Mid-IR Spectroscopy Achieved With Undetected Quantum Photons

Researchers at the Research Center for Non-Destructive Testing and ohannes Kepler University, are developing a new approach to mid-infrared spectroscopy that relies on photons that go undetected as they probe a sample. Utilizing spontaneous parametric down-conversion (SPDC), which usually occurs on the order of 10^-9, the team generates correlated photons, employing the longer-wavelength “idler” photons as the probing mid-IR source while measuring changes via the shorter-wavelength “signal” photons. This quantum nonlinear interferometer offers significant advantages, including room-temperature operation and shot-noise-limited detection, potentially streamlining non-destructive testing applications. In this work, the team demonstrates mid-IR microscopic mapping and QFTIR spectroscopy, alongside mid-IR OCT, as promising techniques for routine, quantum-based inspection.

Quantum Metrology with Undetected Mid-Infrared Photons

A surprising approach to material science is gaining traction, utilizing mid-infrared photons that are never directly detected to probe sample properties. This method, detailed in recent work, hinges on a process called spontaneous parametric down-conversion (SPDC) to generate pairs of “signal” and “idler” photons, with the latter acting as the unseen probes. The core innovation lies in exploiting quantum nonlinear interferometry, replacing traditional beam splitters with nonlinear crystals. When pumped with a laser, these crystals convert photons into correlated pairs; the team explains that “a pump photon could be converted into two photons of lower frequencies.” These longer wavelength idler photons interact with the sample, and changes induced in their properties, absorption, scattering, or phase shift, are read out via interference patterns observed in the shorter wavelength signal photons, which are detected using standard visible light sensors.

This circumvents the need for expensive and technically challenging mid-IR detection systems. The researchers state that this approach offers several key advantages. The team highlights “ultra-low probing powers, room-temperature operation, and shot-noise-limited detection” as benefits over conventional mid-IR techniques like Fourier transform infrared spectroscopy (FTIR). Room-temperature operation is particularly significant, as many quantum experiments require extremely cold environments, limiting their practical application. The potential applications are focused on non-destructive testing, with mid-IR spectroscopy, microscopy, and OCT identified as the most promising avenues for implementation. In this work, they demonstrate mid-IR microscopic mapping and QFTIR spectroscopy as well as mid-IR OCT. Specifically, mid-IR OCT offers a solution for imaging highly porous materials like ceramics or paintings, where traditional methods struggle due to light scattering. The system’s operation relies on carefully manipulating the phase of the photons.

According to the research, the cumulative phase change between the pump, signal, and idler photons, denoted as Δϕ, dictates the interference observed in the signal photons. The team demonstrates that any absorption or scattering of the idler photons by the sample directly affects this interference, allowing for precise material characterization. They note that the probability of SPDC occurring is usually on the order of 10^-9, emphasizing the sensitivity and precision required to implement this technique effectively. The researchers are currently focused on refining the system and exploring its potential for broader applications in materials science and beyond.

The pursuit of sharper, more versatile non-destructive testing methods has increasingly turned to quantum phenomena, specifically leveraging the unusual properties of light at the quantum level. Their work centers on utilizing correlated photons generated through spontaneous parametric down-conversion (SPDC) in a novel way, employing the undetected photons as the probing source for mid-IR analysis. SPDC occurs usually on the order of 10^-9. The team’s system hinges on a nonlinear interferometer, where traditional beam splitters are replaced by nonlinear crystals. Crucially, the idler photons are directed towards the sample, while the signal photons are used for detection. This configuration allows researchers to infer information about the sample, absorption, scattering, or phase shifts, by observing interference patterns in the detected signal photons. This method offers distinct advantages over traditional mid-IR techniques. They demonstrate mid-IR microscopic mapping and QFTIR spectroscopy, alongside mid-IR OCT in this work.

