This PDF file contains the front matter associated with SPIE Proceedings Volume 6893, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing

Dendrimer is a workhorse nanomaterial for a number of important photonic devices. The electro-optic (EO) properties of a chromophore doped and poled dendrimer film exhibits higher electro-optic coefficient ^r33 . Measured refractive index shows significant difference between poled and unpoled dendrimer film. The ^r33 value was determined to be ~130 pm/V at 633 nm that dropped to ~90 pm/V at 1553 nm. Dendrimer waveguide can be used to design several important photonic devices. EO dendrimer is used to generate terahertz radiation via electro-optic route. Here electro-optic rectification and difference frequency generation has been demonstrated. Significantly higher terahertz power can be generated by the higher χ⁽²⁾ dendrimer emitter of the present investigation.

We present here a bird eye view on the basic physical factors limiting the efficiency of nanostructure-based THz lasing. The origin of these limitations is the contradiction between the requirements of THz gaps, high radiative matrix element value, and selective depopulation. Various ways were suggested to go out of these limitations. They include: sophisticated nanostructure layout, the usage of QCLs, and switch to bipolar THz lasing. We present the results of detailed density matrix based calculations comparing these approaches. In theory InAs-GaSb bipolar THz lasers are the most promising.

Terahertz (THz) technology has been demonstrated as a promising tool for detection of explosives and is being developed for aviation screening and sensing of improvised explosive devices. THz radiation is attractive for many applications due to its ability to penetrate through a wide range of dielectric materials including clothing, paper, cardboard, plastics, and wood. Of course, metals block THz waves as is the case for microwave, IR, and visible light. Our work has involved investigating the reflection spectroscopy of a variety of materials including explosives such as RDX and PETN, plastic explosive taggants such as DMDNB, and other organic materials. We have also investigated the changes of the reflection spectra in varying grades of sucrose. Spectral differences are observed between three grades of crystalline sugar in the region from 0.1 to 1 THz. By exploiting the unique spectral features, the discrimination capabilities of THz reflection spectroscopy points to the broad applicability of identifying a wide variety of materials.

We have efficiently generated tunable terahertz (THz) radiation using intracavity parametric down-conversion in gallium arsenide (GaAs). We used three types of micro-structured GaAs to quasi-phase-match the interaction: optically contacted, orientation-patterned, and diffusion-bonded GaAs. The room-temperature GaAs was placed in an optical parametric oscillator (OPO) cavity, and the THz wave was generated by difference-frequency mixing between the OPO signal and idler waves. 250-GHz-bandwidth radiation was generated with frequencies spanning 0.4-3.5 THz. We measured two orders of optical cascading generated by the mixing of optical and THz waves. In a doubly resonant oscillator (DRO) configuration, the efficiency increased by 21 times over the singly resonant oscillator (SRO) performance with an optical-to-THz efficiency of 10^-4 and average THz power of 1 mW.

We review our progress made on the efficient conversion of the broadband terahertz (THz) pulses from ultrafast laser pulses propagating in semiconductor electro-optic materials. By investigating the behaviors of the THz output vs. the pump beam in terms of incident angle, polarization, and azithumal angle, we can precisely determine the contributions made by optical rectification and photocurrent surge. When a material is pumped below its bandgap, optical rectification is the primary mechanism for the THz generation. Above the bandgap, however, the two mechanisms mentioned above compete with each other, depending on the material characteristics and pump intensity. At a sufficiently-high pump intensity, optical rectification becomes the dominant mechanism for a second-order nonlinear material. When a material is pumped above its bandgap, second-order nonlinear coefficients are resonantly enhanced. In such a case, the THz output power and normalized conversion efficiency can be dramatically enhanced. We have also analyzed how the THz generation is affected by some competing processes including two-photon absorption.

Imaging and spectroscopy at terahertz frequencies have great potential for healthcare, plasma diagnostics, and homeland security applications. Terahertz frequencies correspond to energy level transitions of important molecules in biology and astrophysics. Terahertz radiation (T-rays) can penetrate clothing and, to some extent, can also penetrate biological materials. Because of their shorter wavelengths, they offer higher spatial resolution than do microwaves or millimeter waves. We are developing hot electron bolometer (HEB) mixer receivers for heterodyne detection at terahertz frequencies. HEB detectors provide unprecedented sensitivity and spectral resolution at terahertz frequencies. We describe the development of a two-pixel focal plane array (FPA) based on HEB technology. Furthermore, we have demonstrated a fully automated, two-dimensional scanning, passive imaging system based on our HEB technology operating at 0.85 THz. Our high spectral resolution terahertz imager has a total system noise equivalent temperature difference (NEΔT) value of better than 0.5 K and a spatial resolution of a few millimeters. HEB technology is becoming the basis for advanced terahertz imaging and spectroscopic technologies for the study of biological and chemical agents over the entire terahertz spectrum.

