Tomoharu Nakazato

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.

Plasma physics and laser development for the Fast-Ignition Realization Experiment (FIREX) Project

Since the approval of the first phase of the Fast-Ignition Realization Experiment (FIREX-I), we have devoted our efforts to designing advanced targets and constructing a petawatt laser, which will be the most energetic petawatt laser in the world. Scientific and technological improvements are required to efficiently heat the core plasma. There are two methods that can be used to enhance the coupling efficiency of the heating laser to the thermal energy of the compressed core plasma: adding a low-Z foam layer to the inner surface of the cone and employing a double cone. The implosion performance can be improved in three ways: adding a low-Z plastic layer to the outer surface of the cone, using a Br-doped plastic ablator and evacuating the target centre. An advanced target for FIREX-I was introduced to suit these requirements. A new heating laser (LFEX) has been constructed that is capable of delivering an energy of 10 kJ in 10 ps with a 1 ps rise time. A fully integrated fast-ignition experiment is scheduled for 2009.

Temperature dependence of scintillation properties for a hydrothermal-method-grown zinc oxide crystal evaluated by nickel-like silver laser pulses

The scintillation properties of a hydrothermal-method-grown zinc oxide (ZnO) crystal in the extreme ultraviolet region are evaluated using a nickel-like silver laser with an emission wavelength of 13.9 nm. The temperature dependence of the scintillation properties of the ZnO emission at a wavelength of ~380 nm is investigated. The emission exhibits a broad peak at 386 nm with a full width at half-maximum (FWHM) of 15 nm at room temperature (298 K). Decreasing the ZnO crystal temperature to 25 K causes the peak position to be blueshifted by 12 nm while the FWHM becomes narrower by 9 nm as compared with the room temperature case.

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.

Strong enhancement of terahertz emission from GaAs in InAs/GaAs quantum dot structures

We report on the intense terahertz emission from InAs/GaAs quantum dot (QD) structures grown by molecular beam epitaxy. Results reveal that the QD sample emission was as high as 70% of that of a p-type InAs wafer, the most intense semiconductor emitter to date. Excitation wavelength studies showed that the emission was due to absorption in strained undoped GaAs, and corresponds to a two order-of-magnitude enhancement. Moreover, it was found that multilayer QDs emit more strongly compared with a single layer QD sample. At present, we ascribe the intense radiation to huge strain fields at the InAs/GaAs interface.

Development of Vacuum Ultraviolet Streak Camera System for the Evaluation of Vacuum Ultraviolet Emitting Materials

Details of the design and development of a bright vacuum ultraviolet Seya–Namioka type spectrometer and streak camera system with reflection-type input optics is presented. The Seya–Namioka spectrometer has a 40 mm by 45 mm holographic grating with a groove density of 600 grooves/mm, linear dispersion of 8 nm/mm, and effective F-number of 4.2. To eliminate loss of light by absorption, the streak camera imaging unit uses Al and MgF2 coated imaging optics with at least 80% reflection from 125–850 nm and has an effective F-number of 4. It is equipped with a synchronized scan unit that can be tuned to 82 MHz and a slow scan speed unit that can measure up to 2-MHz single event. Temporal response of the system at the 20-ns scanning range is evaluated to be around 260 ps.

Response Time-Shortened Zinc Oxide Scintillator for Accurate Single-Shot Synchronization of Extreme Ultraviolet Free-Electron Laser and Short-Pulse Laser

We report an over one-order-of magnitude improvement in the response time of conventional hydrothermal method-grown zinc oxide (ZnO) scintillator by introducing additional quenching channels via intentional indium ion doping. A 3-ps fluorescence decay time constant is achieved, therefore making it the fastest scintillator operating below 100 nm to date. Using this indium-doped ZnO, relative jitter between extreme ultraviolet free electron laser (EUV-FEL) probe and optical pump pulses is evaluated to be less than 3 ps. Moreover, pulses from these sources can be synchronized with 3-ps accuracy through in-situ observation of relative time difference in single-shot base.

Laser Quality Ce3+:LiCaAlF6 Grown by Micro-Pulling-Down Method

We describe the successful growth of a laser quality Ce3+:LiCaAlF6 crystal using the micro-pulling down method. The crystal is grown using less than one gram of raw material and a pulling rate of 0.1 mm/min. Ultraviolet laser emission at 290 nm with 23% slope efficiency and 1 mJ pulse energy is demonstrated from a Brewster-cut crystal, which is 30 mm in length and 2 mm in diameter. The flexibility and lower cost of this growth technique will markedly reduce the cost of growing laser grade crystals.

Birefringence of β-BaB2O4 crystal in the terahertz region for parametric device design

β-BaB2O4 (BBO) was found to have huge birefringence in the 0.1–1.1THz frequency region. Polarized terahertz transmission spectroscopy revealed a Δn∕n value of about 0.12±0.02. This result could be related to a polarization-dependent absorption that was observed at about 0.65THz; in close agreement with first principles-calculations, showing this to be due to low-frequency phonon modes of the [B3O6]3-rings. These findings suggest the utilization of the birefringent properties of BBO for optical parametric devices including amplifiers and oscillators, operating in the terahertz regime.

Er:LiCAF as Potential Vacuum Ultraviolet Laser Material at 163 nm

We report the vacuum ultraviolet luminescence characteristics of a micro-pulling down method grown Er³⁺:LiCaAlF₆. Emission at 163 nm with a lifetime of 1.3-μs observed with F₂ laser (157-nm) pumping. This is one of the shortest emission wavelengths reported from a rare-earth doped fluoride at room temperature.