Chemical sensing applications utilizing surface enhanced Raman spectroscopy (SERS) have drawn significant
attention recently. However, developing a reliable, high performance SERS platform remains a challenge. A novel
SERS substrate based on nanofingers was successfully demonstrated to provide large enhancement reliably and
showed great promise for practical applications. Capillary forces bring the gold caps on the nanofingers into close
proximity upon exposure to a solution containing molecules of interest, trapping molecules within the gaps and
producing greatly enhanced Raman signals. Transmission electron microscopy (TEM) was used to characterize the
structure of the nanofingers, in particular the gaps between finger tips to improve the fundamental understanding of
the structural-performance relationship.
Metal nanoparticle arrays offer the possibility to considerably surpass the optical field confinement of silicon
waveguides. The properties of directional couplers composed of such plasmonic nanoarrays are analyzed theoretically,
while neglecting material losses. It is found that it is possible to generate very compact, submicron length, high fieldconfinement
and functionality devices with very low switch energies. We further perform a study of spatial losses in Ag
nanoparticle arrays by obtaining the group velocity and the lifetime of the surface plasmon polaritons. The losses are
determined for different host permittivities, polarizations, and for spherical and spheroidal particles, with a minimum
loss of 12 dB/μm. The possibilities to compensate the losses using gain materials, and the added noise associated with
that, is briefly discussed.
Nanoimprint lithography is used to fabricate a metamaterial with the "fishnet" structure composed of Ag/a-Si/Ag layers
that exhibits negative refractive index in the near-infrared. We have carried out a femtosecond pump-probe experiment
to measure the transient photo-induced response of this structure. With a pump fluence of 330μJ/cm2 at 800nm, the
transmission at the magnetic resonance is increased by ~15.4%. The induced change originated from carrier excitation in
the a-Si layer has a fast decay constant of 1.1ps.
A variety of metamaterials has been demonstrated recently that support backward waves and negative refraction
(Negative Index Materials, NIM.) In particular, these materials enable sub-wavelength resolution that makes them even
more interesting, especially in optical domain rather than at microwave frequencies where their unusual properties were
known for decades. We describe below theoretical and experimental studies of the so-called 'fishnet' metal-spacer holearray
metamaterials, which exhibit NIM behavior at optical frequencies, having unit cell size of a few 100s nm. We
demonstrate experimentally that their refractive index can be modulated very fast and very strongly (from -2.4 to -1.5)
around the communication wavelength of λ1.55 um, in good agreement with the FDTD results. We also discuss a
problem of loss compensation in those materials with hefty Ohmic losses by using gain media and local field
enhancement in metallic nanoparticles ensembles that enable SERS.
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