M. B. Byrne

Terahertz vibrational absorption spectroscopy using microstrip-line waveguides

We demonstrate that terahertz microstrip-line waveguides can be used to measure absorption spectra of polycrystalline materials with a high frequency resolution (∼2 GHz) and with a spatial resolution that is determined by the microstrip-line dimensions, rather than the free-space wavelength. The evanescent terahertz-bandwidth electric field extending above the microstrip line interacts with, and is modified by, overlaid dielectric samples, thus enabling the characteristic vibrational absorption resonances in the sample to be probed. As an example, the terahertz absorption spectrum of polycrystalline lactose monohydrate was investigated; the lowest lying mode was observed at 534(±2) GHz, in excellent agreement with free-space measurements. This microstrip technique offers both a higher spatial and frequency resolution than free-space terahertz time-domain spectroscopy and requires no contact between the waveguide and sample.

Simultaneous measurement of orthogonal components of polarization in a free-space propagating terahertz signal using electro-optic detection

We report a polarization-sensitive terahertz time-domain spectroscopy system, which allows the simultaneous measurement of orthogonal components of the polarization of a free-space propagating terahertz beam using a dual electro-optic detection scheme. We demonstrate the operation of our system by measuring the birefringence of lithium niobate, simultaneously obtaining terahertz spectra from two orthogonal crystallographic directions.

On-chip terahertz Goubau-line waveguides with integrated photoconductive emitters and mode-discriminating detectors

We have measured the picosecond time-domain response of Goubau-line waveguides, formed on quartz substrates, by integrating regions of low-temperature-grown gallium arsenide into the waveguides to act both as pulsed current emitters and detectors. Using one pair of photoconductive switches for excitation and a second pair for detection, pulsed signal propagation of a low dispersion electric field mode was demonstrated in the Goubau-lines, with the signal bandwidth extending beyond 800 GHz. Furthermore, it was demonstrated that terahertz bandstop filters can be integrated into a Goubau-line for removal of specific frequencies from the transmitted pulses.

Evanescent-field Terahertz time-domain microscopy

Several modalities have previously been presented for the sub-wavelength imaging of objects at terahertz (THz) frequencies. An important limitation of many such techniques is that they require intense near-infrared radiation to be focused onto the sample being studied, making them unsuitable for measuring light sensitive specimens such as biological substrates and charge carriers contained within semiconductor microstructures. We present an alternative approach based on evanescent-field THz time-domain spectroscopy using on-chip THz circuits. In this case, a pulsed evanescent THz electric field, extending above an on-chip THz waveguide is used to probe specimens. A lithographically-defined microstrip waveguide restricts the THz electric field laterally, while the exponential decay of the field above the waveguide into free-space provides controllable sub-wavelength penetration into specimens. We present an overview of recent results showing the interaction of pulsed THz evanescent electric fields with semiconductor samples, and discuss how such on-chip THz waveguides present a useful route towards THz microscopy.

Terahertz goubau waveguides with integrated photoconductive emitters and mode discriminating detectors

We have measured the picosecond-timescale response of Goubau-line waveguides integrated with regions of low-temperature-grown gallium arsenide, which act as both pulsed photoconductive emitters and detectors. We demonstrate, using two pairs of excitation / detection switches, detection of the radial propagation mode of the Goubau-line, with signal components extending above 800 GHz. This waveguide geometry offers advantages compared to microstrip or coplanar-line geometries for on-chip guiding of terahertz radiation.

Terahertz vibrational absorption resonances observed using on-chip terahertz circuits

A technique for measuring the vibrational absorption spectra of polycrystalline materials in the terahertz (THz) frequency range is demonstrated. We show that the evanescent electric field extending above lithographically-defined on-chip THz microstrip waveguides can be used to measure absorption resonances, offering higher sensitivity than typical free-space THz time-domain spectroscopy systems permit. The results indicate that on-chip THz systems could find new applications in the analysis, quality control, and identification of polycrystalline materials, such as pharmaceutical compounds.

Sub-wavelength imaging of terahertz dielectric permittivity using planar resonant circuits

A new technique for quantitative sub-wavelength mapping of terahertz dielectric permittivity is demonstrated, using on-chip resonant terahertz filters. Imaging results on etched semiconductor samples show the viability of the technique, and future applications are discussed.

Measurement and simulation of the sensitivity of terahertz frequency range passive flter elements to overlaid dielectrics

Resonant terahertz filters in microstrip lines have been proposed as devices suitable for the detection of the properties of overlaid dielectric materials, with potential applications including the on-chip assay of DNA and other biological molecules. Here, we provide a direct comparison of numerical simulations to calibrated experimental data obtained on our resonant structures. We employ two different codes (HFSS and Sonnet) to cross-check the results of our numerical simulations, and conclude that stub-based THz band-stop filters offer an improvement in sensitivity (of order 100x) per unit active area compared with alternative designs. This is a result of the enhanced THz electric field concentration in the overlaid dielectric which this particular geometry produces.