We demonstrate 2.2 microm axial resolution optical coherence tomography (OCT) in 1.1-1.7 microm wavelength regime by using a nonidentical multiple-quantum-well (MQW) superluminescent diode (SLD) with record-bandwidth emission. The compact, low-cost, and reliable light source with extreme broadband emission demonstrates significant potentials for spectroscopic and commercial OCT applications requiring ultrahigh spatial resolution.

We characterize the backward second-harmonic-generation (SHG) emission in biological samples with a SHG microscope. Contributions from different mechanisms, including direct backward emissions, reflection and back-scattering of the forward emissions, are identified and clarified.

We demonstrate a compact and self-starting fiber-delivered femtosecond Cr:forsterite laser for nonlinear light microscopy. A semiconductor saturable absorber mirror provides the self-starting mechanism and maintains long-term stability in the laser cavity. Four double-chirped mirrors are employed to reduce the size of the cavity and to compensate for group velocity dispersion. Delivered by a large-mode-area photonic crystal fiber, the generated laser pulses can be compressed down to be with a nearly transform-limited pulse width with 2.2-nJ fiber-output pulse energy. Based on this fiber-delivered Cr:forsterite laser source, a compact and reliable two-photon fluorescence microscopy system can thus be realized.

We demonstrate what is to our knowledge the first example of four-photon luminescence microscopy in GaN and apply it to quality mapping of bulk GaN. The simultaneously acquired second- and third-harmonic generation can be used to map the distribution of the piezoelectric field and the band-tail state density, respectively. Through spectrum- and power-dependent studies, the fourth power dependence of the band edge luminescence is confirmed. The superb spatial resolution of the four-photon luminescence modality is also demonstrated. This technique provides a high-resolution, noninvasive monitoring and tool for examining the physical properties of semiconductors.

We demonstrate hollow-core and large-core photonic-crystal-fiber-based two-photon fluorescence microscopes at 0.8/spl mu/m and 1.3/spl mu/m. The two-photon excitation efficiencies and image performances are comparable to or better than those acquired by the conventional fiber-free system.

Self-phase-modulation and group velocity dispersion of near IR femtosecond pulses in fibers restrict their use in two-photon fluorescence microscopy (TPFM). Here we demonstrate a hollow-core photonic crystal fiber based two-photon fluorescence microscope with low nonlinearity and dispersion effects. We use this fiber-based TPFM system to take two-photon fluorescence (chlorophyll) images of mesophyll tissue in the leaf of Rhaphidophora aurea. With less than 2mW average power exposure on the leaf at 755nm, the near zero-dispersion wavelength, chloroplasts distribution inside the mesophyll cells can be identified with a sub-micron spatial resolution. The acquired image quality is comparable to that acquired by the conventional fiber-free TPFM system, due to the negligible temporal pulse broadening effect.