This PDF file contains the front matter associated with SPIE Proceedings Volume 8261, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Ultrafast photoconductors have been an enabling device technology in the THz field during the past decade. And their implementation is now worldwide in time- and frequency-domains systems of various types. While the technological push is towards InGaAs or similar photoconductors operating at 1550 nm, the GaAs-based devices operating around 800 nm still provide superior performance and robustness in most cases. This paper contrasts the GaAs and 1550-nm devices in terms of materials design and solid-state metrics such as electron-hole lifetime, carrier mobility, and resistivity. It also summarizes the main materials developed over the past 20 years.

We demonstrate several optical beating sources based on 1.55 μm photonic devices. Broadband antenna-integrated, low-temperature-grown (LTG) InGaAs photomixers for widely tunable continuous-wave THz generation and detection are also verified. The novel optical beat sources show a beat frequency tuning range from 0.3THz to over 1.34 THz. The dual-mode laser diode (DML) consists of one phase and two active sections. Micro-heaters are used to independently tune the wavelengths of the two DML laser modes. Broadband antenna-integrated, LTG InGaAs photomixers are used as THz wave generators and detectors. This use of 1.55 μm photonic devices could connect current THz and InP-based communication technologies because the well-developed InP-based optoelectronic technologies are already expected to enable the integration of tunable LD sources with other optical components such as semiconductor optical amplifiers (SOAs), electro-absorption modulators, and waveguide-type THz photomixers. As well as realizing an optical fibercoupled THz time-domain spectroscopy (TDS) system, we also successfully achieved continuous frequency tuning of the CW THz emissions. Our results show that photomixing using the photonic devices is a promising approach to realize compact, cost-effective, and portable THz spectrometer.

Ballistic characterization of improved materials for Soldier personal protective equipment is an ever-challenging task, requiring precise measurement of materials during ballistic impact. Current dynamic deformation technologies, such as high-speed digital image correlation, and laser velocimetry and vibrometry, are only able to provide surface measurements. However, there is a need to measure the dynamic delamination and mass loss of composite material, allowing calculation of available kinetic energy remaining in the material. A high sensitivity terahertz dynamic scanning reflectometer may be used to measure dynamic surface deformation and delamination characteristics in real-time. A number of crucial parameters can be extracted from the reflectance measurements such as dynamic deformation, propagation velocity, and final relaxation position. As proof of principle, an acrylic plate was struck with a blunt pendulum impactor and dynamic deformation was captured in real-time. Reflectance kinetics was converted to deformation and the velocity was calculated from the kinetics spectrum. Kinetics of a focused pendulum impactor on a steel plate was also acquired, characterizing plate relaxation from maximum deformation to equilibrium with discernible vibrations before reaching stable equilibrium.

We explore terahertz imaging with CMOS field-effect transistors exploiting their plasmonic detection capability and the advantages of CMOS technology for the fabrication of THz cameras with respect to process stability, array uniformity, ease of integration of additional functionality, scalability and cost-effectiveness. A 100×100-pixel camera with an active area of 20×20 mm² is physically simulated by scanning single detectors and groups of a few detectors in the image plane. Using detectors with a noise-equivalent power of 43 pW/√Hz, a distributed illumination of 432 μW at 591.4 GHz, and an integration time of 20 ms (for a possible frame rate of 17 fps), this virtual camera allows to obtain images with a dynamic range of at least 20 dB and a resolution approaching the diffraction limit. Imaging examples acquired in direct and heterodyne detection mode, and in transmission and reflection geometry, show the potential for real-time operation. It is demonstrated that heterodyning (i) improves the dynamic range substantially even if the radiation from the local oscillator is distributed over the camera area, and (ii) allows sensitive determination of object-induced phase changes, which promises the realization of coherent imaging systems.

