Over the past several years there has been an increase in interest in THz technology for a wide variety of applications. While much of the related work has concentrated in the use of short pulse THz transceivers, there has been an interest in CW THz technology as well. Owing to its non-ionizing nature, THz radiation seems to be an ideal choice for applications where people will be exposed or studied. In particular there are potential security applications which have been the subject of a number of studies. In order for these to be practical, the scan time for an entire person must be on the order of 5s or less. To support such a requirement with a single pixel scanner, very high SNR must be obtainable so that this can be traded for scan time (increased noise bandwidth). The combination of high spectral purity and high sensitivity offered by certain types of CW transceivers, makes them a good choice for these types of imaging applications.In this presentation we will provide an overview of CW heterodyne THz transceiver technology and imaging results for the detection of explosives hidden beneath a person’s clothing. Unlike many previous studies which focused on showing detection of hidden metal objects, we will be showing results for detection of hidden dielectrics. Additionally, results and discussion of practical limitations induced by shape effects of clothing reflection will also be discussed.

Gallium arsenide has attracted attention as a nonlinear optical material since the beginning of the laser era, because of its high second-order nonlinear susceptibility and superior transparency, both in the mid- and far-infrared. The main problem was how to achieve phase-matched interaction in this dispersive and optically isotropic material. Now it became possible, using epitaxial and other methods, to create GaAs structures with periodically-flipped orientation, and thus resolve the phase-matching problem. This opened up a variety of new applications in the mid-IR: broadly-tunable optical parametric oscillators, difference frequency and supercontinuum generators and polarization-insensitive frequency converters. I will talk about different methods of photonic THz generation in periodically-inverted quasiphase-matched GaAs – by means of both optical rectification (with femtosecond pulses) and frequency mixing (with picosecond pulses). In the latter case we took advantage of resonance enhancement of the optical fields. GaAs was placed inside the cavity of a high-finesse synchronously-pumped optical parametric oscillator (OPO) where dramatic improvement of the optical-to-THz efficiency was observed and 1 mW of output average THz power was achieved. This approach allows scalability of a THz source to ~ 100 mW of average power.

Far-infrared (THz) production using difference frequency generation (DFG) in an optical waveguide has attracted renewed attention because of the large pump power densities now available with fiber-based optical sources (e.g., rare earth-doped fiber lasers). While it is well understood that an optical waveguide can preserve optical power densities over long interaction lengths without concomitant diffraction, continuous phase matching of pumps and product (β_THz = β₁-β₂, where β_1,2 are the propagation constants of the pumps) has not been achievable without using mixed transverse modes (e.g., fundamental THz mode with higher order pumps). This significantly degrades the pump/product interaction (overlap). A nested waveguide structure – one waveguide carrying the DFG pump light, buried in a second guide carrying the THz product – has been proposed as a method for achieving continuous phase matching with good optical confinement among the fundamental transverse optical modes of the pumps and a fundamental THz mode. Continuously phase matched DFG has recently been demonstrated in a hybrid nested waveguide consisting of quartz/LiNbO₃/polyethylene. Model calculations, however, show that the significantly lower loss of AlGaAs in the THz makes it a more effective guided wave THz source than its LiNbO₃ counterpart – in spite of the latter’s larger nonlinearity. We have calculated an estimated 2000-fold improvement in the normalized conversion efficiency over that obtained in bulk semiconductors, using a guide wave AlGaAs heterostructure. To avoid 2-photon absorption of the near-IR pumps, the aluminum composition needs to be >16%. The AlGaAs nested waveguide, itself, is ~30 μm thick, requiring a very high growth rate (~120-200 nm/min). Metal-organic vapor phase epitaxy (MOVPE), using trimethyl aluminum (TMA), trimethyl gallium (TMG) and arsine (AsH₃) precursors, was carried out at growth temperatures between 750^oC-950^oC. Growth studies have been performed on GaAs and Si substrates oriented along [100] and [111] directions. Smooth morphology has been achieved on GaAs (100) and (111) substrates at 950^oC at growth rates as high as 150 nm/min. Low carrier concentrations were achieved by adjusting growth conditions such as the V/III ratio. Semi-insulating AlGaAs can also be achieved by oxygen doping. The nested waveguide has been transferred from GaAs to Si by lapping and etching off the GaAs substrate and wafer-bonding. Growth and fabrication results will be presented along with preliminary conversion efficiency data.

