A new optical sensing technique has been designed to detect the vibrations of atoms within molecules. [Image: Jonathan Kitchen/ Getty Images]
Researchers in the United States have demonstrated a new optical technology that provides the sensitivity needed to detect the vibrational motion of atoms bound within molecules (Sci. Adv., doi: 10.1126/sciadv.ady7670). The robust and accurate technique operates at room temperature, potentially providing a powerful new tool for detecting disease, monitoring industrial processes and measuring trace amounts of pollutants or hazardous compounds.
Enhancing spectral signals
Optical techniques such as Raman and infrared spectroscopy are already used to detect molecular vibrations, with the movement of atoms between quantized vibrational modes providing specific information about the structure and behavior of molecules. However, these passive measurement methods rely on extremely faint signals that can be difficult to isolate in the recorded spectra, which limits their use in many real-world conditions.
The research team, led by Ishan Barman at Johns Hopkins University, sought to devise a new solution that actively enhances and manipulates the signals generated by molecular vibrations. “Rather than trying to incrementally improve conventional methods, we asked whether we could re-engineer the very way light interacts with matter to create a fundamentally new kind of sensing,” Barman commented.
Their idea was to enclose an ensemble of the target molecules within an optical cavity formed by a pair of gold mirrors. Strong interactions are then generated between one of the cavity modes and a specific vibrational transition, which in turn created hybrid light–matter states that produce distinct peaks in the spectral response.
Numerical simulations by the team show how the presence of the molecules splits the single cavity mode into a pair of hybridized quantum states, which the researchers call “vibro-polaritons.”
Emerged hybrid states
Numerical simulations by the team show how the presence of the molecules splits the single cavity mode into a pair of hybridized quantum states, which the researchers call “vibro-polaritons.” Experiments confirmed the emergence of these hybrid states when a thin polymer film was enclosed within the cavity and when molecules in solution were injected into a cavity that had been fabricated within a microfluidic flow cell.
The researchers also demonstrated that the formation of hybrid states could be used as an indicator of molecular concentration. While the transmission spectra at low concentrations are dominated by the normal cavity modes, the coupling at higher concentrations becomes strong enough to create hybrid states that can be detected in the spectral response.
The transition to this stronger coupling regime occurs at concentrations some three times lower than is needed to detect the same molecular signal using conventional infrared spectroscopy, demonstrating the practical utility of the technique. Even so, the concentrations used in this proof-of-concept study are higher than those typically encountered in molecular sensing. Theoretical analysis suggests that the detection limit of the technique could be significantly reduced, which Barman and colleagues believe could be achieved by optimizing the design of the optical cavity.