Contamination-Resistant Sol-Gel TechnologyNovember, 2008
What started in 2004 as a Summer High School Program research project focusing on the development of contaminant-resistant sol-gel coatings for laser optics, has now evolved into a significant piece of technology that has been installed on the 30-TW, 60-beam OMEGA laser at the University of Rochester’s Laboratory for Laser Energetics (LLE).
Laser optics for the OMEGA Laser System have previously used silica-based sol-gels as antireflection (AR) coatings to improve their transmittance. Water vapor and volatile organic compounds in the laser and target bays eventually leads to the contamination of these coatings. Such chemical contamination reduces the optical throughput and increases the potential for laser damage to these costly optics. As a result, these coatings must be removed, cleaned, and re-coated with new sol-gel every three months, on average, to ensure their efficiency and to reduce the potential for laser damage.
Working together, scientists and students participating in the Summer High School Program at LLE have reached their goal of developing contamination-resistant sol-gel AR coating formulations. The new coatings use dimethyldiethoxysilane (DDS) and methyltriethoxysilane (MTES) to modify standard sol-gel solutions based on tetraethylorthosilicate (TEOS). Test conditions conservatively included exposure to a much more highly saturated contaminant environment than would ever be encountered in use on OMEGA.
In this process, TEOS sol-gel solutions containing 10%, 20%, or 30% of either MTES or DDS, were co-hydrolyzed, aged for three days at room temperature, and then spin-coated onto clean substrates at 4800 rpm for 20 seconds. A control sample of the standard TEOS sol-gel solution was spin-coated on an identical substrate at 4000 rpm for 40 seconds. After drying in air for 24 hours, a spectrophotometer was used to measure the transmittance of each sol-gel. The transmittance values of all samples were measured again after five days exposure to laboratory air. Each of the samples was exposed to a number of organic liquids in the vapor state that might be found in trace quantities in the laser or target bay areas. The exposures were conducted in sealed containers at room temperature for 24 hours with sufficient quantities of the test fluid present to produce a fully saturated vapor-phase environment of the contaminant. Transmittance spectra of the coatings were taken both before and after exposure to monitor the change in optical characteristics of the coatings as a function of both the coating composition and the type of contaminant. One-on-one laser-damage testing at 351 nm (1-ps pulse width) was conducted on a separate set of cohydrolyzed 30% DDS and 30% MTES sol-gels deposited onto 5-cm fused-silica substrates in the LLE Optical Manufacturing (OMAN) Facility by dip-coating. A third substrate coated with the standard TEOS-based sol-gel solution was also tested as a control.
The new coatings can now be applied to OMEGA optics with only minimal changes to the current production process. The results of this research, involving co-hydrolysis of TEOS-based sol-gels with select organosiloxanes, include the following advantages:
- The original sol-gel deposition process requires minimal alterations
- Resistance to specific contaiminants can be fine-tuned using a variety of organosilanes
- The high laser-damage threshold of sol-gel AR coatings formerly deployed on OMEGA is maintained or exceeded.
In addition to reducing maintenance and replacement costs in high-peak-power lasers, the favorable results promise to expand the use of sol-gel-derived products for optical applications related to electronics and photonics, energy, space, (bio) sensors, medicine (e.g., controlled drug release), and separation (e.g., chromatography) technology.