The influence of single-photon absorption on the performance of thetwo-photon waveguide autocorrelator

The propagation loss and the single-photon absorption coefficients in a two-photon waveguide autocorrelator are measured as a function of wavelength. The propagation loss was as low as 1.37 cm^-1 at a wavelength of 940 nm in a device with an Al_0.2Ga_0.8As waveguide core. Using a photocurrent technique, band-to-band absorption was measured for photon energies well below that of the bandgap. It was demonstrated that, although the band-to-band absorption coefficient is small (~10^-2 cm^-1 at a wavelength of 1 μm), it is responsible for reducing the contrast ratio of the waveguide autocorrelator. It is suggested that the single-photon absorption takes place via deep levels with relatively long carrier lifetimes     

All-optical photonic switches using integrated inverted asymmetricFabry-Perot modulators and heterojunction phototransistors

InGaAs/GaAs(100) multiple-quantum-well-based inverted cavity asymmetric Fabry-Perot modulators are vertically integrated with GaAs/AlGaAs heterojunction phototransistors to yield all-optical photonic switches. The photonic switches using `normally on' modulator pixels exhibit an output on-off ratio of 12:1 with internal optical gain of 4 dB. The photonic switches using `normally off' modulator pixels yield similar contrast and gain, but exhibit intrinsic bistable behavior. The inverted cavity modulators employed permit utilizing the transparency of the GaAs substrate at the operating wavelength and offer advantages for fabricating large arrays for optical signal processing     

High-contrast/low-voltage normally on InGaAs/AlGaAs asymmetricFabry-Perot modulator

An asymmetric Fabry-Perot cavity modulator is proposed with a 50-period In_0.15Ga_0.85As/Al_0.30Ga_0.70 As strained-layer superlattice active layer. The back Bragg reflector consists of 25 periods of GaAs/AlAs layers while for the front reflector of the device the natural reflection of the air-semiconductor interface was used. Spectral measurements as a function of the applied reverse voltages showed a change in reflectivity from 33% for 0 V reverse bias to 5% for 7 V, at a wavelength of 969 nm. This gives a maximum contrast ratio of 8.3 dB and an insertion loss of 4.9 dB. For higher voltages applied across the device, the reflectivity increases again     

Low-voltage superlattice asymmetric Fabry-Perot reflection modulator

A normally off transverse superlattice asymmetric Fabry-Perot modulator that has a contrast ratio of more than 26:1 at the Fabry-Perot resonance, and a reflection change of 33% at a wavelength of approximately 20 degrees AA away from Fabry-Perot mode for an operating voltage swing of <3 V is discussed. The structure contains an active region of 52 periods of a 30 AA GaAs-30 AA Al/sub 0.3/Ga/sub 0.7/As superlattice, embedded between top and bottom quarter-wave grating mirrors. The modulation is achieved by reducing the cavity loss at the Fabry-Perot resonance through the field-induced effective absorption edge blue-shift in the superlattice.<>     

Quantum interference control of electrical currents in GaAs

In an earlier publication, preliminary observations of the generation of electrical currents were reported in GaAs and low-temperature-grown GaAs (LT-GaAs) at 295 K using quantum interference control of single- and two-photon band-band absorption of 1.55- and 0.775-μm ultrashort optical pulses. Time-integrated currents were measured via charge collection in a metal-semiconductor-metal (MSM) electrode structure. Here we present detailed characteristics of this novel effect in terms of a simple circuit model for the MSM device and show how the injected current depends on MSM parameters as well as optical coherence, power, and polarization. For picosecond pulse excitation with peak irradiance of only 30 MW/cm^-2 (1.55 μm) and 9 kW/cm^-2 (0.775 μm), peak current densities of ~10 A/cm^-2 at peak carrier densities of 10¹⁵ cm ^-3 are inferred from the steady-state signals. This compares with 50 A/cm^-2 predicted theoretically; the discrepancy mainly reflects inefficient charge collection at the MSM electrodes     

