Enhanced electrorefraction in symmetric narrow coupled quantumwells

The quadratic electrooptic coefficient of narrow symmetric-coupled GaAs-Al_0.2Ga_0.8As quantum wells has been measured for light having a range of detuned wavelengths with TE polarization and is found to be 2-3 times larger than that of single quantum wells equivalent overall width     

Intersubband optical second-harmonic generation in multiple quantumwells: radiative coupling effects

An analysis of intersubband optical second-harmonic generation (SHG) in multiple quantum wells (QW's) is presented, in which special attention has been paid to radiative coupling effects. Starting from the Maxwell-Lorentz equations, the field at the second-harmonic frequency in the QW system is determined. Following the same framework, the field at the fundamental frequency is also obtained. In a reflection geometry, the SH conversion efficiency is derived for a p-polarized incident field. Detailed numerical calculations for different numbers of QW's, angles of incidence, as well as barrier thicknesses show that the SHG conversion efficiency can be significantly modified due to radiative coupling among wells     

Band Tail Modeling of Bulk InGaAsP

The accuracy of waveguide normalized photocurrent spectroscopy is quantified and used to assess the accuracy of the parabolic band Franz-Keldysh model of electroabsorption and refraction in the band tail of bulk InGaAsP     

Carrier recombination rates in strained-layer InGaAs-GaAs quantumwells

The carrier recombination rates in semiconductor quantum wells are found to be structure dependent. Under high levels of excitation they generally do not follow the recombination rule of the bulk material. Through a differential carrier-lifetime measurement in the strained-layer InGaAs/GaAs quantum wells, it is shown that in quantum wells with lower potential barrier or thinner well width, the recombination rates are smaller due to a larger portion of the injected carriers populating the confinement layers where the carriers recombine more slowly owing to dilute carrier volume density     

Optical beam steering using InGaAsP multiple quantum wells

We report an efficient optical beam steering device based on InGaAsP multiple quantum wells. An area-selective zinc in-diffusion process is used to define highly localized p-n junctions through which electrical currents are injected into the quantum wells. The extent of the lateral spreading of the electrical carriers can be optimized by selecting the appropriate diffusion depth. Using a twin-parallel-stripe structure, an optical beam at a wavelength of 1.51 /spl mu/m was steered over a 17-/spl mu/m range using dc electrical currents of less than 13 mA.     

Nonlinear absorption properties of AlGaAs/GaAs multiple quantumwells grown by metalorganic chemical vapor deposition

The nonlinear absorption properties of the excitonic resonances associated with multiple quantum wells (MQWs) in AlGaAs/GaAs grown by metalorganic chemical vapor deposition are reported. The dependence of the saturation properties on growth parameters, especially growth temperature, and the well width are described. The minimum measured saturation intensity for these materials is 250 W/cm², the lowest reported value to date. The low saturation intensities are the result of excellent minority carrier properties. A systematic study of minority carrier lifetimes in quantum wells are reported. Lifetimes range from 50-350 ns depending on growth temperature and well width     

Design of InGaAsP multiple quantum-well Fabry-Perot modulators forsoliton control

The use of synchronous optical modulators is effective in reducing the pulse timing jitter in long-distance soliton transmission. The inherently polarization-insensitive characteristics of the Fabry-Perot multiple quantum-well (MQW) electroabsorption modulator make it a potentially suitable device for this application. We investigate the intensity and phase modulation characteristics of symmetric and asymmetric Fabry-Perot modulators, and show that, by positioning the resonant wavelength <30 nm away from the exciton absorption peak to obtain negative chirp operation, both configurations can be used to successfully reduce timing jitter in a 20 Gb/s soliton system     

Impurity-free intermixing in compressively strained InGaAsP multiple quantum well structures

We report on controlled band gap modification in a compressively strained InGaAsP multi-quantum well-laser structure using different encapsulating layers followed by rapid thermal processing (RTP). The structure used was designed as a 1.55 μm laser with an active region consisting of three In_0.76Ga_0.24As_0.85P_0.15 quantum wells with In_0.76Ga_0.24As_0.52P_0.48 barriers grown by metal organic chemical vapor deposition. The heterostructure is capped with 100 nm thick InGaAs layer. Prior to RTP, the samples were coated with various dielectric layers or a thin film of low temperature (300°C) grown InP. Using a Si_xN_y film deposited by plasma-enhanced chemical vapor deposition with a refractive index of about 2.0, quantum well intermixing (QWI) was effectively suppressed. The suppression effect was independent of the Si_xN_y film thickness for layers of 30–2400 nm. With an e-beam-evaporated SiO₂ film, QWI was enhanced and a net blue shift of about 100 nm can be achieved between the samples covered with SiO₂ and Si_xN_y after RTP at 750°C for 100 s. Furthermore, InP grown at a low temperature by gas-source molecular beam epitaxy was proved to be even more efficient in enhancing QWI. Group V interstitial diffusion is used to explain the enhanced QWI between the wells and adjacent barriers which have the same group III compositions. Two-section tunable laser operated around 1.55 μm based on this laser structure was fabricated using this technique.     

