To better predict optical third-harmonic generation ͑THG͒ in transparent dielectrics, we model a typical ultrashort pulsed Gaussian beam, including both group velocity mismatch and phase mismatch of the fundamental and harmonic fields. We find that competition between the group velocity mismatch and phase mismatch leads to third-harmonic generation that is sensitive only to interfaces. In this case, the spatial resolution is determined by the group velocity walk-off length. THG of modern femtosecond lasers in optical solids is a bulk process, without a surface susceptibility, but bears the signature of a surface enhancement effect in z-scan measurements. We demonstrate the accuracy of the model, by showing the agreement between the predicted spectral intensity and the measured third-harmonic spectrum from a thin sapphire crystal.
We obtain simple analytic solutions for ultrashort pulse generation in a degenerate mode-locked optical parametric oscillator pumped synchronously by a mode-locked laser. Very short pulses (∼1 psec) are predicted at steady state very close to threshold where pump depletion is negligible; but the parametric pulses broaden to the same order as the pumping laser pulses as soon as pump pulse depletion becomes significant. In the transient buildup regime, where pump depletion is again negligible, pulse widths approximately a quarter of the pump pulse width are obtained.
We have observed laser induced damage morphologies and the accumulation dependence of damage on single crystal metal surfaces under Q-switched Nd:YAG laser irradiation (1064 nm, 10 nsec pulses with a 240 μm spot diameter). The samples were chemically-polished copper and electro-polished aluminum of several crystallographic orientations.
Several different damage morphologies have been observed: slip-line formation, ripple patterns, flat-melting, and boiling. Damage probability versus fluence curves and accumulation curves were plotted to investigate damage behavior as it correlates with morphology. Flat-melting damage was observed near the 50% damage fluence on chemically polished copper surfaces, and slip-lines were found near the 50% damage fluence on electropolished aluminum surfaces. Surface defects produced during sample preparation greatly influenced the damage threshold of copper because of its high melting threshold. The influence of surface defects on copper decreased with increasing numbers of laser pulses. Accumulation curves show different damage behavior for crystals of different orientation. Accumulation was the largest on (111) Cu and Al surfaces, and the single shot damage threshold of these surfaces was less than the other crystal orientations for both Cu and Al.
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