Jerry Waldman

An anti-reflection coating for silicon optics at terahertz frequencies

A method for reducing the reflections from silicon optics at terahertz frequencies has been investigated. In this study, we used thin films of parylene as an anti-reflection (AR) layer for silicon optics and show low-loss behavior well above 1 THz. Transmittance spectra are acquired on double-sided-parylene-coated, high-resistivity, single-crystal silicon etalons between 0.45 THz and 2.8 THz. Modeling the optical behavior of the three-layer system allowed for the determination of the refractive index and absorption coefficient of parylene at these frequencies. Our data indicate a refractive index, n, of 1.62 for parylene C and parylene D, and a reasonably modest absorption coefficient make these materials a suitable AR coating for silicon at terahertz frequencies. Coatings sufficiently thick for AR performance reduced the average transmittance of the three-layer system by <10% compared to a lossless AR coating with an ideal refractive index.

Modulated submillimeter laser interferometer system for plasma density measurements

A high resolution submillimeter interferometer system for measurement of electron densities in the 10(14)-cm(-3) </= n(e) </= 2 x 10(15)-cm(-3) range has been developed for use in high density tokamaks. Phase modulation at ~1 MH(z) is accomplished by difference frequency mixing of two cavity tuned laser oscillators. The optically pumped CH(3)OH lasers, which operate on the 118.8-microtm line, feature a novel output coupling design that permits good mode quality and low beam divergence. The beat signals are detected using a newly developed Ge:Li photoconductor, and a direct measurement of the phase shift is obtained from the time lag between probe and reference signals. The sensitivity of the resulting phase measurement is independent of the instantaneous phase and unaffected by fluctuations in the amplitude or in the frequency of the modulation.

Submillimeter lasers optically pumped off resonance

Abstract Submillimeter laser action has been achieved by optically pumping molecules far from their vibrational absorptions. In the case of the P(32) pumped sQ(5,3) transition of NH 3 , the separation between the high power CO 2 TEA laser frequency and the absorption line center is 950 MHz. The ability to optically pump off resonance not only greatly increases the number of available lines, it also implies that significant tuning of the submillimeter radiation by applied electric fields is feasible.

Terahertz Imaging of Subjects With Concealed Weapons

In response to the growing interest in developing terahertz imaging systems for concealed weapons detection, the Submillimeter-Wave Technology Laboratory (STL) at the University of Massachusetts Lowell has produced full-body terahertz imagery using coherent active radar measurement techniques. The proof-of-principle results were readily obtained utilizing the compact radar range resources at STL. Two contrasting techniques were used to collect the imagery. Both methods made use of in-house transceivers, consisting of two ultra-stable far-infrared lasers, terahertz heterodyne detection systems, and terahertz anechoic chambers. The first technique involved full beam subject illumination with precision azimuth and elevation control to produce high resolution images via two axis Fourier transforms. Imagery collected in this manner is presented at 1.56THz and 350GHz. The second method utilized a focused spot, moved across the target subject in a high speed two dimensional raster pattern created by a large two-axis positioning mirror. The existing 1.56THz compact radar range was modified to project a focused illumination spot on the target subject several meters away, and receive the back-reflected intensity. The process was repeated across two dimensions, and the resultant image was assembled and displayed utilizing minimal on-the-fly processing. Imagery at 1.56THz of human subjects with concealed weapons are presented and discussed for this scan type.

Terahertz behavior of optical components and common materials

As short range, ground based, surveillance systems operating at terahertz frequencies continue to evolve, increasing attention is being directed towards the behavior of dielectric materials at terahertz frequencies as well as the behavior of optical components used to control terahertz radiation. This work provides an overview of several terahertz optical components such as frequency selective filters, laser output couplers, artificial dielectrics, and electromagnetic absorbers. In addition, a database was established that contains terahertz properties of common materials that have been largely unexplored in this region of the spectrum. The database consists of transmittance and reflectance spectra of a variety of materials measured using Fourier transform infrared spectroscopy techniques from 175 GHz - 2 THz. In addition, ultra-stable, CO2 optically pumped, far-infrared gas lasers were used to collect fixed-frequency transmittance data at 326 GHz, 584 GHz, and 1.04 THz. A Gunn oscillator was used for measurements at 94 GHz.

