Katsuhiro Ajito

• Ph D
• Dean, Professor, katsuhiro.ajito.jp@ieee.org at International Professional University of Technology in Osaka

Recent studies of saccharides' peculiar anti-freezing and anti-dehydration properties point to a close association with their strong hydration capability and destructuring effect on the hydrogen bond (HB) network of bulk water. The underlying mechanisms are, however, not well understood. In this respect, examination of the complex dielectric consta...

In situ Raman spectroscopic measurements with 785 nm excitation were carried out in aqueous solutions containing bipyridine derivatives. Intense Raman signals were observed when the Ag dimer structure was optimized. The SERS activity was dependent upon on the structure of the Ag dimer with a distinct gap distance, suggesting that the intense SERS o...

Sodium chloride (NaCl) aqueous solution becomes NaCl hydrate, NaCl·2H2O, at low temperature, which is different from potassium chloride and is a typical complex model for studying the freeze-drying process in foods and pharmaceuticals. Here, we detected unit-cell-sized NaCl particles in ice as precursor substances of NaCl·2H2O during freezing of Na...

Whether urea can serve as a kosmotrope or chaotrope has long been a topic of debate. In this study, broadband THz spectroscopy (from 0.2 to 12 THz) of aqueous solutions of urea was used to characterize the hydration state and the hydrogen bond structure of water around urea. Three low-frequency vibration modes of urea were found around 2 THz, 4 THz...

10の11乗から13乗Hzの周波数をもつテラヘルツ（ＴＨｚ）波は光学的不透明な材料への透明性やイメージングにおけるサブミリメートルの空間分解、さらに水素結合や水和など分子間相互作用がテラヘルツ帯のエネルギー準位にあたるため、その分子間指紋を識別することに利用できる。テラヘルツ分光法、分光システム、スペクトル解析方法、そして医薬品分析を主とした応用例として、分子種、結晶多形、共結晶の識別や定量分析について解説する。

Modification of the water hydrogen bond network imposed by disaccharides is known to serve as a bioprotective agent in living organisms, though its comprehensive understanding is still yet to be reached. In this study, aiming to characterize the dynamical slowing down and destructuring effect of disaccharides, we performed broadband dielectric spec...

The Raman spectroscopy of pharmaceutical cocrystals based on caffeine and oxalic acid in nanosized pores of mesoporous silica has been demonstrated at various molar amounts. The Raman peak shifts of caffeine molecules express the existence of pharmaceutical cocrystals in mesoporous silica. The molar amount dependence of the peak shifts describes th...

Terahertz (THz) spectroscopy methods, instrumentation, and valuable applications are introduced. THz spectroscopy can detect molecular interactions in compounds formed by hydrogen bonds and hydration reactions. Although improvements are required in many respects, quantitative analyses based on THz spectra point to directions in the application of T...

The Raman spectroscopy of pharmaceutical cocrystals based on caffeine and oxalic acid in nanosized pores of mesoporous silica has been demonstrated at various molar amounts. The Raman peak shifts of caffeine molecules express the existence of pharmaceutical cocrystals in mesoporous silica. The molar amount dependence of the peak shifts describes th...

Terahertz (THz) spectroscopy can detect molecular interactions in compounds formed by hydrogen bonds and hydra-tion reactions. Although improvements are required in many respects , we are seeing encouraging progress in quantitative analyses based on THz spectra, and promising directions in the application of THz imaging to pharmaceuticals, the life...

We present a compact and stable continuous-wave terahertz spectroscopic system with a planar lightwave circuit-LiNbO 3 phase modulator for terahertz phase control. Thanks to significantly shortened optical paths with temperature control, the system enables fast vector measurement with effective phase stabilization. The phase stability of 2° was obt...

We demonstrate that the photonics-based self-heterodyne system enables us to measure the phase change of the continuous THz-wave with the theoretical uncertainties limited by the signal-to-noise ratio (SNR) of the amplitude measurement. When the SNR was 44 dB, the standard deviation of the phase measurement was 0.37 degree, which is 10 times better...

