Optical damage thresholds

In a crystalline medium, the parametric gain (2) T2 is propor-tionnal to d2 Ip n-3 and the oscillation condition r2A2>aA where a is the signal residual absorption (dramatically increased by any crystalline defect), d the efficient phase-matched nonlinear susceptibility, n an average refractive index, Ip the pump intensity (limited by the optical damage threshold) and A the effective interaction length (also limited by any source of crystalline disorientation). 

The achievement of the corresponding monocrystals of sufficient optical and crystalline quality is made possible only after very thorough purification. Chemical impurities are known to disturb the crystal lattice through the occurence of twins, veils dislocations, rounding-off of faces ultimately quenching further growth. Any crystalline defect dramatically increases the residual absorption coefficient and lowers the optical damage threshold. 

Crystals of BBO and LBO are extensively used for generating visible and UV light from high-power pulsed lasers. The general utility of the materials derives from their high optical damage thresholds and transparency ranges that extend to 190 nm for BBO and 170 mn for LBO. 

However, other considerations are also involved if LB films are to compete eflFectively with materials such as lithium niobate and gallium arsenide. Molecular engineering will also be required to cater to needs such as phase matching, suitable spectral response and refractive index, good optical damage threshold, low scattering coefiScients, and mechanical stability before practical objectives can be accomplished. 

Just as important as the nonlinear refractive index is a collection of physical properties that must be compatible with the high-intensity optical fields that are expected in NLO devices. Among these are optical clarity, high optical damage thresholds, and thermal stability. Candidate materials must also be processable, so appropriate structures can be fabricated. In this contribution, I review some of these materials requirements by using our work on poly(5,7-dodecadiyne-1,12-diolbis(/i-butoxycarbonylmethylurethane)) (poly(4BCMU)) as a prototype material. 

The limiting time resolution of a laser T-Jump arrangement seems to be between 10 and lOO picoseconds. It is dictated by the optical damage threshold (1) of the sample, the state of the art of the very short laser pulse technique handling energies of more than 200 mJ (7), as well as the sensitivity and bandwidth of suitable detection systems. 

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