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Sub-wavelength microscopy techniques in the TeraHertz frequency range
Abstract
Imaging in the Terahertz frequency range at subwavelength resolution has gained a great interest for certain studies which cannot be carried out with other parts of the electromagnetic spectrum. However, classical optical schemes cannot be employed to obtain micrometre-range resolution for THz microscopy as diffraction limits the resolution to about 100 µm. In this thesis, we present two different original subwavelength THz microscopy techniques. In the first technique, the THz beam is screened by a thin metallic sheet in which a subwavelength hole has been made. The sample is placed against the sheet and moved over the hole to perform a raster image. The expected resolution is then equal to the hole size. The second technique presented in this thesis is based on generated a THz signal directly from the sample. When a laser beam is focused in the sample, the illuminated region, if non-centrosymmetric, can generate THz signals through optical rectification. The raster image is obtained by recording this THz signal while the laser beam is moved over the sample. The expected resolution is then close to the laser spot size.Both technique might involve weak THz signals. That is why we investigated on the possibility to measure them with a very sensitive detector, usually used for astronomy, named kinetic inductance detector (KID). This manuscript presents its principle as well as the study that was carried on. On a “classical” time domain spectroscopy setup, signal as low as 0.2 fW were thus recorded, demonstrating the interest of such detectors.The two last chapters are dedicated to the two microscopy techniques themselves. For the first one, a simulation model using a finite element model solver is used to design the most efficient aperture to enhance the transmission through a subwavelength hole. The results show that a conically tapered hole has a higher transmission than a classical cylindrical hole. Our attempts at using the KIDs camera for the first time for THz microscopy are discussed and first encouraging results are presented.Finally, the ORTI (Optical Rectification Terahertz Imaging) technique is investigated. An image with a 10 µm spatial resolution (λ/214 for 0.14 THz) was obtained while scanning the ferroelectric domains of a crystal of PPKTP. We show that the resolution of the image depends only on the laser spot size and not on the generated THz frequency. In addition, we showed that ORTI image can be used to scan a poly-crystalline sample as well as a crystal with different thickness areas. Lastly, the limitations of the spatial resolution of ORTI images are discussed in detail.
... where n 0 is the refractive index of the EO crystal at the frequency of the optical beam, L is the thickness of the EO crystal, r i j is one unit EO coefficient in standard crystallographic coordinate system, which varies for different crystal classes, ω and c are the angular frequency and the speed of light for the optical beam, respectively [163]. The balanced photodiodes measure intensities of x and y components : ...
Due to the wide range of applications of discharges and plasmas, their characterizations have been receiving a revival of interest these last couple of years. Among all the relevant characteristics of a plasma, the spatial distribution and the temporal evolution its associated electric field (E-field) is a subject of major interest and one of the more critical parameters to be analyzed. Firstly, this thesis work concerns the non-invasive vector characterization of the electric field associated to cold plasmas (dielectric barrier discharge, atmospheric plasma jet, ...). The other part is devoted to the nonlinear interaction between a cold plasma and an optical pulse wave for the parametric generation of Terahertz (THz) waves. In both cases (plasma generated with a high voltage source and plasma due to air breakdown), we have detected an electric field (of the order of MV/m for plasma analysis and of a few V/m for high frequency generation) either with a pigtailed electro-optical probe, or by using an experimental free space EO bench (developped at IMEP-LAHC). We have carried out a complete polarimetric study of the E-field generated by electrodes or by ionization of a gas like helium. In particular, we have been interested in the polarization behavior of the observed phenomena. We also studied the possibility to improve the efficiency of the THz generation by adding a helium plasma jet or the gas alone. We have performed a complete characterization of the generated THz signal (in modulus and orientation). The obtained results are perfectly described by a model that we have developed. This model has been experimentally validated by a polarimetric study of the THz pulse using an innovative EO detection.
In this paper, we study terahertz generation through optical rectification in reflection at normal incidence in a dielectric nonlinear crystal. We first analyze, with a nonlinear optical model, the sample parameters (thickness, absorption at both laser and terahertz wavelengths, etc.) for which a terahertz optical rectification reflection scheme is preferable to the common transmission scheme. Then, we report our experimental observations of a reflected terahertz signal generated at the surface of a ZnTe crystal. The reflected terahertz signal shares all the characteristics of a signal generated in transmission but is not limited by absorption losses in the crystal, thereby providing a broader bandwidth. At high pump laser power, the signal exhibits saturation, which is caused by the decrease of the nonlinear susceptibility due to photocarriers generated by two-photon absorption. This reflection scheme could be of great importance for terahertz microscopy of opaque materials like, e.g., humid samples or samples exhibiting strong absorption bands or to study samples for which the transmitted signal cannot be recorded.
