Intraband Dynamics and Terahertz Emission in Biased GaAs/In0.53Ga0.47As Semiconductor Superlattices

Using an excitonic basis, we investigate the intraband polarization, optical absorption spectra, and terahertz emission of semiconductor superlattice with the density matrix theory. The excitonic Bloch oscillation is driven by the dc and ac electric fields. The slow variation in the intraband polarization depends on the ac electric field frequency. The intraband polarization increases when the ac electric field frequency is below the Bloch frequency. When the ac electric field frequency is above the Bloch frequency, the intraband polarization downwards and its intensity decreases. The satellite structures in the optical absorption spectra are presented. Due to excitonic dynamic localization, the emission lines of terahertz shift in different ac electric field and dc electric field.     

Excitonic Absorption of Semiconductor Nanorings under Terahertz Fields

The optical absorption of GaAs nanorings （NRs） under adc electric field and a terahertz （THz） ac electric field applied in the plane containing the NRs is investigated theoretically. The NRs may enclose some magnetic flux in the presence of a magnetic field perpendicular to the NRs plane. Numerical calculation shows that the excitonic effects are essential to correctly describe the optical absorption in NRs. The applied lateral THz electric field, as well as the dc field leads to reduction, broadening and splitting of the exciton peak. In contrast to the presence of a dc field, significant optical absorption peak arises below the zero-field bandgap in the presence ofa THz electric field at a certain frequency. The optical absorption spectrum depends evidently on the frequency and amplitude of the applied THz field and on the magnetic flux threading the NRs. This promises potential applications of NRs for magneto-optical and THz electro-optical sensing.     

Gain Saturation Effects of a Flowing Chemical Oxygen-Iodine Laser

A new oxygen-iodine medium gain model is developed to include pumping and deactivation of the upper laserlevels of the iodine atoms, hyperfine and translation relaxation, as well as the flowing effect. The rate equationsfor gain of a supersonic flowing cw oxygen-iodine laser (COIL) are described when the medium is stimulated bya single-mode field. The general solution of the self-consistency integral equation is obtained. The result showsthat the saturation behaviour in low pressure of the COIL differs from both the inhomogeneous and homogeneousbroadening, and exhibits an ‘anomalous‘ saturation phenomenon.     

Optical response of an inverted InAs/GaSb quantum well in an in-plane magnetic field

The optical response of an inverted InAs/GaSb quantum well is studied theoretically. The influence of an in-plane magnetic field that is applied parallel to the quantum well is considered. This in-plane magnetic field will induce a dynamical polarization even when the electric field component of the external optical field is parallel to the quantum well.The electron–electron interaction in the quantum well system will lead to the de-polarization effect. This effect is found to be important and is taken into account in the calculation of the optical response. It is found that the main feature in the frequency dependence of the velocity–velocity correlation function remains when the velocity considered is parallel to the in-plane magnetic field. When the direction of the velocity is perpendicular to the in-plane magnetic field, the depolarization effect will suppress the oscillatory behavior in the corresponding velocity–velocity correlation function. The in-plane magnetic field can change the band structure of the quantum well drastically from a gapped semiconductor to a no-gapped semi-metal, but it is found that the distribution of the velocity matrix elements or the optical transition matrix elements in the wave vector space has the same two-tadpole topology.     

A PVK—Based High—Performance Photorefractive Polymer Composite Under a Low External Electric Field

We describe a PVK-based photorefractive polymer composite doped with disperse red 1,poly-butylacrylate and TNF.Using the two-beam coupling experiment,we investigate its photorefractive properties as well as the dynamic process of the index gratings in the sample.We observe the photorefractive effect in the absence of an applied external electric field.The two-beam coupling gain coefficient was measured to reach a value as high as 140cm^-1 in the absence of an external applied electric field and a value of 470cm^-1 at a poling dc electric field of 25V/μm.     

An Exactly Solvable Spin-1 Model under External Magnetic Field

A two-fold Cayley tree with fully q-coordinated sites is constructed and the spin-1 Blume-Capel model in the presence of an external magnetic field is solved exactly. The relevant properties such as magnetization m and square magnetic moment q are investigated as functions of temperature and external magnetic fieId both for ferromagnetic and antiferromagnetic couplings. In the study, we propose a possible mechanism of the plateau based on the exchange couplings.     

Gain enhancement in two-stream free electron laser with a planar wiggler and an axial guide magnetic field

The theory for a two-stream free electron laser (FEL) consisting of a relativistic electron beam transported along the axis of a planar wiggler in the presence of an axial guiding magnetic field is proposed and investigated. The electron trajectories and the small signal gain are derived. The characteristic of the linear gain and the normalized maximum gain are studied numerically. The result shows that the normalized maximum gain is considerably enhanced in comparison with that of the single stream. The effect of the difference between the energies of the two beams in this configuration of FEL is also considered, and we find that the gain is affected by the energy differences between groups 1 and 2.     

