This PDF file contains the front matter associated with SPIE Proceedings Volume 7938, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

This paper reviews the design methodology and some of the applications space of standard photomixers and photoconductive switches. The methodology falls into three categories: (1) photoelectrostatics, (2) terahertz (THz) electromagnetics, and (3) laser coupling and thermal management. The applications space of ultrafast photoconductive devices, as for any device technology, is the best measure of their utility. At present photomixers are being used worldwide in at least these two instruments: (1) broadly tunable sweep oscillators for THz diagnostics, and (2) broadly tunable coherent transceivers for high-resolution THz spectroscopy. Photoconductive switches are being used in at least these two systems applications: (1) time-domain spectrometers, and (2) illuminators for THz impulse radars. Each of these applications will be addressed in turn, and some commercialization challenges facing ultrafast photoconductive devices will be addressed.

Spectroscopic studies are useful in a range of areas, from remote sensing and radio astronomy to the medical community. Since spectroscopy can provide information used to identify chemicals, new techniques have been developed for high-resolution measurements of chemical absorption frequencies. These high-resolution measurements not only enhance accuracy of the molecule's energy level transitions, but also allow for environmental information to be gathered through collisional broadening of the spectral lines. High-resolution measurements, made possible by far-infrared lasers coupled with Schottky diodes, were supplemented with data taken by variable pathlength FTIR measurements in the terahertz region. Chemicals investigated include methanol, nitromethane, water and its isotopes.

The enhancement of acoustic waves from a laser-induced plasma, from audible into the ultrasonic range, is a linear function of the terahertz (THz) intensity incident on the plasma, making THz-enhanced acoustics (TEA) useful for THz detection at a standoff distance. When a femtosecond laser of sufficient peak energy is focused into air, the gas is ionized and acoustic waves are launched through the photoacoustic process. THz field-induced energy transfer through acceleration of free electrons, and the following electron-molecule collisions, results in an enhanced acoustic emission from the plasma. A physical understanding of free electron manipulation inside a dual-color laser filament is used to encode THz spectroscopic information into acoustic waves, making it possible to coherently detect the electric field profile of the THz pulse by simply "listening" to the plasma from a distance. Our most recent results utilizing TEA as method for remote THz detection will be presented.

Existing parameter extraction techniques in the terahertz range utilize the magnitude and phase of the transmission function at different frequencies. The number of unknowns is larger than the number of available information creating a nonuniqueness problem. The estimation of the material thickness thus suffers from inaccuracies. We propose a novel optimization technique for the estimation of material refractive index in the terahertz frequency range. The algorithm is applied for materials with arbitrary frequency dependence. Dispersive dielectric models are embedded for accurate parameter extraction of a sample with unknown thickness. Instead of solving N expensive nonlinear optimization problems with different possible material thickness, our technique obtains the optimal material thickness by solving only one optimization problem. The solution of the utilized optimization problem is accelerated by estimating both the first order derivatives (gradient) and second order derivatives (Hessian) of the objective function and supplying them to the optimizer. Our approach has been successfully illustrated through a number of examples with different dispersive models. The examples include the characterization of carbon nanotubes. The technique has also been successfully applied to materials characterized by the Cole-Cole, Debye, and Lorentz models.

