Metastable atomic-ordered configurations for Al1/2Ga1/2N predicted by Monte-Carlo method based on first-principles calculations

Metastability of Aln/12Ga1-n/12N (n= 2-10: integer) with the 1-2 monolayer (ML) in-plane configuration towards thec[0001] direction has been demonstrated recently. To theoretically explain the existence of these metastable structures, relatively large calculation cells are needed. However, previous calculations were limited to the use of small calculation cell sizes to estimate the local potential depth (Δσ) of ordered Al1/2Ga1/2N models. In this work, we were able to evaluate large calculation cells based on the interaction energies between proximate Al atoms (δEAl-Al) in AlGaN alloys. To do this,δEAl-Alvalues were estimated by first-principles calculations (FPCs) using a (5a1× 5a2× 5c) cell. Next, a survey of the possible ordered configurations using various large calculation cell models was performed using the estimatedδEAl-Alvalues and the Monte-Carlo method. Then, various Δσvalues were estimated by FPCs and compared with the configurations previously reported by other research groups. We found that the ordered configuration obtained from the (4a1× 2a2× 1c) calculation cell (C42) has the lowest Δσof -9.3 meV/cation and exhibited an in-plane configuration at thec(0001) plane having (-Al-Al-Ga-Ga-) and (-Al-Ga-) sequence arrangements observed along them11-00planes. Hence, we found consistencies between the morphology obtained from experiment and the shape of the primitive cell based on our numerical calculations.

Tunable vacuum ultraviolet cross-luminescence from KMgF under high pressure as potential fast-response scintillator

We report on the potential of the potassium magnesium fluoride (KMgF) crystal as a fast-response scintillator with tunable cross-luminescence (CL) emission wavelength through high-pressure applications. By performing first-principles density functional theory calculations using the Perdew-Burke-Ernzerhof (PBE) hybrid functional including exact exchange (PBE0) and Green's function and screened Coulomb interaction approximation as implemented in the Vienna Ab initio Simulation Package using plane-wave basis sets within the projector-augmented wave method, we identify the specific valence-to-core band transition that results in the experimentally observed CL emission at 148 nm (8.38 eV) and 170 nm (7.29 eV) wavelengths with intrinsically fast decay times of 290 ps and 210 ps, respectively. Uniform volume compression through hydrostatic high-pressure applications could decrease the energy gap between the valence and core bands, potentially shifting the CL emission wavelength to the ultraviolet (UV) region from 200 nm (6.2 eV) to 300 nm (4.1 eV). The ability to tune and shift the CL emission to UV wavelengths allows for the detection of the CL emission using UV-sensitive photodetectors in ambient atmosphere instead of highly specialized vacuum UV detectors operating in vacuum while maintaining the intrinsically fast CL decay times, thereby opening up new possibilities for KMgF as a fast-response scintillator.

Mid-infrared imaging through up-conversion luminescence in trivalent lanthanide ion-doped self-organizing optical fiber array crystal

We propose a scheme for imaging mid-infrared (MIR) wavelengths via pre-excitation-assisted up-conversion luminescence in lanthanide ion (Ln³⁺)-doped Self-organizing Optical FIber Array (SOFIA) crystal. First, near-infrared pre-excitation wavelength excites an electron from the ground state to an excited state of Ln³⁺. Next, the MIR wavelength to be imaged promotes this excited electron to a higher-lying energy state. Finally, relaxation of the electron from the higher-lying energy state to the ground state emits the up-conversion luminescence in the visible region, completing the MIR-to-visible wavelength conversion. An analysis of the 4f to 4f intra-configurational energy level transitions in Ln³⁺, together with an appropriate selection of the pre-excitation wavelength and the visible luminescence constrained within the 500-700 nm wavelength range, reveals that trivalent erbium (Er³⁺), thulium (Tm³⁺), holmium (Ho³⁺), and neodymium (Nd³⁺) can be used to image MIR wavelengths. Our proposed scheme, called MIR imAging through up-Conversion LuminEscence in a SOFIA crystal, will enable the imaging of MIR wavelengths using low-cost optics and readily available silicon-based detectors in the visible spectral region and will open up new possibilities for MIR wavelength detection and imaging.

