Damage threshold

In air, PTFE has a damage threshold of 200—700 Gy (2 x 10 — 7 x 10 rad) and retains 50% of initial tensile strength after a dose of 10" Gy (1 Mrad), 40% of initial tensile strength after a dose of 10 Gy (10 lad), and ultimate elongation of 100% or more for doses up to 2—5 kGy (2 X 10 — 5 X 10 rad). During irradiation, resistivity decreases, whereas the dielectric constant and the dissipation factor increase. After irradiation, these properties tend to return to their preexposure values. Dielectric properties at high frequency are less sensitive to radiation than are properties at low frequency. Radiation has veryHtde effect on dielectric strength (86). 

For this study, thin films were deposited onto glass substrates. The as-deposited films showed no spectral narrowing at any pump energy up to the damage threshold. Stimulated emission was observed only in annealed films. The spol-to-spot reproducibiliiy of the measured characteristics was good, and we could measure with excitation energies of up to 4 mJ (1.8 mm beam diameter) without visual damage of the illuminated spot. 

On the other hand, we cannot ignore drawbacks in observing fourth-order responses. The desired response is always weak due to the high optical order. The damage threshold of the interface to be analyzed is severe with intense irradiation. The difficulty has been overridden by one-photon resonant enhancement of Raman-pump efficiency. The observable range of materials is somewhat limited as a result. There is still much room for technical improvements and the author is optimistic for the future. 

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. 

LiNbOj is a widely used ferroelectric crystal with various applications in the nonlinear optics and integrated optics (10). Another attractive material for 10 devices is LiTaOs. Its electrooptic (EO) and nonlinear (NL) coefficients are comparable to those of LiNbOj, and its photorefractive damage threshold is more than an order of magnitude higher than that of LiNbOj in the visible range. 

The nitropyridine derivative MBA-NP [(-)-2-(a-methylbenzylamino)-5-nitropyridine] 45,46) is a chiral molecule which crystallizes in the monoclinic form (a 5.392 A, b = 6.354 A, c = 17.924 A, with p= 94.6°). It is very pale yellow, and transparent from 430 nm into the infrared at 1.8 1. Damage thresholds near 1 GW/cm2 are quoted at 1.06 i where Type I phase matching occurs. Excellent crystals (up to 7x5x5 cm3) of this material are reported to grow from methanol by slow cooling. 

Table II lists the nonlinear coefficient, damage threshold and transparency cutoff for the most well known of the inorganic materials. Urea is included for comparison. As in Table I, materials are listed alphabetically. The data in Table II is taken from a tabulation by Bierlein (personal communication) and from one by Lin and Chen.( 103 ...

Compound d Coefficient(s)a Damage Threshold 3 Cutoff FOMd... 

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