Terahertz (THz) imaging is promising for nondestructive evaluation, since many optically opaque dielectrics are transparent to THz waves. Conventional THz imaging systems employ focusing elements such as spherical lenses and off-axis parabolas, but their fixed focal length produces an inherent trade-off between lateral resolution and depth of focus. Furthermore, image quality suffers when imaging objects located inside a dielectric medium. The air-dielectric interface introduces significant spherical aberration that degrades spatial resolution. Bessel beams are known to produce a small spot size over a large depth of focus. The contribution of our work is two-fold: (1) We demonstrate THz imaging with a significantly improved depth of focus using a zero-th order Bessel beam produced by an axicon lens. (2) We also demonstrate, for the first time to our knowledge, that Bessel beams experience reduced spherical aberration when imaging objects embedded in a dielectric medium. Imaging experiments are performed with a time-domain THz system, where a zero-th order quasi-Bessel beam is formed with an axicon lens made from high density polyethylene (HDPE). The HDPE axicon has a 50 mm diameter and an apex angle of 120 degrees. Point spread function (PSF) measurements confirm that lateral resolution is maintained over a 25 mm depth of field in air. The same lateral resolution is achieved over a 35 mm range inside a HDPE substrate. Needle objects embedded inside a thick HDPE substrate are imaged with high spatial resolution. Image contrast is significantly improved by digital filtering to reduce sidelobe levels. These promising results suggest that Bessel beams are well suited for terahertz nondestructive imaging of thick dielectric objects.

We report terahertz time-domain spectroscopy (THz-TDS) system based on Er:fiber laser at 1.55 μm wavelength that integrates an ion-irradiated In_0.53Ga_0.47As photoconductive antenna as an emitter and a DAST electro-optic sensor as a detector. DAST crystal cannot be used in traditional electro-optic sampling set-ups because of its high intrinsic birefringence. We propose here THz detection in DAST crystal by using an optical phase detection technique. The detected bandwidth is 5 THz. The performances of our THz-TDS systems based on electro-optic detection in a DAST crystal using optical phase detection is compared theoretically to the performance of a THz-TDS systems based on electro-optic detection in a GaAs crystal using traditional polarization rotation detection.

This study describes terahertz (THz) imaging of hydration changes in physiological tissues with high water concentration sensitivity. A fast-scanning, pulsed THz imaging system (centered at 525 GHz; 125 GHz bandwidth) was utilized to acquire a 35 mm x 35 mm field-of-view with 0.5 mm x 0.5 mm pixels in less than two minutes. THz time-lapsed images were taken on three sample systems: (1) a simple binary system of water evaporating from a polypropylene towel, (2) the accumulation of fluid at the site of a sulfuric acid burn on ex vivo porcine skin, and (3) the evaporative dehydration of an ex vivo porcine cornea. The diffusion-regulating behavior of corneal tissue is elucidated, and the correlation of THz reflectivity with tissue hydration is measured using THz spectroscopy on four ex vivo corneas. We conclude that THz imaging can discern small differences in the distribution of water in physiological tissues and is a good candidate for burn and corneal imaging.

Quantum cascade lasers (QCLs) employ the mid- and far-infrared intersubband radiative transitions available in semiconducting heterostructures. Through the precise design and construction of these heterostructures the laser characteristics and output frequencies can be controlled. When fabricated, QCLs offer a lightweight and portable alternative to traditional laser systems which emit in this frequency range. The successful operation of these devices strongly depends on the effects of electron transport. Electron-electron scattering is an essential mechanism involved in electron transport and approximations are often made in finding the electron-electron scattering rate in order to simplify calculations. Results will be presented characterizing various effects which are sometimes ignored in calculating electron-electron scattering rates. These effects include state-blocking, electron screening, temperature dependence, as well as the inclusion of all possible transitions that can occur in three periods of the QCL active region. These effects will be presented in the context of several QCL active region designs, including those grown and fabricated at the University of Massachusetts Lowell Photonics Center.

We propose integrated waveguides for terahertz (THz) and mid-infrared (MIR) applications on wafer platform. Based on the prototype of spoof plasmonic waveguides consisting of textured metallic surface, we explore the possibility of coating periodic metallic pattern with silicon (at 0.6 THz) or germanium (at MIR region of 30 THz) to further shrink the relative mode size of propagation spoof plasmonic waves. Numerical modeling via 3D finite-difference time-domain (FDTD) has shown deep sub-wavelength mode confinement in transverse directions to smaller than λ/50 by λ/50, with an estimated propagation loss of less than 0.1 dB for each repetitive unit.

