Free Carrier Gratings Research Articles

Charge carrier trapping by dislocations in single crystal diamond

Charge carrier trapping in diamond crystals containing well-defined concentrations of dislocations was investigated by several complementary techniques. Samples with dislocation densities ndis between <1 × 107 and ≈1 × 109 cm−2 were grown heteroepitaxially on Ir/YSZ/Si(001). In optical pump–probe experiments, ambipolar diffusion coefficients were determined from the decay of light-induced transient free carrier gratings. Modeling their variation with excitation density yielded trapping cross sections σ of 29 and 10 nm for the dislocations and a stress-field-induced reduction in exciton binding energies from 80 to 73 and 60 meV at ndis = 1 × 108 and 1 × 109 cm−2, respectively. The lifetime measured by induced absorption scaled proportional to 1/ndis with absolute values ranging from 0.1 to 10 ns. In the electrical measurements on two sets of detector slices, electron–hole pairs were excited by α-particles and transport was measured separately for electrons and holes. Both types of carriers showed fast transient current signals. The time constant of the additional slow component exclusively seen for holes was in agreement with the activation energy of boron acceptors. Their concentration of ≈0.5 ppb yielded σ = 1.77 × 10−13 cm2 for charged point traps. Schubweg and carrier lifetime due to deep trapping roughly reproduced the 1/ndis trend. For electrons at 3 V/μm, a value σ = 40 nm was deduced. Cross sections for holes were significantly smaller. Differences in hole trapping between the samples are attributed to charging of dislocations controlled by chemical impurities. Increase in lifetime at high voltages is explained by reduced capture cross sections for hot carriers.

Bragg holography in active semiconductor microcavities

High-speed dynamic holography is studied numerically and experimentally in broad-area InGaAs/InP multiple-quantum-well asymmetric Fabry–Perot microcavities with optically pumped gain gratings operating near the lasing threshold. Two numerical approximation methods are developed to predict Bragg diffraction effects in these active microcavities. The Cavity Raman–Nath method assumes multipass Raman–Nath diffraction, while the Cavity Bragg method assumes an effective medium. Both methods predict that Raman–Nath diffraction in a micrometer-thick cavity resonator can exhibit Bragg-like selectivity for angles near the Bragg angle. The angular and spectral selectivities are studied experimentally in optically pumped free-carrier gratings using a tunable 1.55 μm laser in a nondegenerate four-wave mixing configuration. The device operates at gigahertz speeds with a transient diffraction efficiency near 40%.

Active holography in InGaAs/InP quantum-well microcavities

High-efficiency dynamic holography at 1.55 μm is demonstrated in a broad-area InGaAs/InP multiple-quantum-well vertical microcavity. The design places single quantum wells at the cavity antinodes, reducing mode-pulling and enabling a higher Q-factor. The device is pumped by interference fringes through an amorphous mirror that is transparent to a high-energy hologram writing pulse at a wavelength of 1.06 μm. Optically pumped free carrier gratings are probed by a tunable 1.5 μm laser in a four-wave mixing configuration. Diffraction efficiency into both m=±1 diffraction orders of 35% (70% total) has been obtained with a phase grating contribution approaching the maximum π phase shift by combining absorption bleaching with asymmetric Fabry-Perot reflectivity. The diffracted signal exhibits rise/fall times of 5 ns, demonstrating the high speed capabilities of this device.

Comparative Studies of Carrier Dynamics in 3C-SiC Layers Grown on Si and 4H-SiC Substrates

Time-resolved nonlinear optical techniques were applied to determine the electronic parameters of cubic silicon carbide layers. Carrier lifetime, τ, and mobility, μ, were measured in a free-standing wafer grown on undulant Si and an epitaxial layer grown by hot-wall chemical vapor deposition (CVD) on a nominally on-axis 4H-SiC substrate. Nonequilibrium carrier dynamics was monitored in the 80 K to 800 K range by using a picosecond free carrier grating and free carrier absorption techniques. Correlation of τ(T) and μ a(T) dependencies was explained by the strong contribution of diffusion-limited recombination on extended defects in the layers. A lower defect density in the epitaxial layer on 4H-SiC was confirmed by a carrier lifetime of 100 ns, being ~4 times longer than that in free-standing 3C.

Analysis of Conditions for Free-Carrier Grating Formation in InP n+nn+Structures using Monte Carlo Technique

Electron transport in 5 μm long InP nnn structure with the n-region doping of 10 cm−3 is theoretically investigated by the Monte Carlo particle technique at low lattice temperature (T = 10 K), when dominating scattering mechanism is the optical phonon emission. It is shown that at the constant bias a free-carrier grating can be formed inside the n-region. The free-carrier grating formation conditions are analysed by Monte Carlo particle simulation of electric field profiles and noise in the considered InP structure.