New terahertz (THz) optical devices which can control the optical properties of THz waves are needed to broaden the application area of THz technology. Liquid crystals (LCs) are very attractive materials for developing such devices because they have outstanding properties such as sensitivity to applied electric fields, chemical stability, relatively large birefringence and moderate absorption in the THz range. LCs need to be optimized to have a large birefringence and small absorption in the THz range. In this paper, we have investigated optical properties of a set of LCs in the THz range: E7, BL037, and RDP-97304. Optical parameters for the ordinary and extraordinary axis of LCs were acquired using THz time-domain spectroscopy and THz air-biased coherent detection system. We found that RDP-97304 has the largest birefringence and smallest absorption compared to E7 and BL037 in the THz range. It is thus a good candidate to design fast and efficient THz optical devices.

We report on the unique highly-configurable wavelength tuning and switching properties of a tunable external cavity laser based on multiplexed volume holographic gratings (VHGs) and a micromirror device. The ultra-compact laser has a 3 THz bandwidth and exhibits single mode operation in either single or multiple wavelengths with narrow linewidth (<7.5 MHz), and a switching rate of 0.66 kHz per wavelength. A prototype laser exhibited 40 mW of output power for wavelengths from 776 - 783 nm. The unique discrete-wavelength-switching features and low power consumption of this laser make it well suited as a source for continuous-wave (cw) terahertz signal generation in portable photomixing systems.

We report the design, simulation, and fabrication of a miniaturized Golay cell array, implemented with monolayer graphene suspended over a TEM grid as the deflecting membrane. Currently, ultra-thin membranes for Golay cell applications suffer diminishing responsivity as the lateral dimensions are reduced to the microscopic scale. We propose graphene as the ideal membrane material for micro-Golay cell arrays, whereby the minimal elastic stiffness of atomically thin graphene allows membranes to be scaled to microscopic dimensions. We examine how graphene's unique material parameters, such as high mobility, negligible gas permeability, and supreme strength, offer ease of fabrication and improved performance over existing technology. Simulations of graphene membrane deflection versus temperature are presented, with an analysis of the optimal geometry for maximum sensitivity. Cavities with all spatial dimensions under 100 μm are predicted to provide sensitivities of hundreds of nanometres per Kelvin, in good competition with existing research on devices many times larger. Up to a four-fold increase in responsivity of 400 nm/K is predicted for a graphene cell of the same dimensions as current technology, and a three-fold increase for a cell one quarter the diameter. These predictions permit an increased detector density in a focal plane array application while still providing improved responsivity. Furthermore, our fabrication method permits the construction of arrays consisting of thousands of devices, avoiding individual cell assembly and including built-in electrical contacts due to the conductive nature of graphene. We also present a theoretical analysis of interferometric optical read-out of membrane deflection.

A novel approach for the generation of THz radiation that utilizes "interband" transitions and tunneling processes occurring simultaneously within double-barrier (DB) GaSb/InAs/GaSb broken-gap (BG) resonant-tunneling-diodes (RTDs) is discussed. This paper focuses on the architectural and cavity designs for realizing TE polarized emission from single DB-BG-RTD devices and quantum-dot pillar arrays. Design techniques useful for mitigating CB drive current (& the associated thermal heating) while at the same time optimizing output power and power efficiency are discussed.

The generation of terahertz (THz) pulses based on optical rectification effects in GaAs has become more and more attractive and practical due to advances in high power ultrashort pulse fiber lasers. Normally coherence length is a parameter introduced for judging how the phases match by comparing the group velocity of optical pulses with the phase velocity of one of frequency components, like, for example, a component at 2 THz, of THz pulses. It is shown in this paper that the coherence length can not characterize the THz pulse generating process well because it can not count the contribution of all components in the spectrum band of the THz pulses. An energy conversion efficiency calculation model is proposed in this paper by integrating the energy of all THz components generated in the optical rectification process in a planar waveguide device. Based on the calculation model, the evolution of a THz pulse along the longitudinal direction of the waveguide is simulated and the results are used for design of the optimal waveguide structure for which the highest energy conversion efficiency could be reached to 1.5 × 10^-3.

