This PDF file contains the front matter associated with SPIE Proceedings Volume 6772, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

After more than a dozen years of basic research into the submillimeter and far infrared range, terahertz (THz) wave research has finally come into its own, and is recognized by the world scientific community as a new frontier. While femtosecond laser pumped THz wave sources have opened up a new vista in applied research, the ideal THz wave source will likely require high coherence and wide tunability. When this level of quality is finally made available in a userfriendly device, there is little doubt that applied research efforts into the THz region will enjoy a true renaissance. In this direction we have developed a widely tunable injection seeded THz-wave parametric generator (is-TPG) that operates at room temperature. The spectral resolution is the Fourier transform limit of the nanosecond THz wave pulses. In our laboratory, THz-waves continue to broaden their range of applications as following. We have developed a basic technology for THz imaging which allows detection and identification of drugs concealed in envelopes by introducing the component spatial pattern analysis. On the other hand, for inspecting electrical failures in large scale integration circuit, we developed the laser-THz emission microscope, which records the map of THz emission amplitude in a sample upon excitation with fs laser pulses.

In this paper, we present an integrated fabrication process for realizing a switching/modulation mechanism for negative index materials (NIMs) based on photoconductive coupling. The metamaterial element chosen is an array of regular copper split-ring resonator (SRR) that was fabricated on two different substrates: high-resistivity silicon (HRS) and fused silica glass. The switching mechanism proposed can be achieved through tuning the SRR gap and/or substrate conductivity. The photosensitive material of the SRR structure (amorphous silicon for the glass substrate samples and intrinsic silicon for the HRS substrate samples) upon illumination generates excess carriers that essentially shunt the gap capacitance thus diminishing the resonance response significantly. The response in terms of S-parameters is simulated using HFSS under varying magnitude of optical illumination. Our simulation with a single SRR to demonstrate total suppression of resonance amplitude with a high extinction ratio is applicable to NIMs comprising of both negative permeability and negative permittivity without any loss of generality. This method may provide a basis for long-sought practical applications and devices based on NIM in the fields of ultra-fast communications at RF and optical frequencies, sensing and imaging promising a potential of dramatically improving the performance of existing phased array antennas, optical beam-forming networks, antenna remoting and transportation of RF power through fiber-radio.

Compared to the neighboring infrared and microwave regions, the terahertz regime is still in need of fundamental technological advances. This derives, in part, from a paucity of naturally occurring materials with useful electronic or photonic properties at terahertz frequencies. This results in formidable challenges for creating the components needed for generating, detecting, and manipulating THz waves. Considering the promising applications of THz radiation, it is important overcome such material limitations by searching for new materials, or by constructing artificial materials with a desired electromagnetic response. Metamaterials are a new type of artificial composite with electromagnetic properties that derive from their sub-wavelength structure. The potential of metamaterials for THz radiation originates from a resonant electromagnetic response which can be tailored for specific applications. Metamaterials thus offer a route towards helping to fill the so-called "THz gap". In this work we discuss novel planar THz metamaterials. Importantly, the dependence of the resonant response on the supporting substrate enables the creation of active THz metamaterials. We show that the resonant response can be efficiently controlled using optical or electrical approaches. This has resulted in the creation of efficient THz switches and modulators of potential importance for advancing numerous real world THz applications.

We have developed a 120-GHz-band wireless communication link that uses photonic and electronic technologies for the generation, modulation, and amplification of 125-GHz signals. Photonic technologies are suitable for generating ultra-high frequency signals, and enable long-distance wired transmission and the distribution of millimeter-wave signals. All-electronic systems, on the other hand, have the advantages of being compact and inexpensive, especially when the transceiver functions are implemented with monolithic microwave integrated circuits (MMICs). We succeeded in the error-free transmission of a 10-Gbit/s data signal over a distance of 450 m, and estimated the maximum transmission distance to be about 2 km.

