This PDF file contains the front matter associated with SPIE Proceedings Volume 6472, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Optical design in the terahertz (THz) waveband can be challenging, especially for high-precision applications. In this paper we summarise our experience with the quasi-optical design and subsequent performance of astronomical telescopes designed to measure the faint temperature and polarisation properties of the Cosmic Microwave Background Radiation, in particular QUaD¹, the PLANCK Surveyor² and MBI³. These telescopes contain a range of quasi-optical components including corrugated feed horns, on- and off-axis conic mirrors and lenses. Knowledge of their optical performance and beam patterns is critical for understanding systematic effects in the reliable extraction of feeble polarisation signals. Although Physical Optics can be used to characterise electromagnetic systems to high accuracy, it is computationally intensive at these frequencies and often not suitable for the initial design or preliminary analysis of large multi-element optical systems. In general there is a lack of dedicated software tools for modelling the range of components and propagation conditions encountered in typical systems and we have employed a variety of commercial and in-house software packages for this task. We describe the techniques used, their predictions and the performance of the telescopes that have been measured to-date.

The amplitudes of terahertz radiation are measured for a series of GaAs surface intrinsic-n⁺ (SIN⁺) structures with various built-in surface electric fields as the bias. As the surface field is lower than the so-called "critical electric field" related with the energy difference between the &Ggr; to L valley of the semiconductor, the amplitude is proportional to the product of the surface field and the number of photo-excited carriers. As the intensity of surface field exceeds the critical field, the THz amplitude is independent of the surface field but proportional the number of the photo-excited carriers. Our study proposed two optimal conditions for an SIN⁺ structure to serve as a THz emitter: the width of its intrinsic layer is nearly equal to the penetration depth of the pump beam, and the intensity of built-in electric field is nearly equal to the critical electric field. Notably, the critical field determined from the THz amplitude under various electric fields provides one way to estimate the &Ggr; to L valley splitting in semiconductors.

This paper reports on the development of micromachined pillar arrays for the filtering of terahertz radiation. These pillar arrays are fabricated using ultraviolet based processing of thick SU8. This micromachining technique enables the array patterns, dimensions, and consequently the filter characteristics, to be readily defined. In particular, we demonstrate that by combining individual filter arrays with either different periods or pillar diameters we can isolate individual pass bands in the 1 to 2 THz region.

In this report, we will review our recent development on the sub-wavelength plastic fiber for THz waveguiding. The proposed and demonstrated terahertz single-mode sub-wavelength waveguide is similar to an optical taper fiber, having a low attenuation constant (~10^-2 cm^-1), a high coupling efficiency, and a free-space direct coupling capability, comprised with a sub-wavelength PE fiber core with air cladding. The spectral characteristic of the sub-wavelength THz fiber will be discussed, with an effective attenuation minimum of THz waves on the order of or less than 10^-3 cm^-1 at a specific wavelength range which depends on the fiber diameter. More over, the application of the sub-wavelength plastic fiber will also be discussed, including a first demonstration of a singlemode fiber-based THz directional coupler.

This paper presents predictions of classical electromagnetic scattering from granular material illuminated by a terahertz (THz) source. Random media models are created to represent the explosive grains, air voids and filler material commonly found in explosive devices. These constituents can cause significant volume scattering that may alter or obscure the chemical response of the explosive, thus impacting THz detection of explosives. Furthermore, the air-explosive interface may have significant roughness, and scattering from this interface may be a dominant factor - particularly in reflection spectroscopy. The volume scattering is calculated using the Quasi-Crystalline Approximation (QCA) and a Finite Difference Time Domain (FDTD) calculation; the FDTD method is also used to estimate the rough surface scattering. Results from these calculations are provided for mixtures that are representative of common classes of explosives.

