Rodolfo Salas

Growth and characterization of LuAs films and nanostructures

We report the growth and characterization of nearly lattice-matched LuAs/GaAs heterostructures. Electrical conductivity, optical transmission, and reflectivity measurements of epitaxial LuAs films indicate that LuAs is semimetallic, with a room-temperature resistivity of 90 μΩ cm. Cross-sectional transmission electron microscopy confirms that LuAs nucleates as self-assembled nanoparticles, which can be overgrown with high-quality GaAs. The growth and material properties are very similar to those of the more established ErAs/GaAs system; however, we observe important differences in the magnitude and wavelength of the peak optical transparency, making LuAs superior for certain device applications, particularly for thick epitaxially embedded Ohmic contacts that are transparent in the near-IR telecommunications window around 1.3 μm.

Impact of substrate characteristics on performance of large area plasmonic photoconductive emitters

We present a comprehensive analysis of terahertz radiation from large area plasmonic photoconductive emitters in relation with characteristics of device substrate. Specifically, we investigate the radiation properties of large area plasmonic photoconductive emitters fabricated on GaAs substrates that exhibit short carrier lifetimes through low-temperature substrate growth and through epitaxially embedded rare-earth arsenide (ErAs and LuAs) nanoparticles in superlattice structures. Our analysis indicates that the utilized substrate composition and growth process for achieving short carrier lifetimes are crucial in determining substrate resistivity, carrier drift velocity, and carrier lifetime, which directly impact optical-to-terahertz conversion efficiency, radiation power, radiation bandwidth, and reliability of large area plasmonic photoconductive emitters.

Growth and properties of rare-earth arsenide InGaAs nanocomposites for terahertz generation

We explore the electrical, optical, and structural properties of fast photoconductors of In0.53Ga0.47As containing a number of different rare-earth arsenide nanostructures. The rare-earth species provides a route to tailor the properties of the photoconductive materials. LuAs, GdAs, and LaAs nanostructures were embedded into InGaAs in a superlattice structure and compared to the relatively well-studied ErAs:InGaAs system. LaAs:InGaAs was found to have the highest dark resistivities, while GdAs:InGaAs had the lowest carrier lifetimes and highest carrier mobility at moderate depositions. The quality of the InGaAs overgrowth appears to have the most significant effect on the properties of these candidate fast photoconductors.

Growth and characterization of single crystal rocksalt LaAs using LuAs barrier layers

We demonstrate the growth of high‐quality, single crystal, rocksalt LaAs on III‐V substrates; employing thin well-behaved LuAs barriers layers at the III-V/LaAs interfaces to suppress nucleation of other LaAs phases, interfacial reactions between GaAs and LaAs, and polycrystalline LaAs growth. This method enables growth of single crystal epitaxial rocksalt LaAs with enhanced structural and electrical properties. Temperature-dependent resistivity and optical reflectivity measurements suggest that epitaxial LaAs is semimetallic, consistent with bandstructure calculations in literature. LaAs exhibits distinct electrical and optical properties, as compared with previously reported rare-earth arsenide materials, with a room-temperature resistivity of ∼459 μΩ-cm and an optical transmission window >50% between ∼3-5 μm.

Highly Strained Mid-Infrared Type-I Diode Lasers on GaSb

We describe how growth at low temperatures can enable increased active layer strain in GaSb-based type-I quantum-well diode lasers, with emphasis on extending the emission wavelength. Critical thickness and roughening limitations typically restrict the number of quantum wells that can be grown at a given wavelength, limiting device performance through gain saturation and related parasitic processes. Using growth at a reduced substrate temperature of 350 °C, compressive strains of up to 2.8% have been incorporated into GaInAsSb quantum wells with GaSb barriers; these structures exhibited peak room-temperature photoluminescence out to 3.96 μm. Using this growth method, low-threshold ridge waveguide lasers operating at 20°C and emitting at 3.4 μm in pulsed mode were demonstrated using 2.45% compressively strained GaInAsSb/GaSb quantum wells. These devices exhibited a characteristic temperature of threshold current of 50 K, one of the highest values reported for type-I quantum-well laser diodes operating in this wavelength range. This temperature stability is attributable to the increased valence band offset afforded by the high strain values, due to the simultaneously high quantum well indium and antimony mole fractions. Exploratory experiments using bismuth both as a surfactant during quantum well growth, as well as in dilute amounts incorporated into the crystal were also studied. Both methods appear to be promising avenues to surmount current strain-related limitations to laser performance and emission wavelength.