The optical frequency conductance is derived for quantum wells and quantum dots, and the optical frequency conductivity of bulk narrow-gap semiconductors is revisited. The teraHertz (THz) and infrared (IR) response of these semiconductor structures, in both free-space and guided-wave geometries, is described in a simple manner within the optical frequency conductance formalism. Familiar concepts form the microwave domain, including transmission lines and impedance matching, are extended into the THz and IR domains. We show that the fine structure constant of quantum electrodynamics sets the natural scale for the optical conductance of semiconductor structures, from which rules of thumb and physical limits to THz/IR gain and absorption can be derived. The optical conductance formalism can be applied to MCT photodetectors, quantum well IR photodetectors, quantum dot IR photodetectors, and quantum cascade lasers.

This paper will illustrate the potential of InAs/GaSb broken-gap structures for providing a solution to the well-known and long-standing terahertz (THz) frequency gap in source technology. In a double-barrier GaSb/InAs/GaSb heterostructure, the ultrafast heavy-hole interband tunneling can be utilized to achieve electron depopulation of a quasibound, heavy-hole level located in the valence-band of the right GaSb barrier region. A population inversion is then created using electron injection into the conduction-band resonant state of the double-barrier structure. Degrading nonradiative processes such as acoustic phonon, optical phonon and Auger recombination are suppressed in this spatially separated two-level energy system. Hence, heavy-hole interband tunneling prevails over nonradative transition rates and establishes a population inversion at relatively high operating temperatures. Detailed simulations predict a significant optical (total) gain of ~0.001 which is comparable with GaAs/AlGaAs quantum well laser in spite of the small overlap of conduction band and heavy-hole wavefunctions. The TE emission allows for implementation of vertical surface emission in large area, for single or arrayed devices. Also, lateral quantum confinement of arrayed systems can be used to reduce the conduction band current density (and undesired thermal heating) and increase the quantum efficiency.

We are exploring the degree to which one can control the spectral emission of heated photonic crystals (or, more generally, electromagnetic crystal) structures in the THz frequency range. Because THz frequencies are well below the room temperature thermal emission maximum, this configuration may realize a low power but extremely low cost incoherent broadband THz source. Electromagnetic crystals are structures whose periodicity either enhances or reduces the associated photonic density of states over some frequency range. Consequently, they either enhance or reduce its thermal emission over the same frequency range. Thermal radiation from electromagnetic crystals has been studied theoretically and experimentally for higher frequency ranges, but usually for infinite lattices. We have experimentally and theoretically investigated a simple 1D, bi-layered electromagnetic crystal structure composed of air and silicon slabs. We have calculated the emissivity using Kirchhoff's thermal radiation law, as well as by calculating the density of states directly, and have compared successfully those results to the experimental values. Our ultimate goal is to be able to control the spectral emission of an electromagnetic crystal in the THz region (or other wavelength ranges, such as the infrared) by engineering its band structure. Controlled thermal emission, i.e., thermal management, could be used for applications as diverse as solar energy convertors, thermoelectric devices, and integrated circuits.

Onto a double layer, which is made of a Si substrate ( ρ> 1000 Ω·cm ) and a SiO₂ layer 100 nm thick on top of it, a Nb₅N₆ thin film microbridge is deposited and integrated with an aluminum bow-tie planar antenna. With a SiO₂ air-bridge further fabricated underneath the microbridge and operated at room temperature, such a combination behaves very well as a bolometer for detecting signals at 100 GHz, thanks to a temperature coefficient of resistance (TCR) as high as -0.7% K^-1 of the Nb₅N₆ thin film. According to our estimations, the best attainable electrical responsivity of the bolometer is about -400 V/W at a current bias of 0.4 mA. The electrical noise equivalent power (NEP) is 6.9x10^-11 W/Hz^1/2 for a modulation frequency at 300 Hz and 9.8x10^-12 W/Hz^1/2 for a modulation frequency above 10 kHz respectively, which are better than those of commercial products (such as Golay cell and Schottky diode detectors). A quasi-optical receiver based on such a bolometer is constructed and measured.