In this paper, we continue to investigate a theoretical framework based on Gaussian Beam Mode Analysis for modelling standing waves in submillimetre optical systems with experimental verification. Standing waves or multiple reflections have been traditionally difficult to model but this analytical method proves to be very versatile in first order predictions. In previous papers we reported on the underlining theory and described some important examples including reflections between a feed horn and telescope secondary mirror and also reflections between two coupled corrugated horns. This technique can in addition be applied to reflections between components such as lenses and apertures [1], [2]. As our method uses a full multi-moded scattering matrix description of the corrugated horn, which is then transformed to equivalent free space Gaussian modes, multiple reflections between the source/detector device, located at the back of the horn, and any arbitrary surface in the optical path can be accurately analysed. An overview of the technique is presented including experimental measurements to try to verify our theoretical methods. We investigate mechanisms to reduce standing wave ripples experimentally and theoretically often present in submillimeter optics and try to understand more deeply the form and structure of the reflected power component.

The properties of terahertz (THz) radiation potentially make it ideal for medical imaging but the difficulty of producing laboratory sources and detectors has meant that it is the last unexplored part of the electromagnetic spectrum. In this paper we report on near-field reflection and absorption measurements of biological samples at 0.1THz as a first step towards developing THz and millimetre-wave imaging schemes. Variation of the absorption and reflection of THz in these samples is investigated as a means of determining information about the sample structure. Operating at 100 GHz with standard detecting devices we illustrate preliminary results in imaging (transmission and reflection) measurements of meat samples using various optical configurations and draw conclusions on the scope of the techniques. Some encouraging provisional results are discussed as well as limitations in "intensity only" measurements due, primarily, to standing waves and a lack of dynamic range. These experiments were performed as part of a Masters thesis. A discussion on a variety of absorbing materials utilized to reduce reflected radiation from surrounding optical components is also given. In addition we report on initial trials in extracting information about an object's size by sparsely measuring points in the equivalent Fourier plane in a simple optical setup, thus avoiding the need for time consuming raster scanning. This technique has many potential applications in detecting and scanning systems. Here the background theory and preliminary results are presented.

MODAL is an optical design and analysis package targeting the millimetre and sub-millimetre region of the electromagnetic spectrum. It is being developed at NUI Maynooth with the aim of integrating advanced modelling techniques and access to High Performance Computing into a user-friendly and yet very powerful tool for an (quasi-)optical designer. MODAL has been recently extended to allow integrated simulation of custom corrugated horns and dielectric lenses. This made it possible to model an existing instrument (QUaD), with the goal of optimising its performance. Here we present new results from analysis of the predicted performance of the QUaD telescope, with particular emphasis on polarisation information. They were obtained by using MODAL to model the whole telescope, with the distortion of the primary accounted for, for a range of component tilts and separations.

A scanning Fabry-Perot transmission filter composed of a pair of dielectric mirrors has been demonstrated at millimeter and sub-millimeter wavelengths. The mirrors are formed by alternating quarter-wave optical thicknesses of silicon and air in the usual Bragg configuration. Detailed theoretical considerations are presented for determining the optimum design. Characterization was performed at sub-mm wavelengths using a gas laser together with a Golay cell detector and at mm-wavelengths using a backward wave oscillator and microwave power meter. High resistivity in the silicon layers was found important for achieving high transmittance and finesse, especially at the longer wavelengths. A finesse value of 411 for a scanning Fabry-Perot cavity composed of three-period Bragg mirrors was experimentally demonstrated. Finesse values of several thousand are considered to be within reach. This suggests the possibility of a compact terahertz Fabry-Perot spectrometer that can operate in low resonance order to realize high free spectral range while simultaneously achieving a high spectral resolution. Such a device is directly suitable for airborne/satellite and man-portable sensing instrumentation.

We review recent research using amorphous electrooptic (EO) polymers for generation and detection of broadband terahertz radiation (0.3 THz -30 THz). The advantages of amorphous EO polymers over other materials for broadband THz generation (via optical rectification) and detection (via EO sampling) include a lack of phonon absorption (good transparency) in the THz regime, high EO coefficient and good phase-matching properties, and, of course, easy fabrication (low cost). Our ~12-THz, spectral gap-free THz system based on a polymer emitter-sensor pair is an excellent demonstration of the advantages of the use of EO polymers. This system has been employed as a wideband spectrometer to study dielectric materials in the THz regime.

