Photorefractive Multiple Quantum Research Articles

Separate coupled solitons in biased series photorefractive semiconductor circuit

We present, for the first time a theoretical foundation for separately coupled solitons in a biased series circuit consisting of semi-insulating photorefractive multiple quantum well (MQW) waveguides. As an example, a slab semiconductor waveguide based on a GaAs/AlGaAs structure is considered. The coupled steady state space charge field is derived for both the MQW structures using a bipolar band transport model. Hence, a system of two coupled equations is obtained for each MQW structure which governs the dynamical evolution of light beams. We then prove the existence of dark–dark, grey–grey and grey–dark separately coupled soliton pairs. We also see that the soliton propagating individually in one PR semiconductor can affect the other by the light-induced current. We investigate this coupling effect with respect to the soliton beam’s intensity profile and for each of the three cases, namely, dark–dark, grey–grey and grey–dark separately coupled solitons. Some potential applications are discussed.

Coupling of optical spatial solitons in photorefractive multiple quantum well planar waveguide

A comprehensive theory predicting the existence of both incoherently coupled and coherently coupled grey and dark optical spatial solitons in multiple quantum well planar (MQW) waveguides is presented. In addition, we also show the existence of dark Manakov soliton pair and the bright-dark Manakov soliton pair in such Multiple Quantum well waveguides for the first time. In the case of a coherently coupled soliton pair, the mutual phase difference, the overlap of the optical mode on the waveguide, the external bias field and beam intensity ratio have a profound effect on the FWHM of the soliton pairs. As a general case, a theory for the interesting and hitherto uninvestigated case of coherently coupled multicomponent solitons has been presented and analyzed for the first time. The effect of the mutual phase difference, the overlap of the optical mode on the waveguide(overlap parameter), the external bias field and beam intensity ratio on the FWHM is studied for these coherently coupled multicomponent solitons also. The Manakov soliton solutions are found analytically and they exist in the limit of low total intensity of the beams. We find that the dark-dark and bright-dark Manakov soliton pairs exist in such MQW waveguides whereas bright-bright Manakov soliton pair does not exist. The Manakov solitons’ spatial profile depends upon the overlap parameter, the intensity of the components, and the external electric field.

The impact of electron and hole trapping coefficient on nonlinear phenomena in photorefractive multiple quantum well structures

Semiconductor photorefractive quantum wells belong to materials with strong optical nonlinearity. One of the parameters that may affect the course of nonlinear phenomena in these materials is the electron and hole trapping coefficient. We present the results of a numerical analysis aimed to find out, how electric field-dependent trapping coefficients affect the process of space-charge field formation in multiple quantum wells in the phenomenon of photorefractive two-wave mixing. Full Text: PDF ReferencesQ. Wang, R. M. Brubaker, D. D. Nolte and M. R. Melloch, "Photorefractive quantum wells: transverse Franz-Keldysh geometry," J. Opt. Soc. Am. B 9, 1626 (1992) CrossRef D.D. Nolte and M.R. Melloch, in: Photorefractive effects and Materials, Chap.6, ed. by D. D. Nolte (Kluwer Academic, Boston, 1995) CrossRef D.D. Nolte, "Semi-insulating semiconductor heterostructures: Optoelectronic properties and applications"", J. Appl. Phys. 85, 6259 (1999) CrossRef Q.Wang, R. M. Brubaker and D. D. Nolte, "Photorefractive phase shift induced by hot-electron transport: Multiple-quantum-well structures", J. Opt. Soc. Am. B 9 (1994) 1773. CrossRef V. Ya. Prinz, S. N. Rechkunov, "Influence of a Strong Electric Field on the Carrier Capture by nonradiative Deep-Level Centers in GaAs", Phys. Stat. Sol. (b) 118, 159 (1983) CrossRef S.M. Sze, Physics of Semiconductors Devices, second ed., Wiley, New York, 1981 (Chapter 10) DirectLink B. Jablonski, "Impact of donor compensation ratio on photorefractive two-wave mixing dynamics in multiple quantum wells structures", JNOPM 23, 1450029 (2014) CrossRef

