This PDF file contains the front matter associated with SPIE Proceedings Volume 8985 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

This paper summarizes our recent progress on the discovery and THz performance of ErAs:GaAs photoconductive devices driven around 1550 nm. We will present the impulse response of such device in a time-domain spectrometer where the detection is realized with a GaAs electro-optic crystal. The full width at half-maximum of the temporal pulse is 500 fs and the corresponding bandwidth is greater than 2.5 THz. We also present different 1550-nm properties of this material including carrier lifetime by pump-probe phototransmission. All evidence to date suggests that the 1550-nm ultrafast behavior in ErAs:GaAs occurs by extrinsic photoconductivity, not two-photon effect.

The output of seeded, dual periodically poled lithium niobate (PPLN) optical parametric generators (OPG) are combined in the nonlinear crystal 4-dimthylamino-N-methyl-4-stilbazolium-tosylate (DAST) to produce a widely tunable narrowband THz source via difference frequency generation (DFG). We have demonstrated that by employing this type of configuration we are able to tune our system seamlessly, without mode-hops, from 1.5 THz to 21THz with a minimum bandwidth of 3.1 GHz. The bandwidth of the source was measured by using the THz transmission spectrum of water vapor lines over a 3-meter path length. By selecting of the DFG pump wavelength to be at 1380 nm and the signal wavelength to tune over a range from 1380 nm to 1570 nm, we produced several maxima in the output THz spectrum that was dependent on the phase matching ability of the DAST crystal and the efficiency of our pyro-electric detector. Due to the effects of dispersive phase matching, filter absorption of the THz waves, and two-photon absorption multiple band gaps in the overall spectrum occur and are discussed. Employing the dual generator scheme, we have obtained THz images at several locations in the spectrum using an infrared camera that runs at a rate of 35 frames per second. We have demonstrated the ability to image 13 THz to 20 THz under static conditions. We will present images of carbon fibers illuminated at different THz frequencies.

We designed and theoretically investigated nonlinear optical micro-ring resonators for tunable terahertz (THz) emission in 1-10 THz range by using difference frequency generation (DFG) phenomenon with large second order optical nonlinearity (χ(²)). Our design consists of a nonlinear ring resonator and another ring underneath capable of sustaining high-Q resonant modes for infrared pump beams and the generated THz radiation, respectively. The nonlinear ring resonator generates THz through DFG by mixing the input waves carried by a pair of waveguides. The proposed device can be a viable platform for tunable, compact THz emitters and on-chip integrated spectrometers.

A novel type of semiconductor beating source, a monolithically integrated dual-mode laser, and continuous-wave terahertz (THz) system adopting it will be investigated. The combined system of the beating source with broadbandantenna- integrated low-temperature-grown semiconductor photomixers shows the possibility of the realization of the cost-effective and compact continuous-wave THz systems. Such a system is highly-demanded to examine the THz finger prints of specimens without limitations. Since the optimized performance depends not only on the characteristics of functional devices but also module configurations, various approaches such as traveling-wave photomixers, Schottky barrier diodes, and nano-structure contained photomixers have been investigated to implement high-performance THz platforms as the main building blocks of a THz system. Semiconductor-based compact and cost-effective photonics technologies will envisage the bright future of THz systems.

We investigate through numerical simulation and analytic design the electromagnetic coupling properties of a novel, contact-free, antenna-free optoelectronic probe that should operate up to 1 THz and beyond. The probes are based on continuous-wave photomixers and are designed to transmit and receive THz waves propagating along a coplanar waveguide (CPW) containing a device-under-test (DUT). Two kinds of designs have been investigated: a broadband design that performs up to 1300 GHz with coupling exceeding -27 dB over 1 THz, and a narrow band design showing a coupling better than -12 dB over a bandwidth of ≈100 GHz centered about a chosen operating frequency.

Intense Terahertz radiation in organic crystals is typically generated by optical rectification of short wavelength infrared femtosecond lasers between 1.3 and 1.5 μm. In this wavelength range high energy ultrashort pump sources are hardly available. Here we present results on powerful THz generation by using DAST and DSTMS pumped directly by the widely used and well-established Ti:sapphire laser technology, emitting at 0.8 μm. This approach enables straightforward THz generation by optical rectification. We present systematic studies on
nIR-to-THz conversion efficiency, damage threshold, and on the emitted THz spectrum and field strength.

