53 research outputs found
Ideal near-field thermophotovoltaic cells
We ask the question, what are the ideal characteristics of a near-field
thermophotovoltaic cell? Our search leads us to a reformulation of near-field
radiative heat transfer in terms of the joint density of electronic states of
the emitter-absorber pair in the thermophotovoltaic system. This form reveals
that semiconducting materials with narrowband absorption spectra are critical
to the energy conversion efficiency. This essential feature is unavailable in
conventional bulk semiconductor cells but can be obtained using low dimensional
materials. Our results show that the presence of matched van Hove singularities
resulting from quantum-confinement in the emitter and absorber of a
thermophotovoltaic cell boosts both the magnitude and spectral selectivity of
radiative heat transfer; dramatically improving energy conversion efficiency.
We provide a model near-field thermophotovoltaic system design making use of
this idea by employing the van Hove singularities present in carbon nanotubes.
Shockley-Queisser analysis shows that the predicted heat transfer
characteristics of this model device are fundamentally better than existing
thermophotovoltaic designs. Our work paves the way for the use of quantum dots,
quantum wells, two-dimensional semiconductors, semiconductor nanowires and
carbon nanotubes as future materials for thermophotovoltaic cells.Comment: 9 pages, 5 figure
High temperature epsilon-near-zero and epsilon-near-pole metamaterial emitters for thermophotovoltaics
We propose a method for engineering thermally excited far field
electromagnetic radiation using epsilon-near-zero metamaterials and introduce a
new class of artificial media: epsilon-near-pole metamaterials. We also
introduce the concept of high temperature plasmonics as conventional
metamaterial building blocks have relatively poor thermal stability. Using our
approach, the angular nature, spectral position, and width of the thermal
emission and optical absorption can be finely tuned for a variety of
applications. In particular, we show that these metamaterial emitters near 1500
K can be used as part of thermophotovoltaic devices to surpass the full
concentration Shockley-Queisser limit of 41%. Our work paves the way for high
temperature thermal engineering applications of metamaterials.Comment: 15 pages, 8 figure
Broadband super-Planckian thermal emission from hyperbolic metamaterials
We develop the fluctuational electrodynamics of metamaterials with hyperbolic
dispersion and show the existence of broadband thermal emission beyond the
black body limit in the near field. This arises due to the thermal excitation
of unique bulk metamaterial modes, which do not occur in conventional media. We
consider a practical realization of the hyperbolic metamaterial and estimate
that the effect will be observable using the characteristic dispersion
(topological transitions) of the metamaterial states. Our work paves the way
for engineering the near-field thermal emission using metamaterials
Hierarchical Mean-Field Operator Bounds on Electromagnetic Scattering: Upper Bounds on Near-Field Radiative Purcell Enhancement
We show how the central equality of scattering theory, the definition of the
operator, can be used to generate hierarchies of mean-field
constraints that act as natural complements to the standard electromagnetic
design problem of optimizing some objective with respect to structural degrees
of freedom. Proof-of-concept application to the problem of maximizing radiative
Purcell enhancement for a dipolar current source in the vicinity of a
structured medium, an effect central to many sensing and quantum technologies,
yields performance bounds that are frequently more than an order of magnitude
tighter than all current frameworks, highlighting the irreality of these models
in the presence of differing domain and field-localization length scales.
Closely related to domain decomposition and multi-grid methods, similar
constructions are possible in any branch of wave physics, paving the way for
systematic evaluations of fundamental limits beyond electromagnetism
Maximum Electromagnetic Local Density of States via Material Structuring
The electromagnetic local density of states (LDOS) is crucial to many aspects
of photonics engineering, from enhancing emission of photon sources to
radiative heat transfer and photovoltaics. We present a framework for
evaluating upper bounds on LDOS in structured media that can handle arbitrary
bandwidths and accounts for critical wave scattering effects with no heuristic
approximations. The bounds are solely determined by the bandwidth, material
susceptibility, and device footprint, with no assumptions on geometry. We
derive an analytical expression for the maximum LDOS consistent with the
conservation of energy across the entire design domain, which upon benchmarking
with topology-optimized structures is shown to be nearly tight for large
devices. Novel scaling laws for maximum LDOS enhancement are found: the bounds
saturate to a finite value with increasing susceptibility and scale as the
quartic root of the bandwidth for semi-infinite structures made of lossy
materials, with direct implications on material selection and design
applications.Comment: Corrected minor typos throughout paper; corrected mislabel of inverse
designs in Figure 1; added full Supplementary Information; added
acknowledgment
Material Scaling and Frequency-Selective Enhancement of Near-Field Radiative Heat Transfer for Lossy Metals in Two Dimensions via Inverse Design
The super-Planckian features of radiative heat transfer in the near-field are
known to depend strongly on both material and geometric design properties.
