Optical nonlinearities of hypocrellin A with the excitation of nanosecond pulses

Abstract

Nonlinear absorption and nonlinear refraction in the resonant region and the two-photon absorption (TPA) region of hypocrellin A (HA) have been investigated, for the first time to our knowledge, using the Z-scan technique and intensity-dependent transmittance measurement with nanosecond pulses. At 0.53 μm the effective absorption of the triplet state of the normal form of HA is the dominant mechanism causing the reverse saturable absorption. The refractive nonlinearities of HA can be mainly attributed to the thermal effect. With the excitation at 1.06 μm the optical limiting behavior in HA may be caused by the TPA of its ground state. The ground state of HA exhibits a large real part of second-order hyperpolarizability. By establishing the kinetic model for HA, we obtained several nonlinear optical parameters, such as the cross section of TPA, the absorption cross sections of the excited state and the triplet state of the normal form of HA, and the real part of second-order hyperpolarizability of the ground state of HA. The theoretical results are in good agreement with the experimental ones, which not only shows the completeness of the kinetic model but also demonstrates the close relationship between the optical nonlinearities of HA and its dynamic processes. In addition, HA has been proved to be a potential optical limiting material.