Polarization-dependent femtosecond laser filamentation in air

We report on a systematic study of the laser polarization effect on a femtosecond laser filamentation in air.By changing the laser's ellipticity from linear polarization to circular polarization, the onset position of laser filament formation becomes farther from the focusing optics, the filament length is shorter, and less laser energy is deposited. The laser polarization effect on air filaments is supported by a simulation and analysis of the polarization-dependent critical power and ionization rates in air.     

Theoretical study on the lasing plasmon of a split ring for label-free detection of single molecules and single nanoparticles

This paper reports the plasmonic lasing of a split ring filled with gain material in water. The lasing mode(1500 nm)is far from the pump mode(980 nm), which can depress the detection noise from the pump light. The laser intensities of the two modes simultaneously increase by more than 10~3 in amplitude, which can intensify the absorption efficiency of the pumping light and enhance the plasmonic lasing. The plasmonic lasing is a sensitive sensor. When a single protein nanoparticle(n = 1.5, r = 1.25 nm) is trapped in the gap of the split ring, the lasing spectrum moves by 0.031 nm, which is much larger than the detection limit of 10~(-5) nm. Moreover, the lasing intensity is also very sensitive to the trapped nanoparticle. It reduces to less than 1/600 when a protein nanoparticle(n = 1.5, r = 1.25 nm) is trapped in the gap.     

Nuclear fusion from Coulomb explosions of deuterated methane clusters subjected to ultraintense femtosecond laser pulses

This paper reports that Coulomb explosions taken place in the experiment of heteronuclear deuterated methane clusters ((CD₄)₂) in a gas jet subjected to intense femtosecond laser pulses (170mJ, 70fs) have led to table-top laser driven DD nuclear fusion. The clusters produced in supersonic expansion had an average size of about 5nm in radius and the laser intensity used was 3×10¹⁷W/cm².The measured maximum and average energies of deuterons produced in the laser--cluster interaction were 60 and 13.5keV, respectively. From DD collisions of energetic deuterons, a yield of 2.5(±0.4)×10⁴ fusion neutrons of 2.45MeV per shot was realized, giving rise to a neutron production efficiency of about 1.5×10⁵ per joule of incident laser pulse energy. Theoretical calculations were performed and a fairly good agreement of the calculated neutron yield with that obtained from the present experiment was found.     

Intensity-dependent asymmetric photoionization in few-cycle laser pulses

The asymmetric photoionization of atoms irradiated by intense, few-cycle laser pulses is studied numerically. The results show that the pulse intensity affects the asymmetric photoionization in three aspects. First, at higher intensities, the asymmetry becomes distinctive for few-cycle pulses of longer durations. Second, as the laser intensity increases, the maximal asymmetry first decreases then increases after it has reached a minimal value. Last, the value of the carrier-envelope phase corresponding to the maximal asymmetry varies with the pulse intensity. This study reveals that the increasing of pulse intensity is helpful for observing the asymmetric photoionization.     

Jet-like structures in photoelectron angular distributions

The photoelectron angular distributions (PADs) of hydrogen atoms in an intense laser field of linear polarization are studied using the S-matrix theory in the length gauge. The PADs show main lobes along the laser polarization and jet-like structures sticking from the waist of main lobes. Our previous prediction, based on a nonperturbative scattering theory of photoionization developed by Guo et al, showing that the number of jets on one side of PADs may increase by one, three, or other odd numbers and may decrease by one when one more photon is absorbed, is confirmed by this treatment. Within the strong-field approximation, good agreement is obtained between these two quite different treatments. We further study the influence of the Coulomb attraction to PADs, by taking a Coulomb--Volkov state as the continuum state of photoelectrons. We find that under the influence of the Coulomb attraction, the PADs change greatly but the predicted phenomena still appear. This study verifies that the jet-like structures have no relation with the angular momentum of photoelectrons.     

