Andrew J. Gatesman

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.

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.

Frequency stabilization of a single mode terahertz quantum cascade laser to the kilohertz level.

A simple analog locking circuit was shown to stabilize the beat signal between a 2.408 THz quantum cascade laser and a CH(2)DOH THz CO(2) optically pumped molecular laser to 3-4 kHz (FWHM). This is approximately a tenth of the observed long-term (t approximately sec) linewidth of the optically pumped laser showing that the feedback loop corrects for much of the mechanical and acoustic-induced frequency jitter of the gas laser. The achieved stability should be sufficient to enable the use of THz quantum cascade lasers as transmitters in short-range coherent transceivers.

Wet-etch optimization of free-standing terahertz frequency-selective structures

Free-standing frequency-selective surfaces consisting of approximately 10-microm-thick copper films with cross-aperture arrays are found to be tunable toward lower frequencies by means of wet chemical etching. Center frequencies were tuned from 1.57 to 1.53 THz while maintaining high transmittance. Wet etching also adjusts bandwidth, peak transmittance, and sidelobe transmittance. The advantage of the wet-etch technique is demonstrated by employment of these devices as bandpass filters for difluoromethane-based terahertz lasers. Adjustment in aperture dimensions because of etching results in suppression of a competing laser line (133.93 microm) by 15 dB while maintaining high transmittance at the operating wavelength of 192.06 microm.

Terahertz sideband-tuned quantum cascade laser radiation

A compact, tunable, narrowband terahertz source was demonstrated by mixing a single longitudinal mode 2.408 THz, free running quantum cascade laser with a 2–20 GHz microwave sweeper in a conventional corner-cube-mounted Schottky diode. The sideband spectra were characterized with a Fourier transform spectrometer, and the radiation was tuned through several D2O rotational transitions to estimate the longer term (t≥several sec) bandwidth of the source. A spectral resolution of 2 MHz in CW regime was observed.

Terahertz laser based standoff imaging system

Definition and design of a terahertz standoff imaging system has been theoretically investigated. Utilizing terahertz quantum cascade lasers for transmitter and local oscillator, a detailed analysis of the expected performance of an active standoff imaging system based on coherent heterodyne detection has been carried out. Five atmospheric windows between 0.3 THz and 4.0 THz have been identified and quantified by carrying out laboratory measurements of atmospheric transmission as a function of relative humidity. Using the approximate center frequency of each of these windows, detailed calculations of expected system performance vs target distance, pixel resolution, and relative humidity were carried out. It is shown that with 1.5 THz laser radiation, a 10m standoff distance, 1 m times 1 m target area, and a 1cm times 1cm pixel resolution, a viable imaging system should be achievable. Performance calculations for various target distances, target pixel resolution, and laser frequency are presented

Characterization of roughness parameters of metallic surfaces using terahertz reflection spectra

This paper reports on the effect of random Gaussian roughness with rms roughness values of 5-20 microm on the terahertz reflection spectra of metallic aluminum surfaces using Fourier transform IR spectroscopy. By comparing experimental data with a theoretical model based on the Kirchhoff approximation, the rms roughness of a surface is accurately determined. The rms roughness determined by this method is in good agreement with the rms roughness measured using a stylus surface profilometer. In addition, we demonstrate that this method can be used to clearly resolve rough surfaces that differ in rms roughness by approximately 1 microm.