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The quadratic nonlinear optical coefficient in the AlGaAs system has been systematically measured as a function of the alloy composition at the fundamental wavelength of 1.064 μm by the method of reflected harmonics. The harmonic waves... more
The quadratic nonlinear optical coefficient in the AlGaAs system has been systematically measured as a function of the alloy composition at the fundamental wavelength of 1.064 μm by the method of reflected harmonics. The harmonic waves from the thin-film samples are analyzed considering various interferences including multiple reflections. The experimental results show a reduction of the magnitude of the second-harmonic coefficient with increasing Al content in the AlGaAs system. This tendency is consistent with Miller’s rule.
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Recently there has been considerable interest in surface-emitting second-harmonic (SH) generators fabricated from the AlxGa1-xAs system.1,2) No systematic data are available, however, on the d coefficients of AlxGa1-xAs1,3,4) A major... more
Recently there has been considerable interest in surface-emitting second-harmonic (SH) generators fabricated from the AlxGa1-xAs system.1,2) No systematic data are available, however, on the d coefficients of AlxGa1-xAs1,3,4) A major reason for this is that many interesting semiconductors, including AlxGa1-xAs, are obtainable only in the form of a thin epitaxial layer. Furthermore, the substrate is often absorptive at SH frequencies. We have developed a new method for characterizing the optical nonlinearity of such thin-film materials on the basis of the method of reflected harmonics.5) The new method has been applied to the determination of the coefficient d14 of AlxGa1-xAs as a function of x, the AlAs composition of the alloy semiconductor.
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Collision-induced transitions of NH/sub 3/ in the collisions with He and H/sub 2/ are studied by the laser-microwave double and triple resonance methods. The results are summarized in the preference rules. It is confirmed that ..delta..k=... more
Collision-induced transitions of NH/sub 3/ in the collisions with He and H/sub 2/ are studied by the laser-microwave double and triple resonance methods. The results are summarized in the preference rules. It is confirmed that ..delta..k= +- 3n transitions occur, while ortho-para conversion is still strictly forbidden.
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We report on an efficient generation of cw ultraviolet radiation (257 nm) using external resonant doubling. The technique of second- harmonic generation (SHG) for cw tight has been developed by following two different approaches: an... more
We report on an efficient generation of cw ultraviolet radiation (257 nm) using external resonant doubling. The technique of second- harmonic generation (SHG) for cw tight has been developed by following two different approaches: an intracavity method1 and an external resonant cavity method.2 Recently, 257-nm cw radiation of over 1 W was generated1 by the intracavity method, using a libarium borate (BBO) crystal and an argon laser. However, from the viewpoint of practical use, laser-diode-pumped solid-state lasers (such as YAG) are becoming very attractive as fundamental light sources for generating deep ultraviolet light, which is fouth harmonic. In this application, efficient external resonant doubling is required.
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... The decrease of the oscillator strength of the ZPL at higher temperatures was observed and de-scribed by the temperature dependence of a modified Debye-Waller factor with an effec-tive phonon energy and a Huang-Rhys factor. ...
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Optical control of excited states of α-perylene crystal was realized by a femtosecond optimized pulse shaping method using Genetic Algorithm (GA). We succeeded in controlling the emission spectral feature of an α-perylene crystal; the... more
Optical control of excited states of α-perylene crystal was realized by a femtosecond optimized pulse shaping method using Genetic Algorithm (GA). We succeeded in controlling the emission spectral feature of an α-perylene crystal; the intensity of E-emission was increased by a factor of 1.4 without the change of Y-emission intensity. Furthermore, we found a near-infrared pulse shape whose multi-photon excitation efficiency is larger than that of a single femtosecond pulse by a factor of two. On the auto-correlation traces of these shaped pulses, the several satellite peaks appeared beside the main peak. The origin and mechanism of the attained change were discussed.
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The f 3 Pi g state of molecular fluorine, which is assumed to be the upper level of a VUV F2 laser at 157 nm, has been directly observed for the first time by high-resolution electron energy loss spectroscopy at low impact energies (less... more
The f 3 Pi g state of molecular fluorine, which is assumed to be the upper level of a VUV F2 laser at 157 nm, has been directly observed for the first time by high-resolution electron energy loss spectroscopy at low impact energies (less than about 30 eV) and large scattering angles. The new transitions consist of several vibrational components at 11.62, 11.75, 11.86 and 11.99 eV, and show every characteristic feature of spin-forbidden transitions.
