3.9 THz spatial filter based on a back-to-back Si-lens system

Abstract

We present a terahertz spatial filter consisting of two back-to-back (B2B) mounted elliptical silicon lenses and an opening aperture defined on a thin gold layer between the lenses. The beam filtering efficiency of the B2B lens system is investigated by simulation and experiment. Using a unidirectional antenna coupled 3rd-order distributed feedback (DFB) quantum cascade laser (QCL) at 3.86 THz as the source, the B2B lens system shows 72% transmissivity experimentally with a fundamental Gaussian mode as the input, in reasonably good agreement with the simulated value of 80%. With a proper aperture size, the B2B lens system is capable of filtering the non-Gaussian beam from the QCL to a nearly fundamental Gaussian beam, where Gaussicity increases from 74% to 99%, and achieves a transmissivity larger than 30%. Thus, this approach is proven to be an effective beam shaping technique for QCLs, making them to be suitable local oscillators in the terahertz range with a Gaussian beam. Besides, the B2B lens system is applicable to a wide frequency range if the wavelength dependent part is properly scaled.

Roadmap of Terahertz Imaging 2021

“…It is worth noting initiatives to apply spatial filtering in pulse shaping to monitor and control emission of compact THz sources. One can mention, for instance, THz QCLs by design of two back-to-back mounted elliptical silicon lenses and an opening aperture defined on a thin gold layer between the lenses [232] which enables filtering of the non-Gaussian beam from the QCL to a nearly fundamental Gaussian beam. As for multispectral THz imaging utilizing broadband THz pulses, an important innovation can stem from Fresnel apertures and filters that provide possibilities not only for the pulse shaping but also generation of longitudinal THz fields [233], hence exciting relevant applications.…”

An ultrastrongly coupled single terahertz meta-atom

“…1 d). Being very akin to a 4 f arrangement, a similar setup has been recently proposed also as a THz spatial filter 29 . A set of ray optics simulations in COMSOL Multiphysics is done to find the correct lens dimensions and adjust the input and output numerical aperture of the system such that it is compatible with the existing THz-TDS setup.…”

“…The presence of the substrate where the sample is deposited also makes the back lens effectively a hyperhemispherical one. The significant advantage of this asymmetric design compared to the previous symmetric designs 26 , 29 is that there is no need to sandwich the target surface (in our case, the metamaterial metallic thin film) between two semiconducting substrates to have the surface at the focus point of the confocal system. The asymmetry of the lenses accounts for the sample substrate thickness as an active part of the optics.…”

“…Thus the back lens with a larger diameter compensates for the substrate thickness and collects the diverging beam at the back interface of the substrate. The performance of our aSIL setup could in any case be further improved by a careful choice of the lens curvature to optimize the matching with the incoming THz beam and by providing the Si lenses with a broadband anti-reflection coating to further enhance the SNR 29 .

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Low noise MgB2 hot electron bolometer mixer operated at 5.3 THz and at 20 K

“…The optical loss of the AR coated Si lens is simulated using COMSOL Multiphysics by taking the absorption coefficients of parylene-C of 27 cm À1 at 5.25 THz into consideration. 24,25 The power loss from the antenna to the HEB is found to be 0.94 dB due to the lower HEB resistance (51 X) and is 0.31 dB more than the matched case. 20 Figure 1 shows a set of current-voltage (IV) curves of the HEB from zero to fully pumped by the 5.3 THz LO, recorded at 20 K. Within the optimal operating region (2.5-4 mV and 0.19-0.25 mA), the T DSB rec of the HEB, to be described, only increases $5% from its lowest value.…”