Magneto-optical properties of self-assembled InAs quantum dots for quantum information processing

Semiconductor quantum dots have been intensively investigated because of their fundamental role in solid-state quantum information processing. The energy levels of quantum dots are quantized and can be tuned by external field such as optical, electric, and magnetic field. In this review, we focus on the development of magneto–optical properties of single In As quantum dots embedded in Ga As matrix, including charge injection, relaxation, tunneling, wavefunction distribution,and coupling between different dimensional materials. Finally, the perspective of coherent manipulation of quantum state of single self-assembled quantum dots by photocurrent spectroscopy with an applied magnetic field is discussed.     

Magnetic field measurement based on a stimulated two-photon Raman transition

The magnetic field in the microwave interaction zone of the fountain atomic clock was measured by stimulated Raman transitions.By measuring the two-photon transition frequency between the Zeeman levels of the two ground states,we achieved a magnetic field measurement accuracy of the order of 0.28 nT.This method is immune to the Doppler shift and the AC Stark shift.The second order Zeeman shift of the fountain clock is 170.7×10-15,with the uncertainty of 7.2×10-16.     

Berry Phase in an Entangled Spin Cluster with Five Particles

The geometric phase, in particular the Berry phase, in an entangled state of five spin-1/2 particles is studied. A time-dependent magnetic field is applied to control the time evolution of the cluster. Using the method of algebraic dynamics, we calculate the non-adiabatic geometric phase or Berry phase and the degeneracy energy levels when the magnetic rotates around Z axis. Based on the exact analytical solutions, we show how the Berry phase of the entangled state of this cluster depends on the external magnetic field parameters w （the angular velocity of the rotating magnetic field） and θ （the angle between the magnetic field and Z axis）.     

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.     

Measurement of spin-flip raman gain,effectiveg-value and absorption of radiation under transient electric field conditions in insb

We studied the change in Spin-flip Raman (SFR) gain and behaviour of the effectiveg-value as a function of pulsed electric fields inn-type InSb. The samples were held at 1.8 K in magnetic fields up to 1.4 T. Varying the strength of a pulsed electric field applied to the samples parallel to the magnetic field we found a direct correlation between the increase in electron temperature and the decrease of the Raman gain. Measurements of the Raman gain and spontaneous line width at different times after the starting of the electric pulse and during its fall time with a time resolution of 10 ns give no indication of a motional narrowing of the SFR line and of an increase of the gain during the decrease of the electric field pulse. These two predictions were made by Mooradian et al. (1972) to explain the observed transient increase of the intensity of the stimulated SFR Stokes radiation after an electric field across the sample was switched off. With our direct measurement of the Raman gain we could establish that at any time during and after the electric field pulse, the gain is less or equal to the value without electric field. In the electric field region from 0.3 V/cm up to 0.6 V/cm the Raman gain shows several maxima, originating from SFR scattering between different spin splitted conduction band levels. The electron distribution under pulsed electric field conditions give rise to a dynamically varying absorption mechanism of transmitted radiation. Time resolved transmission measurements explain the observed increase of stimulated SFR Stokes radiation during the switch-off time of electric field pulses by a lower absorption of radiation and not by a larger gain.     

Optical pattern formation in alkali metal vapors: Mechanisms, phenomena and use

The phenomena in optical pattern formation are reviewed using a single mirror feedback experiment as an example. The nonlinear medium is sodium metal vapor. We discuss the mechanism responsible for the spontaneous formation of spatial structures and the selection between simple periodic patterns as hexagons and squares. Introducing a polarization changing element in the feedback loop, quasiperiodic patterns with a twelve-fold rotational symmetry are obtained and their properties are studied by filtering methods in Fourier space. Finally, we discuss the formation of localized states as spatial solitons due to a self-induced lensing effect. Their interaction is studied and the formation of clusters provides a link between localized states and extended patterns. Genuine optical features, such as the polarization degrees of freedom, the experimental accessibility of the Fourier space, and the possibility to control the optical susceptibility of atomic vapors by means of an external magnetic field, provide very powerful tools for investigating the principles of pattern formation. Received: 15 July 2000 / Revised version: 7 November 2000 / Published online: 6 December 2000     

光纤喇曼放大泵浦光与信号光的关联研究

分析了信号光入射后可能发生的效应基于电磁场理论,综合光纤受激喇曼散射(SRS)效应、建立了光纤喇曼放大泵浦光与斯托克斯光、信号光的关联方程,给出了系统增益形成正反馈条件的表达式,结果表明:在光纤参量满足增益正反馈的条件下,当SRS增益对应于非饱和增益工作状态时入射信号光得到放大,SRS增益愈高则放大倍数愈高,并有区段上SRS增益相对低时其关联程度相对高.     

