Carrier-induced Change Research Articles

Measurement of carrier-induced changes of the complex refractive index in InGaAsP near the band edge

A new method has been developed to measure the complex refractive index changes of a thin film medium, using a probe light perpendicularly transversing the surface and a small interference-ellipsometry bridge. Carrier-induced changes of the refractive index and absorption have been measured in InGaAsP near the band edge using this method. Experimental results are compared with calculations.

Refractive index and loss changes produced by current injection in InGaAs(P)-InGaAsP multiple quantum-well (MQW) waveguides

We have theoretically estimated the carrier-induced changes in the refractive index /spl delta/n and the optical loss /spl delta//spl alpha/ produced by the injection of free carriers in InGaAs(P)-InP multiple quantum-well (MQW) optical waveguides. MQW structures are specially designed for the tuning layer in carrier injection type tunable laser diodes, such as tunable twin-guide laser diode (TTG-LD), at the lasing wavelength /spl lambda/ of 1.55 /spl mu/m. Anomalous dispersion and plasma effect for /spl delta/n and IVBA (intervalance band absorption) for /spl delta//spl alpha/ were included. A valance-band mixing, an overlap integral of electron-hole, and confinement factors of carriers in the well were considered to include quantum-size effect of carriers in /spl delta/n and /spl delta//spl alpha/. Predictions of /spl delta/n are in reasonably good agreement with the experimental results. Systematic analysis shows that the following InGaAs(P)(well)-InGaAsP (barrier) MQW structures ace promising in order to obtain a large /spl delta/nat a given injection current density J: 1) well materials: InGaAsP alloy with its bulk bandgap wavelength of around 1.55 /spl mu/m, 2) barrier materials: InGaAsP alloy with its bulk bandgap wavelength of around 1.4 /spl mu/m, 3) as many as possible number of wells, typically about 15./spl delta//spl alpha/ is also estimated by calculating the carrier distributions in MQW structures and by fitting experimental data of bulk and MQW waveguides. The maximum well number is limited by the increase of optical loss. According to these results, we have found that 1.55-/spl mu/m InGaAsP (well 1.8-nm thick)/1.40 /spl mu/m InGaAsP (barrier 14-nm thick) MQW, with the well number of around 15, will be the optimum tuning layer structure for 1.55-/spl mu/m band tunable LD's. Over 10 nm continuous tuning range, with moderate output power, is expected for TTG-LD's. >

Carrier-Induced Changes in The Gap States of Undoped a-Si:H Films

ABSTRACTCarrier-induced changes in the gap states of plasma-enhanced-chemical-vapor-deposited (PECVD) undoped a-Si:H films are systematically studied using isothermal capacitance transient spectroscopy (ICTS) method via the novel structure we proposed previously. The density-of-state distribution g (E) and the energy dependence of electron-capture cross section σn (E) of gap states in undoped a-Si:H films before and after injection of electrons are directly measured for the first time. Experimental results show that the density of g (E) increases but the σn (E) has no obvious change after the electron injection. These indicate that the defects created during the electron injection are the same in type as those of the as-deposited films. In addition, it is found the shape of g (E) distorts after the injection, implying that the gap states with different energy levels are associated with distinct types of defects.

Biexciton effects in femtosecond nonlinear transmission of semiconductor quantum dots

The dynamics of carrier-induced absorption changes in CdSe quantum dots are investigated with femtosecond spectroscopy. After excitation with 4-eV photons a redshift of the lowest optical transition is observed in the initial phase of carrier relaxation. This shift is attributed to a biexciton effect where two electron-hole pairs interact via the Coulomb potential.

Influence of reflectivity on femtosecond transmission spectroscopy of thin GaAs films

The femtosecond dynamics of the nonlinear transmission and reflectivity in films of GaAs are investigated. Both nonlinear optical responses are required to determine carrier-induced absorption changes. This has significant implications on the interpretaton of spectral hole burning experiments in semiconductors.

