Lu Ke-Qing

Steady-state spatial solitons in biased photovoltaic-photorefractive crystals for short circuits

A theory on spatial solitons in photovoltaic (PV)-photorefractive (PR) crystals for short circuits is developed that gives rise to spatial solitons due to both a bulk PV effect and spatial nonuniform screening of the external field. When the PV effect is neglected these solitons are screening solitons. When the external field is absent these solitons are PV solitons in short and open circuits and it is predicted that grey solitons exist in PV-PR crystals for short circuits. We also show theoretically that PV nonlinearity can be switched from self-defocusing to self-focusing (or vice versa) by adding the external field.

NONLINEAR EFFECTS OF THIRD-AND FIFTH-ORDER ON POLARIZATION BEATS IN A FOUR-LEVEL SYSTEM

We have employed second-order coherence function theory to study the nonlinear effects of third-and fifth-order on polarization beats in a four-level system (TPBFS and FPBFS). It is found that the different temporal behavior of the beat signal in TPBFS and FPBFS depends on the stochastic properties of the lasers and transverse relaxation rate of the transition. We have considered the cases that pump beams have either narrow band or broadband linewidth and found that for both cases a Doppler-free precision can be achieved in the measurement of the energy-level difference between two excited states which are dipolar forbidden from the ground state. We also discussed the spatiall modulation behavior of the beat signal.

Temporal development of open-circuit bright photovoltaic solitons ⁄

This paper investigates the temporal behaviour of open-circuit bright photovoltaic spatial solitons by using numerical techniques. It shows that when the intensity ratio of the soliton, the ratio between the soliton peak intensity and the dark irradiance, is small, the quasi-steady-state soliton width decreases monotonically with the increase of ?, where ? is the parameter correlated with the time, that when the intensity ratio of the soliton is big, the quasi-steady-state soliton width decreases with the increase of ? and then increases with ?, and that the formation time of the steady-state solitons is not correlated with the intensity ratio of the soliton. It flnds that the local nonlinear efiect increases with the photovoltaic fleld, which behaves as that the width of soliton beams is small and the self-focusing quasi-period is short. On the other hand, we also discuss that both the time and the temperature have an efiect on the beam bending.

Modulation instability of quasi-plane-wave optical beams in biased photorefractive-photovoltaic crystals

We investigate the modulation instability of quasi-plane-wave optical beams in biased photorefractive–photovoltaic crystals by globally treating the space-charge field. The modulation instability growth rate is obtained, which depends on the external bias field, on the bulk photovoltaic effect and on the ratio of the optical beams intensity to that of the dark irradiance. Our analysis indicates that this modulation instability growth rate is identical to the modulation instability growth rate studied previously in biased photorefractive–nonphotovoltaic crystals when the bulk photovoltaic effect is negligible for shorted circuits and predicts the modulation instability growth rate in open- and closed-circuit photorefractive–photovoltaic crystals when the external bias field is absent.

RAMAN-ENHANCED POLARIZATION BEATS WITH PHASE-CONJUGATION GEOMETRY

The Raman-enhanced polarization beats (REPB) with phase-conjugation geometry in carbon disulfide are investigated using chaotic and phase-diffusion models. The overall accuracy of using REPB to measure the resonant frequency of Raman-active vibrational mode is determined by the relaxation rates of the Raman mode and the molecular-reorientational grating. Different stochastic models of the laser field only affect the sixth- and fourth-order coherence function. The difference between the REPB and the fourth-order coherence on ultrafast modulation spectroscopy is discussed as well from a physical viewpoint.

Modulation Instability in Biased Photorefractive?Photovoltaic Crystals

We show the modulation instability of broad optical beams in biased photorefractive–photovoltaic crystals under steady-state conditions. This modulation instability growth rate depends on the external bias field, the bulk photovoltaic effect, and the ratio of the optical beam intensity to that of the dark irradiance. Under appropriate conditions, this modulation instability growth rate is the modulation instability growth rate studied previously in biased photorefractive–nonphotovoltaic crystals, and the modulation instability growth rate in open- and closed-circuit photorefractive–photovoltaic crystals can be predicted.

Co-existence of multi-wave mixing in an opening five-level atomic system

We study the co-existence of multi-wave mixing processes in an open five-level atomic system affected by strong coupling fields. Competition among four-wave mixing （FWM） with two dressing fields, the six-wave mixing （SWM） with one dressing field as well as the eight-wave mixing （EWM） propagating along the same direction can be evoked by adjusting laser beams because these multi-wave mixing signals are induced by atomic coherence between common levels. With the dressing fields properly controlled, the FWM and SWM signals can be suppressed while the EWM signals enhanced in the beam. We also amply analyze the difference between the parallel-cascade and sequentical-cascade doubly dressing mechanisms in doubly-dressed four-wave mixing （DDFWM） processes.

Four-wave mixing and six-wave mixing in a four-level confined atomic system

We have investigated coexisting four-wave mixing and six-wave mixing (SWM) in ultra-thin, micrometre and long vapour cells. There exists competition between Dicke-narrowing features and polarization interference in the micrometre cell. The oscillation behaviour of SWM signal intensities and linewidths results from destructive interference. With a larger destructive interference, the SWM signal in ultra-thin cells shows a narrow spectrum, in contrast to the long cell case. Due to the Dicke-narrowing features, a narrow spectrum can be obtained, and such spectra can be used for high precision measurements and metrological standards.

Grey screening-photovoltaic soliton-induced waveguides

This paper shows that waveguides induced by grey screening-photovoltaic solitons are always single mode for all intensity ratios, which are the ratio between the peak intensity of the soliton and the dark irradiance. It finds that the confined energy near the centre of the grey soliton and the propagation constant of the guided mode increase monotonically with increasing intensity ratio. On the other hand, when the soliton greyness increases, the confined energy near the centre of the grey soliton and the propagation constant of the guided mode reduce monotonically. When the bulk photovoltaic effect is neglected for short circuits, these waveguides become waveguides induced by grey screening solitons. When the external bias field is absent, these waveguides become waveguides induced by grey photovoltaic solitons.

Waveguides induced by grey screening solitons

We investigate the properties of waveguides induced by one-dimensional grey screening solitons in biased photorefractive crystals. The results show that waveguides induced by grey screening solitons are always of single mode for all intensity ratios, i.e. the ratios between the peak intensity of the soliton and the dark irradiance. Our analysis indicates that the energy confined near the centre of the grey soliton and the propagation constant of the guided mode of the waveguide induced by the grey screening soliton increase monotonically with intensity ratio increasing. On the other hand, when the soliton greyness increases, the energy confined near the centre of the grey soliton and the propagation constant of the guided mode of the waveguide induced by the grey screening soliton decrease monotonically. Relevant examples are provided where photorefractive crystal is of the strontium barium niobate type.