Continuously-tunable light-matter coupling in optical microcavities with 2D semiconductors

Purcell gain equalized zero-mode waveguide

We refresh the design of zero-mode waveguides (ZMWs) by introducing metamaterials that makes the zeroth order resonant mode existence. Of particular importance, the resulting electromagnetic field exhibits nearly constant distribution but not a trivial solution of Maxwell's equation, showing great advantage to equalize the excitation rate of molecules throughout the waveguides. A closed form expression for the wave impedance is derived which is verified by the finite-difference time-domain simulations. Benefitted from the cavity Purcell effect which is lacking in existing ZMWs, fluorescence amplification and lifetime reduction are simultaneously enhanced. A practical design where the excitation volume reduced down to sub-zeptoliter and the fluorescence lifetime shortened to picosecond scale is illustrated. This result makes single molecule real time (SMRT) sensing of biochemical reactions at biophysically relevant concentrations (~ μM) possible, combining off-the-shelf ultrafast lasers.

Vibrational Strong Light-Matter Coupling in an Open Microcavity Based on Reflective Germanium Coatings

Open microcavities (OMCs) enable tuning of the optical resonances of a system and insertion of different materials between the mirrors. They are of large scientific interest due to their many potential applications. Using OMCs, we can observe strong light-matter coupling while tuning the cavity wavelength. Typically, dielectric Bragg reflectors (DBRs) and Au mirrors are used to form microcavities and observe vibrational strong coupling (VSC) in the middle-infrared (MIR) spectral region. Here, we make the mirrors of the OMC using thin film coatings of the semiconducting material germanium (Ge) and demonstrate VSC in the MIR region. We deposited a uniform coating of poly(methyl methacrylate) (PMMA) on one of the OMC mirrors' inner surfaces, and then we tuned the cavity to the carbonyl stretch mode resonance at 1731 cm^-1. Comparing VSC using Ge mirrors to DBRs or Au mirrors, we achieve enhanced optical transmission through the polaritonic resonances and large Rabi splitting, with Rabi-splitting values of 8.8 meV for the Ge mirror-based OMC compared to 7.0 and 7.4 meV for the DBR- and Au-based microcavities, respectively. The use of Ge mirror components can simplify the microcavity structure and offer a new and simple alternative for MIR semiconductor mirrors, which may be particularly useful for polariton chemistry applications.

Microcavity phonon polaritons from the weak to the ultrastrong phonon-photon coupling regime

Strong coupling between molecular vibrations and microcavity modes has been demonstrated to modify physical and chemical properties of the molecular material. Here, we study the less explored coupling between lattice vibrations (phonons) and microcavity modes. Embedding thin layers of hexagonal boron nitride (hBN) into classical microcavities, we demonstrate the evolution from weak to ultrastrong phonon-photon coupling when the hBN thickness is increased from a few nanometers to a fully filled cavity. Remarkably, strong coupling is achieved for hBN layers as thin as 10 nm. Further, the ultrastrong coupling in fully filled cavities yields a polariton dispersion matching that of phonon polaritons in bulk hBN, highlighting that the maximum light-matter coupling in microcavities is limited to the coupling strength between photons and the bulk material. Tunable cavity phonon polaritons could become a versatile platform for studying how the coupling strength between photons and phonons may modify the properties of polar crystals.

Strong coupling between light and matter can be engineered to influence their properties and behaviour. Here, the authors demonstrate the evolution from weak to ultrastrong coupling of microcavity modes and optical phonons with hexagonal boron nitride layers in a Fabry-Perot resonator.

Direct Transition from Triplet Excitons to Hybrid Light-Matter States via Triplet-Triplet Annihilation

Strong light–matter coupling generates hybrid states that inherit properties of both light and matter, effectively allowing the modification of the molecular potential energy landscape. This phenomenon opens up a plethora of options for manipulating the properties of molecules, with a broad range of applications in photochemistry and photophysics. In this article, we use strong light–matter coupling to transform an endothermic triplet–triplet annihilation process into an exothermic one. The resulting gradual on–off photon upconversion experiment demonstrates a direct conversion between molecular states and hybrid light–matter states. Our study provides a direct evidence that energy can relax from nonresonant low energy molecular states directly into hybrid light–matter states and lays the groundwork for tunable photon upconversion systems that modify molecular properties in situ by optical cavities rather than with chemical modifications.