The role of leaky-waves in high-performance extraordinary THz transmission quasi-optical filters

The need of micrometer features over cm size areas for high-performance THz frequency selective surfaces and other components has proven to be a major challenge for the field. Direct metal milling usually does not meet resolution requirements for THz quasi-optical filters. Advanced microfabrication based on SU-8 or Silicon such as deep reactive ion etching does meet all the requirements at the expense of complexity (multi-step process) and cost. In the light of this, photolithography has emerged as the preferred choice in most cases, but it is not adequate for thick meshes and, more importantly, involves the need of a dielectric substrate that introduces additional losses. Electroforming is a cost-effective fabrication technique to meet tolerances while dielectric substrates are avoided.
In this contribution we report a systematic analysis of extraordinary THz transmission subwavelength hole arrays fabricated by photolithography and electroforming with varying experimental conditions; namely, array matrix size (i.e., number of holes) and beam-waist of the interrogating Gaussian beam, in addition to the substrate effect. This extensive analysis is underpinned by the use of two different spectroscopy techniques (CW and time-domain) and three different commercial instruments (ABmm millimeter-wave vector network analyser, and TERA K15 and TPS Spectra 3000 time-domain spectrometers). The contribution demonstrates that the periodic nature of the array favours the existence of leaky-waves that play a major role in the resonant phenomenon. Indeed, the contribution establishes undoubtedly that the important parameter for a strong extraordinary THz transmission is the total number of holes explored by the leaky-waves rather than the illuminated area. The contribution also reports an unprecedented 0.04 dB insertion loss for a free-standing quasi-optical filter with central frequency ~0.58 THz.