This PDF file contains the front matter associated with SPIE Proceedings Volume 8363, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Calculations are presented of ground state resonance structure associated with molecular clusters of β-HMX using density functional theory (DFT), which is for analysis using a model representation of the coupling between resonance modes for ground state excitation. With respect to qualitative analysis, this model representation of the coupling among resonance modes provides a reasonable molecular level interpretation of various features of the excitation spectra associated with the ground state of molecular clusters. The DFT software GAUSSIAN was used for the calculations of ground state resonance structure presented.

We present the results of design, fabrication, and characterization of the room-temperature, low electron heat capacity hot-electron THz microbolometers based on two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. The 2DEG sensor is integrated with a broadband THz antenna and a coplanar waveguide. Devices with various patterning of 2DEG have been fabricated and tested. Optimizing the material properties, geometrical parameters of the 2DEG, and antenna design, we match the impedances of the sensor and antenna to reach strong coupling of THz radiation to 2DEG via the Drude absorption. Testing the detectors, we found that the THz-induced photocurrent, ΔI, is proportional to the bias current, I, and the temperature derivative of the resistance and inversely proportional to the area of 2DEG sensor, S. The analysis allowed us to identify the mechanism of the 2DEG response to THz radiation as electron heating. The responsivity of our sensors, normalized to the bias current and to unit area of 2DEG, R*= ΔI•S/ (I∙P), is ~ 10³ W^-1 μm². So, for our typical sensor with an area of 1000 μm² and bias currents of ~ 10 mA, the responsivity is ~ 0.01 A/W. The measurements of mixing at sub-terahertz frequencies showed that the mixing bandwidth is above 2 GHz, which corresponds to a characteristic electron relaxation time to be shorter than 0.7 ps. Further decrease of the size of 2DEG sensors will increase the responsivity as well as allows for decreasing the local oscillator power in heterodyne applications.

To increase the sensitivity of uncooled microbolometer-based THz imagers, absorbing structures (metamaterial films) with resonant absorption that can be tuned to a QCL illuminator frequency are investigated. The metamaterial films are comprised of periodic arrays of aluminum (Al) squares and an Al ground plane separated by a thin silicon-rich silicon oxide (SiO_x) dielectric film. Finite element simulations were performed by varying the structural parameters to establish the design criteria for high absorption, spectral tunability and bandwidth. Several structures with single band and multiband absorption characteristics were fabricated. Measured absorption spectra show absorption up to 100% at designed THz frequencies and the spectral characteristics agree with simulations.

This paper describes a real-time terahertz (THz) imaging system, using the combination of a palm-size THz camera with a compact quantum cascade laser (QCL). The THz camera contains a 320x240 microbolometer focal plane array which has nearly flat spectral response over a frequency range of ca. 1.5 to 100 THz, and operates at 30 Hz frame rate. The QCL is installed in compact cryogen-free cooler. A variety of QCLs are prepared which can cover frequency range from ca. 1.5 to 5 THz. THz images of biochemical samples will be presented, using the combined imaging system. Performance of the imaging system, such as signal-to-noise ratio of transmission-type THz microscope, is predicted.

INO has developed infrared camera systems with microscanning capabilities in order to increase image resolution. It has been shown in previous works that the image quality may be improved even if the pixel pitch is smaller than the point spread function. This paper introduces a catadioptric optics system with fully integrated microscan for improved resolution in the THz band. The design, inspired by the INO's HRXCAM infrared camera core and adapted for terahertz wavelengths, includes two mirrors and one refractive element. It has a 11.9 degree full field of view and an effective F-number of 1.07 over a wide spectral range, from 100 μm to 1.5 mm wavelength. This diffraction limited optics is used to provide video rate high quality THz images. A THz camera, with 160 x 120 pixel and 52 μm pitch detector, is combined with the microscan objective to provide a 320 x 240 pixel image with a 26 μm sampling step. Preliminary imaging results using a THz illumination source at 118 μm wavelength are presented. A comparison between standard and microscanned images is also presented.

We have designed, built, and tested an uncooled THz imager based on optical readout photomechanical imaging technology, in which a MEMS-based sensor chip transduces the THz scene into a visible signal that is captured by a CCD imager. The performance of the 130x90 resolution, 100 μm pitch, 30 fps uncooled THz imager was measured using the λ = 119 μm (2.52 THz) emission line of a CO₂-pumped methanol gas laser. Excellent linearity of the responsivity was observed over a wide range of laser power. The noise equivalent power (NEP), limited by shot noise from the optical readout, was 76 pW/Hz^1/2. Switching to a high-capacity CCD imager to reduce shot noise and tailoring the photomechanical pixel structure for THz absorption will yield an NEP of less than 1 pW/Hz^1/2. In addition, the uncooled THz imager successfully profiled the output beam of a λ=134 um (2.24 THz) quantum cascade laser (QCL) in real time, with performance far superior to a commercial pyroelectric array camera.

