This PDF file contains the front matter associated with SPIE Proceedings Volume 8624, including the Title Page, Copyright Information, Table of Contents, Introduction and Conference Committee listing.

Research in THz imaging is generally focused on three primary application areas: medical, security, and nondestructive evaluation (NDE). While work in THz security imaging and personnel screening is populated by a number of different active and passive system architectures, research in medical imaging in is generally performed with THz time-domain systems. These systems typically employ photoconductive or electro-optic source/detector pairs and can acquire depth resolved data or spectrally resolved pixels by synchronously sampling the electric field of the transmitted/reflected waveform. While time-domain is a very powerful scientific technique, results reported in the literature suggest that desired THz contrast in medical imaging may not require the volume of data accessible from time-resolved measurements and that a simpler direct detection, active technique may be sufficient for specific applications. In this talk we discuss an active direct detection reflectometer system architecture operating at a center frequency of ~ 525 GHz that uses a photoconductive source and schottky diode detector. This design takes advantage or radar-like pulse rectification and novel reflective optical design to achieve high target imaging contrast with significant potential for high speed acquisition time. Results in spatially resolved hydration mapping of burn wounds are presented and future outlooks discussed.

We present the microfabrication and cold test measurement results of serpentine waveguide amplifier circuits at 220 GHz. The circuits were fabricated using a novel embedded polymer monofilament technique combined with Ultraviolet- LIGA to simultaneously create both the beam tunnel and interaction circuits. We find remarkable characteristic matches between the measurements of the best circuits, illustrating that the process developed is able to create repeatable, highly precise circuits with high yield. It was found that slight beam tunnel misalignment can cause very strong stopbands to appear in the operating band due to bi- or quasi-periodicity. The NRL code TESLA-SW/FW has been used to rapidly simulate the as-built structure under a variety of conditions to accurately predict the performance with an electron beam. The tolerances needed on beam tunnel alignment are studied, with implications extending to the THz range.

We demonstrate the tunable continuous-wave (CW) terahertz generator based on the λ/4 phase-shifted 1.3 μm dual-mode laser diode (DML) and travelling-wave photodiode (TWPD). The DML and TWPD operate as an optical beat source and terahertz photomixer, respectively. The laser diodes (LDs) operating at the 1.3 μm have more suitable characteristics as optical beat sources than the LDs operating at 1.55 μm because of their high efficiency and better thermal stability. The micro-heaters are integrated on top of each DFB LD for mode beat frequency tuning. The fabricated DML was continuously tuned from 230 GHz to 1485 GHz by increasing the temperature of each DFB section independently via integrated micro-heaters. The high-speed TWPD with an InGaAs absorber was designed and fabricated to efficiently generate the photomixing terahertz CW. A complementary log-periodic antenna was integrated with the TWPD to radiate the generated terahertz wave with minimum reflection in the wide frequency range. The terahertz characteristics of the tunable CW terahertz generator based on the DML and TWPD were measured in a fiber-coupled, homodyne terahertz photomixing system. Our results of the tunable CW terahertz generator show the feasibility of a compact and highly efficient CW terahertz spectrometer and imager.

Due to the rapid growth of the biomedical and the security applications, the interest in the development of compact and flexible sub-mm/THz radiation sources has grown significantly. In this paper we propose a novel source design to address this need. Taking the advantage of the Photonic Crystal (PC) structure, a metallic defected PC, with no axial discontinuities, was designed as a Slow Wave Structure (SWS). This SWS was used in a high power Cerenkov-transition based oscillator/amplifier working in sub-mm/THz frequency regime. The SWS allows electron beams to travel axially in it, facilitating the beam-wave interaction required for the desired signal generation. Two coaxial transitions are used as input and output for the proposed device. The analysis of the proposed device was done using a combined Finite Difference Time Domain (FDTD) / Particle In Cell (PIC) simulation. Using the FDTD thin wire model, coaxial transitions were modeled to feed/extract the input/generated signals. To show the potential of the proposed device, two design examples; one for the amplifier and the other for the oscillator, are presented.