Dielectric characterization of glasses and amorphous materials has been done with two terahertz time-domain spectroscopies at millimeter wavelength and terahertz frequencies. A quasi-optical free-space spectrometer is equipped with a backward-wave oscillator as a high power tunable source of coherent radiation and utilized in this research. The measurement technique is based on the transverse magneto-optical effect to enable the determination of the real and imaginary parts of complex dielectric permittivity of various glass specimens from 70 to 120 GHz. Above 120 GHz dispersive Fourier transform spectrometer is more effective to provide high resolution continuous spectra of dielectric permittivity up to 800 GHz. The real part of permittivity can be reproduced to 1 part in 100,000. The complex dielectric spectra of high purity fused silica glasses clearly show the existence of boson peaks at terahertz frequencies.

The emergence of terahertz (THz) technology has opened up a new frontier of fundamental research with many novel applications, such as in imaging and spectroscopy. For these fibers, the waveguiding parameters are easily controllable, when compared to conventional all silica optical fibers and recently there has been an initiative to use such waveguides for guiding THz frequencies. In this context, A rigorous full-vectorial finite element method has been used to obtain the modal solutions of simple, high-density polystyrene dielectric waveguides along with their propagation constants, attenuation characteristics, vectorial modal field profiles, the spot-size and the modal hybridness.

Compared to Millimeter waves THz frequencies can provide an almost magnitude higher resolution, which enables detection of small threats at larger distances. Here we present an active THz standoff imaging system based on commercially available BWO sources and a thermal detector. At a excitation frequency of 720GHz an almost diffraction limited resolution was achieved of about 1.6mm at 1.2m distance to the target. With signal to noise ratio of 35dB detection of metal and ceramic weapons covered by different clothing is demonstrated. A detailed system analysis is done, which led to limiting factors of the proposed system.

Finite element analysis, based on the vector H-field formulation and incorporating the perturbation technique, is used to calculate the complex propagation characteristics of silver/polystyrene (PS) coated hollow glass waveguides for terahertz (THz) frequency radiation. The mode profiles, which after emerging from the waveguide travel through either a horizontal or vertical polarizer are also presented and camera images of these profiles are compared to numerically simulated results. The effect of the polystyrene coating thickness on the attenuation characteristics of these waveguides has also been investigated and is shown to be critical to their design optimization.

Dispersive Fourier Transform Spectroscopy (DFTS) provides us with a very precise method of measuring the absorption and refractive index spectra of common pollutant gases. This paper presents the rotational transition lines of Sulfur Dioxide and Carbon Monoxide gas as a function of varying pressure using DFTS for the very first time as a combined study. The relationship between the variation of the pressure and the change in the absorption spectrum is examined and discussed in detail. Sulfur Dioxide and Carbon Monoxide gases are highly toxic, pollutant gases that are major contributors to global pollution and can potentially be used as a chemical threat. The relationship between pressure and rotational transmission lines is discussed in detail in the frequency range of 0.3 THz - 0.9 THz. These findings are crucial in characterizing these gases as well as identifying them in a blind test.

The dielectric properties of common household powders from 0.6 to 1.2 THz are presented in this paper. Terahertz Dispersive Fourier Transform Spectroscopy was used to yield the dielectric properties of powders as a continuous function of frequency. Tests were conducted using a polarized interferometer and two cryogenically-cooled high frequency detectors. Dielectric spectroscopy was utilized to provide high-resolution and precise information on the dielectric spectra of powders including the powder's unique resonance signature. This signature can be employed to detect the presence of a hoax or harmful powder within a baggage or mail package.

