Peifen Lu

Plasma waveguide array induced by filament interaction

We demonstrate that interference-assisted coalescence of two noncollinearly overlapped filaments creates a wavelength-scale periodic plasma density modulation to guide the input pulses equivalently as a photonic crystal plasma waveguide. The periodic self-channeling is evidenced by the direct observation of the filament coalescence, which reveals wavelength-scale spatial widths and periodicity dependent on the crossing angles and intensity ratios between the incident filaments.

Verification of the physical mechanism of THz generation by dual-color ultrashort laser pulses

We experimentally verified the physical mechanism of the dual-color ultrashort laser-driven THz wave generation in air by measuring its dependence on the dual-color field phase difference. Our results evidenced that the THz generation from the laser-induced plasma is dominated by the photo-current mechanism. A continuous rotation of THz polarization was observed by changing the dual-color phase difference, facilitating coherent control of the THz intensity and polarization.

Intense ultrafast light kick by rotational Raman wake in atmosphere

We report that intense ultrafast light can be particlelike kicked through the rotational Raman wake of the impulsively pre-excited diatomic molecules in atmosphere, manifested by controllable repulsion and attraction of intense pulses delayed by molecular alignment revival periods, and by mutual fusion of orthogonally polarized intense pulses nearby zero time delay. The rotational Raman wake facilitates longer ranges of interaction on the intense ultrafast light kicks than the plasma wake and Kerr effect.

Supercontinuum pulse measurement by molecular alignment based cross-correlation frequency resolved optical gating

We demonstrate that complex supercontinuum and few-cycle ultrashort laser pulses can be fully characterized by using a cross-correlation frequency-resolved optical gating with molecular alignment induced birefringence functioned as a gate. The temporal envelope and phase of the broadband supercontinuum pulse are retrieved by the principal component generalized projection algorithm. This technique shows advantages without phase-matching constraint that may limit the measurable spectral bandwidth, experimental robustness in operating through the whole transparent spectral region of the molecular gases, and intensity sensitivity to measure weak pulses which is inherited from the intrinsic linear process in recording the molecular birefringence induced polarization spectroscopy. Experimental measurements of a few-cycle pulse in the visible region of 525-725 nm confirm that the molecular alignment gating supports a full field characterization of the ultrashort pulse around 10 fs in duration.

Interaction of two parallel femtosecond filaments at different wavelengths in air

We study the interaction of two parallel-launched femtosecond filaments at their fundamental-wave and second-harmonic frequencies in air. Besides the Kerr effect within the pulse duration, the impulsive alignment of the diatomic molecules in the fundamental-wave filament leads to controllable and field-free achievable attraction, fusion, or repulsion of the second-harmonic filament. The molecular-alignment-assisted filament interaction is further confirmed by the fluorescence intensity variation and spectral modulation of the second-harmonic filament at various molecular alignment revivals.

Ultrafast optical imaging by molecular wakes

Ultrafast optical imaging is demonstrated by ultrashort laser pulse induced impulsive molecular alignment, involving optical image storage in the created molecular wakes followed by periodic readout and display. For diatomic molecules in air, both raised and intagliated monochromatic images are demonstrated, which is field-free, periodically achievable, and works as an ultrafast buffer memory for the imprinted optical images. Analogous to the holographic imaging, the phase information of a three-dimensional object can be revealed at various time delays.

Cross-correlation frequency-resolved optical gating by molecular alignment for ultraviolet femtosecond pulse measurement

We experimentally demonstrate a molecular-alignment-based cross-correlation frequency-resolved optical gating technique for ultraviolet femtosecond pulse measurement, which use laser-induced impulsive alignment of gaseous molecules as the gate function instead of the phase-matched frequency mixing process in a nonlinear crystal. The spectrogram measurements and retrieving results of the intensity and phase of the second and third harmonic pulses centered at 400 and 267 nm are presented.

Photon Energy Deposition in Strong-Field Single Ionization of Multielectron Molecules

Molecules exposed to strong laser fields may coherently absorb multiple photons and deposit the energy into electrons and nuclei, triggering the succeeding dynamics as the primary stage of the light-molecule interaction. We experimentally explore the electron-nuclear sharing of the absorbed photon energy in above-threshold multiphoton single ionization of multielectron molecules. Using CO as a prototype, vibrational and orbital resolved electron-nuclear sharing of the photon energy is observed. Different from the simplest one- or two-electron systems, the participation of the multiple orbitals and the coupling of various electronic states in the strong-field ionization and dissociation processes alter the photon energy deposition dynamics of the multielectron molecule. The population of numerous vibrational states of the molecular cation as the energy reservoir in the ionization process plays an important role in photon energy sharing between the emitted electron and the nuclear fragments.

Humidity measurement in air using filament-induced nitrogen monohydride fluorescence spectroscopy

Strong fluorescence emissions from nitrogen monohydride (NH) free radical at 336 nm were observed in air, when water vapor was present in a 400 nm femtosecond laser filament. The generated NH radical is ascribed to chemical reaction between nitrogen and water vapor during laser filamentation. The backscattered fluorescence from NH shows an exponential increase with increasing filament length, indicating amplified spontaneous emission. The fluorescence intensity of NH was found to be linearly dependent on the relative humidity around the filament region, and a record of NH fluorescence detection at 2 m was achieved, which opens up the possibility for remote humidity measurements.

Directional bond breaking by polarization-gated two-color ultrashort laser pulses

We experimentally investigate directional bond breaking in dissociative single ionization of H2 driven by circularly polarized two-color ultrashort pulses of counter- and co-rotating laser fields. Trefoil or semilunar patterns of directional proton emission in a two-dimensional space spanned by the laser fields are observed, which can be finely controlled by varying the relative phase of the counter- or co-rotating circularly polarized two-color fields, respectively. Our results open new possibilities to manipulate two-dimensional directional bond breaking of molecules by strong laser fields.