Emission of terahertz (THz) radiations from interdigitated GaN quantum-wells structures under DC-bias has been measured at room temperature. This measurements has been performed by a 4K Si-Bolometer associated with a Fourier Transform Spectrometer. Using an analytical model, we have shown that the observed peak at approximately 3 THz due to 2D ungated plasma-waves oscillations in the quantum well, is emitted by the metallic contacts of our device acting as antennas.

In this paper, the manipulation of light properties in the pure optical domain and its limits are briefly reviewed, followed by the general descriptions of the advantages and the feasibility of altering the light properties by using RF signals. The principles of manipulating light, including continuous wave (CW) light and pulsed light sources, are introduced, respectively. Several research systems which have recently emerged for potential applications in optical communication and optical sensing networks are discussed in order to demonstrate the understanding of how light can be modified by RF signals.

In this paper, we conduct transmission and reflection mode terahertz time-domain spectroscopy (THz-TDS) measurements of organic semiconductors such as ALQ3 and TBADN. THz-TDS is effective for determining the purity of the organic semiconductors based on the refractive index and spectral signatures in THz range. In order to prepare the sample for a custom built sample holder, the powder samples are pressed into pellets of 13 mm diameter and a thickness of 2 mm using a hydraulic press. The organic semiconductor, for example ALQ3 sample, is prepared as a 70% ALQ3 and 30% polyethylene (PE) concentration pellet by mixing ALQ3 and PE. The ALQ3 pellet is measured in a chamber purged with dry nitrogen to avoid the effect of water vapor absorptions in ambient air. The absorption coefficient and index of refraction are measured from the spectra of the reference THz pulse and the THz pulse after transmission through the sample. The THz spectrum is obtained by applying a fast Fourier transform to the THz waveform. Further studies were conducted by reducing the concentration of the organic semiconductor from 70% to 10% ALQ3. We also obtained the spectral signature and absorption coefficient for 50% TBADN 50% PE pellet. The spectral signatures of ALQ3 were found to be at 0.868 THz, 1.271 THz and 1.52 THz, while spectral signature of TBADN was found to be at 1.033 THz.

We present a characterization of THz beams generated in both a two-color air plasma and in a LiNbO₃ crystal. Using a commercial THz camera, we record intensity images as a function of distance through the beam waist, from which we extract 2D beam profiles and visualize our measurements into 3D beam profiles. For the two-color air-plasma, we measure a conical beam profile that is focused to a bell-shape at the beam waist, whereas we observe a Gaussian beam profile for the THz beam generated from the LiNbO₃ crystal.

We report the development and initial results of two Terahertz imaging systems based on monochromatic sources at 0.2 and 2.52 THz. The first is based on a microwave oscillator, whose frequency is multiplied to 0.2 THz, used in conjunction with a zero-bias detector. The sample is scanned across the beam, and transmission images are obtained after processing. The second system allows real-time images, and consists of a methanol gas laser emitting at 119 microns (2.52 THz) and a commercial camera based on a microbolometer array. We describe the construction and performance of the methanol laser and a tunable CO₂ laser, which emits 20 W at the 9P(36) pump line. Due to the high coherence of the laser, this system is particularly suited for diffraction and interference imaging. We have measured the absorption coefficients of a few samples assuming the Beer law.

Terahertz is a field in expansion with the emergence of various security needs such as parcel inspection and through-camouflage vision. Terahertz wavebands are characterized by long wavelengths compared to the traditional infrared and visible spectra. However, it has recently been demonstrated that a 52 μm pixel pitch microscanned down to an efficient sampling pitch of 26 μm could provide useful information even using a 118.83 μm wavelength. With this in mind, INO has developed a terahertz camera core based on a 384x288 pixel 35 μm pixel pitch uncooled bolometric terahertz detector. The camera core provides full 16-bit output video rate.

We present an overview of the theoretical and experimental research activities at the U.S. Naval Research Laboratory in the area of millimeter-wave and sub-millimeter-wave vacuum electronics. Amplifier and related circuit development ranging in frequency from 30 GHz to 1.35 THz at power levels from multi-kilowatts to watts will be described along with electron gun topologies that include single-, multiple-, and sheet electron beams. We also describe new ultra-violet photolithographic techniques that enable the fabrication of thick (up to 1000 μm), high vertical aspect ratio, solid copper structures with integrated beam tunnels with length-to-diameter aspects ratios of <500.