By focusing a femtosecond laser beam onto the lateral face of a periodically poled KTP crystal, we generate a THz pulse through optical rectification of the laser pulse. The THz signal is recorded by means of a common THz time-domain technique, which allows us to obtain both the magnitude and sign of the THz waveform, and then the magnitude and phase of its spectral components thanks to a numerical Fourier-transform. By moving the laser beam along the crystal, we record a THz signal that renders for the alternative orientation of the crystal domains, with a lateral resolution as good as 10 μm, whatever the THz wavelength. This demonstrates the potential of the Optical Rectification TeraHertz Imaging (ORTI) technique to produce sub-wavelength THz images.
Physics is the interplay of energy and matter. Energy, in the form of light, interacting with matter, principally in the solid state, underpins this topical review. The subject is developed carefully and methodically, beginning with basic definitions pertaining to terahertz detectors and terahertz radiation, then proceeding systematically to delineate characteristics of terahertz photons and terahertz detectors in more detail. In-between, the intimate connection linking terahertz sensors and terahertz sources is highlighted—an important aspect unfortunately often overlooked or ignored when terahertz detectors are discussed in isolation. At the centre of this topical review are the various physical mechanisms by which electromagnetic radiation of terahertz frequencies interacts with matter. The logic is to present first the underlying physical principles of detection before presenting the practical implementation in a specific device, rather than the other way around. A taxonomy of terahertz detectors is then proposed based on the underlying physical principles of detection. Following on from this classification, state-of-the-art terahertz detectors are surveyed and appraised; this overview constitutes the longest section of the review. Key detector parameters which inform applications are then presented and tabulated. Finally, the present state-of-the-art is anchored within the wider scientific context of historical developments and future prospects.
Ferroelectric materials have been utilized in a broad range of electronic, optical, and electromechanical applications and hold the promise for the design of future high-density nonvolatile memories and multifunctional nano-devices. The applications of ferroelectric materials stem from the ability to switch polarized domains by applying an electric field, and therefore a fundamental understanding of the switching dynamics is critical for design of practical devices. In this review, we summarize the progress in the study of the microscopic process of ferroelectric domain switching using recently developed in situ transmission electron microscopy (TEM). We first briefly introduce the instrumentation, experimental procedures, imaging mechanisms, and analytical methods of the state-of-the-art in situ TEM techniques. The application of these techniques to studying a wide range of complex switching phenomena, including domain nucleation, domain wall motion, domain relaxation, domain-defect interaction, and the interplay between different types of domains, is demonstrated. The underlying physics of these dynamic processes are discussed.
Since its inception more than 25 years ago, Piezoresponse Force Microscopy (PFM) has become one of the mainstream techniques in the field of nanoferroic materials. This review describes the evolution of PFM from an imaging technique to a set of advanced methods, which have played a critical role in launching new areas of ferroic research, such as multiferroic devices and domain wall nanoelectronics. The paper reviews the impact of advanced PFM modes concerning the discovery and scientific understanding of novel nanoferroic phenomena and discusses challenges associated with the correct interpretation of PFM data. In conclusion, it offers an outlook for future trends and developments in PFM.
Organic crystals with second-order optical nonlinearity feature very high and ultra-fast optical nonlinearities and are therefore attractive for various photonics applications. During the last decade, they have been found particularly attractive for terahertz (THz) photonics. This is mainly due to the very intense and ultra-broadband THz-wave generation possible with these crystals. We review recent progress and challenges in the development of organic crystalline materials for THz-wave generation and detection applications. We discuss their structure, intrinsic properties, and advantages compared to inorganic alternatives. The characteristic properties of the most widely employed organic crystals at present, such as DAST, DSTMS, OH1, HMQ-TMS, and BNA are analyzed and compared. We summarize the most important principles for THz-wave generation and detection, as well as organic THz-system configurations based on either difference-frequency generation or optical rectification. In addition, we give state-of-the-art examples of very intense and ultra-broadband THz systems that rely on organic crystals. Finally, we present some recent breakthrough demonstrations in nonlinear THz photonics enabled by very intense organic crystalline THz sources, as well as examples of THz spectroscopy and THz imaging using organic crystals as THz sources for various scientific and technological applications.
