Stimulated Raman Scattering in a Weakly Polar Ⅲ-Ⅴ Semiconductor： Effect of dc Magnetic Field and Free Carrier Concentration

Using the hydrodynamic model of semiconductor plasmas, we perform an analytical investigation of stimulated Raman scattering （SITS） of an electromagnetic pump wave in a transversely magnetized weakly polar semicon- ductor arising from electron-density perturbations and molecular vibrations of the medium both produced at the longitudinal optical phonon frequency. Assuming that the origin of SItS lies in the third-order susceptibility of the medium, we investigate the growth rate of Stokes mode. The dependence of stimulated Raman gain on the external dc magnetic field strength and free carrier concentration is reported. The possibility of the occurrence of optical phase conjugation via SItS is also studied. The steady-state Raman gain is found to be greatly enhanced by the presence of the strong external dc magnetic field.     

Diffusion-induced parametric dispersion and amplification in doped semiconductor plasmas

Using the hydrodynamic model of semiconductor plasma, the diffusion-induced nonlinear current density and the consequent second-order effective susceptibility are obtained under off-resonant laser irradiation. The analysis deals with the qualitative behaviour of the anomalous parametric dispersion and the gain profile with respect to the excess doping concentration and pump electric field. The analysis suggests that a proper selection of doping level and pump field may lead to either positive or negative enhanced parametric dispersion, which can be of great use in the generation of sequeezed states. It is found that gain maximizes at moderate doping concentration level, which may drastically reduce the fabrication cost of parametric amplifier based on this interaction.      

Acousto-optic modulation in magnetised diffusive semiconductors

The modulation of an intense electromagnetic beam induced by the acousto-optic (AO) effect has been analysed in a transversely magnetised semiconductor-plasma medium. The effect of carrier diffusion on the threshold field and gain profile of the modulated wave has been extremely investigated using coupled mode theory. The origin of the AO interaction is assumed to lie in the induced nonlinear diffusion current density of the medium. By considering the modulation process as a four wave parametric interaction an expression for effective third-order AO susceptibility describing the phenomena has been deduced. The modulation is greatly modified by propagation characteristics such as dispersion and diffraction due to dielectric relaxation of the acoustic mode. The threshold pump field and the steady state growth rates are estimated from the effective third-order polarisation in the plasma medium. Analytical estimation reveals that in the presence of enhanced diffusion due to excess charge carriers the modulated beam can be effectively amplified in a dispersionless acoustic wave regime. The presence of an external dc magnetic field is found to be favourable for the onset of diffusion induced modulational amplification of the modulated wave in heavily doped regime. Received 5 November 2001     

Steady-state and transient Raman gain in magnetoactive narrow band-gap semiconductors

Using the hydrodynamic model of semiconductor-plasmas and following the coupled-mode approach, a detailed analytical investigation is undertaken to study both steady-state and the transient Raman gain in transversely magnetized narrow band-gap semiconductors arising from electron density perturbations and molecular vibrations of the medium. Using the fact that the origin of stimulated Raman scattering (SRS) lies in the third-order susceptibility of the medium, we obtain an expression for the gain coefficient of the backward Stokes mode in steady-state and transient regimes and study the dependence of the magnetic field and pump pulse duration on its growth rate. The threshold pump intensity and optimum pulse duration for the onset of transient SRS are estimated. An externally applied magnetic field substantially enhances the transient SRS gain coefficient in narrow band-gap semiconductors, which can be of great use in the compression of scattered pulses.     

