  Plasmonic optical tweezers are a ubiquitous tool for the precise manipulation
of nanoparticles and biomolecules at low photon flux, while femtosecond-laser
optical tweezers can probe the nonlinear optical properties of the trapped
species with applications in biological diagnostics. In order to adopt
plasmonic optical tweezers in real-world applications, it is essential to
develop large-scale fabrication processes without compromising the trapping
efficiency. Here, we develop a novel platform for continuous wave (CW) and
femtosecond plasmonic optical tweezers, based on gold-coated black silicon. In
contrast with traditional lithographic methods, the fabrication method relies
on simple, single-step, maskless tabletop laser processing of silicon in water
that facilitates scalability. Gold-coated black silicon supports repeatable
trapping efficiencies comparable to the highest ones reported to date. From a
more fundamental aspect, a plasmon-mediated efficiency enhancement is a
resonant effect, and therefore, dependent on the wavelength of the trapping
beam. Surprisingly, a wavelength characterization of plasmon-enhanced trapping
efficiencies has evaded the literature. Here, we exploit the repeatability of
the recorded trapping efficiency, offered by the gold-coated black silicon
platform, and perform a wavelength-dependent characterization of the trapping
process, revealing the resonant character of the trapping efficiency maxima.
Gold-coated black silicon is a promising platform for large-scale parallel
trapping applications that will broaden the range of optical manipulation in
nanoengineering, biology, and the study of collective biophotonic effects.
