Uncertainty evaluation of photoluminescence quantum yield measurement in an integrating hemisphere-based instrument.

We present an integrating hemisphere-based (i.e., a variant of integrating spheres) implementation of the indirect illumination method for absolute photoluminescence quantum yield measurements, which is a recommended method in the international standard IEC 62607-3-1:2014. We rigorously formulated a mathematical model and a measurement procedure for the absolute photoluminescence quantum yield measurement in the integrating hemisphere-based system. The measurement system was calibrated using an Hg-Ar discharge lamp and spectral irradiance standard lamps for wavelength and relative spectral radiant flux scales, respectively. Furthermore, we identified and evaluated uncertainty components involved in the photoluminescence quantum yield (PLQY) measurement. To validate our measurement system, we applied it to the two de facto standard dyes: quinine bisulfate (QBS) and fluorescein (FLS). Consequently, their PLQY values were determined to be 0.563±0.024 (k=2) and 0.876±0.032 (k=2) for, respectively, QBS and FLS, which are consistent with previous reports.

Defect corrections for coherent optical information processing of grayscale images in a DMD-based 4f-system using a collimated light source.

Digital micromirror device (DMD)-based 4f-systems, a type of coherent optical information processing system, have become a powerful tool for optical convolutional neural networks taking advantage of their fast modulation speed and high-resolution capability. However, proper high bit-depth image information processing remains challenging due to the optical diffractions that arise from the binary nature of DMD operation. In this paper, we first characterize the diffraction phenomena that cause irradiance defects, namely the nonlinear grayscale and unintended dark lines. Then to resolve the issues, we propose a DMD operation method and a modified structure of the 4f-system based on blazed diffraction grating theory and numerical calculation of the Rayleigh-Sommerfeld propagation model. As a demonstration, we implement high bit-depth image information processing with an optimized optical 4f-system using DMDs and a collimated coherent light source.

Reflectometers for Absolute and Relative Reflectance Measurements in the Mid-IR Region at Vacuum.

We demonstrated spectral reflectometers for two types of reflectances, absolute and relative, of diffusely reflecting surfaces in directional-hemispherical geometry. Both are built based on the integrating sphere method with a Fourier-transform infrared spectrometer operating in a vacuum. The third Taylor method is dedicated to the reflectometer for absolute reflectance, by which absolute spectral diffuse reflectance scales of homemade reference plates are realized. With the reflectometer for relative reflectance, we achieved spectral diffuse reflectance scales of various samples including concrete, polystyrene, and salt plates by comparing against the reference standards. We conducted ray-tracing simulations to quantify systematic uncertainties and evaluated the overall standard uncertainty to be 2.18% (k = 1) and 2.99% (k = 1) for the absolute and relative reflectance measurements, respectively.

1.1-µm Band Extended Wide-Bandwidth Wavelength-Swept Laser Based on Polygonal Scanning Wavelength Filter.

We demonstrated a 1.1-µm band extended wideband wavelength-swept laser (WSL) that combined two semiconductor optical amplifiers (SOAs) based on a polygonal scanning wavelength filter. The center wavelengths of the two SOAs were 1020 nm and 1140 nm, respectively. Two SOAs were connected in parallel in the form of a Mach-Zehnder interferometer. At a scanning speed of 1.8 kHz, the 10-dB bandwidth of the spectral output and the average power were approximately 228 nm and 16.88 mW, respectively. Owing to the nonlinear effect of the SOA, a decrease was observed in the bandwidth according to the scanning speed. Moreover, the intensity of the WSL decreased because the oscillation time was smaller than the buildup time. In addition, a cholesteric liquid crystal (CLC) cell was fabricated as an application of WSL, and the dynamic change of the first-order reflection of the CLC cell in the 1-µm band was observed using the WSL. The pitch jumps of the reflection band occurred according to the electric field applied to the CLC cell, and instantaneous changes were observed.

Optically controlled in-line graphene saturable absorber for the manipulation of pulsed fiber laser operation.

We demonstrate an optically tunable graphene saturable absorber to manipulate the laser operation in pulsed fiber laser system. Owing to the strongly enhanced evanescent field interaction with monolayer graphene, we could realize an efficient control of modulation depth in the graphene saturable absorber by optical means through cross absorption modulation method. By integrating the tunable graphene saturable absorber into the fiber laser system, we could switch the laser operation from Q-switching through Q-switched mode-locking to continuous wave mode-locking by adjusting only the optical power of the control beam. In addition, we realized a hybrid Q-switching of fiber laser by periodical modulation of the absorption of the graphene saturable absorber, where we observed that the repetition rate of the Q-switched laser could be continuously tuned according to the modulation frequency of the applied external signal.

Alkyl Conformation and π-π Interaction Dependent on Polymorphism in the 1,8-Naphthalimide (NI) Derivative.

The 1,8-naphthalimide (NI) derivative Lumogen F Violet 570 exhibits different photoluminescence (PL) and aggregation-caused quenching properties due to its crystal polymorphism, which depends on the solvent evaporation process in tetrahydrofuran solution. In the slow drying process, molecules aggregated into an energetically more stable form (time-dependent density functional theory calculation), of which the PL peak maximum was 453 nm, corresponding to blue emission at the 365 nm excitation. However, the fast evaporation process induces an energetically less stable form, with a PL peak maximum of 508 nm, corresponding to green emission. The main difference between the two crystal structures is the alkyl conformation, as confirmed by X-ray single-crystal analysis. Due to the different alkyl conformations, NI groups aggregated into more obliquely aligned structures that emit blue PL, which plays a role in weakening the π-π interactions between molecules relative to green PL crystals. We found that the conformational stable molecular stacking induced instability in the electronic energy levels of the blue crystal compared to the green crystal.