Layer thickness dependence of the terahertz emission based on spin current in ferromagnetic heterostructures

The emission with a bandwidth of 1.5 terahertz based on the spin current in the ferromagnetic heterostructure Co/Pt is demonstrated. The spin transient launched by the NIR femtosecond laser pulse in the Co/Pt is converted into the in-plane charge current due to the inverse spin Hall effect, which gives rise to the terahertz emission towards free space. The dependence of the terahertz emission on the Pt-layer thickness is investigated. To optimize the geometry structure of the new type of emitter, we developed the theoretical model by carefully analyzing the spin transport. Our model reveals the importance to take into account the interfacial spin loss. It can be used to analyze more complex heterostructures.

Enhancing terahertz magnetic near field induced by a micro-split-ring resonator with a tapered waveguide

Substantial enhancement of terahertz magnetic near field achieved by the combination of a tapered metallic waveguide and a micro-split-ring resonator is demonstrated. The magnetic near field is probed directly via the magneto-optic sampling with a Tb₃Ga₅O₁₂ crystal. The incident terahertz wave with a half-cycle waveform is generated by using the pulse-front tilting method. The magnetic near field at the resonant frequency is enhanced by more than 30 times through the combination of the waveguide and the resonator. The peak amplitude of the magnetic field with a damped oscillation waveform in the time domain is up to 0.4 T. The resonant frequency can be tuned by adopting different resonator designs. The mechanism of the enhancement is analyzed by performing calculations based on the finite element method. The strong terahertz magnetic near field enables the excitation of large-amplitude spin dynamics and can be utilized for an ultrafast spin control.

Development of multichannel low-energy neutron spectrometer

A multichannel low-energy neutron spectrometer for down-scattered neutron (DSN) measurements in inertial confinement fusion (ICF) experiments has been developed. Our compact-size 256-channel lithium-glass-scintillator-based spectrometer has been implemented and tested in ICF experiments with the GEKKO XII laser. We have performed time calibration of the 256-channel analog-to-digital convertor system used for DSN measurements via X-ray pulse signals. We have clearly observed the DD-primary fusion neutron signal and have successfully studied the detector's impulse response. Our detector is soon to be implemented in future ICF experiments.

Characterizing a fast-response, low-afterglow liquid scintillator for neutron time-of-flight diagnostics in fast ignition experiments

The characteristics of oxygen-enriched liquid scintillators with very low afterglow are investigated and optimized for application to a single-hit neutron spectrometer for fast ignition experiments. It is found that 1,2,4-trimethylbenzene has better characteristics as a liquid scintillator solvent than the conventional solvent, p-xylene. In addition, a benzophenon-doped BBQ liquid scintillator is shown to demonstrate very rapid time response, and therefore has potential for further use in neutron diagnostics with fast time resolution.

The photonuclear neutron and gamma-ray backgrounds in the fast ignition experiment

In the fast-ignition scheme, very hard x-rays (hereinafter referred to as γ-rays) are generated by Bremsstrahlung radiation from fast electrons. Significant backgrounds were observed around the deuterium-deuterium fusion neutron signals in the experiment in 2010. In this paper the backgrounds were studied in detail, based on Monte Carlo simulations, and they were confirmed to be γ-rays from the target, scattered γ-rays from the experimental bay walls (γ'-rays), and neutrons generated by (γ, n) reactions in either the target vacuum chamber or the diagnostic instruments (γ-n neutrons).

Note: Light output enhanced fast response and low afterglow 6Li glass scintillator as potential down-scattered neutron diagnostics for inertial confinement fusion

The characteristics of an APLF80+3Ce scintillator are presented. Its sufficiently fast decay profile, low afterglow, and an improved light output compared to the recently developed APLF80+3Pr, were experimentally demonstrated. This scintillator material holds promise for applications in neutron imaging diagnostics at the energy regions of 0.27 MeV of DD fusion down-scattered neutron peak at the world's largest inertial confinement fusion facilities such as the National Ignition Facility and the Laser Mégajoule.