This paper reports on the development of thin film lithium niobate (TFLN™) electro-optic devices at SRICO. TFLN™ is formed on various substrates using a layer transfer process called crystal ion slicing. In the ion slicing process, light ions such as helium and hydrogen are implanted at a depth in a bulk seed wafer as determined by the implant energy. After wafer bonding to a suitable handle substrate, the implanted seed wafer is separated (sliced) at the implant depth using a wet etching or thermal splitting step. After annealing and polishing of the slice surface, the transferred film is bulk quality, retaining all the favorable properties of the bulk seed crystal. Ion slicing technology opens up a vast design space to produce lithium niobate electro-optic devices that were not possible using bulk substrates or physically deposited films. For broadband electro-optic modulation, TFLN™ is formed on RF friendly substrates to achieve impedance matched operation at up to 100 GHz or more. For narrowband RF filtering functions, a quasi-phase matched modulator is presented that incorporates domain engineering to implement periodic inversion of electro-optic phase. The thinness of the ferroelectric films makes it possible to in situ program the domains, and thus the filter response, using only few tens of applied volts. A planar poled prism optical beam steering device is also presented that is suitable for optically switched true time delay architectures. Commercial applications of the TFLN™ device technologies include high bandwidth fiber optic links, cellular antenna remoting, photonic microwave signal processing, optical switching and phased arrayed radar.

An intensity modulation direct detection RF photonic link using a dual output Mach-Zehnder modulator and a balanced detection scheme has been modeled and simulated. Validation of the model was performed by comparing the optical third-order intercept point, spur free dynamic range, and the gain results to actual provided industry measured output metrics. The model is highly accurate and provides the basis for demonstrating the RF photonics links performance as a function of varied input parameters such as power levels, detector performance, and varied fiber lengths. This allows the system designer to analyze performance parameters that are not possible in a laboratory environment. In addition the designer can analyze the performance of new and improved link designs without having to incur significant fabrication or manufacturing costs associated with prototypes. The link architecture and specific implementation challenges particular to the link are discussed. Performance comparisons are shown between the models to the theoretical calculations as well as to collected experimental data.

A system for generating widely tunable, narrow-line RF signals from a pair of injection-locked lasers has been developed. By injection seeding one laser with a sideband obtained from a second laser that is phase modulated by a spectrally pure low-frequency RF reference oscillator and then mixing these coherent optical outputs on a fast photodiode, RF with the same linewidth and phase noise characteristics as the reference oscillator can be produced. By using harmonics of the reference oscillator the RF output can be tuned over a much wider range than the reference oscillator while maintaining its narrow linewidth.Here we present the results of our efforts to develop an integrated version of this system, based on a silicon-photonic integrated circuit coupled to III-V semiconductor gain chips.Towards that goal we have successfully demonstrated an integrated module and shown tunable RF generation with a 1 Hz linewidth.

We present a photonic integrated circuit, which enables the full control of the THz signal in continuous wave photomixing THz systems via standard electronics. The device comprises two DFB-lasers and an optical phase modulator on a single chip. Due to a unique bidirectional operation technique, the chip provides the optical beat signal for both THz emitter and THz receiver and allows for manipulation of the THz phase via the optical phase modulator. To evaluate the performance of our solution, we realize a coherent cw THz system based on our photonic integrated circuit and compare it to discrete lasers and standard components. As the results show, both setups feature an identical signal-to-noise ratio, reaching 50 dB at a frequency of 1 THz for an integration time of 500 ms. This is the best reported performance of CW photomixing systems running at 1.5 μm optical wavelength.

We present the stabilization of the beatnote of an Er,Yb:glass Dual Frequency Laser at 1.53 μm with optical fiber delay lines. Instead of standard optoelectronics oscillators, this architecture does not need RF filter and offers a wide tunability from 2.5 to 5.5 GHz. Thank to a fine analysis of the laser RIN to phase noise conversion in the photodiodes, the expected RF-amplifiers noise limit is reached with a phase noise power spectral density of -25 dBc/Hz at 10 Hz (respectively -110 dBc/Hz at 10 kHz) from the carrier over the whole tuning range. Implementation of a double fiber coil architecture improves the oscillator spectral purity: the phase noise reaches a level of -35 dBc/Hz at 10 Hz (respectively -112 dBc/Hz respectively 10 kHz) from the carrier.

