Second-Harmonic Generation Imaging

Abstract

Three-dimensional second-harmonic generation imaging using femtosecond pulses is demonstrated. Since second-harmonic generation results from coupling of the optical field to the second-order nonlinearity of the specimen, SHGI is a probe of local anisotropy and the distribution of molecular hyperpolarisability. The technique was implemented using a 3D scanning reflectance microscope together with a femtosecond Ti:sapphire laser. The efficiency of second-harmonic generation is enhanced by the use of short pulses (< 90 fs at 800 nm) since the detected average signal is inversely proportional to the laser pulse width. The method was characterised using crystalline nonlinear optical materials, 2D arrays and molecular crystals. We are currently applying the technique to biological specimens including isolated DNA, chromosomes and human skin.