We report on the detection of weak terahertz pulsed signals, generated by either a commercial photoconductive antenna or an electro-optic crystal pumped by femtosecond laser pulses, using high-sensitive kinetic inductance detectors. Average powers as low as 0.1 fW have been recorded. We observe an unexpected constant signal when the bias voltage of the photoconductive antenna is strongly reduced.
The phenomena and recent progress in the evolution of the basic classes of the THz detectors that exist today are considered. Issues associated with the development and exploitation of THz radiation detectors in human activity applications are briefly addressed. The operation conditions of the THz detectors and their upper limit performance are discussed. Because of lack of space not all the important references will be mentioned. Where reasonable, mainly publications after 2011 are addressed.
We record a sub-wavelength terahertz image of a caster sugar grain thanks to optical rectification in
the sample excited with a femtosecond laser beam. The lateral spatial resolution of this technique is
given by the laser spot size at the sample and here its measured value is 50 μm, i.e. ~λ/12. We give an
estimation of the ultimate resolution that could be achieved with this method.
In this paper, we give an overview of emission and detection of terahertz electromagnetic pulses using, respectively, optical rectification and electro-optic effect in [111] zinc blende crystals. This crystal orientation allows us to generate and read any polarization state of the THz beam only by controlling the polarization state of the laser beam that excites or probes the emitting and receiving crystals. This technique is very useful for polarimetric terahertz spectroscopic studies.
We report for the first time on the observation of an angular anisotropy
of the THz signal generated by optical rectification in a < 111 > ZnTe crys-
tal. This cubic (zinc-blende) crystal in the < 111 > orientation exhibits both
transverse isotropy for optical effects involving the linear χ (1) and nonlinear
χ (2) susceptibilities. Thus the observed anisotropy can only be related to χ (3)
effect, namely two-photon absorption, which leads to the photo-generation of
free carriers that absorb the generated THz signal. Two-photon absorption in
zinc-blende crystals is known to be due to a spin-orbit interaction between
the valence and higher-conduction bands. We perform a couple of measure-
ments that confirm our hypothesis, as well as we fit the recorded data with
a simple model. This two-photon absorption effect makes difficult an efficient
generation, through optical rectification in < 111 > zinc-blende crystals, of THz beams of any given polarization state by only monitoring the laser pump
polarization.
Science and technologies based on terahertz frequency electromagnetic radiation (100GHz-30THz) have developed rapidly over the last 30 years. For most of the 20th century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to “real world” applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2016, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 17 sections that cover most of the key areas of THz Science and Technology. We hope that The 2016 Roadmap on THz Science and Technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.
We demonstrate near-field imaging capabilities of a conical waveguide without cutoff using broadband terahertz (THz) radiation. In contrast to conventional conically tapered waveguides, which are characterized by strong suppression of transmission below the cutoff frequency, the proposed structure consists of two pieces, such that there is an adjustable gap along the length of the waveguide. We also ensure that the sidewalls are thin in the vicinity of the gap. The combination of these geometrical features allow for significantly enhanced transmission at frequencies below the cutoff frequency, without compromising the mode confinement and, consequently, the spatial resolution when used for imaging applications. We demonstrate near-field imaging with this probe simultaneously at several frequencies, corresponding to three regimes: above, near and below the cutoff frequency. We observe only mild degradation in the image quality as the frequency is reduced below the cutoff frequency. These results suggest that further refinements in the probe structure will allow for improved imaging capabilities at frequencies well below the cutoff frequency.
Molecular recognition and discrimination of carbohydrates are important
because carbohydrates perform essential roles in most living organisms for
energy metabolism and cell-to-cell communication. Nevertheless, it is difficult
to identify or distinguish various carbohydrate molecules owing to the lack of
a significant distinction in the physical or chemical characteristics. Although
there has been considerable effort to develop a sensing platform for individual
carbohydrates selectively using chemical receptors or an ensemble array, their
detection and discrimination limits have been as high in the millimolar
concentration range. Here we show a highly sensitive and selective detection
method for the discrimination of carbohydrate molecules using nano-slot-antenna
array-based sensing chips which operate in the terahertz frequency range. This
THz metamaterial sensing tool recognizes various types of carbohydrate
molecules over a wide range of molecular concentrations. Strongly localized and
enhanced terahertz transmission by nano-antennas can effectively increase the
molecular absorption cross sections, thereby enabling the detection of these
molecules even at low concentrations. We verified the performance of
nano-antenna sensing chip by both THz spectra and images of transmittance.