Optimized second-harmonic generation in asymmetric double quantumwells

We present a theoretical analysis of surface-incidence and waveguide-mode second harmonic generation with detuned intersubband transitions in GaAs-AlGaAs, InGaAs-InAlAs and GaSb-InGaSb-AlGaSb asymmetric double quantum wells. The analysis includes the effects of absorption, saturation, pump depletion, optical carrier heating, mode confinement and competition, and the loss of phase coherence due to waveguide, bulk and resonant intersubband contributions to the refractive index mismatch. Optimal structures have been determined for each material system in both surface-incidence and waveguide-mode geometries. A scheme for maintaining phase matching by incorporation of a separate region with an intersubband transition tuned midway between the first and second harmonic frequencies is analyzed. At 10.6 μm, the maximum conversion efficiency for the optimized InGaAs-InAlAs waveguide-mode device is ≈16% at a pump-beam intensity of 40 MW/cm ². Furthermore, the same device can be modulated to vanishing second harmonic output power when an electric field of -32 kV/cm is applied     

Impact of amplified spontaneous emission on carrier density formeasurement of optical nonlinearities in GaAs/AlGaAs multiple quantumwells

It is shown that amplified spontaneous emission (ASE) can affect the measurement of nonlinear optical properties in semiconductors through its reduction of the excess carrier density. When the optical excitation area is large, lateral ASE can reduce the carriers within a much shorter period of time than the normal carrier lifetime. Just after ASE is over, the time-integrated surface photoluminescence signal may be used as a measure of the carrier density, which is the carrier density experienced by an ns probe in ps pump experiments. When the excited carrier density is at 10²⁰ cm^-3, surface ASE is also possible and is observed, even with small spot sizes     

Enhanced intersubband absorption in stepped double barrier quantumwells

Intersubband absorption is measured in the conduction band of GaAs and stepped GaAs/In_xGa_1-xAs multiple-quantum-wells confined by narrow AlAs barriers. Enhanced absorption from n=1 to n=2 is observed in the stepped wells. This is attributed to relaxation of the intersubband polarisation selection rule     

Optical gain of interdiffused InGaAs-As and AlGaAs-GaAs quantumwells

We have analyzed theoretically the effects of interdiffusion on the gain, differential gain, linewidth enhancement factor, and the injection current density of In_0.2Ga_0.8As-GaAs and Al_0.3Ga_0.7As-GaAs quantum-well (QW) lasers. We have calculated the electron and hole subband structures including the effects of valence band mixing and strains. The optical gain is then calculated using the density matrix approach. Our results show that the gain spectrum can be blue-shifted without an enormous increase in the injected current density. Imposing an upper limit (416 A·cm^-2) on the injection current density for a typical laser structure, we find that the InGaAs-GaAs and AlGaAs-GaAs QW lasers can be blue-shifted by 24 and 54 mn, respectively. Our theoretical results compare well with the tuning ranges of 53 and 66 meV found for AlGaAs-GaAs QWs in some experiments. This indicates that the interdiffusion technique is useful for the tuning of laser operation wavelength for multiwavelength applications     

Sum frequency generation by intersubband transition in step quantumwells

Using two CO₂ lasers, sum frequency generation was achieved in step quantum wells via intersubband transition processes. The power from each continuous wave (CW) CO₂ lasers was kept low such that the total power onto the sample was below 2 W. The observed sum frequency signal obeyed the polarization selection rule predicted by the simple effective mass envelope function model. The experimentally determined and calculated second order susceptibility values are in reasonable agreement     

Large Stark shift of the interband transition in two-step quantumwells

Large and near-linear Stark shifts of the electron-heavy-hole ground state excitonic transition were observed in photoluminescence (PL) measurements for a two-step quantum-well (TSQW) structure. The Stark shift was 40 meV while a corresponding square well shifted only 20 meV at a field of 70 kV/cm. The observed Stark shifts agreed well with calculations. The large Stark shift of the TSQW was achieved on a global-to-local state transition realized via tailor-made quantum well (QW) parameters. This structure is an ideal candidate for optoelectronic devices based on the quantum confined Stark effect (QCSE)     

