Mid-infrared (mid-IR) spectra revealing the distinct “fingerprint” of molecules are of critical importance for both science and technology in chemistry, physics, materials science, and biology. Mid-IR sensing and spectroscopy provide unique and powerful tools for crucial applications in greenhouse gas monitoring, hazardous emission detection, and medical breath diagnosis. However, the low-photon energy characteristic of the mid-IR spectrum poses significant challenges for both mid-IR light sources and detectors, hindering their compact and cost-efficient applications in various areas. Here, we experimentally demonstrate mid-IR gas spectroscopy without mid-IR photons solely using convenient near-IR optical components. This is realized by generating photon pairs (i.e., a mid-IR photon and a near-IR one) through a spontaneous parametric downconversion in a microscale optical cavity. The generation rate of near-IR photons strongly relies on the absorption of the mid-IR photon due to their quantum correlation. Through interacting with evanescent fields around the microcavity, the absorptive mid-IR resonances of the surrounding gas greatly affect the detected near-IR photons without harassing any mid-IR photon. This scheme presents a cost-effective and compact solution for on-chip quantum gas spectroscopy without requiring expensive mid-IR lasers and detectors or lasers, thereby paving new avenues for applications in environmental monitoring/safety, healthcare, and agriculture.
Mid-infrared (mid-IR) spectra revealing the distinct “fingerprint” of molecules are of critical importance for both science and technology in chemistry, physics, materials science, and biology. Mid-IR sensing and spectroscopy provide unique and powerful tools for crucial applications in greenhouse gas monitoring, hazardous emission detection, and medical breath diagnosis. However, the low-photon energy characteristic of the mid-IR spectrum poses significant challenges for both mid-IR light sources and detectors, hindering their compact and cost-efficient applications in various areas. Here, we experimentally demonstrate mid-IR gas spectroscopy without mid-IR photons solely using convenient near-IR optical components. This is realized by generating photon pairs (i.e., a mid-IR photon and a near-IR one) through a spontaneous parametric downconversion in a microscale optical cavity. The generation rate of near-IR photons strongly relies on the absorption of the mid-IR photon due to their quantum correlation. Through interacting with evanescent fields around the microcavity, the absorptive mid-IR resonances of the surrounding gas greatly affect the detected near-IR photons without harassing any mid-IR photon. This scheme presents a cost-effective and compact solution for on-chip quantum gas spectroscopy without requiring expensive mid-IR lasers and detectors or lasers, thereby paving new avenues for applications in environmental monitoring/safety, healthcare, and agriculture.
High Resolution Image Download MS PowerPoint Slide Numerous molecules exhibit unique absorption bands in the “fingerprint” infrared (IR) region, allowing chemical identification and detection. This makes IR spectroscopy an essential tool in biomedical diagnostics, sensing, and material characterization. However, measurements in the IR range are challenging due to the relatively poor performance, need for cryogenic cooling, and high cost of IR light sources and photodetectors. Here, we demonstrate that the mid-IR fingerprints of the sample can be revealed from measurements in the near-IR range using conventional silicon photodetectors. Our approach relies on the quantum interference of correlated photon pairs produced in strong frequency nondegenerate parametric down-conversion. Our technique simultaneously measures the absorption coefficient and refractive index in the broad “fingerprint” region with high accuracy. As a proof-of-concept, we perform spectroscopy of nitrous oxide gas in the 7.4 to 8.4 μm wavelength range, with the detection in the 865 to 877 nm range. This work extends the applicability of quantum interferometry to a range of broad practical appeals, such as detection of atmosphere pollutants, breath diagnostics, chemical safety, and others.
