DeChang Dai

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.

Silicon based microfluidic cell for terahertz frequencies

We present a detailed analysis of the design, fabrication and testing of a silicon based, microfluidic cell, for transmission terahertz time-domain spectroscopy. The sensitivity of the device is tested through a range of experiments involving primary alcohol/water mixtures. The dielectric properties of these solutions are subsequently extracted using a Nelder–Mead search algorithm, and are in good agreement with literature values obtained via alternative techniques. Quantities in the order of 2 μmol can be easily distinguished for primary alcohols in solution, even with the subwavelength optical path lengths used. A further display of the device sensitivity is shown through the analysis of commercial whiskeys, where there are clear, detectable differences between samples. Slight absorption variations were identified between samples of the same commercial brand, owing to a 2.5% difference in their alcoholic content. Results from data taken on subsequent days after system realignment are also presented, confirming the robustness of the technique, and the data extraction algorithm used. One final experiment, showing the possible use of this device to analyze aqueous biological samples is detailed; where biotin, a molecule known for its specific terahertz absorptions, is analyzed in solution. The device sensitivity is once again displayed, where quantities of 3 nmol can be clearly detected between samples.

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...

Negative refraction and the spectral filtering of terahertz radiation by a photonic crystal prism

We demonstrate how micromachined photonic crystals can be used to negatively refract terahertz frequency light. The photonic crystals, which are constructed from conventional dielectric materials, manipulate the incident beam via interaction with their photonic bands. Consequently, we show that different components of a broadband beam incident on the structure may be positively or negatively refracted, depending upon its frequency and that the structure can be used as an effective spectral filter of THz radiation.

Negative refracting materials at THz frequencies

We demonstrate here, for the first time, the construction of artificial materials, which theoretically possess two separate pass-bands utilizing the difference between positive and negative refraction in the terahertz frequency regime. Experimental testing of these devices is currently being undertaken using both broadband pulsed sources and narrow frequency quantum cascade lasers.

Negatively refracting plasmonic devices at THz frequencies

In this paper, we present experimental results showing how appropriately designed and constructed artificial materials can be used to refract negatively terahertz frequency light without the need for the materials to have both a negative permeability and a negative permittivity.

Silicon based microfluidic device for THz frequencies

In this paper we report on the design, fabrication and testing of a silicon based, microfluidic device, for transmission THz-TDS. Testing of the device and the associated data extraction algorithms is accomplished by the analysis of primary alcohol/water mixtures, where quantities of 2 µmol can be distinguished between samples. The analysis of commercial whiskeys shows clear absorption differences between brands, owing to the individual composition of the blends. Testing of biotin solutions has displayed sensitivity of the order of 3 nmol.

Micromachining for Terahertz Artificial Materials

In this paper we report on the development of micromachined filters for operation at terahertz frequencies. SU8, a negative photodefinable epoxy, is used to define arrays of high aspect ratio rods which are subsequently sputter coated in gold to form the filter. We fabricate and test a filter with a fixed period but variable diameter along the length of the array. By moving the array in the terahertz beam we demonstrate the ability to mechanically tune specific filter characteristics from a single device.

The Fabrication of THz Photonic Filters Using Ultraviolet Based SU8 Micromachining

Here we report on the development of an ultraviolet (UV) based exposure technique for the fabrication of high aspect ratio metallic pillar arrays. These arrays act as photonic band gap based filters. Completed structures show a relative transmission of up to 97% in the main pass band centred at 1.25 THz.

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.