Mark Woolston

Three-dimensional nanoscale molecular imaging by extreme ultraviolet laser ablation mass spectrometry

Analytical probes capable of mapping molecular composition at the nanoscale are of critical importance to materials research, biology and medicine. Mass spectral imaging makes it possible to visualize the spatial organization of multiple molecular components at a samples surface. However, it is challenging for mass spectral imaging to map molecular composition in three dimensions (3D) with submicron resolution. Here we describe a mass spectral imaging method that exploits the high 3D localization of absorbed extreme ultraviolet laser light and its fundamentally distinct interaction with matter to determine molecular composition from a volume as small as 50 zl in a single laser shot. Molecular imaging with a lateral resolution of 75 nm and a depth resolution of 20 nm is demonstrated. These results open opportunities to visualize chemical composition and chemical changes in 3D at the nanoscale.

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.

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.

Advances in High Average Power, 100 Hz Repetition Rate Table-Top Soft X-Ray Lasers

We discuss new results of the demonstration of 100 Hz table-top soft x-ray lasers operating at wavelengths from 10.9 nm to 18.9 nm and report the highest soft x-ray laser average power to date at sub-20 nm wavelength. The results include the generation of average powers up to 0.2 mW at λ = 18.9 nm, 0.1 mW at λ = 13.9 nm, and 20 μW at λ = 11.9 nm. These lasers are driven by a compact chirped pulse amplification laser system featuring diode-pumped, cryogenically-cooled Yb:YAG power amplifiers that produces 1 J pulses of 5 ps FWHM duration at 100 Hz repetition rate. The driver laser pulse shape was tailored to more efficiently pump soft x-ray plasma amplifiers operating at sub-15 nm wavelengths leading to a threefold increase in the λ = 13.9 nm laser pulse energy and lasing down to 10.9 nm. The pump pulse profile consisting of a nanosecond ramp followed by two closely spaced peaks of picosecond duration, is shown to create a plasma with an increased density of Ni-like lasing ions at the time of peak temperature, resulting in a larger gain coefficient over a temporally and spatially enlarged space. The development of rotating solid targets with high shot capacity has allowed the uninterrupted operation of the λ = 18.9 nm soft x-ray laser for hundreds of thousands of consecutive shots making it suitable for applications in nanoscience and nanotechnology that require high photon flux at short wavelengths. As a proof-of-principle demonstration of the utility of this laser in such applications we have lithographically printed an array of nanometer-scale features through coherent Talbot self imaging. These results open the path to milliwatt average power table-top soft x-ray lasers.

Volumetric Composition Imaging at the Nanoscale by Soft X-Ray Laser Ablation Mass Spectrometry

Single shot soft x-ray laser ablation in combination with mass spectrometry is shown to make it possible to detect intact molecular ions from organic and inorganic materials from atto-liter volumes with high sensitivity. The technique is demonstrated to record 3D composition images of heterogeneous samples.

Soft x-ray laser ablation mass spectrometry for materials study and nanoscale chemical imaging

There are significant advantages for using a compact capillary discharge soft x-ray laser (SXRL) with wavelength of 46.9 nm for mass spectrometry applications. The 26.4 eV energy photons provide efficient single-photon ionization while preserving the structure of molecules and clusters. The tens of nanometers absorption depth of the radiation coupled with the focusing of the laser beam to diameter of ∼100 nm result in the ablation of atto-liter scale craters which in turn enable high resolution mass spectral imaging of solid samples. In this paper we describe results on the analysis of composition depth-profiling of multilayer oxide stack and material studies in photoresists, ionic crystals, and magnesium corrosion products using SXRL ablation mass spectrometry, a method first demonstrated by our group. These materials are used in a variety of soft x-ray applications such as detectors, multilayer optics, and many more.

Demonstration of a 100 Hz Repetition Rate Soft X-Ray Laser and Gain-Saturated Sub-10 nm Table-Top Lasers

We report the first operation of a table-top soft x-ray laser at 100 Hz repetition rate. This gain saturated laser produces 0.15 mW average power in the 18.9 nm line of nickel-like molybdenum in the form of 1.5 μJ pulses. This is the highest average power reported to date from a compact coherent soft x-ray laser source operating at wavelengths shorter than 20 nm. The soft x-ray laser is excited by a diode-pumped chirped pulse amplification Yb:YAG laser that produces 1 J pulses of 5 ps duration. We have also demonstrated the efficient generation of sub-9 nm wavelength laser pulses of microjoule energy at 1 Hz repetition rate with a table-top laser. Gain-saturated lasing was obtained at 8.85 nm in nickel-like lanthanum ions. Isoelectronic scaling along the lanthanide series resulted in lasing at wavelengths as short as 7.36 nm. Simulations show that the collisionally broadened atomic transitions in these dense plasmas can support the amplification of sub-picosecond soft x-ray laser pulses.

Powerful high-repetition-rate tabletop soft x-ray lasers

The generation of high-average-power, coherent, soft x-ray and extreme ultraviolet (EUV) radiation is of great interest for numerous applications, such as sequential single-shot imaging of dynamic nano-scale phenomena, inspection of next-generation EUV lithographic masks at their specific wavelength, and printing of nanostructures without defects. The development of tabletop sources of soft x-ray radiation with increased average power can greatly expand access to high flux, coherent beams of this type of radiation, which at present are only available at dedicated laser facilities. However, achieving the necessary performance has remained a challenge. Plasma-based laser-pumped soft x-ray lasers, in which spontaneous emission is amplified in a highly ionized plasma column, typically have low repetition rates (of a few shots per day to one shot per minute in the first devices), which limits their average power. Several research groups made significant progress in this area, greatly reducing the size of this type of laser1–5 and extending the wavelength ( ) to below 10nm.6 However, the repetition rates of these devices are below ̃10Hz, resulting in average powers that are insufficient for applications requiring high photon flux (such as printing nano-scale features by self-imaging coherently illuminated masks).7 This limitation originates in thermal effects within the flash-lamp-pumped solid-state lasers that drive plasma-based soft x-ray lasers. To address this issue, we developed a compact, ultrashort solid-state laser system driven entirely by laser diodes that produces picosecond duration, D 1:03 m pulses of sufficient energy to efficiently drive D 10–20nm lasers at much higher repetition rates. Our device is the first tabletop soft x-ray laser with a 100Hz repetition rate, and it produces an average power of 0.15mW at D 18:9nm.8 In our experiments, laser diodes Figure 1. Concept of diode-pumped, high-repetition-rate soft x-ray lasers. Laser diodes efficiently pump a ytterbium-doped yttriumaluminum-garnet chirped-pulse-amplification laser that produces picosecond pulses of 1J energy at 100Hz repetition rate. The laser pulses are focused to form a narrow line on a solid target, where they rapidly heat the material creating a plasma with population inversion and gain in highly ionized ions. Amplified spontaneous emission in the narrow plasma column creates intense saturated soft x-ray laser pulses of picosecond duration.

High-average-power 100-Hz repetition rate table-top soft x-ray lasers

The table-top generation of high average power coherent soft x-ray radiation in a compact set up is of high interest for numerous applications. We have demonstrated the generation of bright soft x-ray laser pulses at 100 Hz repetition rate with record-high average power from compact plasma amplifiers excited by an ultrafast diode-pumped solid state laser. Results of compact λ=18.9nm Ni-like Mo and λ=13.9nm Ni-like Ag lasers operating at 100 Hz repetition rate are discussed.