Quantum-entangled source boosts mid-infrared spectroscopy

31 Jan 2024

Kyoto University broadband emitter could lead to more sensitive and compact devices.

Research into spectroscopy in the mid-IR spectral region has made several recent breakthroughs, spurred by potential uses of the technique in biophotonics and medical imaging. But development of instrumental platforms has remained challenging.

Conventional IR sources and detectors are hard to miniaturize while maintaining the sensitivity needed, and Fourier transform infrared spectrometer (FTIR), considered a promising approach, uses a heating element as its light source and as a result suffers from high detector noise.

Efforts have been made to tackle these hurdles, as in a project at the University of Chicago developing colloidal quantum dots able to emit in the mid-IR range as alternative sources, and the design of improved ultrafast mode-locked sources by Shanghai Jiao Tong University, both intended to address "the mid-IR bottleneck."

A team at Kyoto University has now demonstrated another possible answer employing quantum infrared spectroscopy, using visible and infrared photon pairs in a quantum entangled state.

Published in Optica, the breakthrough uses quantum-entangled light created through chirping, gradually changing an element's polarization reversal period to generate quantum photon pairs over a wide bandwidth.

"The bandwidth of conventional quantum entangled light sources is at most 1 micron or less, hindering broadband measurements which are important in spectroscopic applications" commented the project in its published paper. "We have realized an ultra-broadband entangled state of visible-infrared photons with wavelengths from 2 to 5 microns, harnessing a specially designed nonlinear crystal with chirped poling structure inside."

Environmental monitoring, medicine and security

The project incorporated its source into an experimental platform, building a nonlinear quantum interferometer designed to allow broadband IR spectroscopy of inorganic and organic materials using a visible silicon detector. The overall operation is termed quantum FTIR (QFTIR) spectroscopy.

In trials, the device was able to measure characteristic IR spectra for a variety of samples, such as fused silica glass, polystyrene films, and liquid-phase ethanol, performing QFTIR spectroscopy over a range of 2.5 to 4.5 microns. The results were in good agreement with reference spectra from conventional FTIR spectroscopy.

"We can obtain spectra for various target samples, including hard solids, plastics, and organic solutions," commented Shigeki Takeuchi from Kyoto University. "Shimadzu Corporation, our partner that developed the quantum light device, has concurred that the broadband measurement spectra were very convincing for distinguishing substances for a wide range of samples."

These sources could be valuable as a means to reduce the size and complexity of standard FTIR platforms, which are hard to transport to locations where testing is needed and consume large amounts of power in operation. Shigeki Takeuchi foresees Kyoto's source installed in compact, high-performance, battery-operated scanners, leading to easy-to-use applications in fields such as environmental monitoring, medicine, and security.

"Improving the sensitivity of quantum infrared spectroscopy and developing quantum imaging in the infrared region are part of our quest to develop real-world quantum technologies," said Takeuchi.