This PDF file contains the front matter associated with SPIE Proceedings volume 7601, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

Coatings are applied to pharmaceutical tablets (or pills) to for either cosmetic or release control reasons. Cosmetic coatings control the colour or to mask the taste of an active ingredient; the thickness of these coating is not critical to the performance of the product. On the other hand the thickness and uniformity of a controlled release coating has been found affect the release of the active ingredient. In this work we have obtained from a pharmacy single brand of pantoprazole tablet and mapped them using terahertz pulsed imaging (TPI) prior to additional dissolution testing. Three terahertz parameters were derived for univariate analysis for each layer: coating thickness, terahertz electric field peak strength and terahertz interface index. These parameters were then correlated dissolution tested. The best fit was found to be with combined coating layer thickness of the inert layer and enteric coating. The commercial tablets showed a large variation in coating thickness.

Sparse arrays are highly attractive for implementing two-dimensional arrays, but come at the cost of degraded image quality. We demonstrate significantly improved performance by exploiting the coherent ultrawideband nature of singlecycle THz pulses. We compute two weighting factors to each time-delayed signal before final summation to form the reconstructed image. The first factor employs cross-correlation analysis to measure the degree of walk-off between timedelayed signals of neighboring elements. The second factor measures the spatial coherence of the time-delayed delayed signals. Synthetic aperture imaging experiments are performed with a THz time-domain system employing a mechanically scanned single transceiver element. Cross-sectional imaging of wire targets is performed with a onedimensional sparse array with an inter-element spacing of 1.36 mm (over four λ at 1 THz). The proposed image reconstruction technique improves image contrast by 15 dB, which is impressive considering the relatively few elements in the array. En-face imaging of a razor blade is also demonstrated with a 56 x 56 element two-dimensional array, showing reduced image artifacts with adaptive reconstruction. These encouraging results suggest that the proposed image reconstruction technique can be highly beneficial to the development of large area two-dimensional THz arrays.

Continuous wave terahertz imaging has the potential for diagnosing and delineating skin cancers. While contrast has been observed between cancerous and normal tissue at terahertz frequencies, the source mechanism behind this contrast is not clearly understood.¹Transmission measurements of 240μm thick sections of nonmelanoma skin cancer were taken at two frequencies of 1.39 THz and 1.63 THz that lie within and outside the tryptophan absorption band, respectively. Two CO₂ pumped Far-Infrared molecular gas lasers were used for illuminating the tissue while the transmitted signals were detected using a liquid Helium cooled Silicon bolometer. At both THz frequencies 2-dimensional THz transmission images of nonmelanoma skin cancers were acquired with better than 0.5mm spatial resolution. The resulting images were compared to the sample histology and showed a correlation between cancerous tissue and decreased transmission. The results of the imaging experiments will be presented and discussed.

A coherent transceiver using a THz quantum cascade laser as the transmitter and an optically pumped molecular laser as the local oscillator has been used, with a pair of Schottky diode mixers in the receiver and reference channels, to acquire high-resolution images of fully illuminated targets, including scale models. Phase stability of the received signal, sufficient to allow coherent image processing of the rotating target (in azimuth and elevation), was obtained by frequency-locking the TQCL to the free-running, highly stable optically pumped molecular laser. While the range to the target was limited by the available TQCL power (several hundred microwatts) and reasonably strong indoor atmospheric attenuation at 2.408 THz (2.0 dB/m at 40% RH), the coherence length of the QCL transmitter will allow coherent imaging over distances up to several hundred meters. In contrast to non-coherent heterodyne detection, coherent imaging allows signal integration over time intervals considerably longer than the reciprocal of the source, or signal bandwidth, with consequent improvement in the signal-to-noise ratio. Image data obtained with the system will be presented.

Previously, it has been shown that scattering of terahertz waves by surface roughness of a target can alter the terahertz absorption spectrum and thus obscure the detection of some chemicals in both transmission and reflection geometries. In this paper it is demonstrated that by employing Maximal Overlap Discrete Wavelet Transform (MODWT) coefficients, wavelet-based methods can be used to retrieve spectroscopic information from a broadband terahertz signal reflected from a rough surface target. It is concluded that while the commonly used direct frequency domain deconvolution method fails to accurately characterize and detect the resonance in the dielectric constant of rough surface lactose pellets, wavelet techniques were able to successfully identify such features.