Continuous wave terahertz (THz) imaging has the potential to offer a safe, non-ionizing, and nondestructive medical imaging modality for delineating colorectal cancer. Fresh excisions of normal colon tissue were obtained from surgeries performed at the University of Massachusetts Medical School, Worcester. Reflection measurements of thick sections of colorectal tissues, mounted in an aluminum sample holder, were obtained for both fresh and formalin fixed tissues. The two-dimensional reflection images were acquired by using an optically pumped far-infrared molecular gas laser operating at 584 GHz with liquid Helium cooled silicon bolometer detector. Using polarizers in the experiment both co-polarized and cross-polarized remittance form the samples was collected. Analysis of the images showed the importance of understanding the effects of formalin fixation while determining reflectance level of tissue response. The resulting co- and cross-polarized images of both normal and formalin fixed tissues showed uniform terahertz response over the entire sample area. Initial measurements indicated a co-polarized reflectance of 16%, and a cross-polarized reflectance of 0.55% from fresh excisions of normal colonic tissues.

Recent progress in the investigation of millimeter-wave and THz detectors based on plasmon excitation in the twodimensional electron gas (2DEG) of a high electron mobility transistor (HEMT) is reported. A tunable resonant polarized photoresponse to mm-wave radiation in the frequency range of 40 to 110 GHz is demonstrated for a gratinggated InGaAs/InP based device. The gate consisted of a metal grating with period of 9 μm specifically designed for excitation of sub-THz plasmons. The resonant excitation of plasmons, which shifts with gate-bias, changes the channel conductance. This resonant change in channel conductance enables potential applications in chip-scale frequency-agile detectors, which can be scaled to mid-THz frequencies.

We report spatially resolved measurements of frequency, phase, and amplitude of GHz and THz emitters employing an unstabilized THz frequency comb. Different optoelectronic detection methods are compared regarding accuracy and invasiveness. The measurement setup allows for high-precision measurements of the frequency (accuracy 9·10^-14), relative amplitude (standard deviation of the mean 0.1 %) and phase (standard deviation of the mean 0.2 °). By simultaneously measuring the emission of a 30 GHz emitter as well as the emission of a CO₂ laser at 28 THz, a large spectral coverage is demonstrated. Keywords:

The development of miniaturized Golay cell arrays would enable the combination of the high sensitivity of a Golay cell with the imaging capability of focal plane arrays. The critical component of a miniaturized Golay cell is the deflecting membrane, which must simultaneously have a high breaking strength and a low flexural rigidity. Graphene suits this purpose ideally on account of its high strength and atomic thickness, in contrast with thicker polymeric membranes. Low flexural rigidity is critical to deflection sensitivity in response to temperature changes of the gas enclosed within a Golay cell scaled to the 10 μm to 100 μm scale. We report here a simple method for fabrication of suspended graphene membranes suitable for Golay cells. The technique is based on chemical vapour deposition of graphene on copper, followed by a sacrificial etch of the copper substrate. By this organic-free technique, graphene can be suspended over 10 - 20 μm apertures in copper thin films free of surface contamination and with high structural integrity. The cavities are sealed on the back-side with an indium film to produce proof-of-principle miniature cells with a flexible, suspended graphene membrane. Atomic force microscopy enables the force versus deflection curve of a graphene-enclosed cell to be characterized. We further report the temperature dependent equilibrium deflection (up to 60°C) of a graphene-enclosed cell by atomic force microscopy measurements taken with heat directly applied to the cell substrate.

