High-performance Dichroic Filters for Large-Scale Neutrino Detector

The high-performance dichroic filter is the base line and a key optical component not only for photon wavelength sorting devices for Cherenkov and scintillation lights, but also for scintillation light trapping devices in water, liquid Argon (LAr) scintillator-based neutrino detectors. Future mega detectors for THEIA and DUNE programs will require significant amount of large-area dichroic filters at a low cost and with improved transmissive and reflective properties. Dichroic filters are traditionally manufactured by physical vapor deposition (PVD). However, they all have been subject to intrinsic drawbacks of poor large area uniformity, poor thickness control and high cost associated with vacuum technology and thickness monitoring tools. Atomic layer deposition (ALD) on the other hand has been established as a unique coating tool for precise thickness control, excellent large area uniformity and super conformity for coatings even on curved surface. Therefore, it is well suited for manufacturing larger area, low-cost dichroic filters with comparative and even higher performance compared to traditional coating methods. During the Phase I effort, we achieved the following major accomplishments 1. We successfully designed and fabricated by ALD a short-pass dichroic filters on larger area optical glasses (4"?4") with uniformity < 1%, transmission ~85% and cut off ? of 520nm. 2. We demonstrated high optical qualify ALD coatings on larger curved glass tiles designed and fabricated for Winston cone. 3. We also demonstrate High quality ALD coatings on EJ-280 polymer using low temperature (50°C) ALD process. This work demonstrates dichroic filter coating can be directly deposited on temperature sensitive polymers such as WLS slabs with good compatibility.The goal of this SBIR Phase II program is to develop a processing and manufacturing technique which not only meets the require performance of future mega experiment like DUNE or THIEA but also significantly improve the yield of high-performance optical dichroic filters, thus reduce unit cost. A prototype batch ALD reactor using VEECO S200 as the base and be customized to manufacture such filters with large area of more than 4"?4" using a batch reactor capable of handling a minimal volume of 10-20 unit in a prototype batch process in order to reduce unit cost <$100 with a high yield. The optical dichroic filter must meet specs: transmission and reflectance >95% at cut off/or cut on wavelength of 460nm-500nm, ±1.72° acceptance angle. The same batch reactor can be modified to accommodate a whole Winston cone and we will demonstrate the dichroic filter coating over whole Winston cone instead of assembling 6 pieces in Phase I.