Fourier-transform THz spectroscopy based on electric-field interferometry using THz-PMT.

We demonstrate a high dynamic range (DR) Fourier-transform-based terahertz (THz) spectrometer by combining a THz photomultiplier tube (PMT) with a metasurface and a conventional Michelson interferometer. Because the THz-PMT response depends on the incident electric-field strength following the Fowler-Nordheim equation, we can directly obtain an electric field interferogram without any synchronized optical probe pulse in contrast to conventional THz-time-domain-spectroscopy (THz-TDS). The DR of the corresponding power spectrum using the proposed method was 4.6 × 105 without the use of a lock-in amplifier. The complex refractive index of a quartz glass plate obtained using the proposed method was in good agreement with the results of conventional THz-TDS.

Nondestructive and noncontact evaluation of cellulose nanofiber-reinforced composites using terahertz time-domain spectroscopy.

Cellulose nanofibers (CNFs) can be obtained from natural sources, such as plants and wood fibers. Thermoplastic resin composites reinforced with CNFs exhibit various features, including superior mechanical strength. As the mechanical properties of composites reinforced with CNFs are affected by the amount of fiber addition, it is important to determine the concentration of the CNF filler in the matrix after injection or extrusion molding. We confirmed that there was good linearity between the CNF concentration and terahertz absorption. We could distinguish the difference in the 1%-point-CNF concentration with terahertz time-domain spectroscopy. Furthermore, we estimated the mechanical properties of the CNF nanocomposites using terahertz information.

Spectral imaging of pharmaceutical materials with a compact terahertz difference-frequency generation semiconductor source.

Spectral imaging of pharmaceutical material using a compact ultra-broadband (1-4 THz) terahertz semiconductor source was demonstrated. False-color RGB images could be obtained using a simple procedure (calibration free). The ability to distinguish the polymorphism of carbamazepine (CBZ), the hydrate forms of D-(+)-glucose and caffeine, and the crystallinity of nifedipine was demonstrated using the THz DFG source. Crystal forms of pharmaceutical materials can be distinguished using this method.

Measurement of group refractive indices of glass using a color-selective multi-pulse generated with a spatial light modulator.

We demonstrate a method of measuring group refractive indices (ng) by using only a temporal waveform of a color-selective multi-pulse (CSMP) generated with a spatial-light-modulator-based optical pulse shaper. Each pulse of the CSMP is characterized by different spectral components. Thus, ng of a sample can be estimated by calculating the group delay of the spectrally different pulses in the CSMP after passing through the sample. As a proof-of-concept, we measured ng of a glass plate by using a CSMP. We confirmed that the measured ng values were in agreement with those derived from Sellmeier's formula with an accuracy on the order of 10-4.

Detection of single human hairs with a terahertz nonlinear quantum cascade laser.

We report the demonstration of imaging of a single human hair with a terahertz quantum cascade laser (THz-QCL) source based on intracavity difference-frequency generation. A single human hair whose diameter was about 100 µm was detected using the THz-QCL source operating at 240 K, of which the THz beam had a linear polarization. The results show that the THz image of a human hair clearly depends on the polarization direction of the THz beam. The THz QCL sources that are capable of room temperature operation will be useful for detection of small foreign objects like human hairs.

Terahertz optical properties of wood-plastic composites.

Terahertz (THz) optical materials containing polymeric materials have been useful for terahertz technologies. We investigated the THz optical properties of wood-plastic composites (WPCs), which are composed of polystyrene and wood powder, and their suitability as THz optical materials. We found that the refractive indexes and absorption coefficients of the WPCs increased with increasing wood powder content. WPCs are inexpensive and have tunable THz optical properties.

Generation of arbitrarily chirped and CEP-controlled terahertz pulses for dispersion compensation using an optical pulse shaping technique and a fan-out periodically poled crystal.

We constructed a system that can generate phase-controlled terahertz (THz) pulses using a fan-out periodically poled lithium tantalate crystal and an optical pulse shaper containing a spatial light modulator. The phase of each THz frequency components could be controlled by manipulating the delay time of the corresponding optical pulses. Using the system, we generated arbitrarily group-velocity-dispersion-controlled THz pulses, where the chirp parameter was 2.53 ps2/rad between 0.6 and 1.5 THz. In addition, we generated arbitrarily carrier-envelope-phase-controlled THz pulses in the same system. Phase-controlled THz pulses may be useful for applications such as dispersion compensation.

Measurement of Circular Dichroism Spectra without Control of a Phase Modulator using Retardation Domain Analysis.

This study investigated the measurement of circular dichroism (CD) spectra without controlling a phase modulator. In a conventional CD system, the peak retardation of the phase modulator must remain constant over the observed wavelength range. Thus, the phase modulator must be controlled to maintain an appropriate modulation degree at an observed wavelength. In contrast, CD obtained using retardation domain analysis is not affected by peak retardation. Consequently, CD spectra can be measured without control of the phase modulator, which was experimentally demonstrated in this study. Additionally, linear dichroism spectra were obtained using retardation domain analysis.

Photon Upconverted Emission Based on Dye-Sensitized Triplet-Triplet Annihilation in Silica Sol-Gel System.

