Petawatt-class High-Energy Laser Coatings

Coating prevents or avoids such damage to laser systems’ optical materials and coatings and at the same time, delivers higher energies to better transform a target into a high-energy-density plasma quickly emerged as a competing factor in the development of high-energy lasers.

FREMONT, CA: The ability of lasers to deliver ever-higher energy to a target–to attain electric-field strengths greater than those binding electrons and nuclei–was one of the main directions of growth that emerged very shortly following the invention of lasers in the 1960s. In the focal volume of the laser, it was intended to conduct controlled research on high-energy-density plasmas. Such plasmas might be created and investigated using this method without the use of unrestrained above-ground or underground nuclear explosions. The Z-Backlighter petawatt laser's 75-cm forward-optical assembly steering mirror, immediately following the coating run for its laser-damage-resistant optical coating made up of HfO2/SiO2 layer pairs.

Ironically, these efforts swiftly came to an end because optics and optical coatings required to direct and concentrate high-energy laser beams on a target were being damaged by lasers. The need for higher energies to more effectively convert a target into a high-energy-density plasma while also minimising or avoiding damage to optical materials and laser system coatings has quickly emerged as a competing force in the development of high-energy lasers. Since then, high-energy laser research and applications have always included the tension between those two elements.

It is a tension that is both frustrating and exhilarating—annoying when inadequate energy reaches a target or when high-intensity laser radiation in a beam train damages an optic, and exciting when everything functions without such harm. It examines how the field of optical coatings with a high laser-induced damage threshold (LIDT) has developed to support the creation of laser systems that are pushing the boundaries of high-energy physics—and even the potential realisation of inertial confinement fusion (ICF) as a potentially significant energy source.

Due to their little optical absorption, very transparent optical coating layer materials display the highest LIDTs. The best of these materials are metal oxides, which have great transparency due to their wide band gaps. However, a more thorough understanding necessitates a quick review of the ways that lasers can harm optical components and coatings.

Extrinsic and intrinsic forms of damage mechanisms caused by lasers can be distinguished. Each type uses a different method and a different time scale to convert optical light into a coating on the substrate's molecular structure. This results in either catastrophic structural damage or a structural change like a melt, scald, or blister. If the area is exposed to more laser pulses, the damage may or may not continue to spread. However, all damage is irreversible and just serves to further scatter or absorb laser energy. Additionally, the optical performance requirements of the system in a specific high-energy laser application determine the density and severity of damage sites that can be tolerated before an optic in a beam train needs to be replaced.

Extrinsic damage occurs when an otherwise high-LIDT material experiences optical absorption by opaque nanoscale and microscale imperfections, such as impurities, particles, or microstructural faults. These defects—which are common and difficult to prevent or eliminate in optical coating and processing environments—include microstructural flaws within layers or at their interfaces; subsurface microfractures; substrate surface scratches or digs; contamination by trace levels of polishing compounds; and particulates present as a result of improperly enforced cleanroom and optics-handling and cleaning protocols.

Extrinsic damage happens when optical energy that is absorbed in such defect sites combines into phonon excitations through heat-transfer mechanisms, which ultimately results in the irreversible change or catastrophic destruction of the material's structure. Long nanoseconds and longer laser pulse durations are necessary for the optical absorption and heat transfer processes to take place. LIDT of an optical coating must be optimised by reducing extrinsic flaws.

Intense laser electric fields are directly coupled with the molecular electronic structure of the optical coating causing intrinsic damage, which releases free electrons by multiphoton ionisation or excitations into electronic conduction bands. Collisions between the free electrons and the atoms in the material structure can convert the energy into heat and phonon excitations. The material is later damaged in bulk as a result of heat-transfer operations.

In the context of laser-induced damage, relevant pulse lengths sub-picosecond to femtosecond are characterised as short pulses because photon-electron interaction timescales about 10-13 s to 10-15 s correspond to those of electronic mobility and transitions in molecules. However, ensuing heat-transfer processes that result in bulk damage take place on nanosecond and longer time scales, just like with extrinsic damage. It has long been known that an optic suffers laser-induced damage as soon as its coated surface is exposed to even 1J of laser energy across a 1 cm² area.

Intrinsic damage is largely dependent on molecular-level electrical structural flaws that interact significantly with high-energy laser electric fields. These flaws are also commonplace, such as metal impurities that easily provide free electrons to conduction bands or intraband electronic states of high-band-gap coating molecules linked to impurities or molecular gaps that can develop during coating deposition. However, because the multiphoton excitations of intrinsic damage may cross the wide electronic band gaps of transparent materials, they also pose a threat to defect-free regions of very transparent thin-film materials.

Nevertheless, high-transparency coatings' defect sites are more likely than their defect-free counterparts to produce free electrons as a result of photon-electron interactions. Therefore, using ultra-high-purity coating-layer materials is necessary to reduce intrinsic damage, particularly concerning iron and other metallic conductive impurities. Additionally, for the production of stoichiometrically accurate layers with fewer intra-band-defect electronic states for metal-oxide thin-film layers, appropriate oxygen enrichment in reactive coating deposition is crucial.