Operational temperature increase of CW THz QCLs to 77 K has enabled us to employ solid nitrogen (SN2) as the cryogen. A roughing pump was used to solidify liquid nitrogen and when the residual vapor pressure in the nitrogen reservoir reached the pumping system's minimum pressure the temperature equilibrated and remained constant until all the nitrogen sublimated. The hold time compared to liquid helium has thereby increased approximately 70-fold, and at a greatly reduced cost. The milliwatt CW QCL was at a temperature of approximately 60 K, dissipating 5 W of electrical power. To measure the long-term frequency, current, and temperature stability, we heterodyned the free-running 2.31 THz QCL with a CO₂ pumped far-infrared gas laser line in methanol (2.314 THz) in a corner-cube Schottky diode and recorded the IF frequency, current and temperature. Under these conditions the performance characteristics of the QCL, which will be reported, exceeded that of a device mounted in a mechanical cryocooler.

Tunable resonant absorption by plasmons in the two-dimensional electron gas (2DEG) of grating-gated InP- and Graphene-based HEMTs are investigated. Fourier-spectrometer-obtained transmission resonances are observed over a wide spectral band from mm wavelengths to THz frequencies. These results are found to be consistent with grating period and 2DEG sheet charge density dependent theoretical calculations. The temperature dependence of these transmission resonances as a function of temperature is also reported for both devices. Such devices have potential as a chip-scale frequency-agile THz imaging spectrometers for man-portable or space-based spectral-sensing applications.

New developments in waveguide mode matching techniques are considered, in particular the efficient modeling of waveguide cavity coupled detectors. This approach is useful in far-infrared astronomical instrumentation and cosmic microwave background experiments in which bolometers feeding horn antennas or Winston cones are often employed for high sensitivity, good control of stray light and well behaved beam patterns. While such systems can, in theory, be modeled using full wave FEM techniques it would be desirable, especially for large structures in terms of the wavelength, to exploit more efficient mode matching techniques, particularly for initial design optimization. This would also be especially useful for cavities feeding partially coherent multi-mode horns or Winston cones. The mode matching approach also allows for straightforward modeling of the complete coupling structure including the horn, waveguide cavity and absorbing layer of the bolometer, thus marking a significant advance in the ability to predict the optical efficiencies of cavity coupled bolometers. We consider typical single mode and multi-mode examples that illustrate the power of the technique.

We explore guided modes in metallic "spoof-insulator-spoof" (SIS) waveguides: parallel plate structures with subwavelength corrugation on the surfaces of both conductors. A dispersion relation for SIS waveguides is analytically obtained. The modes in the structure arise from the coupling of conventional parallel plate waveguide modes with the localized modes of the grooves. SIS waveguides can be engineered to guide modes with low group velocities and SIS tapers can be used to convert light between photonic modes and plasmonic ones.

Electro-optic dendrimer was used to generate CW terahertz radiation via difference frequency method. In case of electro-optic excitation, the pump-THz conversion is not limited either by emission saturation or by heat dissipation. Especially, the difference frequency generation (DFG) uses two-photon excitation that eliminates the use of a femtosecond pulsed laser and allows for producing both continuous wave (CW) and pulsed terahertz radiation. This report outlines a wideband terahertz spectrometer that is designed around an EO dendrimer terahertz source. This source allows for a wide terahertz range and higher output power. The spectrometer (TeraSpectra) was calibrated with a polyethylene card. It was found that the TeraSpectra reproduces known absorbance peaks of polyethylene with many additional peaks not discovered before. The main origin of these additional peaks is from the fact that the TeraSpectra is sensitive to many resonances possible in a molecule.