Screening and identification of various carbohydrates can be applied to test
even real market beverages with a high sensitivity and selectivity.
The potential of non-destructive imaging techniques, such as optical coherence tomography (OCT), X-ray micro-3D computed tomography (μ-CT) and terahertz (THz) imaging has been considered for structural and age diagnostic tasks in the field of paleontology. In particular OCT, a high-resolution, non-destructive and contactless technology for two and three dimensional (2D, 3D) imaging, was evaluated for the investigation of dental cementum microstructures, exemplified by cave bear teeth. OCT with a depth resolution in the micron range showed its ability to count the annual appositional lines consisting of cementum and thus to determine the age of the individual. As additional method also THz technology is presented which exhibits much larger penetration depths up to centimeters, but with a lower resolution, in contrast to OCT. Thus, THz imaging gives insight into the internal structure of the cave bear teeth on a larger scale. Furthermore, to complete the variety of considered non-destructive imaging techniques, μ-CT has been applied for analysis of the teeth structures, in comparison to OCT and THz imaging results. The combination of these complementary methods is well suited for the non-destructive characterization of teeth.
This paper studies the terahertz optical properties of nonlinear potassium titanyl phosphate crystals with different conductivities in the spectral range of 0.2 to 2.6 THz. The observed properties are characterized by several absorption lines lying along different optical axes which represent the relevant potassium sublattice phonon modes. The peculiarities of these absorption lines are attributed to the structural order of potassium ions.
We report on the emission of terahertz radiation by irradiation of femtosecond laser pulses in non-centrosymmetric paraelectric and ferroelectric phases of Co3B7O13I boracite. The generation of the terahertz waves in both phases is caused by optical rectification via a second-order nonlinear optical effect. In the ferroelectric phase, we successfully visualized ferroelectric domains by analyzing the polarization state of the terahertz wave radiated from the crystal. In a large area of the crystal (~500 × 500 µm2), the observed polarization vector of the radiated terahertz wave was tilted from directions of spontaneous polarization, i.e., , , and directions in cubic setting, which can be explained by the presence of a ferroelectric 90° domain wall of the plane.
Many time-domain terahertz applications require systems with high bandwidth, high signal-to-noise ratio and fast measurement speed. In this paper we present a terahertz time-domain spectrometer based on 1550 nm fiber laser technology and InGaAs photocon-ductive switches. The delay stage offers both a high scanning speed of up to 60 traces / s and a flexible adjustment of the measurement range from 15 ps – 200 ps. Owing to a precise reconstruction of the time axis, the system achieves a high dynamic range: a single pulse trace of 50 ps is acquired in only 44 ms, and transformed into a spectrum with a peak dynamic range of 60 dB. With 1000 averages, the dynamic range increases to 90 dB and the measurement time still remains well below one minute. We demonstrate the suitability of the system for spectroscopic measurements and terahertz imaging.
We directly measured phase-matching directions of second harmonic, sum, and difference frequency generations in the Langatate (LGT) uniaxial crystal. The simultaneous fit of the data enabled us to refine the Sellmeier equations of the ordinary and extraordinary principal refractive indices over the entire transparency range of the crystal, and to calculate the phase-matching curves and efficiencies of LGT for infrared optical parametric generation.
Terahertz and millimeter waves penetrate various dielectric materials, including plastics, ceramics, crystals, and concrete, allowing terahertz transmission and reflection images to be considered as a new imaging tool complementary to X-Ray or Infrared. Terahertz imaging is a well-established technique in various laboratory and industrial applications. However, these images are often two-dimensional. Three-dimensional, transmission-mode imaging is limited to thin samples, due to the absorption of the sample accumulated in the propagation direction. A tomographic imaging procedure can be used to acquire and to render three-dimensional images in the terahertz frequency range, as in the optical, infrared or X-ray regions of the electromagnetic spectrum. In this paper, after a brief introduction to two dimensional millimeter waves and terahertz imaging we establish the principles of tomography for Terahertz Computed tomography (CT), tomosynthesis (TS), synthetic aperture radar (SAR) and time-of-flight (TOF) terahertz tomography. For each technique, we present advantages, drawbacks and limitations for imaging the internal structure of an object.
We report here, for the first time to our
knowledge, on generation of THz waves by optical
rectification of a fs laser pulse in a langatate crystal.
This crystal shows higher laser damage threshold,
which permits to pump it with intense laser pulses.