The “gain-lever" effect in InGaAsP/InP multiple quantum welllasers

The gain-lever-effect has been assessed in both multiple quantum well (MQW) and bulk active region lasers having a range of lengths and split ratios in the top contact. Compared with a conventional single-contact laser, a 500-μm MQW FP device with a top-contact split ratio of 8:1 exhibited >15-dB improvement in AM efficiency and a signal-to-noise ratio improvement of 7.5 dB. With proper impedance matching, these figures may be improved. A reduced noise figure is obtained at the expense of dynamic range with an additional nonlinearity seen experimentally due to thermal and carrier leakage when compared with simulations carried out using a harmonic balance model. Used carefully, these gain-lever lasers are useful components for future analogue fiber-optic systems     

Efficient electroabsorption in InGaAsP/InGaAsP MQW optical waveguide

Efficient electroabsorption in an InGaAsP/InGaAsP MQW optical waveguide modulator structure is reported. A 17 dB extinction ratio is obtained by applying a 3.5 V drive voltage to a 78 mu m long waveguide operating at 1.54 mu m under TE-polarisation mode. The on-state attenuation is only 2 dB.<>     

Observation of multiple resonance frequencies in stripe geometryInGaAs/InGaAsP/InP quantum-well lasers

Intensity fluctuation noise in strained InGaAsP/InP multi-quantum-well lasers is analyzed for both ridge-guided and broad-area gain-guided structures. A single resonance peak is observed in the noise spectrum for the ridge-guided laser, as expected. However, the noise spectrum for the broad-area lasers shows multiple (~2-5) resonance peaks, distinctly spaced, from ~2 to ~5 GHz. Combined with near-field measurements, the experiments show that these peaks originate from lasing filaments having significantly nonuniform optical power. The authors also determined the resonance frequency of the single-mode laser from both small-signal modulation and turn-on relaxation oscillation measurements and found the results to be consistent with the measured peak noise frequency     

Room temperature excitonic nonlinear absorption and refraction in GaAs/AlGaAs multiple quantum well structures

We present detailed experimental studies and modeling of the nonlinear absorption and refraction of GaAs/AlGaAs multiple quantum well structures (MQWS) in the small signal regime. Nonlinear absorption and degenerate four-wave mixing in the vicinity of the room temperature exciton resonances are observed and analyzed. Spectra of the real and imaginary parts of the nonlinear cross section as a function of wavelength are obtained, and these are in excellent agreement with experimental data. A simple model for excitonic absorption saturation is proposed; it accounts qualitatively for the very low saturation intensities of room temperature excitons in MQWS.     

Photoluminescence and interface abruptness in InGaAsP/InGaAsP quantum wells

Quantum well (QW) structures consisting of InGaAsP wells and InGaAsP barriers grown by gas-source molecular beam epitaxy have been examined by low temperature photoluminescence (PL) in order to evaluate the contributions of compositional fluctuations in the quaternary alloy and of interface roughness to the PL linewidth. The well material was InGaAsP with a bandgap corresponding to a wavelength of 1.3 μm and the barrier material was InGaAsP of 1.15 μm. The theory for QW excitonic linewidths as a function of well thickness L_z due to fluctuations in alloy composition has been extended to include the case of the quaternary InGaAsP barrier. If the interfaces are atomically abrupt, the linewidth is dominated by compositional fluctuations in the well at large L_z and compositional fluctuations in the barrier at small L_z. The theory predicts a weak dependence of the linewidth on L_z since the composition of the well and barrier are similar. For rough heterointerfaces, the theory indicates the usual increase in linewidth with decreasing L_z. Photoluminescence measurements at 13K in arrays of single InGaAsP/InGaAsP QWs with L_z from 1.0 to 6.0 nm show only a weak variation of the full width at half maximum (FWHM) with L_z, in agreement with the theory for smooth interfaces. Furthermore, the lowest measured FWHM of 8.9 meV was found for a narrow well of L_z=1.8 nm, indicating the InGaAsP/InGaAsP interfaces are smooth and that the PL linewidth is dominated by compositional fluctuations.     

Experimental and theoretical analysis of the carrier distributionin asymmetric multiple quantum-well InGaAsP lasers

The authors present an experimental and theoretical analysis of the carrier distribution in multiple quantum-well (MQW) lasers and the effect of this carrier distribution on the gain of wells at different locations in the active region. An experimental technique using mirror image asymmetric multiple quantum-well (AMQW) lasers is described which provides quantitative information on the degree to which the carrier distribution affects the gain of quantum wells (QWs) in the active region. A gain model for AMQW lasers is developed and used to explain some important characteristics of AMQW devices. A rate equation model is presented which incorporates the effects of fields across the p-i-n junction active region. The model is able to predict experimental results measured from thirteen AMQW laser structures to within experimental uncertainty