Stimulated raman scattering of fuel droplets

The strong stimulated Raman scattering (SRS) from diesel fuel droplets has the potential of providing the relative concentration of multicomponent fuel and the absolute size of individual droplets. The morphology-dependent resonances (MDRs) of a sphere cause the droplet to act as an optical resonator which greatly lowers the SRS threshold. The number density, quality factor, and frequency shift of several MDRs are calculated as a function of the ratio of the index of refraction of the liquid and the surrounding gas, which approaches unity at the thermodynamic critical condition for the fuel spray. The SRS spectra of monodispersed droplets of toluene, pentane, Exxon-Aromatic-150, and Mobil D-2 are presented. The exponential growth region of the SRS intensity I _1S as a function of the input laser intensity I _input is investigated for the toluene carbon ring breathing mode v ₂ and the pentane C-H stretching region. The I _1S ratio of toluene and pentane is measured as a function of the ratio of the toluene and pentane concentration for monodispersed droplets. The reduced fluctuation in I _1S when I _input is changed from multimode to single-mode is displayed as a histogram of the I _1S of the v ₂ mode of toluene droplets.     

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     

Highly efficient Raman conversion in O2 pumped by a seeded narrow band second-harmonic Nd:YAG laser

The first-Stokes conversion efficiency for a stimulated Raman scattering (SRS) is usually very low in gaseous oxygen media. In 3.0 Mpa O₂, a single longitudinal mode second harmonic Nd:YAG laser pump source gives a typical vibrational first-Stokes conversion efficiency of only 2.5%, In comparison, the accompanying stimulated Brillouin scattering (SBS) attains a reflectivity of 67%. However, by seeding an OPO beam into the Raman cavity, the first-Stokes photon conversion efficiency now attains a peak value of 54%, while the SBS reflectivity reduces to 5% in a 6.1 Mpa 41:59 O₂/ He mixture. This 54% efficiency was obtained for a seeder laser pulse-width less than one half that of pump laser (6.8 ns). A first-Stokes peak power conversion efficiency as high as 88% has been obtained when the pump and seeder pulse peaks coincide. So, we may expect a higher first-Stokes photon conversion efficiency if the seeder pulse-width can be made equal to or larger than that of the pump pulse. On the other hand, the beam quality of the first-Stokes in an O₂/ He mixture excels that of the pump laser for a seeder energy of 5 mJ and pump energy of 50 mJ. However, at pump energies higher than 105 mJ and a pump laser repetition rate of 10 Hz, the thermal defocusing effect worsens the first-Stokes beam quality. This thermal defocusing effect is a result of the Raman heat release and could be eliminated by fast circulating and cooling the Raman gas medium.     

Influencing factors of external fluorescence seeding enhancing stimulated Raman scattering in liquid‐core optical fiber

This paper reports an external fluorescence seeding technology enhancing the stimulated Raman scattering (SRS) in liquid‐core optical fiber (LCOF). By surrounding a small section of LCOF with a glass capillary and a solution of Rhodamine 6G filled between them to separate fluorescent dye and Raman medium, the initial intensity of SRS is linearly amplified by external fluorescence seeding, and then the SRS of LCOF can be enhanced effectively. Experimental results show that both the concentration of fluorescent dye and the seeding position have an influence on enhancement of SRS. The maximum enhancement of Stokes lines is obtained when the concentration of dye solution is optimized at ~10⁻⁶ mol/l and seeding position is located at the input end. Copyright © 2013 John Wiley & Sons, Ltd.     

光纤-氮系统的受激拉曼散射

在实验上,对三种不同长度的石英光纤进行了高温高压扩氮制成了光纤氮拉曼增益介质,给出了测试SRS光谱的实验装置、光谱及传输模式照片.在理论上和实验上讨论了阈值条件和谱线宽度与抽运光功率之间的关系,实验结果和理论计算基本符合.同时,在实验上还观测到微弱的氮分子和石英分子间的耦合模式. 关键词： 光纤-氮系统 受激拉曼散射 传输模式 阈值条件     

Generalized wave coupling theory of linear electro-optic effect in absorbent medium

Starting from Maxwell’s equations and considering the linear electro-optic effect as a perturbation, we present a generalized wave coupling theory of linear electro-optic effect in absorbent medium. We give the rigorous solution of the resultant equations for a light wave propagating along any direction with an external dc electric field along an arbitrary direction. As an application, we use the theory to discuss the influence of absorption on the light wave in a KTP crystal. The results demonstrate that the absorption coefficients influence not only the amplitude but also the phase of the light wave.