20 dB contrast GaAs/AlGaAs multiple quantum-well nonresonantmodulators

The authors report a successful fabrication of nonresonant GaAs/AlGaAs multiple quantum well (MQW) modulators with a contrast ratio of more than 20 dB and a wide bandwidth of 3 nm. The optical path, as long as 8 μm, is attained by fabricating a high-purity thick MQW layer with a residual carrier concentration of less than 10¹⁴ cm^-3 and introducing a high-reflectivity AlAs/AlGaAs quarter-wavelength mirror. The driving wavelengths are located at the absorption edge, resulting in the low-absorption loss     

Measurement of differential carrier lifetime in vertical-cavitysurface-emitting lasers

Measurements of differential carrier lifetimes on gain-guided proton-implanted vertical-cavity surface-emitting lasers with device size as a parameter are reported. The lifetimes were obtained from laser impedance measurements and from small-signal modulation optical response at subthreshold currents. A simple small-signal equivalent circuit was used to correct the optical data and to extract the carrier lifetimes from the impedance data. Carrier lifetimes ranged from 4.2 ns at 0.04 mA, to about 0.6 ns at a bias close to threshold. The measured carrier lifetimes were used to calculate the corresponding threshold carrier density (n_th~6×10¹⁸ cm^-3) and recombination parameters     

Room temperature stimulated emission from a CdTe/CdMgTesuperlattice laser structure in the red spectral range

We have investigated the optical response of a red emitting CdTe/CdMgTe laser structure after optical excitation. For the first time, stimulated emission has been observed in this material system at room temperature, demonstrating the potential for the development of CdTe/CdMgTe laser structures in the visible spectral range. An analysis of our experimental data yields a threshold carrier density of about 3.5·10¹² cm^-2 and a net gain coefficient of about 95 cm^-1 at T=300 K     

Experimental studies of proton-implanted GaAs-AlGaAsmultiple-quantum-well modulators for low-photocurrent applications

We describe the first attempts to control photocurrent, and thus power dissipation, in surface-normal multiple-quantum-well (MQW) modulators. We have made detailed experimental studies of proton-implanted p-i-n GaAs-Al_xGa_1-xAs MQW modulators having barrier layers of x=0.3, 0.45, and 1.0. Structures were implanted to levels of 1×10¹² cm^-2, 1×10¹³ cm^-2, and 1×10¹⁴ cm ^-2. Photocurrent progressively decreased with increasing implant-dose and barrier mole fraction (x). Exciton linewidths showed a strong voltage and implant dose dependence, demonstrating a tradeoff between photocurrent and modulation performance. We obtained our best results with x=1.0 barriers. For example, 1×10¹³ cm^-2-implanted asymmetric Fabry-Perot modulators were realized in which the optical performance was similar to that of unimplanted devices. The photocurrent responsivity was, however, only 0.007 A/W at 12.5 V bias. We report measurements of carrier lifetime in these materials that show the reduction in photocurrent arises from a reduction in lifetime due to implant-induced damage. In addition, the reduced lifetime decreases the optically-excited quantum-well carrier population, leading to an increase in cw saturation intensity. Specifically, 1×10¹³ cm^-2-implanted devices with x=1.0 have a saturation intensity of roughly 45 kW/cm², while unimplanted devices have 3.5 kW/cm². Asymmetric self electro-optic effect devices (A-SEED's) are demonstrated, and power dissipation issues associated with the use of low-photocurrent modulators in integrated systems are discussed     

Analysis and design of surface-normal Fabry-Perot electroopticmodulators

Design equations relating the refractive index shift, absorption change, cavity length, and grating properties of Fabry-Perot modulators are derived. Experimental results of reflection modulators with GaAs/AlGaAs multiple-quantum-well active regions are given. The design equations are applied by investigating the optimization of the reflectivity modulation and the sensitivity of the performance on the operating conditions. The modulation is found to be determined mainly by the index shift and is less sensitive to the accompanying absorption change for a chirp greater than one where the chirp is defined as the ratio of the real part of index change to the imaginary part. Modulation can also be obtained via absorption modulation when the chirp is small. However, the residual loss in the cavity with multiple quantum wells should be kept lower than 500 cm^-1     

