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 nanowire's 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.

Efficiency above the Shockley-Queisser limit by using nanophotonic effects to create multiple effective bandgaps with a single semiconductor.

We present a pure photonic approach to overcome the Shockley-Queisser limit. A single material can show different effective bandgap, set by its absorption spectrum, which depends on its photonic structure. In a tandem cell configuration constructed from a single material, one can achieve two different effective bandgaps, thereby exceeding the Shockley-Queisser limit.

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 cell's 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.

Lossless intensity modulation in integrated photonics.

We present a dynamical analysis of lossless intensity modulation in two different ring resonator geometries. In both geometries, we demonstrate modulation schemes that result in a symmetrical output with an infinite on/off ratio. The systems behave as lossless intensity modulators where the time-averaged output optical power is equal to the time-averaged input optical power.

Experimental demonstration of two methods for controlling the group delay in a system with photonic-crystal resonators coupled to a waveguide.

We measure the group delay in an on-chip photonic-crystal device with two resonators side coupled to a waveguide. We demonstrate that such a group delay can be controlled by tuning either the propagation phase of the waveguide or the frequency of the resonators.

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.

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.

Experimental realization of an on-chip all-optical analogue to electromagnetically induced transparency.

We provide the first experimental observation of structure tuning of the electromagnetically induced transparency-like spectrum in integrated on-chip optical resonator systems. The system consists of coupled silicon ring resonators with 10 microm diameter on silicon, where the coherent interference between the two coupled resonators is tuned. We measured a transparency-resonance mode with a quality factor of 11,800.