific progress over the last two decades in the areas of pharmaceutical crystallization thermodynamics, kinetics, polymorphism, process modeling, and process control were attempted. Effective utilization of such scientific knowledge for optimized control of API crystallization process was discussed. A conceptual PAT framework for API crystallization process control was proposed. Three case studies from the literatures were discussed as hypothetical examples to illustrate the application of PAT framework to pharmaceutical crystallization process.
Preface 1. Introduction 2. Classical propagation 3. Interband absorption 4. Excitons 5. Luminescence 6. Semiconductor quantum wells 7. Free electrons 8. Molecular materials 9. Luminescence centres 10. Phonons 11. Nonlinear optics Appendix A: Electromagnetism in dielectrics Appendix B: Quantum theory of radiative absorption and emission Appendix C: Band theory Appendix D: Semiconductor p-i-n diodes
The authors demonstrate the use of a terahertz quantum cascade laser (QCL) for real-time imaging in transmission mode at a standoff distance of 25 meters. Lasing frequency was selected for optimum transmission within an atmospheric window at ~4.9 THz. Coarse frequency selection was made by design of the QCL gain medium. Finer selection (to within 0.1 THz) was made by judicious choice of laser cavity length to adjust facet losses and therefore lasing threshold bias, in order to overlap the peak frequency of the Stark-shifted gain spectrum with the atmospheric window. Images are shown using an uncooled 320×240 microbolometer camera.
FDA's Process Analytical Technology (PAT) initiative provides an unprecedented opportunity for chemical engineers to play significant roles in the pharmaceutical industry. In this article, the authors provide their perspectives on (1) the need for chemical engineering principles in pharmaceutical development for a thorough process understanding; (2) applications of chemical engineering principles to meet the challenges from the semiconductor and pharmaceutical industries; and (3) the integration of chemical engineering practice into the semiconductor and pharmaceutical industries to achieve process understanding and the desired state of quality-by-design. A real-world case study from the semiconductor industry is presented to demonstrate how a classic chemical engineering concept, mixing homogeneity, can be implemented by inducing forced flow to ensure an excellent copper electrochemical plating process performance and to improve product quality substantially. Further, a case study of brake system design is discussed with the concept of Dr. Taguchi's robust engineering design to illustrate how quality-by-design can be achieved through appropriate experimental design, in conjunction with the discussion on the concept of quality-by-design in pharmaceuticals. Third, a case study of freeze-dried sodium ethacrynate is presented to demonstrate the vital importance of controlling the processing factors to achieve the desired product stability. Finally, the problems of the current pharmaceutical manufacturing mode, the opportunities and engineering challenges during implementation of PAT in the pharmaceutical industry, and the role of chemical engineering in implementation of PAT is discussed in detail.
In this paper we review recent progress in obtaining laser action from semiconductor quantum cascade structures covering the low THz region of the electromagnetic spectrum, from 2 THz (lambda similar or equal to 155 mu m) down to the sub-THz region (lambda > 300 mu m). Particularly, laser active region designs based on bound-to-continuum transition and magnetically assisted intra-well transition are presented. The wide scalability of active region designs is discussed and illustrated with experimental data. Latest results including the demonstration of laser action from quanturn ticterostructure at 950 GHz are presented.
Commercial methylaluminoxane (MAO) solutions were dried under reduced pressure to remove trimethylaluminium (TMA) and solvent. TMA has been added to a solution of such dried MAO, and the response was followed by a special in situ FTIR spectroscopic technique. No reaction was seen between TMA and MAO in toluene solution at room temperature within a time span of up to 4 h TMA and MAO remained in the solution as separate components, as verified by the spectra of the mixtures being superpositions of the spectra of the two components, linear dependence of the main TMA dimer peaks to the added amount, and appearance of isosbestic points. Experiments at 80°C gave the same results.A further important observation is the identification of bridging methyl groups in the TMA-depleted MAO, clearly seen as an IR band at 1257 cm−1. This, in addition to the documented lack of reaction between MAO and TMA, forces a rethinking of the hitherto proposed models of the MAO structure.
