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.
Highlights
Heterodyne detection, which makes use of a mixing detector and a high power local oscillator (LO), measures the light intensity together with an exceptionally high spectral resolution
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
At supra-terahertz frequencies (> 1 THz), the available heterodyne detectors include Schottky diodes [2], hot electron bolometers (HEB) [3], terahertz quantum well photodetectors (QWP) [4], and quantum cascade laser (QCL) detectors, which can be used for dual-comb multiheterodyne detection [5,6,7]
Summary
Heterodyne detection, which makes use of a mixing detector and a high power local oscillator (LO), measures the light intensity together with an exceptionally high spectral resolution. QCL based on the surface plasmon design has a better output beam, but has the disadvantage of a relatively low operation temperature Such an approach has been successfully implemented for the 4.7 THz heterodyne spectrometer GREAT on SOFIA the beam still contains higher-order Gaussian modes [24]. We report a spatial filter based on a back-to-back (B2B) lens system operated at a supra-THz frequency and its characterization using an available high power QCL at 3.9 THz. The beam filtering efficiency of the B2B lens system, which characterizes both the Gaussicity of the output beam and power transmissivity, is studied in both simulation and experiment. The B2B lens system should be applicable for different wavelengths if the wavelength dependent part is properly scaled
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