SummarySeveral SrTiO₃/DyScO₃ (STO/DSO) strained epitaxial heterostructures with variable thickness and number of layers are investigated by time-domain THz spectroscopy as a function of electric bias and temperature. A general model is proposed to explain the high electric-field tunability of SrTiO₃ in the THz range. MotivationThe optical or electric control of the propagation of THz radiation is of high importance in the current THz technology and has received recently a considerable attention [1]. Ferroelectric materials such as STO are suitable for the modulation of the real part of the permittivity by applied bias or temperature. Indeed, the dielectric behavior of STO is controlled by the soft mode whose frequency (around 2.7 THz at room temperature) decreases with cooling. Strain is an important factor affecting the ferroelectric properties of STO because of its strong coupling to the soft mode polarization. The tensile strain in STO films grown on a DSO substrate induces a significant decrease of the soft mode frequency even at room temperature and a dramatic increase of the permittivity [2]. Such material properties can lead to interesting applications in the metamaterials science and in tunable devices.ResultsWe studied epitaxial STO/DSO heterostructures (with 1×100, 2×50, 3×50, 4×50 and 20×10 nm thick STO films) grown on DSO substrates by pulsed laser deposition [3] with a sub-wavelength interdigitated gold electrode structure added on top. We show that it is possible to describe the dielectric behavior of all the samples using a general model which assumes a highly anharmonic behavior of the soft mode. The soft mode is characterized by a bare eigenfrequency ω₀, oscillator strength f and damping Γ and it exhibits an additional low-frequency decay channel (Debye relaxation with a bare frequency γ). The relaxation is silent—it is observable in the spectra only owing to its coupling (δ) to the polar soft mode.The experiment shows that the THz dielectric behavior of the strained STO layers is entirely governed by the soft mode. The parameters f, γ and δ are constant (sample and electric-field independent) and the soft mode damping Γ varies only slightly among samples (inhomogeneous broadening due to the strain distribution within heterostructures).With the investigated structures one can achieve a THz power modulation of more than 40% at room temperature. ConclusionWe have shown outstanding tuning capabilities of high-permittivity STO in the THz range and related them to the ferroelectric soft mode dynamics. The technological challenge for further enhancement of the tunability consists in preparing structures with a high number of STO/DSO bilayers and high STO filling factor while preserving the suitable value of tensile strain in STO films. References[1] H.-T. Chen et al., Nature 444, 597 (2006)[2] J. H. Haeni et al., Nature 430, 758 (2004)[3] P. Kuzel et al., Appl. Phys. Lett. 91, 232911 (2007)

Self-assembled semiconductor quantum dots (QDs), providing a 3D confinement to electrons and holes have been intensively investigated with respect to their material and optical properties, giving deep insight down to a level where only single QDs can be addressed. For optical applications, however, it has proven difficult to achieve for electrons a ground state intraband transition (1s-2p) smaller than 40-60 meV, since it arises from the radial quantization in the QD. Recently we have introduced a novel quantum dot - based nanostructure grown by molecular beam epitaxy [1] called quantum posts (QPs). These structures form short nanowire-like structures (average composition In_.43Ga_.57As) aligned along the growth direction terminated at both ends by a quantum dot. The QPs are imbedded in an In_.1Ga_.9As quantum well (QW) of the same height as the QPs and precise control of their conduction band ground state transition is achieved simply by varying their height during growth. Up to now heights of 23 nm to 60 nm have been achieved, with predicted transitions between 6 and 0.5 THz. Here we report the electronic THz absorption in QP structures and structures having only an InGaAs QW but no QPs, serving as a reference for the absorption that originates from the shallow InGaAs region that laterally surrounds the QPs. For the absorption experiments, a single QP (QW) layer was embedded in between an n-doped back-gate and a Schottky-contact, allowing us to load controllably the QPs with electrons. Prior to the absorption experiments, the loading behavior was studied by capacitance-voltage spectroscopy which reveals distinct and characteristic loading-features of electrons into the QPs and the QW. The absorption measurements were performed using an FTIR at T=4 K and the polarization was set parallel to the growth direction. The absorption spectrum taken from a sample with 30 nm high QPs revealed two clear maxima and a broad absorption lying underneath these two narrow resonances. The QW reference sample showed only the sharp resonances at the same energy. Employing a correctly weighted fit to the QP data resembles the QW-attributed resonances and a broad absorption with a maximum at 5.2 THz. Since this feature is entirely absent in the QW sample, we attribute it to the absorption in the QPs. To further investigate this THz absorption in the QPs, a QP sample with 35 nm height has been grown where a similar behavior was observed with the QP resonance shifted to 3.9 THz. Hence the QPs represent a very promising approach for semiconductor nanostructures that have tunable resonances in the THz frequency region. Tuning the resonance to lower frequencies with taller QPs and a theoretical analysis based on an eight band k●p model that takes into account the 3D shape and material compositions of the QPs are currently under investigation. This work is supported by the NSF and the Alexander von Humboldt Foundation.[1] J. He et al, Nanoletters 7, 802 (2007).

