Fig. 1. Crystal structure and layer-dependent excitonic properties of 2D perovskite. (a) Fundamental unit of octahedral inorganic framework [PbX6]2−, where X=Cl, Br, I; (b) crystal structure of 2D layered perovksite. The inorganic layers are surrounded by insulative organic layers, resulting in self-assembled multiple quantum-well structures, where interlayer organic cations provide both dielectric and quantum confinement for inorgaic stacks. (c) The exciton binding energy of 2D perovskite decreases with inorganic layer number as a result of weak quantum confinement at large inorganic layer numbers [13]. Copyright 2018, Springer Nature. (d) and (e) are the absorpation and PL spectra of 2D perovskite (BA)2(MA)n−1PbnI3n+1 at different n values [14]. Copyright 2020, Springer Nature.

Fig. 2. Unique excitonic behaviors of 2D perovskite. (a) Left, absorpation spectra of (PA)2(MA)2Pb3Br10 and MAPbBr3 thin films; right, optical images of the two perovskite thin films under UV illumination. Adapted from [28] with permission from Royal Society of Chemistry. (b) In-plane and out-of-plane exciton of 2D perovskite. Left, 2D Fourier image of PL in the vertical polarization (white arrow), in which out-of-plane component locates at the TIR angle (k0). Adapted with permission from [33]. Copyright 2018, American Chemical Society. Right, dielectric model of 2D perovskite indicating the in-plane and out-of-plane electric field. Adapted with permission from [34]. Copyright 2019, American Chemical Society. (c) Phonon coherence of (PEA)2PbI4. Left, absorpation and time-resolved differential transmission spectrum at 5 K; right, oscillatory components extracted from the time-resolved differential transmission spectrum indicating the coherent vibrational dynamics. Adapted with permission from [40]. Copyright 2019, Springer Nature. (d) Energy diagram showing the generation of free exciton and self-trapped exciton due to the lattice reorganization; the resulting emission spectra behave as sharp excitonic emission and broad exciton self-trapping emission at low temperature. Adapted from [43]. Published by the Royal Society of Chemistry.

Fig. 3. Fabrication of different 2D perovskite photonic cavities. Upper side shows the synthesis of 2D perovskite materials with different morphology, including bulk crystals, microstructures, nanocrystals, and thin films. From left to right, the first image is adapted with permission from [64]. Copyright 2016, American Chemical Society. Further permissions related to the material excerpted should be directed to the ACS. The second image is adapted with permission from [53]. Copyright 2019, John Wiley and Sons. The third image is adapted with permission from [52]. Copyright 2015, John Wiley and Sons. The fourth image is adapted from [54]. Published by MDPI. Bottom side summarizes the different cavity structures including F-P cavity, WGM cavity, photonic crystal cavity, and DBR cavity.

Fig. 4. Model of exciton polariton inside a cavity. (a) Schematic diagram showing the strong coupling of excitons in the gain medium and F–P cavity mode between two reflectors; (b) dispersion curve of exciton polariton consisting of two polariton branches. The dashed lines are uncoupled exciton and cavity modes.

Fig. 5. Polariton in 2D perovskite self-organized crystal cavity. (a) Schematic diagram showing the oscillation of light between top and bottom interface of 2D perovskite single crystal (green arrow) and in-plane transmission of light inside the perovskite crystal (yellow arrow). (b) In-plane and out-of-plane exciton polariton in (PEAI)2PbI4 crystal. Top left is the PL image of perovskite crystal; right is the real-space intensity image of a resonant injected beam in the perovskite crystal; bottom, energy-momentum resolved reflection spectra of TE (left) and TM (middle) polarizations; the calculated reflection minima with (blue lines) and without (red lines) the interaction of excitonic resonance indicating the strong light-matter coupling. Adapted with permission from [33]. Copyright 2018, American Chemical Society. (c) Energy dispersion of a 2D perovskite (PEAI)2PbI4 single crystal showing the multimode exciton polaritons. The two low polariton modes (blue region) under different pumping power show energy blueshift of different values, indicating the exciton polariton nonlinearity. Adapted from [102]. Copyright 2019, AAAS. (d) Upper side, PL spectra consisting of a series of oscillating modes in a 2D perovskite planar cavity. The relation of oscillating modes versus wave vector can be fitted by a polariton model with a large Rabi splitting. Bottom is the PL mapping of a selected area of 2D perovskite planar crystal cavity. The interference pattern indicates the oscillating emission of exciton polariton. Adapted with permission from [56]. Copyright 2020, American Chemical Society.