Near-infrared (near-IR) spectroscopy was used in the determination of three parameters of theophylline tablets film-coated with ethylcellulose. Spectra of individual intact tablets were collected on two near-IR spectrometers: a grating-based spectrometer, and an acousto-optic tunable filter spectrometer. Calibrations were developed for the prediction of the time to 50% dissolution (t50%) of theophylline for tablets of varying coat thickness, for the determination of the thickness of the ethylcellulose coat applied, and for the prediction of the hardness of coated tablets. Principal component analysis was performed on the spectra prior to calibration development. The standard errors of calibration (SEC) and prediction (SEP) for determination of dissolution rates were 2.8 and 6.6 min, respectively. The SEC for the coating thickness calibration was 0.0002 inches, with an SEP of 0.00024 inches, and the SEC and SEP for the determination of tablet hardness were 0.54 and 0.62 kilopons, respectively.
A device has been invented that produces radiation in the terahertz range. It is a considerable feat of semiconductor fabrication, and could be used in a wide range of applications.
Chemists and engineers involved in the industrial production of solid drugs have to deal with difficult new challenges, including the on-line mastery of the crystal habits and size distribution, the control of polymorphic transitions or the improvement of the chemical purity. A major limitation to improving the control of industrial crystallizers lies in the lack of versatile, accurate and reliable on-line sensors. It is shown that supersaturation measurements can be performed using in situ ATR mid-infrared spectroscopy thus providing valuable real-time information about the crystallization process. Several case studies are presented to illustrate new potential applications of the technique. The reported experimental results outline recent advances in the acquisition of key data characterizing the solute/solvent system in question (i.e. solubility, metastability, phase transformations...), the design of on-line control strategies capable of improving both the crystal size distribution (CSD) and the reproducibility of the quality of the final product, the assessment of improved operating strategies (e.g. seeding batch crystallizers), and the monitoring of polymorphic transitions during cooling crystallization operations. The possibility of evaluating on-line the process impurities, which could allow the reduction of batch-to-batch variations of the quality of the solid product, is also briefly envisaged.
Crystallizations of pharmaceutical active ingredients, particularly those that posses multiple polymorphic forms, are among the most critical and least understood pharmaceutical manufacturing processes. Many process and product failures can be traced to a poor understanding and control of crystallization processes. The Food and Drug Administration's process analytical technology (PAT) initiative is a collaborative effort with industry to introduce new and efficient manufacturing technologies into the pharmaceutical industry. PAT's are systems for design, analysis, and control of manufacturing processes. They aim to assure high quality through timely measurements of critical quality and performance attributes of raw materials, in-process materials, and final products. Implementation of PAT involves scientifically based process design and optimization, appropriate sensor technologies, statistical and information tools (chemometrics), and feedback process control strategies working together to produce quality products. This review introduces the concept of PAT and discusses its application to crystallization processes through review of several case studies. A variety of in situ analytical methods combined with chemometric tools for analysis of multivariate process information provide a basis for future improvements in modeling, simulation, and control of crystallization processes.
Formulators are charged with the responsibility to formulate a product which is physically and chemically stable, manufacturable, and bioavailable. Most drugs exhibit structural polymorphism, and it is preferable to develop the most thermodynamically stable polymorph of the drug to assure reproducible bioavailability of the product over its shelf life under a variety of real-world storage conditions. There are occasional situations in which the development of a metastable crystalline or amorphous form is justified because a medical benefit is achieved. Such situations include those in which a faster dissolution rate or higher concentration are desired, in order to achieve rapid absorption and efficacy, or to achieve acceptable systemic exposure for a low-solubility drug. Another such situation is one in which the drug remains amorphous despite extensive efforts to crystallize it. If there is no particular medical benefit, there is less justification for accepting the risks of intentional development of a metastable crystalline or amorphous form. Whether or not there is medical benefit, the risks associated with development of a metastable form must be mitigated by laboratory work which provides assurance that (a) the largest possible form change will have no substantive effect on product quality or bioavailability, and/or (b) a change will not occur under all reasonable real-world storage conditions, and/or (c) analytical methodology and sampling procedures are in place which assure that a problem will be detected before dosage forms which have compromised quality or bioavailability can reach patients.
