Mid-infrared Fourier ptychographic upconversion imaging

Technical Summary: Mid-infrared Fourier Ptychographic Upconversion Imaging

Problem Statement
Mid-infrared (MIR) imaging is critical for applications ranging from industrial inspection to biomedical diagnosis due to its ability to reveal thermal and molecular characteristics. However, achieving high-resolution, wide-field MIR imaging at room temperature remains challenging. Traditional focal plane arrays suffer from thermal noise and dark current, while emerging sensors often lack single-photon sensitivity or large pixel counts. Frequency upconversion offers a solution by converting MIR signals to visible/near-infrared wavelengths for detection by mature sensors. Nevertheless, the spatial resolution of existing upconversion imagers is severely restricted by the limited transverse aperture of the nonlinear crystals used in the Fourier plane. Periodically poled crystals, essential for high-efficiency quasi-phase-matching (QPM), are typically limited to millimeter-scale thicknesses to maintain domain homogeneity, creating a narrow passband that acts as a low-pass filter and blurs the image. While large-aperture crystals are difficult to fabricate due to high-voltage poling requirements and domain merging, current methods to increase resolution often compromise the field of view (FOV) or require complex interferometric setups.

Methodology
The authors propose a nonlinear Fourier ptychographic imaging system that combines elliptical pumping with computational aperture synthesis to overcome the physical aperture limitations of the nonlinear crystal.

• Elliptical Pumping: Instead of a standard Gaussian pump, the system employs an engineered elliptical pump beam. This shape is designed to match the narrow but wide transverse section of the chirped-poling lithium niobate (CPLN) crystal (1 mm thickness × 3 mm width). This configuration allows the system to capture higher spatial frequency components along the major axis of the ellipse, effectively utilizing the crystal's width rather than its thickness.
• Fourier Ptychography: To recover resolution in the orthogonal direction and synthesize a larger effective aperture, the system performs rotational sampling. The object is rotated at discrete angles, which correspondingly rotates the elliptical passband in the Fourier space. This generates a sequence of low-resolution upconversion images, each capturing different high-frequency information.
• Computational Reconstruction: An iterative alternating projection algorithm is used to computationally recover a high-resolution, wide-field image. This process fuses the sequence of captured intensity patterns, synthesizing a large aperture without requiring phase measurements or reference beams, thereby avoiding the complexity of interferometry.
• Experimental Setup: The system utilizes a 4f configuration with a CPLN crystal at the Fourier plane. It employs temporally synchronized pulsed lasers (1030 nm pump and 3070 nm signal) to drive sum-frequency generation (SFG). The upconverted signal (771 nm) is detected by a silicon-based EMCCD. The setup includes a rotating diffuser for incoherent illumination and a delay line to optimize temporal overlap for efficient conversion.

Key Contributions

• Novel Imaging Paradigm: The work extends Fourier ptychography into the nonlinear upconversion regime, addressing the specific bottleneck of limited crystal apertures in MIR imaging.
• Elliptical Pumping Strategy: The introduction of an elliptical pump beam tailored to the crystal's aspect ratio enables the acquisition of high spatial frequencies that would otherwise be filtered out by the crystal's thickness.
• Computational Aperture Synthesis: By rotating the elliptical passband and applying phase retrieval algorithms, the system computationally reconstructs an image with a significantly enlarged effective numerical aperture.
• Single-Photon Sensitivity: The system maintains ultra-sensitive detection capabilities, operating effectively under low-light illumination conditions of 1 photon/pulse/pixel, leveraging the high conversion efficiency of pulsed coincidence pumping and low-noise photon counting.

Experimental Results

• Resolution and Field of View: The system achieved a spatial resolution of 39 µm within a field of view of approximately 25 mm.
• Space-Bandwidth Product (SBP): The combination of high resolution and wide FOV resulted in an SBP of 3.2×10⁵. This represents a more than tenfold improvement over previously demonstrated values for wide-field MIR upconversion imaging.
• Resolution Enhancement: Comparative tests using USAF resolution charts and Siemens star targets demonstrated that the elliptical pumping significantly enhanced resolution along the major axis compared to Gaussian pumping. The rotational synthesis successfully recovered isotropic high resolution across the entire field of view.
• Penetration Imaging: The system successfully imaged a university emblem inscribed on the back of a 700 µm thick silicon wafer, demonstrating the ability to penetrate semiconductor materials transparent in the MIR range.
• Low-Light Performance: High-resolution images were successfully reconstructed from data acquired at a photon flux of 1 photon/pulse/pixel, confirming the system's single-photon sensitivity.

Significance
The paper claims that this paradigm of nonlinear Fourier ptychographic upconversion imaging provides a scalable pathway toward high-throughput infrared imaging with massive resolvable elements and single-photon sensitivity. By circumventing the physical aperture limits of nonlinear crystals through computational synthesis, the technique enables high-resolution, wide-field MIR imaging at room temperature without the need for complex interferometric hardware. The authors suggest this capability is particularly valuable for applications requiring non-destructive analysis, such as industrial quality control (e.g., semiconductor defect inspection) and biomedical diagnosis, where high sensitivity and large field coverage are simultaneously required. The work establishes a foundation for future high-dimensional MIR imaging that could integrate hyperspectral, phase, and depth-resolved capabilities.