Samuel P. Mickan

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.

Label-free bioaffinity detection using terahertz technology

We report the first use of differential terahertz time-domain spectroscopy for bioaffinity sensing. Binding is observed by measuring the transmission of a thin layer of biotin bound to the sensor protein avidin. We demonstrate the THz wave transmission of a sub-micron-thick film and sensitivity to 0.1 microg cm(-2) of biotin. These results point the way for a host of biosensor applications using T-rays, or pulsed far-infrared (FIR) radiation.

The impact of hydration changes in fresh bio-tissue on THz spectroscopic measurements

We present a study of how residual hydration in fresh rat tissue samples can vastly alter their extracted terahertz (THz) optical properties and influence their health assessment. Fresh (as opposed to preserved) tissue most closely mimics in vivo conditions, but high water content creates many challenges for tissue handling and THz measurement. Our THz measurements of fresh tissue over time highlight the effect of tissue hydration on tissue texture and dimension, the latter directly influencing the accuracy of calculated optical properties. We then introduce lyophilization (freeze drying) as a viable solution for overcoming hydration and freshness problems. Lyophilization removes large amounts of water while retaining sample freshness. In addition, lyophilized tissue samples are easy to handle and their textures and dimensions do not vary over time, allowing for consistent and stable THz measurements. A comparison of lyophilized and fresh tissue shows for the first time that freeze drying may be one way of overcoming tissue hydration issues while preserving tissue cellular structure. Finally, we compare THz measurements from fresh tissue against necrotic tissue to verify freshness over time. Indeed, THz measurements from fresh and necrotic tissues show marked differences.

Analysis of system trade-offs for terahertz imaging

In this paper we analyse the trade-offs for a terahertz imaging system and discuss implementation of a terahertz micro antenna array for imaging. We also describe applications of terahertz imaging and improvements in the signal processing.

Double modulated differential THz-TDS for thin film dielectric characterization

Terahertz differential time-domain spectroscopy (DTDS) is a new technique that uses pulsed terahertz radiation to characterize the optical properties of thin dielectric films. Characterizing thin films in the GHz to THz range is critical for the development of new technologies in integrated circuitry, photonic systems and micro-electro-mechanical systems. There are potential applications for gene and protein chips. This paper shows how DTDS can be combined with double modulation in the pump-probe system to improve sensitivity by an order of magnitude. An iterative algorithm is presented to estimate the optical properties of a given thin film. The technique is experimentally verified using 1-μm-thick samples of silicon dioxide on silicon.

Low noise laser-based T-ray spectroscopy of liquids using double-modulated differential time-domain spectroscopy

Liquid transmission studies at terahertz frequencies (0.1–10 THz) are valuable for understanding solvation dynamics of salts, exploring long-range structure in mixtures and probing biomolecules in suspension. T-ray (or THz) time-domain spectroscopy, based on terahertz pulse generation from ultrafast lasers, is a sensitive technique for measuring material parameters in this frequency range. This paper proposes and demonstrates a novel technique for increasing the sensitivity and repeatability of liquid studies with T-ray time-domain spectroscopy (TDS), reducing relative parameter measurement errors below 0.0001. The proposed technique combines dual-thickness liquid measurement with rapid modulation (double-modulated differential TDS) to reduce the effect of both thickness-measurement errors and T-ray noise errors below 0.0001. The possible reduction in error is calculated and a liquid differential TDS (DTDS) prototype is demonstrated, incorporating amplitude and mean detection for near-simultaneous measurement of two T-ray waveforms.

Terahertz Spectroscopic Differentiation of Microstructures in Protein Gels

We demonstrate that terahertz (THz) spectroscopy can be used to differentiate soft protein microstructures. Differentiation of soft microstructures in gels has to date been performed using optical imaging techniques (e.g. electron microscope), but a non-destructive differentiation tool is lacking. Particulate and fine-stranded (fibrillar) soft protein microstructures are of interest, particularly to medical researchers, because they form from naturally occurring proteins that are thought to be involved in several human diseases, such as Alzheimers disease. In this study, globular beta-lactoglobulin structures with diameters of 2 microm, and fibrillar structures with diameters less than 0.03 microm are observed between 0.8 and 1.5 THz. Results show that the globular structures have a decline in THz transmission when compared to the fibrillar ones. The cause of this decline is possibly due to Rayleigh scattering from the globular microstructures.

2-D Wavelet Segmentation in 3-D T-Ray Tomography

In this letter, segmentation techniques for terahertz (T-ray) computed tomographic (CT) imaging are investigated. A set of linear image fusion and novel wavelet scale correlation segmentation techniques is adopted to achieve material discrimination within a 3-D object. The methods are applied to a T-ray CT image dataset taken from a plastic vial containing a plastic tube. This setup simulates the imaging of a simple nested organic structure, which provides an indication of the potential for using T-ray CT imaging to achieve T-ray pulsed signal classification of heterogeneous layers

NOISE REDUCTION IN TERAHERTZ THIN FILM MEASUREMENTS USING A DOUBLE MODULATED DIFFERENTIAL TECHNIQUE

Differential terahertz (THz) time-domain spectroscopy (TDS) is a technique for decreasing noise levels in THz thin film characterization experiments. Characterizing thin films in the GHz to THz range is critical for the development of fast integrated circuits and photonic systems, and is potentially applicable to biosensors and proteomics. This paper shows how the differential technique, combined with double modulation, enables the study of thin films with noise reduction over normal TDS that improves at the film gets thinner. Double modulated differential THz-TDS has enabled the characterization of films with less than 1-μm thickness.

Limit of Spectral Resolution in Terahertz Time-Domain Spectroscopy

The pulsed nature of terahertz time-domain spectroscopy (THz-TDS) sets a fundamental limit on its spectral resolution. The spectral resolution of THz-TDS can be improved by increasing the duration of the temporal measurement, but is limited by the dynamic range of the system in the time domain. This paper presents calculations and experimental results relating the temporal dynamic range of a THz-TDS system to its spectral resolution. We discuss three typical terahertz sources in terms of their dynamic range and hence achievable spectral resolution.