Herman Bravo

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.

Warm photoionized plasmas created by soft-x-ray laser irradiation of solid targets

We report the first experimental study of plasmas created by photoionization of solid targets with focused soft X-ray laser light. The plasma properties are dominantly determined by material absorption, in agreement with model simulations.

Demonstration of Nanomachining With Focused Extreme Ultraviolet Laser Beams

A major challenge in laser machining of microstructures is that of extending the spatial domain to the smaller dimensions of interest in nanotechnology. We demonstrate the feasibility of directly machining nanoscale structures with a focused extreme ultraviolet (EUV) laser beam. Clean sub-200-nm-wide trenches (130-nm full width at half maximum) were ablated on polymethyl methacrylate photoresist by focusing the 46.9-nm wavelength beam from a Ne-like Ar capillary discharge tabletop laser with a Fresnel zone plate lens. Considering that clean 82-nm holes were also ablated using the same laser, it can be expected that focused EUV laser light will enable the machining of significantly smaller features.

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.

Ablation and transmission of thin solid targets irradiated by intense extreme ultraviolet laser radiation

The interaction of an extreme ultraviolet (EUV) laser beam with a parylene foil was studied by experiments and simulation. A single EUV laser pulse of nanosecond duration focused to an intensity of 3 × 1010 W cm−2 perforated micrometer thick targets. The same laser pulse was simultaneously used to diagnose the interaction by a transmission measurement. A combination of 2-dimensional radiation-hydrodynamic and diffraction calculations was used to model the ablation, leading to good agreement with experiment. This theoretical approach allows predictive modelling of the interaction with matter of intense EUV beams over a broad range of parameters.

Nanopatterning and Nanomachining with Table-top Extreme Ultraviolet Lasers

Nanopatterning and nanomachining of PMMA coated wafers was performed using table top extreme ultraviolet lasers. Features below 100 nm were imprinted with short (50-60 s) exposure times.

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.

Photoionized plasmas created by soft x-ray laser irradiation of solid targets

We report the first experimental study of plasmas created by photoionization of solid targets with focused soft X-ray laser light. The plasma properties are dominantly determined by material absorption, in agreement with model simulations.

Nanoscale Resolution Microscopy and Ablation with Extreme Ultraviolet Lasers

We obtain a spatial resolution down to 38 nm with full field imaging and laser-ablation systems that exploit the short wavelength and high brightness output from compact extreme ultraviolet lasers in combination with zone plate optics.