Large near resonance third order nonlinearity in GaN

We have studied third order nonlinearities, including two-photon absorption coefficient and nonlinear refractive index n ₂, of GaN in below bandgap ultraviolet (UV) wavelength regime by using UV femtosecond pulses. Two-photon absorption was investigated by demonstrating femtosecond UV pulsewidth autocorrelation in a GaN thin film while femtosecond Z-scan measurements revealed information for both n ₂ and . The distribution of n ₂ versus wavelength was found to be consistent with a model described by the quadratic Stark effect, which is the dominant factor contributed to the nonlinear refractive index near the bandgap. Large on the order of 10 cm/GW and large negative n ₂ with a magnitude on the order of several 10^–12 cm²/W were obtained. The at near mid-gap infrared (IR) wavelength was also found to be on the order of several cm/GW by using two-photon-type autocorrelations in a GaN thin film. Taking advantage of the large two-photon absorption at mid-gap wavelengths, we have demonstrated excellent image quality on two-photon confocal microscopy, including two-photon-scanning-photoluminescence imaging and two-photon optical-beam-induced current microscopy, on a GaN Hall measurement sample and an InGaN green light emitting diode.     

Local stimulation of cultured myocyte cells by femtosecond laser-induced stress wave

When an 800 nm femtosecond laser is tightly focused into cell culture medium a stress wave is generated at the laser focal point. Since the stress wave localizes in a few tens of μm, it is possible to locally stimulate single cells in vitro. In this work, several kinds of cultured mammalian cells, HeLa, PC12, P19CL6, and C2C12, were stimulated by the stress wave and the cell growth after the stress loading with the laser irradiation was investigated. In comparison with the control conditions, cell growth after the laser irradiation was enhanced for the cells of C2C12 and P19CL6, which can differentiate into myocytes, and suppressed for PC12 and HeLa cell lines. These results suggest a possibility of cell growth enhancement due to myogenic cells response to the femtosecond laser-induced stress.     

Confocal optical beam induced current microscopy of light-emitting diodes with a white-light supercontinuum source

To improve the efficiency of confocal optical beam induced current (OBIC) and the non-destructive, high-resolution analysis of semiconductor media we report the application of a white-light supercontinuum laser source capable of confocal OBIC across a wide spectral range. To demonstrate the capability of this source, we performed confocal OBIC of light emitting diodes with varying absorption and emission properties in the visible spectrum. Using the wavelength flexibility afforded by the broadband laser source, we were able to determine and apply the optimum excitation wavelength range for efficient confocal OBIC instead of applying inferior fixed wavelength laser sources. PACS 87.64.Tt; 85.30.De     

Monitoring cellular metabolism of 3T3 upon wild type E. coli infection by mapping NADH with FLIM

Fluorescence lifetime imaging microscopy (FLIM) has gained popularity as a sensitive technique to monitor the functional/conformational states of reduced nicotinamide adenine dinucleotide (NADH), one of the main compounds of oxidative phosphorylation. In this letter, we apply the technique to characterize the metabolic changes in mouse embryonic fibroblast 3T3 cells upon bacterial infection. A gradual shortening of the decaying time constants in both the short and the long lifetime components of NADH’s autofluorescence is detected. The ratio of the short and the long lifetime components’ relative contributions, however, shows a rapid increase, indicating the rise of cellular metabolic activity over the course of infection.     

Contribution of stress wave and cavitation bubble in evaluation of cell–cell adhesion by femtosecond laser-induced impulse

When an intense femtosecond laser is focused in a cell culture medium, shock wave, stress wave, and cavitation bubble are generated at the laser focal point. Cell–cell adhesion can be broken at the cellular level by the impacts of these factors. We have applied this breaking of the adhesion to an estimation of the cell–cell adhesion strength. In this application, it is important to identify which of these factors is the dominant factor that breaks the adhesion. Here we investigated this issue using streptavidin-coated microbeads adhering to a biotin-coated substrate as a mimic of the cell–cell adhesion. The results indicated that the break was induced mainly by the stress wave, not by the impact of the cavitation bubble.