arXiv subjects

Zhizhan Xu

Publications and source records attributed to Zhizhan Xu.

53 records · Page 3Linked to original sources

arXiv2013

Scheme for proton-driven plasma-wakefield acceleration of positively charged particles in a hollow plasma channel

A new scheme for accelerating positively charged particles in a plasma wakefield accelerator is proposed. If the proton drive beam propagates in a hollow plasma channel, and the beam radius is of order of the channel width, the space charge force of the driver causes charge separation at the channel wall, which helps to focus the positively charged witness bunch propagating along the beam axis. In the channel, the acceleration buckets for positively charged particles are much larger than in the blowout regime of the uniform plasma, and stable acceleration over long distances is possible. In addition, phasing of the witness with respect to the wave can be tuned by changing the radius of the channel to ensure the acceleration is optimal. Two dimensional simulations suggest that, for proton drivers likely available in future, positively charged particles can be stably accelerated over 1 km with the average acceleration gradient of 1.3 GeV/m.

physics.plasm-ph↗

arXiv2013

Signature of multi-channel interference in high-order harmonic generation from N2 driven by intense mid-infrared pulses

We investigate the multi-electron dynamics in high-order harmonic generation (HHG) from N2 molecules. Clear spectral minima are observed in the cutoff region at all three mid-infrared wavelengths (i.e., 1300, 1400 and 1500 nm) chosen in our experiment. It is found that the positions of the spectral minima do not depend on the alignment angles of molecules. In addition, the spectral minima shift almost linearly with the increasing laser intensity at all three wavelengths, which provides a strong evidence on the dynamic multi-channel interference origin of these minima. The advantages of observation of dynamic multi-channel interference based on HHG driven by long wavelength lasers are discussed.

physics.atom-ph↗

arXiv2013

Reconstruction of a Rotational Wavepacket of Inverted Molecular Ions in an Intense Femtosecond Laser Field

We report on generation of a rotational wavepacket in the ground vibronic state (v = 0) of excited electronic B2Σu+ state of N2+ in a femtosecond laser induced plasma spark. Decoding of the rotational wavepacket is achieved with the frequency-resolved seed-amplified air laser spectrum resulting from the population inversion between the B2Σu+-X2Σg+ states of N2+ in the plasma. We also observe that the rotational wave-packet leads to modulation of the amplified seed signals in the time domain using a pump-probe scheme, which can be well reproduced by theoretical calculation. Our results demonstrate that the air laser provides an ideal probe for remote characterization of molecular rotational states distribution in a femtosecond laser induced filament.

physics.atom-ph↗

arXiv2013

Tomographic reconstruction of molecular orbitals with twofold mirror antisymmetry: overcoming the nodal plane problem

We propose a new method to overcome the nodal plane problem for the tomographic reconstruction of molecular orbitals with twofold mirror antisymmetry in the length form based on high-order harmonic generation. It is shown that, by carrying out the reconstruction procedure in the rotating laboratory frame using the component of the dipole moment parallel to the electron recollision direction, the nodal plane problem is avoided and the target orbital is successfully reconstructed. Moreover, it is found that, the proposed method can completely avoid the additional artificial lobes found in the results from the traditional method in the velocity form and therefore provides a more reliable reproduction of the target orbital.

physics.chem-ph↗

arXiv2013

Influence of large permanent dipoles on molecular orbital tomography

The influence of large permanent dipoles on molecular orbital tomography via high-order harmonic generation (HHG) is investigated in this work. It is found that, owing to the modification of the angle-dependent ionization rate resulting from the Stark shift, the one-side-recollision condition for the tomographic imaging can not be satisfied even with the few-cycle driving pulses. To overcome this problem, we employ a tailored driving pulse by adding a weak low-frequency pulse to the few-cycle laser pulse to control the HHG process and the recollision of the continuum electrons are effectively restricted to only one side of the core. Then we carried out the orbital reconstruction in both the length and velocity forms. The results show that, the orbital structure can only be successfully reproduced by using the dipole matrix elements projected perpendicular to the permanent dipole in both forms.

physics.atom-ph↗

arXiv2012

Role of the Coulomb potential on the ellipticity in atomic high-order harmonics generation

The role of the Coulomb potential on the generation of elliptically polarized high-order harmonics from atoms driven by elliptically polarized laser is investigated analytically. It is found that the Coulomb effect makes a contribution to the harmonic ellipticity for low harmonic orders and short quantum path. By using the strong-field eikonal-Volkov approximation, we analyze the influence of the Coulomb potential on the dynamics of the continuum state and find that the obtained harmonic ellipticity in our simulation originates from the break of symmetry of the continuum state.

physics.atom-ph↗

arXiv2012

Remote creation of strong and coherent emissions in air with two-color ultrafast laser pulses

