Ultrasmall all-optical plasmonic switch and its application to superresolution imaging

Because of their exceptional local-field enhancement and ultrasmall mode volume, plasmonic components can integrate photonics and electronics at nanoscale, and active control of plasmons is the key. However, all-optical modulation of plasmonic response with nanometer mode volume and unity modulation depth is still lacking. Here we show that scattering from a plasmonic nanoparticle, whose volume is smaller than 0.001 μm3, can be optically switched off with less than 100 μW power. Over 80% modulation depth is observed, and shows no degradation after repetitive switching. The spectral bandwidth approaches 100 nm. The underlying mechanism is suggested to be photothermal effects, and the effective single-particle nonlinearity reaches nearly 10−9 m2/W, which is to our knowledge the largest record of metallic materials to date. As a novel application, the non-bleaching and unlimitedly switchable scattering is used to enhance optical resolution to λ/5 (λ/9 after deconvolution), with 100-fold less intensity requirement compared to similar superresolution techniques. Our work not only opens up a new field of ultrasmall all-optical control based on scattering from a single nanoparticle, but also facilitates superresolution imaging for long-term observation

Label-free identification of spore-forming bacteria using ultra-broadband multiplex CARS (coherent anti-Stokes Raman scattering) microspectroscopy

Spore-forming bacteria accumulate dipicolinic acid (DPA) to form spores to survive in extreme environments. Vibrational spectroscopy is widely used to detect DPA to elucidate the existence of the bacteria, while a vegetative cell, another form of spore-forming bacteria, has not studied deeply. Here we applied coherent anti-Stokes Raman scattering (CARS) microscopy to spectroscopically identify both spores and vegetative cells without staining or molecular tagging. The spores were identified by the strong CARS signals due to dipicolinic acid (DPA). Furthermore, we observed bright spots in the vegetative cells in the CARS image at 1735 cm-1. The vegetative cells contained molecular species with C=O bonds because of this vibrational mode being associated with the carbonyl group. One of the candidate molecular species is diketopimelic acid, a DPA precursor. The results indicate that the observed vegetative cell is in the sporulation process. CARS spectra can be used to monitor the maturation or formation of spores

Saturated excitation microscopy using differential excitation for efficient detection of nonlinear fluorescence signals

We report a method to increase the efficiency of detecting nonlinear fluorescence signals in saturated excitation (SAX) microscopy. With this method, we compare fluorescence signals obtained under different degrees of saturated excitation to extract the nonlinear fluorescent signal induced by saturated excitation. Compared to conventional SAX microscopy using the harmonic demodulation technique, the detection efficiency of the fluorescence signal can be increased up to 8 and 32 times in imaging using the second-order and the third-order nonlinear fluorescence signals, respectively. We combined this approach with pulsed excitation, which is effective to reduce photobleaching effects, and achieved super-resolution imaging using third-order nonlinear fluorescence signals induced by saturated excitation of an organic dye. The resolution improvement was confirmed in the observations of fluorescent beads, actin-filaments in HeLa cells, and a spine in mouse brain tissue