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

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

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

Plasmon Fizeau drag in 3D Dirac and Weyl semimetals

There is a need for compact, dynamically tunable nonreciprocal optical elements to enable on-chip-compatible optical isolators and more efficient radiative energy transfer systems. Plasmon Fizeau drag, the drag of electrical current on propagating surface plasmon polaritons, has been proposed to induce nonreciprocal surface modes to enable one-way energy transport. However, relativistic electron drift velocities are required to induce appreciable contrast between the dispersion characteristics of co-propagating and counter-propagating surface plasmon modes. The high electron drift velocity of graphene previously allowed for the experimental demonstration of current-induced nonreciprocity in a two-dimensional (2D) Dirac material. The high electron drift and Fermi velocities in three-dimensional (3D) Dirac materials make them ideal candidates for the effect, however, both the theory of the Fizeau drag effect and its experimental demonstrations in 3D Dirac materials are missing. Here we develop a comprehensive theory of Fizeau drag in DC-biased 3D Weyl semimetals (WSM) or Dirac semimetals (DSM), both under local and non-local approximation and with dissipative losses. We predict that under practical assumptions for loss, Fizeau drag in the DSM Cd$_3$As$_2$ opens windows of pseudo-unidirectional transport. We additionally introduce new figures of merit to rank nonreciprocal plasmonic systems by their potential for directional SPP transport. Further, we propose a new approach for achieving appreciable plasmonic Fizeau drag via optically pumping bulk inversion symmetry breaking WSMs or DSMs

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

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