We experimentally demonstrate generation of strong narrow-bandwidth emissions with excellent coherent properties at ~391 nm and ~428 nm from molecular ions of nitrogen inside a femtosecond filament in air by an orthogonally polarized two-color driver field (i. e., 800 nm laser pulse and its second harmonic). The durations of the coherent emissions at 391 nm and 428 nm are measured to be ~2.4 ps and ~7.8 ps respectively, both of which are much longer than the duration of the pump and its second harmonic pulses. Furthermore, the measured temporal decay characteristics of the excited molecular systems suggest an "instantaneous" population inversion mechanism that may be achieved in molecular nitrogen ions at an ultrafast time scale comparable to the 800 nm pump pulse.

physics.optics↗

arXiv2012

Harmonic-seeded remote laser emissions in N2-Ar, N2-Xe and N2-Ne mixtures: a comparative study

We report on the investigation on harmonic-seeded remote laser emissions at 391 nm wavelength from strong-field ionized nitrogen molecules in three different gas mixtures, i.e., N2-Ar, N2-Xe and N2-Ne. We observed a decrease in the remote laser intensity in the N2-Xe mixture because of the decreased clamped intensity in the filament; whereas in the N2-Ne mixture, the remote laser intensity slightly increases because of the increased clamped intensity within the filament. Remarkably, although the clamped intensity in the filament remains nearly unchanged in the N2-Ar mixture because of the similar ionization potentials of N2 and Ar, a significant enhancement of the lasing emission is realized in the N2-Ar mixture. The enhancement is attributed to the stronger third harmonic seed, and longer gain medium due to the extended filament.

physics.optics↗

arXiv2012

Ultrafast laser-induced subwavelength structures towards nanoscale: the significant role of plasmonic effects

Nowadays, via controlling surface plasmons (SPs) on elaborate man-made structures, plasmonics aiming at manipulating light beyond the diffraction limit has aroused great interest. Here, nevertheless, we demonstrate in short-pulse laser ablation ultrafast active plasmonic structures spontaneously generate in virtue of plasmonic effects rather than human ingenuity. First, the splitting of laser-induced subwavelength gratings that is experimentally evidenced on ZnO, Si, and GaAs, is confirmed to originate in the conversion of SP modes from the resonant to the nonresonant mode and further to the inphase or antiphase asymmetric mode. Further, as pulse number increases the universal scaling-down of laser-induced structures is derived from the conversion of physical regimes of plasmonic interaction from the optical to the electrostatic regime, which may arouse quasistatic SPs with interacting scales far beyond the diffraction limit and result in the ultrafast, non-thermal ablation for extraordinary electrostatic enhancement. Generally, the plasmonic mechanisms reveal the link between the deep-subwavelength space-scale and the ultra-short timescale for ultrafast laser-induced nanostructures: basically, "nanoscale" tends to eliminate electromagnetic retardation effects and bring an instant respond to the incident field, and arouse electrostatic interactions with giant local-field enhancement facilitating ultrafast ablation driven by tremendous electrostatic forces. Thus, active plasmonic structures provided with simultaneous "nanoscale" and "ultrafast" is apt to form in ultrafast ablation. In addition, the plasmonic mechanisms can act as a powerful "evolutionary force" acting in multipulse ablation and promoting the growth of resonant nanostructures-various typical plasmonic structures may be self-generated through multipulse evolution, which can be considered as a kind of natural plasmonics.

physics.optics↗

arXiv2012

Waveform-Controlled Terahertz Radiation from the Air Filament Produced by Few-Cycle Laser Pulses

Waveform-controlled Terahertz (THz) radiation is of great importance due to its potential application in THz sensing and coherent control of quantum systems. We demonstrated a novel scheme to generate waveform-controlled THz radiation from air plasma produced when carrier-envelope-phase (CEP) stabilized few-cycle laser pulses undergo filamentation in ambient air. We launched CEP-stabilized 10 fs-long (~ 1.7 optical cycles) laser pulses at 1.8 μm into air and found that the generated THz waveform can be controlled by varying the filament length and the CEP of driving laser pulses. Calculations using the photocurrent model and including the propagation effects well reproduce the experimental results, and the origins of various phase shifts in the filament are elucidated.

physics.optics↗

arXiv2012

Direct generation of intense extreme ultraviolet supercontinuum with 35 fs, 11 mJ pulses from a femtosecond laser amplifier

We report on the generation of intense extreme ultraviolet (EUV) supercontinuum with photon energies spanning from 35 eV to 50 eV (i. e., supporting an isolated attosecond pulse with a duration of ~271 as) by loosely focusing 35 fs, 11 mJ pulses from a femtosecond laser amplifier into a 10-mm long gas cell filled with krypton gas. The dramatic change of spectral and temporal properties of the driver pulses after passing through the gas cell indicates that propagation effects play a significant role in promoting the generation of the EUV supercontinuum.

physics.optics↗

arXiv2012

On-chip three-dimensional high-Q microcavities fabricated by femtosecond laser direct writing