Modulational interactions in magnetised piezoelectric semiconductor plasmas

Based on hydrodynamic model of plasmas an analytical investigation of frequency modulational interaction between copropagating high frequency pump and acoustic mode and consequent amplification (steady-state and transient) of the modulated waves is carried out in a magnetised piezoelectric semiconductor medium. The phenomenon of modulational amplification is treated as four wave interaction process involving cubic nonlinearity of the medium. Gain constants, threshold-pump intensities and optimum-pulse duration for the onset of modulational instabilities are estimated. The analysis has been performed in non-dispersive regime of the acoustic mode, which is one of the preconditions for achieving an appreciable initial steady-state growth of the modulated signal wave. It is found that the transient gain constant diminishes very rapidly if one chooses the pump pulse duration beyond the maximum gain point. Moreover, the desired value of the gain can be obtained by adjusting intensity and pulse duration of the pump and doping concentration of the medium concerned. Received 12 August 1999 and Received in final form 17 December 1999     

Parametric dispersion and amplification in semiconductor plasmas: Effects of carrier heating

Using the hydrodynamic model of a semiconductor plasma, the influence of carrier heating on the parametric dispersion and amplification has been analytically investigated in a doped III-V semiconductor, viz. n-InSb. The origin of the phenomena lies in the effective second-order optical susceptibility (χ_e⁽²⁾) arising due to the induced nonlinear current density of the medium. Using the coupled-mode theory, the threshold value of pump electric field (|E_0T|_para) and parametric gain coefficient (α_para) are obtained via χ_e⁽²⁾. The relevant experiment has not been performed. Proper selection of the doping level not only lowers |E_0T|_para required for the onset of parametric excitation but also enhances α_para. The carrier heating induced by the intense pump modifies the electron collision frequency and hence the nonlinearity of the medium, which in turn further lowers |E_0T|_para and enhances α_para by a factor of ∼10³ and ∼2×10², respectively. The results strongly suggest that the incorporation of carrier heating by the pump in the analysis leads to a better understanding of parametric processes in solids and gaseous plasmas, which can be of great use in the generation of squeezed states.     

Mechanism of Current Oscillations in Gallium ArsenidePhotoconductive Semiconductor Switches

Semi-insulating photoconductive semiconductor switch with an electrode gap of 4 mm, triggered by a laser pulse with energy of 0.5md, and applied bias of 2.5kV, the periodicity current oscillation with a cycle of 12ns is obtained. It is indicated that the current oscillation is one mode of transferred electron effect, namely quenched domain mode. This mode of trans-electron oscillator is obtained when the instantaneous bias electric field drops below the sustaining field （the minimum electric field required to support the domain） before the domain reaches the anode, which leads to the domain disappears somewhere in the bulk of the switch and away from the ohmic contacts. We mainly analyse the time-dependent characteristic of the mode, the theoretical analysis results are in excellent agreement with the experiment.     

Nonlinear absorption and refractive index of Brillouin scattered mode in piezoelectric semiconductor plasmas by an applied magnetic field

With the aid of hydrodynamic model, a detailed analytical investigation is made of the stimulated Brillouin scattering (SBS) of the Stokes component of the scattered wave in piezoelectric-doped semiconductor plasma subjected to a magnetostatic field. The origin of the SBS process lies in the third-order nonlinear optical susceptibility arising due to the induced nonlinear current density and acoustic perturbations internally generated due to crystal properties such as piezoelectricity and electrostriction. Using the coupled mode theory of plasmas the effective refractive index and absorption coefficient are determined via the effective susceptibility. The influence of piezoelectricity, magnetostatic field and doping concentration has been explored. The analysis has been applied to both noncentrosymmetric and centrosymmetric crystals. Numerical estimates are made for n-type InSb crystal duly irradiated by a frequency doubled 10.6 μm CO₂ lasers. Results are found to be well in agreement with available literature. The analysis establishes that a large nonlinear refractive index and small absorption coefficient can easily be obtained under moderate excitation intensity in piezoelectric doped magnetized semiconducting crystal, which proves its potential as candidate material for the fabrication of cubic nonlinear devices.     

Velocity of pulse propagation in media with amplitude nonlinearity

The propagation velocity of optical wave fronts can be accelerated by the influence of gain saturation. We report systematic measurements for the specific case of Brillouin gain in optical fibers. A simplified analytic rate equation approach permits a qualitative understanding of the observations in terms of a pure amplitude nonlinearity. We point out that there is a close analogy to a mode-locked laser with gain saturation. Pursuing this analogy, we can explain why the changes in propagation velocity are hardly measurable for synchronously pumped lasers, but easily amount to several percent for amplifiers or lasers based on stimulated Brillouin scattering.     

