Keisuke Seto

Development of a multiplex stimulated Raman microscope for spectral imaging through multi-channel lock-in detection

We report the development of a multiplex stimulated Raman microscope for spectral imaging through multi-channel lock-in detection with a single light source. A white pump beam is prepared with a piece of photonic crystal fiber (PCF). The system does not require the synchronization of plural light sources or the scanning of their wavelengths, and thus a jitter-free pair of pump and Stokes beams is obtained, and a high degree of temporal synchronization is attained in the spectra. The multi-channel lock-in detection (extended to 128 channels) enables the observation of pseudo-continuous stimulated Raman spectra, demonstrating the strong ability of qualitative analysis to identify various types of C-H stretching modes such as the symmetric and asymmetric modes of the methylene∕methyl and aromatic groups. Images of a mixed film of polystyrene and polymethylmethacrylate are presented to demonstrate the systems spectral imaging ability. The spatial distribution of these materials is successfully captured through one-time imaging, although the noise of the white light pump beam generated with the PCF limits the systems imaging speed.

Multiplex stimulated Raman imaging with white probe-light from a photonic-crystal fibre and with multi-wavelength balanced detection

A straightforward method for spectral stimulated Raman scattering (SRS) microscopy is to measure the scanned gain/loss spectrum of a white probe light from a photonic crystal fibre (PCF). However, the intensity of the white light noise is a serious problem for SRS microscopy. In this study, we have demonstrated simultaneous two-wavelength SRS microscopy with PCF through balanced detection suitable for spectroscopy with a modification of an auto-balance scheme. The developed auto-balance detection system suppresses the degradation of noise cancellation performance caused by a sample, and is suitable for spectral SRS imaging with simple and robust optics.

Development of a balanced detector with biased synchronous detection and application to near shot noise limited noise cancelling of supercontinuum pulse light

We report on the development of a balanced detector suited for multicolor imaging. The source pulsed light is split into probe and reference pulsed light. The reference pulse is delayed through an optical path and the probe and reference pulses are detected by a single photodetector. The signs of the detected signals of the probe and reference pulses are flipped based on a signal synchronous to the light source. Then, the signals are averaged through a low-pass filter. The output signal is proportional to the intensity difference between the probe and the reference. This balanced detector has two features: (1) both the probe and reference pulsed lights are detected by a single photodetector and (2) a voltage bias on the sign flipping compensates for the optical-intensity unbalance between the probe and reference pulsed lights. The first feature enables the probe and reference pulses to travel along a common optical path from a sample through a spectrograph to the photodetector, which minimizes the intensity unbalance between the probe and reference pulses during imaging and spectroscopy. The second feature ensures the complete balanced-detection in whole wavelength range by compensating for the optical unbalance created by deviations in the splitting ratios of the probe and reference lights at different wavelengths. Although a higher signal to noise ratio (SNR) reached to near shot noise limited SNR is attained by attaching a resonator to the photodetector for pulse repetition, the electrical bias cannot compensate for the optical balance. This unbalance is, however, corrected by adjusting the phase of the synchronous signal. We applied the present balanced detection to a stimulated Raman microscope with supercontinuum probe light and demonstrated its noise cancelling performance through capturing polystyrene beads.

Orientation Control of Hemispherical Janus Particles and Metal Coating on the Selective Surface To Excite Surface Plasmon Polaritons in the Micro-Kretschmann Geometry

Asymmetric-shaped particles (the Janus particle) are difficult to be arranged in a uniform orientation on a solid substrate. This difficulty prevents further modification of the selective surface of the particles for fabrication of the Janus particles with anisotropy of the shape and surface. We successfully arranged hemispherical particles in a uniform orientation at the air-water interface. The particles were arranged on the solid substrate in a uniform orientation by transferring the particle film onto the substrate. This arrangement enabled the fabrication of the Janus particles with anisotropy of the shape and surface by selective deposition of a film on either the equatorial plane or the spherical surface. Additionally, we demonstrated the function of the microscopic Kretschmann geometry for excitation of the surface plasmon polaritons of a thin metal film on the equatorial plane of a single hemispherical particle.

Noise cancellation with phase-detection technique for pump-probe measurement and application to stimulated Raman imaging

Intensity noise on a probe beam is a serious obstacle to highly sensitive and high-speed pump-probe microscopy. In this report, a reference beam of the probe is prepared and delayed. The intensity modulation by the sample is measured as the phase modulation of the superposition of detected electrical signals of the probe and reference beams, and the intensity noise is canceled. We evaluate performance of the noise cancellation using the super-continuum light from a piece of photonic crystal fiber, and find that the noise is canceled by ∼26  dB. We then apply the method to a stimulated Raman microscope. This method contributes to highly sensitive and high-speed pump-probe imaging with various light sources.