Down-scattered neutron imaging detector for areal density measurement of inertial confinement fusion

A custom developed (6)Li glass scintillator (APLF80+3Pr) for down-scattered neutron diagnostics in inertial confinement fusion experiments is presented. (6)Li provides an enhanced sensitivity for down-scattered neutrons in DD fusion and its experimentally observed 5-6 ns response time fulfills the requirement for down-scattered neutron detectors. A time-of-flight detector operating in the current mode using the APLF80+3Pr was designed and its feasibility observing down-scattered neutrons was demonstrated. Furthermore, a prototype design for a down-scattered neutron imaging detector was also demonstrated. This material promises viability as a future down-scattered neutron detector for the National Ignition Facility.

Response-time improved hydrothermal-method-grown ZnO scintillator for soft x-ray free-electron laser timing-observation

For pump and probe experiments in x-ray free-electron laser (XFEL) facilities, accurate timing synchronization between short-wavelength femtosecond pulses from XFELs and short optical pulses from other light sources is required. For this purpose, the response time of a hydrothermal-method-grown ZnO is improved by over one order of magnitude via intentional iron ion doping. The fluorescence rise- and decay-time constants are measured to be less than 10 and 100 ps, respectively. Owing to its intense fluorescence even for single pulse XFEL excitation, the timing jitter of the soft x-ray pulse and timing electronics are evaluated to be less than 70 ps.

Pr3+-doped fluoro-oxide lithium glass as scintillator for nuclear fusion diagnostics

Experimental results are presented on the neutron scintillating properties of a custom-designed Pr3+ (praseodymium)-doped lithium (Li) glass. Luminescence was observed at 278 nm wavelength, originating from the 5d-4f transition. Time-resolved measurements yielded about 20 ns decay times for ultraviolet and x-ray excitation while much faster decay times of about 6 ns were observed for alpha particle and neutron excitation. Actual time-of-flight data in laser fusion experiments at the GEKKO XII facility of the Institute of Laser Engineering, Osaka University reveal that it can clearly discriminate fusion neutrons from the much stronger x-rays signals. This material can promise improved accuracy in future scattered neutron diagnostics.

Transmission of terahertz radiation using a microstructured polymer optical fiber

A hollow-core microstructured polymer optical fiber was analyzed in the terahertz (THz) region. Spectral analysis of time domain data shows propagation of THz waves in both the hollow-core and the microstructured cladding with a time delay of approximately 20 ps. The frequency range and shift of the transmission bands between different sized waveguides suggested photonic bandgap or resonant guidance. Finite-difference time domain calculations agree relatively well to the experimental transmission results. Propagation losses were estimated to be as low as 0.9 dB/cm.

Ratio of low serum zinc levels in elderly Japanese people living in the central part of Japan

OBJECTIVE

Elderly patients often have impaired taste, nausea, anorexia and delayed healing of decubitus. In many of these patients, serum zinc levels are low and they respond to zinc supplementation. To date, no epidemics of zinc deficiency have been reported in Japan. We studied the prevalence of zinc deficiency and its causative factors in a typical local town in Japan.

DESIGN

Cross-sectional study.

SETTING

Nagano Prefecture (central Japan).

SUBJECTS AND METHODS

Serum zinc levels were measured in 1009 habitants (18-96 years old) who participated in an annual mass health examination program of the City in 2003. Of all subjects, 86 with low serum zinc (<65 microg/dl) or high zinc (> or =90 microg/dl) levels were randomly selected, and a dietary survey using 24-h recalling methods for 2 consecutive days was performed in 2004. Among them, blood was collected in the morning from 50 subjects (26-94 years old).

RESULTS

The percentage of those with low serum zinc levels (2.5th percentile of Americans) was 37.9% in the elderly (> or =60 years old). The age-adjusted prevalence of low serum zinc was 21.1% in the aged population. A positive correlation was detected between zinc intake and serum zinc levels in elderly subjects (> or =60 years old).

CONCLUSIONS

The possibility of zinc deficiency in adult inhabitants in central Japan rises with age. The deficiency correlates with dietary zinc intake.