Reflectivity measurements of femtosecond carrier and field dynamics in semiconductors

Carrier and field dynamics in a photoconductive switch are investigated by voltage modulated reflectivity. Measurements are performed with single, femtosecond pulsed and cw probe beams, and with pulsed pump and probe beams. We investigate the spatial dependence of the reflectivity changes and show that for mode-locked probe light the signal at the positive electrode completely dominates the response. The comparison of above and below bandgap excitation and reflectivity changes resolves the contributions of carrier-induced field changes and those due to the modulation of the applied field. Photoconductive switches on p- and n-doped substrates show that field enhancement at the Schottky barriers has only secondary influence. We explain the results in terms of a two-dimensional field distribution which produces the largest field gradient and hence acceleration of carriers at the electrodes and that the higher mobility of electrons accounts for the dominant signal appearing at the positive electrode.

Balanced detection technique to measure small changes in transmission

A novel technique to measure changes in optical transmission of better than 10(-3) with time resolution of better than 10 ns is discussed. The photocurrent from two photodiodes is subtracted with a balanced detection circuit to eliminate the common signal; carrier-induced change in absorption for a semiconductor single quantum well produces a large differential signal.

Optical absorption and refraction spectra in highly excited GaSb

A theoretical estimate of carrier-induced changes in absorption and refraction spectra in GaSb is presented. The partly phenomenological electron-hole plasma model including band filling, band-gap renormalization, screening of the Coulomb interaction and the free-carrier plasma effect is used. A dominant contribution of the band filling and screening of the continuum-state Coulomb enhancement is observed. For the first time the influence of high lying conduction band valleys is taken into account. In addition, the model is applied to calculate the absorption and refraction spectra in doped material. The obtained results may prove to be useful in the design of GaSb optoelectronic devices. >

Carrier-induced change in index, gain, and lifetime for (InAs)1/(GaAs)4 superlattice lasers

The carrier density dependence of gain, index, and carrier lifetime has been measured for (InAs)1/(GaAs)4 short-period superlattice lasers. The 500-μm-long lasers have a threshold current density of 200 A/cm2 and exhibit a small carrier-induced change in refractive index (Δn/ΔN∼2×10−22 cm3). The small Δn/ΔN also results in small linewidth enhancement factor (α∼1.1) of these lasers. The measured dependence of carrier lifetime on carrier density can be fitted to a bimolecular recombination model which suggests that radiative recombination is the dominant recombination mechanism in these structures.

Carrier-induced change in the refractive index of InAs1−xSbx

Abstract An estimate of refractive index change due to the free carriers in InAs1−xSbx alloy is performed. Band filling, bandgap renormalization and the free-carrier plasma effect are included in the theoretical model utilized. The dispersion of Δn near the bandgap edge is investigated for carrier densities of 1015–1018cm-3. The largest change in refractive index is obtained for InAs0.45Sb0.65.

Calculation of the carrier-induced refractive index change in InSb

We report a theoretical model for an estimate of carrier-induced changes in the refractive index of InSb at room temperature. The dispersion of Δn within the photon-energy interval of 0.12–0.24 eV is investigated for nonequilibrium carrier concentrations of 1015–1018 cm−3. An analysis of the influence of the initial carrier density on Δn is performed. The obtained results can help to develop InSb opto-electronic devices.

Optically activated integrated optic Mach–Zehnder interferometer on GaAs

An optically activated guided-wave Mach–Zehnder interferometer has been constructed and used to study photoinduced phase modulation in GaAs channel waveguides. A π radian phase shift at the optical wavelength of 1.15 μm has been achieved with modulating pulse energy of 320 pJ at an interaction length of 22 μm. Our experiment also indicates that for infrared (IR) light with photon energy far below the band-gap energy of the semiconductor material, the free carrier-induced refractive index changes are much more significant than the free carrier-induced absorption changes.

AlGaAs grating surface-emitting beam deflector with ridge structure

A ridge-waveguide-type AlGaAs grating surface-emitting beam deflector was fabricated. The deflection is controlled by carrier-induced refractive-index change of the waveguide. The maximum deflection angle change is about 0.6 degrees , which corresponds to about ten resolvable spots. The temperature raising of the active layer was estimated in comparison with the characteristics for direct and pulsed current operation. It was 6.5 degrees C for the injection current (DC) of 20 mA.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A 1.7 Gb/s ASK and a 622 Mb/s incoherent FSK transmission experiment using a 1.55- mu m multiquantum well distributed feedback laser