Electromagnetic media and metamaterials have been explored in frequency regimes ranging from the acoustic to the visible domain over the past decade. A large part of the design, fabrication and prototyping of such materials has focused on planar structures and devices have been demonstrated primarily for certain propagation directions and/or defined polarization. Here, we present the design of a focusing GRadient INdex (GRIN) lens that operates at RF frequencies and is not polarization constrained. We compare the theoretical and experimental results from this lens designed to operate at X-band and fabricated using 3D printing technology to implement the effective medium. The lens with radially varying refractive index gradient was designed, optimized and analyzed by conducting full-wave simulations finite-element method based software. The permittivity was estimated by effective medium theory and calculated using HFSS®. The optimized design was used to fabricate the GRIN lens with isotropic, inhomogenous dielectric material. The refractive index was designed to match the theoretical results using mixing ratio of air/voids and a polymer. Further, we used the refractive index profile to predict the rays’ trajectories and focus length to compare them to those predicted by the FEM simulations. The field distributions were also analyzed to compare performance of the theoretical design to the fabricated lens and were found to be in good agreement with each other.

Fiber Bragg grating (FBG) sensors have numerous advantages to sense multi-physical quantities such as the temperature and strain simultaneously by monitoring the shift of the returned “Bragg” wavelength resulting from changes in these quantities. Several FBG interrogation systems have been set up using photo detectors instead of an optical spectrum analyzer (OSA) to convert wavelength to time measurements. However, in those systems, it is necessary to use mechanical tuning components to generate fast-speed wavelength-swept light sources for high-precision FBG interrogation. In this paper, a low-cost and delicate wavelength-shift detection system, without any mechanical scanning parts, is proposed and demonstrated. The wavelength scanning system is a recirculating frequency shifter (RFS) which consists of an optical amplifier, an under test FBG sensor and an optical single-sideband (SSB) modulator driven by RF signals at 10 GHz. The measurement accuracy of this system is 0.08nm.

Dual vertical slot modulators leverage the field enhancement provided by the continuity of the normal electric flux density across a boundary between two dielectrics to increase modal confinement and overlap for the propagating optical and RF waves. This effect is achieved by aligning a conventional silicon-based optical slot waveguide with a titanium dioxide RF slot. The TiO₂ has an optical refractive index lower than silicon, but a significantly higher index in the RF regime. The dual slot design confines both the optical and RF modes to the same void between the silicon ribs of the optical slot waveguide. To obtain modulation of the optical signal, the void is filled with an organic electro optic material (OEOM), which offers a high optical non-linearity. The optical and RF refractive index of the OEOM is lower than silicon and can be deposited through spin processing. This design causes an extremely large mode overlap between the optical field and the RF field within the non-linear OEOM material which can result in a device with a low Vπ and a high operational bandwidth. We present work towards achieving various prototypes of the proposed device, and we discuss the fabrication challenges inherent to its design.

Rational harmonic mode locking (RHML) in an active mode-locked fiber laser can increase the output pulse repetition rate a number of times the modulation frequency of an optical modulator in a cavity when driven by gigahertz (GHz) RF. The amplitudes of the output optical pulse train in a high order RHML operation are not equalized and flat due to the GHz RF drive signals. A modified RHML technique using standard instrumentation that generates 1 GHz electrical square wave signals to accomplish up to 6^th order RHML in fiber lasers is presented for improving the flatness of the amplitudes of the output optical pulse train at the pulse repetition rate of up to 12 GHz.

Continued progress in terahertz (THz) research has emphasized the need for both improved THz sources and detectors. One approach to generate a narrowband THz radiation is to use metamaterial absorbers as thermal emitters. We present metamaterial based THz emitters consisting of a 100 nm aluminum layer patterned into squares separated from a ground plane of aluminum by a thin layer of silicon oxide (<2 μm) fabricated using standard microfabrication techniques. These metamaterials were designed to emit in one, two, and three different bands of the 4-8 THz range and demonstrate clearly definable separate peaks with bandwidths of approximately 1 THz. Modifying the multiple band configurations can produce relatively broad emission peak if desired. Single band emitters designed for 4.1, 5.4, and 7.8 THz were observed to emit, respectively, 11, 18, and 36 W/m² at 400 °C in accordance with Kirchhoff's law of thermal radiation. Coating a 4-inch wafer with these materials and heating it to 400 °C would produce an estimated 86, 145, and 280 mW of power, respectively. Additionally, emitted power increased linearly with temperature, as expected from the Planck’s radiation law in the THz spectral region at elevated temperatures. Emissivity of the metamaterial did not change significantly when heated, indicating that the constituent materials did not significantly change their optical or geometric properties.

