Two-photon absorption spectroscopy and anisotropy of bulk zincblende and diamond symmetry semiconductors and quantum dots*

Si quantum dots (SiQD) are potential candidates for biologically-inert theranostic applications. Excitation in the red-near infrared (NIR) by two-photon absorption (2PA) is useful for confocal microscopy and therapy techniques. We studied the spectroscopic response and size-dependence of 2PA in colloidal ligand-passivated SiQDs in the red-NIR by indirect two-photon induced photoluminescence (2PIP) and direct pump-probe modulation spectroscopy (PPMS). We are developing a model to explain the spectra for the nanocrystalline response from ab initio calculations using length gauge theory for bulk Si. We measured the 2PA spectra and anisotropies of bulk Si, GaAs, and GaP in the range 650-2000 nm using PPMS in the femtosecond excitation regime to further verify these calculations.