The optical properties of colloidal ZnO nanoparticle (NP) solutions, with size ranging from several nm to around 200 nm, have been tailored to have high optical nonlinearity for bioimaging with no auto-fluorescence above 750 nm and minimal auto-fluorescence below 750 nm. The high second harmonic conversion efficiency enables selective tissue imaging and cell tracking using tunable near-infrared femtosecond laser source ranging from 750-980 nm. For laser energies exceeding the two-photon energy of the bandgap of ZnO (half of 3.34 eV), the SHG signal greatly decreases and the two-photon emission becomes the dominant signal. The heat generated due to two-photon absorption within the ZnO NPs enable selective cell or localized tissue destruction using excitation wavelength ranging from 710-750 nm.

Localized spatial excitation of a single hexagonal GaN micropyramid with (1 101) facets formed by selective area growth is optimized for nonlinear optical light (NLO) generation due to second harmonic generation (SHG) and multiphoton luminescence (MPL). Multiphoton transition induced ultraviolet and yellow luminescence is observed for excitations above and below half bandgap energy. SHG and MPL observed for excitations below half the bandgap energy are superimposed to realize broadband emission in the UV-visible range. The light generation is optimized by controlling the cavity modes formed by the hexagonal facets and the tip enhanced effects from the pyramid. The MPL is optimum at the apex of the pyramid. The SHG is most efficient within the pyramid (≈4 µm above the base) due to the formation of spatially stable cavity modes within the cavity. The NLO interactions within the pyramid are optimized to realize microphotonic white light sources and coherent tunable UV-visible sources using spatially controlled excitation without any change in material parameters. At the bandgap of GaN, the resonant two-photon emission dominates the nonlinear light generation process compared to the coherent SHG light generated within the cavity.

We investigated the potential for using polydimethylsiloxane microfluidic devices in a biological assay to explore the cellular stress response (CSR) associated with hyperthermia induced by exposure to laser radiation. In vitro studies of laser-tissue interaction traditionally involved exposing a monolayer of cells. Given the heating-cooling dynamics of the cells and nutrient medium, this technique produces a characteristic "bulls-eye" temperature history that plagues downstream molecular analyses due to the nonuniform thermal experience of exposed cells. To circumvent this issue, we devised an approach to deliver single cells to the laser beam using a microfluidic channel, allowing homogeneous irradiation and collection of sufficient like-treated cells to measure changes in CSR after laser heating. To test this approach, we irradiated Jurkat-T cells with a 2-μm-wavelength laser in one branch of a 100-μm-wide bifurcated channel while unexposed control cells were simultaneously passing through the other, identical channel. Cell viability was measured using vital dyes, and expression of HSPA1A was measured using reverse transcription polymerase chain reaction. The laser damage threshold was 25 ± 2 J/cm2, and we found a twofold increase in expression at that exposure. This approach may be employed to examine transcriptome-wide/proteome changes and further comparative work across stressors and cell types.

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