Xi Yu, Naoki Horita, Motoki Bessho, Massaki Sudo, Shingo Ono, Jongsuck Bae
https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10906/109061L/Shape-dependence-of-Terahertz-anti-reflective-structures-fabricated-by-femtosecond/10.1117/12.2507534.short
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.
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