The development of low loss, small size and flexible waveguides is one of the most challenging issues of THz research due to the poor characteristics of both metal and dielectrics in this frequency range. Hollow core tube lattice fibers (HCTLFs) have been recently proposed and experimentally demonstrated to overcome this problem. However, they require very large hollow core size leading to big and hardly flexible fibers. Scaling law analysis plays an important role in determining the best trade-off between low loss and small fiber diameter. The dependence of the confinement on frequency and core radius are here numerically investigated. Results show that confinement loss exhibits a stronger dependence on core size and frequency with respect to other hollow core fibers proposed for THz waveguiding, such as Bragg, Tube, and Kagome fibers.

This paper presents novel a first pass on the thorough analysis of THz optical designs intended for image acquisition of burn wounds in animal models. Current THz medical imaging research typically employs and fixed source detector architecture coupled by a train of off-axis parabolic mirrors. When used individually, parabolic mirrors have near diffraction limited focusing properties, extremely low loss, and are dispersion free. However, when a combination or train of multiple parabolic mirrors are utilized geometric errors can be generated early in the train and exacerbated as the beam propagates to the detector. These errors manifest as significant increases in spot size, asymmetries about the optical axis in beam irradiance and polarization, and the generation of cross polarization components. This work presents a novel configuration of off-axis parabolic mirrors designed to maximize the practicality of beam alignment and image acquisition. Quasi-physical optics simulations of the optical performance are described and significant perturbations in polarization symmetry were observed. The configuration can be described as in between two canonical parabolic mirror configurations. The performance of three different pairs of off-axis parabolic mirror pairs coupled to the novel configuration are presented herein.

Continuous wave terahertz (THz) imaging has the potential to offer a safe, noninvasive medical imaging modality for delineating colorectal cancer. The terahertz reflectance measurements of fresh 3 – 5 mm thick human colonic excisions were acquired using a continuous-wave polarization imaging technique. A CO₂ optically pumped Far- Infrared molecular gas laser operating at 584 GHz was used to illuminate the colon tissue, while the reflected signals were detected using a liquid Helium cooled silicon bolometer. Both co-polarized and cross-polarized remittance from the samples was collected using wire grid polarizers in the experiment. The experimental analysis of 2D images obtained from THz reflection polarization imaging techniques showed intrinsic contrast between cancerous and normal regions based on increased reflection from the tumor. Also, the study demonstrates that the cross-polarized terahertz images not only correlates better with the histology, but also provide consistent relative reflectance difference values between normal and cancerous regions for all the measured specimens.

Metallic rod array is successfully demonstrated to be integrated a parallel plate waveguide and used as a slab-waveguide for sensing applications. Both the waveguide properties are characterized from the transmission spectra using different polarization conditions of terahertz electromagnetic fields, which are parallel and perpendicular to the rod axis. When THz field polarization is parallel to the rod axis, there is high-pass filtering feature with structure-period-dependent threshold frequency, corresponding to the plasma frequency of the rod-array composite. For the polarization perpendicular to the rod axis, there is Bragg diffraction frequency as forbidden band with considerably low transmission power. The metal-rod-array integrated parallel-plate waveguide is connected with a microfluidic channel to sense liquids, and the minimum detectable quantity is about 13μmol/mm³. The metal-rod-array slab waveguide is able to sense thinfilm with nanometer level, and the detectable optical path difference is down to λ/2380. The sensor properties of multiple functions for sensing various analyte conformations, microfluidic integration and high sensitivity would be applied in biomedicine and biochemical applications.

Here we demonstrate for the first time that terahertz time domain spectroscopy (THz-TDS) can be used to distinguish doping profile discrepancies in semiconductor silicon wafers. These proof of concept results suggest the suitability of the technique for in-line process control applications in both IC/photovoltaic (PV) industries. The experimental results show that THz radiation is sensitive to the implant dosage changes in the time domains.

