Szu-yuan Chen

Fabrication of spatial transient-density structures as high-field plasma photonic devices

Fabrication of periodic transient-density structures in a gas jet with a boundary scale length approaching 10μm was demonstrated. This was achieved by passing an ultrashort high-intensity laser pulse through a patterned mask and imaging the mask onto the target plane. Gas/plasma density at the laser-irradiated regions drops as a result of hydrodynamic expansion following ionization and heating by the laser pulse. The fabrication of gas/plasma density structures with such a scheme is an essential step in the development of plasma photonic devices for applications in high-field physics.

Self-starting issues of passive self-focusing mode locking.

An analysis of passive mode locking with the intracavity self-focusing effect is presented, and the self-starting issue is addressed. The analysis shows that the initial pulse-shortening force of this mechanism is too weak to start the mode locking from mode beating or noise and how it can be greatly enhanced with a dilute dye saturable absorber in the cavity. This analysis also shows shortening force in cw mode-locking lasers.

Efficient generation of extended plasma waveguides with the axicon ignitor-heater scheme

By using an axicon lens in conjunction with the ignitor-heater scheme, a 1.2-cm-long high-quality plasma waveguide is generated efficiently, which can extend the range of laser-plasma interaction much beyond the limit of Rayleigh range

Programmable fabrication of spatial structures in a gas jet by laser machining with a spatial light modulator

Programmable fabrication of longitudinal spatial structures in a gas jet was achieved by using laser machining with a liquid-crystal spatial light modulator as the pattern mask. By this technique single-shot fabrication of arbitrary gas and/or plasma structures is demonstrated, which establishes the crucial step toward raising the designs and applications of high-field plasma devices to the level of adaptive feedback optimization.

Tomography of high harmonic generation in a cluster jet.

Tomography of high harmonic generation in a cluster jet was achieved by using laser machining. The growth curves help explain the optimal backing pressure, which is ascribed to the change in phase matching condition.

Dependence of relativistic self-guiding and Raman forward scattering on duration and chirp of an intense laser pulse propagating in a plasma

Relativistic self-guiding and Raman forward scattering of an intense ultrashort laser pulse propagating in an underdense plasma were studied using a terawatt Ti:sapphire laser system. The dependence of these processes on the duration and frequency chirp of the laser pulse was investigated by detuning the pulse compressor. It was observed that the efficiency of Raman forward scattering is enhanced for a positively chirped pulse and diminished for a negatively chirped pulse. In addition, as a result of the dependence on pulse duration and peak power, an optimal duration for Raman forward scattering was found for fixed pulse energy and spectral bandwidth. On the other hand, relativistic self-guiding of the laser pulse was affected by the pulse duration while no dependence on the chirp was observed.

Enhancement of injection and acceleration of electrons in a laser wakefield accelerator by using an argon-doped hydrogen gas jet and optically preformed plasma waveguide

A systematic experimental study on injection of electrons in a gas-jet-based laser wakefield accelerator via ionization of dopant was conducted. The pump-pulse threshold energy for producing a quasi-monoenergetic electron beam was significantly reduced by doping the hydrogen gas jet with argon atoms, resulting in a much better spatial contrast of the electron beam. Furthermore, laser wakefield electron acceleration in an optically preformed plasma waveguide based on the axicon-ignitor-heater scheme was achieved. It was found that doping with argon atoms can also lower the pump-pulse threshold energy in this experimental configuration.

Programmably structured plasma waveguide for development of table-top photon and particle sources

Programmable fabrication of longitudinal spatial structures in an optically preformed plasma waveguide in a gas jet was achieved, by using laser machining with a liquid-crystal spatial light modulator as the pattern mask. Fabrication of periodic structures with a minimal period of 200 μm and density-ramp structures with a minimal slope length of 100 μm was attained. The technique is useful for the optimization of various laser-plasma-based photon and particle sources.

Single-shot soft-x-ray digital holographic microscopy with an adjustable field of view and magnification.

Single-shot digital holographic microscopy with an adjustable field of view and magnification was demonstrated by using a tabletop 32.8 nm soft-x-ray laser. The holographic images were reconstructed with a two-dimensional fast-Fourier-transform algorithm, and a new configuration of imaging was developed to overcome the pixel-size limit of the recording device without reducing the effective NA. The image of an atomic-force-microscope cantilever was reconstructed with a lateral resolution of 480 nm, and the phase contrast image of a 20 nm carbon mesh foil demonstrated that profiles of sample thickness can be reconstructed with few-nanometers uncertainty. The ultrashort x-ray pulse duration combined with single-shot capability offers great advantage for flash imaging of delicate samples.

Seeding of a soft-x-ray laser in a plasma waveguide by high harmonic generation

A strongly saturated waveguide-based optical-field-ionization soft-x-ray laser seeded by high harmonic generation was demonstrated for Ni-like Kr lasing at 32.8 nm. Compared with the same laser seeded only with spontaneous emission, seeding with high harmonics yields much smaller divergence, enhanced spatial coherence, and controlled polarization. The integration of high harmonic seeding, optically preformed plasma waveguide, and optical-field-ionization pumping forms one of the optimal archetypes of an ultrashort-pulse soft-x-ray laser.