Enhanced light extraction in flex-mold nanopatterned scintillators for TOF-PET

Abstract

Objective. Optical photon losses at the scintillator-photosensor interface limit both energy and coincidence time resolution (CTR) in time-of-flight positron emission tomography (TOF-PET) detectors. To address this challenge, we applied diffractive nanopatterns on the surface of scintillation crystals to enhance light extraction. Approach. We utilized flex-mold-based soft lithography to imprint 300 nm hole and line patterns over cm-scale scintillator surfaces with less than 4% relative dimensional variation. Three scintillators (LYSO, BGO, GAGG) were tested under four optical coupling configurations: bare crystal with air (reference), bare with optical grease, nanopatterned crystal with air, and nanopatterned with grease. Main results. A 3 & times; 3 & times; 20 mm3 BGO with an H_hole-plaid nanopattern combined with optical grease achieved a maximum light extraction gain of 2.22-fold compared to the bare crystal with air coupling. Its energy resolution improved from 18% to 11%. The same fabrication process was applied to 3 & times; 3 & times; 20 mm3 LYSO and 5 & times; 5 & times; 5 mm3 GAGG, yielding light extraction gains of 1.58- and 1.72-fold. We also performed CTR measurements on nanopatterned LYSO 3 & times; 3 & times; 20 mm3 crystals, where the average CTR dropped from 250.6 ps (bare crystal with air coupling) to 142.6 ps (P_line with optical grease) at a fixed leading-edge discriminator threshold. Further threshold optimization for P_line with optical grease yielded a best result of 132.0 ps. The nanopatterns retained most of their gain after mechanical loading and 20 cycles of repeated cleaning. Significance. These findings demonstrate that flex-mold-based soft nanoimprint lithography provides a practical, high-fidelity approach for surface engineering of TOF-PET systems.