Pulse Compression Based on Laser-Induced Optical Breakdown in Suspension

Pulse compression based on laser-induced optical breakdown in suspension is investigated. The physical mechanism behind it is analyzed theoretically and validated in the Q-switched Nd：YAG laser system. A 12-ns pump pulse is suppressed to 5ns With good fidelity in the front edge and sharp steepness in the trailing edge. The HT-270, which has a small gain coefficient and absorption coefficient, is used as a solvent, and therefore the disturbance induced by stimulated Brillouin scattering and absorption are minimized and the transmittivity is enhanced.     

Theoretical Analysis of the Critical Phenomena of a Brillouin Laser

The critical phenomena of a Brillouin laser are analyzed theoretically. The results show that the behavior of a Brillouin laser in the threshold region is a second-order phase transition. The critical point of the phase transition is the gain threshold of the Brillouin laser, and the order parameter is the amplitude of the Stokes component in stimulated Brillouin scattering. The critical slow-down phenomenon and the typical characteristics in phase transition are demonstrated. Further work on the combination of nonlinear optics and phase transition in the Brillouin laser may lead to a new view and findings that could be significant for both fields.     

Stimulated brillouin scattering of alfvén wave in a highly collisional magnetoactive compensated semiconductor: Kinetic theory

Summary A rigorous theoretical investigation has been made on stimulated Brillouin scattering of electromagnetic Alfvén wave propagating in a highly collisional magnetoactive compensated semiconductor,viz. compensated germanium. By using the Krook-model solution, the Boltzmann transport equation has been solved to obtained the non-linear response of electrons and holes in the semiconductor sample immersed in an external static magnetic field. It is noticed here that the threshold of this parametric instability is quite low and the growth rate of it is considerably large at moderate power of the incident Alfvén wave. It is also noted that the effect of the electron-phonon collision frequency on this three-wave parametric process is remarkable and its growth rate decreases very rapidly with the electron-phonon collision frequency in the semiconductor.     

Long Optical Delay Lines Enhanced by Ring Configuration in Optical Fibres

A long optically controlled delay line enhanced by ring configuration is demonstrated by using the group-velocity control of signal pulses based on stimulated Brillouin scattering. In experiment, two optical fibre ring cavities are used： one is used as the Brillouin laser, providing single-mode Stokes wave as probe wave; the other is used as the Brillouin amplifier, working as slow light medium. We achieve a maximum time delay of 215ns using the ring Brillouin amplifier, five times larger than the input probe pulse width of 40ns. In the meantime, a considerable pulse broadening is observed, which agrees well with the theoretical prediction based on linear theory.     

Stimulated brillouin scattering in SF6 with a free-running XeCl laser as pump

A XeCl laser system without dispersive elements is used to investigate near threshold reflectivity and phase-conjugation (PC) fidelity of stimulated Brillouin scattering (SBS) mirrors with SF₆ as the active medium. Using different focal-length lenses to focus the broadband laser radiation into the Brillouin medium, it was found that at threshold the effective interaction length for SBS is equal to the confocal parameter and that the SBS gain is equal to its steady-state value for monochromatic pumping. High PC-fidelity values ranging from 0.8–1.0 were found under most of the experimental conditions investigated.     

Induction of an atomically thin ferromagnetic semiconductor in 1T′ phase ReS2 by doping with transition metals

Two-dimensional 1T′ phase ReS₂, a transition metal dichalcogenide, has a unique structure and electronic properties that are independent of thickness. The pure phase is a nonmagnetic semiconductor. Using density functional theory calculations, we show that ReS₂ can be tuned to a magnetic semiconductor by doping with transition metal atoms. The magnetism mainly comes from the dopant transition metal and neighboring Re and S atoms as a result of competition between exchange splitting and crystal field splitting. ReS₂ doped with Co can be considered as a promising diluted magnetic semiconductor owing to its strong ferromagnetism with long-range ferromagnetic interaction, high Curie temperature (above room temperature) and good stability. These findings may stimulate experimental validation and facilitate the development of new atomically thin diluted magnetic semiconductors based on transition metal dichalcogenides.     