We have constructed a continuous wave (CW) terahertz spectroscopy system using photonic integration-compatible technology. This system is intended for use in the medical field as a miniaturized terahertz spectroscopy system, in contrast to the conventional pulsed wave method. This system can simultaneously measure the absorption and phase (dielectr...

Terahertz (THz) spectroscopy is a powerful tool for identifying inter-molecular hydrogen bonds in pharmaceutical materials. Here, we present quantitative chemical imaging of multicomponent pellets composed of pharmaceutical cocrystals and cellulose by THz spectroscopy. The stabilized THz time-domain spectroscope with laser beam monitoring and feedb...

We demonstrate a self-heterodyne terahertz (THz) frequency-domain spectrometer in which commercially available uni-traveling-carrier photodiodes (UTC-PDs) are used as optical-to-electrical (O/E) converter and optoelectronic mixer (OEM). The optical intensity beat generated by mixing two frequency-detuned free-running 1.55 $mu$m laser diodes (LDs) i...

This paper presents 300-GHz step-profiled corrugated horn antennas, aiming at their integration in low-temperature co-fired ceramic (LTCC) packages. Using substrate integrated waveguide technology, the cavity inside the multi-layer LTCC substrate and a surrounding via fence are used to form a feeding hollow waveguide and horn structure. Owing to th...

Molecular networks based on noncovalent bonds have resonant frequencies in the terahertz (THz) region. THz spectroscopy is a powerful tool for identifying molecular bonds, such as intermolecular or intramolecular hydrogen bonds, in pharmaceuticals. A THz chemical imaging (TCI) system was developed by combining a THz time-domain spectrometer with a...

Terahertz radio waves have about the same energy level as the non-covalent bonds between molecules, so they can be used in a spectroscopic technique for identifying differences in molecular networks. We introduce terahertz spectroscopy as a revolutionary technique for visualizing the molecular bonds in various forms of pharmaceutical crystals such...

We present a compact and stable terahertz (THz) vector spectroscopy system using silicon photonics technology. A silicon-based integrated phase control circuit greatly reduces the physical size of the continuous-wave THz spectroscopy system and enhances environmental phase stability. Differential lightwave phase control using two carrier-injection...

We demonstrate direct quadrature modulator and demodulator monolithic microwave integrated circuits for future terahertz communications at 300 GHz based on the quadrature phase-shift keying (QPSK) modulation format. For the modulating and demodulating signal, we employed half-Gilbert cell mixers, which provide balanced signaling and moderate perfor...

We present a continuous-wave terahertz (THz) vector spectroscopy and imaging system based on a 1.5-μm fiber optic unitraveling- carrier photodiode and InGaAs photo-conductive receiver. Using electro-optic (EO) phase modulators for THz phase control with shortened optical paths, the system achieves fast vector measurement with effective phase stabil...

A 120-GHz-band close-proximity wireless system that achieves the first 10-Gbit/s real-time transmission satisfying the regulation for extremely low-power radio stations (ELRS regulation). The use of photonic technologies enables the transmitter to perform 10-Gbit/s quadrature-phase-shift keying (QPSK) modulation and generate an RF spectrum with neg...

A 120-GHz-band close-proximity wireless system that achieves the first 10-Gbit/s real-time transmission satisfying the regulation for extremely low-power radio stations (ELRS regulation). The use of photonic technologies enables the transmitter to perform 10-Gbit/s quadrature-phase-shift keying (QPSK) modulation and generate an RF spectrum with neg...

Molecular networks based on noncovalent bonds have resonant frequencies in the terahertz (THz) region. THz spectroscopy is a powerful tool for identifying molecular bonds, such as intermolecular or intramolecular hydrogen bonds, in pharmaceuticals. A THz chemical imaging (TCI) system was developed by combining a THz time-domain spectrometer with a...