Sensing with undetected photons has become a vibrant, application-driven research domain with a special focus on the mid-infrared (mid-IR) wavelength region. Since the mid-IR contains spectral bands with highly specific and strong molecular absorbance signatures, often referred to as fingerprints, a multitude of different samples and their compositions can be detected and quantified spectroscopically. Enhancing this spectroscopic method with imaging capabilities leads to a powerful technique for environmental monitoring and biomedical applications that enables automated diagnostics while omitting time-consuming and non-reversible labeling steps. To evade the shortcomings of state-of-the-art instruments for mid-IR hyperspectral microscopy—namely cost, complexity, power consumption, and performance, which stem from technological challenges in mid-IR detection and light sources—we construct a proof-of-concept nonlinear interferometer in a wide-field imaging arrangement. This nominally narrowband imaging technique is then expanded to acquire broadband spectral information through capturing images for varying interferometer displacement and applying a pixelwise Fourier-transform of the resulting interferograms, yielding high-resolution infrared spectra for each camera pixel. For the broadband range of 2300−3100cm −1 , covering the important CH-stretch band, we perform hyperspectral imaging that simultaneously resolves 3500 spatial modes, each with a spectral resolution of 10cm −1 , leveraging in total around 10 5 spatio-spectrally entangled photon modes. Our image acquisition uses a commercial sCMOS camera, while a medium-power and compact continuous-wave pump laser is the only necessary light source. For a moderate speed of 360 voxel/s, we obtain a predominantly shot-noise-influenced signal-to-noise ratio (SNR) of 50. We further demonstrate the practicality of our novel hyperspectral imaging technique for microplastics detection and bio-imaging tasks and outline engineering solutions to increase its speed by several orders of magnitude. This shows that our quantum imaging technique is highly promising for applications requiring compact, cost-effective label-free analyses.
Spectroscopy in the mid-infrared region is an indispensable tool for identifying molecular types in various fields, including physics, chemistry, and medical sciences. However, conventional infrared light sources, detectors, and noise from blackbody radiation have been the obstacles to miniaturization and higher sensitivity of infrared spectrometers. Quantum infrared spectroscopy, which uses visible and infrared photon pairs in a quantum entangled state, has attracted attention as a new sensing technology that enables infrared spectroscopy with detectors in the visible range. However, the bandwidth of conventional quantum entangled light sources is at most 1 µm or less, which hinders broadband measurements, which are important in spectroscopic applications. Here we have realized an ultra-broadband entangled state of visible–infrared photons with wavelengths from 2 to 5 µm, harnessing a specially designed nonlinear crystal with chirped poling structure inside. Furthermore, we constructed a nonlinear quantum interferometer using the ultra-broadband quantum entangled photons and realized broadband infrared spectroscopy of inorganic and organic materials using a visible detector made of silicon. Our results show that quantum infrared spectroscopy can achieve ultra-broadband spectroscopic measurements and pave the way for the highly sensitive, ultra-compact infrared spectrometers using quantum entangled photons.
Single-photon creation through parametric downconversion underpins quantum technology for quantum sensing and imaging. Here we numerically study the creation of single photons in the near- and mid-infrared regime from 1.5 - 12 um in a range of novel nonlinear semiconductor and chalcopyrite materials. We identify phase-matching conditions and single out regimes in which group-velocity matching can be achieved with commercially available pump lasers. Finally, we discuss how mid-infrared single photons can be detected. Using our numerical results, we identify materials and pump lasers for up-conversion detection in conventional wavelength bands. Our study provides a complete recipe for mid-IR single-photon generation and detection, opening up quantum enhancements for mid-IR applications such as bio-medical imaging, communication, and remote sensing.
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.
Sensing applications in the mid-infrared (MIR) spectral range are challenging due to the high cost and low performance of the available light sources and detectors. This issue can be bypassed by utilizing the process of spontaneous parametric down-conversion (SPDC) to generate tailored MIR-NIR photon-pairs. The nature of SPDC enforces the generation of frequency-entanglement between these paired photons which can be used for spectroscopy: Within a nonlinear interferometric configuration, the MIR radiation is employed for the illumination of a specific material, whereas the NIR radiation is analyzed using a conventional spectrometer. This technique, which is referred to as sensing with ”undetected photons”, so far relied on a breadboard optical setup that is both space-consuming, heavy and complex. Here we present the design, manufacturing and application of a miniaturized quantum light module, with a footprint of less than 80 x 60 mm², which is powered by a direct emitting diode laser at 720 nm. The module contains the nonlinear crystal as the quantum light source, the optical path for the NIR beam, and mechanical connectors leading to the MIR sensing arm. It also provides an interconnection to a spectrometer for the information-carrying NIR photons. This study presents insights into the optical design of the module, its manufacturing and the nonlinear optics quantum light source. The module emits a MIR-NIR photon-pair rate of 1.7 × 109 s−1 W−1, which allows for the implementation of time-efficient sensing applications. The design is optimized for a first application - the detection of synthetic polymers in aqueous solutions, which will be illustrated by concepts for the detection of micro-plastics in wastewater in the field with a mobile device. The potential for integrating a sophisticated, non-linear optical interference configuration into a user-friendly micro-module paves the way for the development of industry-ready setups. This enables a variety of other applications, which are covered by sensing with undetected photons.