431 research outputs found
Coupling of whispering-gallery modes in size-mismatched microdisk photonic molecules
Mechanisms of whispering-gallery (WG) modes coupling in microdisk photonic
molecules (PMs) with slight and significant size mismatch are numerically
investigated. The results reveal two different scenarios of modes interaction
depending on the degree of this mismatch and offer new insight into how PM
parameters can be tuned to control and modify WG-modes wavelengths and
Q-factors. From a practical point of view, these findings offer a way to
fabricate PM microlaser structures that exhibit low thresholds and directional
emission, and at the same time are more tolerant to fabrication errors than
previously explored coupled-cavity structures composed of identical
microresonators.Comment: 3 pages with 5 figures (to appear in Opt. Lett. 2007
Exceeding the solar cell Shockley-Queisser limit via thermal up-conversion of low-energy photons
Maximum efficiency of ideal single-junction photovoltaic (PV) cells is
limited to 33% (for one sun illumination) by intrinsic losses such as band edge
thermalization, radiative recombination, and inability to absorb below-bandgap
photons. This intrinsic thermodynamic limit, named after Shockley and Queisser
(S-Q), can be exceeded by utilizing low-energy photons either via their
electronic up-conversion or via thermophotovoltaic (TPV) conversion process.
However, electronic up-conversion systems have extremely low efficiencies, and
practical temperature considerations limit the operation of TPV converters to
the narrow-gap PV cells. Here we develop a conceptual design of a hybrid TPV
platform, which exploits thermal up-conversion of low-energy photons and is
compatible with conventional silicon PV cells by using spectral and directional
selectivity of the up-converter. The hybrid platform offers
sunlight-to-electricity conversion efficiency exceeding that imposed by the S-Q
limit on the corresponding PV cells across a broad range of bandgap energies,
under low optical concentration (1-300 suns), operating temperatures in the
range 900-1700K, and in simple flat panel designs. We demonstrate maximum
conversion efficiency of 73% under illumination by non-concentrated sunlight. A
detailed analysis of non-ideal hybrid platforms that allows for up to 15% of
absorption/re-emission losses yields limiting efficiency value of 45% for Si PV
cells.Comment: 28 pages, 9 figure
Band splitting and Modal Dispersion induced by Symmetry braking in Coupled-Resonator Slow-Light Waveguide Structures
We study the dispersion relations in slow-light waveguide structures
consisting of coupled microdisk resonators. A group theoretical analysis of the
symmetry properties of the propagating modes reveals an interesting phenomenon:
The degeneracy of the CW and CCW rotating modes is removed, giving rise to two
distinct transmission bands. This effect induces symmetry-based dispersion
which may limit usable bandwidth of such structures. The properties of this
band splitting and its impact on CROW performance for optical communications
are studied in detail
Search optimization techniques in the creation of an online store website
Business organizations involved in e-commerce recognize the importance of how commerce searches are conducted by individual customers. In 2005, 80% of all Website visits began with Internet searching. Altering a Website so that it ranks higher for particular terms used in Web searches is called \u27search engine optimization.\u27 The present study investigates how search engine marketing activities may result in higher rankings by such search engines as Google, Yahoo, MSN and Ask. One method of search engine marketing is illustrated by optimizing a new Website, www.thelightingdivision.com. The results may interest online marketers and search engine optimizers wishing to increase the visibility of their business enterprise and considering search engine studies as a part of their advertising campaigns
Q-factor and emission pattern control of the WG modes in notched microdisk resonators
Two-dimensional (2-D) boundary integral equation analysis of a notched
circular microdisk resonator is presented. Results obtained provide accurate
description of optical modes, free from the staircasing and discretization
errors of other numerical techniques. Splitting of the double degenerate
Whispering-Gallery (WG) modes and directional light output is demonstrated. The
effect of the notch depth and width on the resonance wavelengths, Q-factors,
and emission patterns is studied. Further improvement of the directionality is
demonstrated in an elliptical notched microdisk. Applications of the notched
resonators to the design of microdisk lasers, oscillators, and biosensors are
discussed.Comment: 7 pages with 11 figures; to appear in IEEE J. Select. Topics Quantum.
Electron., Jan/Feb 200
Spectral shift and Q-change of circular and square-shaped optical microcavity modes due to periodic sidewall surface roughness
Radiation loss and resonant frequency shift due to sidewall surface roughness
of circular and square high-contrast microcavities are estimated and compared
by using a boundary integral equations method. An effect of various harmonic
components of the contour perturbation on the Whispering-Gallery (WG) modes in
the circular microdisk and WG-like modes in the square microcavity is
demonstrated. In both cases, contour deformations that are matched to the mode
field pattern cause the most significant frequency detuning and Q-factor
change. Favorably mode-matched deformations have been found, enabling one to
manipulate the Q-factors of the microcavity modes.Comment: 5 pages with 6 figure
Efficient analysis and design of low-loss whispering-gallery-mode coupled resonator optical waveguide bends
Waveguides composed of electromagnetically-coupled optical microcavities
(coupled resonator optical waveguides or CROWs) can be used for light guiding,
slowing and storage. In this paper, we present a two-dimensional analysis of
finite-size straight and curved CROW sections based on a rigorous Muller
boundary integral equations method. We study mechanisms of the coupling of
whispering gallery (WG) modes and guiding light around bends in CROWs composed
of both identical and size-mismatched microdisk resonators. Our accurate
analysis reveals differences in WG modes coupling in the vicinity of bends in
CROWs composed of optically-large and wavelength-scale microcavities. We
propose and discuss possible ways to design low-loss CROW bends and to reduce
bend losses. These include selecting specific bend angles depending on the
azimuthal order of the WG mode and tuning the radius of the microdisk
positioned at the CROW bend.Comment: 8 pages with 10 figures (to appear in IEEE/OSA J. Lightwave
Technology, 2007
Limiting efficiencies of solar energy conversion and photo-detection via internal emission of hot electrons and hot holes in gold
We evaluate the limiting efficiency of full and partial solar spectrum
harvesting via the process of internal photoemission in Au-semiconductor
Schottky junctions. Our results based on the ab initio calculations of the
electron density of states (e-DOS) reveal that the limiting efficiency of the
full-spectrum Au converter based on hot electron injection is below 4%. This
value is even lower than previously established limit based on the parabolic
approximation of the Au electron energy bands. However, we predict limiting
efficiency exceeding 10% for the hot holes collection through the Schottky
junction between Au and p-type semiconductor. Furthermore, we demonstrate that
such converters have more potential if used as a part of the hybrid system for
harvesting high- and low-energy photons of the solar spectrum.Comment: Proc. SPIE 9608, Infrared Remote Sensing and Instrumentation XXIII,
960816 (September 1, 2015) 7 pages, 4 figure
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