Offset Coupling, Feedback, and Spatial Multiplexing in 4

We experimentally evaluate the performance of incoherent multiple-input multiple-output (MIMO) multimode fiber (MMF) links with different fiber media, link lengths, and modulation techniques. The performance of conventional 62.5 μm diameter silica as well as perfluorinated plastic, graded-index MMF sections of various lengths were evaluated using 1 × 1, 2 × 2, 3 × 3 and 4 × 4 MIMO setups over a range of lengths, with both open-loop equalization and feedback-based spatial multiplexing techniques. The data rate was evaluated over silica fiber sections with lengths ranging between 100 m-3 km using distributed feedback lasers and VCSELs at the transmitters. In addition, plastic optical fiber sections with lengths ranging from 1 m - 100 m were evaluated with Fabry Perot lasers. Axial offsets were introduced while launching into, and detecting from the MMF to enhance modal diversity, and the impact of these offsets on the data rates was measured to obtain optimal launch and detection conditions. With optimized launch conditions and modulation parameters, the silica based systems realized data rates of 16 to 26 Gb/s, representing a bandwidth-length product improvement of 28× over MMF standards such as 10 GBASE-SR. The plastic fiber system reached data rates of 24 to 43 Gb/s, with bandwidth-length product increases of up to 12× of rated parameters. The use of off-axis launch and detection with flexible offsets on all four MIMO streams of the 4 × 4 system would enable further performance improvement.

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We explore the effects of surfactant-mediated epitaxy on the structural, electrical, and optical properties of fast metal-semiconductor superlattice photoconductors. Specifically, application of a bismuth flux during growth was found to significantly improve the properties of superlattices of LuAs nanoparticles embedded in In0.53Ga0.47As. These improvements are attributed to the enhanced structural quality of the overgrown InGaAs over the LuAs nanoparticles. The use of bismuth enabled a 30% increase in the number of monolayers of LuAs that could be deposited before the InGaAs overgrowth degraded. Dark resistivity increased by up to ∼15× while carrier mobility remained over 2300 cm2/V-s and carrier lifetimes were reduced by >2× at comparable levels of LuAs deposition. These findings demonstrate that surfactant-mediated epitaxy is a promising approach to enhance the properties of ultrafast photoconductors for terahert generation.

4 Incoherent-MIMO Multimode Fiber Links

Offset coupling and DSP based multiplexing techniques are evaluated on silica and plastic multimode fibers of various lengths and operating wavelengths. We demonstrate a 10-28× boost in bandwidth-length product with excellent tolerance to fiber misalignments.

Surfactant-assisted growth and properties of rare-earth arsenide InGaAs nanocomposites for terahertz generation

Mid-infrared (3-5 μm) diode lasers are important for a wide range of applications, including gas sensing. GaSb-based type-I quantum well (QW) diode lasers are attractive choices for this wavelength range, due to their temperature stability and relatively lower operating voltage. In turn, these properties yield lower power consumption at threshold than quantum cascade lasers and interband cascade lasers, which is essential for portable systems. Excellent diode lasers, based on GaInAsSb QWs and lattice-matched AlGaAsSb barriers/waveguide layers, have been demonstrated below ~3 μm with high wallplug efficiency and low threshold currents.

Enhancing data rates in graded-index multimode fibers with offset coupling and multiplexing

We investigate the electrical conductivity of GaAs-based tunnel junctions enhanced with semimetallic ErAs nanoparticles. In particular, we examine the effects of digitally-graded InGaAs alloys on the n-type side of the tunnel junction, along with different p-type doping levels. Device characteristics of the graded structures indicate that the n-type Schottky barrier may not be the limiting factor in the tunneling current as initially hypothesized. Moreover, significantly improved forward and reverse bias tunneling currents were observed with increased p-type doping, suggesting p-side limitation.