Free-carrier grating due to the optical phonon emission in InP n + nn + structures

Electron transport in 5 μm long InP n+nn+ structure with the n-region doping of 1015cm−3 is theoretically investigated by the Monte Carlo Particle (MCP) technique at low lattice temperature (T = 10 K), when dominating scattering mechanism is the optical phonon emission. It is shown that at the constant bias a free-carrier grating (FCG) can be formed inside the n-region. The simple model of FCG formation is proposed and verified by MCP simulation of electron transport and noise in the considered InP structure.

Determination of carrier diffusion length in MOCVD‐grown GaN epilayers on sapphire by optical techniques

AbstractTwo optical techniques for the determination of a bipolar diffusion length LD of optically excited carriers in GaN epitaxial layers, namely a time‐resolved picosecond four‐wave mixing (FWM) on free carrier grating and time‐integrated photoluminescence (PL) are presented and examined. The PL technique is based on time‐integrated photoluminescence (PL) spectra measurements from the front and back sides of the sample under cw and nanosecond pulsed laser excitation. The another method utilizes time‐resolved picosecond four‐wave mixing (FWM) at various light‐induced grating periods to extract diffusion coefficient and carrier recombination lifetime. The value of the diffusion length derived by means of FWM decreases with GaN layer thickness from LD = 260 nm (for 1.7 μm‐thick layer) to LD = 100 nm (for 0.3 μm‐thick layer). The integral PL measurements give the value of LD = 120‐130 nm for the 620 nm layer under pulsed excitation intensities up to 200 kW/cm2. It increases to 150‐170 nm at the excitation intensity enhancement to 1 MW/cm2. These values are close to the value of the diffusion length equal to 160 nm obtained using FWM for this layer thickness evidencing the compatibility of both methods. The changes in the value of LD are discussed in terms of the defect distribution in the epitaxial GaN layer. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Small and Large Signal Analysis of Terahertz Generation from InN n+nn+Structures with Free-Carrier Grating

Electron transport in long (up to 15 μm) InN nnn structures is theoretically investigated by the Monte Carlo particle technique at low lattice temperatures when optical phonon emission is the dominating scattering mechanism. It is shown that at constant bias a free-carrier grating can be formed inside the n-region. Such a grating is found to be responsible for microwave power generation in the THz frequency range. The generation mechanism is similar to that in submicron nnn structures under quasiballistic transport conditions.

Evaluation of photoelectric processes in photorefractive crystals via the exposurecharacteristics of light diffraction

We demonstrate a novel way to analyse carrier recombination and transport processes inphotorefractive semiconductors via the exposure characteristics of light induced diffraction.The results of a picosecond four-wave mixing on free carrier gratings in semi-insulatingGaAs crystals at various grating periods and modulation depths of a light interferencepattern are discussed. The role of a deep-trap recharging in carrier diffusion andrecombination is sensitively revealed through a feedback effect of a space–charge field tonon-equilibrium carrier transport.

Free-carrier grating and terahertz generation from InN n+nn+ structures under streaming plasma instability

Electron transport in 3 µm long InN n+nn+ structures is theoretically investigated by the Monte Carlo particle technique at low lattice temperatures when optical phonon emission is the dominating scattering mechanism. It is shown that at constant bias a free-carrier grating can be formed inside the n-region. Such a grating is found to be responsible for microwave power generation in the THz frequency range.

Carrier transport and recombination in InGaN/GaN heterostructures, studied by optical four‐wave mixing technique

Time-resolved four-wave mixing has been performed in InGaN/GaN/sapphire heterostructures using picosecond pulses at 355 nm for carrier excitation. In-plane diffusion and recombination of carriers, confined in the front layer of 50-nm-thick InGaN, were monitored by a delayed probe beam at 1064 nm. Decay times of free carrier gratings with various spatial periods allowed determination of bipolar diffusion coefficient D = 2.1 cm2/s, effective carrier lifetime of 470 ps, and estimate the corresponding hole mobility 40 cm2/Vs at carrier density of about 1018 cm−3. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamic holographic phase gratings in multiple-quantum-well asymmetric Fabry–Perot reflection modulators

Dynamic holographic pi-phase gratings can be written in a multiple-quantum-well asymmetric Fabry-Perot reflection modulator by spatial modulation of the quantum-confined exciton absorption. Operating near the Fabry-Perot resonance, the phase grating quenches the zero-order reflection and enhances the first-order diffraction efficiency. The diffraction efficiency of these structures as diffractive optics elements is not sensitive to the contrast ratio, which eases the constraints for device fabrication. Numerical simulations for multiple-quantum-well structures are presented that fully include refractive-index changes in response to a spatial modulation of the exciton absorption, which can be caused by any of several techniques, including free-carrier gratings, absorption bleaching, electro-optic effects, and photorefractive effects.