Vacuum ultraviolet spectroscopy of Ce(3+)-doped SrMgF(4) with superlattice structure

X-ray diffraction of Ce(3+)-doped SrMgF(4) (SMF:Ce) crystals shows a superlattice structure, reflecting the distribution of Ce(3+) polyhedra centres observed in optical experiments. Optical absorption bands and fluorescence bands from the Ce(3+) polyhedra centres overlap in the vacuum ultraviolet (VUV) and ultraviolet (UV) regions, respectively, so that wide pumping and tuning ranges are expected for laser operation. The SMF:Ce crystals, as well as the isomorphous BaMgF(4), are candidates for a tunable laser gain material with nonlinear properties. The optical absorption, excitation, and fluorescence bands observed in the SMF:Ce crystals at low temperatures are ascribed to five distinct fluorescent centres. Three centres have well-known Ce(3+) optical characters, for example, fluorescence with double peaks separated by 2000 cm(-1) and five resolved absorption/excitation bands. These centres are assigned to Ce(3+)-polyhedra classified by weak and strong crystal fields as a consequence of the superlattice structure. The other two fluorescence bands observed in the visible region have 1.5-2 times larger linewidths than those of the former three bands. These bands are interpreted as optical transitions from complexes consisting of Ce(3+) and one or two electrons trapped at a vacancy of the nearest neighbour F(-) ligand ions.

Terahertz time-domain spectroscopy of amino acids and polypeptides

Frequency-dependent absorption coefficients and refractive indices of amino acids (glycine and l-alanine) and polypeptides (polyglycine and poly-l-alanine) in the wavenumber region from 7 to 55 cm(-1) were measured by terahertz time-domain spectroscopy. A vibrational band was observed at 45.5 cm(-1) for polyglycine, which was assigned as an interchain mode. The reduced absorption cross sections of the amino acids and polypeptides show power-law behavior. The exponents are different between the monomers and polymers, and those of the two polypeptides suggest that the time dependences of the total dipole moments are similar in the timescale of subpico- to picoseconds.

High-energy, all-solid-state, ultraviolet laser power-amplifier module design and its output-energy scaling principle

We demonstrate that a coaxially pumped, large-aperture ultraviolet power-amplifier module with solid-state tunable laser medium Ce(3+):LiCaAlF6 has 98-mJ, 290-nm, and 3-ns output pulses with a sufficient extraction efficiency of 25%. The detailed information of design parameters, including the gain-coefficient dependence on pump condition, is successfully accumulated for further energy scaling for a terawatt-class ultraviolet chirped pulse amplification laser system or a high-pulse-energy laser system.

Terahertz radiation from a shallow incidence-angle InAs emitter in a magnetic field irradiated with femtosecond laser pulses

The optimized incidence angle and magnetic field direction geometry of an InAs terahertz radiation emitter irradiated with femtosecond laser pulses in a magnetic field is reported. The optimum geometric layout is the magnetic field direction parallel to the semiconductor surface and at an incidence angle that is slightly larger than the Brewster angle. Additionally, we also observed a center frequency shift of terahertz radiation spectrum by changing the incidence angle of the excitation laser.

Chirped-pulse amplification of ultraviolet femtosecond pulses by use of Ce(3+):LiCaAlF(6) as a broadband, solid-state gain medium

Chirped-pulse amplification in the ultraviolet region is demonstrated by use of a broadband Ce(3+): LiCaAlF(6) laser medium. A modified bow-tie-style four-pass amplifier pumped by 100-mJ, 266-nm pulses from a Q -switched Nd:YAG laser has a gain factor of 370 and delivers 6-mJ, 290-nm pulses. After dispersion compensation, the output pulses can be compressed to 115 fs.

All-Solid-State Tunable Ultraviolet Subnanosecond Laser with Direct Pumping by the Fifth Harmonic of a Nd:YAG laser

We report what we believe is the first all-solid-state tunable ultraviolet laser pumped by the fifth harmonic of a Q-switched Nd:YAG laser. Our laser based on a Ce(3+):LiLuF(4) active medium stably generates a single, satellite-free, 0.88-ns pulse under 5-ns, 10-Hz repetition rate pumping conditions. A novel tilted-incident-angle side-pumping scheme resulted in a simple laser-cavity design.

