4 research outputs found
Plasmonic Properties of Silicon Nanocrystals Doped with Boron and Phosphorus
Degenerately doped silicon nanocrystals
are appealing plasmonic materials due to siliconâs low cost
and low toxicity. While surface plasmonic resonances of boron-doped
and phosphorus-doped silicon nanocrystals were recently observed,
there currently is poor understanding of the effect of surface conditions
on their plasmonic behavior. Here, we demonstrate that phosphorus-doped
silicon nanocrystals exhibit a plasmon resonance immediately after
their synthesis but may lose their plasmonic response with oxidation.
In contrast, boron-doped nanocrystals initially do not exhibit plasmonic
response but become plasmonically active through postsynthesis oxidation
or annealing. We interpret these results in terms of substitutional
doping being the dominant doping mechanism for phosphorus-doped silicon
nanocrystals, with oxidation-induced defects trapping free electrons.
The behavior of boron-doped silicon nanocrystals is more consistent
with a strong contribution of surface doping. Importantly, boron-doped
silicon nanocrystals exhibit air-stable plasmonic behavior over periods
of more than a year
Ultrafast Silicon Photonics with Visible to Mid-Infrared Pumping of Silicon Nanocrystals
Dynamic optical control
of infrared (IR) transparency and refractive
index is achieved using boron-doped silicon nanocrystals excited with
mid-IR optical pulses. Unlike previous silicon-based optical switches,
large changes in transmittance are achieved without a fabricated structure
by exploiting strong light coupling of the localized surface plasmon
resonance (LSPR) produced from free holes of p-type silicon nanocrystals.
The choice of optical excitation wavelength allows for selectivity
between hole heating and carrier generation through intraband or interband
photoexcitation, respectively. Mid-IR optical pumping heats the free
holes of p-Si nanocrystals to effective temperatures greater than
3500 K. Increases of the hole effective mass at high effective hole
temperatures lead to a subpicosecond change of the dielectric function,
resulting in a redshift of the LSPR, modulating mid-IR transmission
by as much as 27%, and increasing the index of refraction by more
than 0.1 in the mid-IR. Low hole heat capacity dictates subpicosecond
hole cooling, substantially faster than carrier recombination, and
negligible heating of the Si lattice, permitting mid-IR optical switching
at terahertz repetition frequencies. Further, the energetic distribution
of holes at high effective temperatures partially reverses the BursteinâMoss
effect, permitting the modulation of transmittance at telecommunications
wavelengths. The results presented here show that doped silicon, particularly
in micro- or nanostructures, is a promising dynamic metamaterial for
ultrafast IR photonics
Broadband Absorbing ExcitonâPlasmon Metafluids with Narrow Transparency Windows
Optical
metafluids that consist of colloidal solutions of plasmonic and/or
excitonic nanomaterials may play important roles as functional working
fluids or as means for producing solid metamaterial coatings. The
concept of a metafluid employed here is based on the picture that
a single ballistic photon, propagating through the metafluid, interacts
with a large collection of specifically designed optically active
nanocrystals. We demonstrate water-based metafluids that act as broadband
electromagnetic absorbers in a spectral range of 200â3300 nm
and feature a tunable narrow (âŒ100 nm) transparency window
in the visible-to-near-infrared region. To define this transparency
window, we employ plasmonic gold nanorods. We utilize excitonic boron-doped
silicon nanocrystals as opaque optical absorbers (âoptical
wallâ) in the UV and blue-green range of the spectrum. Water
itself acts as an opaque âwallâ in the near-infrared
to infrared. We explore the limits of the concept of a âsimpleâ
metafluid by computationally testing and validating the effective
medium approach based on the BeerâLambert law. According to
our simulations and experiments, particle aggregation and the associated
decay of the window effect are one example of the failure of the simple
metafluid concept due to strong interparticle interactions