We propose a new scheme of attosecond pulse generation by starting from a narrow-band transform-limited highpower solid-state laser and phase lock the fundamental and its first four cascaded harmonics generated by the second-order nonlinear optical processes. The relative phase among the optical fields of the harmonics can be maintained a constant at least for thousands of nanosecond pulses. The worst-case relative phase fluctuation is 0.04π rad. It is shown that sub-single-cycle (∼ 0.37 cycle) sub-femtosecond (360 attosecond) pulses with carrier-envelope phase (CEP) control can be generated in this manner. The peak intensity of each pulse exceeds 10 14 W/cm 2 when it is focused to a spot size of 20 μm. We also demonstrate synthesis of square and saw-tooth waveforms. Correlation signal, a.u. 1 2 3 0 2 4 8 6 Time delay, fs CEP = 0 4 2 0 -2 -4 1 0 -1 The electric field synthesized waveform retrieving by the method of shaper-assistant linear cross-correlation at CEP = 0. The inset figures show the simulation of electric field waveform in black solid line and pulse envelope in blue dash line
We investigate generation of the third harmonic (TH; λ = 355 nm) signal by two-colour excitation (λ = 1064 nm and its second harmonic, λ = 532 nm) in argon gas, with emphasis on the influence of relative phases and intensities of the two-colour pump on the third-order nonlinear frequency conversion process. Perturbative nonlinear optics predicts that the TH signal will oscillate periodically with the relative phases of the two-colour driving laser fields due to the interference of TH signals from a direct third-harmonic-generation (THG) channel and a four-wave mixing (FWM) channel. For the first time, we show unequivocal experimental evidence of this effect. A modulation level as high as 0.35 is achieved by waveform control of the two-colour laser field. The modulation also offers a promising way to retrieve the relative phase value of the two-colour laser field.
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