Browsing by Author "Seto, Keisuke"

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Development of a balanced detector with biased synchronous detection and application to near shot noise limited noise cancelling of supercontinuum pulse light
(2014-02-01)
Seto, Keisuke
;
Tsukada, Toshiaki
;
Okuda, Yoshinao
;
Tokunaga, Eiji
;
Kobayashi, Takayoshi
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. (C) 2014 AIP Publishing LLC.
Development of a multiplex stimulated Raman microscope for spectral imaging through multi-channel lock-in detection
(2013-08-01)
Seto, Keisuke
;
Okuda, Yoshinao
;
Tokunaga, Eiji
;
Kobayashi, Takayoshi
;
電子物理學系
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 multichannel 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 system's 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 system's imaging speed. (C) 2013 AIP Publishing LLC.
Noise cancellation with phase-detection technique for pump-probe measurement and application to stimulated Raman imaging
(2015-05-01)
Seto, Keisuke
;
Tsukada, Toshiaki
;
Okuda, Yoshinao
;
Tokunaga, Eiji
;
Kobayashi, Takayoshi
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 supercontinuum light from a piece of photonic crystal fiber, and find that the noise is canceled by similar to 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. (C) 2015 Optical Society of America
Sub-10 fs spectroscopy of K-TCNQ crystal for observation of intramolecular vibration modulation in melting of the Peierls dimer
(2014-08-14)
Nakata, Kazuaki
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Tokunaga, Eiji
;
Du, Juan
;
Xue, Bing
;
Miyazaki, Jun
K-TCNQ crystals show photoinduced phase transition (PIPT) between a Mott insulator and a spin Peierls (SP) phase, having T-c = 395 K (= and the crystal has T-c = 395 K of the PIPT). We report the intra-molecular vibrations in the excited K-TCNQ crystal after impulsive excitation using a sub 9.4 fs pulse laser in the reflection spectrum. The frequencies of the vibrations are close to the Raman frequencies of molecules. The frequencies increase with melting of the SP phase, and then decrease with rearrangement of the dimerization. In addition, the frequencies of the vibrations are time-resolved to be modulated by the intermolecular optical phonon with 510 fs period corresponding to 65 cm(-1). This modulation is associated with the periodical change of coupling strengths of intermolecular charge transfer and/or the dipolar coupling between the counterpart molecules in the neighboring chains.
Sub-diffraction-limit imaging using mode multiplexing
(2015-05-15)
Wang, Nan
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Miyazaki, Jun
;
He, Jinping
;
Seto, Keisuke
;
Kobayashi, Takayoshi
Pixel-by-pixel processed fluorescence difference microscopy is experimentally demonstrated by multiplexing excitation laser beams with Gaussian and donut spot shapes and then demultiplexing the fluorescent signals using lock-in amplifiers. With this scheme, a fixed sample of fluorescent spheres and a slice of mouse brain tissue are imaged with resolutions that exceed the diffraction limit. Compared to previously reported subtraction imaging techniques, this pixel-by-pixel scan can be applied to improve the resolution of a moving sample without introducing subtraction errors. The synchronized signal detection feature makes this method extendible to various applications. (C) 2014 Elsevier B.V. All rights reserved.
Thermal Relaxation Spectra for Evaluating Luminescence Quantum Efficiency of CASN:Eu2+ Measured by Balanced-Detection Sagnac-Interferometer Photothermal Deflection Spectroscopy
(2020-02-01)
Chima, Hiromichi
;
Shiokawa, Naoyuki
;
Seto, Keisuke
;
Takahashi, Kohsei
;
Hirosaki, Naoto
Highly sensitive broadband photothermal spectroscopy with a white-light lamp as the excitation source was developed by combining a Sagnac interferometer and balanced detection with a photothermal deflection method. A probe beam was split by a birefringent crystal CaCO3 into signal and reference beams with a balanced intensity. This balanced detection enabled the measurement of photoexcited thermal relaxation spectra of materials in the air over the whole visible range in the weak excitation limit 50 mu W/cm(2). The photothermal excitation spectrum of Eu2+-doped CaAlSiN3 phosphors (CASN:Eu2+) with a high luminescent quantum efficiency was measured to be distinctly different from the photoluminescence excitation spectrum which reflects the absorption spectrum, revealing the thermal relaxation mechanism of the phosphor. Assuming a typical non-radiative relaxation from the higher excited states to the lowest excited state and successively to the ground state, it is demonstrated that the photoluminescence efficiency of the phosphors is readily evaluated simply by comparing the photothermal and photoluminescence excitation spectra.