3.9 THz spatial filter based on a back-to-back Si-lens system
Yuner Gan (SRON–Netherlands Institute for Space Research, Rijksuniversiteit Groningen)
Behnam Mirzaei (SRON–Netherlands Institute for Space Research, TU Delft - ImPhys/Optics)
Sebastiaan Van Der Poel (TU Delft - QN/van der Zant Lab, Kavli institute of nanoscience Delft)
Jose R.G. Silva (SRON–Netherlands Institute for Space Research, Rijksuniversiteit Groningen)
Matvey Finkel (SRON–Netherlands Institute for Space Research)
Martin Eggens (SRON–Netherlands Institute for Space Research)
Marcel Ridder (SRON–Netherlands Institute for Space Research)
Ali Khalatpour (Massachusetts Institute of Technology)
Jian Rong Gao (SRON–Netherlands Institute for Space Research, TU Delft - ImPhys/Optics)
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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.