Valence orbital method of calculating harmonic emission from diatomic molecule

We present a valence orbital method of calculating high-order harmonic generation from a diatomic molecule with arbitrary orientation by using a space rotation operator. We evaluate the effects of each valence orbital on harmonic emissions from N2 and O2 molecules in detail separately. The calculation results confirm the different properties of harmonic yields from N2 and O2 molecules which are well consistent with available experimental data. We observe that due to the orientation dependence of ／sigma and ／pi orbitals, the bonding orbital （π2pz）^2 of N2 determines the maximum of harmonic emission when the molecular axis of N2 is aligned parallel to the laser vector, and the magnitude of the high harmonic signal gradually weakens with the orientation angle of molecular axis increasing. But for O2 molecule the antibonding orbitals （π2pz）^1 and （π2pz）^1 contribute to the maximum of harmonic yield when O2 is aligned at 45° and bonding orbitals （π2pz）^2 and （π2pz）^2 slightly influence the orientation angle of maximum of harmonic radiation not exactly at 45°.

Red Shift and Broadening of Backward Harmonic Radiation from Electron Oscillations Driven by Femtosecond Laser Pulse

The characteristics of backward harmonic radiation due to electron oscillations driven by a linearly polarized fs laser pulse are analysed considering a single electron model. The spectral distributions of the electron＇s backward harmonic radiation are investigated in detail for different parameters of the driver laser pulse. Higher order harmonic radiations are possible for a sufficiently intense driving laser pulse. We have shown that for a realistic pulsed photon beam, the spectrum of the radiation is red shifted as well as broadened because of changes in the longitudinal velocity of the electrons during the laser pulse. These effects are more pronounced at higher laser intensities giving rise to higher order harmonics that eventually leads to a continuous spectrum. Numerical simulations have further shown that by increasing the laser pulse width the broadening of the high harmonic radiations can be controlled.

Enhancement of third harmonic generation in air filamentation using obstacles

The intensity of third harmonic emission in air filamentation disturbed by copper fibers and alcohol droplets has been investigated experimentally. Enhancement of the third harmonic emission up to more than one order of magnitude has been observed. The physical mechanism of third harmonic enhancement is attributed to suppression of the destructive interference by comparison of the experimental results and it is closely related to the type, size, and relative position of the obstacles.

Harmonic Dark Pulse Emission in Erbium-Doped Fiber Laser

A harmonic dark pulse generation in an erbium-doped fiber laser is demonstrated based on a figure-of-eight configuration. It is found that the harmonic dark pulse can be shifted from the fundamental to the 5th order harmonic by increasing the pump power with an appropriate polarization controller orientation. The fundamental repetition rate of 2O kHz is obtained at the pump power of 29 m W. The highest pulse energy of 42.6 n3 is obtained at the fundamental repetition rate. The operating frequency of the dark pulse trains shifts to 2nd, 3rd, 4th and 5th harmonic as the pump powers are increased to 34mW, 50mW, 59mW and 137mW, respectively.

Generation of third harmonic of femtosecond laser emission in propagation pulses in air

The main characteristics of the third harmonic emission generated by femtosecond laser pulses propagating in air are investigated by numerically solving the coupled nonlinear Schrodinger equations. Strong third harmonic emission is observed with a maximum conversion efficiency as high as 0.43%. The on-axis phase difference between fundamental beam and third harmonic is investigated. The result is in good agreement with the phase-locking mechanism. Dependence of the conversion of third harmonic emission on focusing conditions is also studied. The results are also compared with those of experiments.