This paper presents a terahertz (THz) tomographic imaging system using monochromatic 400~800-GHz signals, which have been generated and swept by photonic techniques. Measurable thickness of plastic plates has been experimentally verified as small as 0.37 mm, which corresponds to a depth resolution of 0.61 mm in the air.

Download Citation Email Print Request Permissions Save to Project This paper presents 300-GHz horn antennas aiming at their integration in low-temperature co-fired ceramic (LTCC) packages. Using substrate integrated waveguide technology, we fabricated vertical horn antennas with an LTCC multi-layer process. A cavity inside the multi-layer LTCC sub...

We demonstrate a self-heterodyne terahertz (THz) frequency-domain spectrometer in which commercially available uni-traveling-carrier photodiodes (UTC-PDs) are used as an optical-to-electrical (O/E) converter and a photonic THz mixer. Optical intensity beat generated by mixing two frequency-detuned 1.55 μm laser diodes is used as a source for the ph...

We present a phase-sensitive continuous-wave THz homodyne spectroscopy system for simultaneous measurement of the THz intensity and phase responses in frequency domain. Using this system, complex permittivity evaluation of dielectric materials was successfully demonstrated.

We present a 325 GHz quadrature voltage controlled oscillator (QVCO) using 0.25 InP HBT technology and su-perharmonic coupling of two differential Colpitts VCOs. The indi-vidual VCOs exhibit about single-ended output power and 10 GHz frequency tuning range. The quadrature oscillation is successfully demonstrated with dc power consumption of 92.4 mW...

We present a 325 GHz quadrature voltage controlled oscillator (QVCO) using 0.25 InP HBT technology and su-perharmonic coupling of two differential Colpitts VCOs. The indi-vidual VCOs exhibit about single-ended output power and 10 GHz frequency tuning range. The quadrature oscillation is successfully demonstrated with dc power consumption of 92.4 mW...

An ASK receiver MMIC operating at 300 GHz for future terahertz communications is presented. In the receiver IC, we fully integrated all necessary components-a receiving dipole antenna, high gain RF amplifier, envelop detector for demodulating ASK signal and output differential data amplifier-in a 1000×2500 μm2 area. A silicon lens was used to compe...

Molecular networks based on hydrogen bonds have resonant frequencies in the terahertz (THz) region. THz spectroscopy is a powerful tool for identifying molecular networks created by intermolecular or intra-molecular hydrogen bonds in pharmaceutical and biomedical samples such as drugs, proteins, and cancer cells. A THz chemical imaging (TCI) system...

Terahertz (THz) spectroscopic imaging is a promising technique for distinguishing pharmaceuticals of similar molecular composition but differing crystal structures. Physicochemical properties, for instance bioavailability, are manipulated by altering a drug's crystal structure through methods such as cocrystallisation. Cocrystals are molecular comp...

We present a direct quadrature modulator MMIC based on InP HBTs for generating QPSK signals at 300 GHz. The modulator consists of two single balanced mixers and passive baluns and couplers. In this work, two different transmission line structures - thin-film microstrip lines and inverted microstrip lines - were simultaneously utilized in the single...

We present a 300-GHz-band fundamental voltage controlled oscillator (VCO) for wireless communications using 0.25-μm InP HBT technology. The VCO exhibits about -2-dBm differential output power and 10-GHz frequency tuning range with dc power consumption of 46.2 mW. The oscillation frequency band of the VCO can be extended over 360 GHz in the same str...

We propose and demonstrate a technique for phase change measurement of a continuous terahertz (THz) wave, based on 1.55-µm photonic techniques. The generation of the THz wave is based on photomixing using a uni-traveling-carrier photodiode. The detection of the THz wave is based on heterodyne mixing of the THz wave, using a low-temperature-grown Ga...

We present a continuous-wave coherent homodyne detection system based on terahertz (THz) phase control using balanced electro-optic phase modulation for high dynamic range. Phase-sensitive and fast THz measurement is demonstrated with a dynamic range of over 55 dB in 1-ms integration time.