We have observed coherent synthesis of spatial profiles of THz radiation emitted from a photoconductive antenna array. The antenna array is composed of seven independently addressable photoconductive antenna units having interdigitated electrodes. The antennas were pumped by amplified 800 nm femtosecond optical pulses. Emitted THz radiation was focused, and the time-resolved spatial profile of the THz radiation on the focal plane was observed using a newly developed real-time imaging apparatus, which can correct the terahertz field images for nonuniformity in birefringence of the EO crystal. By scanning the delay time, frequency-resolved images were also obtained, which exhibit frequency-dependent field profiles. Field profiles observed were coherent superpositions of terahertz waves emitted from the antenna units constituting the array. By inverting the bias voltage to the central unit of the emitter array, we observed super-resolution beam size of terahertz waves, which are smaller than the diffraction limit.

We have designed and fabricated our original terahertz plasmon-resonant emitter incorporating doubly interdigitated grating gates and a vertical cavity into an InGaP/InGaAs/GaAs high-electron mobility transistor (HEMT) structure. The fabricated device is subjected to 1550-nm, 1-mW (a) a single CW-laser, (b) 4-THz photomixed dual CW-laser, and (c) a 70-fs pulsed-laser illumination at room temperature. In case of (a), terahertz emission due to the plasmon modes of self oscillation is detected by a Si bolometer under certain bias conditions. In case of (b), a resonant peak of injection-locked 4-THz oscillation is clearly observed on the device photoresponse. In case of (c), an impulsive radiation followed by relaxation oscillation is observed by electrooptic sampling, whose Fourier spectrum exhibited resonant peaks of plasmons' harmonic modes up to 4 THz. Estimated radiation power exceeds 0.1 μW, resulting in excellent conversion efficiency of the order of 10^-4.

We report an experimental study on THz-emission from narrow band gap semiconductors excited by femtosecond nearinfrared laser pulses. In particular, we studied GaSb, Ga_1-xIn_xSb and InN. The Ga_1-xIn_xSb material system enables the study of the influence of carrier concentrations on THz emission process in narrow band gap semiconductors. The study demonstrates the existence of a compromise between the positive effect of high electron temperature provided by narrow band gap materials and the negative effect of high intrinsic carrier concentrations. The influence of the majority and minority carrier types and concentrations on THz emission strength was investigated using GaSb:Te. By varying the majority and minority carrier type and carrier concentrations over three orders of magnitude the THz emission mechanism in GaSb can be tuned from being dominated by the photo-Dember effect to being dominated by surface field acceleration. Within each regime photo-Dember based THz emission and surface field acceleration based THz emission are maximized under specific majority and minority carrier concentrations. Strong advantages of InN as THz-emitter over other narrow band gap materials, are strong intrinsic electric fields, low intrinsic carrier concentration and most importantly, very low probability of intervalley scattering.

Imaging of styrofoam with the help of ultrashort Terahertz pulses is investigated. With a combination of pulse amplitude and time delay imaging it is possible to speed up the measurement about two orders of magnitudes.

We report the characterization of superconducting niobium microbolometers designed for time-resolved terahertz spectroscopy on nanosecond to millisecond timescales. Coupling of the incident signal is achieved via a planar antenna mounted on a hyperhemispherical silicon lens. We have integrated these detectors into a custom Fourier-transform spectrometer. The spectrometer optics are frequency independent over the spectral range 0.1-3 terahertz and thus the system bandwidth is set by the detector antenna. We have fabricated devices with two different antenna geometries, the double-dipole and the log spiral, and have characterized the spectral response of each. This detector will enable a variety of novel spectroscopy applications.