An understanding of the finished structure of complex pharmaceutical coating is becoming desirable, because tablet coatings are now one of the preferred routes to control the release of active pharmaceutical ingredients. There are few nondestructive techniques capable of examining the coatings of compressed tablets; for example laser induced breakdown spectroscopy has been used but this is a destructive method. Terahertz pulsed imaging offers a potential technique to examine coatings quickly and nondestructively. In the study reported herein, it was possible to distinguish between two brands of across-the-counter ibuprofen tablets. The terahertz maps obtained were compared with obtained photographs of cut-through sections; there was good agreement. The technique is fast: a waveform can be obtained in <20 ms allowing the technique to be considered as a candidate for on-line or at-line analysis in a process analytical environment. The lateral resolution of the technique is limited by diffraction of the terahertz focus to about 150 microm at 3 THz, whereas the axial resolution is limited by the terahertz pulse duration, which is <200 fs, to about 30 microm.
Near-infrared spectroscopy (NIRS) has become a widely used analytical technique in the pharmaceutical industry, serving for example to determine the active substance or water content of tablets. Its great advantage lies in the minimal sample preparation required and speed of measurement. In a study designed to detect the effects of process on tablet dissolution, we describe the application of NIRS to the detection and identification of changes in uncoated and coated tablets in response to pilot-scale changes in process parameters during melt granulation, compression, and coating. Beginning with a qualitative comparison between pharmaceutical batches, we show that NIRS and principal component analysis can separate batches produced with different melt granulation parameters and differentiate between cores compressed with different compaction forces. Complementary infrared imaging can also explain the difference in dissolution properties between samples produced with different melt granulation parameters. NIRS is sensitive to changes in coating formulation, the quality of a coating excipient (hydroxypropyl methylcellulose), and coating time. In a concluding quantitative analysis, we demonstrate the feasibility of NIRS in a manufacturing context for predicting coating time and detecting production cores failing to meet dissolution test specifications.
We report a novel approach to the measurement of colored tablet coating thickness, which employs Raman spectroscopy with univariate and multivariate data analysis. Our results suggest that Raman sensing can serve as a viable non-invasive means to quantify tablet coating thickness in the presence of a fluorescent ingredient in the coating formulation (food colorant Alphazurine FG or D&C Blue No. 4). This study comparatively tests the advantage of several data transformation approaches, including mean centering, standard normal variate, and Savitzky-Golay smoothed second derivative as means of improving predictive models in the presence of fluorescence. By application of the partial least squares (PLS) calibration algorithm to establish optimum covariance between transformed spectral data and measured tablet coating thicknesses, we have been able to create predictive models with calibration errors as small as 4 microm for a training set that spans colored coating thicknesses from 50 to 151 microm.
Three dimensional terahertz pulsed imaging (TPI) was evaluated as a novel tool for the nondestructive characterization of different solid oral dosage forms. The time-domain reflection signal of coherent pulsed light in the far infrared was used to investigate film-coated tablets, sugar-coated tablets, multilayered controlled release tablets, and soft gelatin capsules. It is possible to determine the spatial and statistical distribution of coating thickness in single and multiple coated products using 3D TPI. The measurements are nondestructive even for layers buried underneath other coating structures. The internal structure of coating materials can be analyzed. As the terahertz signal penetrates up to 3 mm into the dosage form interfaces between layers in multilayered tablets can be investigated. In soft gelatin capsules it is possible to measure the thickness of the gelatin layer and to characterize the seal between the gelatin layers for quality control. TPI is a unique approach for the nondestructive characterization and quality control of solid dosage forms. The measurements are fast and fully automated with the potential for much wider application of the technique in the process analytical technology scheme.
Abuse of amphetamine-type stimulant (ATS) is increased in Japan. Most ATS has optical isomers and their stimulatory effects are different between optical active forms and racemic form. We applied terahertz spectroscopy to differentiate the optical active form and racemic form in solid phase. Most of spectra of two different forms showed different spectral pattern and terahertz spectroscopy can be useful for differentiation of optical isomers from racemic form.
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Fig. 4 e Terahertz image of men with hidden knife. 33
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