New quantum infrared spectroscopy method developed by Kyoto University: Realization of compact devices is expected

Atoms in molecules vibrate at specific frequencies and absorb and emit infrared light that matches those frequencies. This principle is used in infrared spectroscopy, which is indispensable in the pharmaceutical and chemical industries. However, the energy efficiency of a light source is poor and nitrogen cooling is required to increase the sensitivity of infrared photodetectors, making it difficult to reduce size and cost. In this context, quantum infrared spectroscopy, which utilizes quantum entanglement, allowing infrared spectroscopy using only a visible light source and a detector, is attracting attention. However, in quantum infrared spectroscopy, the sample is placed in a quantum interferometer. The sample must be prepared with an extremely smooth surface or quantum interference will not occur.

A research group including Special Researcher (at the time of the research) Torataro Kurita, Assistant Professor Yu Mukai, Special Researcher Toshiyuki Tashima, Associate Professor Ryo Okamoto, and Professor Shigeki Takeuchi of the Department of Electronic Science and Engineering at Kyoto University, in collaboration with a research group led by Senior Researcher Katsuhiko Tokuda of Shimadzu Corporation, have developed quantum infrared spectroscopy based on the attenuated total reflection (ATR) method, which allows the analysis of samples with uneven surfaces by pressing them against a prism. This enables easy measurement of a variety of samples, including thick samples that were previously difficult to analyze. This development is expected to lead to the realization of a small, portable quantum infrared spectroscopy system that can be used for a wider range of applications. The results were published in Physical Review Applied.

The ATR method is utilized in existing infrared spectroscopy and widely adopted because it allows easy measurement by simply pressing a sample against a prism. Further, it can be used for analyzing samples that cannot be analyzed using transmission measurements because they involve the absorption of most of the infrared light. The newly developed attenuated total reflection quantum infrared spectroscopy (ATR-QIRS) applies the total reflection phenomenon, in which a small amount of light seeps out of the prism surface when it is totally reflected, to quantum infrared spectroscopy. When a continuous-wave (CW) laser for excitation with a wavelength of 532 nm radiates on a nonlinear crystal of lithium niobate, quantum-entangled photon pairs are generated, consisting of a visible photon and an infrared photon.

In the experiment, the wavelengths of quantum-entangled photon pairs were varied from 700 to 620 nm for visible photons and from 2.2 to 3.8 µm for infrared photons by adjusting the angle of incidence of the excitation light on the nonlinear crystal. The infrared photons, visible photons, and excitation light were then separated to travel along different paths, and the infrared photons were incident on a prism made of calcium fluoride, causing total reflection on the top surface of the prism. This process ceased the quantum entanglement of the visible and infrared photons by the amount of seeping out photons. The infrared photons were then estimated by measuring the visible light and the excitation light. By measuring the absorption spectrum of a water sample using quantum Fourier transform infrared spectroscopy (QFTIR) with ATR-QIRS, the researchers succeeded in obtaining a broadband OH-vibration absorption spectrum of water.

This time, the researchers demonstrated for the first time that the ATR method, which utilizes light seeping through the prism-sample interface (near field), can be applied to quantum infrared spectroscopy.

Takeuchi said, "Two years ago, we launched a consortium and are currently in discussions with several companies with the aim of implementing the developed technology for societal benefit. We are hoping to have it ready for use in around 2028."

Journal Information
Publication: Physical Review Applied
Title: Quantum infrared attenuated total reflection spectroscopy
DOI: 10.1103/PhysRevApplied.23.014061