Optical properties of high-quality flux-grown potassium titanyl phosphate (KTiOPO4, KTP) crystals were studied at 81 K in the spectral range of 0.2–2.4 THz by terahertz time-domain spectroscopy and compared with the room temperature results. No strong phonon peaks were found at frequencies below 2.15 THz in the absorption spectra of αx ≈ αy components. At 81 K they do not exceed that of widely used GaSe crystal. Experimental data on the dispersion of refractive index at 81 K for THz waves polarized parallel to optical axes were approximated in the form of Sellmeier equations. Phase matching for collinear difference-frequency generation of visible and near-IR emission into the THz domain was found possible, as well as second harmonic generation within THz region that can be a basis for the new THz sources designs. Phase matching in KTP was found insensitive to temperature variation. These properties together with the exceptional set of other known physical properties render KTP crystal amongst the most prospective crystals for high-power THz wave generation under intense laser pumping.
We developed THz-resonant scanning probe tips, yielding strongly enhanced and nanoscale confined THz near fields at their tip apex. The tips with length in the order of the THz wavelength (λ = 96.5 µm) were fabricated by focused ion beam (FIB) machining and attached to standard atomic force microscopy (AFM) cantilevers. Measurements of the near-field intensity at the very tip apex (25 nm radius) as a function of tip length – via graphene-based (thermoelectric) near-field detection - reveal their first and second order geometrical antenna resonances for tip length of 33 and 78 µm, respectively. On resonance, we find that the near-field intensity is enhanced by one order of magnitude compared to tips of 17 µm length (standard AFM tip length), which is corroborated by numerical simulations that further predict remarkable intensity enhancements of about 107 relative to the incident field. Because of the strong field enhancement and standard AFM operation of our tips, we envision manifold and straightforward future application in scattering-type THz near-field nanoscopy and THz photocurrent nano-imaging, nanoscale nonlinear THz imaging or nanoscale control and manipulation of matter employing ultrastrong and ultrashort THz pulses.
Laser terahertz emission microscopy (LTEM) has become a powerful tool for studying ultrafast dynamics and local fields in many different types of materials. This technique, which relies on acceleration of charge carriers in a material upon femtosecond excitation, can provide insight into the physics of charge transport, built-in fields, grain boundaries or surface states. We describe a new implementation of LTEM with a spatial resolution in the nanoscale regime based on a scattering-type near-field tip-based approach. We observe a spectral reshaping of the signal compared to conventional LTEM, which is analyzed using a resonant antenna model. Our experimental and computational results clarify the importance of the mechanisms for both the plasmonic in-coupling of the near infrared pulses into the near field and the out-coupling of generated terahertz pulses. We demonstrate a tip-size-limited spatial resolution of ~20 nanometers by imaging a gold nanorod using terahertz emission from the underlying substrate. This work enables for the first time the possibility of performing LTEM measurements on individual nanostructures.
Watching a single molecule move on its intrinsic time scale—one of the central goals of modern nanoscience—calls for measurements that combine ultrafast temporal resolution1–8 with atomic spatial resolution9–30. Steady-state experiments achieve the requisite spatial resolution, as illustrated by direct imaging of individual molecular orbitals using scanning tunnelling microscopy9–11 or the acquisition of tip-enhanced Raman and luminescence spectra with sub-molecular resolution27–29. But tracking the dynamics of a single molecule directly in the time domain faces the challenge that single-molecule excitations need to be confined to an ultrashort time window. A first step towards overcoming this challenge has combined scanning tunnelling microscopy with so-called ‘lightwave electronics”1–8, which uses the oscillating carrier wave of tailored light pulses to directly manipulate electronic motion on time scales faster even than that of a single cycle of light. Here we use such ultrafast terahertz scanning tunnelling microscopy to access a state-selective tunnelling regime, where the peak of a terahertz electric-field waveform transiently opens an otherwise forbidden tunnelling channel through a single molecular state and thereby removes a single electron from an individual pentacene molecule’s highest occupied molecular orbital within a time window shorter than one oscillation cycle of the terahertz wave. We exploit this effect to record ~100 fs snapshot images of the structure of the orbital involved, and to reveal through pump-probe measurements coherent molecular vibrations at terahertz frequencies directly in the time domain and with sub-angstrom spatial resolution. We anticipate that the combination of lightwave electronics1–8 and atomic resolution of our approach will open the door to controlling electronic motion inside individual molecules at optical clock rates.
