Sunil Sandhu

Wireless energy transfer with the presence of metallic planes

We numerically demonstrated that efficient wireless energy transfer can be achieved between two high Q resonators in a complex electromagnetic environment. In particular, in the close proximity of metallic planes, efficient wireless energy transfer can be achieved with proper system designs.

Dynamically tuned coupled-resonator delay lines can be nearly dispersion free

We investigate dispersion effects in dynamically tuned, coupled-resonator delay lines. Provided that the system is tuned to a zero-bandwidth state, a signal can be delayed indefinitely with almost no dispersion. We present a theoretical analysis of such a light-stopping system and verify the results using numerical simulations.

Advances in Theory of Photonic Crystals

In this paper, the authors review some of the recent advances in the theory of photonic crystals, drawing examples from their own work in magnetooptical and dynamic photonic crystals. The combination of theory and simulations shows that these crystal structures exhibit rich optical physics effects and can provide new ways to accomplish sophisticated optical information-processing tasks

Detailed balance analysis and enhancement of open-circuit voltage in single-nanowire solar cells.

We present a detailed balance analysis of current density-voltage modeling of a single-nanowire solar cell. Our analysis takes into account intrinsic material nonidealities in order to determine the theoretical efficiency limit of the single-nanowire solar cell. The analysis only requires the nanowires absorption cross-section over all angles, which can be readily calculated analytically. We show that the behavior of both the current and voltage is due to coherent effects that arise from resonances of the nanowire. In addition, we elucidate the physics of open-circuit voltage enhancement over bulk cells in nanowires, by showing that the enhancement is related to the removal of resonances in the immediate spectral vicinity above the bandgap.

Tuning the coherent interaction in an on-chip photonic-crystal waveguide-resonator system

We experimentally demonstrated that the characteristics of the coherent interaction between two waveguide-coupled resonators can be drastically tuned by changing the propagation phase in the waveguide. In particular, the transmission line shape can be tuned between an electromagnetically induced transparency like optical resonance and a flat-top reflection filter.

Aligning microcavity resonances in silicon photonic-crystal slabs using laser-pumped thermal tuning

We report the postfabrication alignment of multiple microcavity resonances in silicon photonic-crystal (PhC) slabs using laser-pumped thermal tuning. The thermal gradient resulting from a focused laser spot was used to differentially tune the resonant wavelengths of two microcavities spaced about 50μm apart. The resonant wavelengths could be brought closer together, over a tunable range of more than 5nm. A cross over in the resonant wavelengths was demonstrated, showing that two microcavities can be tuned to the identical wavelength. The results show that differential thermal tuning can be used to remove slight fabrication differences in nominally identical microcavities, relaxing the fabrication tolerances that will be required to realize coupled-resonator structures in PhCs.

Detailed balance analysis of nanophotonic solar cells

We present a detailed balance based approach for performing current density-voltage characteristic modeling of nanophotonic solar cells. This approach takes into account the intrinsic material non-idealities, and is useful for determining the theoretical limit of solar cell efficiency for a given structure. Our approach only requires the cells absorption spectra over all angles, which can be readily calculated using available simulation tools. Using this approach, we elucidate the physics of open-circuit voltage enhancement over bulk cells in nanoscale thin film structures, by showing that the enhancement is related to the absorption suppression in the immediate spectral region above the bandgap. We also show that with proper design, the use of a grating on a nanoscale thin film can increase its short-circuit current, while preserving its voltage-enhancing capabilities.

Fundamental bounds on decay rates in asymmetric single-mode optical resonators.

We derive tight upper and lower bounds of the ratio between decay rates to two ports from a single resonance exhibiting Fano interference, based on a general temporal coupled-mode theory formalism. The photon transport between these two ports involves both direct and resonance-assisted contributions, and the bounds depend only on the direct process. The bounds imply that, in a lossless system, full reflection is always achievable at Fano resonance, even for structures lacking mirror symmetries, while full transmission can only be seen in a symmetric configuration where the two decay rates are equal. The analytic predictions are verified against full-field electromagnetic simulations.

Stopping and time reversing a light pulse using dynamic loss tuning of coupled-resonator delay lines

We introduce a light-stopping process that uses dynamic loss tuning in coupled-resonator delay lines. We demonstrate via numerical simulations that increasing the loss of selected resonators traps light in a zero group velocity mode concentrated in the low-loss portions of the delay line. The large dynamic range achievable for loss modulation should increase the light-stopping bandwidth relative to previous approaches based on refractive index tuning.

Condition for perfect antireflection by optical resonance at material interface

Reflection occurs at an air–material interface. The development of antireflection schemes, which aims to cancel such reflection, is important for a wide variety of applications including solar cells and photodetectors. Recently, it has been demonstrated that a periodic array of resonant subwavelength objects placed at an air–material interface can significantly reduce reflection that otherwise would have occurred at such an interface. Here, we introduce the theoretical condition for complete reflection cancellation in this resonant antireflection scheme. Using both general theoretical arguments and analytical temporal coupled-mode theory formalisms, we show that in order to achieve perfect resonant antireflection, the periodicity of the array needs to be smaller than the free-space wavelength of the incident light for normal incidence, and also the resonances in the subwavelength objects need to radiate into air and the dielectric material in a balanced fashion. Our theory is validated using first-principles full-field electromagnetic simulations of structures operating in the infrared wavelength ranges. For solar cell or photodetector applications, resonant antireflection has the potential for providing a low-cost technique for antireflection that does not require nanofabrication into the absorber materials, which may introduce detrimental effects such as additional surface recombination. Our work here provides theoretical guidance for the practical design of such resonant antireflection schemes.