However, the relative importance and interplay of these two facets, and the
degree to which they can be used to ultimately control energy flow, remains an
open question. Recently derived bounds suggest that enhancements as large as
are possible between extended structures (compared to blackbody);
but neither geometries reaching this bound, nor designs revealing the predicted
material () scaling, have been previously reported. Here, exploiting
inverse techniques, in combination with fast computational approaches enabled
by the low-rank properties of elliptic operators for disjoint bodies, we
investigate this relation between material and geometry on an enlarged space
structures. Crucially, we find that the material proportionality given above
does indeed emerge in realistic structures. In reaching this result, we also
show that (in two dimensions) lossy metals such as tungsten, typically
considered to be poor candidate materials for strongly enhancing heat transfer
in the near infrared, can be structured to selectively realize flux rates that
come within of those exhibited by an ideal pair of resonant lossless
metals for separations as small as of a tunable design wavelength.Comment: 6 pages, 2 figure
-operator bounds on angle-integrated absorption and thermal radiation for arbitrary objects
We derive fundamental per-channel bounds on angle-integrated absorption and
thermal radiation for arbitrary bodies---for any given material susceptibility
and bounding region---that simultaneously encode both the per-volume limit on
polarization set by passivity and geometric constraints on radiative
efficiencies set by finite object sizes through the scattering
-operator. We then analyze these bounds in two practical settings,
comparing against prior limits as well as near optimal structures discovered
through topology optimization. Principally, we show that the bounds properly
capture the physically observed transition from the volume scaling of
absorptivity seen in deeply subwavelength objects (nanoparticle radius or thin
film thickness) to the area scaling of absorptivity seen in ray optics
(blackbody limits).Comment: 9 pages including appendices, 2 figures, 1 tabl
Fundamental limits to attractive and repulsive Casimir--Polder forces
We derive upper and lower bounds on the Casimir--Polder force between an
anisotropic dipolar body and a macroscopic body separated by vacuum via
algebraic properties of Maxwell's equations. These bounds require only a coarse
characterization of the system---the material composition of the macroscopic
object, the polarizability of the dipole, and any convenient partition between
the two objects---to encompass all structuring possibilities. We find that the
attractive Casimir--Polder force between a polarizable dipole and a uniform
planar semi-infinite bulk medium always comes within 10% of the lower bound,
implying that nanostructuring is of limited use for increasing attraction. In
contrast, the possibility of repulsion is observed even for isotropic dipoles,
and is routinely found to be several orders of magnitude larger than any known
design, including recently predicted geometries involving conductors with sharp
edges. Our results have ramifications for the design of surfaces to trap,
suspend, or adsorb ultracold gases.Comment: 6 pages, 3 figure
Fundamental limits to radiative heat transfer: theory
Near-field radiative heat transfer between bodies at the nanoscale can
surpass blackbody limits on thermal radiation by orders of magnitude due to
contributions from evanescent electromagnetic fields, which carry no energy to
the far-field. Thus far, principles guiding explorations of larger heat
transfer beyond planar structures have assumed utility in surface
nanostructuring, which can enhance the density of states, and further assumed
that such design paradigms can approach Landauer limits, in analogy to
conduction. We derive fundamental shape-independent limits to radiative heat
transfer, applicable in near- through far-field regimes, that incorporate
material and geometric constraints such as intrinsic dissipation and finite
object sizes, and show that these preclude reaching the Landauer limits in all
but a few restrictive scenarios. Additionally, we show that the interplay of
material response and electromagnetic scattering among proximate bodies means
that bodies which maximize radiative heat transfer actually maximize scattering
rather than absorption. Finally, we compare our new bounds to existing Landauer
limits, as well as limits involving bodies maximizing far-field absorption, and
show that these lead to overly optimistic predictions. Our results have
ramifications for the ultimate performance of thermophotovoltaics and nanoscale
cooling, as well as related incandescent and luminescent devices.Comment: 12 pages including appendices, 1 figure; SM and PSV contributed
equall
Fundamental Limits on Second Harmonic Generation
Recent advances in fundamental performance limits for power quantities are
proving to provide a powerful theoretical tool for understanding the
optimization of wave phenomena. To date, however, in any approach allowing
power conservation to be enforced locally, the linearity of the wave equation
plays a critical role. In this manuscript, we generalize the current
quadratically constrained quadratic program framework for evaluating linear
photonics limits to incorporate nonlinear processes under the undepleted pump
approximation. Via the exemplary objective of enhancing second harmonic
generation in a (free-form) wavelength-scale structure, we illustrate a model
constraint scheme that can be used in conjunction with standard convex
relaxations to bound performance in the presence of nonlinear dynamics.
Representative bounds are found to anticipate features observed in optimized
structures discovered via computational inverse design. Extensions of our
results to other frequency-conversion processes, including Raman scattering and
four-wave mixing, are straightforward.Comment: 10 pages, 4 figure
- …