M. H. Xu

Control of filamentation induced by femtosecond laser pulses propagating in air

Filamentation formed by self-focusing of intense laser pulses propagating in air is investigated. It is found that the position of filamentation can be controlled continuously by changing the laser power and divergence angle of the laser beam. An analytical model for the process is given.

Effective fast electron acceleration along the target surface

The dependence of angular distributions of fast electrons generated in the interaction of p-polarized femtosecond laser pulses with foil targets on laser intensities is investigated. A novel fast electron beam along the front target surface is observed for high laser intensity. It is found that the electron acceleration along the target surface is more efficient than those in other directions.

Orthogonal polynomials describing polarization aberration for rotationally symmetric optical systems

Optical lithography has approached a regime of high numerical aperture and wide field, where the impact of polarization aberration on imaging quality turns to be serious. Most of the existing studies focused on the distribution rule of polarization aberration on the pupil, and little attention had been paid to the field. In this paper, a new orthonormal set of polynomials is established to describe the polarization aberration of rotationally symmetric optical systems. The polynomials can simultaneously reveal the distribution rules of polarization aberration on the exit pupil and the field. Two examples are given to verify the polynomials.

Propagation of a short-pulse laser-driven electron beam in matter

We studied the transport of an intense electron beam produced by high intensity laser pulses through metals and insulators. Targets were irradiated at two different intensities, 1017 W/cm2 and 1019 W/cm2, at the laser facility Xtreme Light XL-III in Beijing, a Ti:Sa laser source emitting 40 fs pulses at 800 nm. The main diagnostic was Cu-Kα fluorescence imaging. Images of Kα spots have been collected for those two laser intensities, for different target thickness, and for different materials. Experimental results are analyzed taking into account both collisional and collective effects as well as refluxing at the edge of the target. The target temperature is evaluated to be Tc ∼ 6 eV for intensity I = 1017 W/cm2 (for all the tested materials: plastic, aluminium, and copper), and Tc ∼ 60 eV in aluminium and 120 eV in titanium for intensity I = 1019 W/cm2.

Orthonormal polynomials describing polarization aberration for M-fold optical systems

Polarization aberration (PA) is a serious issue that affects imaging quality for optical systems with high numerical aperture. Numerous studies have focused on the distribution rule of PA on the pupil, but the field remains poorly studied. We previously developed an orthonormal set of polynomials to reveal the pupil and field dependences of PA in rotationally symmetric optical systems. However, factors, such as intrinsic birefringence of cubic crystalline material in deep ultraviolet optics and tolerance, break the rotational symmetry of PA. In this paper, we extend the polynomials from rotationally symmetric to M-fold to describe the PA of M-fold optical systems. Two examples are presented to verify the polynomials.

Polarization aberration function for perturbed lithographic lens

A comprehensive polarization aberration function is proposed to evaluate the imaging quality of a perturbed high NA lithographic lens. In this function, the system polarization aberration, i.e. Jones matrix, is decomposed into several basic parts as wavefront aberration, apodization, diattenuation, retardance and rotation by single value decomposition (SVD). The wavefront aberration is described by field-Zernike polynomials (FZP), and the diattenuation, as well as retardance, is described by field-orientation Zernike polynomials (FOZP). The relationship of system polarization aberration with pupil, field and manufacturing errors is established by an approximately analytical equation, which provides a possible way to analyze lens tolerance for polarization aberration.

Multi-peak emission of the fast electron beams along the target surface in ultrashort laser interaction with solid targets

The spatial and energy distributions of fast electrons emitted from foil targets irradiated by ultrashort intense laser pulses are measured. Four groups of collimated emissions of fast electrons along the front and rear target surfaces are observed for an incidence angle of < 60°. This multi-peak characteristic is found to be independent of the polarization states. Numerical simulations reveal that the electron beams are formed due to the deformation of the target surface and then guided by the induced quasistatic electromagnetic fields.

Fast electron transport in high-intensity laser-plasma interactions diagnosed by optical and ion emission

Light and proton emission from rear surface of sandwich-like targets, which are irradiated by intense femtosecond laser pulses, have been measured. It is found that both of them depend on the transport material and density inside the targets. For metal transport layers the patterns of the optical images and the proton beams are uniform. While for the transport layers of dielectric, low-density foam and vacuum gap, the patterns are broken up. This indicates that the fast electron beams are also broken up probably due to instabilities.

Effect of target shape on fast electron emission

Fast electrons emission from the interaction of femtosecond laser pulses with shaped solid targets has been studied. It is found that the angular distributions of the forward fast electrons are closely dependent upon the target shape. The important role played by the electrostatic fields built up near the target surfaces in the confinement of fast electrons is identified. Our two-dimensional particle-in-cell simulations can reproduce the main observations.

Study of Cu Kα emission from frequency doubled intense femtosecond laser-plasma interactions

A laser-driven hard x-ray source with a small spatial size, high contrast characteristic Kα emission has been developed by a frequency doubled (~400nm) high contrast intense laser pulse focused on 50μm thick Cu foil. Simulations demonstrate the vacuum heating may be the main mechanism to generate hot electrons.