Mid-Infrared Upconversion Imaging Techniques

Mid-infrared upconversion imaging harnesses nonlinear optical processes to translate radiation in the 3–12 µm band into shorter wavelengths, where mature silicon-based detectors offer superior sensitivity, speed and spatial resolution. By mixing a mid-infrared signal with a pump beam in a suitably engineered nonlinear crystal, researchers achieve sum-frequency generation that preserves both spatial and spectral information. Advances in crystal design, phase-matching schemes and pump technologies have collectively expanded field of view, enhanced conversion efficiency and reduced background noise, enabling applications that range from real-time chemical mapping and 3D profilometry to in-vivo biomedical diagnostics and environmental sensing. Recent breakthroughs have addressed long-standing limitations in acceptance angle, detector noise and imaging speed, while novel pulse shaping and gating strategies have extended the capabilities of mid-infrared upconversion to photon-starved and time-resolved scenarios.

Research from Nature Portfolio

One study introduced a scan-free hyperspectral chemical imager employing chirped-pulse upconversion of sub-cycle mid-infrared pulses directly at the image plane. The system delivers 640 × 480-pixel images spanning 640–3015 cm⁻¹ in under 8 s, with selectable fields of view from 0.8 × 0.6 mm² to 12 × 9 mm² and discrete-frequency frame rates up to 5 kHz. Demonstrations included mapping of microfluidic channels, plant cell components and tissue sections, illustrating the method’s potential for rapid, label-free chemical analysis. Another work achieved wide-field, single-photon mid-infrared imaging using an aperiodic quasi-phase-matched crystal. By expanding the angular acceptance to approximately 30°, the system attains one-shot snapshot imaging at up to 216 kHz and room-temperature single-photon sensitivity. A picosecond gating module further enables time-of-flight 3D reconstructions, opening pathways to non-destructive inspection, in-vivo examinations and high-speed volumetric tomography.

Research from all publishers

A time-of-flight mid-infrared 3D imager operating at single-photon sensitivity was realised by optically gating back-scattered infrared photons with femtosecond pump pulses. Upconverted photons recorded on a silicon camera, combined with a spatio-temporal denoiser, permit 3D reconstruction under photon-starving conditions below 0.05 photons pixel⁻¹ s⁻¹. In another development, a cavity-enhanced pump scheme boosted the intracavity power of a 1064 nm source by a factor of 36, yielding upconversion efficiencies of 22% and a noise-equivalent power of 0.3 fW Hz⁻¹/². The approach supports high-fidelity spectral mapping with single-frequency pumping, tailored for precision mid-infrared spectroscopy. A further demonstration of mid-infrared hyperspectral imaging in the 6–8 µm range employed an AgGaS₂ crystal and angular scanning to retrieve series of monochromatic images on a silicon CCD. By comparing broadband globar and narrowband quantum-cascade laser illumination, the work highlighted trade-offs in spectral coverage, acquisition speed and spatial resolution, underscoring routes to optimised chemical imaging.

Mid-Infrared Upconversion Imaging Techniques publication trend

The graph below shows the total number of articles in mid-infrared upconversion imaging techniques across all publications each year (not limited to Nature Index journals).

Technical terms

Phase matching: alignment of the propagation constants of interacting waves in a nonlinear medium to maximise frequency-conversion efficiency.

Aperiodic quasi-phase-matching: nonlinear poling pattern with varying periodicity to broaden angular or spectral acceptance.

Chirped-pulse upconversion: technique that temporally stretches pump pulses to encode spectral content into the upconverted signal.

Time-of-flight imaging: depth-mapping method measuring the travel time of light pulses reflected from a scene.

Noise equivalent power: incident optical power required to produce a signal equal to the detector’s noise within a 1 Hz bandwidth.

References

1. Mid-infrared single-photon 3D imaging. Light: Science & Applications (2023).
2. High-speed scanless entire bandwidth mid-infrared chemical imaging. Nature Communications (2023).
3. Highly sensitive mid-infrared upconversion detection based on external-cavity pump enhancement. Advanced Photonics Nexus (2024).
4. Wide-field mid-infrared single-photon upconversion imaging. Nature Communications (2022).
5. Mid-infrared upconversion based hyperspectral imaging.. Optics Express (2018).