J. Balakrishnan

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.

Sensing the hygroscopicity of polymer and copolymer materials using terahertz time-domain spectroscopy

We present the hygroscopicity of polymer and copolymer materials in the low terahertz (THz) frequency range using a linear absorption model. We identify COC 6013 and COC 5013 as optimal THz window materials, possessing both low hygroscopicity and high transmission in the THz regime. The correct choice of window material is of significance for transmission THz spectroscopy and of particular interest for THz liquid spectroscopy.

Fixed Dual-Thickness Terahertz Liquid Spectroscopy Using a Spinning Sample Technique

The conventional double-modulated terahertz differential time-domain spectroscopy (double-modulated THz-DTDS) of liquids requires linear dithering of the sample to rapidly vary the sample thickness in order to produce the required sample and reference signals. Linear dithering, however, imposes a fundamental limitation as it introduces mechanical noise into the system, thereby contributing to measurement uncertainty. In this paper, we address this limitation for the terahertz spectroscopy of liquids by using a fixed dual-thickness sample mounted on a spinning wheel. The concept of spinning the sample allows rapid switching between two fixed sample thicknesses in order to produce sample and reference signals without the introduction of added mechanical noise. We validate this new technique by measuring the dielectric properties of a number of liquids and confirm the results against the Debye relaxation model.

Investigation on improving the noise performance of T-ray liquid spectroscopy via double-modulated differential time-domain spectroscopy

Liquid spectroscopy allows analysis of chemical composition and provides a better understanding of the solvation dynamics of various types of liquids. Although it has been shown that liquid spectroscopy using T-rays is feasible, liquid water absorption is still considered to be one of the most challenging problems facing THz imaging and spectroscopy in biomedical applications. The absorption coefficient for liquid water shows a very high THz absorption, 200 cm-1 at 1 THz. This paper describes a promising novel liquid double-modulated differential time-domain spectroscopy (Double-modulated DTDS) technique to extract the optical parameters with a dual-thickness measurement. The described technique improves on the previous work, by replacing the required sample dithering technique with a rotating spinning wheel resulting in an improved noise performance up to two orders of magnitude.

Novel T-ray Liquid Spectroscopy via Double Modulated Differential Time-Domain Spectroscopy

Liquid water absorption is one of the most enduring problems facing THz imaging and spectroscopy in biomedical applications. The absorption coefficient for liquid water shows a very high THz absorption, 200 cm-1 at 1 THz (Mickan et al., 2004). This strong absorption is a big hurdle for T-ray spectroscopy and imaging of liquid or even wet samples (Zhang, 2002). This paper proposes a novel method for parameter extraction of a liquid, using a double modulated differential time-domain spectroscopy (DTDS) technique with dual-thickness measurement. The proposed technique improves on the previous work by replacing the required sample dithering technique (Mickan et al., 2004) with a rotating spinning wheel resulting in an improved noise performance up to two orders of magnitude.

Terahertz detection of substances for security related purposes

Terahertz (THz) radiation has many far reaching applications - of specific interest is that many non-metallic and non-polar substances are transparent in the THz frequency range. This provides many practical uses for security purposes, where it is possible to detect and determine various substances that may be hidden or undetectable via conventional methods such as X-rays. In addition to this property, terahertz radiation can either be used in reflection or transmission modes. This paper will look into the use of transmission techniques to detect various substances using a terahertz system. Common materials used in bags and suitcases such as nylon, polycarbonate (PC), and polyethylene (PE) are tested for transparency. These materials then sandwich various illicit substances, and are scanned by the terahertz system to obtain spectral data, simulating the probing of a suitcase. The sample materials are then subtracted from the obtained data, which is then compared with previously obtained data of known substances, and an examination of features in the sample is carried out to determine if a particular substance is present in the sample.

Double-modulated DTDS-THz liquid spectroscopy using a novel spinning wheel technique

A fast and reliable novel technique for a terahertz liquid spectroscopy based on the double-modulated differential time-domain spectroscopy (Double-modulated DTDS) is presented. This technique is implemented using a robust stainless-steel wheel with a dual-thickness cyclic olefin copolymer (COC) window driven by a high speed brushless 3 phase motor.

Low-noise terahertz material parameter extraction using a spinning wheel

We demonstrate measurement of material optical parameters, in the THz regime, by mounting the sample under test on a spinning wheel. The proposed measurement technique is implemented using double-modulated terahertz differential timedomain spectroscopy (double-modulated THz-DTDS). Spinning sample addresses fundamental limitations imposed by linear sample dithering. The concept of spinning the sample allows rapid switching between the reference and sample signals with a single mechanical delay scan. We validate this technique by measuring the dielectric properties of thick polymer material.

Hygroscopicity of window materials using terahertz time-domain spectroscopy (THz-TDS)

Liquid spectroscopy allows analysis of chemical composition and provides a better understanding of the solvation dynamics of various types of liquid [1]. In liquid spectroscopy, window material is required to contain the liquid samples. A number of papers have been published on absorption coefficient of different types of window materials for liquid spectroscopy [1], [2], [3], however the hygroscopicity of the window material which could potentially increase the absorption coefficient of the window material has not been reported. This paper measures the hygroscopicity of different types of window materials used in terahertz liquid spectroscopy.

Substance Detection for Security Screening Using Terahertz Imaging Technology

The ability of terahertz radiation to render many non-metallic and non-polar substances transparent provides for many security applications. In this paper we will focus on the possible application of terahertz radiation scanning the contents for illicit substances in common use suitcases in secure areas. Suitcases and contents were simulated by sandwiching a cotton sheet between nylon or plastic, with the cotton being laced with illicit substances of varying concentrations. Data was then obtained by use of terahertz spectroscopy and the substances were identified based on reference samples.