Lu Pei-Xiang

Coloured Conical Emission in BBO Crystal Pumped by Second Harmonic Femtosecond Pulses

Coloured conical emission (CCE) is investigated experimentally in a β-barium borate crystal excited by intense second harmonic femtosecond pulses. Contrary sequences of green and red conical emission with variable diameters are observed at different incidence angles, which is consistent with the calculation results based on the phase matching condition. As its broad range spectrum, CCE offers an alternative means to produce an ultrafast broadband light source. It is found that the spectrum of green CE shifts toward longer wavelengths as the length of BBO crystal increased.

All-Fibre Ytterbium-Doped Photonic Crystal Fibre Laser with High Efficiency

A double-cladding ytterbium-doped photonic crystal fibre (PCF) with a 350-μm2 effective area is fabricated. The measurement results show that the PCF has high absorption peak at 915 nm. Its fluorescence lifetime is 840μs. Laser experiments with all-fibre configurations are carried out with this fibre. A continuous-wave output power of 3.96 W is achieved with a 5.2 W launched pump power. The central wavelength of the output spectrum is 1080.22nm. The results show that the PCF laser has a high slope efficiency of 79.6% and light conversion efficiency of 76.2%.

Dual-Peak Bragg Gratings Inscribed in an All-Solid Photonic Bandgap Fiber for Sensing Applications

Bragg gratings are inscribed in an all-solid photonic bandgap fiber by use of femtosecond laser irradiation. Dual-peak structure is observed in the transmission spectrum of the induced grating, which is formed by the coupling between the forward-propagating fundamental core mode and the backward-propagating core mode or supermode. Sensing characteristics of the device are investigated experimentally by employing strain and temperature tests, and similar behavior is obtained for both resonant peaks. The strain and temperature sensitivities are 0.968pm/μ and 12.01pm/°C, and 0.954pm/μ and 12.04pm/°C, for the two peaks, respectively. This device would find potential applications in real optical fiber sensing without extra reference gratings.

Classical Effects of Carrier–Envelope Phase on Nonsequential Double Ionization

A classical microcanonical 1+1-dimensional model is used to investigate the ion momentum distributions in nonsequential double ionization with linearly polarized few-cycle pulses. We find that the ion momentum distribution has a strong dependence on the carrier–envelope phase of the few-cycle pulse, which is consistent with the experimental results qualitatively. Back analysis shows that the ionization probability of the first electron at different phases and its returning kinetic energy play the main role on the ion momentum distributions.

Further Acceleration of MeV Electrons by a Relativistic Laser Pulse

With the development of photocathode rf electron gun, electrons with high-brightness and mono-energy can be obtained easily. By numerically solving the relativistic equations of motion of an electron generated from this facility in laser fields modelled by a circular polarized Gaussian laser pulse, we find the electron can obtain high energy gain from the laser pulse. The corresponding acceleration distance for this electron driven by the ascending part of the laser pulse is much longer than the Rayleigh length, and the light amplitude experienced on the electron is very weak when the laser pulse overtakes the electron. The electron is accelerated effectively and the deceleration can be neglected. For intensities around 10(19) W(.)mu m(2)/cm(2), an electrons energy gain near 0.1 GeV can be realized when its initial energy is 4.5 MeV, and the final velocity of the energetic electron is parallel with the propagation axis. The energy gain can be up to 1 GeV if the intensity is about 10(21) W(.)mu m(2)/cm(2). The final energy gain of the electron as a function of its initial conditions and the parameters of the laser beam has also been discussed.

