The research team made progress in mid-infrared single-photon three-dimensional imaging

The team of Professor Zeng Heping and researcher Huang Kun of the State Key Laboratory of Precision Spectroscopy Science and Technology of East China Normal University has made progress in the field of mid-infrared 3D imaging, developed wide-field, ultra-sensitive and high-resolution mid-infrared upconversion 3D imaging technology, obtained single-photon imaging sensitivity and femtosecond optical gating accuracy, which can provide strong support for important applications such as chip non-destructive testing, remote infrared remote sensing and biomedical diagnosis The title “imaging” was published online on Light: Science & Applications (IF=20.3) on June 9, 2023. ECNU is the first completion unit of the paper, doctoral student Fang Canan is the first author of the paper, and Professor Zeng Heping and Professor Huang Kun are co-corresponding authors.

Figure 1: Light: Science & Applications publishes the research results of the research team of ECNU

Laser 3D imaging technology has the advantages of high imaging resolution, long measurement distance, and rich detection information, and is widely used in automatic driving, satellite remote sensing, industrial production testing and many other fields. In particular, the mid-infrared band is located in the molecular fingerprint spectral region, covering a variety of functional group absorption peaks, which can chemically specifically identify three-dimensional targets, and has attracted much attention in the fields of non-damaging material identification, label-free biological tissue imaging, and non-invasive medical pathological diagnosis. In addition, this band contains multiple atmospheric transmission windows, and has better ability to penetrate smoke and haze than near-infrared light, and has unique advantages in topography mapping and remote sensing recognition. For a long time, how to achieve detection sensitivity close to the single-photon level has been an international research hotspot in the field of mid-infrared 3D imaging, which is of positive significance for promoting its application in low-light detection scenarios with low luminous flux and sparse photons.

Figure 2: Conceptual image of mid-infrared single-photon upconversion 3D imaging

However, single-photon laser 3D imaging has long been limited to the visible/near-infrared band, and the main limitation factor is the lack of photon detection and imaging devices with high detection sensitivity and high temporal resolution in the mid-infrared band. In recent years, with the advancement of infrared device technology and the emergence of new materials, the performance of mid-infrared detectors has been greatly developed, but they still face urgent problems such as enhancing sensitivity, improving response bandwidth, expanding pixel scale, and increasing operating temperature. Mid-infrared 3D measurement can be achieved by optical coherence tomography, photothermal imaging, photoacoustic imaging and other technical solutions, but often need to scan point by point, and it is impossible to obtain large area array imaging with high noise ratio at a single time. Therefore, it is still challenging to achieve mid-infrared single-photon 3D imaging with a large field of view and high resolution.

Figure 3: Mid-infrared single-photon 3D imaging device

To this end, the research team of ECNU developed mid-infrared upconversion measurement and control technology based on high-precision nonlinear optical sampling, which realized ultra-sensitive, high-resolution, large-field of view mid-infrared three-dimensional imaging, and demonstrated single-photon detection sensitivity, femtosecond gating time accuracy and megapixel wide format. Specifically, the researchers used nonlinear optics and frequency processes to efficiently convert the signal wavelength to the visible band, and used high-performance silicon-based cameras to achieve infrared imaging, thus avoiding the technical bottleneck of insufficient sensitivity of existing infrared focal plane arrays. At the same time, the upconversion imaging system adopts a synchronous pulse pumping scheme, which can limit the background noise to a very narrow time window, and combined with precision spectrum filtering, it can effectively improve the detection signal-to-noise ratio, and then achieve imaging sensitivity at the single-photon level. In addition, the researchers followed the nonlinear wide-angle imaging technique previously developed by the group[Nature Commun. 13, 1077 (2022)]Large field of view imaging can be obtained with a single exposure, eliminating the point-by-point mechanical scanning process and greatly improving imaging speed.

Figure 4: Mid-infrared 3D stereoscopic imaging with a measured signal intensity of about 1 photon/pixel/second

Further, the researchers used ultrafast optical gating technology to accurately measure the relative time-of-flight of mid-infrared signals, so as to obtain the topography information of the surface of the measured object. The time-resolved ability of the time-flight imaging system depends on the optical pulse width, which can achieve the time marking accuracy at the femtosecond level, and through high-speed time-lapse scanning and wide-field full-width acquisition, the measured scene is quickly time-domain sliced, and then the reflectivity, transmittance, absorption, refractive index, dispersion and other rich information of the target interface are reflected, transmitted, absorbed, refractive index, and dispersion of materials. Figure 4 shows the three-dimensional morphology of the measured target reconstructed by the fusion of three-dimensional data information under multi-angle mid-infrared illumination, in which the measured signal strength is about 1 photon/pixel/second.

Figure 5: Spatial-temporal associative denoising algorithm with signal and noise levels of approximately 0.05 and 1000 photons/pixel/sec, respectively

In sparse photonic scenarios, the effective signal is often drowned out in severe background noise, and it is often difficult to identify the measured target from the intensity information alone. Therefore, how to effectively distinguish between signal and noisy light has become a key difficulty in single-photon imaging. In order to simulate extremely low illumination and high noise scenarios, the research team attenuated the infrared signal to 0.05 photons/pixel/second, corresponding to a signal-to-noise ratio as low as 1:20000. As shown in Figure 5a-c, traditional intensity peak recognition algorithms are not effective in identifying signals. In active imaging, the signal photons received by the imaging system have a certain continuity in the space-time space, while the background noise photons are randomly distributed across the entire time axis and spatial pixels. Based on this feature, the researchers developed an accurate, efficient and robust point cloud denoising algorithm to effectively extract and identify signal photons by correlating and enhancing the intensity of adjacent pixels in space and adjacent time frames, so as to realize mid-infrared single-photon three-dimensional imaging under high background noise (Figure 5D-I).

The developed mid-infrared three-dimensional imaging technology has the unique advantages of high sensitivity and high resolution, combined with the superior anti-scattering interference ability of this band, which is of great significance for the recovery of infrared scenes in complex environments, and can develop mid-infrared scattering imaging and mid-infrared non-field imaging. In addition, by tuning the mid-infrared signal wavelength, four-dimensional hyperspectral imaging can be realized, which can provide strong support for innovative applications such as material detection, non-destructive testing, and biological imaging.

In recent years, Professor Zeng Heping and researcher Huang Kun’s research group have carried out a series of innovative research in infrared single-photon measurement and control, and have successively developed mid-infrared nonlinear wide-angle imaging [Nature Commun. 13, 1077 (2022)], mid-infrared single-photon single-pixel imaging[Nature Commun. 14, 1073 (2023)], and high-frame rate mid-infrared single-photon spectroscopy [Laser Photonics Rev. 2300149 (2023)]Wait. The relevant work was supported by the Ministry of Science and Technology, the Foundation Committee, Shanghai Municipality, Chongqing Municipality and East China Normal University. (Source: East China Normal University)

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