Pixelated color conversion of luminescent materials facilitates X-ray to visible light field detection

Light field detection involves measuring the intensity of light and its orientation in free space. However, current light field detection techniques require complex arrays of microlenses and cover a wavelength range limited to ultraviolet to visible light. In order to cope with this problem, Professor Liu Xiaogang from the Department of Chemistry of the National University of Singapore proposed a new idea of light field detection, which encodes the direction of incident light to the luminous color of perovskite nanocrystals, and through ingenious geometric design patterned nanocrystal arrays, the radiation vector of incident rays from X-rays to visible light can be determined, so as to construct a complete 3D form of light field. The results, “X-ray-to-visible light-field detection through pixelated colour conversion,” were published in the May 10, 2023 issue of the journal Nature. The corresponding author of the paper is Professor Liu Xiaogang, and the first author is Dr. Yi Luying.

Although advances in materials and semiconductor processes have revolutionized the design and manufacture of photodetectors, most sensors are limited to detecting the intensity of light, resulting in the loss of all phase information about objects and diffracted light waves. While intensity information alone is sufficient for traditional applications such as 2D photography and microscopic imaging, this limitation hinders 3D and 4D imaging applications such as phase contrast imaging, light field imaging, virtual reality, and space exploration. Microlenses or photonic crystals with integrated photodiode arrays are often used to measure the distribution of light fields or light directions to characterize phase information. However, integrating these elements into CCD or CMOS semiconductor architectures is complex. Optical resonance in subwavelength semiconductor structures offers the possibility of developing angle-sensitive structures by manipulating light to interact with matter. However, they are mostly wavelength or polarization dependent and require high refractive index materials. In addition, the detection and control of light vectors is currently limited to ultraviolet and visible wavelengths. While some sensors using Shack-Hartmann or Hartmann structures are capable of measuring phases in the extreme ultraviolet range, X-ray and gamma-ray measurements remain challenging because conventional mirrors or microlenses cannot focus high-energy beams.

Figure 1: X-ray to visible light vector detection using pixelated perovskite nanocrystal arrays.

(Yi, Luying, et al. Nature(2023):

In order to verify the idea of encoding the direction of light to the luminescent color of nanomaterials, the authors selected inorganic perovskite nanocrystals with high quantum yield, wide excitation spectrum, and narrow luminescence spectrum as luminescent materials. Perovskite nanocrystals emit light under X-ray to visible light irradiation with high color saturation in the visible spectral range. In addition, the optical absorption range of tin (Sn)-based perovskite nanocrystals can extend to the near-infrared region. The basic design of a 3D light field imaging sensor involves patterning perovskite nanocrystalline lithography onto a transparent substrate. A 3D light field imaging sensor can then be constructed by integrating the patterned substrate with a color CCD, which can convert the angle of the incident light into a specific color output. As a proof of concept, three perovskite nanocrystals with emission peaks of 445 nm, 523 nm and 652 nm were used to construct a single light vector sensor. When light is incident from 0 to 360 degrees relative to the reference direction, the detected gamut forms a large triangle on the CIExy chromaticity diagram (Figure 1c). The position of the color output on the chromaticity diagram determines the angle of incidence of the ray, and larger the triangle indicates higher angular resolution. Compared to detectors made of ZnS:Cu2+/Mn2+ and SrAl2O4:Eu2+/Dy3+ phosphors, the color gamut of an azimuth detector made of perovskite nanocrystals forms a larger triangle in the chromaticity diagram, i.e. has a higher angular resolution (Figure 1d). This is due to the wider color coverage and higher color saturation of perovskite nanocrystals.

Figure 2: Construction of a 3D imaging system based on triangulation ranging.

(Yi, Luying, et al. Nature (2023):

Figure 3: 3D imaging of different scenes by color-coded 3D light field imaging sensors.

(Yi, Luying, et al. Nature (2023):

The direct application of light field imaging sensors based on pixelated perovskite nanocrystal arrays is 3D LiDAR imaging. The imaging system is based on the principle of triangulation ranging and consists of a multi-line structured light source, two concentrating lenses, and a 3D light field imaging sensor with integrated nanocrystalline arrays (Figure 2). The authors show the imaging effect of different objects at different depths and light intensities (Figure 3). Another important application of nanocrystalline light field imaging sensors is phase contrast imaging over a wide wavelength range from X-rays to visible light (0.002–550 nm). In phase contrast imaging of a conventional Shack-Hartmann wavefront sensor, a microlens array records the angle of a particular wavefront over a series of grid points (Figure 4). Light field sensors based on nanocrystal arrays can directly measure the angle of visible light or X-rays, allowing wavefront reconstruction without the need for microlens arrays. The authors present phase contrast imaging results based on X-ray (0.089 nm) and visible light (405 nm), respectively.

Figure 4: Phase contrast imaging of X-rays (0.089 nm) and visible light (405 nm) by a pixelated color conversion strategy.

(Yi, Luying, et al. Nature (2023):

The authors propose a strategy for pixelated angle-to-color conversion based on perovskite nanocrystal arrays for light field detection, absolute spatial positioning, 3D imaging, and visible/X-ray phase contrast imaging. With the current design, a resolution of 0.0018° of light incidence angle and a wavelength response range of 0.002–550nm are achieved. Further improvements can further improve angle detection accuracy and spatial resolution by integrating high-end color sensors and processing finer perovskite nanocrystalline structures. (Source: Science Network)

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