Energy spectrum and quantum magnetotransport in type-II heterojunctions

Specific features of the energy spectrum of a separated type-II heterojunction in an external magnetic field are studied theoretically and experimentally. It is shown that, due to hybridization of the states of the valence band of one semiconductor and the conduction band of the other semiconductor at the heterointerface, there are level anticrossings, which produce quasigaps in the density of states in a nonzero magnetic field. The experimental results of magnetotransport studies for the GaInAsSb/p-InAs quaternary solid solutions with different doping levels are shown to agree well with the results of simulation, and specific features of the energy spectrum of separated type-II heterojunctions are established.     

Directivity influence of signals propagation through EDFA gain medium of Brillouin-erbium fiber laser

We experimentally demonstrate a multiwavelength Brillouin-erbium fiber laser in two configurations; uni-directional and bi-directional propagation of Brillouin pump and Brillouin Stokes signals through an Erbium-doped fiber gain. The influence of these configurations on the performance of the output parameters in terms of lasing threshold, output channel generation and tuning range of the generated output channels are investigated. We discovered that there is a trade-off between these two fiber laser configurations. The uni-directional amplifier configuration provides greater tuning range of 46.8 nm against 23 nm at maximum Brillouin pump power of 2 mW and 1480-nm pump power of 130 mW. On the other hand, the bi-directional amplifier configuration provides 13 output channels against 6 output channels obtained from the uni-directional amplifier configuration at the same pumping powers. Nevertheless, the bi-directional amplifier configuration requires much lower pump power to initiate lasing.     

Formation and dynamics of a virtual cathode in a tubular electron beam placed in a magnetic field

Mechanisms underlying the formation and dynamics of a virtual cathode (VC) in a tubular electron beam subjected to a magnetic field are studied using a numerical 2D model. Two qualitatively different competing types of space charge dynamics near the VC are discovered. Which of them predominates depends on the magnetic field strength. The beam current critical density at which a nonstationary VC forms in the system is also strongly dependent on the magnetic field. It is shown that the optimal strength of the magnetic field that minimizes the beam current critical density depends on the Brillouin magnetic field.     

Current Oscillation and dc-Voltage-Controlled Chaotic Dynamics in Semiconductor Superlattices

We report a detailed theoretical study of current oscillation and dc-voltage-controlled chaotic dynamics in doped GaAs/AlAs resonant tunneling superlattices under crossed electric and magnetic fields. When the superlattice is biased at the negative differential velocity region, current self-oscillation is observed with proper doping concentration. The current oscillation mode and oscillation frequency can be affected by the dc voltage bias, doping density, and magnetic field. When an ac electric field with fixed amplitude and frequency is also applied to the system, different nonlinear properties show up in the external circuit with the change of dc voltage bias. We carefully study these nonlinear properties with different chaos-detecting methods.     

Waveguides induced by steady-state gray solitons in biased photorefractive-photovoltaic crystals

We carry out a theoretical investigation of the properties of waveguides induced by photorefractive one-dimensional steady-state gray spatial solitons (i.e., screening solitons, photovoltaic solitons, and screening-photovoltaic solitons). We demonstrate that waveguides induced by photorefractive steady-state gray spatial solitons are only a single guided mode for both all soliton graynesses and all values of ρ, where ρ is the ratio between the soliton peak intensity and the dark irradiance, and moreover, waveguides induced by gray photovoltaic solitons for closed-circuit condition are also only a single guided mode for all electric current densities. We find that the confined energy near the center of a photorefractive steady-state gray spatial soliton increases with ρ and decreases with an increase in the soliton grayness. We also find that the confined energy near the center of a gray photovoltaic soliton for closed-circuit condition increases with the electric current density. On the other hand, waveguides induced by gray screening-photovoltaic solitons are gray screening soliton-induced waveguides when the bulk photovoltaic effect is neglectable and are gray photovoltaic soliton-induced waveguides when the external bias field is absent.