Eureka in a Cavity: Measuring Molecular Densities with Quantum Optics
Gerrit Groenhof
Recent years have seen a surge of claims that placing molecules between the mirrors of a Fabry-Pérot cavity can alter their chemical reactivity. Because such cavities confine photons at discrete wavelengths determined by the mirror separation, these effects are often attributed to the collective interaction of many molecules with a cavity mode. Knowing how many molecules participate is therefore a key step toward understanding cavity-modified chemistry. To address this, we fabricated a series of cavities containing BODIPY molecules. Combining nanofabrication, optical spectroscopy, theory, and computation, we find that approximately 10^7 molecules are collectively coupled to the cavity mode, while the field experienced by each individual molecule is on the order of 0.1 meV. This energy scale is two orders of magnitude smaller than the thermal energy at room temperature, suggesting that previously reported changes in reactivity are unlikely to originate from light-matter coupling alone. In this talk, I will present our approach and discuss its implications for cavity-modified chemistry. While our findings cast doubt on some of the more ambitious claims in the field, they do offer something more robust: a new, quantum-optical way to measure molecular densities—no buoyancy required ...
