2,943 research outputs found
Leveraging Continuous Material Averaging for Inverse Electromagnetic Design
Inverse electromagnetic design has emerged as a way of efficiently designing
active and passive electromagnetic devices. This maturing strategy involves
optimizing the shape or topology of a device in order to improve a figure of
merit--a process which is typically performed using some form of steepest
descent algorithm. Naturally, this requires that we compute the gradient of a
figure of merit which describes device performance, potentially with respect to
many design variables. In this paper, we introduce a new strategy based on
smoothing abrupt material interfaces which enables us to efficiently compute
these gradients with high accuracy irrespective of the resolution of the
underlying simulation. This has advantages over previous approaches to shape
and topology optimization in nanophotonics which are either prone to gradient
errors or place important constraints on the shape of the device. As a
demonstration of this new strategy, we optimize a non-adiabatic waveguide taper
between a narrow and wide waveguide. This optimization leads to a non-intuitive
design with a very low insertion loss of only 0.041 dB at 1550 nm.Comment: 20 pages, 9 figure
Intermediate Mirrors to Reach Theoretical Efficiency Limits of Multi-Bandgap Solar Cells
Creating a single bandgap solar cell that approaches the Shockley-Queisser
limit requires a highly reflective rear mirror. This mirror enhances the
voltage of the solar cell by providing photons with multiple opportunities for
escaping out the front surface. Efficient external luminescence is a
pre-requisite for high voltage. Intermediate mirrors in a multijunction solar
cell can enhance the voltage for each cell in the stack. These intermediate
mirrors need to have the added function of transmitting the below bandgap
photons to the next cell in the stack. In this work, we quantitatively
establish the efficiency increase possible with the use of intermediate
selective reflectors between cells in a tandem stack. The absolute efficiency
increase can be up to ~6% in dual bandgap cells with optimal intermediate and
rear mirrors. A practical implementation of an intermediate selective mirror is
an air gap sandwiched by antireflection coatings. The air gap provides perfect
reflection for angles outside the escape cone, and the antireflection coating
transmits angles inside the escape cone. As the incoming sunlight is within the
escape cone, it is transmitted on to the next cell, while most of the
internally trapped luminescence is reflected
Angular constraint on light-trapping absorption enhancement in solar cells
Light trapping for solar cells can reduce production cost and improve energy
conversion efficiency. Understanding some of the basic theoretical constraints
on light trapping is therefore of fundamental importance. Here, we develop a
general angular constraint on the absorption enhancement in light trapping. We
show that there is an upper limit for the angular integration of absorption
enhancement factors. This limit is determined by the number of accessible
resonances supported by an absorber
Light Trapping Textures Designed by Electromagnetic Optimization for Sub-Wavelength Thick Solar Cells
Light trapping in solar cells allows for increased current and voltage, as
well as reduced materials cost. It is known that in geometrical optics, a
maximum 4n^2 absorption enhancement factor can be achieved by randomly
texturing the surface of the solar cell, where n is the material refractive
index. This ray-optics absorption enhancement limit only holds when the
thickness of the solar cell is much greater than the optical wavelength. In
sub-wavelength thin films, the fundamental questions remain unanswered: (1)
what is the sub-wavelength absorption enhancement limit and (2) what surface
texture realizes this optimal absorption enhancement? We turn to computational
electromagnetic optimization in order to design nanoscale textures for light
trapping in sub-wavelength thin films. For high-index thin films, in the weakly
absorbing limit, our optimized surface textures yield an angle- and
frequency-averaged enhancement factor ~39. They perform roughly 30% better than
randomly textured structures, but they fall short of the ray optics enhancement
limit of 4n^2 ~ 50
Lithographic band gap tuning in photonic band gap crystals
We describe the lithographic control over the spectral response of three-dimensional photonic crystals. By precise microfabrication of the geometry using a reproducible and reliable procedure consisting of electron beam lithography followed by dry etching, we have shifted the conduction band of crystals within the near-infrared. Such microfabrication has enabled us to reproducibly define photonic crystals with lattice parameters ranging from 650 to 730 nm. In GaAs semiconductor wafers, these can serve as high-reflectivity (> 95%) mirrors. Here, we show the procedure used to generate these photonic crystals and describe the geometry dependence of their spectral response
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