We report on the fabrication of a microelectromechanical systems (MEMS) based bi-material terahertz (THz) detector integrated with a metamaterial structure to provide high absorption at 3.8 THz. The absorbing element of the sensor was designed with a resonant frequency that matches the quantum cascade laser illumination source, while simultaneously providing structural support, desired thermomechanical properties and optical read-out access. It consists of a periodic array of aluminum squares separated from a homogeneous aluminum (Al) ground plane by a silicon-rich silicon oxide (SiO_x) layer. The absorbing element is connected to two Al/SiO_x microcantilevers (legs), anchored to a silicon substrate, which acts as a heat sink, allowing the sensor to return to its unperturbed position when excitation is terminated. The metamaterial structure absorbs the incident THz radiation and transfers the heat to the legs where the significant difference between thermal expansion coefficients of Al and SiO_x causes the structure to deform proportionally to the absorbed power. The amount of deformation is probed optically by measuring the displacement of a laser beam reflected on the Al ground plane of the metamaterial absorber. Measurement showed that the fabricated absorber has nearly 95% absorption at 3.8 THz. The responsivity and time constant were found to be 1.2 deg/μW and 0.65 s, respectively. The minimum detectable incident power including the readout noise is around 9 nW. The obtained high sensitivity and design flexibility indicate that sensor can be further tuned to achieve the required parameters for real time THz imaging applications.

Frequency dispersion and damping mechanisms of two-dimensional plasmons in graphene are studied by the numerical simulation based on the Boltzmann equation. The fundamental plasmon mode in a single-grating-gate structure is studied, and the gate-voltage tunability of its frequency as well as the coupling effect of plasmons in the gated and ungated regions are revealed. It is demonstrated that damping rates due to the acoustic-phonon scattering at room temperature and due to the short- and finite-range disorder scattering can be on the order of 10¹¹ s^-1, depending on the level of disorders. In addition, the damping due to the source and drain contacts is reported and its mechanism is discussed.

Astronomical observations in the far-infrared are critical for investigation of cosmic microwave background (CMB) radiation and the formation and evolution of planets, stars and galaxies. In the case of space telescope receivers a strong heritage exists for corrugated horn antenna feeds to couple the far-infrared signals to the detectors mounted in a waveguide or cavity structure. Such antenna feeds have been utilized, for example, in the Planck satellite in both single-mode channels for the observation of the CMB and the multi-mode channels optimized for the detection of foreground sources. Looking to the demands of the future space missions, it is clear that the development of new technology solutions for the optimization and simplification of horn antenna structures will be required for large arrays. Horn antennas will continue to offer excellent control of beam and polarization properties for CMB polarisation experiments satisfying stringent requirements on low sidelobe levels, symmetry and low cross polarization in large arrays. Similarly for mid infrared systems multi-mode waveguide structures will give high throughput to reach the required sensitivities. In this paper we present a computationally efficient approach for modelling and optimising horn characteristics. We investigate smooth-walled profiled horns that have a performance equivalent to that of the corrugated horns traditionally used for CMB measurements. We discuss the horn optimisation process and the algorithms available to maximise performance of a merit parameter such as low cross polarisation or high Gaussicity.

The terahertz (THz) region occupies a large portion of the electromagnetic spectrum, located between the microwave and optical frequencies and normally is defined as the band ranging from 0.1 to 10 THz. In recent years, this intermediate THz radiation band has attracted considerable interest, because it offers significant scientific and technological potential for applications in many fields, such as sensing [1], imaging [2] and spectroscopy [3]. However, waveguiding in this intermediate spectral region is a major challenge and strong dielectric and conductive losses in the terahertz frequency range have been a major problem for waveguiding. The conventional guiding structures exemplified by microstrips, coplanar striplines and coplanar waveguides [4] are highly lossy and dispersive. However, so far the most promising dielectric waveguides have been the use of photonic crystal fibers at terahertz frequencies [5, 6] and metal coated guides [7] at terahertz frequencies. In this paper, various types of practical dielectric and metal coated waveguides are evaluated and design optimization of Quantum Cascade Lasers, MMI-based power splitters and narrow-band filters are presented, by using full-vectorial finite element method [8].