We present novel nanoscale bolometers made of lithographically defined platinum wires. The cores of our structures are narrow wires with fixed width of 300 nm and length ranging from 300 nm to 17 μm. Some are significantly smaller in size than the wavelengths they are exposed to from a 1200 K blackbody source. The response of the wire's resistance to the external radiation reflects its temperature and can be monitored in real-time. Previously, we have reported a steep rise in responsivity and detectivity with decreasing wire length under such infrared exposure, for a constant Joule power dissipation in the wire (drive power). In this work, we aim to enhance the performance of the bolometers by changing physical and driving parameters, i.e. the insulating layer thickness or the external bias. We find that after such optimization, structures can reach a responsivity R of 4.5x10⁵ V/W and a detectivity D* of 2.3x10¹⁰ cmHz^1/2/W. With a reduced size and a high performance, these devices could improve the infrared sensors technology.

THz semi-insulating surface plasmon waveguide QCL's based on the bound-to-continuum design have been developed with 2.31 THz output powers of ~0.5 milliwatt from a single facet, input powers of ~5 watts, and threshold current densities of 117 A/cm² operating continuous wave at 77K. These results were achieved by depositing alloy metal on both contact layers, only annealing the bottom metal layers, and thinning the substrate thickness to ~170 μm to assure good heat dissipation. The structure was based on a previously published 2.83 THz design that was scaled to emit at 2.31 THz. The demonstration of this high temperature, high power laser with low input power enables its use in compact, coherent THz transceivers for heterodyne detection with liquid nitrogen cooling.

Solid state frequency multipliers extend the operating frequency range of Backward Wave Oscillators (BWOs) to 2.6 THz, enabling continuously tunable, narrow linewidth THz sources across the 0.1-2.6 THz range. Power conversion efficiency of frequency multipliers can be improved substantially by optimizing impedance matching between millimeter wave BWOs and frequency multipliers. Performance of sub-millimeter wave BWOs combined with frequency multipliers is limited by multi-mode output of these BWOs and lower power conversion efficiency of solid state multipliers operating above 1 THz.

We present the realization of active photonic crystal terahertz lasers operating in higher photonic bands. The resonator consists of an array of isolated pillars which are embedded in a metallic waveguide. These devices reduce the overlap with gain region and increase the effect of the surrounding medium. Thereby, it is either possible to directly manipulate the lasing mode or to sense variations in the environment.

A portable, stand-alone, real-time THz imaging system for high resolution is presented. The total weight of the apparatus is less than 15 kg and its physical dimension is of approximately (65 cm)³. A quantum cascade laser emitting at 3.4 THz based on a third-order distributed feedback cavity is used as radiation source for transmission and reflection imaging modes. We report real-time THz imaging with a bolometric camera operating at 15 Hz producing movies with a resolution of 120 x 160 pixels. With the help of a Stirling motor cryocooler the laser operates in continuous-wave at 40 K with more than 1 mW output power and less than 300 mW of power consumption. We were able to image small objects employing refractive elements that we manufactured in high density polyethylene achieving a resolution of twice the wavelength.

Low-loss hollow flexible metal/dielectric coated polycarbonate waveguides have been designed and fabricated for the maximal transmission of Terahertz radiation (THz). Attenuation characteristics of Terahertz radiation in Ag/Au coated waveguides with bore diameters 4.1 mm, 3.2 mm, 2 mm were studied at 215 µm wavelength and the maximal transmission was obtained by coupling the lowest loss TE₁₁ mode from an optically pumped terahertz laser. Transmission loss can be reduced substantially by adding a dielectric layer to the metal coated waveguide and by coupling HE₁₁ mode into it. Polystyrene (PS) was chosen to be the dielectric, due to its low extinction coefficient, which enhances the transmission through the waveguide. A propagation loss of less than 1 dB/m was achieved with these metal/dielectric coated waveguides.

