Raghu Dharmavarapu, Soon Hock Ng, Shanti Bhattacharya, Saulius Juodkazis,
https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10456/104561W/All-dielectric-metasurface-for-wavefront-control-at-terahertz-frequencies/10.1117/12.2283090.short?SSO=1
Recently, metasurfaces have gained popularity
due to their ability to offer a spatially varying phase response, low intrinsic
losses and high transmittance. Here, we demonstrate numerically and
experimentally a silicon meta-surface at THz frequencies that converts a
Gaussian beam into a Vortex beam independent of the polarization of the
incident beam. The metasurface consists of an array of sub-wavelength silicon
cross resonators made of a high refractive index material on substrates such as
sapphire and CaF2 that are
transparent at IR-THz spectral range. With these substrates, it is possible to
create phase elements for a specific spectral range including at the molecular
finger printing around 10 μm as well as at longer THz wavelengths where
secondary molecular structures can be revealed. This device offers high
transmittance and a phase coverage of 0 to 2π. The transmittance phase is tuned
by varying the dimensions of the meta-atoms. To demonstrate wavefront
engineering, we used a discretized spiraling phase profile to convert the
incident Gaussian beam to vortex beam. To realize this, we divided the
metasurface surface into eight angular sectors and chose eight different
dimensions for the crosses providing successive phase shifts spaced by π/4
radians for each of these sectors. Photolithography and reactive ion etching
(RIE) were used to fabricate these silicon crosses as the dimensions of these
cylinders range up to few hundreds of micrometers. Large 1-cm-diameter optical
elements were successfully fabricated and characterised by optical profilometry.
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