This paper presents three-dimensional tomographic imaging using swept-source optical-coherence-tomography (SS-OCT) techniques in the terahertz (THz) region. The system employs photonically generated frequency-tunable THz signals. Operational principle and system configuration of the THz SS-OCT are described, and the first successful tomographic ima...

Three-dimensional (3D) terahertz (THz) imaging or THz tomography has recently proven to be powerful for non-de-structive testing of industrial materials and structures. In order to reduce complexity and cost of conventional THz tomography systems, we propose a new approach using broadband THz noise sources based on amplified spontaneous emission no...

Presented is 24 Gbit/s wireless data transmission at 300 GHz using a uni-travelling carrier photodiode (UTC-PD) emitter and Schottky barrier diode detector, which were designed and fabricated for larger bandwidth. Both the emitter and the detector were fabricated on the same epi-layer of the UTC-PD. At the link distance of around 50 cm, a bit error...

In this letter, we demonstrate over 1 mW power generation at 300 GHz with a uni-travelling-carrier photodiode (UTC-PD) packaged in a WR-3 waveguide module. To increase the maximum power, two identical UTC-PDs were monolithically integrated along with a T-junction to combine the power from the two PDs. The UTC-PD module exhibited peak saturated outp...

Terahertz (THz) absorption spectra have been applied to measure the compositions of amino acid samples. Because the concentration information about a specific composition is involved in its spectrum, keeping the spectrum length and dynamic range as wide as possible is essential for precise measurements. It is well known that a spectrum is always co...

Terahertz spectroscopy allows the detection and identification of various molecular networks of chemical bond interactions, including those of hydrogen bonds. In this article, we explain terahertz timedomain spectroscopy, the principles of terahertz chemical imaging, and a multicomponent quantitative analysis method for amino acids that can visuali...

Terahertz spectroscopy allows the detection and identification of various molecular networks of chemical bond interactions, including those of hydrogen bonds. In this article, we explain terahertz timedomain spectroscopy, the principles of terahertz chemical imaging, and a multicomponent quantitative analysis method for amino acids that can visuali...

テラヘルツ分光法は水素結合や水和結合，そのほか分子ネットワークに関 する化学結合などに関して，分子間の相互作用の検出や同定を可能にします． 最近の超短パルスレーザや光伝導アンテナなどのテラヘルツ部品の性能向上によって，テラヘルツ時間領域分光法（THz-TDS）の感度が向上し，テラヘルツ化学イメージング（TCI）が可能となってきました．本稿ではTHz-TDSおよびTCIの原理について説明し，アミノ酸を例に多成分の定量分析法について解説します．さらに，その応用例として医薬品の結晶多形の可視化技術について紹介します．

We demonstrate homodyne detection of microwaves using low-temperature-grown (LTG) GaAs with 1.55-µm optical local oscillator (LO). The sensitivity of the detection is almost proportional to the average power of the optical LO, which indicates that the photoresponse of the LTG-GaAs is linear process in our case. The dynamic range of the detection is...

We present a potential of photonic technologies originally developed for fiber-optic communications for use in contemporary terahertz-wave applications and demonstrate several examples, including a high-precision time-continuous terahertz-wave signal and terahertz-wave band noise generators, sensing and imaging with CW and noise signals, and wirele...

THz spectroscopy is advantageous in analytical chemistry because it can detect and identify intermolecular interactions in chemical compounds, such as hydrogen bonds and hydrations, and molecular networks. Recent advances in THz components, such as ultrashort pulsed lasers and photoconductive antennas, have improved the sensitivity of THz time-doma...

A terahertz spectroscopic imaging system was fabricated for the detection of two-dimensional molecular distributions in pharmaceutical tablets based on absorption peaks of intermolecular or intramolecular hydrogen bonds in crystals. The terahertz imaging system consists of a terahertz time-domain spectrometer and a three-dimension translational sta...

