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A low-cost type terahertz time-domain spectroscopic system can be constituted using a multimode laser diode instead of a pulse laser. To suppress noise, a window function is usually used in the Fourier transformation. When this system is used to measure an optically thick sample, the obtained refractive index shows sinusoidal structures. This is caused by ingress of a signal fragment into the measured range and egress of another signal fragment out of the measured range. In addition, a broad positive background component appears in the imaginary part of the refractive index. This is because of the decreased amplitude of the central structure of the signal, which results from the time shift under the window function caused by sample insertion. These false structures can be eliminated by using a modified window function that is shifted with the signal when the sample is inserted.

Background: Hydrogen (H) and Carbon (C) are major elements that occur in various materials, including organic matter. The identification and analysis of C and H are necessary for several fields. LIBS is an excellent method for such analysis because it is rapid, and can be conducted remotely. The technique has been employed for the analysis of H in zircaloy metals. However, few studies on the emission characteristics of H and C in various gases have been undertaken because of the difficulty of identifying H and C using standard LIBS techniques. In this work, the emission characteristics of H and C were studied using pulsed CO2 LIBS. H and C elements were obtained from ethanol vapor. Various gas environments were employed, including Nitrogen (N2), Argon (Ar), and Helium (He) gases, in order to study the stability of the laser-induced plasma, the plasma lifetime, and the excitation mechanisms of H and C. Methods: The LIBS system used in this work consisted of a pulsed TEA CO2 laser (Shibuya SQ 2000), pulse generator, and optical multichannel analyzer. In this work, the laser with a wavelength of 10.6 μm, pulse duration of 200 ns, and pulse energy of 3 J, was used as the irradiation source. The laser energy used was 1.5 J. The laser was irradiated, and focused, using a 200 mm zinc selenide (ZnSe) lens, onto a metal surface in order to initiate and induce a luminous plasma. The sample used in this study was ethanol vapor, obtained from ethanol (99.5%, Merck). For this purpose, 10 mL ethanol was poured into a glass beaker, and this was placed into a closed chamber that could be evacuated by ambient gases including N2, Ar, and He gases. Results: Identification of H emission line has been successfully carried out using this present technique demonstrated in various gases including N2, Ar, and He. From the results, it was observed that in N2 gas, the Hα I 656.3 nm emission line was clearly expressed, with a wide, full-width halfmaximum, and quite a low emission intensity. Conclusion: The emission characteristics of H and C in laser-induced plasma, produced in various ambient gases, including N2, Ar, and He, were studied. The emission spectra of Hα and Hβ were expressed clearly, with high intensity and low background emission, in He, while they were broad and had high background emissions in N2 and Ar. Based on the time-resolved emissions, the Hα emission had a longer lifetime in Ar and He. It was assumed that the metastable atoms of Arand He were predominant in the excitation process of H and C. The characteristics of the H and C emissions in various gases are important in selecting a suitable ambient gas for the study of light atomic emission in the medical field, which mostly deals with organic materials containing H, C, and oxygen.

•Accurate and direct analysis of human blood serums for elemental composition using a metal mesh-supported CO2laser-induced breakdown spectroscopy under a helium gas environment at atmospheric pressure was carried out.•A significant increment of elemental signal intensity occurs by introducing a metal mesh covering the serum film.•Qualitative and quantitative analyses have been conducted on the human blood serum's major, minor, and trace elements such as C, Ca, K, Mg, P, Na, and Fe.•The CO2LIBS method demonstrates a high level of concurrence with the results obtained by the conventional XRF method.

A sub-terahertz time-domain spectroscopic system with spiral antennas, which have high efficiency in the sub-THz region, was applied to imaging. In the time-domain signals, there were several peaks with various widths. Images were obtained by translating samples at the focus of the sub-THz beam with the time delay fixed at certain positions while monitoring the signal. At the edge of the metal patterns, the signal basically showed step-like variations, which were sometimes accompanied by overshoot structures (peaks and dips) depending on the time-delay position. The overshoot can be utilized to make the images sharper. The time-delay position can be adjusted to make the step height zero and produce images that contain only the overshoot, which can be transformed into images of the edge lines of the metal pattern. In addition, an image of the watermark on a 1000-yen bill was obtained. The image contrast caused by the small variation of the bill paper thickness was enhanced by locating the time delay at the position where the signal shows maximum sensitivity to the phase shift.

To study the mechanism of atomic excitation in gas plasma, a transversely excited atmospheric pressure (TEA) CO2laser (750 mJ, 200 ns) was focused on a metal subtarget through a hole (2 mm in diameter) produced in a glass slide sample (1.5 mm in thickness); the glass slide sample was placed in close contact with a nickel metal plate, which functions as the metal subtarget. It was demonstrated that a strong gas plasma can be induced in various gases including He, N2, and CO2and that plasma can be employed as the excitation source for spectrochemical analysis. It has been proved that, in He gas plasma, strong Ca ionic emissions (Ca II 393.3 nm and Ca II 373.7 nm) with a long lifetime can be observed regardless of excitation energy. On the other hand, in N2and CO2gases, the emissions are very weak and have a short lifetime. We assumed that, in He gas plasma, He metastable atoms play significant role in excitation. In the case of the Ca atom, the double ionization of Ca (Ca2+) is directly induced when a Ca atom collides with He metastable atoms through the Penning effect. The ion produced then recombines with an electron to produce an ionic excitation state, from which ionic emission is induced. In neutral Ca and Mg emissions, unique emission characteristics were observed in the He gas plasma case, namely, the triplet state is the main product and the singlet state is a minor product. This result can be explained by our proposed model.

