Baozhen Zhao

Near-infrared femtosecond laser machining initiated by ultraviolet multiphoton ionization

We report on the experimental study of microstructures fabricated on the surface of fused silica by two femtosecond laser pulses, a tightly focused 266 nm beam followed by a loosely focused 800 nm beam. By setting the fluence of each pulse below the damage threshold, visible microstructures are fabricated using the combined beams. Our results suggest that the ultraviolet pulse generates seed electrons through multiphoton absorption, and the near-infrared pulse utilizes these electrons to cause damage by avalanche ionization.

Ellipticity dependence of 400 nm-driven high harmonic generation

We studied the dependence of high harmonic generation efficiency on the ellipticity of 400 nm driving laser pulses at 7.7 × 1014 W/cm2 and compared it with the 800 nm driving laser under the same conditions. The measured decrease of high harmonic yield with the ellipticity of the 400 nm laser is ∼1.5 times slower that of the 800 nm, which agrees well with theoretical predictions based on a semi-classical model. The results indicate that it is feasible to use the generalized double optical gating with 400 nm lasers for extracting single attosecond pulses with high efficiency.

Probing AC Stark shift with attosecond transient absorption

The dynamics of helium 1snp excited states in an intense, few-cycle laser pulse are probed using isolated attosecond pulses in a transient absorption scheme. In addition to the dynamic Stark shift, we observe half-cycle oscillations.

Attosecond Absorption Spectroscopy

Photoabsorption spectroscopy with isolated attosecond pulses provides a means for all-optical time-domain measurement of ultrafast dynamics in bound and quasi-bound states of atoms and molecules, as well as near absorption edges in condensed matter systems. Due to the strong oscillator strength in atoms and molecules in the extreme ultraviolet and soft X-ray spectral regions as well as the high detection efficiency of the transmitted photon signal, attosecond transient absorption spectroscopy is an attractive alternative to traditional photoelectron spectroscopy techniques. We present measurements of the laser-perturbed photoabsorption of helium 1snp singly excited states and quasi-bound autoionizing states of argon with a few-femtosecond autoionization lifetime. In both systems, we observe laser-induced changes in the photoabsorption spectrum which evolve on the single-femtosecond timescale, faster than the laser light oscillation period.

Generation of ultra-broadband infrared pulses through difference frequency mixing

Ultra-broadband infrared pulses were generated through difference frequency mixing with BBO crystal. The spectra range of the generated infrared pulse was above one octave, with pulse duration 39fs and pulse energy of 15μJ.

Broadband high harmonic generation from 400 nm sub-10 fs driving pulses

Broadband high harmonic generation produced from 400 nm sub-10 fs driving pulses is reported. The results indicate an efficient method of generating intense single attosecond pulses from 400 nm driving lasers.

Generation of high flux attosecond pulses with phase-matched Double Optical Gating

Generalized Double Optical Gating is an effective scheme for generating single isolated attosecond pulses with multi-cycle driving lasers. It is to be implemented using a >;100 mJ Ti:Sapphire laser to upscale the attosecond photon flux.

Ellipticity dependence of high harmonics from 400 nm driving pulses

First experimental measurement of ellipticity dependence of high harmonics produced from 400 nm driving pulses is reported and compared with harmonics produced from 800 nm driving pulses in an argon filled gas cell.

Femtosecond laser patterning of Mo thin film on flexible substrate for CIGS solar cells

Finding ways to scribe Mo back conductor plays an important role in the fabrication and assembly of CIGS thin film solar cells. Using a femtosecond (fs) laser, we selectively removed the Mo thin films of 800 ∼850 nm thick on flexible Polyamide (PI) substrates. In particular, we studied the effect of laser fluence and laser scanning speed on the ablation quality. The single pulse ablation thresholds at various pulse durations were determined. Clean removal of the Mo layer was observed when laser fluence was above the threshold of 0.08-0.1 J/cm2. Furthermore, the morphologies and structure of the grooves were characterized using scanning electron microscope (SEM) and KLA Tencor P-16 Profiler. The femtosecond laser provides a unique scheme to ablate the metal layer on flexible substrates for the fabrication of thin film solar cells.Finding ways to scribe Mo back conductor plays an important role in the fabrication and assembly of CIGS thin film solar cells. Using a femtosecond (fs) laser, we selectively removed the Mo thin films of 800 ∼850 nm thick on flexible Polyamide (PI) substrates. In particular, we studied the effect of laser fluence and laser scanning speed on the ablation quality. The single pulse ablation thresholds at various pulse durations were determined. Clean removal of the Mo layer was observed when laser fluence was above the threshold of 0.08-0.1 J/cm2. Furthermore, the morphologies and structure of the grooves were characterized using scanning electron microscope (SEM) and KLA Tencor P-16 Profiler. The femtosecond laser provides a unique scheme to ablate the metal layer on flexible substrates for the fabrication of thin film solar cells.