T-Ray Sensing and Imaging

T-ray wavelengths are long enough to pass through dry, nonpolar objects opaque at visible wavelengths, but short enough to be manipulated by optical components to form an image. Sensing in this band potentially provides advantages in a number of areas of interest to security and defense such as screening of personnel for hidden objects and the retection of chemical and biological agents. Several private companies are developing smaller, reliable cheaper systems allowing for commercialization and this motivates us to review a number of promising applications within this paper. While there are a number of challenges to be overcome there is little doubt that T-ray technology will play a significant role in the near future for advancement of security, public health, and defense.     

A Review on Thin-film Sensing with Terahertz Waves

In the past two decades, the development and steady improvement of terahertz technology has motivated a wide range of scientific studies designed to discover and develop terahertz applications. Terahertz sensing is one such application, and its continued maturation is virtually guaranteed by the unique properties that materials exhibit in the terahertz frequency range. Thin-film sensing is one branch of this effort that has enjoyed diverse development in the last decade. Deeply subwavelength sample thicknesses impose great difficulties to conventional terahertz spectroscopy, yet sensing those samples is essential for a large number of applications. In this article we review terahertz thin-film sensing, summarizing the motivation, challenges, and state-of-the-art approaches based predominately on terahertz time-domain spectroscopy.     

Simple material parameter estimation via terahertz time-domain spectroscopy

A simple and precise method based on fixed-point iteration is used to estimate dielectric parameters using terahertz time-domain spectroscopy (THz-TDS). The method converges and gives correct parameters when the sample thickness is greater than 200 /spl mu/m at a frequency of 0.1 THz or 20 /spl mu/m at a frequency of 1.0 THz. The technique in validated using measured terahertz data, obtained by probing a sample of high-resistivity silicon.     

Fundamentals of Measurement in Terahertz Time-Domain Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) has emerged as a main spectroscopic modality to fill the frequency range between a few hundred gigahertz to a few terahertz. This spectrum has been known as “terahertz gap” owing to limited accessibility by conventional electronic and optical techniques. Over the past two decades, THz-TDS has evolved substantially with enhanced compactness and stability. Since THz-TDS is becoming an industrial standard, the performance and precision of the system are of prime importance. This article provides an overview on terahertz metrology, including parameter estimation, signal processing, measurement characteristics, uncertainties, and calibrations. The overview serves as guidance for metrology and further developments of THz-TDS systems.     

Quarter-wavelength multilayer interference filter for terahertz waves

This work presents a multilayer interference filter, suitable for operation with terahertz waves (T-rays). An analysis of the effect of the number of layers on the spectral response is given, with full measurement data including time-resolved signals, transmittances, and phase spectra. The silicon-air structure with a submillimeter thickness shows a stop-band between 0.2 and 0.5 THz, and the attenuation inside the stop-band increases in proportion to the number of layers in the structure. The measurement of the fabricated structure is in agreement with a characteristic matrix analysis.     

Recent Progress in Terahertz Metasurfaces

In the past decade, the concept of metasurfaces has gradually dominated the field of metamaterials owing to their fascinating optical properties and simple planar geometries. At terahertz frequencies, the concept has been driven further by the availability of advanced micro-fabrication technologies that deliver sub-micron accuracy, well below the terahertz wavelengths. Furthermore, terahertz spectrometers with high dynamic range and amplitude and phase sensitivity provide valuable information for the study of metasurfaces in general. In this paper, we review recent progress in terahertz metasurfaces mainly in the last 5 years. The first part covers nonuniform metasurfaces that perform beamforming in reflection and transmission. In addition, we briefly overview four different methodologies that can be utilized in realizing high-quality-factor metasurfaces. We also describe two recent approaches to tuning the frequency response of terahertz metasurfaces using graphene as an active medium. Finally, we provide a brief summary and outlook for future developments in this rapidly progressing field.