High-performance Dichroic Filters for Large-Scale Neutrino Detector

The dichroic filter is a key optical component in water- and scintillator-based neutrino detectors for high energy physics. Future detectors such as THEIA and Dune in Brookhaven National Lab (BNL) will require large-area (tens of cm2) dichroic filters at a low cost and with improved transmissive and reflective properties (both >95%) for wavelengths 300-600 nm. Dichroic filters in precise optics are traditionally manufactured by various physical vapor deposition (PVD) techniques such as eBeam, sputtering, thermal evaporators, Pulsed lased deposition, etc. with ion assistance and enhancement. However these traditional PVD techniques are subject to intrinsic drawbacks of poor large uniformity, difficulty in precise thickness control and high cost associated with vacuum technology and thickness monitoring tools. Raytum Photonics propose a state-of-the-art coating technology, atomic layer deposition (ALD) to be used to make high performance dichroic filter devices with significant cost effectiveness and excellent large area uniformity. ALD is one of rapidly developing coating technology in the past 10-15 years and has been proved very successful in semiconductor industry and is very active in optical coating R&D. Compared to many PVD techniques, ALD is well known for precise, reproducible thickness control, excellent large area uniformity and conformity, and low cost and easy to scale –up. It is thus well suitable for manufacturing multiple dielectric layer coatings for achieving precise wavelength positioning and steepness of wide bandpass dichroic filter devices. In addition, ALD is able to deposit a wide range of high quality dielectric materials (oxides, nitride fluorides, carbides, etc.) with a large range of options for high, medium and low refractive indexes. Other advantages of ALD include defect free, and low thermal stress because of low growth temperature. In recent years, there have been some breakthroughs in high volume production ALD tool in industry, especially with innovation of special ALD, and roll to roll ALD to overcome some shortcomings of traditional temporal ALD like low growth rate and low volumes. Production ALD tools for handing large area up to meter size, and multiple wafers up to hundreds per hour were demonstrated commercially. During the Phase I, we are going to concept prove if ALD coating materials are suitable for high performance dichroic filter devices in term of optical quality, multiple layer stress control and reproducibility in a large area substrate. Dichroic filter reflectivity will be designed using commercial design tools such as TFcal and materials selections will based on most popular filter materials used. During Phase II a suitable ALD production tool for such dichroic filter will be identified and further developed and a prototype of dichroic filter devices will be made and evaluated for cost effectives. Raytum Photonics will collaborate with researchers in Brookhaven National Lab to test the dichroic filter in liquid Argon detector.