We fabricated and studied a planar composite material consisting of sub-wavelength double split ring resonator structures made of Gold on a Silicon substrate. Our measurements reveal a strong transmission dip at 0.6 THz. Experimental and numerical results indicate that there is an Inductor-Capacitor resonance at 0.6 THz, characterized by enhanced electric field strength across the ring gap. Our results also indicate a shift in the resonance to higher frequencies as thickness is increased. Spectral properties of the composite material were measured using THz Time Domain Spectroscopy in the range from 0.1 THz to 3.5 THz. Simulations were carried out using the commercially available electromagnetic solver, Microwave Studio^TM. Fabrication of the structures was done with Proton Beam Writing, a nanolithography technique based on focused MeV protons. The direct-write technique allowed us to fabricate structures much thicker than otherwise possible. For this work, the ring resonator structures had overall dimensions of 38 μm and a thickness of 8 μm with highly vertical and smooth sidewalls with minimum critical dimensions of 2 μm.

Research in the terahertz regime of the electromagnetic spectrum has become increasingly prominent in photonics for a number of reasons, including the abilities to temporally resolve ultrashort pulses (~ ps), extract polarization information, and easily create structures with size on the order of the wavelength. In a conventional time-domain terahertz spectroscopy system, when a pulse of radiation is incident on a sample, the system detects the radiation transmitted along the incident beam axis. While this approach has proven to be effective for numerous applications, it is often desirable to detect radiation scattered from a sample at off-axis angles. The realization of a pump-probe terahertz spectroscopy system capable of time- and polarization-resolved detection at arbitrary angles is nontrivial, however, since the sampling probe pulse must retain its spatial alignment, polarization, and timing at all detection angles. In this work, we unveil a system capable of angular- and time-resolved detection of terahertz radiation, with detection angles spanning 360°. Moreover, the polarization of the radiation can be determined at any detection angle. The performance of the system is demonstrated experimentally by detecting the radiation scattered from a spherical sample, which exhibits a radiation pattern that is symmetric about the incident beam axis. To the best of our knowledge, an angularly resolved terahertz system with such versatility has not been previously realized, and its development introduces a new platform for explorations into angular dependent phenomena.

The existence of unique absorption spectrum patterns for many chemicals at terahertz frequencies has opened an exciting avenue in non-contact safe detection of such materials by terahertz spectroscopy. However, scattering of THz waves, which have wavelengths on the order of material grain sizes, by surface roughness challenges the sensitivity of this detection scheme in practice. In this work, we present terahertz time domain spectroscopy results for materials with rough surfaces. Both reflection from and transmission through lactose, which has sharp absorption peaks in the terahertz regime, are studied and the effect of increasing scattering through controlled surface roughness is investigated. Such electromagnetic scattering can alter the terahertz absorption spectrum and thus obscure the detection of chemicals. Furthermore we examine electro-optic detection of terahertz signals reflected from randomly rough targets with a theoretical electromagnetic system perspective and provide a method to retrieve coherent reflection responses from rough surface targets.

The Atacama Large Millimetre Array will be a single research instrument composed of up to 50 high precision antennas, located at the Chajnantor plain in the district of San Pedro de Atacama, 5000m above sea level. Each ALMA telescope will contain 10 frequency channels/bands, ranging from 30 to 950GHz. Radiation from the secondary reflector is collected to the receivers of each wavelength channel through their accompanying front end optics. We present a full electromagnetic treatment of the front end optics for band 5 (163 - 211 GHz) and band 9 (602 - 720 GHz). A full quasi optical and physical optics analysis of the band 5 front end optics, using the antenna analysis tool, GRASP9 [1] is presented. Potential optimisation for the system is presented, namely a reflector edge taper and a comparison of two surface geometries. A similar analysis of the band 9 system is presented. Full electromagnetic simulations are compared with cold beam pattern measurements made at the Space Research Organisation of the Netherlands [2, 3]. Analysis of the effect of the polarizing grid is presented, with suggested modifications to improve cross polar levels.

