Image reconstruction method for non-synchronous THz signals was developed for a combination of THz Free Electron Laser (THz-FEL) developed by Osaka University with THz imager. The method employs a slight time-difference between repetition period of THz macro-pulse from THz-FEL and a plurality of frames for THz imager, so that image can be reconstructed out of a predetermined number of time-sequential frames. This method was applied to THz-FEL and other pulsed THz source, and found very effective. Thermal time constants of pixels in 320x240 microbolometer array were also evaluated with this method, using quantum cascade laser as a THz source.

Nowadays, the detection and identification of dangerous substances at long distance (several meters, for example) by using of THz pulse reflected from the object is an important problem. In this report we demonstrate possibility of THz signal measuring reflected from investigated object that is placed before a flat metallic mirror. A distance between the flat mirror and the parabolic mirror this mirror is equal to 3.5 meters. Therefore, at present time our measurements contain features of both transmission and reflection modes. The reflecting mirror is used because of weak average power of used femtosecond laser. Measurements were provided at room temperature and humidity about 60%. The aim of investigation was the detection of a substance in real condition. Chocolate and Cookies were used as samples for identification. We also discuss modified correlation criteria for the detection and identification of various substances using pulsed THz signal in the transmission and reflection mode at short distances of about 30-40 cm. These criteria are integral criteria in time and they are based on the SDA method. Proposed algorithms show both high probability of the substance identification and a reliability of realization in practice. We compare P-spectrum and SDA- methods in the paper and show that P-spectrum method is a partial case of SDAmethod.

We have developed a cost-effective, quad-frequency band THz imager for real-time THz imaging applications operating at 220GHz, 320GHz, 420GHz, and 520GHz frequency bands. The new sensor is based on antimonide-based heterostructure backward diodes impedance matched and monolithically integrated with high-gain, narrowband planar antennas. The antennas are dual-linearly polarized to allow direct measurement of beam polarization. This paper details the fabricated THz detector array and the high-speed, low-noise readout electronic chain. Experimental results on the performance of the readout chain and simulations of the expected THz detector performance are presented.

Numerous applications require fast and accurate range measurement of concealed objects. As THz waves penetrate almost all materials except metals, they are a candidate to perform this task. In this paper a system consisting of an illumination THz source and a THz detector array is presented. A solid state 300GHz THz source signal is frequency modulated and guided to a detector array using a beam splitter. The detector array consists of ultra-sensitive bolometers with 1000V/W sensitivity and NEP of 5pW/√Hz. Their square law sensitivity characteristic allows mixing of the reflected wave with the illumination wave, resulting in a mixed product with a difference frequency of few kHz proportional to the target distance, the frequency modulation span, and its rate. Parallel signal processing of all 16 signals results in fast range detection of the target. All 16 detectors provide four readings per second for all X and Y positions of the image. A compact and portable system with 40GHz frequency modulated source provides Δf/Δt = 160GHz/sec. This corresponds to a 1kHz difference frequency per one meter distance of the target. A resolution accuracy in the micrometer range has been achieved using advanced signal processing. In the paper the processing algorithms and the obtained results are presented and discussed. The complete hardware structure of the system is described together with the required signal processing procedures. The advantage of the presented system is that it operates at room temperature and is therefore cost effective and very robust.

We report a technique using photo-induced coded-aperture arrays for potential real-time THz imaging at roomtemperature. The coded apertures (based on Hadamard coding) were implemented using programmable illumination on semi-insulating Silicon wafer by a commercial digital-light processing (DLP) projector. Initial imaging experiments were performed in the 500-750 GHz band using a WR-1.5 vector network analyzer (VNA) as the source and receiver. Over the entire band, each array pixel can be optically turned on and off with an average modulation depth of ~20 dB and ~35 dB, for ~4 cm² and ~0.5 cm² imaging areas respectively. The modulation speed is ~1.3 kHz using the current DLP system and data acquisition software. Prototype imaging demonstrations have shown that a 256-pixel image can be obtained in the order of 10 seconds using compressed sensing (CS), and this speed can be improved greatly for potential real-time or video-rate THz imaging. This photo-induced coded-aperture imaging (PI-CAI) technique has been successfully applied to characterize THz beams in quasi-optical systems and THz horn antennas.

