Patrick Poole

Femtosecond laser damage threshold of pulse compression gratings for petawatt scale laser systems

Laser-induced femtosecond damage thresholds of Au and Ag coated pulse compression gratings were measured using 800 nm laser pulses ranging in duration from 30 to 200 fs. These gratings differ from conventional metal-on-photoresist pulse compression gratings in that the gratings patterns are generated by etching the fused silica substrate directly. After etching, the metal overcoating was optimized based on diffraction efficiency and damage threshold considerations. The experiment on these gratings was performed under vacuum for single-shot damage. Single-shot damage threshold, where there is a 0% probability of damage, was determined to be within a 400-800 mJ/cm(2) range. The damage threshold exhibited no clear dependence on pulse width, but showed clear dependence on gold overcoat surface morphology. This was confirmed by electromagnetic field modeling using the finite element method, which showed that non-conformal coating morphology gives rise to significant local field enhancement near groove edges, lowering the diffraction efficiency and increasing Joule heating. Large-scale gratings with conformal coating have been installed successfully in the 500 TW Scarlet laser system.

Microengineering Laser Plasma Interactions at Relativistic Intensities

We report on the first successful proof-of-principle experiment to manipulate laser-matter interactions on microscales using highly ordered Si microwire arrays. The interaction of a high-contrast short-pulse laser with a flat target via periodic Si microwires yields a substantial enhancement in both the total and cutoff energies of the produced electron beam. The self-generated electric and magnetic fields behave as an electromagnetic lens that confines and guides electrons between the microwires as they acquire relativistic energies via direct laser acceleration.

Liquid crystal films as on-demand, variable thickness (50–5000 nm) targets for intense lasers

We have developed a new type of target for intense laser-matter experiments that offers significant advantages over those currently in use. The targets consist of a liquid crystal film freely suspended within a metal frame. They can be formed rapidly on-demand with thicknesses ranging from nanometers to micrometers, where the particular value is determined by the liquid crystal temperature and initial volume as well as by the frame geometry. The liquid crystal used for this work, 8CB (4′-octyl-4-cyanobiphenyl), has a vapor pressure below 10−6 Torr, so films made at atmospheric pressure maintain their initial thickness after pumping to high vacuum. Additionally, the volume per film is such that each target costs significantly less than one cent to produce. The mechanism of film formation and relevant physics of liquid crystals are described, as well as ion acceleration data from the first shots on liquid crystal film targets at the Ohio State University Scarlet laser facility.

Experiment and simulation of novel liquid crystal plasma mirrors for high contrast, intense laser pulses.

We describe the first demonstration of plasma mirrors made using freely suspended, ultra-thin films formed dynamically and in-situ. We also present novel particle-in-cell simulations that for the first time incorporate multiphoton ionization and dielectric models that are necessary for describing plasma mirrors. Dielectric plasma mirrors are a crucial component for high intensity laser applications such as ion acceleration and solid target high harmonic generation because they greatly improve pulse contrast. We use the liquid crystal 8CB and introduce an innovative dynamic film formation device that can tune the film thickness so that it acts as its own antireflection coating. Films can be formed at a prolonged, high repetition rate without the need for subsequent realignment. High intensity reflectance above 75% and low-field reflectance below 0.2% are demonstrated, as well as initial ion acceleration experimental results that demonstrate increased ion energy and yield on shots cleaned with these plasma mirrors.

Moderate repetition rate ultra-intense laser targets and optics using variable thickness liquid crystal films

Liquid crystal films are variable thickness, planar targets for ultra-intense laser matter experiments such as ion acceleration. Their target qualities also make them ideal for high-power laser optics such as plasma mirrors and waveplates. By controlling parameters of film formation, thickness can be varied on-demand from 10 nm to above 50 μm, enabling real-time optimization of laser interactions. Presented here are results using a device that draws films from a bulk liquid crystal source volume with any thickness in the aforementioned range. Films form within 2 μm of the same location each time, well within the Rayleigh range of even tight F/# systems, thus removing the necessity for realignment between shots. The repetition rate of the device exceeds 0.1 Hz for sub-100 nm films, facilitating higher repetition rate operation of modern laser facilities.