Perturbative approach to space-charge field dynamics in photorefractive semiconductors

A general perturbative approach to solving photorefractive material equations for a two-wave mixing geometry is developed. The method allows the study of both stationary and transient states of the space-charge field. As an example, solutions were given for GaAs–AlGaAs photorefractive multiple quantum well structures working in the Franz–Keldysh geometry. Approximate explicit expressions describing the recording and erasure dynamics of photorefractive gratings have been derived. The results are compared with numerical solutions of a complete set of material equations.

Slow and fast light in photorefractive GaAs–AlGaAs multiple quantum wells in transverse geometry

We show theoretically that, based on the dispersive phase coupling effect during the wave mixing process, both slow and fast light can be achieved in GaAs–AlGaAs photorefractive multiple quantum wells (PRMQWs) films applied with a transverse direct-current electric field. The general formula for the group velocity of the diffracted beams in the Raman–Nath regime during the wave mixing process in a nonlinear thin film is derived and is then applied to the case of the PRMQWs films in the transverse geometry. The simulation results in the transverse-geometry PRMQWs films show that the group velocity and bandwidth of slow light can be on the order of centimeter per second and 100 kHz, respectively. The extremely low group velocity and the relatively broad bandwidth are mainly originated from the strong quadratic electro-optic effect and the fast response rate of the PRMQWs films, respectively. Our results show that the delay-bandwidth product of slow light can be significantly improved in PRMQWs films as compared to the reported results in other photorefractive materials.

Space-charge electric field in photorefractive multiple quantum wells

We have developed a graphical method to determine the space-charge electric field in photorefractive multiple quantum wells when the photocurrent is dominated by the drift current. The minima and maxima of the space-charge electric field due to space modulated light intensity are obtained from photocurrent measurements under uniform illumination. In addition, we find that the space-charge field depends nonlinearly on the applied electric field and the intensity of light, and changes sharply with the intensity up to a critical intensity beyond which it remains unchanged.

Temporal response of photorefractive multiple quantum wells in Franz–Keldysh geometry

Transient behavior of the photorefractive grating generated in multiple quantum well structure working in the transverse Franz–Kieldysh geometry is considered. Analysis was performed in the framework of the band transport model with material parameters of GaAs/AlGaAs system. The influence of carrier transport nonlinearity caused by the intervalley scattering of electrons was taken into account. An analytical expression for the grating formation time is derived and compared with numerical solutions as well as with the available experimental results. The results show that in the case of stationary interference pattern the nonlinear transport of electrons does not considerably improve the response time of the structure. This conclusion, checked for parameters used in majority of experiments, does not confirm the statements of earlier papers.

Angle dependence of two-wave mixing efficiency in photorefractive multiple quantum wells

The authors have investigated how two-wave mixing efficiency changes as a function of angle tuning while crossing over from the Raman-Nath diffraction to the Bragg diffraction in AlGaAs∕GaAs photorefractive multiple quantum wells (PRQWs). They have shown that by changing the inclination angle of the PRQW, the apparent thickness of the PRQW (as seen by the mixing beams) can become larger than the grating fringe spacing. This moves the wave mixing from Raman-Nath diffraction to Bragg diffraction and causes an enhancement in the mixing efficiency at smaller fringe spacing.

Surface breaking crack evaluation with photorefractive quantum wells and laser-generated Rayleigh waves

An adaptive laser ultrasound system using optical two-wave mixing technique with photorefractive quantum wells is developed for crack characterization in noisy environments. AlGaAs∕GaAs photorefractive multiple quantum wells featuring a quick response time of 9.1μs cancel disturbances below 100kHz. A laser-generated Rayleigh wave in a thermoelastic regime is used for crack characterization. The positions and depths of artificial cracks, 3 and 5mm in depth and 0.2mm in width, were evaluated. The crack orientations were also determined using a shear wave that was generated through mode conversion of a Rayleigh wave at the crack tip.