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Collision-induced transitions of NH3 in the collisions with He and H2 are studied by the laser-microwave double and triple resonance methods. The results are summarized in the preference rules. It is confirmed that Δk=±3n transitions... more
Collision-induced transitions of NH3 in the collisions with He and H2 are studied by the laser-microwave double and triple resonance methods. The results are summarized in the preference rules. It is confirmed that Δk=±3n transitions occur, while ortho–para conversion is still strictly forbidden.
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Abstract Second harmonic generation (SHG) spectroscopy using a tunable femtosecond laser has been demonstrated as an effective and a practical in situ monitor of surface electronic states during adsorption processes. We have successfully... more
Abstract Second harmonic generation (SHG) spectroscopy using a tunable femtosecond laser has been demonstrated as an effective and a practical in situ monitor of surface electronic states during adsorption processes. We have successfully shown this technique to be suitable for the study of surface electronic states, not only those induced by adsorbed molecules but also those associated with clean surfaces. By observing the change in the SH signals from a Cu(1 1 1) surface during exposure to benzene or CO molecules, we discovered new resonances of the clean Cu surface that could not be ascribed to the well-known surface state (SS)–image state (IS) transition. One of these was from a surface site that is less likely to adsorb benzene, where the SH signal intensity was kept constant until the Cu surface was covered by the sub-monolayer.
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ABSTRACT
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Differential excitation cross sections have been measured for Kr 4${\mathit{p}}^{6}$${(}^{1}$${\mathit{S}}_{0}$)\ensuremath{\rightarrow}4${\mathit{p}}^{5}$${(}^{2}$${\mathit{P}}_{1/2,3/2}$)5s transitions at 300- and 500-eV impact energies... more
Differential excitation cross sections have been measured for Kr 4${\mathit{p}}^{6}$${(}^{1}$${\mathit{S}}_{0}$)\ensuremath{\rightarrow}4${\mathit{p}}^{5}$${(}^{2}$${\mathit{P}}_{1/2,3/2}$)5s transitions at 300- and 500-eV impact energies and for 1.5\ifmmode^\circ\else\textdegree\fi{}\char21{}10\ifmmode^\circ\else\textdegree\fi{} scattering angles by electron-energy-loss spectroscopy. The integrated cross sections for these impact energies are reported here. The generalized oscillator strengths have also been obtained to determine the optical oscillator strengths. The errors are estimated to be less than 15%.
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Abstract The possibility of Bose condensation of excitons has been predicted in a system with spatially separated electron hole pairing (SEHP), e.g. a GaAs/AlGaAs double quantum well separated by a thin quantum barrier. Coherent... more
Abstract The possibility of Bose condensation of excitons has been predicted in a system with spatially separated electron hole pairing (SEHP), e.g. a GaAs/AlGaAs double quantum well separated by a thin quantum barrier. Coherent recombination emission has been expected, due to the transfer of coherence from the condensed excitons to the photon field.