Ultraviolet short pulses from an all-solid-state Ce:LiCAF master-oscillator power-amplifier system

We have developed an all-solid-state master-oscillator-power-amplifier system employing Ce:LiCAF, a new degradation-free tunable ultraviolet laser medium pumped by the fourth harmonic of conventional 10-ns Q -switched Nd:YAG lasers. The low- Q , short-cavity Ce:LiCAF master oscillator produced a satellite-free 1-ns pulse under appropriate pumping-fluence control. 18% energy extraction with sufficient gain was achieved in a single-stage, confocal, double-pass amplifier. As a result, 289-nm, 1-ns, 14-mJ pulses were efficiently obtained from this simple laser system.

Direct and passive subnanosecond pulse-train generation from a self-injection-seeded ultraviolet solid-state laser

We propose a passive self-injection-seeding scheme for the generation of short-pulse trains from various pulsed lasers. In this scheme a single subnanosecond pulse is generated from a short-cavity seeding laser. The pulse is then returned to the same gain medium and amplified regeneratively until the gain is quenched completely. Cw operation capability or an external short-pulse seeding laser is not required for generation of short-pulse trains. Based on this simple scheme an ultraviolet subnanosecond pulse train is directly and passively generated from a solid-state laser medium (Ce(3+):LuLiF(4)) pumped by a standard 10-ns laser.

Ce(3+):LuLiF(4) as a broadband ultraviolet amplification medium

Ce(3+):LuLiF(4) is shown to have a broad gain spectrum, which is attractive for short-pulse applications. A large small-signal gain over 6 dB/cm and a sufficiently large saturation fluence of 50 mJ/cm(2) were observed for 10-ns probe pulses under KrF excimer laser pumping. In a confocal four-pass configuration, 17-dB amplification was demonstrated for 325-nm picosecond pulses, and 20-dB gain was demonstrated for cw laser light.

Terawatt KrF/Ti:sapphire hybrid laser system

A terawatt KrF/Ti:sapphire hybrid laser system with a high contrast between amplified spontaneous emission and the main pulses has been developed with an output energy of 130 mJ in 130-fs pulses. An efficient frequencytripling scheme with Ti:sapphire is demonstrated with an efficiency of 11% in the 100-fs regime with LiB(3)O(5) and beta-BaB(2)O(4).

Pulse evolution dynamics of a femtosecond passively mode-locked Ti:sapphire laser

The pulse-formation process in a femtosecond passively mode-locked Ti:sapphire laser with a saturable absorber is investigated. The time to reach the steady state is 200 micros. The formation time dependence on the dye concentration and the coincidence of the steady-state pulse width with the self-mode-locked state without a saturable absorber indicate that the function of the saturable absorber is mainly to induce the initial modulation and to shorten the pulse-formation time.

Generation of 50-fsec pulses from a pulse-compressed, cw, passively mode-locked Ti:sapphire laser

Stable pulses of less than 150 fsec are generated directly from a tunable cw passively mode-locked Ti:sapphire laser, through a balance of self-phase modulation in the Ti:sapphire rod and negative group-velocity dispersion produced by a prism pair. After external fiber compression, 50-fsec pulses are obtained at approximately 750 nm.

Multiterawatt subpicosecond KrF laser

A 390-fsec pulse with a peak power of 4 TW has been obtained in KrF, yielding an output energy of 1.5 J from an electron-beam-pumped amplifier with an active cross section of 320 cm(2). The amplified spontaneous emission is as low as 1.8% in energy. An initial pulse width of 210 fsec is increased to 390 fsec in the amplifier, mainly as a result of linear dispersion in the thick windows and lenses used throughout the system. Nonlinear absorption and self-phase modulation are not significant in the 2.5-cm-thick CaF(2) output window at an intensity of 13 GW/cm(2).

Single-shot measurement of subpicosecond KrF pulse width by three-photon fluorescence of the XeF visible transition

The intensity of the XeF C-A transition induced by a subpicosecond KrF laser is shown to have a cubic dependence on KrF laser intensity. The third-order autocorrelation technique for measuring the duration of a single KrF subpicosecond pulse has been developed utilizing this visible transition. A pulse width of 220 fsec was successfully measured with a high contrast of ~10. The visible fluorescence is more useful to researchers than vacuum-UV fluorescences. Furthermore, this simple technique may be applied over a wide UV wavelength region from 204 to 306 nm.