A terahertz (THz) chemical imaging system was fabricated for the detection of two-dimensional molecular networks in pharmaceutical tablets. Frequency-dependent THz images of homemade tablets containing polymorphic forms enables us to determine the distribution of two polymorphic forms. The system also provided THz images of polymorphic form distrib...

We successfully analyzed the concentrations of five amino acids in commercially available dietary amino acid supplements by using terahertz time-domain spectroscopy (THz-TDS) with an error of ± 12% for the best reproduced components. We also succeeded in analyzing tablets of the supplements wrapped in paper, and thus showed the merit of using THz w...

An underpinning software is devoted to THz spectrum analyzing and 3-D imaging. The paper describes the software's outline, structure, functions and some of considerations. Users in LAN (local area network) can access it and implement some basic and advanced works such as files operation, echoes cutting, spectrum calculation, baseline cancelling, pe...

We present a terahertz wave wireless link operating at 300 GHz which has a potential for use in ultra fast future wireless services in short range. Terahertz wave was generated and modulated with photonic technologies in the transmitter, allowing us to use radio on fiber system concept as well. For the receiver, we used a Schottky barrier diode det...

This paper describes our recent challenges for ultrahigh-speed wireless communications technologies using millimeter and terahertz waves. Instrument-limited 14-Gbit/s transmission has been demonstrated at 300 GHz.

Spectroscopic images were obtained using a system combining a tunable continuous-wave terahertz laser with an InP-based Schottky barrier diode detector integrating a broadband log-periodic antenna. The distribution of polymorphic forms in pharmaceutical tablets was observed in obtained at images over 1 THz.

This paper discusses practical applications of the frequency band starting at 100 Ghz-a band that has yet to be fully exploited. It considers the feasibility of future applications driven by the sub-terahertz and terahertz bands, examining this feasibility from two perspectives: (1) market needs and the standards being released to meet these needs...

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Photodiodes have been commonly used for generation of continuous terahertz (THz) waves. In this paper, we proposed the use of photodiodes also for detection of THz waves in order to realize CW THz spectroscopy system based on 1.55-μm fiber-optics. We experimentally demonstrated two detection schemes based on the square-law detection and downconvers...

8 Gbit/s wireless data transmission using a 250 GHz millimetre-wave carrier signal is demonstrated. Photonic devices and technologies, including an electro-optic modulator and J-band uni-travelling-carrier photodetector (UTC-PD) module, were used for the transmitter. In addition, a high-efficiency Schottky barrier diode detector monolithically inte...

This paper presents photonic generation of high-power continuous terahertz-wave signals at frequencies from 300 GHz to 400 GHz, and their application to giga-bit wireless communication systems.

This paper describes a novel imaging system using incoherent millimeter waves generated by photonic techniques, which improves an image quality by reducing interference effect.

Error-free data transmission at 8 Gbps in the 250-GHz millimeter-wave band over 0.5-m distance was demonstrated. A J-band uni-travelling-carrier photodetector (UTC-PD) module and an InP-Schottky barrier diode detector integrated with a slot ring antenna and an output low-pass filter were used for the transmitter and receiver, respectively. Taking t...

In this article, we introduce the lates terahertz spectroscopy technology. Recent advances such us ultrashort pulsed lasers have dramatically improved measurement lime and sensitivity, making it possi-ble- to investigate phenomena, such as molecular networks, which have previously been difficult to study. The last few years have seen rapid progress...

Recent progress in analytical terahertz (THz) spectroscopy is reviewed with illustrative examples showing that it is an effective method for detecting and identifying intermolecular interactions in chemical compounds, such as hydrogen bonds. The unique and characteristic properties of THz waves, their significance to both science and industry, and...

The spatial resolution of transmitted terahertz (THz) images of metal slits and protein [myoglobin (Mb)] thin films was analyzed at 1.40, 1.63, and 2.55 THz by using continuous THz waves from a tunable gas laser. The resolution of the THz images of the metal slit was 0.40, 0.35, and 0.45 mm at 2.55, 1.63, and 1.40 THz, respectively. The spatial res...

