We show the effects of hole shape, size, and periodicity variation on extraordinary transmission through a tuned
metamaterial of 4 pairs of alternating layers of Ag-Al2O3 with 20 nm thicknesses under illumination of TM light.
The advantage of a metamaterial over a Ag film of similar thickness is the tunability of the surface plasmons
coupling k-vector. Because their cross sections appear the same in fourier space, incident TM light sees the
same structure for circular, square, and rectangular holes transmission is unaffected by these shape variations.
Further, as we increase the hole size, the transmission does not exhibit the expected enhancement as both the
250 nm and 150 nm diameter holes are both too small to have their maxima overlap with the tuned transmission
enhancement of the metamaterial. For the metamaterial, we are able to tune it to support surface plasmons
from periodicities ranging from 130 nm to 215 nm, which effectively shifts the transmission peaks from the blue
end of the spectrum in Ag films across the visible and into the red.
Vertically aligned bundles of TiO2 nanocrystals were fabricated by pulsed laser deposition (PLD) and tested as a
photoanode material in dye sensitized solar cells (DSSC) using scanning electron microscopy (SEM), light absorption
spectroscopy (UV-Vis), and incident photon-to-current efficiency (IPCE) experiments. An optimal background pressure
of oxygen during deposition was discovered to produce a photoanode structure that simultaneously achieves high surface
area and improves charge transport for enhanced photoelectrochemical performance. UV-Vis studies show that there is
a 1.4x enhancement of surface area for PLD-TiO2 photoanodes compared to the best sol-gel films. PLD-TiO2 DSSC
IPCE values are comparable to 3x thicker sol-gel films and nearly 92% APCE values have been observed for optimized
structures.
The minority carrier transport length (L) is a critical parameter limiting the performance of inexpensive Cu2O-ZnO photovoltaic devices. In this work, this length is determined for electrochemically deposited Cu2O by linking the optical carrier generation profile from front and back incident-photon-to-electron conversion efficiency (IPCE) measurements to a one dimensional carrier transport model. A transport length of ~ 400 nm is estimated. This critical length explains the losses typically presented by these devices. The consequences of this L on device design with the aim of improving solar cell performance are described.
We present experimental and theoretical studies of a nanopatterned photonic crystal formed between the bulk heterojunction blend,
poly-3-hexylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and nanocrystalline zinc oxide (nc-ZnO). The nanopattern is fabricated using the Pattern Replication in Non-wetting
Templates (PRINT) technique. We summarize the fabrication method and show how it can be used to make a highly ordered hexagonal array of photovoltaic P3HT:PCBM posts. We also discuss theoretical studies of optical absorption for the nanopattern design that result in a 22% enhancement over a conventional planar cell. Spectroscopic ellipsometry is also used to determine the optical constants of solar cell materials that are used in the optical model. Finally, we
calculate the local exciton creation profile within the photoactive nanopattern to relate the nanostructured geometry to electrical performance.
We describe experiments aimed at distinguishing possible mechanisms of second-harmonic generation (SHG) in
lithographically prepared arrays of metal nanoparticles. It is well-known that even-order harmonics cannot be
generated by electric dipole-dipole interactions in centrosymmetric systems. The experiment employs two basic
sample geometries. In our first geometry, as in our previous work, the NPs are left exposed to air, producing
an asymmetric local dielectric environment with ITO on one side and air on the other. In the second geometry,
we propose coating the arrays with the same material as they are created on, thus producing a centrosymmetric
environment in which any SHG observed can not be due to asymmetry in the medium, but to nonlocal or
retardation mechanisms in the particles. The arrays are fabricated using focused ion-beam lithography and vapor
deposition of the metal, followed by standard lift-off protocols. This procedure yields typical NP dimensions
between 60 nm and 200 nm in diameter, and between 15 nm and 30 nm in height, as characterized by scanning
electron and atomic-force microscopy. By tuning the NP resonances to the excitation wavelength the SHG signal
can be substantially enhanced. Surface melting effects are minimized by the use of ultra-short (50-fs) pulses
which give high intensity while allowing us to work at relatively low fluence.
Closely spaced pairs or "dimers" of elongated gold nanoparticles may be expected to exhibit electric field hotspots. We investigate the possible influence of hotspots on second harmonic generation. Preliminary results show that arrays of nanoparticle dimers exhibit reduced second-harmonic generation compared with arrays of single nanoparticles having similar extinction spectra, contradicting a simple model of second-harmonic generation (varying as the fourth power of the local fundamental field) if hotspots can be shown to exist in such gaps.
We present experimental results from second-harmonic generation studies of lithographically-prepared arrays of centrosymmetric gold nanorods, extending a previous treatment. The arrays serve as diffraction gratings, allowing control over the emission directions. The intrinsic radiation patterns from the nanoparticles are superimposed on the diffraction pattern, creating a unique angular distribution of second-harmonic light. The surface plasmon resonance mode of the particles is tuned to match the wavelength of the ultrafast Ti:sapphire excitation laser, dramatically enhancing the second-harmonic intensity but also increasing photodesorption effects. The details of the diffracted peak intensities depend sensitively on the geometry of the system and require a complex normalization of the data.
Z-scan and pump-probe measurements with ultra-fast 800 nm laser pulses were used to compare the ultrafast third-order optical nonlinearities of VO2 nanoparticles and thin films in both semiconducting and metallic states. It is found that when the samples are hold at temperatures above 67oC in their metallic state, both nanocrystals and thin films present a positive intensity-dependent nonlinear index of refraction. In this metallic state the nanocrystals exhibit a saturable optical nonlinearity and enhancement of the nonlinear effects larger than those found in thin films. Below the transition temperature, the optical nonlinearities are more complex, since they arise from alterations in the VO2 that arise both from the phase transition and from unrelated third-order nonlinear effects. Under these conditions, thin films exhibit a complete reversal to a negative nonlinear index of refraction while the nanocrystals, remarkably, show a smaller but still positive index. Pump-probe measurements on vanadium dioxide nanocrystals and thin films show they both exhibit an ultrafast response, undergoing the phase transition induced by a single laser shot in less than 120 fs. The speed of the solid-solid transformation, along with the striking reversal of the nonlinear properties across the phase transition, puts vanadium dioxide in a unique category among nonlinear materials.
Vanadium dioxide undergoes a structural (monoclinic to tetragonal) insulator-to-metal transition at 70°C, accompanied by large changes in electrical and optical properties. By combining focused ion-beam lithography and pulsed laser depo-sition, patterned arrays of vanadium dioxide nanoparticles are created that can be used for studies of linear and nonlinear optical physics, as well as demonstrating the potential for a variety of applications.
We study the resonant excitation of the electromagnetic modes in a planar waveguide of metallic walls - light incident on the guide from the air can transfer energy through the walls exciting normal modes of propagation. It is found theoretically that radiation propagates along the guide while the reflectivity presents a minimum. The energy of the incident radiation can be transferred to the guide almost completely when the thickness dm of the metallic wall is around two times the skin depth. Experimental evidence of the injection of light is presented for the system Ag/Al2O3/Ag that was grown by pulsed laser deposition.
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