The purpose of this research is to introduce and validate novel sub-THz resonance spectroscopy combined with molecular dynamics (MD) computation as a promising approach for optical analysis and potential quantification of molecular biomarkers in ovarian (OC) cancer cells. The ability of sub-THz spectroscopy to identify and quantify biological molecules is demonstrated by interrogation of resonance features caused by atomic vibrations within biological molecules in cancer and normal samples. In vitro human cell cultures of two ovarian cancer subtypes, SK-OV-3 human epithelial and ES-2 human clear cell carcinoma, were characterized in comparison with a normal nontransformed cell line (FT131-human fallopian tube epithelial cell line). A dramatic difference between the THz absorption spectra of cancer and normal cells and cell free samples is observed with much higher absorption intensity and a strong absorption peak at frequency of ~13 cm⁻¹ dominating the spectra from cancer samples. Comparison of experimental spectra with MD predictions of spectroscopic signatures of microRNA molecules from the miR-200 family, known to be overexpressed in ovarian cancer, suggests an origin for this pronounced spectral peak. The rest of the cancer samples' signature is in part similar to the signatures of normal cells and represents contributions from proteins and nucleic acid polymer molecules. Even though ovarian cancer is utilized for this proof of concept, the sub-THz spectroscopy method is very general and can be as well applied to other cancer types.
Future astrophysics and cosmic microwave background space missions operating in the far-infrared to millimetre part of the spectrum will require very large arrays of ultra-sensitive detectors in combination with high multiplexing factors and efficient low- noise and low-power readout systems. We have developed a demonstrator system suitable for such applications. The system combines a 961 pixel imaging array based upon Microwave Kinetic Inductance Detectors (MKIDs) with a readout system capable of reading out all pixels simultaneously with only one readout cable pair and a single cryogenic amplifier. We evaluate, in a representative environment, the system performance in terms of sensitivity, dynamic range, optical efficiency, cosmic ray rejection, pixel-pixel crosstalk and overall yield at an observation frequency of 850 GHz. The overall system has an excellent sensitivity, with an average detector sensitivity NEP=2.8 +- 0.8 x 10^-19 W/rt(Hz) measured using a thermal calibration source. The dynamic range would allow the detection of ~ 1 Jy bright sources within the field of view. The expected dead time due cosmic ray interactions, when operated in an L2 or a similar far-Earth orbit, is found to be <4%. Additionally, the achieved pixel yield is 83% and the crosstalk between the pixels is <-30dB. This demonstrates that MKID technology can provide multiplexing ratios on the order of a 1000 with state-of-the-art single pixel performance, and that the technology is now mature enough to be considered for future space based observatories and experiments.
Kinetic Inductance Detectors (KID) represent a novel detector technology based on superconducting resonators. Since their first demonstration in 2003, they have been rapidly developed and are today a strong candidate for present and future experiments in the different bands of the electromagnetic spectrum. This has been possible thanks to the unique features of such devices: in particular, they couple a very high sensitivity to their intrinsic suitability for frequency domain multiplexed readout, making the fabrication of large arrays of ultrasensitive detectors possible. There are many fields of application that can profit of such detectors. Here, we will briefly review the principle of operation of a KID, and give two sample applications, to mm-wave astronomy and to particle physics.
NIKA2 (New IRAM KID Array 2) is a camera dedicated to millimeter wave
astronomy based upon kilopixel arrays of Kinetic Inductance Detectors (KID).
The pathfinder instrument, NIKA, has already shown state-of-the-art detector
performance. NIKA2 builds upon this experience but goes one step further,
increasing the total pixel count by a factor 10 while maintaining the
same per pixel performance. For the next decade, this camera will be the
resident photometric instrument of the Institut de Radio Astronomie
Millimetrique (IRAM) 30m telescope in Sierra Nevada (Spain). In this paper we
give an overview of the main components of NIKA2, and describe the achieved
detector performance. The camera has been permanently installed at the IRAM 30m
telescope in October 2015. It will be made accessible to the scientific
community at the end of 2016, after a one-year commissioning period. When this
happens, NIKA2 will become a fundamental tool for astronomers worldwide.
This chapter gives a review of bolometer use for THz detection, drawing specific attention to such sensors for THz imaging. First, the detection principles are presented highlighting the technological solutions that ensure the main functions of such thermal sensors. The two last sections are devoted to the review of technological approaches that are encountered depending on the operation temperature. Examples of developed cooled bolometer arrays are reviewed with emphasis on illustrating different approaches applied to perform the main bolometer functions. Finally, a focus is made on uncooled array of microbolometers, that is the field of R&D activity of the author.
