PULSED TERAHERTZ REFLECTION-SCATTERING MEASUREMENTS

Shu-Zee A. Lo and Edwin J. Heilweil

The terahertz spectrum has recently been the focus of many research groups due to its potential applications in spectroscopy, imaging and communications. Extensive studies have been conducted using terahertz spectroscopic tools for nonmetallic, non-polar chemical and biological molecules.  Generally, the loss mechanisms of these samples have been mostly attributed to sample absorption. While this is true for most isotropic and uniform samples, the same conclusion is ambiguous for samples that are composite and porous. This distinction is important for applications such as security inspection, where most common materials of interest (e.g., cloth, paper, cardboard) are composite and porous. In spectroscopy, one common method of measuring absorption spectra of biomolecules is to mix the sample of interest with finely ground plastic materials and compressing the mixture into pellets that are then inserted into the terahertz beam path and measured in transmission. While individual sample and plastic powder crystals are miniscule compared to the terahertz wavelength, the pellet sample under test might still scatter the probing terahertz beam due to irregularities or inhomogeneities caused by inadequate mixing of the two materials.

The aim of this project is to modify an existing conventional pulsed time domain terahertz spectroscopy system with fixed generation and detection optical components to include rotation stages for collecting scattered light from a sample. We have successfully developed a light scattering apparatus that automatically scans the output angle from a sample, redirecting the output terahertz beam from the sample to the fixed terahertz detection arm. Hence, we are able to perform conventional time delay scans and angular dependent reflection/scattering measurements at the same time. Using the fast Fourier transform algorithm, we are also able to obtain angular dependent terahertz reflection spectra for different samples.  A complete schematic diagram of the apparatus and scattering data from several samples will be presented in this poster.