Pyroelectric thermal detectors are excellent candidates for detection of broadband radiation. Such detectors utilize permanently poled ferroelectric single crystal lithium tantalate to generate a charge as the crystal heats up by absorbing radiation. The charge, which results in a current output when connected to an external electrical circuit, is directly proportional to the rate of change of temperature of the crystal. The fundamental approach toward enhancing pyroelectric detector response is to form the pyroelectric material into a thin film. An elegant approach for producing bulk quality thin films of pyroelectric materials is by crystal ion slicing. In this paper, we report on the formation of thin film lithium tantalate (TFLT™) pyroelectric detector devices using the ion slicing process. The devices incorporate films less than 9 microns thin and feature apertures as large as 5 mm in diameter. To make functional detectors, ion sliced films were transferred to ceramic carriers in TO-type can test packages. Test results have shown improvement in room temperature detectivity about 20 times higher than the state-of-the-art lithium tantalate pyroelectric detectors.

In this paper, we present a design for a narrowband absorber based on metamaterials in the infrared wavelengths for wavelength-selective uncooled hyperspectral imaging systems. The proposed narrowband absorber integrated with microbolometer focal plane arrays has the potential to increase the detection sensitivity of the microbolometers. The design of the metamaterial unit cell consists of a resonant metallic 'cross' structure which has a resonance in the IR wavelengths and is placed on a dielectric substrate with a metal back plate. In order to achieve a very high absorption of the electromagnetic radiation, the designed metamaterial needs to have minimal transmission and reflection within its spectral response window. Minimal reflection is achieved through impedance matching of the metamaterial with the free space whereas zero transmission is ensured through the metal back plate. Moreover, for the purpose of hyperspectral imaging, the metamaterial structure is combined with a tunable electro-optic material, namely, liquid crystal. Tunability can be achieved upon applying a voltage across the combined liquid crystal and metamaterial structure thus bringing about a shift in the resonant frequency. In our simulated model, where losses of metal and dielectric substrate materials were taken into account, we noted more than ninety percent of absorption can be achieved in a narrow spectral window for the designed metamaterial structure.

Application of terahertz radiation for the creation of medical equipment and solving of biological problems has become widely spread. From this point of view, the influence of THz radiation on the nerve fibers is of primary concern. In addition, several studies indicated both stimulating and depressive effects on nerve cells. However, the mechanism of this effect has not yet been studied, including the dose and exposure time. Our research was devoted to the impact of broadband pulsed THz radiation in the frequency range of 0.05 to 2 THz on the neurite growth in the sensory ganglia of 10-12-day chicken embryos. Dependence of changes in functional responses of cells on the average output power has been found. An increase in the stimulating effect was observed at the lowest power density used (0.5 μW/cm²). Through non-destructive process and choosing the correct parameters of THz radiation, potential control of neural response becomes possible, which can subsequently lead to new medical treatments.

Terahertz (THz) hydration sensing and image has been a topic of increased interest recently due largely to improvements in source and detector technology and the identification of applications where current hydration sensing techniques are insufficient. THz medical imaging is an expanding field of research and tissue hydration plays a key role in the contrast observed in THz tissue reflectance and absorbance maps. This paper outlines the most recent results in burn and corneal imaging where hydration maps were used to assess tissue status. A 3 day study was carried out in rat models where a THz imaging system was used to assess the severity and extent of burn throughout the first day of injury and at the 24, 48, and 72 hour time points. Marked difference in tissue reflectance were observed between the partial and full thickness burns and image features were identified that may be used as diagnostic markers for burn severity. Companion histological analysis performed on tissue excised on Day 3 confirms hypothesized burn severity. The results of these preliminary animal trials suggest that THz imaging may be useful in burn wound assessment where current clinical modalities have resolution and/or sensitivity insufficient for accurate diagnostics.

sonant cavity enhancement results in substantial improvement in the efficiency of photonic THz-wave generation via difference frequency generation (DFG). A nearly degenerate optical parametric oscillator (OPO) was pumped by 6 ps pulses at 1064 nm, producing signal and idler pulses with average total power in excess of 80 W. By placing a sample of quasi-phasematched gallium arsenide (QPM-GaAs) at a focus of a ring cavity OPO, multicycle, narrowband THz radiation was produced, with average powers in excess of 100 μW and peak powers exceeding 150 mW. The dependence of the THz power on pump power shows no signs of saturation, so with higher power pump lasers, mW levels of average THz should be obtainable.

