Coherent Coupling between a Molecular Vibration and Fabry–Perot Optical Cavity to Give Hybridized States in the Strong Coupling Limit

Abstract

The coherent coupling between an optical transition and a confined optical mode, when sufficiently strong, gives rise to a new pair of mixed modes separated in frequency by the vacuum Rabi splitting. Such systems have been widely investigated for electronicstate transitions such as molecular excitons coupled to surfaceplasmons and optical microcavities. However, only very recently have vibrational transitions been considered. Here we experimentally investigate the coupling between a Fabry−Perot cavity and the carbonyl stretch at an infrared frequency near 1730 cm −1 in polymethyl methacrylate. As is requisite for the "strong coupling" regime, the measured vacuum-Rabi-splitting of 132 cm −1 is much larger than the full width of either the cavity resonance (34 cm −1 ) or the inhomogeneously broadened carbonyl-stretch absorption (24 cm −1 ). With the assistance of quantitative analysis using transfermatrix methods, we provide evidence that the mixed-state resonances are relatively immune to inhomogeneous vibrational broadening and demonstrate the ability to extract splittings by convenient angle tuning of the Fabry−Perot cavity to match the vibrational frequency. Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied both statically (as here) and dynamically with ultrafast methods. Moreover, microfluidic cavities will permit the study of liquids, greatly expanding the range of assessable molecules. W hen an optical transition (e.g., a molecular excitonic absorption) is excited by a confined optical-mode oscillating at the same frequency (e.g., an optical microcavity), a strong coupling between the two resonances can occur. This coupled-oscillator system exhibits a hybridization of the two resonances to create two new eigenstates of mixed character that are each shifted from the original resonant frequency by half the vacuum Rabi splitting, Ω. Such coupled systems are variously referred to as cavity polaritons, 1 normal coupled modes, 2 more specific terms (e.g., plexitons 3 ), or simply coupled-modes. These coupled-mode systems have received widespread attention in part because the split states possess new properties such as immunity to inhomogeneous broadening 4 or altered relaxation pathways. 5,6 The coupled systems can have marked implications for emission and relaxation rates and, accordingly, have found utility in the pursuit of nanoscale organic lasing materials 7 and nearly threshold-less lasers. 8,9 Cavities strongly coupled to single optical-emitters have also been considered for use in quantum information systems. 10−13 To date, a wide variety of strategies spanning microwave to visible frequencies have been employed or proposed to produce the confined-mode, including surface-plasmon polaritons propagating on films 14,15 and on nanowires, 16 local surface plasmons, 17,18 Fabry−Perot cavities, 8 whispering gallery cavities, 11 photonic crystals, 10,19 and microwave cavities. The species coupled to...

Modified relaxation dynamics and coherent energy exchange in coupled vibration-cavity polaritons

“…Analysing the cavity-coupled results requires a more sophisticated approach than outside the cavity. Briefly, we use an analytical expression for FP cavity transmission, T cav , which treats the oscillators as an effective medium to calculate transient spectra with excited-state population152743.…”

“…The independent works by Shalabney, et al 26. and Long and Simpkins27 report Fabry–Pérot (FP) cavity modes coupled to vibrational resonances in polymer films and were rapidly extended to vibrational polaritons in molecular liquids28 and small molecules in solution29. Vibrational strong coupling has also recently been the subject of theoretical treatments by del Pino et al 3031.…”

Spanning Strong to Weak Normal Mode Coupling between Vibrational and Fabry–Pérot Cavity Modes through Tuning of Vibrational Absorption Strength

“…15,16,17,18 While strong normal mode coupling has been widely investigated for electronic-state transitions, it is only very recently that cavity coupling to vibrational transitions were proposed and experimentally realized. 19,20,21 Work in this field has compared measured Rabi splittings to classical 19 and quantum mechanical descriptions 21 and the work by George et al 22 greatly extended the experimental landscape by utilizing molecular liquids and higher order cavity modes. Vibrational modes are particularly interesting because they involve nuclear motion along coordinates related to bond 3 formation and cleavage.…”

Strong light‐matter coupling between a molecular vibrational mode in a PMMA film and a low‐loss mid‐IR microcavity

“…The combination of high index contrast from the Ge layer, relative to PMMA, and linewidth matching between cavity and organic layer lead to an observed Rabisplitting for a DBR-based cavity that is on-par with the splitting previously reported in metallic cavities [19,20,22].…”

“…Very recently, it was shown that strong light-matter coupling can be achieved in the mid-IR using dipole-allowed vibrational transitions associated with specific chemical bonds in organic solids [19][20][21] and liquids [22]. These are essentially molecularly-localized phonons of the material; hence the resulting strongly-coupled states can be considered as collective microcavity-based molecular phonon-polaritons.…”