Mid-IR Spectroscopy & Microscopy via Quantum FTIR

Paul Gattinger and colleagues are developing quantum-based spectroscopic and imaging methods that utilize undetected mid-IR photons to probe sample characteristics. Metrology with undetected photons was first introduced in 2014 [14]. The principle of this technology is based on nonlinear interferometry and utilizes spontaneous parametric down-conversion (SPDC), where photons of a shorter (pump) wavelength are converted to correlated photon pairs of longer wavelengths. These benefits include “ultra-low probing powers, room-temperature operation, and shot-noise limited detection,” a significant leap forward considering many quantum experiments demand extremely cold environments. The ability to operate at room temperature is particularly impactful, streamlining the technology for wider adoption and real-world deployment. The underlying principle hinges on the fact that information about the mid-IR photons, absorption, scattering, or phase shifts induced by the sample, is encoded within the correlations with their signal photon partners.

By carefully manipulating the phase of these photons, changes in the signal photon rate reveal details about the sample’s composition and structure. “OCT at longer wavelengths, for example, in the mid-IR domain, is a potential solution,” they state, adding that this technique “opens up a range of formerly inaccessible applications.” The system’s design, utilizing two sequentially pumped crystals, allows for the observation of interference in the signal domain, revealing sample characteristics through subtle changes in photon behavior. In the study, the researchers demonstrate mid-IR microscopic mapping and QFTIR spectroscopy as well as mid-IR OCT with undetected photons, and are currently focused on benchmarking performance and identifying further avenues for development.

The demand for increasingly detailed non-destructive testing across industries like aerospace, materials science, and cultural heritage preservation is driving innovation in imaging techniques. Researchers at the Research Center for Non-Destructive Testing in Linz, Austria are now leveraging the subtle principles of quantum mechanics to overcome longstanding limitations in mid-infrared (mid-IR) optical coherence tomography (OCT), a method particularly well-suited for analyzing highly scattering materials. Central to this advancement is the utilization of spontaneous parametric down-conversion (SPDC). This process generates correlated pairs of photons, a “signal” photon detectable in the visible or near-infrared range, and an “idler” photon in the mid-IR, designed to probe the material. SPDC occurs usually on the order of 10^-9. Unlike conventional OCT, this system relies on subtle changes induced in the undetected idler photons. In the study, the researchers demonstrate mid-IR microscopic mapping and QFTIR spectroscopy as well as mid-IR OCT. Since mid-IR light is less prone to scattering, the technique promises clearer sub-surface imaging. This innovative approach represents a significant step toward practical, quantum-enhanced material analysis.

ppKTP & ppLN Crystals in Photon Pair Generation

The pursuit of detailed mid-infrared (mid-IR) spectroscopic and imaging techniques often encounters a fundamental hurdle: generating and detecting light at those wavelengths is notoriously difficult. While conventional approaches rely on increasingly complex and expensive technologies, a surprising alternative is gaining traction, leveraging quantum effects to probe with “undetected” photons. Central to this innovation are nonlinear crystals, specifically periodically poled potassium titanyl phosphate (ppKTP) and periodically poled lithium niobate (ppLN), which facilitate the generation of correlated photon pairs essential for this quantum metrology. The process hinges on spontaneous parametric down-conversion (SPDC), which usually occurs on the order of 10^-9. These aren’t just any photon pairs; they are intrinsically linked, allowing information gleaned from the longer-wavelength “idler” photons, those used to probe the sample, to be read out via measurements of the shorter-wavelength “signal” photons in the visible or near-IR range.

The choice between ppKTP and ppLN isn’t arbitrary; their crystal structures are engineered to maximize the efficiency of this down-conversion process and tailor the generated wavelengths. The design of these crystals is crucial. Periodic poling, a technique involving reversing the polarization of the crystal domains, allows for quasi-collinear emission of the signal and idler photons, expanding the range of accessible wavelengths. Simulations demonstrate how these crystals can generate narrowband pump, signal, and idler photons, individually normalized for clarity. This precise control over wavelength is vital for targeting specific absorption bands in mid-IR spectroscopy, enabling detailed chemical analysis. This indirect detection method, coupled with the use of readily available visible light detectors, represents a significant simplification of mid-IR measurements, potentially unlocking new applications in non-destructive testing and materials science.