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The Potassium Titanyl Phosphate, KTi0PO4 (KTP) is crystalline ferroelectric nonlinear material suitable for optoelectronics particularly for frequency doublers and waveguides. The symmetry change (mm2-mmm) at the... more
The Potassium Titanyl Phosphate, KTi0PO4 (KTP) is crystalline ferroelectric nonlinear material suitable for optoelectronics particularly for frequency doublers and waveguides. The symmetry change (mm2-mmm) at the high-temperatureferroelectric phase transition is such, that the crystal in the room-temperatureferroelectric phase is divided into the 180-degree domains with the collinear crystal lattices. For some practical usage, in particularly in the second harmonic generators the presence of domains is very undesirable because it radically reduces effectiveness of SHG . On the other hand, some other applications need regular domain structure. _
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When the 14NH3 molecules in the upper inversion level of the J=8, K=7 (para) rotational state are pumped by using the N2O P (13) laser line, many microwave inversion lines change their intensities because of collision-induced populational... more
When the 14NH3 molecules in the upper inversion level of the J=8, K=7 (para) rotational state are pumped by using the N2O P (13) laser line, many microwave inversion lines change their intensities because of collision-induced populational transfer. Pressure dependence of this effect has been studied for several tens of inversion transitions in the ground vibrational state. The non-Boltzmann distribution introduced into the pumped levels is transfered to other levels by two mechanisms: (i) collision-induced transitions into or out of the pumped levels and succeeding cascading processes and (ii) selective energy transfer mechanisms to collision partners. In the former mechanism the transfer of populational change is caused by the transition of pumped molecules whereas in the latter it is caused by collision partners. Although the first mechanism is more efficient than the second, the second mechanism is not negligible and causes the populational change in also ortho rotational state. We used mixtures of 15NH3 and 14NH3 and monitored the 15NH3 inversion lines to discriminate between the two mechanisms. Since 15NH3 is not pumped by the laser line, the double resonance signal is caused only by the second process. The result on 15NH3 is similar to that for the ortho-14NH3, which clearly indicates that the signal previously observed for the ortho-14NH3 is not due to the collision-induced ortho–para conversion but due to the second mechanism. Double resonance signals for para-14NH3 show different pressure dependence from those of ortho-14NH3 and 15NH3. The results are analyzed consistently by taking account of various collision mechanisms. In the course of this analysis it was found that the V–V energy transfer becomes much more inefficient when the energy discrepancy becomes several tens of wavenumbers.
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Collision‐induced transitions of NH3 in the collisions with He and H2 are studied by the laser‐microwave double and triple resonance methods. The results are summarized in the preference rules. It is confirmed that Δk=±3n transitions... more
Collision‐induced transitions of NH3 in the collisions with He and H2 are studied by the laser‐microwave double and triple resonance methods. The results are summarized in the preference rules. It is confirmed that Δk=±3n transitions occur, while ortho–para conversion is still strictly forbidden.
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Research Interests:
It is often stated, and with justification, that the concepts underlying coherent optical transient phenomena can be found in the literature of spin transients. For example, the Bloch vector [1] description of spin echoes [2] and free... more
It is often stated, and with justification, that the concepts underlying coherent optical transient phenomena can be found in the literature of spin transients. For example, the Bloch vector [1] description of spin echoes [2] and free precession [3] applies equally well to photon echoes [4] and optical free induction decay [5]. While this conclusion remains largely true, it is not equally valid to say that the techniques are the same or to prophesy that the applications of optical and rf transients are identical also. In fact, the decay channels of optically excited atoms, molecules and solids usually involve a variety of interactions which contrast with the magnetic interactions encountered in spin resonance. This is a major distinction, one which has permitted new studies at optical frequencies. Moreover, it is also true that some of the practioners of magnetic resonance are beginning to learn a few lessons from quantum optics as well. In this paper, we have selected some of the new developments which are representative of the field of coherent optical transients.
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The maximum entropy method (MEM) is one of the key techniques for spectral analysis. The major feature is that spectra in the low frequency part can be described by the short time-series data. Thus, we applied MEM to analyse the spectrum... more
The maximum entropy method (MEM) is one of the key techniques for spectral analysis. The major feature is that spectra in the low frequency part can be described by the short time-series data. Thus, we applied MEM to analyse the spectrum from the time dependent dipole moment obtained from the time-dependent density functional theory (TDDFT) calculation in real time. It is intensively studied for computing optical properties. In the MEM analysis, however, the maximum lag of the autocorrelation is restricted by the total number of time-series data. We proposed that, as an improved MEM analysis, we use the concatenated data set made from the several-times repeated raw data. We have applied this technique to the spectral analysis of the TDDFT dipole moment of ethylene and oligo-fluorene with n = 8. As a result, the higher resolution can be obtained, which is closer to that of FT with practically time-evoluted data as the same total number of time steps. The efficiency and the characteristic feature of this technique are presented in this paper.
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It is often stated, and with justification, that the concepts underlying coherent optical transient phenomena can be found in the literature of spin transients. For example, the Bloch vector [1] description of spin echoes [2] and free... more
It is often stated, and with justification, that the concepts underlying coherent optical transient phenomena can be found in the literature of spin transients. For example, the Bloch vector [1] description of spin echoes [2] and free precession [3] applies equally well to photon echoes [4] and optical free induction decay [5]. While this conclusion remains largely true, it is not equally valid to say that the techniques are the same or to prophesy that the applications of optical and rf transients are identical also. In fact, the decay channels of optically excited atoms, molecules and solids usually involve a variety of interactions which contrast with the magnetic interactions encountered in spin resonance. This is a major distinction, one which has permitted new studies at optical frequencies. Moreover, it is also true that some of the practioners of magnetic resonance are beginning to learn a few lessons from quantum optics as well. In this paper, we have selected some of the new developments which are representative of the field of coherent optical transients.
