J. R. Fletcher

Propagation of terahertz radiation through random structures: An alternative theoretical approach and experimental validation

A model describing the propagation of terahertz frequency radiation through inhomogeneous materials is proposed. In such materials (e.g., powders or clothing), the size of the scattering centers, their separation, and the wavelength of the radiation are all commensurate. A phase distribution function is used to model the optical properties of a randomly structured transmitting layer. The predictions of the model are compared with exact (Mie) theory for isolated spherical scatterers and with previously published experimental data. Measurements of the transmission of terahertz radiation through a variety of samples in order to validate the present model are also reported. These include arrays of cylinders, textiles, powders, and glass balls. Overall, satisfactory agreement between the experimental data and theoretical predictions is obtained.

Pulsed Terahertz Signal Reconstruction

A procedure is outlined which can be used to determine the response of an experimental sample to a single, simple broadband frequency pulse in terahertz frequency time domain spectroscopy (TDS). The advantage that accrues from this approach is that oscillations and spurious signals (arising from a variety of sources in the TDS system or from ambient water vapor) can be suppressed. In consequence, small signals (arising from the interaction of the radiation with the sample) can be more readily observed in the presence of noise. Procedures for choosing key parameters and methods for eliminating further artifacts are described. In particular, the use of input functions which are based on the binomial distribution is described. These binomial functions are used to unscramble the sample response to a simple pulse: they have sufficient flexibility to allow for variations in the spectra of different terahertz sources, some of which have low frequency as well as high frequency cutoffs. The signal processing proce...

A simple interferometer for the characterization of sources at terahertz frequencies

The design and performance of a simple and compact far-infrared Fourier-transform spectrometer for the characterization of sources is described. The spectrometer works as a Michelson interferometer without a beam splitter and employs a Golay cell or a pyroelectric detector. A pair of flat lamellar mirrors with a computer-controlled displacement introduces a precisely defined phase difference between two parallel beams of nearly equal radiated power. The interferometer is intended for operation at frequencies between 0.1 and 3 THz with a frequency resolution of 6 GHz, and is used to characterize both continuous-wave and pulsed sources.

Propagation of electromagnetic waves through a system of randomly placed cylinders : the partial scattering wave resonance.

Propagation of electromagnetic waves through a system of randomly placed cylinders has been modelled. It was found that there is a dip in the ballistic transmission spectra for both the E and H polarizations, which is associated with scattering of the partial wave with angular momentum equal to zero by a single cylinder.

Scattering in THz imaging

A new mathematical method, the Phase Distribution Model, is devised for the calculation of attenuation and scattering of THz radiation in random materials. The accuracy of the approximation is tested by comparison with exact calculations and with experimental measurements on textiles and specially constructed phantoms.

Frequency calibration of THz time-domain spectrometers using an etalon

We present an etalon-based method of calibrating the frequency of THz time-domain spectrometers (TDS). The method utilizes the etalon effect produced by multiple reflections in non-absorbing wafers or in narrow air-gaps. The technique provides frequency calibration across the measurement band with uncertainties comparable with the typical THz TDS resolution.

THz pulse reconstruction

A method of post-detection signal processing is described that removes the effects of water vapour, system echoes and propagation dispersion from the recorded signals of a pulsed THz system. The processed signal shows the response the sample would produce to a chosen form of input pulse, well localised in time and free from after-runners. The method requires recording a reference signal subject to all the undesired effects that affect the sample signal. Experimental results are presented for pulses transmitted and reflected by layered structures.