Optical damage

Materials. NLO materials possess a high degree of optical nonlinearity, physical durabiHty and chemical inertness, and high threshold to optical damage. In addition, it must be possible to process the materials into the form required by the intended appHcation. 

Collier, D., and Pantley, W., Deep Ultraviolet Coatings Resist Optical Damage, Laser Focus World, pp. 77-82 (Sept. 1997)... 

In general, molecular crystals are too soft for them to be of interest as structural materials. Also, they fracture readily. Because of their transparencies and non-linear properties some of them are of interest for optical applications, but most of them suffer from optical damage at low intensities of light. 

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. 

To determine optical damage in bulk benzil crystals a Q-switched Nd YAG laser with 1KW peak power, pulse width of 0.1 ps and pulse repetition rate of 500Hz was used. The laser power was attenuated using a set of neutral density filters and focussed onto a bulk benzil crystal using a x10 microscope objective. No optical damage was observed with optical intensities of upto 100MW/cm - Also, no optical damage was observed in benzil cored fibres with similar optical intensities. 

Figure 1. Diagram of the intensity / (W/cm2) vs. duration of laser pulse tp(s) with various regimes of interaction of the laser pulse with a condensed medium being indicated very qualitatively. At high-intensity and high-energy fluence 4> = rpI optical damage of the medium occurs. Coherent interaction takes place for subpicosecond pulses with tp < Ti, tivr. For low-eneigy fluence (4> < 0.001 J/cm2) the efficiency of laser excitation of molecules is very low (linear interaction range). As a result the experimental window for coherent control occupies the restricted area of this approximate diagram with flexible border lines.

Both the intrinsic and extrinsic defects in these materials can affect the properties of interest for applications. An example of this is the observed decrease in the damage susceptibility or photorefractivity of LiNb03 with additions of H, Li or Mg to Li-deficient congruently grown crystals (Table I).(26) The additions produce a reduction in the octahedral vacancy concentrations in the crystals. Therefore, at least some photorefractive optical damage in LiNbC>3 is believed to be related to the concentration of octahedral vacancies present in crystals. 

The use of femtosecond pulses in nonlinear optics has one obvious advantage high peak power, necessary to observe the nonlinear effect, can be obtained from low total power in the pulse. This made samples with a low damage threshold amenable for HRS measurements. One example of such sample is amorphous polymer films. These films do not have the optical quality of single crystals. They are more susceptible to optical damage. With the femtosecond pulses, we have been able to perform HRS measurements on solid thin films, to study the orientational correlation between nonlinear optical chromophores in the film.10 13... 

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. 

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