Keisuke Nagashima

Hydrothermal method grown large-sized zinc oxide single crystal as fast scintillator for future extreme ultraviolet lithography

The scintillation properties of a hydrothermal method grown zinc oxide (ZnO) crystal are evaluated for extreme ultraviolet (EUV) laser excitation at 13.9nm wavelength. The exciton emission lifetime at around 380nm is determined to be 1.1ns, almost identical to ultraviolet laser excitation cases. This fast response time is sufficiently short for characterizing EUV lithography light sources having a few nanoseconds duration. The availability of large size ZnO crystal up to 3in. is quite attractive for future lithography and imaging applications.

Characterization of a high-brilliance soft x-ray laser at 13.9 nm by use of an oscillator-amplifier configuration.

We have improved a highly coherent x-ray laser at 13.9 nm using an oscillator-amplifier configuration. To improve a high-brilliance x-ray laser, we adopted traveling wave pumping for the amplifier target and rotated the amplifier target 3-4 mrad in the counterclockwise direction. Thereby, a seed x-ray laser can be amplified by medium plasma of the amplifier target with a high gain coefficient. The amplified x-ray laser has the output energy of approximately 1.3 microJ, corresponding to a large photon flux of 6.5 x 10(10) photons/pulse and a high peak brilliance of 5 x 10(26) photons/(s x mm(2) x mrad(2) x 0.01% bandwidth).

Temporal contrast enhancement of petawatt-class laser pulses

We demonstrate the temporal contrast enhancement in a petawatt-class Ti:sapphire chirped-pulse amplification (CPA) laser system. An extra saturable absorber, introduced downstream after a low-gain optical parametric chirped-pulse amplification (OPCPA) preamplifier, has improved the temporal contrast in the system to 1.4×10(12) on the subnanosecond time scale at 70 TW power level. We have achieved 28 J of uncompressed broadband output energy with this system, indicating the potential for reaching peak powers near 600 TW.

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.

Development of a chirped pulse amplification laser with zigzag slab Nd:glass amplifiers dedicated to x-ray laser research

A chirped pulse amplification laser with zigzag slab Nd:glass amplifiers dedicated to x-ray laser research is described. The laser provides a 1.6 ps duration pulse with approximately 7 J energy at a repetition rate of 0.1 Hz. In the power amplifier system, laser light is amplified in a two-step manner: The first step is image-relayed multipass amplification up to approximately 1 J with a 10 mm x 10 mm beam. The second step is double-pass amplification up to >10 J with a 10 mm x 90 mm beam. By using this laser system, the saturated amplification of the Ni-like Ag laser at a wavelength of 13.9 nm has been successfully demonstrated.

Demonstration of SubmicroJoule, Spatially Coherent Soft-X-ray Laser Pumped by 0.1 Hertz, 10 Joule, Picosecond Laser

We have demonstrated a spatially coherent 13.9 nm laser with submicroJoule output energy using a double-target geometry with a new driver system. The new driver was a chirped-pulse amplification laser with zigzag slab Nd:glass power amplifiers providing two beams with an energy of 10 J and picosecond duration at a shot rate of 0.1 Hz. The obtained 13.9 nm laser pulse with 1010 coherent photons will enable us to carry out single-shot pump-probe observation of nonperiodic ultrafast phenomena.

Erratum: “Design of Rectangular Transmission Gratings Fabricated in LiNbO3 for High-Power Terahertz-Wave Generation”

Precise parameters are investigated for rectangular transmission gratings fabricated in a LiNbO3 substrate for high-power terahertz-wave generation. Here, a simple setup is considered, where the pumping laser is injected into the grating surface from air or fused silica. The maximum diffraction efficiencies are 0.4 for air and 0.9 for fused silica. The designed gratings have deep grooves with a depth of about 0.5 µm and a width of 0.12–0.14 µm. It was found that the diffraction efficiency is much less sensitive to the groove depth than to the groove width.

Contact grating device with Fabry-Perot resonator for effective terahertz light generation.

A novel design for a contact grating device with an incorporated Fabry-Perot resonator is proposed for high-power terahertz (THz) light generation. We deposited a multilayer consisting of Ta(2)O(5) and Al(2)O(3) on a magnesium-doped stoichiometric LiNbO(3) substrate and fabricated grating grooves on the outermost layer. The multilayer was designed such that conditions for a Fabry-Perot resonator were satisfied for light diffracted by the grating. Consequently, the fraction of light transmitted into the LiNbO(3) substrate, i.e., the diffraction efficiency, was enhanced by the resonator. The diffraction efficiency of the fabricated device was 71%, which is close to the calculated value of 78% from the optimized design. THz light generation was also demonstrated with the contact grating device. The THz output of 0.41 μJ was obtained using near-infrared pump light of 2.7 mJ.

Yb:YAG thin-disk chirped pulse amplification laser system for intense terahertz pulse generation.

We have developed a 1 kHz repetition picosecond laser system dedicated for intense terahertz (THz) pulse generation. The system comprises a chirped pulse amplification laser equipped with a Yb:YAG thin-disk amplifier. At room temperature, the Yb:YAG thin-disk regenerative amplifier provides pulses having energy of over 10 mJ and spectral bandwidth of 1.2 nm. The pulse duration achieved after passage through a diffraction grating pair compressor was 1.3 ps. By employing this picosecond laser as a pump source, THz pulses having a peak frequency of 0.3 THz and 4 µJ of energy were generated by means of optical rectification in an Mg-doped LiNbO3 crystal.

Improvement of diffraction efficiency of dielectric transmission gratings using anti-reflection coatings.

A novel method for increasing diffraction efficiency of transmission gratings is proposed. In this method, dielectric multilayers are inserted between a grating region and a substrate. These multilayers work as an anti-reflection coating for the transmission grating. It is presented that a grating with 1740 grooves/mm has the diffraction efficiency over 99% using this anti-reflection coating.