Photonic Crystal Structures for Transformational Gain in Scintillator Performance B. Singh (P. I.), M. S. J. Marshall, and V. V. Nagarkar, RMD, Inc. S. Li and G. Barbastathis, MIT M. Salomoni and P. Lecoq, CERN Phase III QPR DNDO BAA Contract # HSHQDC-13-C-B0040 DNDO Program Manager: Dr. Alan Janos Period of Performance: 9/30/2013 to 6/29/2016 April 4th, 2016

Outline Executive Overview • Conceptual Overview • Capabilities; Performance targets; Goals • Development Approach (technical tasks, teams, schedule) • Key Challenges / Issues to Address • Risks; Mitigation • Accomplishments & Highlights • Milestones: List all and progress towards each • Remaining Issues • Schedule: Gantt chart with milestones & deliverables indicated • Cost (planned, actual, commitments) Detailed Technical Discussions

Executive Overview

Conceptual Overview Effect of refractive index mismatch at interface From Snell’s law Extracted Light (θ < θc) nair = 1 Larger RI mismatch  Smaller c Smaller RI mismatch  Larger c Reflected Light (θ > θc) nGYGAG = 1.82 θc Isotropic Light Emission Critical angle for scintillator with RI=1.82 coupled to RI=1.5: 55 A large fraction of light hits the air/scintillator interface with angle > c Due to total internal reflection, all light which hits the crystal–air interface with an angle larger than c cannot escape the crystal.

Conceptual Overview Develop adiabatic gradient-index photonic crystal (GRIN) structures which will enable efficient optical coupling of high index scintillators to detectors. Enable realization of the next-generation of high sensitivity, high performance detectors using existing scintillation materials.

Capabilities Strong multi-disciplinary team of collaborators. RMD Dr. Bipin Singh: Principal Investigator, expertise in scintillators and nanotechnology Dr. Matthew S. J. Marshall: Senior Scientist, expertise in nanotechnology and characterization Dr. Vivek V. Nagarkar: Program Manager, expertise in scintillator technology development Massachusetts Institute of Technology Prof. George Barbastathis: expertise in optics and nanotechnology Shuai Li : Doctoral Student with expertise in nanofabrication and nanoimprinting CERN Dr. Paul Lecoq, expertise in particle physics detectors, scintillators and medical imaging Matteo Salomoni, Doctoral student with expertise in modeling and nanoimprinting aBeam Technologies Dr. Keiko Munechika Dr. Carlos Pina-Hernandez

Performance targets/Goals The ultimate goal of this project is to improve energy and timing resolution of current scintillators through the use of super-transmissivity properties of nanostructured photonic crystals. Quarterly Targets: Imprint nanostructures on window of can for SrI2 with new 1"×1" mold and CP4 polymer Demonstrate 20 – 25% gain in Light output for SrI2 (1 inch diameter, 1 inch high) Imprint on GYGAG with CP4 polymer, and new 1"×1" conical photonic crystal mold Demonstrate 20 -25% gain in light output on GYGAG Demonstrate 20 – 25% gain in energy resolution

Review from October 2015 QPR: Light Yield with old mold, aBeam polymer Extraction for LYSO, with n = 2.0 14% gain 14% gain Using a COTS mold from TelAztec, a COTS polymer (CP4a from aBeam) we have exceeded the theoretical predictions Implication: Our manufacturing approach is sound

QPR Review: Challenges of Stamp and Repeat Large Mold Stamp and repeat is challenging, need larger molds.

QPR Review: ALD Simulations Angle Diameter of cone base (y-axis), period of cones on x-axis. Max gain of 16% for L =1 and D = 1. Light Extraction as a function of angle

Higher Light extraction efficiences Where were we (Oct. 2015)? Developed a state-of-the art nanoimprinting Measurements from LYSO, CsI, CLYC Reliably enhance the light yield Reliably achieve (and exceed) the values predicted by theory Where are we now (April 2016)? Measurements from GYGAG, SrI2 as well as LYSO Large increases in light yield, energy resolution In progress: new mold design and manufacture Two new higher refractive index polymers (aBeam) In progress: high refractive index ALD process development

Increasing the Polymer Refractive Index Transmission measured with UV-vis spectrometer Refractive index measured with ellipsometry

Performance for different scintillators Conical PhC mold performance for different scintillators Currently using CP2 (RI = 1.875 @ 420 nm) and CP5 (RI = 1.875 @ 420 nm)

Preliminary results from nanoimprinted GYGAG Nanoimprinted GYGAG achieved 33.6% increase in light yield with the first attempt CP2 n = 1.82 @ 550 nm Energy resolution stayed constant

Characterization of Nanoimprinted GYGAG Scintillator CP2 (n = 1.80) Mold for imprinting Polymer (n=1.50) Glass slide cover Schematic of nanoimprinted GYGAG to solve delamination issues. Replica mold in 1.5 index polymer. Cones from old mold after peeling off the scintillator.