Correlated electron dynamics in strong-field double ionization

In intense laser-atom/molecule interaction, there are many new phenomena involving multielectron effects. As one of the most important strong-field process, strong-field double ionization of atoms and molecules, especially the strong-field nonsequential double ionization (NSDI), exhibits highly correlated behaviors, providing a clean way to study electron correlation in nature. We review the microscopic electron dynamics of NSDI revealed by recent experimental studies based on the COLTRIMS and the related theory contributions. We summarized the laser intensity-dependent microscopic electron dynamics of NSDI. At low laser intensities, NSDI is dominated by the recollision-excitation with subsequent field ionization, and the time delay between the final ionizations of the two electrons could results in the experimental observed back-to-back emission of the electron pairs. As the laser intensity increases, recollision induced direct ionization dominates NSDI and two electrons ionized almost simultaneously after recollision. In this process the final-state ion-electron attraction and electron repulsion significantly affect the details of the electron correlation pattern. As the laser intensity further increases, the role of final-state ion-electron and electron-electron interactions becomes less important, and the asymmetric energy sharing at recollision is prevalent which accounts for the strange V-like shape in the correlated electron momentum spectra at very high laser intensity. We also introduced the control of the correlated electron dynamics. It has been theoretically predicted that the electron pairs can be controlled to exhibit correlated or anticorrelated behavior with the two-color field, which has been proved by experiment. We also review the progress on strong-field sequential double ionization (SDI). Recent experiments have observed many phenomena which conflicted with the predictions of the previous theoretical models of SDI. Here we introduced a recent developed model of strong-field SDI, which successfully explained the new experimental results and predicted many interesting results.

Coupled Optical Tamm States in a Planar Dielectric Mirror Structure Containing a Thin Metal Film

The coupling between two optical Tamm states (OTSs) with the same eigenenergy is numerically investigated in a planar dielectric mirror structure containing a thin metal film. The reflectivity map in this structure at normal incidence is obtained by applying the transfer matrix method. Two splitting branches appear in the photonic bandgap region when both adjacent dielectric layers of metal film are properly set. The splitting energy of two branches strongly depends on the thickness of the metal film. According to the electric field distribution in this structure, it is found that the high-energy branch corresponds to the antisymmetric coupling between two OTSs, while the low-energy branch is associated with the symmetric coupling between two OTSs. Moreover, the optical difference frequency of two branches is located in a broad terahertz region.

Electron Correlation in Nonsequential Double Ionization of Helium by Two-Color Pulses

We investigate the momentum and energy correlations between the two electrons from nonsequential double ionization (NSDI) of helium by strong two-color pulses with the classical three-dimensional ensemble model. The correlated momentum distribution in the direction parallel to the laser field exhibits an arc-like structure and the sum-energy spectrum shows a sharp peak for the NSDI of helium in the two-color fields. Back analysis reveals that the narrow time interval during which recollisions occur, the low returning energy and the short time delay between recollision and double ionization lead to the novel momentum and energy correlations.

Infrared Femtosecond Laser Direct-Writing Digital Volume Gratings in Fused Silica

We demonstrate that digital volume gratings can be fabricated in fused silica glass conveniently by direct femtosecond laser writing. The diffraction efficiencies of volume gratings can be essentially modulated by simply stacking and offsetting the unit structure. A series of volume gratings, which have the pitches of 5 μm and the size of 1 mm × 1 mm, have been fabricated with the writing speed of 500 μm/s, with which the processing period of each grating layer could be reduced to several minutes with a 1-kHz femtosecond laser system. Results show that the power spectrum of the diffracted waves of the volume gratings are dependent on the layer gap and layer offsetting.

Off-Resonant Third-Order Optical Nonlinearity of Au Nanoparticle Array by Femtosecond Z-scan Measurement

A periodic triangular-shaped Au nanoparticle array is fabricated on a quartz substrate using nanosphere lithography and pulsed laser deposition, and the linear and nonlinear optical properties of metal particles are studied. The morphology of the polystyrene nanosphere mask (D = 820 nm) and the Au nanoparticle array are investigated by scanning electron microscopy. The surface plasmon resonance absorption peak is observed at 606 nm, which is in good agreement with the calculated result using the discrete dipole approximation method. By performing the Z-scan method with femtosecond laser (800 nm, 50 fs), the optical nonlinearities of Au nanoparticle array are determined. The results show that the Au particles exhibit negative nonlinear absorption and positive nonlinear refractive index with the effective third-order optical nonlinear susceptibility χ(3)eff can be up to (8.8 ± 1.0) × 10−10 esu under non-resonant femtosecond laser excitation.