Panming Fu

Diode-pumped self-Q-switched single-frequency 946-nm Nd 3+ ,Cr 4+ :YAG microchip laser

A stable low-threshold self-Q-switched diode-pumped 946-nm Nd(3+), Cr(4+):YAG microchip laser operating in single frequency and fundamental transverse mode is reported. The output characteristics of the microchip laser were investigated under cw and pulsed pumping. By combination of the infrared microchip and an external frequency doubler, 473-nm pulses with a conversion efficiency of 18% were achieved.

Resonant photorefractive AlGaAs/GaAs multiple quantum wells grown by molecular beam epitaxy at low temperature

The resonant photorefractive devices using low temperature AlGaAs/GaAs multiple‐quantum‐well structures in a parallel field geometry are demonstrated. The samples are semi‐insulating as grown. The AsGa‐related defects incorporated into the samples during low temperature growth provide the required deep centers. No proton implantation, Cr doping, or annealing is needed for device fabrication. In the photorefractive wave mixing experiment, an output diffraction efficiency higher than 0.84% and a two‐wave‐mixing gain of more than 3000 cm−1 are obtained under a dc electric field of 15 kV/cm.

Self-trapping of Bose-Einstein condensates in optical lattices

The self-trapping phenomenon of Bose-Einstein condensates (BECs) in optical lattices is studied by numerically solving the Gross-Pitaevskii equation. Our numerical results reproduce the self-trapping that was observed in a recent experiment [Anker et al., Phys. Rev. Lett. 94, 020403 (2005)]. However, we do not find that the appearance of the steep edges on the boundaries of the wave packet is the critical signal of the self-trapping. More importantly, we discover that the self-trapping breaks down at long evolution times; that is, the self-trapping in optical lattices is only temporary and has a lifetime. This temporariness is caused by the tunneling of atoms at the edge of the BEC wave packet towards outside wells. Our analysis shows that the phenomena observed numerically can all be understood by regarding the optical lattice as a train of double-well potentials.

The effects of a static electric field on high-order harmonic generation

Based on the full quantum theory of high-order harmonic generation (HOHG) (Lewenstein et al 1994 Phys. Rev. A 49 2117-32), we study the effects of the static electric field on HOHG. It is found that the presence of the static electric field breaks the inversion and reflection symmetry, as a result, the HOHG spectrum exhibits a double-plateau structure. Furthermore, the cut-off of the HOHG no longer corresponds to the maximum kinetic energy the ionized electron can obtain from the laser field. Instead, it corresponds to the maximum kinetic energy of the electron with one return. Finally, for ultra-high static electric field, we find a monotonic decrease of the harmonic intensity as the harmonic order is increased.

Pulse-duration dependence of high-order harmonic generation with coherent superposition state

We make a systematic study of high-order harmonic generation (HHG) in a He+-like model ion when the initial state is prepared as a coherent superposition of the ground state and an excited state. It is found that, according to the degree of the ionization of the excited state, the laser intensity can be divided into three regimes in which HHG spectra exhibit different characteristics. The pulse-duration dependence of the HHG spectra in these regimes is studied. We also demonstrate evident advantages of using coherent superposition state to obtain high conversion efficiency. The conversion efficiency can be increased further if ultrashort laser pulses are employed.

The calculation of photoelectron angular distributions with jet-like structure from scattering theory

Photoelectron angular distributions produced in above-threshold ionization (ATI) are analysed using a nonperturbative scattering theory. The numerical results are in good qualitative agreement with recent measurements. Our study shows that the origin of the jet-like structure arises from the inherent properties of the ATI process and not from the angular momentum of either the initial or the excited states of the atom.

Time-delayed laser-induced double gratings

We have employed chaotic and phase-diffusion models to study the effect of laser coherence on time-delayed laser-induced double gratings (TDLIDG). It is found that, although the temporal behavior of the four-wave mixing signal depends on the stochastic properties of the lasers and the relaxation time of the grating, the modulation of the signal is always damped out when the relative time delay between two pump beams is much longer than the laser coherence time. We performed a TDLIDG experiment mediated by thermal effects to study the temporal behavior of the modulation, and we obtained the frequency difference between two light beams with an accuracy limited by the laser linewidth. We also studied the influence of the phase change in the optical path on TDLIDG.

High-order harmonic generation with Rydberg atoms by using an intense few-cycle pulse

We demonstrate that high-order harmonic generation (HHG) with both high cutoff frequency and high conversion efficiency can be realized by using a Rydberg atom in a few-cycle laser pulse. This is because a Rydberg state has a large electron orbital radius and small binding energy; therefore an electron in the Rydberg state can be ionized easily and accelerated directly toward the core under the interaction of a few-cycle laser pulse, leading to emission of harmonic photons. In this case, the tunneling process of the electron is not involved and, hence, the conversion efficiency and the cutoff frequency of harmonic generation can be higher than that predicted by the conventional three-step model.

Fourth-order interference on polarization beats in a four-level system

We have employed chaotic and phase-diffusion models to study the effects of laser fourth-order coherence on polarization beats with phase-conjugation geometry in a four-level system (PBFS). We found that the temporal behavior of the beat signal depends on the stochastic properties of the lasers and the transverse relaxation rate of the transition. The modulation terms of the beat signal depend on the second-order coherence function, which is determined by the laser line shape. Inasmuch as different stochastic models of the laser field affect only the fourth-order coherence function, they have little influence on the general temporal modulation behavior of the beat signal. The different roles of phase fluctuation and amplitude fluctuation are pointed out. The cases that pump beams have either narrow-band or broadband linewidth are considered, and it is found that for both cases a Doppler-free precision in the measurement of the energy-level difference between two states that are dipolar forbidden from the ground state can be achieved. We also discuss the difference between the PBFS and ultrafast modulation spectroscopy from a physical viewpoint.

Observation of the beat between two independent light sources by a method of time-delayed laser-induced double gratings

A novel method of time-delayed laser-induced double gratings has been proposed to study the beat between two independent light sources. Our experimental results indicate that we can obtain good interference patterns even when the coherence time of the laser source is only a few picoseconds. This new technique can be used to measure the frequency difference between two laser fields.