Polaritonic & plasmonic control
Tunable plasmonic metasurfaces for vibrational coupling
Using self-assembled doped-oxide nanocrystal metasurfaces, tunable across the mid-infrared, as an open-faced alternative to high-Q microcavities for coupling light to molecular vibrations — and using broadband 2D IR to measure what that coupling does to molecular dynamics.
The appeal of the metasurface is that its resonance is a synthetic parameter. Changing nanocrystal doping and packing moves the collective plasmon continuously across the mid-infrared, so the coupling condition can be tuned to a target vibration rather than the target being chosen to fit an available cavity. Because the surface is open, the molecular film sits directly on it and mass transport is unobstructed.
The result that surprised me is that the effect does not wait for strong coupling. In the lossy Fano regime — where the metasurface linewidth exceeds the coupling strength and no polariton bands are resolved — the polymer’s structural dynamics still accelerate by up to sevenfold, and energy transport acquires a direction it does not have on a bare substrate. Whatever mechanism is operating is not contingent on the clean strong-coupling picture.
That matters for how I plan to approach the contested rate experiments. If a lossy, weakly coupled metasurface can change vibrational dynamics this much, then distinguishing genuine resonant strong-coupling effects from more mundane near-field and thermal effects requires exactly the detuning controls and internal comparisons that the reproducibility critiques of this field have called for.
- Scientific motivation
- Nearly all vibrational strong coupling chemistry has used high-Q dielectric microcavities. Their narrow mode volume and limited tunability restrict which molecules and conditions can be studied, and they provide no direct window on dynamics — only on the rate that emerges at the end. Plasmonic metasurfaces are open-faced, broadly tunable and deeply sub-wavelength, with local fields large enough that the same enhancement producing the coupling also amplifies the third-order signal used to read it out.
- The research question
- Can a lossy plasmonic metasurface measurably alter the structural dynamics, vibrational coherence and energy transport of a molecular film coupled to it — and does that hold even outside the strong-coupling regime?
- My contribution
- I designed and performed the broadband 2D IR measurements on polymer films coupled to indium tin oxide nanocrystal metasurfaces, extracted the structural dynamics and coherence lifetimes, and identified the directional energy-transport signature. Nanocrystal synthesis and metasurface fabrication, and the theoretical treatment, were contributed by collaborators.
- Results and current status
- Even in the lossy, weak-coupling Fano regime, a plasmonic metasurface accelerates the interfacial vibrational dynamics of an overlying poly(methyl methacrylate) film by up to sevenfold, reshapes vibrational coherence, and opens directional energy-transport channels. This establishes tunable plasmonic metasurfaces as a scalable, chemically agnostic platform for engineering vibrational dynamics — the coupling does not require a high-finesse cavity or a specific molecular system to produce a large, measurable effect.
- Future direction
- Engineer nanoantenna arrays and surface-lattice-resonance structures that reach genuine vibrational strong coupling of a target vibration, and establish unambiguous signatures — anticrossing, concentration-scaling of the Rabi splitting, resolved polariton bands — cross-validated by surface-enhanced 2D IR. This platform bridges the high-Q microcavity and lossy-cavity regimes and gives direct dynamical readout of polariton formation, which is the missing measurement in most reported cavity chemistry.