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arXiv:2303.08185 · Physical Review A

Eliminating thermal infrared background noise by imaging with undetected photons

Yue Ma, Nathan R. Gemmell, Emma J. Pearce, Rupert F. Oulton, Chris C. Phillips

Published Sep 21, 2023• 8 citations
arXivHTMLPDF

Abstract

Spectroscopy and imaging in the mid-infrared ($2.5\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{m}\ensuremath{\sim}\ensuremath{\lambda}\ensuremath{\sim}25\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{m}$) is bedeviled by the presence of a strong 300-K thermal background at room temperature that makes infrared (IR) detectors decades noisier than can be readily achieved in the visible. The technique of imaging with undetected photons (IUP) exploits the quantum correlations between entangled photon pairs to transfer image information from one spectral region to another, and here we show that it does so in a way that is immune to the thermal background. This means that IUP can be used to perform high-speed photon-counting measurements across the mid-IR spectrum, using uncooled visible detectors that are many times cheaper, faster, and more sensitive than their IR counterparts.

DOI
10.1103/physreva.108.032613

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Spectroscopy and imaging in the mid-infrared (2.5\textmum\ensuremath∼\ensuremathλ\ensuremath∼25\textmum2.5\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{m}\ensuremath{\sim}\ensuremath{\lambda}\ensuremath{\sim}25\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{m}2.5\textmum\ensuremath∼\ensuremathλ\ensuremath∼25\textmum) is bedeviled by the presence of a strong 300-K thermal background at room temperature that makes infrared (IR) detectors decades noisier than can be readily achieved in the visible. The technique of imaging with undetected photons (IUP) exploits the quantum correlations between entangled photon pairs to transfer image information from one spectral region to another, and here we show that it does so in a way that is immune to the thermal background. This means that IUP can be used to perform high-speed photon-counting measurements across the mid-IR spectrum, using uncooled visible detectors that are many times cheaper, faster, and more sensitive than their IR counterparts.
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Quantum imaging with undetected photons (QIUP) allows an object to be probed at mid-infrared frequencies by only measuring interference in the visible range, thus leveraging silicon camera technology. We show that non-collinear phase-matching in a silver thiogallate (AgGaS2_22​) crystal enables wide-field QIUP in the wavelength range of 6-10 μμμm (1670-1000 cm−1^{-1}−1). A combination of coherent detection and infrared photons being ``undetected'' enables imaging at ${\sim}$100 times better than the background-limited infrared photodetection (BLIP) limit. At 8 μμμm, our images have over 8000 ±\pm± 100 resolvable elements with a 297 ±\pm± 5 μμμm resolution, and 10 s acquisition time. Our results pave the way to fast, background-noise-free, room-temperature, spectrally-selective mid-infrared imaging.
Quantum imaging with undetected photons (QIUP) has recently emerged as a new powerful imaging tool. Exploiting the spatial entanglement of photon pairs, it allows decoupling of the sensing and detection wavelengths, facilitating imaging in otherwise challenging spectral regions by leveraging mature silicon-based detection technology. All existing implementations of QIUP have so far utilised the momentum correlations within the biphoton states produced by spontaneous parametric downconversion. Here, for the first time, we implement and examine theoretically and numerically the complementary scenario - utilising the tight position correlations formed within photon pairs at birth. This image plane arrangement facilitates high resolution imaging with comparative experimental ease, and we experimentally show resolutions below 10 μm at a sensing wavelength of 3.7 μm. Moreover, we present a quantitative numerical model predicting the imaging capabilities of QIUP for a wide range of parameters. Finally, by imaging mouse heart tissue at the mid-IR to reveal morphological features on the cellular level, we further demonstrate the viability of this technique for the life sciences. These results offer new perspectives on the capabilities of QIUP for label-free widefield mid-IR microscopy, enabling real-world biomedical as well as industrial imaging applications.
Sensing with undetected photons has enabled new, unconventional approaches to Fourier transform infrared spectroscopy (FT-IR). Leveraging properties of non-degenerate entangled photon pairs, mid-infrared (mid-IR) information can be accessed in the near-infrared (near-IR) spectral domain to perform mid-IR spectroscopy with silicon-based detection schemes. Here, we address practical aspects of vibrational spectroscopy with undetected photons using a quantum FT-IR (QFT-IR) implementation. The system operates in the spectral range from around 3000 cm −1 to 2380 cm −1 (detection at around 12 500 cm −1 ) and possesses only 68 pW of mid-IR probing power for spectroscopic measurements with a power-dependence of the signal-to-noise ratio of 1.5 × 10 5 mW −1/2 . We evaluate the system’s short- and long-term stability and experimentally compare it to a commercial FT-IR instrument using Allan–Werle plots to benchmark our QFT-IR implementation’s overall performance and stability. In addition, comparative qualitative spectroscopic measurements of polymer thin films are performed using the QFT-IR spectrometer and a commercial FT-IR with identical resolution and integration times. Our results show under which conditions QFT-IR can practically be competitive or potentially outperform conventional FT-IR technology.
Metrology with undetected photons is an emerging technique that leverages quantum effects and photon correlations (entanglement) to retrieve valuable information in a target spectral range (e.g., mid-infrared, mid-IR) using measurements in an easily accessible domain (e.g., visible, near-IR). The underlying quantum process of spontaneous parametric down-conversion (SPDC) is utilized to generate non-degenerate correlated signal and idler photons to serve as detection and probing photons, respectively. Sensing with undetected photons enables important advantages, such as ultra-low probe powers, room-temperature operation, and shot-noise-limited detection. In this contribution, we apply a quantum nonlinear interferometer based on an SPDC source to perform applied mid-IR spectroscopy, mid-IR microscopy, and mid-IR optical coherence tomography (OCT) as among the most promising techniques for quantum-based routine non-destructive testing. Moreover, we characterize the system, benchmark it against classical systems, and provide a prospective outlook for this new technology.