New Breakthrough in Quantum Imaging Technology: Photon Pair Generation and Applications Based on Nonlinear Metasurfaces

Research Background and Problem

In recent years, quantum imaging technology has attracted significant attention due to its potential advantages in low photon flux, resolution beyond the classical diffraction limit, and high security. However, traditional quantum imaging systems rely on bulk nonlinear crystals (such as BBO or PPKTP), which typically have thicknesses at the millimeter level. This results in a limited emission angle range under transverse phase matching conditions, thereby restricting the field of view (FOV) and resolution. Additionally, conventional crystals have limited tunability, making it difficult to achieve multi-wavelength operation or fast beam scanning.

To address these issues, researchers have turned their attention to metasurfaces. Metasurfaces are planar optical devices with subwavelength thickness that can enhance and tailor nonlinear optical processes through designed nanostructures. Previous studies have demonstrated that nonlinear metasurfaces can significantly enhance the generation efficiency of entangled photon pairs and enable precise control over spatial, polarization, and spectral entanglement. However, the practical application potential of these technologies has not been fully explored.

The research team of this paper aims to reveal the unique advantages of nonlinear metasurfaces in quantum imaging and develop a new protocol combining “ghost imaging” and all-optical scanning imaging. This study not only expands the application scope of quantum imaging but also demonstrates the enormous potential of metasurfaces in quantum technology.

Source of the Paper and Author Information

This paper was co-authored by Jinyong Ma, Jinliang Ren, Jihua Zhang, and others, with Andrey A. Sukhorukov as the corresponding author. The research team is from the Australian National University (ANU), the University of Melbourne, and the Songshan Lake Materials Laboratory in China. The paper was published in the 2025 issue of the journal eLight (official journal CIOMP, DOI: 10.1186/s43593-024-00080-8).

Research Content and Methods

a) Research Process and Experimental Design

1. Design and Fabrication of Metasurfaces

The research team designed a nonlinear metasurface based on a lithium niobate (LN) thin film covered with a silica grating. The thickness of the metasurface is only 300 nanometers, far smaller than traditional nonlinear crystals. This design supports two non-local optical resonance modes: one exhibits near-flat dispersion characteristics along the grating direction (z-direction), while the other shows quadratic dispersion characteristics perpendicular to the grating direction (y-direction).

During fabrication, the research team used electron beam lithography and inductively coupled plasma etching techniques to create periodic grating structures on the lithium niobate thin film. The final metasurface size is 400 microns × 400 microns.

2. Photon Pair Generation and Characterization

In the experiment, the research team pumped the metasurface with a tunable laser (779–791 nm) to generate spatially entangled signal and idler photon pairs. By adjusting the pump wavelength, the emission angle of the photon pairs could be scanned in the y-direction. Meanwhile, the emission angle of the photon pairs in the z-direction is broad and anti-correlated, suitable for ghost imaging.

To verify the entanglement properties of the photon pairs, the research team measured the two-photon coincidence counting and second-order correlation function ( g^{(2)}(\tau) ). The results showed that the ( g^{(2)}(0) ) value reached as high as 7000, far exceeding the classical limit (2), indicating strong entanglement of the photon pairs.

3. Quantum Imaging Experiment

The research team designed an experimental setup combining ghost imaging and all-optical scanning imaging. Signal photons passing through the target object were collected by a single-pixel detector (bucket detector), while idler photons were captured by a one-dimensional detector array. By recording the photon coincidence counts at different pump wavelengths, the research team successfully reconstructed two-dimensional images of the target objects.

The experiment was divided into two parts: - All-Optical Scanning Imaging: In the y-direction, the emission angle of the photons was scanned by adjusting the pump wavelength to achieve line-by-line imaging of the target object. - Ghost Imaging: In the z-direction, the spatial anti-correlation of the photon pairs was utilized to reconstruct the image of the target object using a one-dimensional detector array.

4. Numerical Simulation and Performance Evaluation

To further validate the potential of this method, the research team conducted numerical simulations. The results showed that when the aperture of the metasurface is increased to 10 mm, the imaging FOV can reach 1.4 radians/micron (y-direction) and 1 radian/micron (z-direction), with minimum resolvable distances of 1 mrad/micron and 0.1 mrad/micron, respectively. Compared to traditional crystals, this method improves the number of resolution cells by four orders of magnitude.

b) Main Results

1. Photon Pair Generation Efficiency and Entanglement Properties

Experimental data showed that the photon pair generation efficiency of the metasurface is 75 MHz/mW, 65 times higher than that of an unpatterned lithium niobate thin film. This is attributed to the resonance enhancement effect and optimized design of the metasurface.

2. Imaging Resolution and Field of View

The two-dimensional images reconstructed in the experiment were highly consistent with those captured by an optical camera, and the processed image reconstruction success rate reached 100%. Numerical simulations further indicated that this method has significant advantages in large FOV and high resolution. For example, when the metasurface aperture is 10 mm, the imaging resolution approaches the diffraction limit, and the FOV range is significantly expanded.

3. Multi-Wavelength Imaging and Beam Scanning

The research team also explored methods for generating non-degenerate photon pairs, including tilted metasurfaces and quasi-periodic grating designs. These methods provide new possibilities for multi-wavelength quantum imaging and fast beam scanning.

c) Conclusions and Significance

This study shows that quantum imaging technology based on nonlinear metasurfaces surpasses traditional methods in terms of FOV, resolution, and device compactness. Specifically: - Scientific Value: Reveals the unique advantages of nonlinear metasurfaces in quantum imaging, providing new tools for quantum optics and quantum information science. - Application Value: This technology can be applied in fields such as quantum radar, quantum communication, and biomedical imaging, with broad application prospects.

d) Research Highlights

1. Innovative Design: First application of nonlinear metasurfaces in quantum imaging, achieving a combination of large FOV and high resolution.
2. Efficient Protocol: Proposes a new method combining ghost imaging and all-optical scanning imaging, simplifying the experimental setup.
3. Multi-Wavelength Operation: Demonstrates the possibility of generating non-degenerate photon pairs, laying the foundation for multi-wavelength quantum imaging.

e) Other Valuable Information

The research team pointed out that in the future, the efficiency of photon pair generation can be further improved by using materials with higher nonlinear coefficients (such as III-V semiconductors or two-dimensional materials) and optimizing triple resonance designs. Additionally, the flexibility of metasurfaces allows the introduction of polarization, spectral, and spatial engineering to enrich imaging data.

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Summary

This paper demonstrates the revolutionary potential of nonlinear metasurfaces in quantum imaging. By combining ghost imaging and all-optical scanning imaging, the research team successfully achieved high-resolution, large FOV two-dimensional image reconstruction. This achievement not only advances the development of quantum imaging technology but also opens new avenues for applications in quantum radar, quantum communication, and other fields.