Photon upconverted emission based on dye-sensitized triplet-triplet annihilation was observed in silica gel systems containing Pt(II) octaethylporphyrin (triplet sensitizer) and 9,10-diphenylanthracene (singlet emitter). The triplet sensitizer was encapsulated and highly dispersed in the silica gels prepared by the sol-gel method. The singlet emitter was adsorbed on the silica gel pores accessible to the outside. Phosphorescence of the triplet sensitizer was partially quenched, and the singlet emission was enhanced with an increase in the concentration of the singlet emitter. The emission intensity increased in proportion to the approximate square of the irradiation power. The triplet energy transfer from some of the encapsulated triplet sensitizer molecules to the adsorbed singlet emitter molecules was observed in the silica gels followed by the triplet-triplet annihilation and upconverted singlet emission. The phosphorescence quenching and upconverted singlet emission were more significantly observed in the gel dried at a lower temperature (wetter gel). The wetter gel contained higher amounts of solvent and water molecules, in which the triplet sensitizer and singlet emitter should collide and then the sensitized emitters should collide between themselves during their excited-state lifetime. The photon upconversion process required the triplet sensitizer and singlet emitter molecules to be in an environment similar to the solvents.

Spatially Heterogeneous Nature of Self-Catalytic Reaction in Hetero-Double Helix Formation of Helicene Oligomers.

A 1:1 mixture of pseudoenantiomeric aminomethylenehelicene oligomers, (P)-tetramer and (M)-pentamer, in fluorobenzene show a self-catalytic phenomenon in the formation of hetero-double helices from random coils. This study visualizes the spatially heterogeneous nature of the self-catalytic reaction in dilute solution. UV/Vis imaging analysis of the mixture at 70 °C, containing random coils, exhibits a homogeneous bright area. When the solution is cooled from 70 to 30 °C and held at that temperature, dark domains of approximately 1 mm in size appear, which move approximately at a rate of 1 mm min-1 . The dark domains indicate that weaker UV/Vis absorption results from the formation of hetero-double helices, which is supported by circular dichroism (CD) imaging experiments. Then a homogeneous mixture is regenerated upon heating to 55 °C, as shown by CD imaging. Under self-catalytic conditions, a homogeneous solution spontaneously changed to a heterogeneous solution in the process of hetero-double-helix formation.

Elimination of artifacts derived from the residual birefringence of a phase modulator for circular dichroism by retardation domain analysis.

It is well known that various artifacts interfere with circular dichroism (CD) measurement using the polarization modulation technique. In this report, we propose a simple analysis method for eliminating the artifacts derived from the residual birefringence of a phase modulator. We consider the origin of the artifacts and find that those in CD can be distinguished by analyzing signals as a function of the retardation of the phase modulator using a Mueller matrix method. We experimentally obtain pure CD from a sample with CD and linear dichroism by applying retardation domain analysis.

Titanium complex formation of organic ligands in titania gels.

Thin films of organic ligand-dispersing titania gels were prepared from titanium alkoxide sols containing ligand molecules by steam treatment without heating. The formation of the ligand-titanium complex and the photoinduced electron transfer process in the systems were investigated by photoelectrochemical measurements. The complex was formed between the 8-hydroxyquinoline (HQ) and titanium species, such as the titanium ion, on the titania nanoparticle surface through the oxygen and nitrogen atoms of the quinolate. A photocurrent was observed in the electrodes containing the complex due to the electron injection from the LUMO of the complex into the titania conduction band. A bidentate ligand, 2,3-dihydroxynaphthalene (DHN), formed the complex on the titania surface through dehydration between its two hydroxyl groups of DHN and two TiOH groups of the titania. The electron injection from the HOMO of DHN to the titania conduction band was observed during light irradiation. This direct electron injection was more effective than the two-step electron injection.

Simultaneous monitoring of superoxides and intracellular calcium ions in neutrophils by chemiluminescence and fluorescence: evaluation of action mechanisms of bioactive compounds in foods.

We have developed a measuring system for simultaneous monitoring of chemiluminescence and fluorescence, which indicate respectively, (i) generation of superoxide anion radicals (O2(-•)) and (ii) change in the intracellular calcium ion concentration ([Ca(2+)]i) of neutrophils triggered by the mechanism of innate immune response. We applied this measuring system for establishing a method to distinguish between anti-inflammatory actions and antioxidant actions caused by bioactive compounds. We evaluated anti-inflammatory agents (zinc ion [Zn(2+)] and ibuprofen) and antioxidants (superoxide dismutase [SOD] and ascorbic acid). It was shown that ibuprofen and Zn(2+) were anti-inflammatory while SOD and ascorbic acid were anti-oxidative. We conclude that it is possible to determine the mechanism of action of bioactive compounds using this method.

Simultaneous measurement of superoxide generation and intracellular calcium ion of neutrophil-like culture cells.

We have developed a novel method for simultaneously measuring fluorescence and chemiluminescence. The generation of superoxide anion and the intracellular Ca(2+) ion concentration of neutrophil-like cells stimulated by agonists were measured in real time by our method. Our results were in agreement with the intracellular signalling in the neutrophils. We also found that the presence of Zn(2+) ion inhibited both the generation of superoxide anion and the influx of Ca(2+) ions.

Liquid chromatographic determination of dicarboxylic acids based on intramolecular excimer-forming fluorescence derivatization.

A highly sensitive and selective fluorimetric determination method for dicarboxylic acids (C5-C12) has been developed. This method is based on an intramolecular excimer-forming fluorescence derivatization with a pyrene reagent, 4-(1-pyrene)butyric acid hydrazide (PBH), followed by reversed-phase liquid chromatography (LC). The carboxylic acids were converted to the corresponding dipyrene-labeled derivatives by reaction with PBH in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The derivatives afforded intramolecular excimer fluorescence (450-550 nm) which can clearly be discriminated from the normal fluorescence (370-420 nm) emitted from PBH and monopyrene-labeled derivatives of monocarboxylic acids. The structures of the derivatives and the emission of excimer fluorescence were studied by LC with mass spectrometry and with spectrofluorimetry, respectively. The PBH derivatives of the carboxylic acids could be separated by reversed-phase LC on an ODS column with isocratic elution. The detection limits (signal-to-noise ratio = 3) were 1.3 fmol to undetectable for a 20-microl injection.