A multiple-quantum-well distributed-feedback (MQW-DFB) laser with narrow linewidth and low frequency chirp at low output power may experience linewidth rebroadening at high output power. the rebroadening is mostly due to a large carrier-induced change of refractive index, which also causes a large frequency modulation response for the MQW-DFB lasers. Using a 1.55- mu m MQW-DFB laser, a 622-Mb/s amplitude-shift-keying (ASK) transmission experiment employing 200-km of fiber and an erbium-doped fiber amplifier has been demonstrated having a dispersion power penalty less than 9.8 dB. The receiver sensitivities at BER=10/sup -9/ of the ASK system are -34.5 dBm and -42.5 dBm for 1.7-Gb/s and 622-Mb/s modulation, respectively. A 622-Mb/s incoherent frequency-shift-keying (FSK) transmission experiment using the same laser has also achieved a receiver sensitivity of -42.5 dBm. >

Diatomic-hydrogen-complex dissociation: A microscopic model for metastable defect generation in Si.

Using first-principles calculations, we demonstrate an explicit reaction pathway for carrier-induced changes in hydrogenated Si through dissociation of paired-H complexes. The complex dissociates through the capture of charge carriers with calculated kinetic barriers for electron- and hole-induced dissociations of 1.1 and 0.9 eV, respectively, in crystalline Si, in general agreement with the experimentally observed activation energies for single-carrier defect creation in hydrogenated a-Si. Exciton lowering of the barrier accounts for the low activation energy of light-induced defect formation.

Carrier-induced refractive-index change, mode gain and spontaneous-emission factor in AlGaInP SQW-SCH laser diodes

The carrier-induced refractive-index change, mode gain and spontaneous emission factor are evaluated for AlGaInP single-quantum-well laser diodes with separate-confinement heterostructures. The results are compared with those of AlGaInP double-heterostructure lasers. The values of the α parameter, defined as the ratio of the real part of the refractive index to the imaginary part of the refractive index, are estimated as 4.9 and 12.8.

Carrier-induced change in refractive index of InP, GaAs and InGaAsP

The change in refractive index Delta n produced by injection of free carriers in InP, GaAs, and InGaAsP is theoretically estimated. Bandfilling (Burstein-Moss effect), bandgap shrinkage, and free-carrier absorption (plasma effect) are included. Carrier concentrations of 10/sup 16//cm/sup 3/ to 10/sup 19//cm/sup 3/ and photon energies of 0.8 to 2.0 eV are considered. Predictions for Delta n are in reasonably good agreement with the limited experimental data available. Refractive index changes as large as 10/sup -2/ are predicted for carrier concentrations of 10/sup 8//cm/sup 3/ suggested that low-loss optical phase modulators and switches using carrier injection are feasible in these materials.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Carrier-induced shift and broadening of optical spectra in an AlxGa1-xAs/GaAs quantum well with a gate electrode.

We have investigated the carrier-induced changes in the photoluminescence (PL) and excitation spectra (PLE) of an n-type ${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As/GaAs quantum-well structure at 10 K by controlling the electron concentration (${N}_{s}$=0--${10}^{12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) via gate electric fields. By comparison with local-density-functional theory, the red shift of the PL peak and the blue shift of the PLE spectra could be ascribed to band-gap renormalization (\ensuremath{\sim}18 meV at ${N}_{s}$=1\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) and to band-filling effect. While appreciable portions of the PL and the PLE linewidths are attributed to thermal broadening, the presence of additional broadening is noted and discussed in terms of the localization of carriers, the Auger effect, and other mechanisms.

Optically controlled GaAs/AlAs multiple quantum well modulators employing integrated dielectric reflectors

We investigate all-optical switching in GaAs/AlAs multiple quantum well devices with integrated dielectric reflector. A 7 mW control beam at λc=790 nm wavelength produces a 15% reflection change for a test beam at λt=858 nm which is close to the lowest energy transition in the quantum wells (λ=868 nm). Switching dynamics are shown to be limited by excess carrier lifetimes of τ≊4 ns corresponding to a 3 dB frequency of 70 MHz. A basic analysis in terms of carrier-induced absorption change by dynamic band filling provides a good description of the experiments.

Carrier-induced refractive-index change in quantum-well lasers

We investigated the loaded gain and the carrier-induced refractive-index change in quantum-well lasers. A quantum-well laser is found typically to have a higher gain and a smaller refractive-index change than a conventional diode laser. We also found that the gain and the refractive-index change for these two types of semiconductor lasers saturate similarly, with the refractive index being a much weaker function of laser intensity.