Optimisation of photorefractive multiple quantum wells for biomedical imaging

A novel method has been proposed and tested to optimize the photorefractive multiple quantum wells device for biomedical imaging. The device acts as a dynamic holographic beam combiner in a holographic optical coherence imaging. By choosing an appropriate duty cycle of the external electric field, the Joule heat limit and saturation of exciton are decreased. We examine several characteristics that govern the use of the device.

Simultaneous determination of the index and absorption gratings in multiple quantum well photorefractive devices designed for laser ultrasonic sensor

A method to determine complex amplitudes of index and absorption gratings in photorefractive multiple quantum wells is presented. Usually to evaluate these parameters, spectral measurement of absorption change due to the applied field over a wide wavelength range is required and the index change is calculated through the Kramers–Krönig’s relationship. In our method, only a set of measured data for a photorefractive two-wave mixing set-up with small sinusoidal phase modulation for fixed wavelength is needed. This is thus well adapted to in situ characterization of systems that operate at a fixed wavelength. Real and imaginary part of the gains owing to the index and the absorption gratings are calculated from the output signals of the system. We apply this method to our photorefractive multiple quantum well devices operating at 1064 nm, and obtain consistent results.

High-speed wide-field coherence-gated imaging via photorefractive holography with photorefractive multiple quantum well devices

The parallel pixel acquisition inherent in wide-field coherence-gated imaging techniques offers the possibility of high-frame-rate imaging and volumetric imaging, including through scattering media. We discuss photorefractive holography using sensitive, dynamic photorefractive multiple quantum well (PRQW) films as a promising approach and, after reviewing the factors that determine the sensitivity of this technique, we demonstrate that high-frame-rate wide-field coherence gated imaging is possible at 830 frames per second with PRQW devices using a non-intensified camera to record the diffracted signal.

Femtosecond Response of Diffraction Efficiency of GaAs/AlGaAs Photorefractive Multiple Quantum Well

Ultrafast response of the diffraction efficiency of a photorefractive multiple quantum well (PRMQW) has been investigated by pump-probe spectroscopy. We evaluated the temporal evolution of the diffraction efficiency of the PRMQW using a differential transmission spectrum. The results were compared with the output-widening factor of diffracted pulses. A good agreement obtained between our evaluation results and the results of electric-field cross correlation has confirmed the applicability of our technique in characterizing the diffraction efficiency of the PRMQW.

High-speed depth-sectioned wide-field imaging using low-coherence photorefractive holographic microscopy

Low-coherence photorefractive holography has the potential to acquire wide-field coherence-gated images at frame rates approaching 1000 frames/s, including through scattering media. We present a quantitative analysis of the system optimization and limits of performance for coherence-gated imaging through scattering media using photorefractive holography and compare this performance to direct CCD detection. We show that, for high optical quality recording photorefractive multiple quantum well devices, photorefractive holography has the potential to provide a higher dynamic range than is possible with direct CCD-based detection.

Photorefractive Multiple Quantum Well Waveguide as a Bragg Reflector

An interaction between the signal wave propagating in a planar waveguide and a photorefractive grating is analysed. The waveguide contains an AlGaAs/GaAs multiple quantum well guiding layer biased with an external electric field applied along the wells plane. The grating is formed by two external beams interfering in the multiple quantum well layer. The dependence of the steady-state grating properties on the external waves parameters and applied electric field intensity is presented. Possible application of the grating as an optically controlled Bragg reflector with memory is analysed.

High Spatial Resolution of an Optical Addressing Spatial LightModulator Made by Photorefractive Semi-Insulting Multiple Quantum Wells

We use nondegenerate four-wave mixing to study the spatial resolutionof photorefractive semi-insulating multiple quantum wells grown bymolecular beam epitaxy. By optimizing the experimentalconditions, we have demonstrated that our sample has spatial resolutionup to 2.5 µm, which approaches the theoretical limit. We also analysethe factors that affect the spatial resolution of multiple quantumwells.