Wavelength conversion (WC) imaging is a methodology that employs temperature sensitive detectors to convert photoinduced termperature into a detectable optical signal. One specific method is to use molecular detectors such as thermochromic liquid crystals (TLC), which exhibits thermochromism to observe the surface temperature of an area by observing the apparent color in the visible spectrum. Utilizing this methodology, an ultra-broadband room temperature imaging system was envisioned and realized using off the shelf thermochromic liquid crystals. The thermochromic properties of the sensor were characterized to show a thermochromic coefficient α = 10%/°K and a noise equivalent power (NEP) of 64 μW. With the TLC camera, images of both pulsed and continuous wave (CW) sources spanning 0.6 μm to 150 μm wavelengths were captured to demonstrate its potential as a portable, low-cost, and ultra-broadband imaging tool.

We present the realization of high electron mobility transistors on GaN-heterostructures usable for mixing and rectification in the THz range. Device fabrication is fully compatible with industrial processes employed for millimetre wave integrated circuits. On-chip, integrated, polarization-sensitive, planar antennas were designed to allow selective coupling of THz radiation to the three terminals of field effect transistors in order to explore different mixing schemes for frequencies well above the cutoff frequency for amplification. The polarization dependence of the spectral response in the 0.18-0.40 THz range clearly demonstrated the possible use as integrated heterodyne mixers.

Femtosecond optoelectronic techniques are routinely employed for the generation and detection of ultrashort voltage pulses. However, so far, not much effort has been spend to determine the exact shape of such voltage pulses over a very broad frequency range. For this purpose, i.e., for the realization of a broadband voltage pulse standard, it is essential to (i) know the transfer function of the detection technique and (ii) be able to separate forward and backward propagating signals from each other. Here we report the realization of a voltage pulse standard with frequency components exceeding 500 GHz and a 500 MHz frequency spacing.

We present the design of a compact and highly sensitive electric field sensor based on a bowtie antenna-coupled slot photonic crystal waveguide (PCW). An electro-optic (EO) polymer with a large EO coefficient, r₃₃=100pm/V, is used to refill the PCW slot and air holes. Bowtie-shaped electrodes are used as both poling electrodes and as receiving antenna. The slow-light effect in the PCW is used to increase the effective in-device r₃₃>1000pm/V. The slot PCW is designed for low-dispersion slow light propagation, maximum poling efficiency as well as optical mode confinement inside the EO polymer. The antenna is designed for operation at 10GHz.

Terahertz uncooled antenna-coupled microbolometer focal plane arrays are being developed at CEA Leti for real time THz imaging and sensing. This detector relies on LETI amorphous silicon uncooled infrared bolometer technology that has been deeply modified to optimize sensitivity in the THz range. The main technological key lock of the pixel structure is the quarter wavelength cavity that consists in a thick dielectric layer deposited over the metalized CMOS wafer; such cavity improves significantly the optical coupling efficiency. Copper plugs connect the microbolometer level down to the CMOS readout circuit (ROIC) upper metal pads through this thick dielectric cavity. This paper explains how we have improved the copper vias technology and the challenges we have faced to customize the microbolometer while keeping a monolithically above IC technology fully compatible with standard silicon processes. The results show a very good operability and reproducibility of the contact through this thick oxide cavity. Due to these good results, we have been able to characterize a very efficient THz absorption that enables real time imaging with high sensitivity in the 1-3 THz range.

In this paper, we report novel designs of tunable THz plasmonic devices. The designed devices will be able to dynamically control and change the spectrum responses of extraordinary THz wave transmissions. The tuning of the devices is accomplished by electrowetting-controlled liquid metals morphing. Different THz device configurations are investigated and numerical simulations have been conducted to theoretically predict the feasibility of proposed structures. Since all of these devices will be constructed by liquid metals, their geometrical shapes can be actively modulated by electrowetting. In this way, THz devices with tunable wave transmission property can be realized. These new THz devices are expected to be applied in various areas of sensing, communication, and imaging.