The next generation of laser technology demands coatings with much higher laser damage resistance than what’s currently available, suggests Roger Qiu, a physicist in LLNL’s Materials Science Division.
“The existing technology cannot produce highly reflective, multilayer dielectric coatings able to sustain the operational conditions that the next generation of laser systems requires,” Qui says. Coatings are applied to beam-steering mirrors to make them more resilient and more reflective in laser systems like NIF.
To prepare for the next generation, Qiu and his team were tasked with two goals: gaining a fundamental understanding of how lasers can induce damage in multilayer dielectric coatings, and developing reliable ways to make them last under extreme conditions. They knew they would need a system that was tunable, controllable, and flexible in process parameter, environment, and materials selection. Veeco’s commercially available SPECTOR-HT system fit the bill.
“People have spent decades proving that dielectric coatings provide the best spectral performance with the highest damage resistance, but we know there is more opportunity for improvement,” says Qiu. “We know what the ideal performance of these materials is; this machine will help us get there.”
SPECTOR-HT is a high-throughput ion-beam sputtering system that uses an ion source to sputter target materials onto a substrate to make either a metal film, a dielectric film, or multilayers for various optical applications. These thin coatings add reflectivity to beam-steering mirrors and must maintain a high standard of beam propagation and prevent too much laser energy from damaging the optic. Because these new lasers are planned to be significantly more powerful than those currently in use, stronger coatings are a necessity to avoid destroying these optical components.
The key to SPECTOR-HT is its dual-beam configuration, which allows additional in situ tunability. The first beam fires argon atoms at a target that is made of either metal or dielectric materials such as hafnium, tantalum, silica, or alumina. As the beam strikes the surface of the target, it produces a plume of vapor which deposits upon a sheet of glass under an oxygen environment as a thin layer of dielectric coating. When each layer is complete, the target can rotate to enable a different oxide vapor to deposit as the next layer.
Throughout the process, a second beam can help scientists “tune” qualities such as structure, density, deposition rate, and the chemistry of the coatings. The second beam also can be used to eliminate impurities and contamination. To ensure the vapor deposits without air resistance and ambient contamination, the system operates in a vacuum with a base pressure of 10-7 torr.
“It was very important for the Laboratory to acquire a dedicated system that enables the study of coating process science,” Qiu says. “It was the only way to explore different designs and materials with high throughput to solidify our results with statistical significance.”
Using the dielectric oxides at different thicknesses, densities, and structures can have real benefits for the reflectivity and durability of the end product. Each new coating configuration lends clues to the success or failure of past configurations.
Using different target materials, backfill environments, and deposition parameters, SPECTOR-HT gives scientists “the ability to play,” as Qiu puts it.
LLNL has long been a leader in laser damage testing of optical materials. The development of high laser performance coatings at LLNL has relied on the evaluation of optical coatings from private industry, where the throughput and flexibility can be challenging for out-of-the-norm coating designs and configurations.
“With the combined in-house capabilities in state-of-the-art laser testing and coating fabrication,” Qiu says, “we now have the total package.”
The Laboratory’s SPECTOR-HT system was installed in June 2017, funded under institutional funds to support a Laboratory Directed Research and Development (LDRD) strategic initiative aimed at developing advanced multilayer systems for national security. The work being done will be critical to the Laboratory’s missions in stockpile stewardship, Discovery Science, and energy security.
With the customizable capabilities of SPECTOR-HT and the industry-leading laser work going on throughout the Laboratory, the next generation of lasers will continue to progress—for maximum energy delivery as well as for achieving LLNL’s national missions.