John F. Federici

THz imaging and sensing for security applications—explosives, weapons and drugs

Over the past 5 years, there has been a significant interest in employing terahertz (THz) technology, spectroscopy and imaging for security applications. There are three prime motivations for this interest: (a) THz radiation can detect concealed weapons since many non-metallic, non-polar materials are transparent to THz radiation; (b) target compounds such as explosives and illicit drugs have characteristic THz spectra that can be used to identify these compounds and (c) THz radiation poses no health risk for scanning of people. In this paper, stand-off interferometric imaging and sensing for the detection of explosives, weapons and drugs is emphasized. Future prospects of THz technology are discussed.

Review of terahertz and subterahertz wireless communications

According to Edholm’s law, the demand for point-to-point bandwidth in wireless short-range communications has doubled every 18 months over the last 25 years. It can be predicted that data rates of around 5–10 Gb/s will be required in ten years. In order to achieve 10 Gb/s data rates, the carrier frequencies need to be increased beyond 100 GHz. Over the past ten years, several groups have considered the prospects of using sub-terahertz (THz) and THz waves (100–2000 GHz) as a means to transmit data wirelessly. Some of the reported advantages of THz communications links are inherently higher bandwidth compared to millimeter wave links, less susceptibility to scintillation effects than infrared wireless links, and the ability to use THz links for secure communications. Our goal of this paper is to provide a comprehensive review of wireless sub-THz and THz communications.

Coherent terahertz radiation detection: Direct comparison between free-space electro-optic sampling and antenna detection

We compare the use of free-space electro-optic sampling (FSEOS) with photoconducting antennas to detect terahertz (THz) radiation in the range of 0.1–3 THz. For the same average THz power and low-frequency modulation, signal-to-noise ratio and sensitivity are better with antenna detection at frequencies smaller than 3 THz. When the modulation frequency is increased to more than 1 MHz in FSEOS, both detection schemes have comparable performance. Using a singular-electric-field THz emitter, we demonstrate the feasibility of a THz imaging system using real-time delay scanning in FSEOS and only 20 mW of laser power.

Design and performance of singular electric field terahertz photoconducting antennas

We present new designs of more efficient terahertz (THz) radiation emitters and detectors enhanced by electric field singularities using sharp and laterally offset electrodes. We compare the performances of the terahertz emission and different polarization properties resulting from these structures. An average THz radiation power of 3 μW is achieved under 20 mW excitation, calibrated by free space electro-optic sampling. We also study the gap size dependence of the THz radiation, and find an absence of a positive electrode effect in the small gap limit.

Terahertz study of 1,3,5-trinitro-s-triazine by time-domain and Fourier transform infrared spectroscopy

This letter describes the use of THz time-domain spectroscopy (TDS) applied in transmission to the secondary explosive 1,3,5 trinitro-s-triazine. Samples were also subjected to Fourier transform infrared spectroscopy over the same range for comparison. A detailed spectroscopy study is presented. General agreement between results from both methods confirms the absorption features found. A comparison study with computer molecular simulations shows that THz-TDS is sensitive to collective modes or vibrational modes of material.

Terahertz pulse propagation through small apertures

Transmission of single-cycle terahertz pulses through subwavelength apertures is experimentally studied in the near-field zone. Measurements of throughput for aperture sizes d as small as λ/300 show that the theoretical d3 law requires a correction term which takes into account the physical thickness of the aperture screen. Frequency dependent transmission of the broad band pulses explains changes in their spectral and temporal characteristics. Practical application of small apertures in near-field microscope probes allows achievement of spatial resolution of 7 μm for pulses with a broad spectral content of λ=120–1500 μm.

Interferometric characterization of 160 fs far‐infrared light pulses

We report the first interferometric characterization of freely propagating, subpicosecond, far‐infrared (FIR) light pulses. FIR light was generated via short pulse photoexcitation of a semi‐insulating InP wafer. The half width of the intensity interferogram was 230 fs. The FIR light contained frequency components from 3 to 150 cm−1.

Terahertz near-field microscopy based on a collection mode detector

We report on the development of a collection mode near-field probe for the terahertz spectral range. The near-field detector is based on an aperture type probe with dimensions of 30×30 μm2. The collection mode technique provides higher sensitivity and higher resolution than the similar illumination mode approach. Spatial resolution better than 40 μm is demonstrated for a broad spectrum of 300–600 μm, which equals to λ/15 for the longest wavelength. The observed resolution is determined by the size of the probe aperture.

Terahertz imaging using an interferometric array

Most methods of imaging in the terahertz (THz) spectral region utilize either pulsed-laser sources or require the THz generation and detection sources to be phase coherent. The application of interferometric imaging to the THz range is described. Interferometric imaging offers considerable advantages in this regard due to its ability to image with only a handful of detector elements, image many sources of THz radiation at once, image incoherent as well as coherent sources, and provide spectral information as well as spatial imaging information. The THz interferometric imaging method is potentially useful for remote detection of explosives.

Intervalley scattering in GaAs and InP probed by pulsed far‐infrared transmission spectroscopy

The dynamics of photoexcited electrons in GaAs and InP were studied using the transmission of 200‐fs pulses of far‐infrared radiation in the spectral range 15–100 cm−1. Kinetic traces of the infrared transmission as a function of delay between optical excitation and infrared probe show a probe‐limited decrease in transmission followed by a more gradual (0.7–2 ps) drop to a steady value, consistent with the slow return of electrons from high‐mass satellite valleys. Infrared transmission spectra, analyzed in the context of a Drude model, reveal density‐dependent electron mobilities 3–4 times below equilibrium n‐doped values. Electron‐hole collisions likely account for the lower mobility.