In this paper, we present description of new methods developed for imaging with terahertz(THz) waves - methods of THz pulse holography. We present two methods - referenceless THz pulse holography and THz pulse holography with reference beam. Modeling and theoretic results already presented earlier ^1,2 are followed by setup for THz holography design and obtained experimental results on referenceless THz pulse holographic object imaging.

Recent improvements in sensing technology have driven new research areas within the terahertz (THz) portion of the electromagnetic (EM) spectrum. While there are several promising THz applications, several outstanding technical challenges need to be addressed before robust systems can be deployed. A particularly compelling application is the potential use of THz reflection spectroscopy for stand-off detection of drugs and explosives. A primary challenge for this application is to have sufficient signal-to-noise ratio (SNR) to allow spectroscopic identification of the target material, and surface roughness can have an impact on identification. However, scattering from a rough surface may be observed at all angles, suggesting diffuse returns can be used in robust imaging of non-cooperative targets. Furthermore, the scattering physics can also distort the reflection spectra, complicating classification algorithms. In this work, rough surface scattering effects were first isolated by measuring diffuse scattering for gold-coated sandpaper of varying roughness. Secondly, we measured scattering returns from a rough sample with a spectral signature, namely α-lactose monohydrate mixed with Teflon and pressed with sandpaper to introduce controlled roughness. For both the specular and diffuse reflection measurements, the application of traditional spectroscopy techniques provided the ability to resolve the 0.54 THz absorption peak. These results are compared with results from a smooth surface. Implications of the results on the ability to detect explosives with THz reflection spectroscopy are presented and discussed. In addition, the Small Perturbation Method (SPM) is employed to predict backscatter from lactose with a small amount of roughness.

The terahertz region (1-10 THz) has potential applications in many areas, such as chemical sensing, medical imaging and free-space optical communications. With the demonstration of terahertz sources, it is quite necessary to develop the detection technology in terahertz. Here we propose an electrically tunable quantum dot infrared photodetector to detect the terahertz region. The proposed detector applies a lateral electrical confinement on the quantum wells and forms a quantum disk in the quantum well area. The two-dimensional quantum confinement of quantum disk combining the vertical confinement from the quantum barrier forms a quantum dot structure. Using the energy states and intersublevel energy spacing in the quantum dot, the detector can be used to detect the terahertz region. Changing the lateral electrical confinement, the intersublevel energy spacing can also be tuned and in hence different wavelengths can be detected. Our modeling and simulation results show the tunability of peak detection wavelength of the photodetector from ~3.3 to ~6.0 THz with a gate voltage applied on the detector from -2 to -5 V. The peak absorption coefficients of the detection are shown in the range of 10³ cm^-1. Compared with current quantum dot photodetectors produced by self-assembled growth method the detector proposed here is easier to be tuned and the effective sizes have a much higher uniformity, because of using electrical confinement.

The next generation of space missions targeting far-infrared bands will require large-format arrays of extremely lownoise detectors. The development of Transition Edge Sensors (TES) array technology seems to be a viable solution for future mm-wave to Far-Infrared (FIR) space applications where low noise and high sensitivity is required. In this paper we concentrate on a key element for a high sensitivity TES detector array, that of the optical coupling between the incoming electromagnetic field and the phonon system of the suspended membrane. An intermediate solution between free space coupling and a single moded horn is where over-moded light pipes are used to concentrate energy onto multimoded absorbers. We present a comparison of modeling techniques to analyze the optical efficiency of such light pipes and their interaction with the front end optics and detector cavity.