Characterization of photorefractive GaAlAs multiple quantum well structures by ac photoelectromotive force and modulated photocarrier grating techniques

Simultaneous utilization of ac photoelectromotive (photo-EMF) and modulated photocarrier grating (MPG) techniques for characterization of bipolar transport in semi-insulating photorefractive GaAlAs multiple quantum well structure is reported. From the ac photo-EMF experiment the electrons were determined as the dominant photocarriers for interband optical excitation at λ=633 nm. From the self-consistent data obtained from photo-EMF and MPG experiments the diffusion length of the photoelectrons was evaluated as LD≃1.4±0.2 μm, and that of the photoholes (minority carriers)—as LDm≃0.6±0.1 μm. In the MPG configuration, the contrast of the illuminating interference pattern was controlled by electro-optic modulation of the signal beam polarization, which allowed us to measure dielectric relaxation time of the photoconductor as well. As evaluated by both techniques it proved to be about τdi≃0.05 μs for the average light intensity I0≃0.3 mW/mm2. The proposed combination of the characterization techniques seems to be especially promising for amorphous a–Si and polymer films.

Diffraction response of a low-temperature-grown photorefractive multiple quantum well modulator

The transient diffraction response of a low-temperature-grown (LTG) photorefractive multiple quantum well (MQW) spatial light modulator (SLM) is studied experimentally as a function of the frequency and the on/off ratio of a single-sided applied voltage when a grating is written using a continuous-wave sinusoidal illumination. LTG MQW modulators have high spatial resolution, but their diffraction efficiency and speed are reduced. By varying the on/off ratio of the applied voltage, it is shown that the frequency response of the SLM can be optimized while maintaining its high spatial resolution. The flatness of the frequency response with respect to the peak diffracted signal is thus shown to increase by an order of magnitude. By temporally gating the diffracted signal, low and bandpass filtering capabilities of the SLM are experimentally demonstrated for applications such as speckle elimination and novelty filtering. When the diffracted signal is measured over an adjustable time window, it is shown that high frame rate operations in integrated optoelectronic systems can be achieved. A numerical model that includes the effect of high trap densities on the space-charge dynamics is developed. It is shown that the space-charge field grows nonuniformly along the MQW growth direction with the largest modulation in lateral field occurring only near the entrance face of the incident light, thus accounting partially for the low diffraction efficiency of LTG materials.

InGaAs/GaAs photorefractive multiple quantum well device in quantum confined Stark geometry

We have demonstrated photorefractive InGaAs/GaAs multiple-quantum-well devices in the quantum confined Stark geometry. A four-wave-mixing diffraction efficiency up to 0.3% is obtained at a wavelength of 949 nm. Proton implantation strongly reduces the maximum diffraction efficiency although it saturates the diffraction efficiency at smaller grating period comparing to as-grown device. We have also observed higher order diffractions. It is found that the space-charge field changes its pattern temporally from a sinusoidal pattern to a rectangular one with decreasing its modulation depth.

The resolution of distributed photorefractive multiple quantum well devices

Photorefractive multiple quantum well (PRMQW) devices can achieve simultaneously higher resolution and greater sensitivity by distributing several highly trapping low temperature growth (LTG) layers at different depths in their intrinsic region. This article extends a previously derived analytical model of PRMQW devices to distributed photorefractive devices (DPDs) that consist of a cascade of smaller MQW regions sandwiched between LTG layers. This enables an understanding of how multiple trapping layers affect the resolution and sensitivity in these DPDs. In addition to an improvement in resolution, the model predicts an enhancement in diffraction efficiency at small grating spacings as the number of subdevices increases for a fixed total DPD length. This result is of significance in designing compact image processing systems that can operate at small grating periods but still be able to achieve a large signal-to-noise ratio for image processing and sensing applications.