Plasma-Assisted Molecular Beam Epitaxy Growth of Mg3N2 and Zn3N2 Thin Films.

This article describes a procedure for growing Mg3N2 and Zn3N2 films by plasma-assisted molecular beam epitaxy (MBE). The films are grown on 100 oriented MgO substrates with N2 gas as the nitrogen source. The method for preparing the substrates and the MBE growth process are described. The orientation and crystalline order of the substrate and film surface are monitored by the reflection high energy electron diffraction (RHEED) before and during growth. The specular reflectivity of the sample surface is measured during growth with an Ar-ion laser with a wavelength of 488 nm. By fitting the time dependence of the reflectivity to a mathematical model, the refractive index, optical extinction coefficient, and growth rate of the film are determined. The metal fluxes are measured independently as a function of the effusion cell temperatures using a quartz crystal monitor. Typical growth rates are 0.028 nm/s at growth temperatures of 150 °C and 330 °C for Mg3N2 and Zn3N2 films, respectively.

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.

Nanoplasmonic terahertz photoconductive switch on GaAs.

Low-temperature (LT) grown GaAs has a subpicosecond carrier response time that makes it favorable for terahertz photoconductive (PC) switching. However, this is obtained at the price of lower mobility and lower thermal conductivity than GaAs. Here we demonstrate subpicosecond carrier sweep-out and over an order of magnitude higher sensitivity in detection from a GaAs-based PC switch by using a nanoplasmonic structure. As compared to a conventional GaAs PC switch, we observe 40 times the peak-to-peak response from the nanoplasmonic structure on GaAs. The response is double that of a commercial, antireflection coated LT-GaAs PC switch.

Epitaxial Nd-doped α-(Al(1-x)Ga(x))2O3 films on sapphire for solid-state waveguide lasers.

Single-crystal aluminum-gallium oxide films have been grown by molecular beam epitaxy in the corundum phase. Films of the (Al(1-x)Ga(x))(2)O(3) alloys doped with neodymium have favorable properties for solid-state waveguide lasers, including a high-thermal-conductivity sapphire substrate and a dominant emission peak in the 1090-1096 nm wavelength range. The peak position is linearly correlated to the unit cell volume, which is dependent on film composition and stress. Varying the Ga-Al alloy composition during growth will enable the fabrication of graded-index layers for tunable lasing wavelengths and low scattering losses at the interfaces.

Optical wave propagation in epitaxial Nd:Y2O3 planar waveguides.

Optical wave propagation in neodymium-doped yttrium oxide (Nd:Y(2)O(3)) films grown on R-plane sapphire substrates by molecular beam epitaxy has been studied by the prism coupler method. The measurements yield propagation loss data, the refractive index, and the dispersion relation. The refractive index of the Nd:Y(2)O(3) at 632.8 nm is found to be 1.909, and the lowest propagation loss measured is 0.9 +/- 0.2 cm(-1) at 1046 nm with a polymethyl methacrylate top cladding layer on a film with 6 nm root mean square surface roughness. The loss measurements suggest that the majority loss of this planar waveguide sample is scatter from surface roughness that can be described by the model of Payne and Lacey [Opt. Quantum Electron. 26, 977 (1994)].

Epitaxial neodymium-doped sapphire films, a new active medium for waveguide lasers.

Epitaxial films of neodymium-doped sapphire have been grown by molecular beam epitaxy on R-, A-, and M-plane sapphire substrates. The emission spectrum features sharp lines consistent with single-site doping of the Nd(3+) ion into the host crystal. This material is believed to be a nonequilibrium phase, inaccessible by conventional high-temperature growth methods. Neodymium-doped sapphire has a promising lasing line at 1096 nm with an emission cross section of 11.9x10(-19) cm(2), similar to the 1064 nm line of Nd:YVO(4).