GYGAG imprinted with CP5 Large gain in light output 40.5% gain for polymer CP5 n = 1.88 33.6% gain for polymer CP2 n = 1.82 Experimental Polymer: Non-uniformity during imprinting. This is a process engineering issue, may be leading to degradation of energy resolution.

Imprinting large areas, hygroscopic scintillators TOP: Photo of a 1 × 1 × 1 cm LYSO scintillator, with the imprinted nanostructured surface displaying optical interference effects. RIGHT: Photo of a 30 mm diameter quartz window used for packaging hygroscopic scintillators. Pitch: 400 nm Diameter nanohole: 200 nm Height: 180 nm

Hygroscopic Scintillators: SrI2 : Eu SrI2:Eu light output & energy resolution measured through: a regular window; a pattern that is not optimized, imprinted in CP4 Critical angle for SrI2 is 46°

Hygroscopic Scintillator: SrI2: Eu Transmission of imprinted film (CP5) + oil, losses compared to specifications, improvement in energy resolution Emission of SrI2:Eu Transmission of Imprinted Layer

Mold from aBeam : Progress Hexagonally close-packed array of cones, with an aspect ratio of approx. 1:1.25 (radius:height) would produce an optimal gain Our new manufacturer is attempting to make our design For SrI2, rounded cones give 5% gain, pointy cones give 10% Significant performance reduction: we have not accepted delivery Manufacturing Attempt: Rounded Cones Rounded Cones only 5% gain

Optimized Process: Imprinting on LYSO 50% improvement in energy resolution Nanoimprinted LYSO exhibits large gains in energy resolution Large gains in light output This is using the TelAztec mold, not our optimized design, and the CP4 polymer

Increasing Refractive Index with ALD Key Challenge: Developing high refractive index polymer without degrading the optical properties, must still be imprintable (in progress) Solution: Use atomic layer deposition (ALD) to cover the nanoimprinted structure with very high index of refraction material Atomic layer deposition (ALD): Widely used in industry, allows conformal coating of nanocrystals, low-T polymer-compatible process. TiO2 layer n = 2.6 Name Composition Refractive Index (n) Alumina Al2O3 1.7 Hafnia HfO2 2.2 Titania TiO2 2.6 CP4a polymer n = 1.78

Increasing the refractive index: 210 nm TiO2 n = 2.6 CP4a polymer n = 1.78 Simulations show the gain as a function of layer thickness for ALD-grown thin films of TiO2, which has a very high RI; Potential gains of 44% for a 210 nm thick film

ALD Coating of Nanocones: ALD coating of high refractive index TiO2 over the imprinted nanocones; Process engineering challenges : TiO2 is oxygen deficient, which increases absorption ALD Strategy: Dark areas where ALD film have poor transmission Light areas with less film have better T TiO2 layer needs more oxygen, requires tweaking the process; AT-400 from Anric Technologies installation shortly

Metrics Publications Presentations Students 1 peer-reviewed paper published in Optics Express 1 manuscript submitted to peer-reviewed IEEE Trans. Nucl. Sci. 1 manuscript under preparation for submission to IEEE Trans. Nucl. Sci. Presentations Oral Presentation in Joint Session at the IEEE NSS/MIC, San Diego, Nov 2015 (BS) Oral Presentation at the SCINT 2015 Conf., LBNL, June 2015 (AK) Students 3 Ph.D. students graduated from MIT, 1 Ph.D. student currently supported 1 Ph.D. student supported at CERN

Capital Equipment Scanning electron microscopy (SEM) is extensively used to characterize the morphology of fabricated nanostructures. ~$6,000 used to partially pay for an SEM equipment purchased by RMD. This is ~15% of the total cost of the equipment purchased.

Summary Imprinted on windows for hygroscopic SrI2:Eu; nominal gain in energy resolution Imprinted directly on GYGAG > 40% increase in light yield Optimized process on LYSO: >25% gain in light yield, >50% gain in energy resolution. Next steps: Delivery of new mold with optimized design Installation of ALD system for the deposition of high-RI layers on top of polymer Optimization of process for SrI2 and GYGAG Demonstrate improvements in timing resolution