Limiting efficiency of indoor silicon photovoltaic devices.

Energy harvesting from ambient light can be used to power wireless sensors and other standalone electronic devices. The intensity of light used for illumination is 300-3000x lower than sunlight and the spectrum of artificial light is normally narrowly concentrated in the visible range. As a result, the optimal design of photovoltaic devices for energy harvesting from ambient light differs from conventional solar cells. We calculate the maximum efficiency for Si photovoltaic devices operating under conditions expected indoors as a function of the cell thickness, taking into account the relevant properties of Si. The optimum thickness for devices operating under 250 lux illumination produced by white LED's is 1.8 µm and the efficiency is 29.0%, whereas for direct sunlight, the optimum thickness is much larger at 109 µm, while the maximum efficiency is almost the same (29.7%). The relative efficiency increases logarithmically with light intensity at 8.5% per decade.

Plasmon-enhanced LT-GaAs/AlAs heterostructure photoconductive antennas for sub-bandgap terahertz generation.

Photocurrent generation in low-temperature-grown GaAs (LT-GaAs) has been significantly improved by growing a thin AlAs isolation layer between the LT-GaAs layer and semi-insulating (SI)-GaAs substrate. The AlAs layer allows greater arsenic incorporation into the LT-GaAs layer, prevents current diffusion into the GaAs substrate, and provides optical back-reflection that enhances below bandgap terahertz generation. Our plasmon-enhanced LT-GaAs/AlAs photoconductive antennas provide 4.5 THz bandwidth and 75 dB signal-to-noise ratio (SNR) under 50 mW of 1570 nm excitation, whereas the structure without the AlAs layer gives 3 THz bandwidth, 65 dB SNR for the same conditions.

Plasmon-Enhanced below Bandgap Photoconductive Terahertz Generation and Detection.

We use plasmon enhancement to achieve terahertz (THz) photoconductive switches that combine the benefits of low-temperature grown GaAs with mature 1.5 µm femtosecond lasers operating below the bandgap. These below bandgap plasmon-enhanced photoconductive receivers and sources significantly outperform commercial devices based on InGaAs, both in terms of bandwidth and power, even though they operate well below saturation. This paves the way for high-performance low-cost portable systems to enable emerging THz applications in spectroscopy, security, medical imaging, and communication.

Nanoplasmonics enhanced terahertz sources.

Arrayed hexagonal metal nanostructures are used to maximize the local current density while providing effective thermal management at the nanoscale, thereby allowing for increased emission from photoconductive terahertz (THz) sources. The THz emission field amplitude was increased by 60% above that of a commercial THz photoconductive antenna, even though the hexagonal nanostructured device had 75% of the bias voltage. The arrayed hexagonal outperforms our previously investigated strip array nanoplasmonic structure by providing stronger localization of the current density near the metal surface with an operating bandwidth of 2.6 THz. This approach is promising to achieve efficient THz sources.