Evaluation of Third-Order Susceptibility of Vacuum-Deposited Polydiacetylene Thin Films

Polydiacetylene (PDA) has high potential for use in nonlinear optical devices, such as optically gated switches and optical bistable devices, because it has large third-order nonlinearity due to one-dimensional conjugated π-electrons in its backbone chains. This material has the advantage of very fast response time and fairly small absorption loss. Nonlinear optical measurements have been performed for various sample forms, single crystals1) solvent-cast films, Langumuir-Blodgett (LB) films2), and vacuum-deposited films3),4). Among these, vacuum-deposited films are easiest to prepare because the process is dry and not time-consuming. However, the measured χ(3) values for films deposited by the usual method were not so large (≃ 1 × 10−11 esu) compared with those of single crystals1) and LB films2). This paper reports on the fairly large χ(3) value that has been observed in a highly-oriented epitaxial PDA thin film prepared by a successful vacuum-deposition method. This χ(3) value is larger than LB films and comparable to that of single crystals. To properly evaluate the χ(3) value, a third-harmonic generation (THG) measurement system with high accuracy has been set up.