Giant Ultrafast All-Optical Modulation Based on Exceptional Points in Exciton-Polariton Perovskite Metasurfaces.

Ultrafast all-optical modulation with optically resonant nanostructures is an essential technology for high-speed signal processing on a compact optical chip. Key challenges that exist in this field are relatively low and slow modulations in the visible range as well as the use of expensive materials. Here we develop an ultrafast all-optical modulator based on MAPbBr3 perovskite metasurface supporting exciton-polariton states with exceptional points. The additional angular and spectral filtering of the modulated light transmitted through the designed metasurface allows us to achieve 2500% optical signal modulation with the shortest modulation time of 440 fs at the pump fluence of ∼40 µJ/cm2. Such a value of the modulation depth is record-high among the existing modulators in the visible range, while the main physical effect behind it is polariton condensation. Scalable and cheap metasurface fabrication via nanoimprint lithography along with the simplicity of perovskite synthesis and deposition make the developed approach promising for real-life applications.

Perovskite Nanowire Laser for Hydrogen Chloride Gas Sensing.

Detection of hazardous volatile organic and inorganic species is a crucial task for addressing human safety in the chemical industry. Among these species, there are hydrogen halides (HX, X = Cl, Br, I) vastly exploited in numerous technological processes. Therefore, the development of a cost-effective, highly sensitive detector selective to any HX gas is of particular interest. Herein, we demonstrate the optical detection of hydrogen chloride gas with solution-processed halide perovskite nanowire lasers grown on a nanostructured alumina substrate. An anion exchange reaction between a CsPbBr3 nanowire and vaporized HCl molecules results in the formation of a structure consisting of a bromide core and thin mixed-halide CsPb(Cl,Br)3 shell. The shell has a lower refractive index than the core does. Therefore, the formation and further expansion of the shell reduce the field confinement for experimentally observed laser modes and provokes an increase in their frequency. This phenomenon is confirmed by the coherency of the data derived from XPS spectroscopy, EDX analysis, in situ XRD experiments, HRTEM images, and fluorescent microspectroscopy, as well as numerical modeling for Cl- ion diffusion and the shell-thickness-dependent spectral position of eigenmodes in a core-shell perovskite nanowire. The revealed optical response allows the detection of HCl molecules in the 5-500 ppm range. The observed spectral tunability of the perovskite nanowire lasers can be employed not only for sensing but also for their precise spectral tuning.

Polaron-Enhanced Polariton Nonlinearity in Lead Halide Perovskites.

Exciton-polaritons offer a versatile platform for realization of all-optical integrated logic gates due to the strong effective optical nonlinearity resulting from the exciton-exciton interactions. In most of the current excitonic materials there exists a direct connection between the exciton robustness to thermal fluctuations and the strength of the exciton-exciton interaction, making materials with the highest levels of exciton nonlinearity applicable at cryogenic temperatures only. Here, we show that strong polaronic effects, characteristic for perovskite materials, allow overcoming this limitation. Namely, we demonstrate a record-high value of the nonlinear optical response in the nanostructured organic-inorganic halide perovskite MAPbI3, experimentally detected as a 19.7 meV blueshift of the polariton branch under femtosecond laser irradiation. This is substantially higher than characteristic values for the samples based on conventional semiconductors and monolayers of transition-metal dichalcogenides. The observed strong polaron-enhanced nonlinearity exists for both tetragonal and orthorhombic phases of MAPbI3 and remains stable at elevated temperatures.

Incorporation of Perovskite Nanocrystals into Polymer Matrix for Enhanced Stability in Biological Media: In Vitro and In Vivo Studies.

The outstanding optical properties and multiphoton absorption of lead halide perovskites make them promising for use as fluorescence tags in bioimaging applications. However, their poor stability in aqueous media and biological fluids significantly limits their further use for in vitro and in vivo applications. In this work, we have developed a universal approach for the encapsulation of lead halide perovskite nanocrystals (PNCs) (CsPbBr3 and CsPbI3) as water-resistant fluorescent markers, which are suitable for fluorescence bioimaging. The obtained encapsulated PNCs demonstrate bright green emission at 510 nm (CsPbBr3) and red emission at 688 nm (CsPbI3) under one- and two-photon excitation, and they possess an enhanced stability in water and biological fluids (PBS, human serum) for a prolonged period of time (1 week). Further in vitro and in vivo experiments revealed enhanced stability of PNCs even after their introduction directly into the biological microenvironment (CT26 cells and DBA mice). The developed approach allows making a step toward stable, low-cost, and highly efficient bioimaging platforms that are spectrally tunable and have narrow emission.

