Two-photon imaging is a powerful tool used to examine molecular and cellular functions in living tissues. In particular, calcium imaging can quantitatively measure neuronal activity i.e. action potential fi ring. Two-photon calcium imaging can detect the multicellular activity of neuronal circuits in the brain at the single cell level while animals perform behavioral tasks. Normal two-photon microscopy can be applied to head-restrained mice, while fi ber-optic, head-mounted miniaturized two-photon microscopes can be used in unrestrained mice. Here, we review the general mechanisms and methodologies for twophoton calcium imaging in awake behaving mice. In addition, we also briefl y discuss the manipulation of photoactivatable proteins (optogenetics), which can be used to activate or inhibit specifi c types of neurons and animal behaviors with millisecond precision.

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Behavior is controlled by complex network of neurons located in the brain. It has been impossible to study neural regulatory mechanism of behavior since there is no technique to control the activity of specifi c type of neurons in vivo with high time accuracy. Recently, new technique called “optogenetics” is developed. Optogenetics enable control the activity of specifi c type of neurons by illuminating light which is less inevasible and permeable by expressing light activated molecules. To achieve optogenetics many knowledge and technique were required, such as molecular biology, physiology, electrophysiology, genetic engineering and photonics. However, it become much easier to introduce optogenetics since many equipments specialized for optogenetics are commercially available. This article explains outline of manipulation of orexin neural activity involved in sleep/wakefulness in vivo using optogenetics.

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Many people avoid sudden death by ischemic brain strokes but suffer from such disabilities as aphasia and/or paralysis. For such patients, we must study the mechanisms underlying the remodeling of neuronal circuits during the recovery phase to achieve good clinical cares. For this purpose, an in vivo imaging technique, which uses two-photon laser microscopy (TPLM), is currently the only tool powerful enough to visualize the morphology and activity of individual neurons buried deep in opaque brain tissue. Here, I review recent studies of brain stroke that use TPLM and other laser techniques. Techniques that would be ideal for in vivo imaging study are also presented. By presenting the ideas that biomedical researchers dream of, experts of laser techniques could try to develop new techniques that would be used in realizing such dream.

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Light-sheet microscopy is an emerging technology that is suitable for observing living tissue or whole organism with its deep penetration depth, low photodamages, and fast acquisition rate. This article reviews the principle, advantages, limitations, and the recent progresses of this microscopy.

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In vivo two-photon microscopy has revealed vital information about neural activity for brain functions, despite its limitations when imaging events at depths greater than several hundred micrometers from the brain surface. To break the limit of this penetration depth, we introduced a novel photon detector that successfully visualizes not only the cortex layer V pyramidal neurons spreading to all cortex layers at a superior S/N ratio but also visualizes the hippocampal CA1 neurons in young adult mice. In addition, we developed liquid crystal devices to convert linearly polarized beams (LP) to vector beams. A liquid device generated a vector beam called a higher-order radially polarized (HRP) beam for identifying individual fluorescent beads whose diameters were 170 nm, which is smaller than the classical PSF width. HRP beams also visualized the fi ner structures of microtubules in fi xed cells. Here, we discuss these improvements and future applications based on our recent data.

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The spatial resolution in optical microscopy is limited by the diffraction limit of light. Here we report the use of nonlinear fluorescence response emerging under saturated excitation (SAX) conditions to improve the spatial resolution of confocal fl uorescence microscopy beyond the diffraction limit. Since the nonlinear fl uorescence response is localized in the excitation focus, it gives the information of sample structures in a region smaller than the focal volume. To detect the nonlinear fl uorescence signals from fl uorescence probes, the sample is excited by an intensity-modulated laser light, and the fl uorescence intensity is demodulated at harmonic frequencies of the laser modulation. SAX microscopy also possesses high background signal rejection capability because the saturated excitation of fl uorescence probes is spatially confined in the excitation focus. Using a SAX microscope, we demonstrate fl uorescence imaging of a single fl uorescent nanodiamond and stained HeLa cell with a spatial resolution beyond the diffraction limit.

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An actual spark plug-sized laser with ignitable, three beam output was successfully demonstrated for simultaneous multi-point ignition of automobile engines. The closed parallel laser array consisted of a single active source of a composite, 10-mm long all-ceramics Nd:YAG/Cr:YAG monolithic laser cavity and three independent pump and focus optics lines. Laser pulses with energy of 3.2 mJ and a duration of 500 ps (> 6 MW peak power) were obtained from each laser at 20-Hz repetitions.

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This paper describes the fi rst demonstration of the time-domain measurements of terahertz (THz)-wave pulses generated from a picosecond optical parametric oscillator (ps-OPO). Unlike pyroelectric detectors, photoconductive antennas allow us to observe the time-discrete THz-wave pulses that depend on arrayed Si-prism couplers. By replacing them with a larger single type, the phase distortion of THzwave pulses was compensated and a single THz-wave pulse with the peak frequency of ～0.2 THz was obtained.

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We investigated the interaction of an axially symmetric polarized light with a vacuum at electromagnetic-fi eld strengths far below the Schwinger limit. Although polarization and magnetization in a vacuum induced by the axially symmetric polarized light are clearly different from those for a linearly polarized light, the number of photons radiated from the vacuum is not very dependent on the light polarization.

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The grain size dependence of the surface hardness of laser-peened carbon steels is investigated. Commercial S35C carbon steel annealed at various temperatures to control the grain size were used as samples. A femtosecond laser was used as a laser driver. Vickers microhardness tests were used to estimate work hardening due to plastic deformation caused by laser peening. The hardness of laserpeened samples varied with laser irradiation parameters and average grain size. The hardness measurements revealed the desirable average grain size for laser peening of carbon steel.

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