A new class of laser, which harnesses coherence in both light and atoms, is possible with ultracold alkaline-earth-metal atoms trapped in an optical lattice inside an optical cavity. Different lasing regimes, including superradiance, superradiant, and conventional lasing, are distinguished by the relative coherence stored in the atoms and in the cavity mode. We analyze the physics in two different experimentally achievable regions of the superradiant lasing regime. Our calculations confirm the narrow linewidth of superradiant lasing for the doubly forbidden clock transition ${}^3{P}_0\to {}^1{S}_0$ of ${}^{87}\mathrm{Sr}$ atoms. Under strong driving of the dipole-forbidden transition ${}^3{P}_1\to {}^1{S}_0$ of ${}^{88}\mathrm{Sr}$ atoms, the superradiant linewidth narrows further due to the coherent excitation of the cavity field.

The quest for solar-blind photodetectors with outstanding optoelectronic properties and weak signals detection capability is essential for their applications in the field of imaging, communication, warning, etc. To date, Ga₂O₃ has demonstrated potential for high-performance solar-blind photodetectors. However, the performance usually decays superlinearly at low light intensities due to carrier-trapping effect, which limits the weak signal detection capability of Ga₂O₃ photodetectors. Herein, a Ga₂O₃ solar-blind photodetector with ultra-thin absorbing medium has been designed to restrain trapping of photo-generated carriers during the transporting process by shortening the carrier transport distance. Meanwhile, multiple-beam interference is employed to enhance the absorption efficiency of the Ga₂O₃ layer using an Al/Al₂O₃/Ga₂O₃ structure. Based on the ultra-thin absorbing medium with enhanced absorption efficiency, a 7 × 7 flexible photodetector array is developed, and the detectivity can reach 1.7 × 10¹⁵ Jones, which is among the best values ever reported for Ga₂O₃ photodetectors. Notably, the performance of the photodetector decays little as the illumination intensity is as weak as 5 nW/cm², revealing the capacity to detect ultra-weak signals. In addition, the flexible photodetector array can execute the functions of imaging, spatial distribution of light source intensity, real-time light trajectory detection, etc. Our results may provide a route to high-performance solar-blind photodetectors for ultra-weak light detection.

Molecular vibrations couple to visible light only weakly, have small mutual interactions, and hence are often ignored for non-linear optics. Here we show the extreme confinement provided by plasmonic nano- and pico-cavities can sufficiently enhance optomechanical coupling so that intense laser illumination drastically softens the molecular bonds. This optomechanical pumping regime produces strong distortions of the Raman vibrational spectrum related to giant vibrational frequency shifts from an optical spring effect which is hundred-fold larger than in traditional cavities. The theoretical simulations accounting for the multimodal nanocavity response and near-field-induced collective phonon interactions are consistent with the experimentally-observed non-linear behavior exhibited in the Raman spectra of nanoparticle-on-mirror constructs illuminated by ultrafast laser pulses. Further, we show indications that plasmonic picocavities allow us to access the optical spring effect in single molecules with continuous illumination. Driving the collective phonon in the nanocavity paves the way to control reversible bond softening, as well as irreversible chemistry.

We simulate the superradiant dynamics of ensembles of atoms in the presence of collective and individual atomic decay processes. We apply the Monte-Carlo wave-function method and identify quantum jumps in a reduced Dicke state basis, which reflects the permutation symmetry of the system. While the number of density matrix elements in the Dicke representation increases polynomially with atom number, the quantum jump dynamics populates only a single Dicke state at the time and thus efficient simulations can be carried out for tens of thousands of atoms. The superradiant pulses from initially excited atoms agree quantitatively with recent experimental results of strontium atoms but rapid atom loss in these experiments does not permit steady-state superradiance. By introducing an incident flux of new atoms to maintain a large average atom number, our theoretical calculations predict lasing with a millihertz linewidth despite rapid atom number fluctuations.