Abstract: Our previous work on an innovative method of hydrogen (H) analysis using the specific characteristics of a TEA CO2 laser, “selective detection method of H”, has been improved to realize a high H sensitive analysis with a detection limit of several µg/g. For this purpose, first, we clarified the origin of the H emission disturbance coming from H2O molecules; namely, we showed that most of the H emission came from H2O on the metal surface and not from H2O existing in the surrounding gas when we formed a laser-induced gas plasma. Second, the difference in the emission characteristics between the H emission from H2O on the metal surface and H emission from inside in sample was studied to determine the optimum gating time of the optical multi-channel analyzer (OMA). Third, the gas plasma was totally covered by fresh helium gas using a big pipe (5 mm in diameter) and by flowing a high amount of He (10 l/min). Also, we demonstrated that our methods could potentially be applied to H analysis in steel samples, where an H analysis with a sensitivity of less than 1 µg/g is required without employing a heating process, by removing H2O on the sample surface with the aid of defocused TEA CO2 laser irradiation. Thus, we stress that our method can be used for a highly sensitive, in-situ analysis of H for metal samples.

Taking advantage of the specific characteristics of a transversely excited atmospheric (TEA) CO_2 laser, a sophisticated technique for the analysis of chromated copper arsenate (CCA) in wood samples has been developed. In this study, a CCA-treated wood sample with a dimension of 20??mm×20??mm and a thickness of 2 mm was attached in contact to a nickel plate (20??mm×20??mm×0.15??mm), which functions as a subtarget. When the TEA CO_2 laser was successively irradiated onto the wood surface, a hole with a diameter of approximately 2.5 mm was produced inside the sample and the laser beam was directly impinged onto the metal subtarget. Strong and stable gas plasma with a very large diameter of approximately 10 mm was induced once the laser beam had directly struck the metal subtarget. This gas plasma then interacted with the fine particles of the sample inside the hole and finally the particles were effectively dissociated and excited in the gas plasma region. By using this technique, high precision and sensitive analysis of CCA-treated wood sample was realized. A linear calibration curve of Cr was successfully made using the CCA-treated wood sample. The detection limits of Cr, Cu, and As were estimated to be approximately 1, 2, and 15??mg/kg, respectively. In the case of standard LIBS using the Nd:YAG laser, the analytical intensities fluctuate and the detection limit was much lower at approximately one-tenth that of TEA CO_2 laser.

To clarify the excitation mechanism of hydrogen in transversely excited atmospheric-pressure (TEA) CO2laser-induced helium gas plasma, atomic emission characteristics of H, C, F, and He were studied using a Teflon sheet (thickness of 2 mm) attached to a metal subtarget. The TEA CO2laser (750 mJ, 200 ns) was focused on the Teflon sheet in the surrounding He gas at 1 atm. Atomic emissions of H, C, F, and He occurred with a long lifetime, a narrow spectrum width, and a low-background spectrum. The correlation emission intensity curves of H--He and F--He indicated a parabolic functions. To explain the emission characteristics, we offered a model in which helium metastable atoms (He*) play an important role in the excitation processes; namely, atoms collide with helium metastable atoms (He*) to be ionized by the Penning effect, and then recombine with electrons to produce excited states, from which atomic emissions occur.

Diffraction gratings that can be used to convert the Hermite-Laguerre-Gaussian (HLG) mode into the Gaussian mode were obtained; these modes are space modes of light beams. The HLG mode is intermediate between the Hermite-Gaussian and the Laguerre-Gaussian modes. Generally, gratings produced from interfering two beams as holograms contain information of not only the phase modulation factor but also the amplitude modulation factor. Thus, they cannot be expected to provide high conversion efficiency. To produce high-efficiency gratings, the interference equation is divided into two factors. The gratings produced by considering only the phase factors act as phase converters.

A fiber-optic mirror magneto-optical trap (mirror-MOT) that uses a pair of circularly polarized light-emitting optical fibers as an optical access is demonstrated. The fiber is fabricated so that a length of birefringence fiber, designed to be a quarter wave retarder at both wavelengths of 780 and 852 nm, is attached directly onto a polarization-maintaining normal fiber. The polarization states of light emitted from the fibers are sufficiently circular for the operation of a mirror-MOT with ^87Rb atoms. The mirror-MOT is able to capture approximately the same number of atoms obtainable with a conventional mirror-MOT. The technique makes it possible to fabricate a compact MOT apparatus by introducing the optical fibers directly into an ultrahigh-vacuum chamber.

We report on increasing the sensitivity of electro-optic sampling detection of terahertz pulses by using the effect of terahertz field enhancement in a tapered metallic waveguide. A thin Si prism, placed in the narrower section of the waveguide, is used to synchronize the enhanced terahertz field with the probe laser pulse propagating in a LiNbO3crystal attached to the obliquely cut end of the waveguide. A 40-µm-thick MgO-doped layer on the surface of LiNbO3is used for guiding the probe pulse. The developed structure demonstrates a 20-fold higher detection sensitivity than a no-waveguide, Si-prism-coupled LiNbO3crystal.