Terahertz imaging and spectroscopy have attracted a lot of attention in recent years, because monocycle terahertz radiation can be generated using an ultra-short pulse laser and semiconductor device technologies. The availability of monocycle terahertz radiation sources has encouraged innovative research and development activities worldwide in an extremely wide range of applications, from security to medical systems. However, the fundamental device technology, namely the semiconductor emitter, amplifier, modulator, focal plane array detector, and optical thin film among others, in the terahertz frequencies has not yet been fully established. Therefore, a measurement system in the terahertz range remains a costly alternative. We report in this paper our recent developments of a terahertz quantum cascade laser (THz-QCL) and a terahertz quantum well photo-detector (THz-QWIP). We believe that the combination of a semiconductor emitter (THz-QCL) and a semiconductor detector array (THz-QWIP) is a good choice for developing a cost-effective measurement system for a given terahertz range (from 1.5 THz to 5.0 THz), because both of these items are based on mass-production semiconductor fabrication techniques. We fabricated the THz-QCLs using a resonant longitudinal-optical phonon depopulation (RPD) scheme, which is made up of both a GaAs/AlGaAs material system and a GaSb/AlGaSb material system. The GaAs/AlGaAs THz-QCL has already successfully demonstrated a high peak power (about 30 milliwatts in pulsed operation) operation at 3.1 THz and a high operating temperature (123K). On the other hand, we have fabricated a THz-QWIP structure consisting of 20 periods of GaAs/Al_0.02Ga_0.98As quantum wells with a grating coupler on the top of detector devices, and successfully operated it at 3 THz with a responsivity of 13mA/W. We now believe we are ready to make a cost-effective measurement system, although both of the devices still require cryogenic coolers.

In this work we present the results on combined experimental and computational study of sub-THz spectra of liquid water. The important new result is the detection of hydrogen bonds in liquid water. The experimental study was performed by employing Fourier Transform terahertz (THz) spectroscopy with spectral resolution of 0.25 cm^-1. Resonance features in transmission spectra of water layers between thin film substrates are demonstrated in the sub-THz range. The theoretical approach for computer simulation of THz absorption spectra from liquid water is also discussed. The molecular dynamical (MD) simulations of water were performed using Amber 8 and the TIP3P, SPCE (Extended Single Point Charge) and TIP4P water models. Several examples of modeling results are presented. The experimental spectra are compared with the theoretical predictions. The SPCE model better correlates with experimental spectra compare to two other models.

The effects of surface scattering on terahertz reflection spectrum for explosive detection are studied by measuring terahertz reflection pulses from sandpapers with different roughness coated with gold. The experimental results show that the amplitude decrease and pulse broadening of the detected signal caused by the surface scattering result in the width reduction of Gaussian distribution of the specular scattering coefficient spectrum. A simple analytical model is applied to the analysis of experimental results and good agreements are obtained.

We have conducted the first study of the use of terahertz radiation to precisely identify pre-melting, melting, polymerization, depolymerization and the influence of polar water in sulfur by scanning frequency as a parametric function of temperature, and including identifying precursor and intermediate states. This spectroscopic study has also identified the orthorhombic-monoclinic phase transformation, and the melting of the superheated orthorhombic phase. This work also reports detection of a water absorption indicating a perturbation of the water molecules, associated with solvation spheres of the inter-chain dynamics, as a precursor to a transition, and supporting our earlier results showing the transducing capabilities of conglomerates of water molecules. Through a study of the fine structure of the water absorption, we are able to determine information about local polarization effects which contribute to the transducing properties of water relative to a ligand. The above inorganic polymer study is applied to the understanding of the response of biomolecules to thermal and chemical influences, and data are included giving optical, electrical, and pH properties of the DNA-water system, showing a major conformational transition at ~43°C, and various forms of reconformation of DNA macromolecule due to chemical perturbation. Our results include findings aimed at complementing existing inhibitors that are intended to prevent retrovirus/phage invasion of the host cell DNA.

We report on the demonstration of a dc to ac radiation converter (DARC) source as a THz emitter. THz radiation is generated through the spatiotemporal change in electron plasma density induced by a relativistic ionization front propagating in a ZnSe crystal. The central emission frequency is 1.2 THz with a bandwidth of 0.7 THz [full width at half-maximum (FWHM)]. The central frequency is attributed to production of the electron plasma density of the order of 10¹⁵ cm^-3 and is in good agreement with the expected theoretical value.