Terahertz uncooled antenna-coupled microbolometer focal plane arrays are being developed at CEA-LETI for THz imaging and sensing. This detector technology relies on amorphous silicon bolometer know-how and aims at opening the way to real-time video rate 2D imaging, with potential low cost either in fabrication and in operation. First prototypes of 320x240 pixel arrays have been designed for 1-3 THz sensing. Sensors have been fabricated monolithically above CMOS Integrated Circuits while applying only full Si standard silicon processes. We present the results of extensive work of characterization and simulations made to estimate the sensitivity and spectral absorption of these prototypes. Tests of latest real-time imaging with active illumination by QCLs are then reported while explosives samples were placed in an optical set-up in reflection configuration.

The present study proposes a fully integrated, semi-automatic and near real-time mode-operated image processing methodology developed for Frequency-Modulated Continuous-Wave (FMCW) THz images with the center frequencies around: 100 GHz and 300 GHz. The quality control of aeronautics composite multi-layered materials and structures using Non-Destructive Testing is the main focus of this work. Image processing is applied on the 3-D images to extract useful information. The data is processed by extracting areas of interest. The detected areas are subjected to image analysis for more particular investigation managed by a spatial model. Finally, the post-processing stage examines and evaluates the spatial accuracy of the extracted information.

The design of an open-path 320 GHz - 340 GHz coherent transmissometer for experimental measurements of amplitude scintillation, phase scintillation, angle-of-arrival (AoA) fluctuations, and transverse coherence near the 325.1529 GHz water absorption resonance is presented. The system uses a uni-directional transmitter and two phase-coherent receivers with adjustable transverse. The objective of the experiment is to verify and improve existing propagation models for use by designers of applied THz systems for remote sensing, radiolocation, or communications. System stability will be verified using a short range near-ground test path of several ~10's of meters length using a cable for locking the transmitter local oscillator (LO) to the receivers' LOs. This short range configuration, similar to tests conducted at Flatville, Illinois during the 1980s, permits characterization of system errors in all of the above parameters, thus yielding a baseline for the long range experiments. Characterization of the phase-coherent RF link will be studied vis-à-vis anticipated theoretical performance based on the Rytov approximation. The system will then be configured for long term open-path measurements on a 1.78 km elevated link between the University of Colorado at Boulder (CU) and the National Telecommunications and Information Administration (NTIA) Mesa site at the NOAA-NIST campus in Boulder, Colorado. The system will provide long range coherent THz propagation statistics during continuous longduration study of turbulent atmospheric propagation effects over an extensive array of atmospheric conditions in a realistic operational environment.

In this paper a low-loss hollow-core rectangular plasmonic waveguide with a dielectric coating of Te on is analyzed for terahertz propagation using a full-vectorial nite element method (FEM). It has been identied that, in contrast to the fundamental H^x₁₀ mode, the H^x₁₂ mode shows interesting modal properties and oers the lowest possible loss for the structure after introducing the dielectric coating. This mode also tends to yield a near-Gaussian eld prole when the dielectric coating thickness is optimized and then it will be easy to couple to a Gaussian shaped source. The optimization of the loss values has been evaluated by comparing the loss characteristics for dierent dielectric materials and also by using dierent metal claddings.

In recent years a great amount of research has been focused on metamaterials, initially for fabrication of left-handed materials for use in devices such as superlenses or electromagnetic cloaking. Such devices have been developed and demonstrated in regimes from the radio frequency all the way to infrared and near optical frequencies. More recently, it has been shown that, by careful adjustment of the effective permittivity and permeability, near perfect electromagnetic absorbers can be realized. High absorption occurs when transmission and reflection are simultaneously minimized. With some clever tuning of the electric and magnetic responses, the electric and magnetic energy can therefore both be absorbed by the same metamaterial structure. In this work we present the design, simulation and characterization of a novel thin, flexible, polarization insensitive metamaterial absorber. Finite-element simulation results show that this device achieves almost perfect absorption at THz frequencies. Each unit cell of the absorber is made up of two metallic structures separated by a dielectric filler material. The electric response can be tuned by adjusting the geometry of the top metallic electric ring resonator structure. We demonstrate that a rotation about the axis of THz wave propagation at normal incidence does not change the absorption or the resonance frequency by a significant amount. A value of absorption of 99.6 % at a resonance frequency of 0.84 THz can be achieved. We also demonstrate the characteristics of this absorber structure under various THz wave incidence angles, with respect to both the incident electric and magnetic fields.