Introduction to THz Wave Photonics examines the science and technology related to terahertz wave technologies, taking a dual approach between presenting the field's history while simultaneously providing an overview of existing technology. The latest research in developing THz areas such as electromagnetic waves are presented, along with an introduction to continuous wave THz technology. Authors X.-C. Zhang and Jingzhou Xu place particular emphasis on pulsed THz technology, among many other facets of THz technology including: •Complete coverage of THz wave spectroscopy and imaging •A discussion of 3D THz wave imaging •Applications of THz technology in the security field as related to explosives and hazardous materials •THz applications in bio-engineering and biomedicine Introduction to THz Wave Photonics is the perfect book for academic researchers, practicing engineers and students interested in learning more about the subject. © Springer Science+Business Media, LLC 2010. All rights reserved.
We report on the feasibility of terahertz time domain spectroscopy (THz-TDS) for discriminating regular petroleum products and their mixtures. Basic optical properties of gasoline, diesel, kerosene and organic solvents, such as xylene, toluene and benzene, used as additives in producing a similar gasoline were analyzed in the spectral range of 0.2 - 2 THz. A quantitative analysis on the mixtures of gasoline and the additives clearly shows that the optical properties of the mixtures differ from that of regular petroleum products in the THz spectral range. Furthermore, a variation in the mixing ratio results in systematic changes in the refractive index and the dielectric constant, indicating that THz-TDS is an effective means to analyze gasoline and its mixtures.
We report quasi-three-dimensional imaging of the ferroelectric domains in a hydrogen-bonded supramolecular ferroelectric 1:1 salt of 5,5′-dimethyl-2,2′-bipyridine and deuterated iodanilic acid by mapping out terahertz waves radiated from the crystal upon femtosecond laser irradiation. Polarization dependence of the effective depth radiating the terahertz waves due to the absorption anisotropy for terahertz waves allows us to distinguish domains in the inside and surface regions of the crystals. In an as-grown crystal, a large domain covering almost all the area is discerned in the inside region, while multidomains are discerned in the surface regions. We observed polarization switching via domain boundary motion in the direction perpendicular to the direction of hydrogen-bonded chains in the surface regions under an external electric field. Obtained images suggest that an uncharged 180° domain wall (DW) parallel to the stacking plane of hydrogen-bonded chains covers the whole area at coercive fields. We argue that the DW dynamics stems from the anisotropic crystal structure, i.e., stacking of hydrogen-bonded chains.
An experimental and numerical study of the laser-induced damage of the surface of optical material in the femtosecond regime is presented. The objective of this work is to investigate the different processes involved as a function of the ratio of photon to bandgap
energies and compare the results to models based on nonlinear ionization processes. Experimentally, the laser-induced damage threshold of optical materials has been studied in a range of wavelengths from 1030 nm (1.2 eV) to 310 nm (4 eV) with pulse durations of 100 fs with the use of an optical parametric amplifier system. Semi-conductors and dielectrics materials, in bulk or thin film forms, in a range of bandgap from 1 to 10 eV have been tested in order to investigate the scaling of the femtosecond laser damage threshold with the bandgap and photon
energy. A model based on the Keldysh photo-ionization theory and the description of impact ionization by a multiple-rate-equation system is used to explain the dependence of laser-breakdown with the photon
energy. The calculated damage fluence threshold is found to be consistent with experimental results. From these results, the relative importance of the ionization processes can be derived depending on material properties and irradiation conditions. Moreover, the observed damage morphologies can be described within the framework of the model by taking into account the dynamics of energy deposition with one dimensional propagation simulations in the excited material and thermodynamical considerations.
Time domain THz spectroscopy measurements were performed on a series of undoped and Mg-doped congruent lithium niobate crystals with 1.2, 6.1, and 8.4 mol% Mg concentrations and on undoped and Mg-doped stoichiometric lithium niobate crystals with 0.7, 1.5, and 4.2 mol% Mg concentrations with polarization parallel (extraordinary) and perpendicular (ordinary) to the z axis of the crystal at 300 K. The absorption coefficient and refractive index spectra were determined in the THz frequency range from 0.25 to ~2.5 THz. In the case of congruent samples for both polarizations, both the refractive index and the absorption coefficient have minimal values for compositions close to the photorefractive threshold. In the case of stoichiometric samples, similar tendencies close to the photorefractive threshold at lower Mg concentration were observed but only for extraordinary polarization, while for ordinary polarization the measured values, especially for the absorption coefficient, were only weakly dependent on the Mg content.
