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Strong exciton–photon coupling results in the formation of hybrid light–matter states whose relaxation dynamics often experience a bottleneck due to a large manifold of dark states. This bottleneck results in dynamics like those of the uncoupled material, hampering the observation and utilization of polaritonic properties. A possible solution to this problem is to use cavities with fewer dark states, suggesting a need for an increased understanding of the influence of the cavity platform on the polariton photophysics. Here, we use pump-probe spectroscopy to compare Fabry–Perot cavities and plasmonic nanorods coupled under identical conditions and analyze how the cavity influences relaxation kinetics. Fabry–Perot cavities host a large number of dark states, leading to dynamics fully dominated by the bottleneck. However, the relaxation from polaritonic states in nanorod systems is distinctly different compared to the uncoupled materials, with a sub-100 fs decay that dominates the kinetics. This fast relaxation emerges only when selectively exciting within the polaritonic region: off-resonance excitation yields plasmonic hot-carrier dynamics. These findings show that the cavity critically defines polariton relaxation. Moreover, this work suggests that plasmonic cavities can operate as either polaritons or plasmons, offering tunable functionality for applications in, for instance, polaritonic chemistry.

We investigated ultrafast defect-lattice dynamics in diamond using the N_s:±á−C⁰ defect, an analog of bond-centered hydrogen in semiconductors. Combining synthesis, ultrafast vibrational spectroscopy, and ab initio calculations, we show that excitation of the defect’s stretch mode leads to the generation of localized phonons and the formation of a hot ground state, where the interatomic potential is transiently modified. Our results reveal unexpected nonequilibrium phonon effects despite diamond’s exceptionally high thermal conductivity, with implications for quantum defect engineering.

We report on the terahertz (THz) harmonic generation in an active graphene/metamaterial device by using powerful ultrafast table-top THz time-domain spectroscopic systems. Complex nonlinear transmission spectra, comprising even and odd harmonics, emerge when the devices are tested with intense ultrafast THz pulses with peak electric fields in the range of 1–150 kV/cm. The odd and even harmonic features show a positive correlation with carrier concentration, allowing for efficient frequency tuning on top of tunable group delay dispersion. Interestingly, before the onset of saturation, observed at 20–25 kV/cm, the main resonance and the harmonic features exhibit an anti-crossing trend that can be further exploited for active frequency tuning. These results report a key milestone for the fundamental investigation of the nonlinearity of 2D materials. At the same time, they represent an important advance in the design of future integrated THz optoelectronics by providing novel functionalities for THz light generation and manipulation.

Monolayer molybdenum disulfide (1L MoS2), a promising optoelectronic material, emits strong visible photoluminescence (PL). Systematic control of the intensity, energy, and spectral width of PL from 1L MoS2 on silicon dioxide/silicon (SiO2/Si) is demonstrated via simple external treatments. Treating MoS2 with solutions formed from the superacid bis-(trifluoromethanesulfonyl)amide (TFSA) enhances, blueshifts, and sharpens the PL. Treatments with solutions from structurally analogous chemicals that lack sulfur, in the case of bis(trifluoroacetamide) (BTFA), or lack fluorine, in the case of methanesulfonamide (MSA), show the same trend, suggesting a two-component mechanism for TFSA involving the presence of electronegative species and sulfur vacancy passivation. Up to ≈100× enhancement of the PL intensity is achieved, with the peak blueshifted by ≈30 meV and the spectral linewidth halved. Conversely, direct thermal atomic layer deposition (ALD) of aluminum oxide (Al2O3) or hafnium oxide (HfO2) is found to suppress the PL by up to a factor of ≈3, redshift by up to ≈70 meV, and broaden by ≈3×. Single-spot and mapping Raman/PL techniques are combined in a robust characterization process to associate changes in the PL character to charge doping. This work demonstrates the convenient tunability of the optical behavior of 1L MoS2 by varying the electron density.