Abstract
An ongoing limitation of terahertz spectroscopy is that the technique is generally limited to the study of relatively large samples of order 4 mm across due to the generally large size of the focal beam spot. We present a nested concentric parabolic reflector design which can reduce the terahertz focal spot size. This parabolic reflector design takes advantage of the feature that reflected rays experience a relative time delay which is the same for all paths. The increase in effective optical path for reflected light is equivalent to the aperture diameter itself. We have shown that the light throughput of an aperture of 2 mm can be increased by a factor 15 as compared to a regular aperture of the same size at low frequencies. This technique can potentially be used to reduce the focal spot size in terahertz spectroscopy and enable the study of smaller samples.
Highlights
We present a nested concentric parabolic reflector design which can reduce the terahertz focal spot size
We have demonstrated an effective way of increasing the light throughput of a small aperture
It can be concluded that the novel nested concentric parabolic reflector significantly reduces the effective focal spot size of low frequency terahertz radiation by coupling it through a small aperture
Summary
Time-domain terahertz spectroscopy (TDTS)[1] has seen tremendous growth is recent years and has found use in many different areas including security applications and biohazard detection,[2,3] detection of protein conformational changes,[4] structural and medical imaging,[5,6,7,8,9] monitoring of pharmaceutical ingredients,[10] and the characterization of materials.[11,12,13] In particular time-domain terahertz spectroscopy has been proven very useful in studying different material systems at low temperatures including superconductors,[14,15] quantum magnets,[16,17,18,19] exciton states in TiO2 nanotubes,[20] and topological insulators.[13,21,22,23]With the photoconductive switches method of TDTS, one typically splits an infrared femtosecond laser pulse along two paths and sequentially excites a pair of photoconductive “Auston”switch antennae. An approximately 1 ps long THz range pulse is emitted by one antenna, focused by mirrors or lenses, transmitted through the sample under test, refocused, and measured at the other antenna. By varying the path length difference of the two paths, the electric field of the transmitted pulse is measured in the time domain. Ratioing the Fourier transform of the transmission through the sample to that of a reference gives the frequency dependent complex transmission function in a range that typically of order 100 GHz - 3 THz. The complex optical parameters of interest e.g. complex dielectric function or the complex index of refraction can be obtained from the complex transmission function. Measurements are generally performed in transmission, but in order to get adequate signal to noise ( at low frequencies) samples must be typically large in the transverse direction. In most cases sample dimensions must be of order 4 mm across, which is well in excess of the estimates of the diffraction
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