A stochastic process of structural finite element simulation of a two mirror telescope has been combined with vector electromagnetic analysis of the thermally stressed structure to determine the influence of material properties on performance at terahertz frequencies. The output gives a reliable understanding of the correlation between properties of the materials used in the building of the telescope, on a component by component basis, and the aberrations of the optics under operating conditions. In this study the only stresses considered were thermo-elastic and the optics were simple, but the method is generally applicable to any optical system and can take account of both stress induced deformation and assembly tolerances.

The main idea of suggested THz holographic system is to measure amplitude-phase diffraction patterns of an object at THz range and further digital processing of obtained data. For check of the method verifying numerical calculations have been carried out and the holograms of amplitude and phase arbitrary shaped transparencies were obtained and reconstructed numerically. Possible advances in computational object reconstruction as well as reconstruction resolution are discussed and implemented.

Gaussian Beam Mode Analysis can be applied as a powerful technique approach in the development of phase gratings for use at terahertz wavelengths, providing a physically intuitive approach relating Fourier and Fresnel diffraction patterns to the scattering of the illumination beam at the grating. Fourier gratings in particular offer the possibility of generating sparse arrays image of a single input beam, useful, for example, in active heterodyne systems with an LO power source. The feasibility of the application of such gratings in real systems was investigated both by simulation and experimental measurements.

An analysis of the single point reproducibility of TD-THz based paint thickness measurements demonstrated a precision of 130 nm, corresponding to 0.1% of the measured thickness. A detailed model of the anticipated TD-THz waveforms from samples of varying thickness indicates that an intrinsic uncertainty of 0.09% is anticipated in the absence of environmental fluctuations. Therefore, the influence of oscillations in the THz field associated with the initial reflection does not adversely impact the ability to extract accurate paint thickness information, and the noise associated with these oscillations could limit the measurement uncertainty of a calibrated instrument under optimum laboratory conditions. In the case of a deployed sensor, we anticipate that the accuracy will be degraded by environmental fluctuations.

The ALMA project (Atacama Large Millimeter Array) is a collection of 50 radio telescopes that will be installed and operated in the high altitude desert of Atacama in northern Chile. The array is phased locked using a microwave reference generated from highly stable phase locked laser signals transmitted over 15km single mode fiber. In this paper we demonstrate the ability of generating and distributing high frequency photonic based microwave reference in a single mode fiber of 14 km to 2 of the ALMA radio telescopes. 81GHz photonic LO reference is generated and transmitted over the single mode fiber to the receivers of the 2 antennas. The paper will also present the method used to measure and dynamically stabilize the phase of the microwave photonics reference transmitted to 2 antennas while simulating antenna motion. The photonic phase correction system will null the unwanted phase in the microwave reference resulted from the motion of the telescope, maintaining coherence between the 2 antennas.

A Terahertz imaging system intended to demonstrate identification of objects concealed under clothing was designed, assembled, and tested. The system design was based on a 2.5 m standoff distance, with a capability of visualizing a 0.5 m by 0.5 m scene at an image rate of 2 frames per second. The system optical design consisted of a 1.56 THz laser beam, which was raster swept by a dual torsion mirror scanner. The beam was focused onto the scan subject by a stationary 50 cm-diameter focusing mirror. A heterodyne detection technique was used to down convert the backscattered signal. The system demonstrated a 1.5 cm spot resolution. Human subjects were scanned at a frame rate of 2 frames per second. Hidden metal objects were detected under a jacket worn by the human subject. A movie including data and video images was produced in 1.5 minutes scanning a human through 180° of azimuth angle at 0.7° increment.

Real-time imaging in the terahertz (THz) spectral range was achieved using a 3.6-THz quantum cascade laser (QCL) and an uncooled, 160×120 pixel microbolometer camera fitted with a picarin lens. Noise equivalent temperature difference of the camera in the 1-5 THz frequency range was calculated to be at least 3 K, confirming the need for external THz illumination when imaging in this frequency regime. After evaluating the effects of various operating parameters on laser performance, the QCL found to perform optimally at 1.9 A in pulsed mode with a 300 kHz repetition rate and 10-20% duty cycle; average output power was approximately 1 mW. Under this scheme, a series of metallic objects were imaged while wrapped in various obscurants. Single-frame and extended video recordings demonstrate strong contrast between metallic materials and those of plastic, cloth, and paper - supporting the viability of this imaging technology in security screening applications. Thermal effects arising from Joule heating of the laser were found to be the dominant issue affecting output power and image quality; these effects were mitigated by limiting laser pulse widths to 670 ns and operating the system under closed-cycle refrigeration at a temperature of 10 K.