3,502 research outputs found
Optical Properties of Bismuth Nanostructures Towards the Ultrathin Film Regime
Bulk bismuth presents outstanding optical properties, such as a giant
infrared refractive index (n near 10) and a negative ultraviolet visible
permittivity induced by giant interband electronic transitions. Although such
properties are very appealing for applications in nanophotonics, the dielectric
function of bismuth nanostructures has been scarcely studied. Here, we
determine by spectroscopic ellipsometry the far infrared to ultraviolet
dielectric function of pulsed laser deposited bismuth thin films with nominal
thickness tBi varied from near 10 nm to several tens of nm. For tBi above 15
nm, the films display a continuous structure and their dielectric function is
comparable with that of bulk bismuth. For tBi below 15 nm, the film structure
is discontinuous, and the dielectric function differs markedly from that of
bulk bismuth. It is proposed from FDTD simulations that this marked difference
arises mainly from effective medium effects induced by the discontinuous film
structure, where quantum electronic confinement does not play a dominant role.
This suggests that ultrathin and continuous bismuth films should present the
same outstanding optical properties as bulk bismuth for high performance
nanophotonic devices
The optical properties of bismuth nanowires
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.Includes bibliographical references (p. 170-177).The optical absorption of bismuth nanowires in the energy (wavenumber) range of 600 - 4000cm-1 is studied. Optical reflection and transmission spectra reveal that bismuth nanowires have a large and intense absorption peak as well as several smaller absorption peaks which are not measured in bulk bismuth. The smaller absorption peaks fit reasonably well to theoretical models for intersubband absorption in bismuth nanowires. The wire diameter, polarization, and doping dependencies as well as the spectral shape of the dominant peak agree with simulations of the optical absorption resulting from an L-point valence to T-point valence band electronic transition. The large absorption peak is present even for nanowires too large to exhibit quantum confinement, thus showing that the absorption results from a surface-induced effect and not from quantum confinement. The enhanced optical absorption in nanowires over bulk bismuth is attributed to a surface term in the matrix element which results from the spacial gradient of the dielectric function and the large dielectric mismatch between bismuth and the surrounding alumina or air. A comparison of the measured spectra with simulations of optical absorption resulting from direct L-point electronic transitions demonstrated that this absorption mechanism is not dominant in our materials. In order to explore the optical properties of bismuth nanowires, two methods were developed. First, effective medium theory applied in reverse was used to deduce the dielectric function of materials smaller than the wavelength of light. Second, a technique to fabricate nanowires with diameters above 200nm was transfered into our laboratory.(cont.) The enhanced coupling between the L-T point valence bands in nanowires may lead to a very accurate measurement of the band gap and band overlap in bismuth as a function of doping and temperature. In addition, the discovery of the enhanced interband coupling resulting from the surface contribution to the matrix element has many implications, especially if this result is applicable to other systems.by Marcie R. Black.Ph.D
Composite THz materials using aligned metallic and semiconductor microwires, experiments and interpretation
We report fabrication method and THz characterization of composite films
containing either aligned metallic (tin alloy) microwires or chalcogenide
As2Se3 microwires. The microwire arrays are made by stack-and-draw fiber
fabrication technique using multi-step co-drawing of low-melting-temperature
metals or semiconductor glasses together with polymers. Fibers are then stacked
together and pressed into composite films. Transmission through metamaterial
films is studied in the whole THz range (0.1-20 THz) using a combination of
FTIR and TDS. Metal containing metamaterials are found to have strong
polarizing properties, while semiconductor containing materials are
polarization independent and could have a designable high refractive index.
Using the transfer matrix theory, we show how to retrieve the complex
polarization dependent refractive index of the composite films. We then detail
the selfconsistent algorithm for retrieving the optical properties of the metal
alloy used in the fabrication of the metamaterial layers by using an effective
medium approximation. Finally, we study challenges in fabrication of
metamaterials with sub-micrometer metallic wires by repeated stack-and-draw
process by comparing samples made using 2, 3 and 4 consecutive drawings. When
using metallic alloys we observe phase separation effects and nano-grids
formation on small metallic wires
Evidence of robust 2D transport and Efros-Shklovskii variable range hopping in disordered topological insulator (Bi2Se3) nanowires
We report the experimental observation of variable range hopping conduction
in focused-ion-beam (FIB) fabricated ultra-narrow nanowires of topological
insulator (Bi2Se3). The value of the exponent in the hopping equation was
extracted as ~ 1/2 for different widths of nanowires, which is the proof of the
presence of Efros-Shklovskii hopping transport mechanism in a strongly
disordered system. High localization lengths (0.5nm, 20nm) were calculated for
the devices. A careful analysis of the temperature dependent fluctuations
present in the magnetoresistance curves, using the standard Universal
Conductance Fluctuation theory, indicates the presence of 2D topological
surface states. Also, the surface state contribution to the conductance was
found very close to one conductance quantum. We believe that our experimental
findings shed light on the understanding of quantum transport in disordered
topological insulator based nanostructures.Comment: 14pages, 4 figure
High-Q bismuth silicate nonlinear glass microsphere resonators
The fabrication and characterization of a bismuth-silicate glass microsphere resonator has been demonstrated. At wavelengths near 1550 nm, high-modes can be efficiently excited in a 179 µm diameter bismuth-silicate glass microsphere via evanescent coupling using a tapered silica fiber with a waist diameter of circa 2 µm. Resonances with Q-factors as high as were observed. The dependence of the spectral response on variations in the input power level was studied in detail to gain an insight into power-dependent thermal resonance shifts. Because of their high nonlinearity and high- factors, bismuth-silicate glass microspheres offer the potential for robustly assembled fully integrated all-optical switching devices
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