Electric field enhancement has been actively studied recently and many metallic structures that are capable of locally enhancing electric field have been reported. The Babinet's principle can be utilized, especially in the form of Booker's extension, to transform the known electric field enhancing structures into magnetic field enhancing structures. The authors explain this transformation process and discuss the regime in which this principle breaks down. Unless the metals used can be well approximated with a PEC model, the principle's predictions fails to hold true. Authors confirm this aspect using numerical simulations based on realistic material parameters for actual metals. There is large discrepancy especially when the structural dimensions are comparable or less than the skin-depth at the wavelength of interest. An alternative way to achieve magnetic field enhancement is presented and the design of a connected bow-tie structure is proposed as an example. FDTD simulation results confirm the operation of the proposed structure.

Dendrimers are a polymeric nanomaterial whose molecular size varies from ~4 nm to ~13 nm based on its generation; within a generation these molecules are highly monodispersed. As the molecular size increases, its number of end groups increase too, resulting in an increased number of available sites for attaching foreign molecules. Generation 3 dendrimer was doped with a commercially available chromophore and formed thin film via spin coating. The cured dendrimer was corona poled. The resulting stand alone film exhibits an electro-optic coefficient ^r₃₃ ~130 pm/V leading to a high χ ⁽²⁾- dendrimer. This EO dendrimer was used for terahertz generation via difference-frequency technique (DFG). Two CW diode lasers were used to pump the EO dendrimer emitter to provide a combined pump power of 5.5W that generated a continuous wave DFG power of ~3.4 mW.

We developed a high-sweeping-speed optically synchronized dual-channel terahertz signal generator, in which the frequency difference between the two terahertz signals is independent of the frequency of the terahertz signals themselves. This feature is essential for heterodyne detection of terahertz signals with various frequencies. With this generator, a frequency-sweepable terahertz transmitter (Tx)/receiver (Rx) system with a wide dynamic range can be realized without sacrificing the high frequency-sweeping speed. Absorption line measurements for water vapor and nitrous oxide show that the developed Tx/Rx system can detect gas absorption with the optical depth of 0.04 or less. This result indicates the potential of the system as a remote gas sensor and gas analyzer.

The potential of Terahertz technology for various applications in security or material inspection has now been demonstrated widely. However, THz systems have to shrink in size and price to become acceptable for real-world applications. Our way to approach this target is to exploit technologies developed and matured originally for 1.55 μm fiber-optic telecom applications. Key devices are photoconductive antennas for 1.55 μm operation. They were developed using a novel InGaAs/InAlAs multi-layer structure. The antennas are packaged into fiber coupled modules. These handheld THz heads are driven by a 1.55 μm pulsed fiber laser. A tunable delay with both an oscillating mirror and a precise linear drive allow for real-time measurements as well as for high resolution spectroscopy. The system performance up to 4 THz is demonstrated.

We propose to integrate a high-gain open-ended rampart slot array antenna in our edge-coupled terahertz (THz) photonic transmitters. By locating the open circuits at the end of each quarter-wavelength slot antenna, the reflected terahertz wave can constructively interference with the input wave and hence produce higher gain. In addition, the folded feeding networks turn more propagating waves into the effective radiating segments rather than the dispersive surface waves. A THz resonant cavity is also designed under our monolithic THz-wave circuits. A 300-nm gold layer is coated below the substrate to serve as a reflecting mirror to reflect back the THz waves to the antenna. The resonant cavity design not only provides a robust base for post device process but also greatly enhances the gain of antenna. With the design of openended rampart slot array antenna and THz resonant cavity, we demonstrate that the gain of antenna can reach 8~9 dBi.

Solid state frequency multipliers extend the operating frequency range of millimeter wave Backward Wave Oscillators (BWOs) to 2.2 THz, enabling continuously tunable, narrow linewidth THz sources across the 0.1- 2.2 THz range. Power conversion efficiency of frequency multipliers can be improved substantially by optimizing impedance matching between the BWOs and multipliers. This impedance matching helps to extract more THz power out of BWOs coupled to frequency multipliers, than is emitted from BWOs radiating into free space.

