10 research outputs found
Scanning optical homodyne detection of high-frequency picoscale resonances in cantilever and tuning fork sensors
Higher harmonic modes in nanoscale silicon cantilevers and microscale quartz
tuning forks are detected and characterized using a custom scanning optical
homodyne interferometer. Capable of both mass and force sensing, these
resonators exhibit high-frequency harmonic motion content with picometer-scale
amplitudes detected in a 2.5 MHz bandwidth, driven by ambient thermal
radiation. Quartz tuning forks additionally display both in-plane and
out-of-plane harmonics. The first six electronically detected resonances are
matched to optically detected and mapped fork eigenmodes. Mass sensing
experiments utilizing higher tuning fork modes indicate >6x sensitivity
enhancement over fundamental mode operation.Comment: 3 pages, 3 figures, submitted to Applied Physics Letter
Quantum Holographic Encoding in a Two-dimensional Electron Gas
The advent of bottom-up atomic manipulation heralded a new horizon for
attainable information density, as it allowed a bit of information to be
represented by a single atom. The discrete spacing between atoms in condensed
matter has thus set a rigid limit on the maximum possible information density.
While modern technologies are still far from this scale, all theoretical
downscaling of devices terminates at this spatial limit. Here, however, we
break this barrier with electronic quantum encoding scaled to subatomic
densities. We use atomic manipulation to first construct open
nanostructures--"molecular holograms"--which in turn concentrate information
into a medium free of lattice constraints: the quantum states of a
two-dimensional degenerate Fermi gas of electrons. The information embedded in
the holograms is transcoded at even smaller length scales into an atomically
uniform area of a copper surface, where it is densely projected into both two
spatial degrees of freedom and a third holographic dimension mapped to energy.
In analogy to optical volume holography, this requires precise amplitude and
phase engineering of electron wavefunctions to assemble pages of information
volumetrically. This data is read out by mapping the energy-resolved electron
density of states with a scanning tunnelling microscope. As the projection and
readout are both extremely near-field, and because we use native quantum states
rather than an external beam, we are not limited by lensing or collimation and
can create electronically projected objects with features as small as ~0.3 nm.
These techniques reach unprecedented densities exceeding 20 bits/nm2 and place
tens of bits into a single fermionic state.Comment: Published online 25 January 2009 in Nature Nanotechnology; 12 page
manuscript (including 4 figures) + 2 page supplement (including 1 figure);
supplementary movie available at http://mota.stanford.ed
Strong coupling of plasmon and nanocavity modes for dual-band, near-perfect absorbers and ultrathin photovoltaics
When optical resonances interact strongly, hybridized modes are formed with mixed properties inherited from the basic modes. Strong coupling therefore tends to equalize properties such as damping and oscillator strength of the spectrally separate resonance modes. This effect is here shown to be very useful for the realization of near perfect dual-band absorption with ultrathin (~10 nm) layers in a simple geometry. Absorber layers are constructed by atomic layer deposition of the heavy-damping semiconductor tin monosulfide (SnS) onto a two-dimensional gold nanodot array. In combination with a thin (55 nm) SiO2 spacer layer and a highly reflective Al film on the back, a semi-open nanocavity is formed. The SnS coated array supports a localized surface plasmon resonance in the vicinity of the lowest order anti-symmetric Fabry-Perot resonance of the nanocavity. Very strong coupling of the two resonances is evident through anti-crossing behavior with a minimum peak splitting of 400 meV, amounting to 24% of the plasmon resonance energy. The mode equalization resulting from this strong interaction enables simultaneous optical impedance matching of the system at both resonances, and thereby two near perfect absorption peaks which together cover a broad spectral range. When paired with the heavy damping from SnS band-to-band transitions, this further enables approximately 60% of normal incident solar photons with energies exceeding the bandgap to be absorbed in the 10 nm SnS coating. Thereby, these results establish a distinct relevance of strong coupling phenomena to efficient, nanoscale photovoltaic absorbers and more generally for fulfilling a specific optical condition at multiple spectral positions
Self-Assembly Based Plasmonic Arrays Tuned by Atomic Layer Deposition for Extreme Visible Light Absorption
Achieving
complete absorption of visible light with a minimal amount
of material is highly desirable for many applications, including solar
energy conversion to fuel and electricity, where benefits in conversion
efficiency and economy can be obtained. On a fundamental level, it
is of great interest to explore whether the ultimate limits in light
absorption per unit volume can be achieved by capitalizing on the
advances in metamaterial science and nanosynthesis. Here, we combine
block copolymer lithography and atomic layer deposition to tune the
effective optical properties of a plasmonic array at the atomic scale.
Critical coupling to the resulting nanocomposite layer is accomplished
through guidance by a simple analytical model and measurements by
spectroscopic ellipsometry. Thereby, a maximized absorption of light
exceeding 99% is accomplished, of which up to about 93% occurs in
a volume-equivalent thickness of gold of only 1.6 nm. This corresponds
to a record effective absorption coefficient of 1.7 × 10<sup>7</sup> cm<sup>–1</sup> in the visible region, far exceeding
those of solid metals, graphene, dye monolayers, and thin film solar
cell materials. It is more than a factor of 2 higher than that previously
obtained using a critically coupled dye J-aggregate, with a peak width
exceeding the latter by 1 order of magnitude. These results thereby
substantially push the limits for light harvesting in ultrathin, nanoengineered
systems