Terahertz (THz) frequency band lies between the microwave and infrared region of the electromagnetic spectrum. Molecules having strong resonances in this frequency range are ideal for realizing "Terahertz tags" which can be easily incorporated into various materials. THz spectroscopy of molecules, especially at frequencies below 10 THz, provides valuable information on the low frequency vibrational modes, viz. intermolecular vibrational modes, hydrogen bond stretching, torsional vibrations in several chemical and biological compounds. So far there have been very few attempts to engineer molecules which can demonstrate customizable resonances in the THz frequency region. In this paper, Diamidopyridine (DAP) based molecules are used as a model system to demonstrate engineering of THz resonances (< 10 THz) by fine-tuning the molecular mass and bond strengths. Density Functional Theory (DFT) simulations have been carried out to explain the origin of THz resonances and factors contributing to the shift in resonances due to the addition of various functional groups. The design approach presented here can be easily extended to engineer various organic molecules suitable for THz tags application.

Concrete, a mixture of cement, coarse aggregate, sand and filler material (if any), is widely used in the construction industry. Cement, mainly composed of Tricalcium Silicate (C3S) and Dicalcium Silicate (C2S) reacts readily with water, a process known as hydration. The hydration process forms a solid material known as hardened cement paste which is mainly composed of Calcium Silicate Hydrate (C-S-H), Calcium Hydroxide and Calcium Carbonate. To quantify the critical hydration level, an accurate and fast technique is highly desired. However, in conventional XRD technique, the peaks of the constituents of anhydrated and hydrated cement cannot be resolved properly, where as Mid-infrared (MIR) spectroscopy has low penetration depth and hence cannot be used to determine the hydration level of thicker concrete samples easily. Further, MIR spectroscopy cannot be used to effectively track the formation of Calcium Hydroxide, a key by-product during the hydration process. This paper describes a promising approach to quantify the hydration dynamics of cement using Terahertz (THz) spectroscopy. This technique has been employed to track the time dependent reaction mechanism of the key constituents of cement that react with water and form the products in the hydrated cement, viz., C-S-H, Calcium Hydroxide and Calcium Carbonate. This study helps in providing an improved understanding on the hydration kinetics of cement and also to optimise the physio-mechanical characteristics of concrete.

In this paper, we present a new compact and versatile spectrometer system operating at 1.55 μm for research and industrial application. The system is capable of testing solid, powder, thin film, gas, and liquid samples for material sensing and characterization applications. A high efficient system with bandwidth up to 1.2 THz is realized by using a fiber coupled terahertz chip packaging technology. The key components are the fiber-coupled THz transmitter and receiver modules, where the laser beam is directly coupled to the THz chip using optical fibers to provide stable and movable transmitter and receiver heads. The antennas are excited by 100 fs optical pulses at 1550 nm telecom wavelength and average power of 10mW. As femtosecond pulses are required on the antenna, the linear dispersion and nonlinear effect resulting from the propagation of the high power optical pulse along the fiber are taken into account and compensated using dispersion compensation fiber. A fast scan optical delay module is employed to realize real-time THz signal and spectrum measurement. The optical delay module also has a long delay scan unit to allow the user to adjust the distance between the transmitter and receiver heads by up to 1m to use the system for characterization of materials in different industrial applications.

In this paper, a novel design of broadband monopole optical nano-antennas is proposed. It consists of a corrugated halfelliptical patch inside an elliptical aperture. Full-wave electromagnetic simulations have been used to investigate the performance of the nano-antenna. The predicted performance of the proposed monopole nano-antenna is remarkably broadband. Moreover, the proposed broadband nano-antenna can respond to light waves with different polarizations. The proposed optical antenna will pave the way towards the development of high performance optical antennas and optical systems.