To exploit the great potential of room-temperature ionic liquids (RTILs) as solvents that offer both low environmental impact and product selectivity, an understanding of the liquid structure, the microscopic dynamics, and the way in which the pertinent macroscopic properties, such as viscosity, thermal conductivity, ionic diffusion, and solvation dynamics depend on these properties, is essential. We have measured the intermolecular dynamics of the 1,3-dialkylimidazoliumbased RTILs [emim][BF₄], [emim][DCA], and [bmim][DCA], at 25 °C from below 1 GHz to 10 THz by ultrafast optical Kerr effect (OKE) spectroscopy and dielectric relaxation spectroscopy (DRS) augmented by time-domain terahertz and far-infrared FTIR spectroscopy. This concerted approach allows a more detailed analysis to be made of the relatively featureless terahertz region, where the higher frequency diffusional modes are strongly overlapped with librations and intermolecular vibrations. In the terahertz region, the signal-to-noise ratio of the OKE spectra is particularly high and the data show that there is a greater number of librational and intermolecular vibrational modes than previously detected. Of greatest interest though, is an intense low frequency (sub-alpha) relaxation that we show is in strong accordance with recent simulations that observe mesoscopic structure arising from aggregates or clusters; structure that explains the anomalous and inconveniently-high viscosities of these liquids.

The design and characterization of a floating gate GaAs/AlGaAs double quantum well long-infrared photoconductor is reported, with record operating temperature and modulation bandwidth for a device of this type. The principle of device operation relies on the photoionization of one quantum well, which functions as a floating gate, to modulate the conductance of the underlying quantum well, which functions as a transistor channel. Photoionization under normal incidence illumination is facilitated by a metal grating. Responsivity of 80-160 A/W for 12-20 μm long-infrared radiation has been observed at a 10 K device temperature, and photoresponse is clearly discernable up to 30 K. The modulation bandwidth was measured to exceed 1 kHz. An NEP = 2×10^-11 W/ square root Hz, and D* = 4×10⁸ cm square root Hz/W were measured, as limited by quantum well conductance fluctuations. Device performance is in fair competition with conventional QWIPs and we also propose a method to increase input saturation power in a photodetector of this structure.

In general, a reflective spectrometer is more suitable for the spectroscopic measurement for highly absorptive samples. We report the design and evaluation of a reflective terahertz time-domain spectroscopy (R-THz-TDS), using air as THz wave emitter and sensor, together with air-biased-coherent-detection (ABCD) method. With an 85 fs pulse amplified laser, we demonstrate a usable bandwidth from 0.5 THz to 12 THz, together with a peak dynamic range (DR) better than 2000:1 and a peak THz electrical field greater than 30 kV/cm. With a 32 fs pulse amplified laser, the usable bandwidth is further expanded to a continuous 35 THz. Several far-infrared optical properties such as phonon resonance and plasma resonance in various samples are reported. We also compared both transmission and reflection measurements. Furthermore, the time-resolved optical pump-THz probe experiment is performed. The evolution of carrier dynamics of GaAs and InSb samples are demonstrated in this study. Finally, the uniqueness and advantage of this R-THz-TDS spectrometer are comprehensively compared with traditional THz-TDS and Fourier transform infrared (FTIR) spectroscopy, including radiation source, detector, DR, bandwidth, resolution, peak power, and other features. In terms of signal-to-noise ratio (SNR), this study provides the SNR variation with frequency in both broadband R-THz-TDS and FTIR spectroscopy.

A GaAs waveguide emitter for terahertz radiation generation based on phase-matched optical rectification process pumped by 1550-nm fiber lasers is proposed as a potential terahertz source. The device consists of two waveguide structures which are adjacent to each other along the longitudinal direction of the waveguide. One of these is a segment of asymmetric dielectric planar waveguide in which a core-layer of GaAs is contacted with a substrate containing an epilayer of Al_xGa_1-xAs. The other is a segment of corrugated waveguide with GaAs second-order rectangular grating teeth layer above the same substrate containing an epilayer of Al_xGa_1-xAs. The terahertz waves could be generated in the non-grating section by a pulsed fiber laser based on an optical rectification process under the circumstance of phase matching by exploring the waveguide mode dispersion, which then could be coupled into the air and the substrate transversely by the following grating section to form the terahertz radiation. The coupling efficiency is estimated to be about 60% for an optimal designed terahertz wave emitter.