Terahertz time-domain spectroscopy (THz-TDS) is used to study the intra- and intermolecular vibrational modes of aromatic carboxylic acids, for example, o-phthalic acid, benzoic acid, and salicylic acid, which form either intra- or intermolecular hydrogen bond(s) in different ways. Incorporating the target molecules in nano-sized spaces in mesoporo...

The hydrogen bond resonance of a sodium chloride (NaCl) ion pair trapped in aqueous ice has been observed by transmission terahertz time-domain spectroscopy. The absorption peak of a sodium chloride ion pair in ice is 1.65 THz at 83 K. By investigating the interaction of the cation and anion with other chemical compounds, we deduce that the absorpt...

We have generated terahertz (THz) waves by difference frequency generation (DFG) with a 1.55 μm pump laser. 1.55 μm pump beams were effectively converted to 1 THz waves through a periodically χ(2)-inverted GaP stack fabricated by direct wafer bonding. The output efficiency was an order of magnitude larger than that of typical χ(2) materials such as...

Novel approach for the detection of small number of molecules was investigated using metal coupled dot structures showing strong surface-enhanced Raman scattering (SERS). The SERS activities of Ag and Au nano-dot array were controlled via choosing appropriate gap distance between metal nano-dots and their structural anisotropy. Intense signals due...

The measurement of absorption spectra using angle-dependent terahertz (THz) time-domain spectroscopy for amino acid single crystals of l-cysteine and l-histidine is reported for the first time. Linearly polarized THz radiation enables us to observe angle-dependent far-infrared absorption spectra of amino acid single crystals and determine the direc...

We propose a novel method to realize widely tunable terahertz-wave generation from GaP by collinearly phase-matched difference-frequency generation. Our method makes it possible to achieve a wavelength-tunable output without decreasing the output efficiency, which can be used for wavelength sweeping in spectroscopy.

The inter- and intramolecular hydrogen-bonding modes of two steric isomers, namely fumaric acid and maleic acid, are observed by using terahertz time-domain spectroscopy (THz-TDS). The intermolecular modes observed with fumaric acid crystals are inactivated by incorporating the molecules in the nano-sized pores of a mesoporous silicate (SBA-16) due...

The quantitative analysis of amino acids by terahertz (THz) time-domain absorption spectroscopy is demonstrated. The optical densities of the amino acids were found to be linearly proportional to the concentration. The molar absorption coefficients of L-glutamic acid (L-Glu), L-glutamic acid sodium salt (Na-L-Glu), L-glutamic acid hydrochloric salt...

We propose a THz notch filter that uses the absorption of the intermolecular hydrogen bonds in a molecular crystal such as sucrose. Terahertz (THz) time-domain spectroscopy was used to investigate the absorption characteristics of a rotationally oriented sucrose single crystal in the 0.3–3.0 THz frequency range. The crystal was set so that the (100...

We have realized a terahertz (THz)-wave source employing difference frequency generation (DFG) from a quasi-phase-matched GaP stack pumped at 1.55 mum. We observed THz waves with enhanced power by quasi-phase matching (QPM) in the direction of GaP with a polarization direction for the incidence of two pump lights with the same propagation and polar...

Near-infrared SERS activity of the Ag film under electric polarization was evaluated in aqueous solution containing 1 mM glutamic acid. Spectra were obtained in situ from the near infrared laser Raman microscope system with an excitation wavelength of 785 nm. Intensity of the SERS increased significantly upon application of an external electric fie...

The near infrared surface-enhanced Raman scattering (NTR-SERS) activity of an Ag on Au thin film on an insulating substrate was controlled by application of an external field to the thin metal films in solution. In the case of glutamic acid molecules adsorbed on the surface of nano-structure-controlled films, the empirical signal enhancement factor...