Hanchen Wang

1 J, 05 kHz repetition rate picosecond laser

We report the demonstration of a diode-pumped chirped pulse amplification Yb:YAG laser that produces λ=1.03  μm pulses of up to 1.5 J energy compressible to sub-5 ps duration at a repetition rate of 500 Hz (750 W average power). Amplification to high energy takes place in cryogenically cooled Yb:YAG active mirrors designed for kilowatt average power laser operation. This compact laser system will enable new advances in high-average-power ultrashort-pulse lasers and high-repetition-rate tabletop soft x-ray lasers. As a first application, the laser was used to pump a 400 Hz λ=18.9  nm laser.

Study of Gd/Tb LPP emission near λ = 6.7nm for beyond EUV lithography

Plasmas emitting near λ=6.7nm are of interest for beyond EUV lithography (BEUVL). We have conducted a comprehensive study of the spectral characteristics, conversion efficiency, and source size of Gd and Tb laser-produced plasmas over a broad range of laser pulsewidths (120ps-4ns) and irradiation intensities (1.4×1011-6.1×1013W/cm2). The data over the entire parameter range was acquired using a single diode-pumped Yb: YAG laser. The angular distribution of the BEUV emission was measured using an array of calibrated energy-monitors, allowing for an accurate estimate of the BEUV yield. A similar conversion efficiency of 0.47% into a 0.6% bandwidth in 2π solid angle was measured for both Gd and Tb plasma.

In situ 3-D temperature mapping of high average power cryogenic laser amplifiers

We demonstrate an accurate, in situ, noninvasive optical technique to generate 2-D and 3-D maps of the temperature profile within cryogenic amplifiers operating at high average power.

Progress in high repetition rate soft x-ray laser development and pump lasers at Colorado State University

We will review recent progress in the development of high repetition, high average power rate soft x-ray lasers at 10-20 nm wavelength at Colorado State University, and the compact diode- pumped solid state lasers that drive them. The latter includes the development of a 1 J picosecond laser capable of operating at 500 Hz repetition rate. Results that demonstrate soft x-ray laser operation at the highest repetition rate reported to date: 400 Hz, and prospects of the use of these lasers in applications are discussed.

Demonstration of a 1 Joule, 500 W average power picosecond laser

We report the demonstration of a chirped pulse amplification laser system that produces 1.5 J pulses at 0.5 kHz repetition rate and 0.75 kW average power. These pulses are subsequently compressed resulting 1 J, ~5 ps duration pulses at 500 Hz repetition rate. The 8-pass main amplifier consists of two diode-pumped, cryogenic-temperature Yb:YAG active mirrors cooled by a thermally efficient, high capacity cryogenic-cooling system. This amplifier operates with an opticalto- optical efficiency of 37%. The amplified pulses have excellent beam quality with a measured M2 factor of ~ 1.3. Over 30 minutes of continuous operation, we measured a shot-to-shot pulse energy fluctuation of only 0.75% RMS over the nearly 1 million shots fired. This laser was employed to make the first demonstration of a compact, plasma-based EUV/soft x-ray laser operating at a repletion rate of 400 Hz. In this proof-of-principle demonstration, shaped 1 J pulses of picosecond duration were focused onto a rotating molybdenum target at grazing incidence. The resulting plasma is collisionally ionized to the Ni-like ionic stage where a large, transient population inversion results in production of bright λ = 18.9 nm laser pulses.

Development of a kilowatt-class, joule-level ultrafast laser for driving compact high average power coherent EUV/soft x-ray sources

Our recent progress in the development of high energy / high average power, chirped pulse amplification laser systems based on diode-pumped, cryogenically-cooled Yb:YAG amplifiers is discussed, including the demonstration of a laser that produces 1 Joule, sub-10 picosecond duration, λ = 1.03μm pulses at 500 Hz repetition rate. This compact, all-diodepumped laser combines a mode-locked Yb:KYW oscillator and a water-cooled Yb:YAG preamplifer with two cryogenic power amplification stages to produce 1.5 Joule pulses with high beam quality which are subsequently compressed. This laser system occupies an optical table area of less than 1.5x3m2. This laser was employed to pump plasma-based soft x-ray lasers at λ = 10-20nm at repetition rates ≥100 Hz. To accomplish this, temporally-shaped pulses were focused at grazing incidence into a high aspect ratio line focus using cylindrical optics on a high shot capacity rotating metal target. This results in an elongated plasma amplifier that produces microjoule pulses at several narrow-linewidth EUV wavelengths between λ = 109Å and 189Å. The resulting fraction of a milliwatt average powers are the highest reported to date for a compact, coherent source operating at these wavelengths, to the best of our knowledge.

Impacts of SiO2 planarization on optical thin film properties and laser damage resistance

Lawrence Livermore National Laboratory (LLNL) and Colorado State University (CSU) have co-developed a planarization process to smooth nodular defects. This process consists of individually depositing then etching tens of nanometers of SiO2 with a ratio of 2:1, respectively. Previous work shows incorporating the angular dependent ion surface etching and unidirectional deposition reduces substrate defect cross-sectional area by 90%. This work investigates the micro-structural and optical modifications of planarized SiO2 films deposited by ion beam sputtering (IBS). It is shown the planarized SiO2 thin films have ~3x increase in absorption and ~18% reduction in thin film stress as compared to control (as deposited) SiO2. Planarized SiO2 films exhibit ~13% increase in RMS surface roughness with respect to the control and super polished fused silica substrates. Laser-induced damage threshold (LIDT) results indicate the planarization process has no effect on the onset fluence but alters the shape of the probability vs fluence trace.