Generation And Applications Of DUV And VUV Femtosecond Laser

Ultraviolet (UV) lasers with center wavelength below 400 nm has high single photon energy, which involves rich physical mechanisms such as nonlinear broadening, frequency up-conversion, dense plasma, sum-frequency, chemical decomposition in nonlinear optical physics. With unique advantage of the UV lasers, femtosecond UV laser pulses have shown bright prospects in high order harmonics generation, plasma transparent devices, green chemical processing, gas trace detection, laser induced breakdown spectroscopy and such areas. Moreover, UV lasers also have low multi-photon ionization coefficient, high electron density in filamentation, good spatial resolution and other advantages. Currently, there are still some obstacles in applications of UV lasers, which lays much on efficient generation and control of femtosecond UV pulses with high energy, so there are not quite rich explorations in depth and range of such direction.This dissertation mainly discusses on the generation and applications of high energy deep UV and vacuum ultraviolet (VUV) lasers, with center wavelengths at 266 nm,200 nm,89 nm and energy of 2.2 mJ,0.1 mJ,190 nJ, respectively. And it also explores filamentation and physical mechanisms in laser pulse with different colors, which is summarized as follows:1) By employing a regenerative Ti:Sapphire laser with center wavelength at 800 nm, its third order harmonic is generated by using three BBO crystals. And the 400 nm pulses are guided into KBBF-PCD to produce intense 200 nm pulses, whose pulse duration is subsequently compressed to femtosecond after passing through a pair of prisms. This is the first time that 200 nm laser with energy of 0.1 mJ is compressed to femtosecond, and the FROG technique is also successfully used to diagnose its pulse duration.2) By using the 800 nm Ti:Sapphire laser with repetition of 10 Hz and BBO crystal group, the second harmonic at 400 nm and third one at 266 nm are induced to wave-mixing in argon, and the multicolor field at 200 nm,133 nm,114 nm,100 nm and 89 nm is collected. The highlights are inducing UV laser to participate into four-wave-mixing for the first time, and it is optimized by adjusting the pressure to control the phase-matching conditions. Additionally, the phase-locking theory is also verified in this experiment.3) By splitting strong 266 nm beam into two beams and introducing them into the argon cell, there are double UV filament generated when focused tightly. Both of them has a length of 2 cm, and when cut off the filament at the focal point with abrupt vacuum environment, a series of spectrum at 89 nm are collected by a UV CCD, which is enhanced by over 5000 times when interrupted by another UV filament at focal point. There are two key technique employed here:one is that during the third harmonic generation, there is phase mismatching for π between fundamental wave and the harmonic, which was dismissed by abrupt vacuum pressure gradient; the other is that the introduced second 266 nm filament interrupts the first one, which efficiently prevent back-conversion of the 89 nm wave by enhancing the plasma density. Finally the conversion efficiency was enhanced by over three magnitudes, it lays the technique foundation of converting the EUV into deeper range.4) By focusing two 400 nm laser beams in the air and crossing them at small angle, with the help of induced 800 nm filament at Bragg angle, the enhanced plasma grating was efficiently generated. Electrons ionized from the local plasma are further accelerated by the infrared pulses, which impact more neutral atoms to generated denser plasma, so in the experiment of laser ablation, the dual-color plasma grating presented higher efficiency by near 6 times in ablation of the same sample plate. This experiment highlights at:(1) an invented dual-color plasma grating scheme as compared with former longer pulse duration; (2) verification of advantage on laser ablation from enhanced plasma grating; (3) obtaining efficient excitation fluorescence signal from high energy particles; (4) verification of different effect on laser ablation when conducted in various gas medium, these all reflected feasibility of material composition detection by femtosecond laser ablation. Finally, the DUV and EUV femtosecond lasers will also have more practices on hydrodynamic analysis, chemical reactions, intense deeper EUV generation, presenting more applications on optical precise micromachining, attosecond pulse generation, high-energy state molecular excitation, fast precision spectroscopy and other areas.