Terahertz inverse synthetic aperture radar (ISAR) imaging with a quantum cascade laser transmitter

A coherent transceiver using a THz quantum cascade (TQCL) 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 and concealed objects. 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, the coherence length of the TQCL transmitter will allow coherent imaging over distances up to several hundred meters. Image data obtained with the system is presented.

Transformation of the multimode terahertz quantum cascade laser beam into a Gaussian, using a hollow dielectric waveguide

We demonstrate that a short hollow dielectric tube can act as a dielectric waveguide and transform the multimode, highly diverging terahertz quantum cascade laser beam into the lowest order dielectric waveguide hybrid mode, EH(11), which then couples efficiently to the free-space Gaussian mode, TEM(00). This simple approach should enable terahertz quantum cascade lasers to be employed in applications where a spatially coherent beam is required.

Power and spatial mode measurements of sideband generated, spatially filtered, submillimeter radiation

The first coherent measurement of submillimeter-wave sideband generator (SBG) output power is reported here. This SBG utilizes a submillimeter laser, microwave synthesizer, and high frequency Schottky diode to produce tunable radiation. Record efficiency and output power (10.5 /spl mu/W) at a drive frequency of 1.6 THz has been obtained, and SBG radiation was efficiently separated from the laser driver with Si etalons. The power measurements were made using a dual CO/sub 2/-submillimeter laser system and two Schottky diodes, one as the sideband generator and one as the receiver. The SBG efficiency of four different models of University of Virginia (UVa) diodes were studied and the first measurement of the output mode of the sideband (without the unshifted laser present) was also performed. Finally, confirmation of the optimal parameters for coupling a Gaussian beam into a corner-reflector mounted Schottky diode is presented. >

High-precision reflectometer for submillimeter wavelengths

A high-precision reflectometer has been designed and implemented to measure directly the specular reflectance (R) of materials in the submillimeter (SM) region of the spectrum (300 GHz < ν < 3000 GHz). Previous laser-based measurement systems were limited to an uncertainty in R of ± 1.0% because of a number of issues such as lack of an absolute reflection standard, difficulties in the interchange of sample and standard in the laser beam, and instabilities in the laser system. We realized a SM reflection standard by ellipsometrically characterizing the complex index of refraction of high-purity, single-crystal silicon to a precision such that its SM reflectivity could be calculated to better than ±0.03%. To deal with alignment issues, a precision sample holder was designed and built to accommodate both sample and silicon reflection standard on an air-bearing rotary stage. The entire measurement system operated under computer control and included ratioing of the reflected signal to a reference laser signal, measured simultaneously, to help to eliminate short-term laser instabilities. Many such measurements taken rapidly in succession helped to eliminate the effects of both source and detector drift. A liquid-helium-cooled bolometer was modified with a large area detecting element to help to compensate for the slight residual misalignment between sample and reflection standard as they were positioned into and out of the laser beam. These modifications enabled the final measurement precision for R to be reduced to less than 0.1%. The major contribution to this uncertainty was the difficulty in precisely exchanging the positions of sample and standard into and out of the laser beam and was not due to laser or detector noise or instabilities. In other words, further averaging would not help to reduce this uncertainty. This order-of-magnitude improvement makes possible, for the first time to our knowledge, high-precision reflectance measurements of common metals such as copper, gold, aluminum, and chromium whose predicted reflectivities exceed 99% in the SM region. Furthermore, precise measurement of the high-frequency losses in high-temperature superconducting materials is now also possible. Measurements reported here of metals at a laser wavelength of λ = 513.01 μm (ν ≈ 584 GHz) indicate a slight discrepancy between experimental and theoretically predicted values, with measured results falling 0.1–0.3% below predicted values.

A terahertz focal plane array using HEB superconducting mixers and MMIC IF amplifiers

We present a focal plane array (FPA) designed for operation at terahertz frequencies. The FPA is based on NbN phonon-cooled hot electron bolometer mixers directly coupled to wide-band microwave monolithic integrated circuit IF amplifiers. The array incorporates all the required dc-bias and IF circuitry in a compact split-block design. We present new experimental results describing the optical coupling efficiency to the array, as well as receiver noise temperature measurements. The measurements were performed at 1.6 THz, showing good agreement with theoretical predictions. This is the first low-noise heterodyne focal plane array to be reported for any frequency above 1 THz.