We describe degenerate four-wave mixing experiments on ZnSe and CdTe semiconductor samples with picosecond laser pulses at wavelengths below the bandgap. Nonlinearities of third, fifth, and seventh order are observed and the mechanisms for each are identified. In all of our measurements, we observe a fast third order nonlinearity. For two-photon absorbers, this is attributed to contributions from both the real (refractive) and imaginary (absorptive) parts of the thirdorder susceptibility. Below the two-photon absorption edge, the nonlinearity is purely refractive. The higher order effects are due to carriers generated by multiphoton excitation. In ZnSe at 0.532 f!m, carriers are generated by two-photon absorption such that a fifth order nonlinearity arises from the change in index due to these carriers, a sequential x(J): x(l) nonlinearity. From such measurements we determine the refractive index change per photoexcited carrier pair and the density dependence of the carrier diffusion coefficient. Analogous signals are observed in CdTe at 1.064 f!m. The seventh order nonlinearity observed in ZnSe at 1.064 f!m results from the refractive index contribution of carriers generated by three-photon absorption. I. INTRODUCTION W E repo~ ~ series of picose~ond degenerate fourwave mixmg (DFWM) studies conducted in ZnSe and CdTe at wavelengths of 0.532 and 1.064 ,urn. The DFWM signal shows a fast third order nonlinearity, as well as higher order slowly decaying nonlinearities due to multi photon absorption generated carriers. We attribute this signal to the combined effects of the real and imaginary parts of the third order susceptibility x (JJ. The imaginary part corresponds to two-photon absorption (2PA), while the real part is due to bound-electronic nonlinear refraction (index n 2), as opposed to a free-carrier effect fl]. From our measurements, we obtain the absolute value of the third order nonlinear susceptibility for both ZnSe and CdTe. This, combined with independent 2PA mea-Manuscript

We demonstrate theoretically and experimentally compensation for positive Kerr phase shifts with negative phases generated by cascade quadratic processes. Experiments show correction of small-scale self-focusing and whole-beam self-focusing in the spatial domain and self-phase modulation in the temporal domain.

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Here we demonstrate high-pulse-energy multiphoton microscopy (MPM) for intravital imaging of neurons and oligodendrocytes in the murine brain. Pulses with an order of magnitude higher energy (~ 10 nJ) were employed from a ytterbium doped fiber laser source at a 1-MHz repetition rate, as compared to the standard 80-MHz Ti:Sapphire laser. Intravital imaging was performed on mice expressing common fluorescent proteins, including green (GFP) and yellow fluorescent proteins (YFP), and TagRFPt. One fifth of the average power could be used for superior depths of MPM imaging, as compared to the Ti:Sapphire laser: A depth of ~ 860 µm was obtained by imaging the Thy1-YFP brain in vivo with 6.5 mW, and cortical myelin as deep as 400 µm ex vivo by intrinsic third-harmonic generation using 50 mW. The substantially higher pulse energy enables novel regimes of photophysics to be exploited for microscopic imaging. The limitation from higher order phototoxicity is also discussed.

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