Cory Baumgarten

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.

Hour-long continuous operation of a tabletop soft x-ray laser at 50-100 Hz repetition rate.

We report the uninterrupted operation of an 18.9 nm wavelength tabletop soft x-ray laser at 100 Hz repetition rate for extended periods of time. An average power of about 0.1 mW was obtained by irradiating a Mo target with pulses from a compact diode-pumped chirped pulse amplification Yb:YAG laser. Series of up to 1.8 x 10(5) consecutive laser pulses of ~1 µJ energy were generated by displacing the surface of a high shot-capacity rotating molybdenum target by ~2 µm between laser shots. As a proof-of-principle demonstration of the use of this compact ultrashort wavelength laser in applications requiring a high average power coherent beam, we lithographically printed an array of nanometer-scale features using coherent Talbot self-imaging.

1 Joule, 100 Hz Repetition Rate, Picosecond CPA Laser for Driving High Average Power Soft X-Ray Lasers

A diode-pumped cryogenic Yb:YAG CPA laser that produces 1J, 5ps pulses allowed for the first time the uninterrupted generation of 1.8×105 sub-20nm wavelength laser pulses with microjoule energy at 100Hz repetition rate on a table-top.

Improvements in the laser damage behavior of Ta2O5/SiO2 interference coatings by modification of the top layer design

We have investigated the properties and laser damage behavior of Ta2O5/SiO2 quarter wave stacks designed for λ=1 μm operation by substituting the Ta2O5 layer by either Y2O3 or HfO2 and the SiO2 by Al2O3 in the top 3 pairs of the multilayer stack. The high reflectors were deposited by dual ion beam sputtering. Laser damage at 1 μm using 350 ps showed enhanced performance when the Ta2O5/SiO2 stack had HfO2 or Y2O3 in its top few pairs.

Modification of multilayer mirror top-layer design for increased laser damage resistance

This work focuses on the optimization of a high reflector design for operation at 1 μm wavelength to achieve a high laser damage threshold when tested at pulse durations of 0.19 and 4 ns. Different designs that modify the standing wave electric field distribution of a quarter wave Ta2O5/SiO2 multilayer dielectric coating are considered. It is found that the addition of an extra SiO2 quarter wave to reduce the peak electric field in the coating, increases the 50% damage probability by over 100% at both pulse durations.

Strategies to increase laser damage performance of Ta_2O_5/SiO_2 mirrors by modifications of the top layer design

Ta₂O₅/SiO₂ high reflection (HR) interference coatings for λ∼1  μm offer superior performance at high irradiance conditions. However, these coatings are not good candidates for high peak power conditions in comparison to HfO₂/SiO₂ multilayer stacks. Here we show that the modification of the top layers design of a quarter wave Ta₂O₅/SiO₂ high reflector leads to 4-5 fold increase in the laser damage fluence compared to a quarter wave (Ta₂O₅/SiO₂)¹⁵ when tested at λ=1.03  μm using pulse durations of 0.19 and 4 ns and peak power densities of 43.5 and 216  GW/cm². One of the designs achieved a laser damage threshold fluence of 174  J/cm² at 4 ns, which is 10% higher than that of a HfO₂/SiO₂ quarter wave design.

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.