In this paper we report on the improvements in holographic techniques developed for applications in the millimeter-wave and terahertz range of the electromagnetic spectrum. An experimental arrangement, adapted from off-axis near-field holography at visible wavelengths, was employed that utilizes a raster scanning detector to record the holograms digitally. The object and reference fields were based on the beams from a pair of radiating antennas fed by a single coherent source via a cross-guide coupler. Using phase retrieval methods, the recorded holographic interference pattern can be used to determine the effective phase centers of radiating feed antennas, including non standard radiators such as planar lens antennas. By numerically propagating the recovered object beam back to the source plane the object beam in the vicinity of the waist (the effective phase center) can be recovered. Among the issues investigated was improvement in the accuracy of the phase retrieval process by taking account of the non-perfect reference beam. The technique has also been applied to the investigation of increased co-polarisation levels in the scattering of radiation from surface features of dielectric materials on millimeter-wave radiation.

Various all-dielectric electromagnetic crystal (EMXT) based THz components, including filter/reflector, waveguide, antenna, and transition structure to planar circuits are proposed and simulated. Several of them have been fabricated via a THz rapid prototyping technique, and the measurements show very good consistency with the simulations. Potential integrated THz micro-systems could be constructed using these components. The layer-by-layer printing virtue of the rapid prototyping technique may enable the integration and packaging of various THz components in a systematic manner.

Future Far-IR space telescopes, such as the SAFARI instrument of the proposed JAXA/ESA SPICA mission, will use horn antennas to couple to cavity bolometers to achieve high levels of sensitivity for Mid-IR astronomy. In the case of the SAFARI instrument the bolometric detectors susceptibility to currents coupling into the detector system and dissipating power within the bolometers is a particular concern of the class of detector technology considered.¹ The simulation of such structures can prove challenging. At THz frequencies ray tracing no longer proves completely accurate for these partially coherent large electrical structures, which also present significant computational difficulties for the more generic EM approaches applied at longer microwave wavelengths. The Finite Difference Time Domain method and other similar commercially viable approaches result in excessive computational requirements, especially when a large number of modes propagate. Work being carried out at NUI-Maynooth is utilising a mode matching approach to the simulation of such devices. This approach is based on the already proven waveguide mode scattering code "Scatter"² developed at NUI-Maynooth, which is a piece of mode matching code that operates by cascading a Smatrice while conserving power at each waveguide junction. This paper outlines various approaches to simulating such Antenna Horns and Cavities at THz frequencies, focusing primarily on the waveguide modal Scatter approach. Recently the code has been adapted to incorporate a rectangular waveguide basis mode set instead of the already established circular basis set.

A dielectric hollow-core tube utilized as a terahertz anti-resonant reflecting hollow-core waveguide (THz-ARRHW) sensor has been demonstrated to detect the minute variation of both refractive index and thickness in macromolecule layers, deposited on the tube wall, and to identify liquid vapors from the various core indices. The minimal quantity of macromolecule layers loaded on the tube wall of a polypropylene tube can be detected at 1.2picomole/mm² and 0.2%, corresponding to the variation of 2.9μm-thickness and 0.001-refractive-index. And the sensing performance of a THz- ARRHW to detect core index variation for identifying volatile liquids is also realized at 0.0001g/cm³- vapor density.

This paper presents some potentially interesting aspects of spectroscopic transmission measurements of explosive materials in Far-Infrared (Terahertz) range: preparation of the samples, influence of covering by clothes and influence of phlegmatization of explosives (addition an agent to an explosive material to stabilize or desensitize it). Moreover, two commonly used techniques - Far Infrared Fourier Spectroscopy and Time Domain Spectroscopy are presented and compared. We also shown and compared spectra of materials obtained in two Time Domain Spectroscopy reflection configurations: specular (45° incident angle) and stand-off with distance 30 and 40 cm to a sample.