  The transparency of silicon in the infrared region enables the design of
nano/microstructures for implementation in devices to harvest the infrared (IR)
part of the solar spectrum. Herein we report a strategy that uses arrays of
inverted silicon pyramids covered with a thin gold film, which exhibit
substantial light absorption in the infrared spectral range (below the gap of
Si). The absorption stems from the resonant excitation at infrared wavelengths
of surface-plasmon polaritons at the metal/dielectric interface mainly by
tuning size and separation of the inverted pyramids. The array-parameters
optimization proceeded by iteration of the calculation and measurement of the
infrared response using finite difference time-domain simulations and
Fourier-transform IR spectroscopy, respectively. We show two fabrication routes
for this kind of metal/silicon metamaterials either by photolithography or
scalable nanoimprint techniques for a seamless integration in optoelectronic
fabrication processes.