Nonmelanoma skin cancers are the most common form of cancer. Continuous wave terahertz imaging has the potential to differentiate between nonmelanoma skin cancers and normal skin. Terahertz imaging is non-ionizing and offers a high sensitivity to water content. Contrast between cancerous and normal tissue in transmission mode has already been demonstrated using a continuous wave terahertz system. The aim of this experiment was to implement a system that is capable of reflection modality imaging of nonmelanoma skin cancers. Fresh excisions of skin cancer specimens were obtained from Mohs surgeries for this study. A CO₂ optically pumped far-infrared molecular gas laser was used for illuminating the tissue at 584 GHz. The reflected signal was detected using a liquid Helium cooled Silicon bolometer. The terahertz images were compared with sample histology. The terahertz reflection images exhibit some artifacts that can hamper the specificity. The beam waist at the sample plane was measured to be 0.57 mm, and the system's signal-to-noise ratio was measured to be 65 dB.

The direct metallic or semiconducting characterization of carbon nanotubes (CNTs) in the high-frequency is one of the key issues to use them in the different state-to-the-art applications. In this work, the terahertz surface conductivity and transmission of carbon nanostructures thin-film utilizing terahertz time-domain spectroscopy (THz-TDS) have been studied. We have also compared the achieved results of single-walled carbon nanotubes thin-film surface conductivity with pervious study as a function of frequency. However, we have improved the obtained conductivity of carbon nanostructures from the microwave to terahertz range by THz-TDS technique with high signal-to-noise ratio.

We report on laser-based, high power single-cycle THz source. The THz radiation is generated by four-wave mixing in plasma and by optical rectification in organic salt crystal pumped by powerful optical parametric amplifier. The first approach permits the generation of electric field of hundreds of kV/cm at central frequency of 0.7 THz. The second technique allows the synthesis of an electric field exceeding 1 MV/cm paired with an unprecedented conversion efficiency of more than 2%, at frequency of 2 THz. The presented sources can be focused to a diffraction-limited spot and are suitable-versatile tool for time resolved THz experiment.

The elements for manipulating THz beams can be designed both on the basis of the microwave and on the optical technology. Similarly to typical optical components, refractive lenses and diffractive structures can be used in the THz range. For many practical applications the passive THz systems require sophisticated optical elements with a large numerical aperture (NA). The expected resolution of the optical setup is close to the diffractive limit. Therefore the aperture diameter of such optical elements is mostly in the range between 100 and 250 mm or even more and their focal length is often equal to the diameter. A standard refractive high NA spherical lens for the THz range exhibits high signal attenuation due to significant thickness. For typical converging lenses their attenuation is higher near the optical axis (low geometrical aberrations) than in the peripheral regions (high geometrical aberrations). This additionally boosts overall geometrical aberrations of the lens. Here we propose a sophisticated Fresnel-type structure. It should be thin enough to provide low attenuation and thick enough (high order kinoform) to avoid chromatic aberration. Due to special design process the spherical aberration of the structure can be significantly decreased. Computer modeling and experimental results are presented.

We report on measurements of radiation transmission in the 0.220-0.325 THz and 0.75-1.1 THz frequency ranges through GaN quantum wells grown on sapphire substrates at nitrogen and room temperatures. Significant enhancement of the transmitted beam intensity with applied voltage is found at nitrogen temperature. This effect is explained by changes in the mobility of two-dimensional electrons under electric bias. We have clarified which physical mechanism modifies the electron mobility and we suggest that the effect of voltage-controlled sub-terahertz transmission can be used for the development of electro-optic modulators operating in the sub-THz frequency range.