218 research outputs found
Plasmonic Enhancement Mechanisms in Solar Energy Harvesting
Semiconductor photovoltaics (solar-to-electrical) and photocatalysis (solar-to-chemical) requires sunlight to be converted into excited charge carriers with sufficient lifetimes and mobility to drive a current or photoreaction. Thin semiconductor films are necessary to reduce the charge recombination and mobility losses, but thin films also limit light absorption, reducing the solar energy conversion efficiency. Further, in photocatalysis, the band edges of semiconductor must straddle the redox potentials of a photochemical reaction, reducing light absorption to half the solar spectrum in water splitting. Plasmonics transforms metal nanoparticles into antennas with resonances tuneable across the solar spectrum. If energy can be transferred from the plasmon to the semiconductor, light absorption in the semiconductor can be increased in thin films and occur at energies smaller than the band gap.;This thesis investigates why, despite this potential, plasmonic solar energy harvesting techniques rarely appear in top performing solar architectures. To accomplish this goal, the possible plasmonic enhancement mechanisms for solar energy conversion were identified, isolated, and optimized by combining systematic sample design with transient absorption spectroscopy, photoelectrochemical and photocatalytic testing, and theoretical development. Specifically, metal semiconductor nanostructures were designed to modulate the plasmon\u27s scattering, hot carrier, and near field interactions as well as remove heating and self-catalysis effects. Transient absorption spectroscopy then revealed how the structure design affected energy and charge carrier transfer between metal and semiconductor. Correlating this data with wavelength-dependent photoconversion efficiencies and theoretical developments regarding metal-semiconductor interactions identified the origin of the plasmonic enhancement.;Using this methodology, it has first been proven that three plasmonic enhancement routes are possible: i) increasing light absorption in the semiconductor by light trapping through scattering, ii) transferring hot carriers from metal to semiconductor after light absorption in the metal, and iii) non-radiative excitation of interband transitions in the semiconductor by plasmon-induced resonant energy transfer (PIRET). The effects of the metal on charge transport and carrier recombination were also revealed. Next, it has been shown that the strength and balance of the three enhancement mechanisms is rooted in the plasmon\u27s dephasing time, or how long it takes the collective electron oscillations to stop being collective. The importance of coherent effects in plasmonic enhancement is also shown. Based on these findings, a thermodynamic balance framework has been used to predict the theoretical maximum efficiency of solar energy conversion in plasmonic metal-semiconductor heterojunctions. These calculations have revealed how plasmonics is best used to address the different light absorption problems in semiconductors, and that not taking into account the plasmon\u27s dephasing is the origin of low plasmonic enhancement Finally, to prove these guidelines, each of the three enhancement mechanisms has been translated into optimal device geometries, showing the plasmon\u27s potential for solar energy harvesting.;This dissertation identifies the three possible plasmonic enhancement mechanisms for the first time, discovering a new enhancement mechanism (PIRET) in the process. It has also been shown for the first time that the various plasmon-semiconductor interactions could be rooted in the plasmon\u27s dephasing. This has allowed for the first maximum efficiency estimates which have combined all three enhancement mechanisms to be performed, and revealed that changes in the plasmon\u27s dephasing leads to the disparity in reported plasmonic enhancements. These findings are combined to create optimal device design guidelines, which are proven by fabrication of several devices with top efficiencies in plasmonic solar energy conversion. The knowledge obtained will guide the design of efficient photovoltaics and photocatalysts, helping usher in a renewable energy economy and address current needs of climate change
Terahertz Waveguiding in Silicon-Core Fibers
We propose the use of a silicon-core optical fiber for terahertz (THz)
waveguide applications. Finite-difference time-domain simulations have been
performed based on a cylindrical waveguide with a silicon core and silica
cladding. High-resistivity silicon has a flat dispersion over a 0.1 - 3 THz
range, making it viable for propagation of tunable narrowband CW THz and
possibly broadband picosecond pules of THz radiation. Simulations show the
propagation dynamics and the integrated intensity, from which transverse mode
profiles and absorption lengths are extraced. It is found that for 140 - 250
micron core diameters the mode is primarily confined to the core, such that the
overall absorbance is only slightly less than in bulk polycrystalline silicon.Comment: 6 pages, 3 figures, journal submissio
Transient extreme ultraviolet measurement of element-specific charge transfer dynamics in multiple-material junctions
