Vibrational strong coupling is a phenomenon in which a vibrational transition in a material placed inside a photonic structure is hybridized with its optical modes to form composite light–matter excitations known as vibro-polaritons. Here we demonstrate a new concept of vibrational strong coupling: we show that a monolithic photonic crystal, made of a resonant material, can exhibit strong coupling between the optical modes confined in the structure and the terahertz vibrational excitations of the same material. We study this system both experimentally and numerically to characterize the dispersion of the photonic modes for various sample thicknesses and reveal their coupling with the vibrational resonances. Finally, our time-domain THz measurements allow us to isolate the free induction decay signal from the grating modes as well as from the vibro-polaritons.

Rayleigh scattering is usually considered to be the elastic scattering of photons from subwavelength physical objects, such as small particles or molecules. Here, we present the spectroscopic study of the scattering properties of molecules embedded in an optical cavity under strong coupling conditions, where the collective interaction between the molecules and the cavity gives rise to composite light-matter excitations known as cavity polaritons. We show that the polaritonic states exhibit strong resonant Rayleigh scattering, reaching ∼ 25% efficiency. Since the polaritonic wavefunctions in such systems are delocalized, our observations correspond to the collective scattering of each photon from a large ensemble of molecules.When quantum emitters are placed inside an optical cavity, their interaction with the quantized electromagnetic mode of the cavity may become large enough to overcome the incoherent processes taking place in the system [1]. This regime, which is known as strong light-matter coupling, has been extensively studied in hybrid molecular-photonic systems over the past decade, as it offers exciting possibilities for controlling the photophysical and chemical properties of molecules [2,3]. The coupling between an ensemble of molecules and an optical resonator is quantified by the vacuum Rabi frequency, which is roughly given byHere d is the transition dipole element of the molecules, is Planck's constant, ω c is the cavity resonance frequency, is the background dielectric constant inside the cavity, V c the cavity mode volume and N is the number of molecules inside the cavity. The √ N dependence of the coupling strength indicates that, under strong coupling conditions, the interaction between the molecules and the optical mode is collective. As a result, the eigenstates of the coupled system, known as cavity polaritons, represent a coherent superposition of a photon and a material excitation which is delocalized across a macroscopically large ensemble of molecules [1,4]. The collective nature of strong coupling in molecular systems is currently attracting considerable attention [5][6][7][8][9][10], as it gives rise to fascinating effects. These include, for instance, long-range spatial coherence [11][12][13], enhanced transport [14-18] and energy transfer [19][20][21], and even collective molecular reactivity [22][23][24][25].The progress in this evolving field is intimately linked to the extensive spectroscopic study of organic strongly coupled systems, which has gradually revealed the properties of the polari- * Corresponding author talschwartz@tau.ac.il

Engineering viscoelastic and biocompatible materials with tailored mechanical and microstructure properties capable of mimicking the biological stiffness (<17 kPa) or serving as bioimplants will bring protein-based hydrogels to the forefront in the biomaterials field. Here, we introduce a method that uses different concentrations of acetic acid (AA) to control the covalent tyrosine–tyrosine cross-linking interactions at the nanoscale level during protein-based hydrogel synthesis and manipulates their mechanical and microstructure properties without affecting protein concentration and (un)­folding nanomechanics. We demonstrated this approach by adding AA as a precursor to the preparation buffer of a photoactivated protein-based hydrogel mixture. This strategy allowed us to synthesize hydrogels made from bovine serum albumin (BSA) and eight repeats protein L structure, with a fine-tailored wide range of stiffness (2–35 kPa). Together with protein engineering technologies, this method will open new routes in developing and investigating tunable protein-based hydrogels and extend their application toward new horizons.

We demonstrate that strongly coupled organic microcavities exhibit strong resonant scattering at wavelengths corresponding to the hybrid polaritonic states. Interestingly, we observe that the scattering strength increases linearly with the photonic weight of the polaritons.

We demonstrate and study experimentally strong coupling in a monolithic, 1D terahertz photonic crystal, in which the guided modes are strongly coupled with the vibrational excitation of the organic material comprising the photonic structure.