Analysis and demonstration of an integrated optical switch based on dynamic free carrier gratings: a blueprint for a 100×100 element switch array

We propose, analyze, and demonstrate a new type of integrated optical switch based on dynamic free carrier gratings at the intersection of crossing waveguides. A low efficiency, nonregrowth device was fabricated to demonstrate the dynamic free carrier grating concept. Experimental dynamic diffraction efficiency results measured as a function of wavelength for various polarization cases are analyzed using the models developed here, and are found to be in good agreement with theory. Theoretical calculations for higher efficiency designs with more involved fabrication requirements indicate that high speed 100/spl times/100 element switch arrays, with good polarization independent diffraction efficiency (40%), and thus low insertion loss, can be fabricated on a single substrate.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Integrated-optical switch arrays in GaAs based on electrically controlled dynamic free carrier gratings

A new high density integrated-optical switch design using electrically controlled diffraction gratings formed in the intersection of crossing waveguides is proposed and demonstrated. The gratings are formed, in one implementation, by implanting donor atoms in a periodic pattern. Free carriers from the implanted ions modify the index of refraction resulting in Bragg diffraction between the waveguides. Depletion of the free carriers using a reverse bias voltage switches off the diffraction. High speed switch arrays of 100×100 elements on a single substrate may be feasible. Fabrication and demonstration of a low efficiency proof-of-concept switch array in GaAs for use at 1.55 μm are reported. Other designs capable of much higher diffraction efficiencies are described.

Time-resolved build-up and decay of photorefractive and free-carrier gratings in CdTe:V

Laser-induced picosecond transient gratings are used to study carrier transport via free carrier and photorefractive nonlinearities. The feedback effect of a light-induced space charge electric field is found responsible for diffracted beam intensity oscillations during the grating recording and its decay.

Free-carrier and thermal grating effects in gallium phosphide

We report the first observation of the simultaneous creation and competition between a free-carrier light-induced diffraction grating and an accompanying thermal grating in gallium phosphide. The lifetimes of the two gratings are widely different and the nonlinear index n2 = 1 × 10−11 cm2/W for the free-carrier grating is almost an order of magnitude larger than the thermal effect of a 100-psec mode-locked pulse. © 1992 John Wiley & Sons, Inc.

Degenerate four-wave mixing measurements of high order nonlinearities in semiconductors

Degenerate four-wave mixing experiments on ZnSe and CdTe semiconductor samples with picosecond laser pulses at wavelengths below the bandgap are described. The authors identify the dominant nonlinearities in ZnSe and CdTe. They determine these to be fast third-order nonlinearities, due to the same processes which give rise to the effects of bound-electronic refraction and two-photon absorption, while higher order effects are due to free-carrier refraction. Measurements of the absolute magnitude of the combined third order susceptibilities are described. Studies of higher order effects due to free-carrier gratings are discussed. To obtain a quantitative measurement of the carrier induced nonlinearities, an expression for the diffraction efficiency of these carrier gratings was developed, and a value for the free-carrier refractive index coefficient in ZnSe was found. By measuring the angular dependence of the grating decay, the carrier diffusion coefficient was determined as a function of carrier density. >

Picosecond pulse amplification by wave mixing in GaAs

Pulses at 1.064 μm were amplified up to 102 times by nonlinear mixing with a coherent pump wave in GaAs. The observed amplification shows a strong dependence on the intersection angle and time delay between pump and probe pulses. The effect is discussed in terms of self-diffraction by a laser-induced free-carrier grating.

Dynamical behaviour of two-wave coupling in undoped GaAs on a picosecond time scale

Transient two-wave coupling gain as a function of delay time between pump beam and probe beam in undoped GaAs has been measured. In addition to the photorefractive effect, induced absorption and the gain due to the transient free-carrier grating are also observed, and have been discussed respectively. By processing our experimental data, we find that the photorefractive gain saturate at 54%, when the pump fluence increase to 2.27 mJ/cm 2, in the case of our experiment.

Theory of transient energy transfer in gallium arsenide

Numerical calculations are given for optical two-beam coupling in undoped, semi-insulating GaAs using picosecond pulses. Absorption at the intrinsic defect EL2, two-photon absorption, free-carrier absorption, photorefractive gratings, free-carrier gratings, and absorption gratings are included. Results for normalized probe transmission as a function of pump fluence and as a function of pump-probe delay show that the major effects are energy transfer from pump to probe or from probe to pump (depending on crystal orientation) due to the photorefractive effect, two-photon absorption, and free-carrier transient energy transfer from the pump to the probe. >