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Optimized pulse shaping experiments were carried out on the control of two-photon excitation efficiency of an α-perylene crystal in the temperature region from 30 to 290 K. It was found that a pulse train with a pulse interval of 90 fs... more
Optimized pulse shaping experiments were carried out on the control of two-photon excitation efficiency of an α-perylene crystal in the temperature region from 30 to 290 K. It was found that a pulse train with a pulse interval of 90 fs and an alternately reversing phase relation increased the excitation efficiency by a factor of 2 for the whole temperature region. The pulse shape characteristic for effective efficiency increase was educed by double pulse experiments in which the dependence of the emission intensity on the pulse interval and relative phase between pulses were measured. The mechanism of the efficiency increase is briefly discussed using a sliding-window Fourier transform of the pulse shape.
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We demonstrated that the two-photon excitation efficiency of perylene in chloroform solution as well as that of crystalline perylene was dramatically increased by optimizing the shape of intense femtosecond laser pulses of a regenerative... more
We demonstrated that the two-photon excitation efficiency of perylene in chloroform solution as well as that of crystalline perylene was dramatically increased by optimizing the shape of intense femtosecond laser pulses of a regenerative amplifier output. The efficiency was three times higher than for an unshaped single femtosecond pulse with the pulse width of shorter than 50fs. The pulse shape optimized for the solution sample was a pulse train with a repetition frequency of about 340cm−1, and the pulse shape optimized for crystalline perylene was very similar. These results supported our previous findings on α-perylene crystals using weak femtosecond pulses from a mode-locked laser oscillator [T. Okada et al. J. Chem. Phys. 121, 6385 (2004)]. Furthermore, it was confirmed that the shaped pulse optimized for the liquid sample could also increase the two-photon excitation efficiency of α-perylene crystals and vice versa. We concluded that the mechanism for the increase in excitatio...
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The quadratic nonlinear optical coefficient in the AlGaAs system has been systematically measured as a function of the alloy composition at the fundamental wavelength of 1.064 μm by the method of reflected harmonics. The harmonic waves... more
The quadratic nonlinear optical coefficient in the AlGaAs system has been systematically measured as a function of the alloy composition at the fundamental wavelength of 1.064 μm by the method of reflected harmonics. The harmonic waves from the thin-film samples are analyzed considering various interferences including multiple reflections. The experimental results show a reduction of the magnitude of the second-harmonic coefficient with increasing Al content in the AlGaAs system. This tendency is consistent with Miller’s rule.
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It was demonstrated that the 1S state of sulfer atoms could be produced by discharge dissociation of OCS; the 1S state atoms were identified by observing the 772.5 nm auroral line fluorescence emitted from the afterglow of short pusle... more
It was demonstrated that the 1S state of sulfer atoms could be produced by discharge dissociation of OCS; the 1S state atoms were identified by observing the 772.5 nm auroral line fluorescence emitted from the afterglow of short pusle discharge of He gas containing 0.1% to 1.2% of OCS. The yield increased linearly with E/p up to 300 V cm-1 Torr-1. The fluorescence showed a single exponential decay. The decay rate constant was measured as a function of the buffer gas pressure and no evidence of quencher formation during the discharge was found.
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We have investigated the exciton dynamics in InGaAs/GaAs single quantum wells using time-resolved photoluminescence spectroscopy. The temperature dependence of the decay time shows that the scattering rate of two-dimensional excitons is... more
We have investigated the exciton dynamics in InGaAs/GaAs single quantum wells using time-resolved photoluminescence spectroscopy. The temperature dependence of the decay time shows that the scattering rate of two-dimensional excitons is reduced in the narrower wells compared with that of AlGaAs/GaAs quantum wells. The results suggest the dominant contribution of the alloy disorder scattering to the exciton scattering processes over the interface roughness scattering.