The precise and accurate determination of optical properties at THz frequencies is essential for the development of increasingly advanced THz optical systems and a prerequisite for the design and manufacturing of optical elements such as windows, focusing optics, optical antireflection coatings, etc.Today, ellipsometry is widely employed as a tool for the accurate, quantitative determination of a material’s complex-valued optical constants in a spectral region ranging from the far-infrared to the VUV. Generalized ellipsometry furthermore allows the determination of the materials anisotropic dielectric tensor properties as they occur in functional metamaterials for the THz frequency domain. Recently, we have reported on the use of high-brilliance THz synchrotron and Smith-Purcell-effect radiation sources for the determination of the optical constants of layered condensed matter samples in the spectral range from 0.7 to 8 THz.Here we employ for the first time a high power backward-wave oscillator type source for ellipsometric measurements in the spectral range from 0.1 to 1.6 THz. The base resonator of the backward-wave oscillator which ranges from 0.1 to 0.18 THz is augmented with Schottky-diodes which allow the extension of the spectral range up to 1.6 THz. Exemplarily, we report on the determination of complex-valued optical constants of several different infrared optical coating materials like Parylene-C, SiO2, and Si3N4. We furthermore demonstrate that our frequency domain generalized ellipsometer in combination with strong magnetic fields at low temperatures allows the investigation of low energy electron dynamics in semi- and superconducting materials.Our research will provide understanding of optical properties for novel materials, inspire new designs, and accelerate development of optical Terahertz devices.

Rapid advancement of solid state transistor technologies enables the possibility of THz circuits for the first time. These will revolutionize future systems and applications such as high speed communications, spectroscopy and remote sensing. InP based High Electron Mobility Transistor (HEMT), utilizing epitaxially grown high mobility InGaAs channel layers on an InP substrate has shown rapid advancement toward THz frequencies through aggressive scaling of device dimensions and increasing the indium concentration in the channel layer. In our latest groundbreaking work, we have optimized the channel composition and thickness, as well as barrier thickness, to maximize the channel-electron mobility while retaining good modulation of the channel-electron current. This work was done in conjunction with device gate length scaling from 100 nm to 35 nm to increase the transistor frequency of operation. Specifically to enhance channel mobility, we developed a composite channel of InGaAs/InAs/InGaAs to reduce alloy scattering and exploit the high mobility of InAs (~30,000 cm2/V/s). The thicknesses of the composite channel layers were optimized to achieve the highest mobility while avoiding channel relaxation and minimize short-channel effects. The optimization of this structure has so far resulted in mobilities as high as 15,400 cm2/V/s (25% higher than previously grown and measured structures) and the 35 nm InP HEMTs have achieved Ft of 500 GHz and Fmax of 1.2 THz. This represents the first time transistors have surpassed 1 THz Fmax. These devices were used for the first time in broadband submillimeter-wave integrated circuit (S-MMIC) amplifiers with 16-20 dB gain from 290-340 GHz. Further optimization of the material structures to target even higher device operating frequencies is underway and will result in future transistor circuits operating above 500 GHz to beyond 1 THz. These advancements will enable a new generation of military and commercial applications up to 1 THz with extremely high bandwidth, reduced aperture size, and narrower beam widths.

