This Review focuses on the integration of plasmonic and dielectric metasurfaces with emissive or stimuli-responsive materials for manipulating light–matter interactions at the nanoscale. Metasurfaces, engineered planar structures with rationally designed building blocks, can change the local phase and intensity of electromagnetic waves at the subwavelength unit level and offers more degrees of freedom to control the flow of light. A combination of metasurfaces and nanoscale emitters facilitates access to weak and strong coupling regimes for enhanced photoluminescence, nanoscale lasing, controlled quantum emission, and formation of exciton–polaritons. In addition to emissive materials, functional materials that respond to external stimuli can be combined with metasurfaces to engineer tunable nanophotonic devices. Emerging metasurface designs including surface-functionalized, chemically tunable, and multilayer hybrid metasurfaces open prospects for diverse applications, including photocatalysis, sensing, displays, and quantum information.

The ability for a magnetic field to penetrate biological tissues without attenuation has led to significant interest in the use of magnetic nanoparticles for biomedical applications. In particular, active research is ongoing in the areas of magnetically guided drug delivery and magnetic hyperthermia treatment. However, the difficulties in tracing these optically nonactive magnetic nanoparticles hinder their usage in medical research or treatment. Here, a new perovskite‐based magneto‐fluorescent nanocomposite that allows the in situ, real‐time optical visualization of magnetically induced cellular movements is reported. A swelling–deswelling technique is employed to capture a cesium lead halide perovskite and magnetite nanoparticles within a biocompatible polyvinylpyrrolidone matrix, to produce a water‐dispersible composite that possesses a combination of strong magnetic response and intense fluorescence. The wavelength‐tunability of perovskite nanocrystals is taken advantage of to demonstrate simultaneous multicolor fluorescent tagging of cancer stem cells. The magneto‐directed motion of the cancer stem cells through a microfluidic channel is also imaged as a proof‐of‐concept toward an optically traceable magnetic manipulation of biological systems. These dual‐functional nanocomposites could find promising applications in advanced biotechnologies, such as in optogenetics, cellular separation, and drug delivery studies.

Solution-based optical amplification affords a host of benefits ranging from flexibility in the choice of cavity size and shape to high photostability afforded by the constant replenishment of gain media. Works reporting solution-based optical amplification in colloidal semiconductor nanocrystals, however, remain sparse due to the difficulty in achieving high particle number densities required for sustained optical gain. In this work, we demonstrate highly stable amplified spontaneous emission (ASE) from a solution of green-emitting CsPbBr3 perovskite nanocrystals dispersed in a nonpolar solvent after a facile postsynthesis processing step. This processing step not only allows for the purification of nanocrystals from their growth solution, but also allows for long-term colloidal stability at high particle concentrations. Although it is widely reported that perovskite nanocrystals suffer from poor chemical stability, our nanocrystal solutions retain their ASE properties despite long-term storage in excess of five months under ambient conditions. Photostability tests show steady ASE intensities in excess of three hours under constant photoexcitation from a femtosecond pulsed laser beam (>107 shots), far exceeding those of thin films by an order of magnitude. This work opens the possibility of harnessing colloidal CsPbBr3 nanocrystals as highly robust, solution-based optical gain media.

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Solution‐processed optical gain media such as thin films of colloidal semiconductor nanocrystals promise ease of fabrication and scalable production while offering a spectrally wide range of emission colors. However, depositing such gain media in a size‐ and shape‐specific manner at a precise location on a substrate can be highly challenging. In this work, a facile approach for fabricating solution‐processed cesium lead halide perovskite structures of any arbitrary shape and size from their nanocrystal counterparts via a pulsed laser photopatterning process is demonstrated. The photopatterned structures resist solvation in both polar and nonpolar solvents, allowing for the straightforward removal of unpatterned regions. Their robustness is attributed to the ligand‐removal, sintering, and photoannealing of the nanocrystals at the site of irradiation. Concomittantly, the photopatterning process results in lengthened Auger‐dominated biexciton lifetimes and larger absorption cross‐sections that permit relatively low amplified spontaneous emission thresholds. It is shown that the photopatterning process may be used to fabricate cesium lead halide based gain media capable of multiwavelength emission, orientation‐dependent wavelength of emission, as well as functional operation while fully immersed in various solvents. It is envisioned that the photopatterning process may be extended to other perovskite systems to include applications beyond those requiring light‐emission.