Aspheric silicon lenses for terahertz photoconductive antennas

We report on the design and characterization of aspheric focusing silicon lenses for terahertz photoconductive antennas. The lenses are engineered using ray-tracing software and characterized using an optical fiber terahertz time-domain spectroscopy system. We find that using aspheric lenses improves terahertz radiation coupling from the emitter and enables improved collection by the detector. The signal-to-noise ratio and the cutoff frequency of measured terahertz spectra are improved. Minimized aberrations also reduce the focal spot size. Simulations based on Fresnel–Kirchhoff diffraction theory, taking into account the radiation pattern of the emitter and aberrations of the lenses, show good agreement with our measurements.