We theoretically investigated and designed a tunable, compact THz source in 1-10 THz range based on a nonlinear optical microdisk resonator. The lack of tunable THz source operating at room temperature is still one of the major impediments for the applications of THz radiation. The proposed device on an insulated borosilicate glass substrate consists of a nonlinear optical disk resonator on top of another disk capable of sustaining THz modes. A pair of Si optical waveguides is coupled to the nonlinear microdisk in order to carry the two input optical waves. Another pair of Si THz waveguides is placed beneath the input optical waveguides to couple out the generated THz radiation from the disk to receiver antenna. Both optical and THz disks are engineered optimally with necessary effective mode indices in order to satisfy the phase matching condition. We present the simulation results of our proposed device using a commercial finite element simulation tool. A distinguished THz peak coincident exactly with the theoretical calculations involving DFG is observed in frequency spectrum of electric field in the microdisk resonator. Our device has the potential to enable tunable, compact THz emitters and on-chip integrated spectrometers.

A method to generate an optical metasurface is developed. In our experimental setup, we use a pump-probe technique, where the pump beam is used to project patterns of v-shaped antennas on the surface of a silicon substrate. In the areas illuminated with the images of v-shaped antennas electron-hole pairs are created. Therefore, the antenna structures on silicon will have metallic-like properties, we classify this structure as a metasurface. The THz beam probes refraction and reflection on the metasurface generated on the silicon substrate. The dynamic change of these patterns of metasurface causes the beam steering effects of THz radiation.

We present results on the comparison of different THz technologies for the detection and identification of a variety of explosives from our laboratory tests that were carried out in the framework of NATO SET-193 “THz technology for stand-off detection of explosives: from laboratory spectroscopy to detection in the field” under the same controlled conditions. Several laser-pumped pulsed broadband THz time-domain spectroscopy (TDS) systems as well as one electronic frequency-modulated continuous wave (FMCW) device recorded THz spectra in transmission and/or reflection.

The terahertz frequency regime is often used as the ‘chemical fingerprint’ region of the electromagnetic spectrum due to the large number of rotational and vibrational transitions of many molecules of interest. This region of the spectrum has particular utility for applications such as pollution monitoring and the detection of energetic chemicals using remote sensing over long path lengths through the atmosphere. Although there has been much attention to atmospheric effects over narrow frequency windows, accurate measurements across a wide spectrum are lacking. The water vapor continuum absorption is an excess absorption that is unaccounted for in resonant line spectrum simulations. Currently a semiempirical model is employed to account for this absorption, however more measurements are necessary to properly describe the continuum absorption in this region. Fourier Transform Spectroscopy measurements from previous work are enhanced with high-resolution broadband measurements in the atmospheric transmission window at 1.5THz. The transmission of broadband terahertz radiation through pure water vapor as well as air with varying relative humidity levels was recorded for multiple path lengths. The pure water vapor measurements provide accurate determination of the line broadening parameters and experimental measurements of the transition strengths of the lines in the frequency region. Also these measurements coupled with the atmospheric air measurements allow the water vapor continuum absorption to be independently identified at 1.5THz. Simulations from an atmospheric absorption model using parameters from the HITRAN database are compared with the current and previous experimental results.

The H-field finite element method (FEM) based full-vector formulation is used in the present work to study the vectorial modal field properties and the complex propagation characteristics of Surface Plasmon modes of a hollow-core dielectric coated rectangular waveguide structures, and graphene based structures. Additionally, the finite difference time domain (FDTD) method is used to estimate the dispersion parameters and the propagation loss of such waveguides and devices.

Receivers based on superconducting Hot-Electron Bolometers (HEBs) are widely used for terahertz (THz) sensing owing to their advantages of high sensitivity, low noise, and low LO power requirement. Balanced HEB mixers are superior to single-element ones since the thermal noise and AM noise from the LO injection can be effectively suppressed. Although a 1.3 THz balanced waveguide HEB mixer has been reported, waveguide mixer configurations offer relatively narrow RF bandwidths. We report on the development, fabrication and characterization of a THz quasioptical balanced superconducting HEB mixer utilizing a dual-polarization sinuous antenna that can potentially achieve both multiband operation and ultra-high sensitivity. In the balanced mixer configuration, a lens-coupled four-arm sinuous antenna was designed for operation from 0.2-1.0 THz with a nearly frequency-independent embedding impedance of ~106 Ω. Two identical superconducting niobium HEB devices have been integrated at the antenna feedpoints, connecting each opposing pair of antenna arms to form a balanced mixer configuration. An air-bridge was also fabricated to separate the two mixer branches. The HEB devices were fabricated from 10 nm thick niobium film sputtered on semi-insulating silicon substrates. Each HEB device has dimensions of 80 nm × 240 nm (3 squares) for approaching a resistance of 105 Ω for impedance matching. Mixer properties including antenna radiation patterns, broadband operation and polarization isolation have been characterized. Finally, in order to achieve multiband mixer operation, electronically reconfigurable THz quasi-optical mesh filters are needed. Frequency-tunable antenna elements using Schottky varactor diodes suitable for the above applications have been designed, simulated and demonstrated at Gband (140-220 GHz) showing 50 GHz tuning range.