Military operations require the ability to locate and identify electronic emissions in the battlefield environment. However, recent developments in radio detection and ranging (RADAR) and communications technology are making it harder to effectively identify such emissions. Phased array systems aid in discriminating emitters in the scene by virtue of their relatively high-gain beam steering and nulling capabilities. For the purpose of locating emitters, we present an approach realize a broadband receiver based on optical processing techniques applied to the response of detectors in conformal antenna arrays. This approach utilizes photonic techniques that enable us to capture, route, and process the incoming signals. Optical modulators convert the incoming signals up to and exceeding 110 GHz with appreciable conversion efficiency and route these signals via fiber optics to a central processing location. This central processor consists of a closed loop phase control system which compensates for phase fluctuations induced on the fibers due to thermal or acoustic vibrations as well as an optical heterodyne approach for signal conversion down to baseband. Our optical heterodyne approach uses injection-locked paired optical sources to perform heterodyne downconversion/frequency identification of the detected emission. Preliminary geolocation and frequency identification testing of electronic emissions has been performed demonstrating the capabilities of our RF receiver.

A 1 × 16 Nb₅N₆ microbolometer array for a terahertz (THz) imaging system has been demonstrated. The system consists of an objective lens and an extended hemispherical silicon lens. A finite difference time domain (FDTD) method was used to analyze the imaging system in detail. The calculated field-of-view of the system is about 7° and the half-power beam width is about 160 μm. The microbolometer array chip is attached to the silicon lens for 0.3 THz detection. The preliminary results for the actual system shows that the mutual coupling among these antenna integrated elements can be ignored when the spacing is larger than 500 μm. The calculated results agree with the experimental data well, which means that the FDTD method can be used to evaluate and optimize such a compact THz imaging system. This linear imaging system should find direct application in active THz imaging.

We report on the development of a novel class of nanowire-based THz detectors in which the field effect transistor (FET) is integrated in a narrow-band antenna. When the THz field is applied between the gate and the source terminals of the FET, a constant source-to-drain photovoltage appears as a result of the non-linear transfer characteristic of the transistor. In order to achieve attoFarad-order capacitance we fabricate lateral gate FET with gate widths smaller than 100 nm. Our devices show a maximum responsivity of 110 V/W without amplification, with noise equivalent power levels ≤ 1 nW/√Hz at room temperature. The 0.3 THz resonant antenna has bandwidth of ~ 10 GHz and opens a path to novel applications of our technology including metrology, spectroscopy, homeland security, biomedical and pharmaceutical applications. Moreover the possibility to extend this approach to relatively large multi-pixel arrays coupled with THz sources makes it highly appealing for a future generation of THz detectors.

An innovative method of examining properties of metasurfaces is presented. A pump-probe technique is used to create a metasurface composed of conductive shapes on a silicon surface. A wave-front of intense pulse of 82 fs from Ti:Sa laser with wavelength of 800 nm is shaped by a spatial light modulator and then focused into a preprogrammed array of vshaped features on a high purity float zone silicon substrate. The laser pulse generates electron-hole pairs on the silicon substrate, thus a metasurface consisting of an array of metal-like v-shaped antennas is inscribed on the silicon substrate. The lifetime of v-shaped antennas is in millisecond time range. In the meantime, the second, less intense pulse, also of wavelength 800 nm is converted to a pulse of terahertz radiation with a peak-power at wavelength approximately 800 μm and used to probe the metasurface inscribed in the silicon. Tracing the position of the refracted terahertz beam is achieved with a specially designed INO video camera for terahertz radiation.

At the National Ignition Facility (NIF), home of the world’s largest laser, a critical pulse screening process is used to ensure safe operating conditions for amplifiers and target optics. To achieve this, high speed recording instrumentation up to 34 GHz measures pulse shape characteristics throughout a facility the size of three football fields—which can be a time consuming procedure. As NIF transitions to higher power handling and increased wavelength flexibility, this lengthy and extensive process will need to be performed far more frequently. We have developed an accelerated highthroughput pulse screener that can identify nonconforming pulses across 48 locations using a single, real-time 34-GHz oscilloscope. Energetic pulse shapes from anywhere in the facility are imprinted onto telecom wavelengths, multiplexed, and transported over fiber without distortion. The critical pulse-screening process at high-energy laser facilities can be reduced from several hours just seconds—allowing greater operational efficiency, agility to system modifications, higher power handling, and reduced costs. Typically, the sampling noise from the oscilloscope places a limit on the achievable signal-to-noise ratio of the measurement, particularly when highly shaped and/or short duration pulses are required by target physicists. We have developed a sophisticated signal processing algorithm for this application that is based on orthogonal matching pursuit (OMP). This algorithm, developed for recovering signals in a compressive sensing system, enables high fidelity single shot screening even for low signal-to-noise ratio measurements.