Alex K-Y. Jen, Department of Materials Science and Engineering, Box 352120, University of Washington, Seattle, WA 98195-2120E-mail: [email protected] radiation from 0-30 THz can be efficiently generated and detected using the NLO processes of differential frequency mixing and EO sampling. To facilitate the development of broadband THz sources, we focus on the development and processing of second-order NLO material systems for use with 1300 and 1500 nm laser sources. Our research is primarily targeted at three levels: 1) design and synthesis at the molecular level of NLO chromophores with large EO activity, excellent thermal stability, and high solubility; 2) employment of covalent and supramolecular interactions between chromophore molecules and polymeric chains to improve macroscopic properties such as EO polymer homogeneity, dipole alignment, and alignment stability; 3) development of simple and efficient processing strategies for constructing poled thick films (>100 μm) with large EO coefficients (>200 pm/V) and easily incorporating these thick films into optical device structures. In this talk, we will review our recent synthetic and processing efforts to adapt our NLO chromophores designs for terahertz generation and detection device structures and how these efforts are intertwined. The future success of using NLO materials to generate and detect broadband terahertz radiation depends not only on the EO activity of the materials, but on the ability to process such materials in a high throughput and efficient manner conducive for commercial production and applications. Invited Talk: Symposium K- Materials Research for Terahertz Technology Development

Layered chalcogenides, GaSe, GaTe, and GaSe_1-xTe_x (0.1≤x≤0.9) crystals have been studied extensively at EIC Laboratories, Inc. for various applications including tunable THz wave generation and broadband THz detection. The crystals were grown for optimum performance under various crystal growth conditions from in-house zone refined ultra pure precursor materials using vertical Bridgman furnaces. The growth process has been monitored, controlled and optimized by a computer simulation and modeling program. Integrated numerical models have been developed combining global heat transfer and elastic thermal stress sub-models. The heat transfer sub-model accounts for heat transfer in the multiphase system, convection in the melt, and interface dynamics. Low temperature photoluminescence (PL), deep-level transient spectroscopy (DLTS), Raman spectroscopy, optical absorption/transmission, anisotropic electrical charge transport properties and terahertz time domain spectroscopy (THz-TDS) have been used to characterize the grown crystals. It is observed that In- and Cr-doping enhances the hardness of the grown crystals, which is very useful in processing and fabricating large-area devices. GaSe crystals have demonstrated promising characteristics with good optical quality (absorption coefficient ≤0.1 cm^-1 in the spectral range of 0.62-18 µm), high dark resistivity (≥10⁹ Ω-cm), wide band gap (2.01 eV at 300K), good anisotropic (∥ and ⊥) electrical transport properties (μ_e/h, τ_e/h, and μτ_e/h) and long term stability. The THz-TDS measurements have shown that the GaSe crystals are highly efficient for broadband tunable THz sources (up to 40 THz) and sensors (up to 100 THz). Additionally, new terahertz wave generations (0.1-3 THz) have been observed for the first time from anisotropic binary and ternary semiconductor crystals. Detailed characterization as well as optimum crystal growth conditions including simulation and computer modeling will be presented.

Rapidly evolving plasmas represent a challenging environment for both study and control. Density, collision frequency and temperature fluctuations can change over orders of magnitude on time scales of one ns with spatial features less than one cm and thus are not amenable to conventional continuous-wave diagnostic techniques such as microwave or mm-wave interferometry. We have developed a new technique for studying plasmas undergoing rapid nonequilibrium changes that uses THz time-domain spectroscopy in conjunction with optical fluorescence imaging. The advantages of using THz pulses lie in the fact that the broad bandwidth of a THz pulse contains frequency components both above and below the plasma frequency allowing a single ps-duration pulse to carry away information about the complex path-integrated susceptibility. Transverse fluorescence gives us a model of the longitudinal plasma distribution and using a novel rms error-minimization technique we can recover the real and imaginary parts of the susceptibility with <5 mm spatial and, potentially, ps time resolution (we are currently limited by S/N considerations to averaging over several THz pulses and thus obtain 40 ns resolution). From this we obtain the electron density and collision frequency, spatially and temporally resolved, with dynamic range >10³. The principle of this new technique will be discussed along with results on a pulsed DC-discharge plasma. We will also present some new ideas such as concurrent molecular spectroscopy and computed tomography.

TBD