A wideband and compact solid state power amplifier MMIC is simulated around 220 GHz. It utilizes 6 μm emitter length common base HBTs from a 250 nm InP HBT technology. Specific power cells and power combiners are simulated in order to minimize the width of the die, which must not exceed 300 μm to avoid multimode propagation in the substrate. Four stages are implemented over a total area of the (275x1840) μm². Simulations of this power amplifier indicate a minimum output power of 14 dBm associated with 16 dB of power gain from 213 GHz to 240 GHz.

Interband photoexcitation in monolayer graphene can produce a weak gain in the terahertz range by only up to 2.3%, but exciting the surface plasmon polaritons mediates the light-matter interaction, resulting in a giant terahertz gain. Nonlinear carrier relaxation/recombination dynamics and resultant stimulated terahertz (THz) photon emission with excitation of surface plasmon polaritons (SPPs) in photoexcited monolayer graphene has been experimentally studied using optical-pump/THz-probe and optical-probe measurement. We observed the spatial distribution of the THz probe pulse intensities under linear polarization of optical pump and THz probe pulses. It was clearly observed that intense THz probe pulse was detected only at the area where the incoming THz probe pulse takes a TM mode being capable of exciting the SPPs. The observed gain factor is in fair agreement with theoretical calculations.

Bridging the gap in scale between the THz wavelength and the biomolecule sample sizes to be sensed is a challenging task. We tackle this mismatch by developing sensing platforms based on the concepts of designer surface plasmon polaritons and localized plasmons. We show that corrugated metallic surfaces, complementary split ring resonators and arrays of micro-dipoles provides enhanced THz-matter interaction times and strong interrogating evanescent fields. We will also demonstrate how transformation optics can be used to design broadband plasmonic semiconductor and metallic gap micro-antennas for terahertz-to-visible applications.

Analytical and numerical studies of the dispersion properties of grating gated THz plasmonic structures show that the plasmonic crystal dispersion relation can be represented in terms of effective index of the dielectric medium around the 2DEG for the plasmons. Forbidden energy band gaps are observed at Brillion zone boundaries of the plasmonic crystal. FDTD calculations predict the existence of the plasmonic modes with symmetrical, antisymmetrical and asymmetrical charge distributions. Breaking the translational symmetry of the crystal lattice by changing the electron concentration of the two dimensional electron gas (2DEG) under a single gate line in every 9th gate induces a cavity state. The induced cavity state supports a weekly-coupled cavity mode.

The device applications of plasmonic systems such as graphene and two dimensional electron gases (2DEGs) in III-V heterostructures include terahertz detectors, mixers, oscillators and modulators. These two dimensional (2D) plasmonic systems are not only well-suited for device integration, but also enable the broad tunability of underdamped plasma excitations via an applied electric field. We present demonstrations of the coherent coupling of multiple voltage tuned GaAs/AlGaAs 2D plasmonic resonators under terahertz irradiation. By utilizing a plasmonic homodyne mixing mechanism to downconvert the near field of plasma waves to a DC signal, we directly detect the spectrum of coupled plasmonic micro-resonator structures at cryogenic temperatures. The 2DEG in the studied devices can be interpreted as a plasmonic waveguide where multiple gate terminals control the 2DEG kinetic inductance. When the gate tuning of the 2DEG is spatially periodic, a one-dimensional finite plasmonic crystal forms. This results in a subwavelength structure, much like a metamaterial element, that nonetheless Bragg scatters plasma waves from a repeated crystal unit cell. A 50% in situ tuning of the plasmonic crystal band edges is observed. By introducing gate-controlled defects or simply terminating the lattice, localized states arise in the plasmonic crystal. Inherent asymmetries at the finite crystal boundaries produce an induced transparency-like phenomenon due to the coupling of defect modes and crystal surface states known as Tamm states. The demonstrated active control of coupled plasmonic resonators opens previously unexplored avenues for sensitive direct and heterodyne THz detection, planar metamaterials, and slow-light devices.

Quasi-three-dimensional invisibility cloaks, comprised of either homogeneous or inhomogeneous media, are experimentally demonstrated in the terahertz regime. The inhomogeneous cloak was lithographically fabricated using a scalable Projection Microstereolithography process. The triangular cloaking structure has a total thickness of 4.4 mm, comprised of 220 layers of 20 μm thickness. The cloak operates at a broad frequency range between 0.3 and 0.6 THz, and is placed over an α-lactose monohydrate absorber with rectangular shape. Characterized using angular-resolved reflection terahertz time-domain spectroscopy, the results indicate that the terahertz invisibility cloak has successfully concealed both the geometrical and spectroscopic signatures of the absorber, making it undetectable to the observer. The homogeneous cloaking device made from birefringent crystalline sapphire features a large concealed volume, low loss, and broad bandwidth. It is capable of hiding objects with a dimension nearly an order of magnitude larger than that of its lithographic counterpart, but without involving complex and time-consuming cleanroom processing. The cloak device was made from two 20-mm-thick high-purity sapphire prisms. The cloaking region has a maximum height 1.75 mm with a volume of approximately 5% of the whole sample. The reflected TM beam from the cloak shows nearly the same profile as that reflected by a flat mirror.