The objective of our work [1] was to determine the opportunities and challenges for Terahertz application development for the next years with a focus on systems: for homeland security and for Non Destructive Testing (NDT). Terahertz radiation has unique abilities and has been the subject of extensive research for many years. Proven concepts have emerged for numerous applications including Industrial NDT, Security, Health, Telecommunications, etc. Nevertheless, there has been no widely deployed application and Businesses based on THz technologies are still in their infancy. Some technological, market and industrial barriers are still to be broken. We summarize the final analysis and data: study of the technology trends and major bottlenecks per application segment, main challenges to be addressed in the next years, key opportunities for THz technologies based on market needs and requirements.

We propose a novel stabilization technique for two 1550-nm band external cavity laser diodes (ECLDs) used in optical generation of microwave and millimeter wave signals. Using FM sideband technique, those two ECLDs are simultaneously locked to two resonant modes of a single Fabry-Perot cavity. In the scheme, a new Υ-type optical configuration is used for simultaneous phase modulation of orthogonally polarized two wavelengths transmitted through slow and fast axis of polarization maintaining fiber. The Υ-type optical configuration, which consists of a phase modulator and a Faraday rotator mirror combined with an optical circulator, is a simple and compact apparatus to achieve double-pass phase modulation with the same modulation index . In this paper, we show the results of frequency stabilization of two ECLDs using Υ-type configuration, and compare with the results obtained in conventional non-Υ-type configuration. Short-term stability of 200 kHz at an averaging of 10 ms is achieved in the simple Υ-type configuration.

In this paper we present two vertical transitions, in multilayer LCP substrates for millimeter wave (mmW) imaging application. The first transition is from conductor-backed co-planar waveguide (CBCPW) to strip line, and the second one connects CBCPW to substrate integrated waveguide (SIW). The multilayer structure consists of three LCP layers and four metal claddings. The CBCPW is designed on the top LCP layer, the strip line is sandwiched by the top and middle layers, and the SIW is built within the middle and bottom layers. Micro vias construct the side wall for the SIW, and electrically connect the transmission lines and waveguides. Both of the transitions perform low loss and low reflection at 77 GHz. They can efficiently connect the passive and active components in the front-end RF module of our mmW imager. Additionally, they may have promising application in high-performance systems, requiring high density, low size, weight, and power (SWaP).

A compact microwave photonic filter (MPF) with continuous tunability of central frequency based on a microdisk (or cascaded microring) resonator on a single silicon-on-insulator (SOI) chip is proposed and experimentally demonstrated. Assisted by the optical single side-band (OSSB) modulation, the optical frequency responses of microring and microdisk resonators are mapping to the microwave frequency response to form an MPF whose central frequency is continuously tunable. Different SOI resonators, including two microdisk chips whose Q factors are 1.07×10⁵, 1.5×10⁴ and a cascaded microring chip whose Q factor is 2.9×10^4, are used to implement the MPFs. The performance of these MPFs is compared in terms of bandwidth, tuning range and rejection ratio. Our approach will allow the implementation of very compact, low-cost, low-consumption and integrated notch MPFs in a silicon chip.

