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Shape dependence of Terahertz anti-reflective structures fabricated by femtosecond laser processing

Silicon and GaAs are typical materials used in terahertz components because of their high transparency and low dispersion, but their high refractive indices will cause about 30% reflection loss in power. Fabricating gradient index photonic structures (also known as moth-eye structure) on the surface of components have been examined suitable especially to terahertz system. In order to achieve high performance on anti-reflection in these structures, the gradient profile of refractive index, this is, the shape of tapers is important as well as high aspect ratio (= depth/pitch in tapers). We theoretically calculated anti-reflection characteristics of tapered structures with different refractive index taper profiles of Klopfenstein, linear and exponential, and the results showed that the Klopfenstein taper profile has the best performance on anti-reflection at the THz frequencies. Moreover, Femtosecond laser is reported as a strong method to perform microfabrication on Silicon substrate. However, if femtosecond pulses are continuously irradiated to the same point, the ablated material will absorb the incident pulse again, and reattach to the original point, which hindered the obtaining of high aspect ratio and shape control of the taper. By increasing the scan speed of laser beam, we decreased the pulse number continuously irradiated in unit time to reduce the thermal influence, and obtain a higher aspect ratio and smooth surface of the tapers. We employed femtosecond laser processing to fabricate anti-reflective structures formed by periodic tapers with different pitches. We also evaluated their anti-reflection characteristics experimentally and theoretically at terahertz frequencies (0.1 THz ~ 2 THz). The experimental results showed that the Fresnel reflection is almost decreased to zero at a wider frequency band, and the measured frequency dependence of reflection in the grating structures is good agreement with theoretical ones. These results showed that the laser processing is very useful to fabricate anti-reflection structures with precise dimensions.