Interest in array based imaging of terahertz energy (T-Rays) has gained traction lately, specifically using a CMOS process due to its ease of manufacturability and the use of MOSFETs as a detection mechanism. Incident terahertz radiation on to the gate channel region of a MOSFET can be related to plasmonic response waves which change the electron density and potential across the channel. The 0.35 μm silicon CMOS MOSFETs tested in this work contain varying structures, providing a range of detectors to analyze. Included are individual test transistors for which various operating parameters and modes are studied and results presented. A focus on single transistor-antenna testing provides a path for discovering the most efficient combination for coupling 0.2 THz band energy. An evaluation of fabricated terahertz band test detection MOSFETs is conducted. Sensitivity analysis and responsivity are described, in parallel with theoretical expectations of the plasmonic response in room temperature conditions. A maximum responsivity of 40 000 V/W and corresponding NEP of 10 pW/Hz^1/2 (±10% uncertainty) is achieved.

Exelis Geospatial Systems and its CEIS partners at the University of Rochester and Rochester Institute of Technology are developing an active THz imaging focal plane for use in standoff detection, molecular spectroscopy and penetration imaging. This activity is focused on the detection of radiation centered on the atmospheric window at 215.5 GHz. The pixel consists of a direct coupled bowtie antenna utilizing a 0.35 μm CMOS technology MOSFET, where the plasmonic effect is the principle method of detection. With an active THz illumination source such as a Gunn diode, a design of catadioptric optical system is presented to achieve a resolution of 3.0 mm at a standoff distance of 1.0 m. The primary value of the initial system development is to predict the optical performance of a THz focal plane for active imaging and to study the interaction of THz radiation with various materials.

The radar cross section of spherical retroreflectors operating at terahertz frequencies is investigated. Several spherical retroreflectors with diameters ranging from 2 mm to 8 mm were fabricated and their radar cross section was measured at 100 GHz, 160 GHz, and 350 GHz. A frequency selective surface was applied to the retroreflectors to demonstrate proof of concept of narrow-band terahertz retroreflection.

Detection of concealed dangerous objects is a very demanding problem of public safety. So far, the problem of detecting objects hidden under clothing was considered only in the case of airports but it is becoming more and more important for public places like metro stations, and government buildings. The development of imaging devices and exploration of new spectral bands is a chance to introduce new equipment for assuring public safety. It has been proved that objects hidden under clothing can be detected and visualized using terahertz (THz) cameras. However, passive THz cameras still offer too low image resolution for objects recognition. On the other hand new infrared cameras offer sufficient parameters to detect objects covered with fabrics in some conditions, as well as high image quality and big pixel resolutions. The purpose of the studies is to investigate the possibilities of using various cameras operating in different spectral ranges for detection of concealed objects. In the article, we present the measurement setup consisting of medium wavelength infrared (MWIR), long wavelength infrared (LWIR), THz and visible cameras and the initial results of measurements with various types of clothing and test objects.

We describe the design and simulation of several non-imaging concentrators designed to couple submillimeter wavelength radiation from free space into highly overmoded, rectangular, WR-10 waveguide. Previous designs are altered to improve the uniformity of efficiency rather than the efficiency itself. The concentrators are intended for use as adapters between instruments using overmoded WR-10 waveguide as input or output and sources propagating through free space. Previous simulation and measurement have shown that the angular response is primarily determined by the Winston cone and is well predicted by geometric optics theory while the efficiencies are primarily determined by the transition section. Additionally, previous work has shown insensitivity to polarization, orientation and beam size. Several separate concentrator designs are studied, all of which use a Winston cone (also known as a compound parabolic concentrator) with an input diameter ranging from 4 mm to 16 mm, and “throat” diameters of less than 0.5 mm to 4 mm as the initial interface. The use of various length adiabatic circular-to-rectangular transition sections is investigated, along with the effect of an additional, 25 mm waveguide section designed to model the internal waveguide of the power meter. Adapters without a transition section and a rectangular Winston cone throat aperture and double cone configurations are also studied. Adapters are analyzed in simulation for consistent efficiency across the opening aperture.