Microemitter-Based IR Spectroscopy and Imaging with Multilayer Graphene Thermal Emission.

IR analyses such as Fourier transform infrared spectroscopy (FTIR) are widely used in many fields; however, the performance of FTIR is limited by the slow speed (∼10 Hz), large footprint (∼ millimeter), and glass bulb structure of IR light sources. Herein, we present IR spectroscopy and imaging based on multilayer-graphene microemitters, which have distinct features: a planar structure, bright intensity, a small footprint (sub-µm2), and high modulation speed of >50 kHz. We developed an IR analysis system based on the multilayer-graphene microemitter and performed IR absorption spectroscopy. We show two-dimensional IR chemical imaging that visualizes the distribution of the chemical information. In addition, we present high-spatial-resolution IR imaging with a spatial resolution of ∼1 µm, far higher than the diffraction limit. The graphene-based IR spectroscopy and imaging can open new routes for IR applications in chemistry, material science, medicine, biology, electronics, and physics.

High-Speed and On-Chip Optical Switch Based on a Graphene Microheater.

Graphene is a promising material for producing optical devices because of its optical, electronic, thermal, and mechanical properties. Here, we demonstrated on-chip optical switches equipped with a graphene heater, which exhibited high modulation speed and efficiency. We designed the optimal structure of the optical switch with an add/drop-type racetrack resonator and two output waveguides (the through and drop ports) by the electromagnetic field calculation. We fabricated the optical switch in which the graphene microheater was directly placed on the resonator and directly observed its operation utilizing a near-infrared camera. As observed from the transmission spectra, this device exhibited high wavelength tuning efficiency of 0.24 nm/mW and high heating efficiency of 7.66 K·µm3/mW. Further, we measured the real-time high-speed operation at 100 kHz and verified that the graphene-based optical switch achieved high-speed modulation with 10%-90% rise and fall response times, 1.2 and 3.6 µs, respectively, thus confirming that they are significantly faster than typical optical switches that are based on racetrack resonators and metal heaters with response times of ∼100 µs. These graphene-based optical switches on silicon chips with high efficiency and speed are expected to enable high-performance silicon photonics and integrated optoelectronic applications.

Determination of the Faraday rotation perpendicular to the optical axis in uniaxial CeF3 crystal by using the Generalized-High Accuracy Universal Polarimeter.

Many single crystals have been developed and commercialized for optical isolators. However, optical isolator materials have been limited to isotropic crystals or to the isotropic direction (optic axis) of anisotropic crystals. This study investigates the wavelength dependences of linear birefringence, linear dichroism, Faraday rotation and magnetic-circular dichroism in a single crystal rare-earth fluoride, namely CeF3. Measurements were made in the direction parallel and perpendicular to the optic axis under an applied magnetic field. The magnetic field was generated by Nd-Fe-B magnets installed in the generalized-high accuracy universal polarimeter (G-HAUP). The first application of G-HAUP to a magneto-optical material is presented. In the CeF3 crystal, the Verdet constants along directions parallel and perpendicular to the optic axis were positive over the measured wavelength region (300-680 nm), and their magnitudes were nearly equal. The success in the accurate measurement on Faraday rotation along anisotropic directions has opened the way to study on optical isolators along the direction other than optic axis.

A light emitter based on practicable and mass-producible polycrystalline graphene patterned directly on silicon substrates from a solid-state carbon source.

We developed a procedure for direct patterning of graphene with arbitrary position, size, and shape on silicon substrates from a solid-state carbon source without dry etching processing. Our light emitting graphene devices perform on a par with those based on high crystallinity graphene obtained via mechanical exfoliation or chemical vapor deposition.

A high-accuracy universal polarimeter study of optical anisotropy and optical activity in laminated collagen membranes.

A novel method for fabricating anisotropic collagen membranes, which regulate the preferred orientation, is developed. The high-accuracy universal polarimeter (HAUP) method is used for simultaneous and quantitative evaluations of the optical anisotropy and optical activity. The evaluation of these optical quantities, enabled by the HAUP method, is beneficial to the field of tissue engineering.

Metastable liquid crystal as time-responsive reaction medium: aging-induced dual enantioselective control.

A metastable liquid crystal (LC) was found to serve as a time-responsive reaction medium, in which the enantioselectivity of a photoreaction was perfectly switched through isothermal annealing of the reaction system. When the LC salt of an enantiopure amine with a photoreactive acid was irradiated with UV/vis light, in situ photodimerization of the acid moiety proceeded smoothly to afford the (+)-isomer of the photodimer with high enantioselectivity (+86% ee). In contrast, photoirradiation of an aged sample, isothermally annealed for 20 h, gave predominantly the (-)-isomer (-94% ee). Systematic studies revealed that the reversal in selectivity originated from metastability of the LC system, which gradually transformed into a crystalline phase during annealing. This finding demonstrates the potential use of metastable aggregates as dynamic time-responsive media, reminiscent of biological systems.

Fluorescence imaging of mitochondria in living cells using a novel fluorene derivative with a large two-photon absorption cross-section.

We have examined optical properties of a fluorene derivative with two positively charged substituents, 1,1'- diethyl-4,4'-(9,9-diethyl-2,7-fluorenediyl-2,1-ethanediyl)dipyridinium perchlorate (1), in water. The photoluminescence quantum yield of 1 was relatively high (35%) for use as a fluorescent probe in water. We also examined two-photon absorption (TPA) properties of 1 in methanol. The maximum value of the TPA cross-section (730 GM at 750 nm, 1 GM = 10(-50) cm4 s photon-1 molecule-1) was larger than that for most two-photon-excited fluorescent dyes including a classical mitochondria-selective fluorescent dye rhodamine 123. Preliminary fluorescence imaging experiments of the mitochondria in living Paramecium caudatum and HeLa cells were carried out with 1. Bright green fluorescence was observed from the mitochondria in both living cells loaded 1 without toxicity effects. These our results indicate that water-soluble fluorene derivative 1 is a promising candidate as a two-photon-excited fluorescence probe for mitochondria in living cells.