Shiqun Li

Monolithic self-Q-switched Cr,Nd:YAG laser.

We report the operation of a diode-pumped monolithic self-Q-switched Cr,Nd:YAG laser in which the codoped ions create saturable absorption for Nd(3+) laser emission at 1064 nm. With a 70-microm beam diameter in the gain medium, the Q-switched pulse has a duration of 3.5 ns and a peak power of 2 kW. The output is linearly polarized with an extinction ratio of 600:1. The pulse-to-pulse intensity fluctuation is less than the instrument resolution of 0.25%. A 5-mm-long KTP crystal butted against the monolithic cavity produces 2-ns-long pulses at 532nm with a peak power-conversion efficiency of 30%.

Self-Q-switched diode-end-pumped Cr,Nd:YAG laser with polarized output

We report self-Q-switching operation of a miniature diode-end-pumped Cr,Nd:YAG laser in which the chromium ions act as a saturable absorber for the Nd3+ laser emission at 1064 nm. The laser output is highly polarized, with an extinction ratio of 600:1. The use of laser host crystal doped with saturable absorber can lead to the development of monolithic Q-switched solid-state lasers.

Self-stabilized single-longitudinal-mode operation in a self-Q-switched Cr,Nd:YAG laser

It is shown, both theoretically and experimentally, that stable single-longitudinal-mode operation, with transform-limited spectral linewidth and without pulse-to-pulse mode competition, can be obtained in a monolithic self-Q-switched Cr,Nd:YAG solid-state laser with a distributed saturable absorber. In this system, the lasing mode establishes a loss grating and thereby stabilizes itself.

Phase locking in a two-element laser array: a test of the coupled-oscillator model.

The steady-state and transient dynamics of phase locking in a two-element Nd:YAG laser array have been studied. By creating two evanescent-coupled lasers in a Nd:YAG étalon using diode end pumping, the coupling strength between the laser elements in the array can be continuously varied by adjusting the positions of the pumping beams. This allows the observation of the phase-locking process over a wide range of coupling strength. We have found that the development of the phase-locked state is as fast as the onset of lasing without an evolutionary process. The instantaneous locking is also independent of the coupling strength once the coupling is strong enough to ensure phase locking. These phenomena disagree with the predictions based on the time-dependent coupled-mode theory of laser-array dynamics. Our experimental study and theoretical analysis have led to the conclusion that all predictions of optical instability in laser arrays need to be reexamined.

Critical Behavior of a Strongly Interacting 2D Electron System

With decreasing density n(s) the thermopower S of a low-disorder two-dimensional electron system in silicon is found to exhibit a sharp increase by more than an order of magnitude tending to a divergence at a finite disorder-independent density n(t) consistent with the critical form (-T/S) is proportional to (n(s)-n(t))(x) with x=1.0±0.1 (T is the temperature). Our results provide clear evidence for an interaction-induced transition to a new phase at low density in a strongly interacting 2D electron system.

Stable and bistable SQUID metamaterials

We investigate the permeability of metamaterials composed of superconducting quantum interference devices for a microwave probe field and derive the frequency spectrum of permeability and physical conditions for negative (real part) permeability, which may be essential to future experimental research on the new kinds of metamaterial. We find that both the resonance frequency and the permeability of the structure can be smoothly tuned over a large range via a DC bias field in the sub-hysteresis case, whereas in the hysteresis case bistability on both permeability and resonance frequency can occur. In addition, negative permeability at extremely low frequency (much lower than the resonance frequency of LC oscillation) is possible.

The new phenomena of orthogonally polarized lights in laser feedback

Some new phenomena of orthogonally polarized lights are observed in the laser-feedback system and in the new system that combines the frequency splitting technique with laser feedback. Results show that there are obvious mode competition between orthogonally polarized lasers throughout the laser-feedback process. This mode competition shows randomness. Further analysis shows that this mode competition between orthogonally polarized lights can effectively increase the laser intensity changing rate relative to the piezoelectric transducer voltage, which provide a new way to improve the axial resolution of a laser-feedback microscope.

Grey spatial solitons in photorefractive polymer

Abstract It is predicted that grey spatial solitons can exist in biased photorefractive polymer. The functional relationships of the transverse velocity, spatial width and phase profile of these grey solitons with their normalized intensity and degree of ‘greyness’ are obtained. The polarization properties of these grey solitons are discussed. The intensity full width at half maximum of these grey solitons is found to be inversely proportional to the absolute value of the external bias field.

Tilt engineering of exchange coupling at G-type SrMnO3/(La,Sr)MnO3 interfaces

With the recent realization of hybrid improper ferroelectricity and room-temperature multiferroic by tilt engineering, “functional” octahedral tilting has become a novel concept in multifunctional perovskite oxides, showing great potential for property manipulation and device design. However, the control of magnetism by octahedral tilting has remained a challenging issue. Here a qualitative and quantitative tilt engineering of exchange coupling, one of the magnetic properties, is demonstrated at compensated G-type antiferromagnetic/ferromagnetic (SrMnO3/La2/3Sr1/3MnO3) interfaces. According to interfacial Hamiltonian, exchange bias (EB) in this system originates from an in-plane antiphase rotation (a−) in G-type antiferromagnetic layer. Based on first-principles calculation, tilt patterns in SrMnO3 are artificially designed in experiment with different epitaxial strain and a much stronger EB is attained in the tensile heterostructure than the compressive counterpart. By controlling the magnitude of octahedral tilting, the manipulation of exchange coupling is even performed in a quantitative manner, as expected in the theoretical estimation. This work realized the combination of tilt engineering and exchange coupling, which might be significant for the development of multifunctional materials and antiferromagnetic spintronics.

High-temperature thermodynamics of strongly interacting s-wave and p-wave Fermi gases in a harmonic trap

We theoretically investigate the high-temperature thermodynamics of a strongly interacting trapped Fermi gas near either s-wave or p-wave Feshbach resonances, using a second-order quantum virial expansion. The second virial coefficient is calculated based on the energy spectrum of two interacting fermions in a harmonic trap. We consider both isotropic and anisotropic harmonic potentials. For the two-fermion interaction, either s-wave or p-wave, we use a pseudopotential parametrized by a scattering length and an effective range. This turns out to be the simplest way of encoding the energy dependence of the low-energy scattering amplitude or phase shift. This treatment of the pseudopotential can be easily generalized to higher partial-wave interactions. We discuss how the second virial coefficient and thermodynamics are affected by the existence of these finite-range interaction effects. The virial expansion result for a strongly interacting s-wave Fermi gas has already been proved very useful. In the case of p-wave interactions, our results for the high-temperature equation of state are applicable to future high-precision thermodynamic measurements for a spin-polarized Fermi gas near a p-wave Feshbach resonance.