The visible and near-UV emission spectroscopy of methane （CH4） induced by a femtosecond intense laser field （800 nm, 40 fs, 1014 W/cm2） is studied. By measuring the decay profiles of the neutral fragment product CH （A2△→X2П）, two reaction pathways, i.e., the electron-ion recombination through e- ＋ CH＋ and the direct disintegration of CH＋ are found to be responsible for populating the electronic excited states of the neutral fragment product CH, which gives rise to the photoemissions. Our results provide complementary information on previous understanding of the strong-field-induced photoemission mechanism of CH4 through neutral dissociation of superexcited states.
We experimentally demonstrate N2^＋ lasing actions at the wavelengths of 353.3, 353.8, and 354.9 nm using a circularly polarized femtosecond laser. The three laser lines correspond to the B2∑u^＋（v＇ = 5, 4, 3） → X2∑g^＋（v = 4, 3, 2） transitions, respectively. Particularly, we reveal the pressure-dependent gain dynamics of these lasing actions from highly excited vibrational states with a pump-probe scheme. Our experimental results confirm that electron eollisional excitation plays an important role in the establishment of a population inversion of N2^＋ lasing at these wavelengths.
We demonstrate that the filamentation process is strongly influenced by the polarization state of the driver laser. When the laser polarization changes from linear to circular, the critical power for the self-focusing of a Ti：Sapphire laser （800 nm, 40 fs） in air increases from about 9.6 ± 1.0 to 14.9± 1.5 GW, while the second nonlinear refractive index n2 of air decreases from 9.9 × 10-2o to 6.4 ×10-20 cm2/W. We also demonstrate that the luminescence from the neutral nitrogen molecules at 337 nm is dependent on both the laser intensity and plasma density inside the filament.
We experimentally investigate the generation of above-threshold harmonics completely from argon atoms on an excited state using mid-infrared femtosecond laser pulses. The highly nonlinear dependences of the observed signal on the pulse energy and polarization of the probe laser pulses indicate its nonperturbative characteristic.
We report on second harmonic generation(SHG) in on-chip high-Q(>105) lithium niobate(Li Nb O3, LN) microresonators fabricated by femtosecond laser micromachining. We examine the efficiency of SHG with either a continuous-wave(CW) or an ultrashort pulsed pump laser. The normalized conversion efficiencies of SHG obtained with the CW and pulsed pump lasers are measured to be 1.35×10?5 m W?1 and 2.30×10?6 m W?1, respectively.
LIN JinTianXU YingXinFANG ZhiWeiWANG MinWANG NengWenQIAO LingLingFANG WeiCHENG Ya