A miniature neon indicator lamp, also known as a Glow Discharge Detector (GDD), costing about 50 cents was found to be an excellent room temperature THz radiation detector. Down conversion detection using the GDD for 300 GHz radiation is demonstrated in this study. Previous results with the GDD at 10 GHz showed 40 times better sensitivity using down conversion detection compared to direct detection. Preliminary results at 300 GHz showed better sensitivity by at least one order of magnitude using down conversion compared to direct detection. This can be improved by increasing reference beam power. In order to realize a down-conversion set up we used two synchronized THz sources based on RF multipliers. The first is a 300 GHz source and the second is a 300 GHz+Δf source, where Δf stands for the frequency difference between the two sources. Using a beam splitter configuration we combine the two frequencies for Δf=20 kHz and directed them to the GDD. Due to the unique detection mechanism of the GDD and its linear response, the difference frequency Δf was detected by the electronics circuits. We anticipate significant improvement in detection performance for higher values of Δf due to lower detector noise at higher frequencies.

We report on sub-wavelength THz plasmonic lenses based on 2 dimensional electron gas (2DEG) at AlGaN/GaN interface and also on few-layer graphene sheets. Circular gratings investigated in this study concentrate THz electric field into deep sub-wavelength scale by plasmonic excitations polarization independently. Propagation of a broadband pulse of EM waves in 0.5-10 THz was simulated by using a commercial FDTD simulation tool. The results show that concentric plasmonic grating structures can be used to concentrate THz into deep sub-wavelength down to λ/350 spot size and achieve very large field enhancements by plasmonic confinement which can be used for THz detection and possibly for sub-wavelength imaging. Electric field intensity under the central point can be orders of magnitude higher than the outer grating area. Moreover, plasmonic lens modes supported by system can be tuned with an applied voltage to gratings.

The SDA (Spectral Dynamics Analysis) method was applied for the detection and identification of ceramic explosives (a mixture of Al₂O₃ with Hexogen or Penthryte) hidden under different coverings - thin, rough, thick layers of Polyethylene foils and a layer of cotton. We analyzed THz pulses reflected from the samples at different angles - nearly 90° (Stand-Off reflection) and 45° (Specular reflection). We showed that in some cases the presence of covering can significantly distort the spectral properties of the reflected THz signal. Nevertheless, it is possible to find the identifiers characterizing the presence of explosive under the covering analyzing the spectrograms and dynamics of spectral lines of the main pulses and the sub-pulses following the main pulse.

The evolution of surface plasmon supermodes through the effective coupling of isolated surface plasmon modes in a semi-insulating quantum cascade laser (QCL) waveguide is thoroughly discussed here. The effect of varying the material and geometric parameters of GaSb/AlGaSb QCL operating at 3.0 THz are thoroughly studied using a full-vectorial finite element method.

Polarimetry is a well-developed technique in radar based applications and stand-off spectroscopic analysis at optical frequencies. Extension to terahertz (THz) frequencies could provide a breakthrough in spectroscopic methods since the THz portion of the electromagnetic spectrum provides unique spectral signatures of chemicals and biological molecules, useful for filling gaps in detection and identification. Distinct advantages to a THz polarimeter include enhanced image-contrast based on differences in scattering of horizontally and vertically polarized radiation, and measurements of the dielectric response, and thereby absorption, of materials in reflection in real-time without the need of a reference measurement. To implement a prototype THz polarimeter, we have developed low profile, high efficiency metamaterial-based polarization control components at THz frequencies. Static metamaterial-based half- and quarter-wave plates operating at 0.35 THz frequencies were modeled and fabricated, and characterized using a MHz resolution, continuous-wave spectrometer operating in the 0.09 to 1.2 THz range to verify the design parameters such as operational frequency and bandwidth, insertion loss, and phase shift. The operation frequency was chosen to be in an atmospheric window (between water absorption lines) but can be designed to function at any frequency. Additional advantages of metamaterial devices include their compact size, flexibility, and fabrication ease over large areas using standard microfabrication processing. Wave plates in both the transmission and reflection mode were modeled, tested, and compared. Data analysis using Jones matrix theory showed good agreement between experimental data and simulation.