Absorption loss coefficient of the active layer for 1.48-μm bulkand MQW lasers

It is shown that the absorption loss coefficient of the active layer for 1.48-μm bulk lasers is 66 cm^-1 which is between 45 and 107 cm^-1 for 1.3-μm bulk lasers and for 1.55-μm bulk lasers, respectively. It is also described that the absorption loss coefficient of the active layer for 1.48-μm multiple-quantum-well (MQW) lasers is 28 cm^-1 which is about two-fifths of that for 1.48-μm bulk lasers. Therefore, the high slope efficiency of the 1.48-μm MQW lasers is attributed not only to the small optical confinement factor but also to the small absorption loss coefficient of the active layer     

Improved performances of a S-SEED using extremely shallowquantum-wells and asymmetric Fabry-Perot cavity structure

The authors report significantly improved performances of a symmetric self-electrooptic-effect device (S-SEED), with high on-off contrast ratio (>30:l) and large optical bistability loop widths (ΔP=44%) at an applied bias of V_a=0 V, i.e., with no power supply. The S-SEED is made of extremely shallow quantum wells (ESQWs) in an asymmetric Fabry-Perot (AFP) cavity structure. At V_a =5 V ΔP increased by up to 95%, preserving the high contrast ratio. The reflectivity changes at V_a=0 and 5 V were about 15% and 30%, respectively. These are believed to be the largest values ever reported for such structures     

Characterization of ErAs:GaAs and LuAs:GaAs Superlattice Structures for Continuous-Wave Terahertz Wave Generation through Plasmonic Photomixing

We investigate the impact of ErAs:GaAs and LuAs:GaAs superlattice structures with different LuAs/ErAs nanoparticle depositions and superlattice geometries on terahertz radiation properties of plasmonic photomixers operating at a 780-nm optical wavelength. Our analysis indicates the crucial impact of carrier drift velocity and carrier lifetime on the performance of plasmonic photomixers. While higher carrier drift velocities enable higher optical-to-terahertz conversion efficiencies by offering higher quantum efficiencies, shorter carrier lifetimes allow achieving higher optical-to-terahertz conversion efficiencies by mitigating the negative impact of destructive terahertz radiation from slow photocarriers and preventing the carrier screening effect.     

Measurement of field-induced refractive index variation in GaAs/AlGaAs superlattice using monolithic Fabry-Perot etalon

The electrorefractive effect in a GaAs/AlGaAs superlattice was measured by using a monolithic Fabry-Perot structure, where the superlattice active region is embedded inside a cavity, formed by two quarter-wave stacks, all grown by molecular beam epitaxy. At the wavelength approximately 7675 AA, there is a large absorption change ( approximately 5400/cm) and an accompanying index change ( approximately 0.015) when the applied field change is approximately 100 kV/cm. In addition, the chirp parameter at this wavelength is less than 1. This result shows that superlattice electroabsorption optical modulation can offer relatively very low chirp.<>     

A comparison of optoelectronic properties of lattice-matched andstrained quantum-well and quantum-wire structures

The k·p formalism is used to study the absorption spectra, material and differential gain in quantum wires as a function of orientation, built-in strain, and wire dimensions. The results for material and differential gain are compared with those for an optimized quantum-well structure. We find that for quantum wires at 300 K, the gain becomes positive at a carrier density of 1.27·10¹⁸ cm^-3, while in quantum wells this density is calculated to be 1.82·10¹⁸ cm^-3. Incorporating tensile strain in the wires reduces the transparency carrier concentration to 0.96·10¹⁸ cm^-3 while compressive strain allows one to obtain positive gain for densities greater than 1.08·10¹⁸ cm^-3. Orienting the wire along the [111] direction reduces the transparency carrier density to 0.60·10¹⁸ cm^-3. The differential gain in quantum-well structures for injections near the threshold is on the order of 10^-14 cm^-4, while for 50 Å·100-Å quantum wires the differential gain near the threshold is found to be on the order of 10^-13 cm^-4 . The differential gain in wires whose wire axis is parallel to the [111] direction has also been found to be on the order of 10^-13 cm^-4 for carrier injections close to the threshold     

High-Efficiency 808-nm InGaAlAs–AlGaAs Double-Quantum-Well Semiconductor Lasers With Asymmetric Waveguide Structures

The InGaAlAs-AlGaAs double-quantum-well semiconductor lasers grown by molecular beam epitaxy show high quantum efficiency and high power conversion efficiency at continuous-wave power output using asymmetric waveguide structures. The threshold current density and slope efficiency of the device are 180 A/cm² and 1.4 W/A, respectively. The internal loss and the internal quantum efficiency are 1.1 cm^-1 and 97%, respectively. The 75% maximum power conversion efficiency is achieved in 100-mum stripe widths 808-nm-emitting laser diodes with 1000-mum cavity length.     