Experimental capabilities of 04 PW, 1 shot/min Scarlet laser facility for high energy density science

We report on the recently completed 400 TW upgrade to the Scarlet laser at The Ohio State University. Scarlet is a Ti:sapphire-based ultrashort pulse system that delivers >10  J in 30 fs pulses to a 2 μm full width at half-maximum focal spot, resulting in intensities exceeding 5×1021  W/cm2. The laser fires at a repetition rate of once per minute and is equipped with a suite of on-demand and on-shot diagnostics detailed here, allowing for rapid collection of experimental statistics. As part of the upgrade, the entire laser system has been redesigned to facilitate consistent, characterized high intensity data collection at high repetition rates. The design and functionality of the laser and target chambers are described along with initial data from commissioning experimental shots.

A confocal microscope position sensor for micron-scale target alignment in ultra-intense laser-matter experiments

A diagnostic tool for precise alignment of targets in laser-matter interactions based on confocal microscopy is presented. This device permits precision alignment of targets within the Rayleigh range of tight focusing geometries for a wide variety of target surface morphologies. This confocal high-intensity positioner achieves micron-scale target alignment by selectively accepting light reflected from a narrow range of target focal planes. Additionally, the design of the device is such that its footprint and sensitivity can be tuned for the desired chamber and experiment. The device has been demonstrated to position targets repeatably within the Rayleigh range of the Scarlet laser system at The Ohio State University, where use of the device has provided a marked increase in ion yield and maximum energy.

Damage testing of critical optical components for high power ultra-fast lasers

Mirrors and gratings used in high power ultra fast lasers require a broad bandwidth and high damage fluence, which is essential to the design and construction of petawatt class short pulse lasers. Damage fluence of several commercially available high energy broad band dielectric mirrors with over 100 nm bandwidth at 45 degree angle of incidence, and pulse compression reflection gratings with gold coating with varying processing conditions is studied using a 25 femtosecond ultra-fast laser.

Liquid Crystal Targets and Plasma Mirrors For Laser Based Ion Acceleration

Practical application of laser based ion acceleration will require advances across a wide range of technologies extending from the laser system itself to the delivery of the ion beam. We have recently shown that the liquid crystal 8CB provides an effective and relatively inexpensive new approach to target and plasma mirror fabrication and insertion for ion acceleration. 8CB is primarily hydrogen and carbon and forms in layers approximately 3 nm thick in its smectic phase. Taking advantage of these properties, we have developed a device we call the Linear Slide Target Inserter (LSTI) that can form films in situ from under 10 nm in thickness to over 50 μm. We describe this new technology and its operation as a target inserter and as a high-power plasma mirror. For proton acceleration, the LSTI readily achieves energies of 25 MeV using pulses of only a few joules by tuning the target thickness for the specific laser pulse characteristics and pre-pulse contrast. For plasma mirrors, we have demonstrated a weak field reflectivity below 0.2% and a high field reflectivity above 75%, yielding a potential pulse contrast improvement over two orders of magnitude. The LSTI can form films at a rate of several per minute for the thinnest films and we have developed a prototype based on a rotary geometry that has demonstrated a film formation rate up to 3 Hz for ultrathin films (approximatly 10 nm). We also propose the use of high repetition rate liquid crystal based plasma mirrors for debris mitigation. Taken together, these ideas and results suggest that liquid crystal technology could play a key role in the development of robust, high repetition rate, laser based ion sources.

Pulse Width dependent Damage testing of critical components in Vacuum for Petawatt class short pulse lasers

Vacuum damage testing of novel pulse compression gratings and mirrors have been damage tested with a 30 fs 200 fs, 800 nm laser, and found that damage threshold increases weakly as pulse duration shortens.