Abstract
A new type of substrate lens for photoconductive antennas (PCA’s) based on sub-wavelength microstructuring is presented and studied theoretically by the use of Greens function integral equation methods (GFIEM’s). By etching sub-wavelength trenches into a flat substrate, the effective dielectric constant can be designed to function like a gradient index (GRIN) lens. The proposed GRIN substrate lenses have sub-mm dimension, which is smaller than the dimensions of a typical hyper-hemispherical substrate lens (HSL), and could enable fabrication of arrays of closely packed PCA’s with individual lenses integrated directly into the PCA substrate. The performance of different GRIN lenses is compared to a HSL and shown to be comparable with regards to the terahertz radiation extraction efficiency, and it is shown that the collimating properties of these GRIN lenses can be tailored by changing the parameters used for microstructuring.
Highlights
Imaging and spectroscopy in the THz range of the electro-magnetic spectrum has, since the demonstration of the first THz time-domain spectrometer (THz-TDS),[1] become an increasingly popular method to study a wide range of biological, chemical and physical systems.[2]
The proposed gradient index (GRIN) substrate lenses have sub-mm dimension, which is smaller than the dimensions of a typical hyper-hemispherical substrate lens (HSL), and could enable fabrication of arrays of closely packed photoconductive antennas (PCA’s) with individual lenses integrated directly into the PCA substrate
The performance of different GRIN lenses is compared to a HSL and shown to be comparable with regards to the terahertz radiation extraction efficiency, and it is shown that the collimating properties of these GRIN lenses can be tailored by changing the parameters used for microstructuring
Summary
Imaging and spectroscopy in the THz range of the electro-magnetic spectrum has, since the demonstration of the first THz time-domain spectrometer (THz-TDS),[1] become an increasingly popular method to study a wide range of biological, chemical and physical systems.[2]. Due to the inherent design of a typical PCA, a large part of the emitted THz radiation is trapped within the semiconductor substrate. The large size of typical HSL’s limits the integrability by adding considerable bulk to the PCA and prevents close packing of many individual PCA’s into PCA micro-arrays. In this paper we propose and theoretically study a compact GRIN lens suitable for fabrication of dense arrays of PCAs with individual integrated substrate lenses.
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