We report on ultrahigh sensitive, broadband terahertz (THz) detectors based on asymmetric double-grating-gate (A-DGG) high electron mobility transistors, demonstrating a record responsivity of 2.2 kV/W at 1 THz with a superior low noise equivalent power of 15 pW/√Hz using InGaAs/InAlAs/InP material systems. When THz radiation is absorbed strong THz photocurrent is first generated by the nonlinearity of the plasmon modes resonantly excited in undepleted portions of the 2D electron channel under the high-biased sub-grating of the A-DGG, then the THz photovoltaic response is read out at high-impedance parts of 2D channel under the other sub-grating biased at the level close to the threshold. Extraordinary enhancement by more than two orders of magnitude of the responsivity is verified with respect to that for a symmetric DGG structure.

We propose Terahertz (THz) plasmonic devices based on linearly integrated FETs (LFETs) on Graphene. LFET structures are advantageous for (THz) detection since the coupling between the THz radiation and the plasma wave is strongly enhanced over the single gate devices and accordingly higher-order plasma resonances become possible. AlGaN/GaN heterostructure LFETs with their high sheet carrier concentration and high electron mobility are promising for plasmonic THz detection. Nevertheless, our numerical studies show that room temperature resonant absorption of THz radiation by the plasmons in AlGaN/GaN LFETs is very weak even if the integration density is sufficiently large. Our simulations also demonstrate that similar LFETs on Graphene, which has very large electron mobility, can resonantly absorb THz radiation up to 5th harmonic at room temperature. Additionally, we investigated LFETs with integrated cavities on Graphene. Such Periodic Cavity LFETs substantially enhance the quality factor of the resonant modes.

The two-dimensional plasma resonance excited in the channel of a field effect transistor has recently been utilized as the frequency-selective absorber in a monolithic far infrared plasmonic cavity detector. In this article we discuss the relevant parameters pertaining to engineering the plasmonic cavity and an integrated detection element as constituent elements of a resonant far infrared detector. The spectra of low-order plasmon modes in 18 μm and 34 μm long two-dimensional plasmonic cavities with 4 μm period grating gates have been measured. When the length of the plasma cavity is significantly larger than the gate length or period, the cavity length rather than grating period defines the plasmon wavevector. Electronic noise sources are considered; random telegraph noise is suggested as a dominant noise source when the device is operated as a highly resistive bolometric detector.

Advances in dielectric resonators and materials have arisen in the context of research into new particle acceleration techniques. In the wakefield accelerator, electromagnetic fields excited by an electron beam in a low loss dielectric structure are used to accelerate a second, trailing beam to high energy. Energy can be efficiently extracted from the beam in this manner and thus the accelerating structure can also be used as an RF source, with frequencies extending into the THz. New ferroelectrics are also finding significant uses in this technology; some of the applications discussed are nonlinear frequency multiplication and frequency agile (tunable) cavities.

Imaging with electromagnetic radiation in the THz frequency regime, between 0.2 THz and 10 THz, has made considerable progress in recent years due to the unique properties of THz radiation, such as being non-ionizing and transparent through many materials. This makes THz imaging and sensing promising for a plethora of applications; most notably for contraband detection and biomedical diagnostics. Though many methods of generation and detection terahertz radiation exist, in this study we utilize Terahertz Time Domain Spectroscopy (THz TDS) and THz digital holography using a coherent, tunable CW THz source. These methods enable access to both the amplitude and phase information of the traveling THz waves. As a result of the direct time-resolved detection method of the THz electric field, unique spectroscopic information about the objects traversed can be extracted from the measurements in addition to being able to yield intensity imaging contrast. Utilizing such capabilities for THz based imaging can be useful for both screening and diagnostic applications. In this work, we present the principles and applications of several reconstruction algorithms applied to THz imaging and sensing. We demonstrate its ability to achieve multi-dimensional imaging contrast of both soft tissues and concealed objects.

Terahertz imaging proves advantageous for metamaterials characterization since the interaction of THz radiation with the metamaterials produces clear patterns of the material. Characteristic "finger prints" of the crystal structure help locating defects, dislocations, contamination, etc. TDS-THz spectroscopy is one of the tools to control metamaterials design and manufacturing. A computational technique is suggested that provides a reliable way of calculation of the metamaterials structure parameters, spotting defects. Based on missing data analysis, the applied signal processing facilitates a better quality image while compensating for partially absent information. Results are provided.

The availability of light and robust structures has led to an increased use of composite materials in the aircraft industry. In order to verify and guarantee the high quality of the conventional and new composite elements, innovative approaches for non-destructive testing of these parts are required. The European research project "DOTNAC" proposes to develop a fast, high resolution, non-invasive and non-contact inspection system for assessing aeronautic composite parts during production using terahertz waves. Conventionally two categories of systems can be discussed: pulsed and continuous wave terahertz systems. Both will be realized and their respective potential as a non-destructive inspection tool will be evaluated against the performance of X-ray testing, ultrasound non-destructive testing, and infra-red imaging.