The finite-difference time-domain (FDTD) method is employed to numerically study the transmission characteristics of an H-shaped nano-aperture in a metal film in the optical frequency range. It is demonstrated that the fundamental TE10 mode concentrated in the gap between the two ridges of the H-shaped aperture provides a high transmission efficiency above unity and the size of the gap determines the sub-wavelength resolution. Fabry-Perot-like resonance is observed. Localized surface plasmon (LSP) is excited on the edges of the aperture in a silver film but has a negative effect on the signal contrast and field concentration, while aluminum acts similar to an ideal conductor if the film thickness is several times larger than the finite skin depth. In addition, it is shown that two other ridged apertures, C-shaped and bowtie-shaped apertures, can also be used to achieve a sub-wavelength resolution in the near field with a transmission efficiency above unity and a high contrast.
Phase-locked ultrashort pulses in the rich terahertz spectral range1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 have provided key insights into phenomena as diverse as quantum confinement7, first-order phase transitions8, 12, high-temperature superconductivity11 and carrier transport in nanomaterials1, 6, 13, 14, 15. Ultrabroadband electro-optic sampling of few-cycle field transients1 can even reveal novel dynamics that occur faster than a single oscillation cycle of light4, 8, 10. However, conventional terahertz spectroscopy is intrinsically restricted to ensemble measurements by the diffraction limit. As a result, it measures dielectric functions averaged over the size, structure, orientation and density of nanoparticles, nanocrystals or nanodomains. Here, we extend ultrabroadband time-resolved terahertz spectroscopy to the sub-nanoparticle scale (10 nm) by combining sub-cycle, field-resolved detection (10 fs) with scattering-type near-field scanning optical microscopy (s-NSOM)16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26. We trace the time-dependent dielectric function at the surface of a single photoexcited InAs nanowire in all three spatial dimensions and reveal the ultrafast (<50 fs) formation of a local carrier depletion layer.
Combined with THz time-domain spectroscopy, THz near-field microscopy based on an atomic force microscope is a technique that, while challenging to implement, is invaluable for probing low-energy light-matter interactions of solid-state and biomolecular nanostructures, which are usually embedded in background media. Here, we experimentally demonstrate a broadband THz pulse near-field microscope that provides sub-surface nanoimaging of a metallic grating embedded in a dielectric film. The THz near-field microscope can obtain broadband nanoimaging of the sub-surface grating with a nearly frequency-independent lateral resolution of 90 nm, corresponding to ~λ/3300, at 1 THz, while the AFM only provides a flat surface topography.
In this chapter, we discuss different techniques to solve numerically wave propagation phenomena in unbounded domains. We present in two ways a unified and simple way to restrict the computation to a finite domain: absorbing (or artificial) boundary conditions and perfectly matched layers. The intent is to give the possibility to the reader to grasp easily similarities and differences between these two truncation tecnhiques. It should also allow the reader to adapt a truncation technique to the peculiarities of his physical modeling.
Using a terahertz-radiation imaging, visualizations of ferroelectric domains were made in a room-temperature organic ferroelectric, croconic acid. In as-grown crystals, observed are ferroelectric domains with sizes larger than 50-μm square, which are separated by both 180° and tail-to-tail domain walls (DWs). By applying an electric field along c axis (the polarization direction), a pair of 180° DWs is generated and an each 180° DW oppositely propagates along a axis, resulting in a single domain. By cyclic applications of electric fields, a pair of 180° DWs repeatedly emerges, while no tail-to-tail DWs appear. We discuss the usefulness of the terahertz-radiation imaging as well as the observed unique DW dynamics.
We demonstrate photon-noise limited performance at sub-millimeter wavelengths
in feedhorn-coupled, microwave kinetic inductance detectors (MKIDs) made of a
TiN/Ti/TiN trilayer superconducting film, tuned to have a transition
temperature of 1.4~K. Micro-machining of the silicon-on-insulator wafer
backside creates a quarter-wavelength backshort optimized for efficient
coupling at 250~\micron. Using frequency read out and when viewing a variable
temperature blackbody source, we measure device noise consistent with photon
noise when the incident optical power is ~0.5~pW, corresponding to noise
equivalent powers ~3 W/. This
sensitivity makes these devices suitable for broadband photometric applications
at these wavelengths.