We report on the fabrication of three-dimensional (3D) high-Q whispering gallery microcavities on a fused silica chip by femtosecond laser microfabriction, enabled by the 3D nature of femtosecond laser direct writing. The processing mainly consists of formation of freestanding microdisks by femtosecond laser direct writing and subsequent wet chemical etching. CO2 laser annealing is followed to smooth the microcavity surface. Microcavities with arbitrary tilting angle, lateral and vertical positioning are demonstrated, and the quality (Q)-factor of a typical microcavity is measured to be up to 1.07x10^6, which is currently limited by the low spatial resolution of motion stage used during the laser patterning and can be improved with motion stages of higher resolutions.

physics.optics↗

arXiv2011

Comparative investigation of third and fifth harmonic generation in atomic and molecular gases driven by mid-infrared ultrafast laser pulses

We report on the comparative experimental investigation on third and fifth harmonic generation (THG and FHG) in atomic and molecular gases driven by mid-infrared ultrafast laser pulses at a wavelength of ~1500 nm. We observe that the conversion efficiencies of both the THG and FHG processes saturate at similar peak intensities close to ~1.5\times1014 W/cm2 for argon, nitrogen, and air, whose ionization potentials are close to each other. Near the saturation intensity, the ratio of yields of the FHG and THG reaches ~10-1 for all the gases. Our results show that high-order Kerr effects do exist, whereas they are insufficient to balance the Kerr self-focusing without the assistance of plasma generation.

physics.optics↗

arXiv2011

Laser-polarization-dependent photoelectron angular distributions from polar molecules

Photoelectron angular distributions (PADs) of oriented polar molecules in response to different polarized lasers are systematically investigated. It is found that the PADs of polar CO molecules show three distinct styles excited by linearly, elliptically and circularly polarized lasers respectively. In the case of elliptical polarization, a deep suppression is observed along the major axis and the distribution concentrates approximately along the minor axis. Additionally, it is also found that the concentrated distributions rotate clockwise as the ellipticity increases. Our investigation presents a method to manipulate the motion and angular distribution of photoelectrons by varying the polarization of the exciting pulses, and also implies the possibility to control the processes in laser-molecule interactions in future work.

physics.chem-ph↗

arXiv2011

High-brightness switchable multi-wavelength remote laser in air

Remote laser in air based on amplified spontaneous emission (ASE) has produced rather well-collimated coherent beams in both backward and forward propagation directions, opening up possibilities for new remote sensing approaches. The remote ASE-based lasers were shown to enable operation either at ~391 and 337 nm using molecular nitrogen or at ~845 nm using molecular oxygen as gain medium, depending on the employed pump lasers. To date, a multi-wavelength laser in air that allows for dynamically switching the operating wavelength has not yet been achieved, although this type of laser is certainly of high importance for detecting multiple hazard gases. In this Letter, we demonstrate, for the first time to our knowledge, a harmonic-seeded switchable multi-wavelength laser in air driven by intense mid-infrared femtosecond laser pulses. Furthermore, population inversion in the multi-wavelength remote laser occurs at an ultrafast time-scale (i.e., less than ~200 fs) owing to direct formation of excited molecular nitrogen ions by strong-field ionization of inner-valence electrons, which is fundamentally different from the previously reported pumping mechanisms based either on electron recombination of ionized molecular nitrogen or on resonant two-photon excitation of atomic oxygen fragments resulting from resonant two-photon dissociation of molecular oxygen. The bright multi-wavelength laser in air opens the perspective for remote detection of multiple pollutants based on nonlinear spectroscopy.

physics.atom-ph↗

arXiv2009

Wavelength scaling of high-order harmonic yield from an optically prepared excited state atom

Wavelength scaling law for the yield of high-order harmonic emission is theoretically examined for excited state atoms which are optically prepared by simultaneously exposing to an extreme ultraviolet pulse at the resonant wavelength and an infrared pulse at a variable wavelength in the range of 0.8\mum-2.4\mum. Numerical simulations are performed based on the three-dimensional time-dependent Schrodinger equation (3D TDSE) for Ne and H. We confirm that the harmonic yield follows a λ^-{4-6} scaling with the single fundamental driving laser pulse; whereas for the optically prepared excited state atoms, a λ^-{2-3} scaling for the harmonic yield is revealed.

physics.atom-ph↗

arXiv2001

Mass spectrometric and first principles study of Al$_n$C$^-$ clusters

We study the carbon-dope aluminum clusters by using time-of-flight mass spectrum experiments and {\em ab initio} calculations. Mass abundance distributions are obtained for anionic aluminum and aluminum-carbon mixed clusters. Besides the well-known magic aluminum clusters such as Al$_{13}^-$ and Al$_{23}^-$, Al$_7$C$^-$ cluster is found to be particularly stable among those Al$_n$C$^-$ clusters. Density functional calculations are performed to determine the ground state structures of Al$_n$C$^-$ clusters. Our results show that the Al$_7$C$^-$ is a magic cluster with extremely high stability, which might serve as building block of the cluster-assembled materials.

physics.atm-clus↗