Anisotropy of hardness and laser damage threshold of unidirectional organic NLO crystal in relation to the internal structure

Unidirectional benzophenone crystals were grown along 〈1 1 0〉, 〈0 1 0〉 and 〈0 0 1〉 directions by uniaxially solution crystallization method at ambient temperature. The growth rate of the grown crystals was varied with orientation. The optical absorption coefficients of benzophenone were measured as a function of wavelength. The optical absorption study reveals that the benzophenone crystal has very low absorption in the wavelength range of interest. Moreover, the laser damage threshold and micro hardness for 〈1 1 0〉, 〈0 1 0〉 and 〈0 0 1〉 oriented unidirectional benzophenone crystals were measured using a Q-switched Nd:YAG laser operating at 1064 nm radiation and Vickerʹs micro hardness tester, respectively. The laser damage threshold is larger for the 〈1 1 0〉 and 〈0 1 0〉 oriented crystals compared to 〈0 0 1〉 oriented crystal at 1064 nm wavelength. The result is consistent with the hardness variation observed for the three different crystallographic directions of benzophenone crystal. The relation between the laser damage profile and mechanical hardness anisotropy is discussed based on the crystal structure of benzophenone.