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

Abstract

Designing coupled vibrational-cavity polariton systems modify chemical reaction rates and paths requires an understanding of how this coupling depends on system parameters (i.e. absorber strength, modal distribution, and vibrational absorber and cavity linewidths). Here, we evaluate the impact of absorption coefficient and cavity design on normal mode coupling between a FabryPérot cavity and a molecular vibration. For a vibrational band of urethane in a polymer matrix, the coupling strength increases with its concentration so that the system spans the weak and strong coupling regimes. The experimentally-determined Rabi splitting values are in excellent agreement with an analytical expression derived for classical coupled oscillators that includes no fitting parameters. Also, the cavity mode profile is altered through choice of mirror type with metal mirrors resulting in stronger confinement, and thus coupling, while dielectric stack mirrors provide higher transmission for a given cavity quality factor, and decreased coupling due to greater mode penetration into the dielectric mirror. In addition to polymers, the cavities can couple to molecular vibrational bands of dissolved species in solution, which greatly expands the range of systems that can be explored. Finally, longer pathlength cavities are used to demonstrate the pathlength-independence of the coupling strength. The ability to adjust the cavity linewidth, through the use of higher order modes, represents a route to match the cavity dephasing time to that of the molecular vibration and may be applied to a range of molecular systems. Understanding the roles of cavity design and validating empirical and analytical descriptions of absorber properties on coupling strength will facilitate application of these strong coupling effects to enable currently unreachable chemistries.Coupling between an optically-driven material excitation (e.g., a semiconductor quantum dot excitonic absorption) and a confined optical-mode (e.g., an optical microcavity) can drastically alter the behavior of both modes. Strong coupling, which occurs when the two modes are in resonance and the interaction between the two exceeds the damping rate, produces two hybridized states whose fundamental character is a quantum-mechanical superposition of the two original modes. Each of these mixed-character eigenstates is shifted from the original resonant frequency by half the Rabi splitting, Ω. 1 Such coupled systems are variously referred to as cavity polaritons, 2 coupled normal modes, 1 plexcitons 3 , or simply coupled normal modes. If the system consists of a single quantum oscillator (twolevel atom, single quantum dot, etc.) interacting with a single cavity photon, nonlinear effects such as photon blocking and climbing of the Jaynes-Cummings ladder may occur. The splitting in such a system is referred to as vacuum Rabi splitting 1,4,5 as opposed to simply Rabi or polariton splitting associated with ensembles of individual oscillators, as described herein. Relaxation lifetimes, 6,7 linewidt...

Two-dimensional infrared spectroscopy of vibrational polaritons of molecules in an optical cavity

“…Vibrational polaritons were experimentally reported 6,8,9 and calculated 7,25 recently. Multidimensional spectroscopic studies for electronically excited states in semiconductor cavity for nano-particles like In 0.04 Ga 0.96 As has been demonstrated 12,27 .…”

“…Electronic polaritons in molecules have been extensively studied both experimentally and theoretically [3][4][5] . Vibrational polaritons in the infrared has been recently demonstrated in molecular aggregates [6][7][8][9] and in semiconductor nano-structures [10][11][12] . The radiation matter coupling is related to cavity frequency ω c by, g i = √ N µ i · e c ( ω c /2 0 V ), where, N is the number of molecules.…”

Cavity Femtochemistry: Manipulating Nonadiabatic Dynamics at Avoided Crossings

“…12 Its potential application to molecular cooling, 13,14 as a tool to probe larger molecules, 15,16 to enhance vibrational spectra, [17][18][19][20] for use with electromagnetically induced transparency, 21 and to expedite cavity-modified photo chemistry 22,23 have been objects of intensive studies. Strong cavity coupling in molecular systems has been demonstrated recently for electronic transitions 24 and for vibrational transitions.…”

Discriminating cascading processes in nonlinear optics: A QED analysis based on their molecular and geometric origin

“…The n-body cascading terms similarly behave like vertex insertions with 2n free branches. An interesting future extension of this work would be to consider manipulating the cascading signal from a system of molecules embedded in an optical cavity, systems which have drawn recent interest [39][40][41][42][43][44]. Optical cavities alter the density of electromagnetic field modes from its free-space value, suppressing cascading in all but the cavity mode.…”