Real-time monitoring of singlet–triplet transitions is an effective tool for studying room-temperature phosphorescent molecules. For femtosecond transient absorption (TA) spectroscopy of a 2,6-di­(9H-carbazol-9-yl) pyridine molecule in dimethyl sulfoxide (DMSO), the stimulated emission signal (380 nm) and the excited-state absorption signal (650 nm) reach their maximum intensity within 397 fs. Subsequently, the two signals decay with time and the triplet–triplet absorption (TTA) signal (400 nm) is enhanced synchronously, accompanied by an isosbestic point at 491 nm. These results confirm intersystem crossing (ISC) within 2.5 ns. Moreover, the TTA signal (400 nm) in nanosecond TA spectroscopy gradually disappeared, accompanied by a phosphorescence lifetime of 4.1 μs. As the solvent polarity decreases (DMSO > N,N-dimethylformamide > 1,4-dioxane > toluene), similar spectral dynamic processes are observed, while the durations of ISC processes and phosphorescence lifetimes are shortened. This combined femtosecond and nanosecond transient absorption spectroscopy study presents the ultrafast excited-state dynamics of organic phosphorescent molecules.

Resonant dielectric metasurfaces have been demonstrated to hold a great promise for manipulation of light-wave dispersion at the nanoscale due to their resonant photonic environment and high refractive index. However, the efficiency of devices based on dielectric nanostructures is usually limited by the quality (Q) factor of their resonant modes. The physics of the bound sates in the continuum (BICs) provide an elegant solution for control over the Q factor of resonant modes. Here, by engineering the substrate of Si-based metasurfaces, we demonstrate two eigenmodes that exhibit an intrinsic magnetic dipole character and have an infinite radiation lifetime. We reveal that they are characterized by in-plane and out-of-plane magnetic dipole modes and respectively correspond to two groups of BICs, that is, Fabry–Pérot BICs and symmetry-protected BICs. Using temporal coupled-mode theory and numerical simulations, we show that these BIC modes can transform into high-Q quasi-BIC resonances with near-unity absorption under normal incidence through tuning structural parameters. Our work provides a promising route to use BIC-inspired metasurfaces for designing ultra-narrowband absorbers which can be used as absorption filters, photodetectors, and sensors.

Solar energy is a clean and renewable energy source and solves today's energy and climate emergency. Near-perfect broadband solar absorbers can offer necessary technical assistance to follow this route and develop an effective solar energy-harvesting system. In this work, the metamaterial perfect absorber operating in the ultraviolet to the near-infrared spectral range was designed, consisting of a periodically aligned titanium (Ti) nanoarray coupled to an optical cavity. Through numerical simulations, the average absorption efficiency of the optimal parameter absorber can reach up to 99.84% in the 200–3000 nm broadband range. We show that the Ti pyramid's localized surface plasmon resonances, the intrinsic loss of the Ti material, and the coupling of resonance modes between two neighboring pyramids are highly responsible for this broadband perfect absorption effect. Additionally, we demonstrate that the absorber exhibits some excellent features desirable for the practical absorption and harvesting of solar energy, such as precision tolerance, polarization independence, and large angular acceptance.

Infrared selective emitters are attracting more and more attention due to their modulation ability of infrared radiance, which provides an efficient ability to blend objects into the surrounding environment. In this paper, an Ag/ZnS/Si/Ag/Si multilayered emitter is proposed by virtue of impedance matching as well as Fabry-Perot cavity effect to achieve selective radiation in the infrared band. The emissivity of the fabricated selective emitter is measured to be ε3–5μm = 0.16 and ε8–14μm = 0.23 in the atmosphere windows, respectively, meeting the requirements of infrared stealth. Meanwhile, the emissivity at the non-atmospheric window (5–8 μm) is as high as 0.78, which allows efficient heat dissipation to achieve radiative cooling. Furthermore, the selective emitter maintains excellent stealth performance until 350 °C, indicating its good heat resistance and dissipation at medium temperature. The proposed emitter with spectral selectivity provides a new strategy for the facile fabrication of mid-/low-temperature infrared stealth devices.

Self-coupled photonic resonators made of exciton materials have recently provoked great interest in the context of light–matter interactions due to their ability to produce large normal mode splittings. In order to obtain giant Rabi energy, it is rather necessary to ensure large electromagnetic fields within exciton materials. Here, using two independent numerical algorithms, namely, the finite-element method and the rigorous coupled wave analysis, we demonstrate that, even with a moderate oscillation strength, giant Rabi splittings in excess of 250 meV can be achieved in subwavelength perovskite-based photonic crystals. This can be attributed to the fact that quasi-guided resonance modes supported by photonic systems are strongly confined inside the exciton material, highly conducing to increasing the volume of light-matter interaction. We reveal how the oscillator strength of excitons and the thickness of perovskite photonic crystals influence photon–exciton couplings. Moreover, the perovskite nanostructures investigated allow us to engineer polaritonic dispersions with linear or slow-light characters. These findings show that perovskite-based photonic crystals could be an appealing and promising platform in realizing polaritonic devices.