Optimization control of modulation-instability gain in photonic crystal fibres with two-zero dispersion wavelengths

We design three kinds of photonic crystal fibres (PCF) with two zero-dispersion wavelengths (ZDWs) using the improved full vector index method (FVIM) and finite-difference frequency domain (FDFD) techniques. Based on these designed fibres, the effect of fibre structure, pump power and wavelength on the modulation instability (MI) gain in the anomalous dispersion region close to the second ZDW of the PCFs is comprehensively analysed in this paper. The analytical results show that an optimal MI gain can be obtained when the optimal pump wavelength (1530~nm) is slightly shorter than the second ZDW (1538~nm) and the optimal pump power is 250~W. Importantly, the total MI gain bandwidth has been increased to 260~nm for the first time, so far as we know, for an optimally-designed fibre with Λ = 1.4~nm and d/Λ = 0.676, and the gain profile became much smoother. The optimal pump wavelength relies on the second ZDW of the PCF whereas the optimal pump power depends on the corporate operation of the optimal fibre structure and optimal pump wavelength, which is important in designing the most appropriate PCF to attain higher broadband and gain amplification.     

An Ultra-Broadband Spatially Dispersed Regenerative Amplifier Free from Spatial Chirp

An ultra-broadband Ti：sapphire regenerative amplifier based on spatially dispersed amplification is demonstrated experimentally. Departing from previous reports, a new design of the cavity gets the amplified pulse free from spatial chirp. Utilizing this new regenerative amplifier, chirped pulses with bandwidth （FWHM） of about 80 nm are obtained, and the bandwidth is limited only by that of the incident seed pulses.     

Experimental Investigation on Selective Excitation of Two-Pulse Coherent Anti-Stokes Raman Scattering

Selective excitation of coherent anti-Stokes Raman scattering from the benzene solution is achieved by adaptive pulse shaping based on genetic algorithm, and second harmonic generation frequency-resolved optical gating （SHG-FROG） technique is adopted to characterize the original and optimal laser pulses. The mechanism for two-pulse coherent mode-selective excitation of Raman scattering is experimentally investigated by modulating the pump pulse in the frequency domain, and it is indicated that two-pulse coherent mode-selective excitation of Raman scattering mainly depends on the effective frequency components of the pump pulse related to specific vibrational mode. The experimental results suggest that two-pulse CARS has good signal-to-background ratio and high sensitivity, and it has attractive potential applications in the complicated molecular system.     

The photoluminescence of ZnSe bulk single crystals excited by femtosecond pulse

This paper reports on the photoluminescence spectra of ZnSe single crystal with trace chlorine excited by the femtosecond laser pulse. Three emission bands, including second-harmonic-generation, two-photon-excited peak and a broad band at 500--700nm, were detected. The thermal strain induced by femtosecond pulse strongly influences the photoluminescence of ZnSe crystal. The corresponding strain \va in ZnSe crystal is estimated to be about 8.8 \ti10^-3 at room temperature. The zinc-vacancy, as the main point defect induced by femtosecond pulse, is successfully used to interpret the broad emission at 500--700nm. The research shows that self-activated luminescence possesses the recombination mechanism of donor--vacancy pair, and it is also influenced by a few selenium defects and the temperature. The rapid decrease in photoluminescence intensity of two-photon-excited fluorescence and second-harmonic generation emission at lower temperature is attributed to the fact that more point defects result in the thermal activation of the two-photo-absorption energy converting to the stronger recombination emission of chlorine--zinc vacancy in 500--700nm. The experimental results indicate that the femtosecond exciting photoluminescence shows a completely different emission mechanism to that of He--Cd exciting luminescence in ZnSe single crystal. The femtosecond laser exhibits a higher sensitive to the impurity in crystal materials, which can be recommended as an efficient way to estimate the trace impurity in high quality crystals.     

低频激光场中二能级原子的高次谐波

在外场频率远远低于能级跃迁频率的情况下，推导出了二能级原子在低频激光场作用下的诱导极化所满足的运动方程，并通过在弱场和超强场两种情况下对该方程的解析求解，得到了在这两种相应情况下产生于二能级原子的高次谐波的特性. 关键词：