Rei Hasegawa

Two-Dimensional Infrared Spectroscopy of Electric-Field-Induced Reorientation Dynamics of a Mixed Nematic Liquid Crystal

A two-dimensional infrared (2D IR) spectrometer has been modified by the following methods to solve several problems on the 2D IR spectroscopy of electric-field-induced reorientation dynamics of a mixed nematic liquid crystal: (1) The response of the liquid crystal (LC) to an electric field was stabilized by an amplitude-modulated ac electric field. (2) Formulation of an anti-reflection coating (ARC) has been proposed to suppress the interference caused by multiple reflections between IR liquid crystal cell windows. (3) The Fourier transform procedure has been modified to correct the phase of the dynamic difference interferograms without spectral anomalies. These modifications can provide a better signal-to-noise ratio and reproducible 2D IR spectrum of a mixed nematic liquid crystal.

Probability Distribution of After Pulsing in Passive-Quenched Single-Photon Avalanche Diodes

The resolution and accuracy of the number of photons detected by a silicon photomultiplier (SiPM) are limited by the excess noise, which consists mainly of after pulsing and optical crosstalk. Since these excess noises are correlated with each other, it is challenging to determine the after pulsing and the optical crosstalk probabilities independently of the output signal of the SiPM. Here we examine the dynamic response of passive-quenched single-photon avalanche diodes to clarify the characteristics of the after pulsing. Optical crosstalk is not present in the output signals of single-photon avalanche diodes; therefore, we can obtain the after-pulsing probability directly from the experimental results. We derive an analytical expression for the probability density function of after-pulse timing and analyze the experimental data based on this expression. We confirm that two types of trap levels with emission lifetimes on the order of 10 and 100 ns are responsible for the after pulsing in our devices. The dependence of the after-pulsing probability on the device recovery time is also well explained by our model.

Electrical Control of LSPR from Gold Nanoparticles Using Electrochemical Oxidation

Large shift of localized surface plasmon resonance (LSPR) spectrum of gold nanoparticles was attained by electrochemical oxidation of the nanoparticle surface. This oxidation occurred in a cell consisting of a pair of indium tin oxide (ITO) electrodes with water medium between the electrodes. On one side of the ITO electrode, the gold nanoparticles were adsorbed. The LSPR spectrum was moved consecutively to the red by increasing the applied positive voltage. By the application of 5 V to the cell, the spectrum shift as large as 55 nm was obtained. Though the spectrum shift has already been observed by changing liquid crystal (LC) orientation surrounding gold nanoparticles, the amount of the shift was not large (11 nm). That was because the variation of the effective refractive index of LC was rather small. Our large shift due to electrochemical oxidation resulted from the large refractive index of Au-O. The upper limit of the LSPR spectrum shift by our method is estimated to be 138 nm.