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Scientific Reports

Broadband infrared light source by simultaneous parametric down-conversion

Masayuki Hojo, Kōichiro Tanaka

Published Sep 9, 2021• 16 citations
Publisher

Abstract

Spontaneous parametric down-conversion is an essential tool for a quantum light source in the infrared region ranging 2-5 µm for the purpose of material identification, chemical analysis, and gas sensing. So far, photon pairs from the process in a nonlinear crystal have low tunability and a narrow spectral range because of the phase-matching condition. Here, we propose a novel type of spontaneous parametric down-conversion processes that overcomes these challenges, where two photon pairs are simultaneously produced in the visible and infrared regions in periodically poled stoichiometric lithium tantalite. It allows broadband and tunable generation of infrared photon pairs that can be employed as an alternative light source for quantum infrared spectroscopy.

DOI
10.1038/s41598-021-97531-w

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Spontaneous parametric down-conversion is an essential tool for a quantum light source in the infrared region ranging 2-5 µm for the purpose of material identification, chemical analysis, and gas sensing. So far, photon pairs from the process in a nonlinear crystal have low tunability and a narrow spectral range because of the phase-matching condition. Here, we propose a novel type of spontaneous parametric down-conversion processes that overcomes these challenges, where two photon pairs are simultaneously produced in the visible and infrared regions in periodically poled stoichiometric lithium tantalite. It allows broadband and tunable generation of infrared photon pairs that can be employed as an alternative light source for quantum infrared spectroscopy.
Abstract The nonlinear process of spontaneous parametric down-conversion in the silver-gallium-sulfide crystal can generate highly non-degenerate photon pairs in near-infrared and mid-infrared spectral ranges. In this work, we demonstrate and characterize the tunable mid-infrared photon source in silver-gallium-sulfide. By angle tuning the phase matching condition of the nonlinear crystal, we can tune the wavelength of the generated photons in mid-infrared from 5.661 to 10.885 μm (from 1766 to 919 cm −1 ), while the correlated photons in the near-infrared are from 747.1 to 702.6 nm. The phase matching curve and the frequency-angular spectrum of mid-infrared photons were experimentally validated by measuring the properties of near-infrared photons. The brightness of the photon pair source was found to be (29.2± 1.1) MHzmW\frac{\text{MHz}}{\text{mW}}mWMHz​ in the low-gain regime, exceeding the values reported in previous studies using silver-gallium-sulfide crystals. The wide tunability of the photon source in the mid-infrared range is important for hyperspectral imaging and sensing, and it is of significant interest for biomedical, chemical, and environmental applications.
Spontaneous parametric down-conversion (SPDC) is one of the most important sources for entangled or correlated photon pairs, which are the foundation of the emerging fields of quantum-optical imaging and spectroscopy. Using periodically poled materials as an SPDC-source allows broad emission spectra in widely tunable wavelength ranges in an efficient and walk-off-free process. For applications that employ nonlinear interference setups, maximum interferometric contrast is only achieved when signal and idler modes are overlapped precisely in the second crystal. Thus, accurate knowledge of the spectral-angular characteristic of SPDC is crucial for designing such devices.
Nonlinear interferometers based on non-degenerate spontaneous parametric down-conversion (SPDC) create a link between separate spectral ranges. This allows for measurements in remote spectral regions while detecting light in easily accessible wavelengths. In our work, we use periodically poled lithium niobate to create correlated signal (visible or near-infrared) and idler (mid-infrared) photon pairs. Using a nonlinear interferometer in Michelson geometry, we obtain broadband mid-infrared spectra from light detected with a silicon avalanche photodiode. Combining the nonlinear interferometer with a measurement scheme in close analogy to classical Fourier-transform infrared spectroscopy allows for sub-wavenumber spectral resolution, which opens up possibilities for applications such as precise spectroscopic gas analysis.
We report an experimental demonstration of wavelength variable generation and detection of photon pairs in the visible and mid-infrared (MIR) regions over a wide spectral range of 2–5 µm via spontaneous parametric downconversion. Despite the recent increase in interest in such a photon-pair source, there have been few detailed evaluations of emitted photons generated via the downconversion process in the low gain regime, due to the lack of suitable single-photon detectors. By changing the angle of the nonlinear crystal, we continuously tune the phase-matching condition and generate photon pairs as signal photons in the wavelength range of 600–965 nm and idler photons in the wavelength range of 1186–4694 nm. We evaluate the generated photon pairs using single and coincidence counts by superconducting nanowire single-photon detectors up to a wavelength of 2 µm and detect the intensity using an InSb detector with a lock-in detection system up to 5 µm. From this analysis, a pair generation rate of 10 5 s − 1 per mW of pump power is experimentally obtained for this wavelength range. This work provides a basis for the realization of applications such as heralded MIR single-photon sources, infrared imaging, and infrared spectroscopy based on quantum technologies in the MIR region.
Entangled photon pairs have attracted many interests from the view of quantum science and technologies. However, there are still several technical issues remaining in the conventional generation method, such as complexity and inefficiency. Here we propose a simple scheme to generate frequency-entangled photon pairs via spontaneous parametric down-conversion in periodically poled stoichiometric lithium tantalite. Our method will open a way to construct a compact and efficient source of entangled photons for quantum sensing.
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.