Coherent cw-THz-radiation allows access to new applications in the field of medicine, industrial process control, data communication and security applications. Major advantages of radiation in this spectral range are that it penetrates through e.g. plastics but is strongly reflected by metals and that molecules show distinct and distinguishable spectra so that a selective sensing of single species is possible. However, existing THz-sources are either very bulky or expensive. THz sources can require cryogenic temperatures or emit only low power radiation. Furthermore the setup is often very complicated and sensitive so that field measurements are not possible. Generation of THz radiation based on the technology of frequency mixing requires laser radiation with a difference frequency in the order of 0.1-2 THz. Due to the low efficiency of frequency mixers, high optical power is required for pumping frequency mixers. Furthermore, the small efficiency requires short optical pulses for avoiding a high heat dissipation of the frequency mixers. We investigated an ultra stable 1W two colour THz pump source for the generation of a THz beat signal with rapid single mode tuning over several THz. The system consist of a fixed wavelength and a motorized tuneable laser pump sources which are optical amplified within a pulse operation module. One laser is stabilized to an atomic reference while the other is locked to an optical cavity which can be tuned continuously. This signal is pump source for a state of the art frequency mixer, which is typically realized as LT-GaAs crystal with an antenna design.

We have developed a first generation of electro-optic polymer modulators, designed specifically for passive millimeter-wave detection. The advantages of utilizing electro-optic polymers for modulator fabrication are their economical and simple fabrication, potential for large scale array fabrication, and well matched RF and optical indices, which provide the potential for an excellent high-frequency response. The current drawbacks of these devices include long term device stability due to oxidation and the relative immaturity of the RF designs for the modulator and interconnects, which lead to unacceptable internal losses and low sensitivity. These are both items we expect remedied in the upcoming year. We provide a brief overview on the opto-electronic method of detecting millimeter waves and our design and fabrication of the polymer modulator. Current measured results for the modulator response at 95GHz are presented and an analysis of the required performance for imaging is presented.

We report the design, fabrication, and characterization of high-speed LiNbO₃ modulator for the millimeter-wave (MMW) detection system at W band covering atmospheric window at 94 GHz. The LiNbO₃ modulator is used to convert the collected MMW power into optical frequency, and hence predominantly determines the system sensitivity. The high sensitivity of detection requires the modulator a broad-band response and a small driving voltage. The ridged traveling-wave structure has been used in the modulator design. The effects of velocity matching, impedance matching, and MMW attenuations in this structure on the device's MMW conversion efficiency are investigated. A numerical model has developed to optimize the device geometric parameters and the fabrication processes. The fabricated modulator achieved the 3-dB optical bandwidth of 67 GHz and the conversion efficiency of ~0.7 W^-1 at 94 GHz. The detection pixel based on it has shown a high sensitivity with a noise equivalent temperature difference of ~6 K at a refreshing rate of 30 Hz.

There are over 2 million reported burn injuries each year in the United States with 75,000 of these incidents resulting in hospitalization. Current medical imaging modalities have limited capabilities to assess initial burn damage and monitor healing progress. Some of these limitations can be attributed to modality occlusion from bandages, dried tissue and/or blood and inflammation. Since terahertz radiation can see through textiles and bandages¹, previous studies^2,3 suggested that terahertz radiation, in a reflectance configuration, could be used for non-invasive analysis of tissue thermal damage and healing status. In this study, we perform an analysis of the terahertz absorption and reflection properties of the tissue constituents comprising a wound area, and provide a feasibility assessment of the capabilities of terahertz imaging to provide a clinical tool for initial burn analysis and healing progress.

Terahertz (THz) spectroscopy of a biomolecule with spatial resolution below the diffraction limit of the radiation has been achieved by use of an all-optical, contactless transient mirror technique. A resolution of around 50 &mgr;m is determined by the use of a test sample of gold strip lines deposited on GaAs, and the differential THz time-domain spectroscopy (THz-TDS) response of biotin has been measured in both the presence and absence of the transient mirror at room temperature. These preliminary results demonstrate the potential for use of the technique for the chemical identification and characterisation of biomolecules in small volumes with the ultimate goal being microscopic imaging of live cells. The technique may find applications in quality control for semiconductor processing, and in identifying material imperfections, i.e. small cracks in non-destructive testing. We discuss the limitations of the transient mirror technique along with several advantages over other related techniques.