The expansion of the universe has red-shifted remnant radiation, called the Cosmic Microwave Background (CMB) radiation, to the terahertz band, one of the last areas of the electromagnetic spectrum to be explored. The CMB has imprinted upon it extremely faint temperature and polarisation features that were present in the early universe. The next ambitious goal in CMB astronomy is to map the polarisation characteristics but their detection will require a telescope with unprecedented levels of sensitivity and systematic error control. The QUBIC (Q&U Bolometric Interferometer for Cosmology) instrument has been specifically designed for this task, combining the sensitivity of a large array of wideband bolometers with the accuracy of interferometry. QUBIC will observe the sky through an array of horns whose signals will be added using a quasi-optical beam combiner (an off-axis Gregorian dual reflector designed to have low aberrations). Fringes will be formed on two focal planes separated by a polarising grid. MODAL (our in house simulation package) has been used to great effect in achieving a detailed level of understanding of the QUBIC combiner. Using a combination of scalar (GBM) and vector (PO) analysis, MODAL is capable of high speed and accuracy in the simulation of quasi-optical systems. There are several technical challenges to overcome but the development of MODAL and simulation techniques have gone a long way to solving these in the design and analysis phase. In this paper I outline the quasi-optical modelling of the QUBIC beam combiner and work envisaged for the future.

Low-loss, hollow, flexible, metal-coated waveguides were designed and fabricated for the maximal transmission of terahertz radiation. Since recent terahertz skin, colon, and breast cancer studies showed a contrast between normal and diseased tissues between 500 to 600GHz frequencies, flexible metal-coated waveguides with various bore diameters were studied at both 584GHz and 1.4THz frequencies for endoscopic applications. Attenuation characteristics of 2μm thick silver-coated waveguides with 99% reflective inner surface were measured as a function of wavelength, bore diameter, bending angle and bend radius. Though the theoretical attenuation coefficient in metal-coated waveguide varies directly as square of wavelength, the propagation loss was found to be smaller at higher wavelengths. This study demonstrates that flexible waveguides with bore diameters less-than 10λ preserve the linearly polarized mode and hence exhibit low bending losses even at smaller bend radii. Also, in contrast to the lower propagation losses in larger bore tubes, the analysis shows higher transmission in smaller bore tubes at larger bending angles. Finally, the dual-frequency investigation of bending and modal characteristics confirms the feasibility of using these metal-coated flexible waveguides at various terahertz frequencies, to obtain low transmission losses even at greater flexures, in addition to the Gaussian mode preservation.

Generating RF signals over the entire spectrum, from hundreds of MHz to the hundreds of GHz, typically requires the use of multiple oscillators which operate only for specific bands within that spectrum. By mixing two lasers together, it is possible to generate RF signals over that entire band. Through the use of a narrow linewidth low frequency oscillator, optical modulator, and injection locking, much higher frequency outputs can be produced that still retain the narrow linewidth of the low frequency oscillator. 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 effort, we have successfully demonstrated an integrated module and shown tunable RF generation with a 1 Hz linewidth.

This paper presents an analog front-end circuit for a 60-GHz wireless communication receiver. The feature of the proposed analog front-end circuit is a bandwidth more than 1-GHz wide. To expand the bandwidth of a low-pass filter and a voltage gain amplifier, a technique to reduce the parasitic capacitance of a transconductance amplifier is proposed. Since the bandwidth is also limited by on-resistance of the ADC sampling switch, a switch separation technique for reduction of the on-resistance is also proposed. In a high-speed ADC, the SNDR is limited by the sampling jitter. The developed high resolution VCO auto tuning effectively reduces the jitter of PLL. The prototype is fabricated in 65nm CMOS. The analog front-end circuit achieves over 1-GHz bandwidth and 27.2-dB SNDR with 224 mW Power consumption.

We investigate the phase noise performance of an optoelectronic oscillator based on a calcium fluoride high quality factor whispering gallery mode optical resonator (WGMR). In the oscillator setup, a particular attention is given to the stabilization of the laser lightwave onto an optical resonance of the WGMR. Once the laser is stabilized, different resonant optical modes are characterized in the microwave domain. Afterwards, phase noise spectra of different oscillations at different modes of the WGMR are measured. Phase noise levels below -93 dBc/Hz and around -90 dBc/Hz at 10 kHz offset frequency from 6.35 GHz and 12.7 GHz carriers are respectively obtained.