The absorption of solid state materials in complex photonic and optoelectronic devices overlap in the visible spectrum. Due to the overlap of spectral features, ultrafast measurements of charge carrier dynamics and transport is obscured. Here, the element specificity of transient extreme ultraviolet (XUV) spectroscopy is advanced as a probe for studying photoexcited charge transport in multiple-material junctions. The core-hole excited by the XUV transitions also imparts structural information on to the probed electronic transition. Transient XUV can therefore measure electron and averaged phonon dynamics for each elemental species in a junction. Application to polaron measurement in α-Fe_2O_3, valley-specific scattering in Si, and charge transfer in a nanoscale Ni-TiO_2-Si junction will be discussed
Molecular hot spots in surface-enhanced Raman scattering
The chemical and electromagnetic (EM) enhancements both contribute to surface-enhanced Raman scattering (SERS). It is well-known that the EM enhancement is induced by the intense local field of surface plasmon resonance (SPR). This report shows that the polarizability of the molecules adsorbed on the metal surface can lead to another channel for the EM field enhancement. When aromatic molecules are covalently bonded to the Au surface, they strongly interact with the plasmon, leading to a modification of the absorption spectrum and a strong SERS signal. The effect is seen in both 3 nm-Au nanoparticles with a weak SPR and 15 nm-Au nanoparticles with a strong SPR, suggesting that the coupling is through both EM field and chemical means. Linear-chain molecules on the 3 nm-Au nanoparticles do not have a SERS signal. However, when the aromatic and linear molecules are co-adsorbed, the strong SPR/molecular polarizability interaction spatially extends the local EM field, leading to a strong SERS signal from the linear-chain molecules. The results show that aromatic molecules immobilized on Au can create “hot spots” just like plasmonic nanostructures
Entangled light-matter interactions and spectroscopy
Entangled photons exhibit non-classical light–matter interactions that create new opportunities in materials and molecular science. Notably, in entangled two-photon absorption, the intensity-dependence scales linearly as if only one photon was present. The entangled two-photon absorption cross section approaches but does not match the one-photon absorption cross section. The entangled two-photon cross section also does not follow classical two-photon molecular design motifs. Questions such as these seed the rich but nascent field of entangled light–matter interactions. In this perspective, we use the experimental developments in entangled photon spectroscopy to outline the current status of the field. Now that the fundamental tools are outlined, it is time to start the exploration of how materials, molecules, and devices can control or utilize interactions with entangled photons
Layer-Resolved Ultrafast XUV Measurement of Hole Transport in a Ni-TiO2-Si Photoanode
Metal-oxide-semiconductor junctions are central to most electronic and
optoelectronic devices. Here, the element-specificity of broadband extreme
ultraviolet (XUV) ultrafast pulses is used to measure the charge transport and
recombination kinetics in each layer of a Ni-TiO2-Si junction. After
photoexcitation of silicon, holes are inferred to transport from Si to Ni
ballistically in ~100 fs, resulting in spectral shifts in the Ni M2,3 XUV edge
that are characteristic of holes and the absence of holes initially in TiO2.
Meanwhile, the electrons are observed to remain on Si. After picoseconds, the
transient hole population on Ni is observed to back-diffuse through the TiO2,
shifting the Ti spectrum to higher oxidation state, followed by electron-hole
recombination at the Si-TiO2 interface and in the Si bulk. Electrical
properties, such as the hole diffusion constant in TiO2 and the initial hole
mobility in Si, are fit from these transient spectra and match well with values
reported previously
Antiadiabatic Small Polaron Formation in the Charge Transfer Insulator ErFeO3
Small polaron formation is dominant across a range of condensed matter
systems. Small polarons are usually studied in terms of ground-state transport
and thermal fluctuations, but small polarons can also be created impulsively by
photoexcitation. The temporal response of the lattice and local electron
correlations can then be separated, such as with transient XUV spectroscopy. To
date, photoexcited small polaron formation has only been measured to be
adiabatic. The reorganization energy of the polar lattice is large enough that
the first electron-optical phonon scattering event creates a small polaron
without significant carrier thermalization. Here, we use transient XUV
spectroscopy to measure antiadiabatic polaron formation by frustrating the
iron-centered octahedra in a rare-earth orthoferrite lattice. The small polaron
is measured to take several picoseconds to form over multiple coherent charge
hopping events between neighboring Fe3+-Fe2+ sites, a timescale that is more
than an order of magnitude longer compared to previous materials. The measured
interplay between optical phonons, electron correlations, and on-site lattice
deformation give a clear picture of how antiadiabatic small polaron transport
would occur in the material. The measurements also confirm the prediction of
the Holstein and Hubbard-Holstein model that the electron hopping integral must
be larger than the reorganization energy to achieve antiadiabaticity. Moreover,
the measurements emphasize the importance of considering dynamical electron
correlations, and not just changes in the lattice geometry, for controlling
small polarons in transport or photoexcited applications
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