Terahertz cavity hybridization of collective proteins vibrations

Abstract

Hybrid light-matter states have transformed photonics, yet their realization with driven collective vibrations in biological systems remains an open challenge. Here we show that optically pumped R-phycoerythrin proteins at room temperature support coherent sub-terahertz vibrational modes consistent with Frohlich condensation, and that these modes hybridize with confined terahertz cavity photons in a microfluidic cavity platform. The resulting spectra exhibit a resolved doublet, power- and concentration-dependent redistribution of spectral weight, and linewidth narrowing indicative of cavity-modified dissipation. Quantitative analysis reveals collective square-root of N-scaling of the coupling strength, with cooperativity and splitting-to-linewidth ratios exceeding unity, consistent with the onset of strong collective coupling driven by the vibrational molecular mode. A microscopic nonequilibrium analysis further indicates that the relaxation timescale toward the Frohlich polariton state is on the order of 1-10 microseconds. These findings identify terahertz cavities as a platform for stabilizing and controlling collective molecular vibration dynamics and open opportunities for cavity-engineered vibrational spectroscopy, label-free biosensing and photonic control of energy transport in complex biomolecular systems.

Figures (8)

• Figure 1: Terahertz microcavity platform for collective vibrational light–matter coupling.(a) Schematic illustration of the experimental configuration used to probe optically driven R-phycoerythrin proteins embedded in a THz microcavity. (b) Simulated electromagnetic field distribution within the cavity geometry. Color-shaded areas corresponds to the protein bath (pink), cavity (grey) and detector (gold) postilions, respectively. (c) Normalized THz transmission spectra, recorded at 55 mW optical excitation, for protein concentrations (from top to bottom) of 2.0, 5.0, 7.2 and 10 $\mu$M, under cavity coupling, revealing concentration-dependent mode evolution and hybridization.
• Figure 2: Concentration-dependent vibrational--cavity coupling dynamics. (a--d) Resonance frequencies (extracted form transmission spectra) as a function of laser power, at protein concentrations of 2.0, 5.0, 7.2 and 10 $\mu$M. For same concentration: (e--h) Integrated spectral areas of the vibrational and hybrid modes versus excitation power. (i--l) Mode amplitudes as a function of laser power. Color code: bare Fröhlich condensate (black), lower hybrid branch $\omega_{-}$ (open circles), upper hybrid branch $\omega_{+}$ (fill circles).
• Figure 3: Quantitative signatures of collective vibrational–cavity coupling. (a) Spectral line $\Gamma$ narrowing. Bare FC mode ($\Gamma_{\mathrm{p}}$, black stars), $\omega_-$ branch ($\Gamma_-$, empty circles) and $\omega_+$ branch ($\Gamma_+$, fill circles). (b) Collective coupling criteria $\mathcal{C} = 4~g_{coll}^2/\kappa \gamma$ variation satisfying the $\sqrt{N}$-scaling law for concentration lower than 10 $\mu$M; before deviating. (c) SLR coefficient $\Omega/(\gamma + \kappa)\geq 1$. All results are given for concentration ranging from 2-15 $\mu$M.
• Figure 4: Power-dependent evolution of hybrid vibrational–photonic modes . (a) Evolution of normalized transmission spectra at C = 10 $\mu$M with optical laser-powers. (b) Amplitude ratio $A_{-} / A_{+}$ of the two hybridized states as a function of laser power. Stars are experimental data for concentration $C=7$ and 10 $\mu$M. Dashed lines are obtained from $\langle n\rangle/\langle N\rangle$ in Eqs.(\ref{['nd']},\ref{['Nd']}). Notice that the two lines share the parameters $\alpha, M, k$.
• Figure S.1: Frequency–air-gap maps showing resonance trajectories as $d_{gap}$ varies. (a) Readout amplitude $|A|$ at Si-FET THz-detector position. (b) Field-magnitude at the water interface. (c) Total reflectance.