Directional Lasing from Nanopatterned Halide Perovskite Nanowire.

Halide perovskite nanowire-based lasers have become a powerful tool for modern nanophotonics, being deeply subwavelength in cross-section and demonstrating low-threshold lasing within the whole visible spectral range owing to the huge gain of material even at room temperature. However, their emission directivity remains poorly controlled because of the efficient outcoupling of radiation through their subwavelength facets working as pointlike light sources. Here, we achieve directional lasing from a single perovskite CsPbBr3 nanowire by imprinting a nanograting on its surface, which provides stimulated emission outcoupling to its vertical direction with a divergence angle around 2°. The nanopatterning is carried out by the high-throughput laser ablation method, which preserves the luminescent properties of the material that is typically deteriorated after processing via conventional lithographic approaches. Moreover, nanopatterning of the perovskite nanowire is found to decrease the number of the lasing modes with a 2-fold increase of the quality factor of the remaining modes.

Halide Perovskite Nanocrystals with Enhanced Water Stability for Upconversion Imaging in a Living Cell.

Halide perovskite nanomaterials are widely used in optoelectronics and photonics due to their outstanding luminescent properties, whereas their strong multiphoton absorption makes them prospective for bioimaging. Nonetheless, instability of perovskites in aqueous solutions is an important limitation that prevents their application in biology and medicine. Here, we demonstrate fluorescence and upconversion imaging in living cells by employing CsPbBr3 nanocrystals (NCs) that show an improved water-resistance (at least for 24 h) after their coating as individual particles with various silica-based shells. The obtained phTEOS-TMOS@CsPbBr3 NCs possess high quality, which we confirm with high-resolution transmission and scanning transmission electron microscopy, X-ray diffraction analysis, Fourier-transform infrared and energy-dispersive X-ray spectroscopies, as well as with fluorescence optical microscopy. The developed platform can make the halide perovskite NCs suitable for various bioimaging applications.

Giant Enhancement of Radiative Recombination in Perovskite Light-Emitting Diodes with Plasmonic Core-Shell Nanoparticles.

The integration of nanoparticles (NPs) into functional materials is a powerful tool for the smart engineering of their physical properties. If properly designed and optimized, NPs possess unique optical, electrical, quantum, and other effects that will improve the efficiency of optoelectronic devices. Here, we propose a novel approach for the enhancement of perovskite light-emitting diodes (PeLEDs) based on electronic band structure deformation by core-shell NPs forming a metal-oxide-semiconductor (MOS) structure with an Au core and SiO2 shell located in the perovskite layer. The presence of the MOS interface enables favorable charge distribution in the active layer through the formation of hole transporting channels. For the PeLED design, we consider integration of the core-shell NPs in the realistic numerical model. Using our verified model, we show that, compared with the bare structure, the incorporation of NPs increases the radiative recombination rate of PeLED by several orders of magnitude. It is intended that this study will open new perspectives for further efficiency enhancement of perovskite-based optoelectronic devices with NPs.

Strongly Luminescent Composites Based on Carbon Dots Embedded in a Nanoporous Silicate Glass.

Luminescent composites based on entirely non-toxic, environmentally friendly compounds are in high demand for a variety of applications in photonics and optoelectronics. Carbon dots are a recently developed kind of luminescent nanomaterial that is eco-friendly, biocompatible, easy-to-obtain, and inexpensive, with a stable and widely tunable emission. Herein, we introduce luminescent composites based on carbon dots of different chemical compositions and with different functional groups at the surface which were embedded in a nanoporous silicate glass. The structure and optical properties of these composites were comprehensively examined using electron microscopy, Fourier transform infrared transmission, UV-Vis absorption, and steady-state and time-resolved photoluminescence. It is shown that the silicate matrix efficiently preserved, and even enhanced the emission of different kinds of carbon dots tested. The photoluminescence quantum yield of the fabricated nanocomposite materials reached 35-40%, which is comparable to or even exceeds the values for carbon dots in solution.