1.3 μm InGaAs/GaAs strained quantum well lasers with InGaPcladding layer

Using MOVPE, we fabricated strained quantum well 1.3 μm lasers with an InGaP cladding layer on a GaAs substrate. The lasers had a high gain coefficient of 60 cm^-1. Lasers with high reflection facets had a low threshold current density of 500 A/cm², and a high characteristic temperature of 100 K     

Low Dit, thermodynamically stable Ga2O3-GaAs interfaces: fabrication, characterization, and modeling

Thermodynamically stable, low D_it amorphous Ga₂ O₃-(100) GaAs interfaces have been fabricated by extending molecular beam epitaxy (MBE) related techniques. We have investigated both in situ and ex situ Ga₂O₃ deposition schemes utilizing molecular beams of gallium oxide. The in situ technique employs Ga₂O₃ deposition on freshly grown, atomically ordered (100) GaAs epitaxial films in ultrahigh vacuum (UHV); the ex situ approach is based on thermal desorption of native GaAs oxides in UHV prior to Ga₂O₃ deposition. Unique electronic interface properties have been demonstrated for in situ fabricated Ga₂O₃-GaAs interfaces including a midgap interface state density D_it in the low 10¹⁰ cm^-2 eV^-1 range and an interface recombination velocity S of 4000 cm/s. The existence of strong inversion in both n- and p-type GaAs has been clearly established. We will also discuss the excellent thermodynamic and photochemical interface stability. Ex situ fabricated Ga₂O₃-GaAs interfaces are inferior but still of a high quality with S=9000 cm/s and a corresponding D_it in the upper 10¹⁰ cm^-2 eV^-1 range. We also developed a new numerical heterostructure model for the evaluation of capacitance-voltage (C-V), conductance-voltage (G-V), and photoluminescence (PL) data. The model involves selfconsistent interface analysis of electrical and optoelectronic measurement data and is tailored to the specifics of GaAs such as band-to-band luminescence and long minority carrier response time τ_R. We will further discuss equivalent circuits in strong inversion considering minority carrier generation using low-intensity light illumination     

Heterostructure injection lasers

A state-of-the-art discussion of heterostructure injection lasers is presented. These lasers are first considered as configurations of uniformly pumped semiconductor slabs that constitute the transmission medium of multilayered slab waveguides, whose boundaries are trapless heterojunctions. Emphasis is on lasers whose electrically active and waveguiding layers are AlxGa1-xAs or GaAs, although several other semiconductors are discussed. Single, double, and separate confinement hetrostructure lasers, all of which yield carrier and optical confinement normal to the plane of the heterojunctions, are described. Lateral electrical and optical confinement in various stripe geometry double-hetrostructure lasers is illustrated. In addition to the more common Fabry-Perot cavity lasers, there are now also several new varieties that use corrugated surfaces as Bragg gratings for internal distributed feedback or reflection. Some of these heterostructure lasers have permitted the demonstration of CW-room temperature injection lasers with low threshold current densities (< 1000 A/cm²), low dc current (∼15 mA), long lives (>10⁵) h), and a considerable degree of control over modes and beam divergence. In addition, the use of heterostructures has permitted the demonstration of the first integratedoptical structures that include both active light generating and passive elements on a single chip.     

Spectroscopic evaluation of Yb3+-doped glasses foroptical refrigeration

The absorption and emission properties of Yb³⁺-doped ZBLANP, BIGaZYT and QX/Yb phosphate glasses are studied to evaluate their potential for laser-induced fluorescent cooling or optical refrigeration. The efficiency of optical refrigeration increases with pump wavelength in the anti-Stokes region. The cooling efficiencies of the three glasses as a function of temperature are evaluated at the wavelength λp corresponding to the absorption coefficient of 10 ^-3 cm^-1. For temperatures <110 K, the cooling efficiency of the BIGaZYT glass may be more than twice that of the ZBLANP