Directional Control and Enhancement of Light Output of Scintillators by Using Microlens Arrays

Bright Innovations: Review of Next-Generation Advances in Scintillator Engineering

This review focuses on modern scintillators, the heart of ionizing radiation detection with applications in medical diagnostics, homeland security, research, and other areas. The conventional method to improve their characteristics, such as light output and timing properties, consists of improving in material composition and doping, etc., which are intrinsic to the material. On the contrary, we review recent advancements in cutting-edge approaches to shape scintillator characteristics via photonic and metamaterial engineering, which are extrinsic and introduce controlled inhomogeneity in the scintillator's surface or volume. The methods to be discussed include improved light out-coupling using photonic crystal (PhC) coating, dielectric architecture modification producing the Purcell effect, and meta-materials engineering based on energy sharing. These approaches help to break traditional bulk scintillators' limitations, e.g., to deal with poor light extraction efficiency from the material due to a typically large refractive index mismatch or improve timing performance compared to bulk materials. In the Outlook section, modern physical phenomena are discussed and suggested as the basis for the next generations of scintillation-based detectors and technology, followed by a brief discussion on cost-effective fabrication techniques that could be scalable.

Polymeric microlens array formed on a discontinuous wetting surface using a self-assembly technique

In this paper, we demonstrate a facile way to prepare polymeric microlens arrays (MLAs) based on a discontinuous wetting surface using a self-assembly technique. A patterned hydrophobic-octadecyltrichlorosilane (OTS) surface was prepared by U V/O 3 irradiation through a shadow mask. The area exposed to U V/O 3 irradiation turned highly hydrophilic, whereas the area protected by the mask remained highly hydrophobic, generating the patterned OTS surface. The surface energy of the OTS/glass surface changed from 23 to 72.8 mN/m after 17 min of U V/O 3 treatment. The scribing of the optical glue-NOA 81 onto the microhole array enabled one to obtain the MLAs due to the generation of the NOA 81 droplet array via the surface tension. After UV light curing, the cured NOA 81 droplet array with uniform dimensions within a large area exhibited excellent MLA characteristics. Moreover, the method developed in this study is simple in operation, low-cost, and requires neither a clean room nor expensive equipment.

Microfluidic Fabrication of a PDMS Microlens for Imaging Tunability

A microfluidic system was created to fabricate polydimethylsiloxane (PDMS) microspheres, whose shape, surface smoothness, and size were controlled. Resulting from their excellent optical properties and elasticity prepared by the apparatus, each PDMS microsphere could act as a microlens and separate imaging unit. The focal length of the microlens was simply tuned by the forces posed on the beads. For the microlens array (MLA) application, it was constructed simply through the assembly of the monodisperse PDMS beads.

Asymmetrical interface design for unidirectional light extraction from spectrum conversion films

In this study, we propose a micro-sized photonic structure that extracts 89% of the intrinsic trapped photons from the spectrum conversion film into free space using the Monte-Carlo ray-tracing method. Furthermore, the spectrum of the spectral-shifting film can be accurately simulated based on a mean free path concept, providing the estimation of its overall performance including the external quantum efficiency and the self-absorption efficiency. The simulations show that the spectrum conversion film with micro-structures shows a two-fold increase in the total external quantum efficiency and a four-fold increase in the external quantum efficiency in the forward viewing direction compared to the planar spectrum conversion films without micro-structures.

Enhancement of Luminous Intensity Emission from Incoherent LED Light Sources within the Detection Angle of 10° Using Metalenses

In this work, we present metalenses (MLs) designed to enhance the luminous intensity of incoherent light-emitting diodes (LEDs) within the detection angles of 0° and 10°. The detection angle of 0° refers to the center of the LED. Because the light emitted from LEDs is incoherent and expressed as a surface light source, they are numerically described as a set of point sources and calculated using incoherent summation. The titanium dioxide (TiO2) and amorphous silicon (a-Si) nanohole meta-atoms are designed; however, the full 2π phase coverage is not reached. Nevertheless, because the phase modulation at the edge of the ML is important, an ML is successfully designed. The typical phase profile of the ML enhances the luminous intensity at the center, and the phase profile is modified to increase the luminous intensity in the target detection angle region. Far field simulations are conducted to calculate the luminous intensity after 25 m of propagation. We demonstrate an enhancement of the luminous intensity at the center by 8551% and 2115% using TiO2 and a-Si MLs, respectively. Meanwhile, the TiO2 and a-Si MLs with the modified phase profiles enhance the luminous intensity within the detection angle of 10° by 263% and 30%, respectively.

Light output enhancement of scintillators by using mixed-scale microstructures

Scintillators play an important role in the field of nuclear radiation detection. However, the light output of the scintillators is often limited by total internal reflection due to the high refractive indices of the scintillators. Furthermore, the light emission from scintillators typically has an approximately Lambertian profile, which is detrimental to the collection of the light. In this paper, we demonstrate a promising method to achieve enhancement of the light output from scintillators through use of mixed-scale microstructures that are composed of a photonic crystal slab and a microlens array. Simulations and experimental results both show significant improvements in the scintillator light output. The X-ray imaging characteristics of scintillators are improved by the application of the mixed-scale microstructures. The results presented here suggest that the application of the proposed mixed-scale microstructures to scintillators will be beneficial in the nuclear radiation detection field.

Improved light output from thick β-Ga2O3 scintillation crystals via graded-refractive-index photonic crystals

β-Ga₂O₃ is a promising candidate as a fast scintillation crystal for radiation detection in fast X-ray imaging and high-energy physics experiments. However, total internal reflection severely limits its light output. Conventional photonic crystals can improve the light output, but such improvement decreases dramatically with increased scintillator thickness due to the strong backward reflection by the photonic crystals. Here, graded-refractive-index photonic crystals composed of nanocone arrays are designed and fabricated on the surfaces of β-Ga₂O₃ crystals with various thicknesses. Compared to the conventional photonic crystals, there is still an obvious light output improvement by using the graded-refractive-index photonic crystals when the thickness of the crystals is increased by three times. The effect of thickness on the improved light output is investigated with numerical simulations and experiments. Overall, the graded-refractive-index photonic crystals are beneficial to the improvement of light output from thick scintillators.