The integration of quantum cascade lasers with devices capable of efficiently manipulating terahertz light, represents a fundamental step for many different applications. Split-ring resonators, sub-wavelength metamaterial elements exhibiting broad resonances that are easily tuned lithographically, represent the ideal route to achieve such optical control of the incident light. We have realized a design based on the interplay between metallic split rings and the electronic properties of a graphene monolayer integrated into a single device. By acting on the doping level of graphene, an active modulation of the optical intensity was achieved in the frequency range between 2.2 THz and 3.1 THz, with a maximum modulation depth of 18%.

We study the use of polymeric components to achieve efficient outcoupling of radiation from double-metal-waveguide THz quantum cascade lasers. We employ cyclicolefincopolymer to realize optical fibers with diameters between 60mm and 1000mm, and characterize their absorption coefficient at 2.9THz. We show that assembling a fiber on the facet of a singleplasmon quantum cascade laser improves its far field radiation pattern, making it narrower. We also provide a design of a broadband directional coupler that can be used to couple radiation from a doublemetal quantum cascade laser waveguide into the tapered end of a metal coated optical fiber.

A major limitation in terahertz (THz) imaging applications is the relatively poor diffraction limited spatial resolution. A common approach to achieve subwavelength resolution is near-field imaging using a subwavelength aperture, but the low transmission efficiency through the aperture limits the sensitivity of this method. Bullseye structures, consisting of a single subwavelength circular aperture surrounded by concentric periodic corrugations, have been shown to enhance transmission through subwavelength apertures. At optical wavelengths, the fabrication of bullseye structures has been traditionally achieved by lithographic or chemical processes. Since the scale of plasmonic structures depends on the incident wavelength, precision micromilling techniques are well suited for THz applications. In this paper we describe a diamond micromilling process for the fabrication a plasmonic lenses operating at 325 GHz. Theoretical simulations are obtained using an FDTD solver and the performance of the lens is measured using a customized THz test bed.

We introduce an optically controlled microwave phase stabilizer based on polarization interference technique using single semiconductor optical amplifier (SOA). A prototype with a frequency of 10 GHz is experimentally demonstrated. It provides a stable phase drift that can be linearly compensated over 10 km single-mode fiber by controlling the SOA injection current.

We propose a dispersion flattened and high birefringence terahertz photonic crystal fiber using roll up method with PTFE. The PCF made of this method shows dispersion flatness by the air region between PTFE tubes around core and high birefringence by asymmetric mode distributions. When using rod with D_out = 1mm, dispersion of the proposed THz PCF has a slope of 0.4ⅹ10^-2 near 0.848 THz and the value of the then is about 0.21 ps/THz.cm. In this structure, the order of birefringence is 10^-3 and the order of confinement loss is about 10^-11 dB/m.

We measured the nonlinear response of field effect transistors fabricated with GaAs-based heterostructures by performing direct detection, heterodyne and subharmonic mixing measurements. The study of the spectral responsivity as a function of different antenna coupling is presented in the 0.18-0.4 GHz range. We also verified the subharmonic and heterodyne mixing at 0.6 THz in a HFET detector with a broadband antenna.

Imaging in long wavelength regimes holds huge potential in many fields, from security to skin cancer detection. However, it is often difficult to image at these frequencies – the so called ‘THz gap¹’ is no exception. Current techniques generally involve mechanically raster scanning a single detector to gain spatial information², or utilization of a THz focal plane array (FPA)³. However, raster scanning results in slow image acquisition times and FPAs are relatively insensitive to THz radiation, requiring the use of high powered sources. In a different approach, a single pixel detector can be used in which radiation from an object is spatially modulated with a coded aperture to gain spatial information. This multiplexing technique has not fully taken off in the THz regime due to the lack of efficient coded apertures, or spatial light modulators (SLMs), that operate in this regime. Here we present the implementation of a single pixel THz camera using an active SLM. We use metamaterials to create an electronically controllable SLM, permitting the acquisition of high-fidelity THz images. We gain a signal-to-noise advantage over raster scanning schemes through a multiplexing technique⁴. We also use a source that is orders of magnitude lower in power than most THz FPA implementations^3,5. We are able to utilize compressive sensing algorithms to reduce the number of measurements needed to reconstruct an image, and hence increase our frame rate to 1 Hz. This first generation device represents a significant step towards the realization of a single pixel THz camera.