Fig. 6. Polariton in vertical F–P cavities coupled with 2D perovskite. (a) Sketch of a microcavity containing 2D perovksite thin film. Anticrossing is observed in angle-resolved reflectivity spectra as two series of dips. Adapted from [81]. Copyright IOP Publishing and Deutsche Physikalische Gesellschaft. Reproduced by permission of IOP Publishing. All rights reserved. (b) Vertical F–P cavity of 2D perovskite thin flim formed by top and bottom silver films. Two polariton branches can be observed in angle-resolved transmission spectra with Rabi splitting of 160 meV. Adapted with permission from [101]. Copyright The Optical Society. (c) Energy dispersion of micro PL spectrum in a 2D perovskite cavity with relatively high Q factor; the emission of MPB is as narrow as 26 meV. Adapted with permission from [59]. Copyright The Optical Society. (d) Strong coupling of exciton with vertical cavity mode and Bragg modes in a DBR cavity of 2D perovskite. Left, cavity structure consisting of exfoliated 2D perovskite microflakes between top and bottom Bragg mirrors; right, angle-resolved reflection and emission spectra. Adapted with permission from [60]. Copyright 2018, American Chemical Society. (e) Polarization-sensitive polariton nonlinearity of 2D perovskite. Left, cavity structure of a DBR cavity; middle, energy momentum k emission; right, polariton blueshift at different pump powers by using a linearly polarized (L) laser and a circularly polarized (C) laser. Adapted from [102]. Copyright 2019, AAAS. (f) Formation of biexciton laser (left) and polariton condensate (right) in a (PEA)2PbI4 vertical cavity at liquid helium temperature. Adapted from [106]. Copyright 2020, John Wiley and Sons.

Fig. 7. Polariton in DFB and photonic crystal cavities coupled with 2D perovskite. (a) Structure of a DFB microcavity containing (PEA)2PbI4 (upper) and energy dispersion of transmission dips indicating the strong coupling of exciton and grating modes. Adapted with permission from [100]. Copyright 1998, American Physical Society. (b) Plasmon-exciton strong coupling in silver grating overcoated with 2D perovskite. Top left, cross section of the grating structure; top right, possible emission routes of SPP-mediated image biexciton; bottom, TM-polarized reflectivity at different incident angles. Adapted with permission from [71]. Copyright 2015, American Physical Society. (c) Exciton polariton of perovskite-based 2D lattice cavity structure: top, cavity design and scanning electron microscope (SEM) images of metasurface; bottom, angle-resolved reflection and emission (left) and corresponding simulation of photonic crystal polariton dispersion (right) with a Rabi splitting of 205 meV. Adapted with permission from [62]. Copyright 2020, American Chemical Society.

Fig. 8. Excitonic and free-carrier recombination of 2D and 3D perovskites. (a) The fabrication of free carriers over the total excitation density at different binding energies by using the Saha–Langmuir equation. Adapted with permission from [116]. Copyright 2016, American Chemical Society. Further permissions related to the material excerpted should be directed to the ACS. (b) The relation of emission intensity and injected carrier concentration of 3D perovskite MAPbI3 and 2D perovskite BA2PbBr4 at different temperatures. The slope indicates dominant bimolecular recombination (n=2) or excitonic recombination (n=1). Adapted from [120] with permission from the Royal Society of Chemistry.

Fig. 9. Optical gain in 2D perovskites. (a) Cascade energy transfer from wide-bandgap QWs to large QWs with increasing n-value; (b) wavelength tunability of 2D perovskite with different y values by mixing precursor solutions and the stability of ASE over 30 h. The inset shows the ASE spectra at the start and after 30 h. Adapted with permission from [121]. Copyright 2018, John Wiley and Sons. (c) Proposed relaxation channels of excitons in (PEA)2PbI4. Adapted from [123]. Published by the PCCP Owner Societies. (d) Schematic shows the biexciton Auger recombination in 2D perovskites with n values of 2 and 4. Auger recombination becomes stronger at small n values as a result of narrow well and larger spatial confinement of exciton. Adapted with permission from [18]. Copyright 2019, John Wiley and Sons.

Fig. 10. Lasing behaviors of 2D perovskite self-organized crystal cavity. (a) Top, lasing of a single 2D perovskite microplate (left) and dual-wavelength lasing (right) from 2D perovskite with n=6 and n≥7. The insets show the corresponding optical images, PL images above laser threshold, and PL intensity as a function of pump fluence. Adapted with permission from [124]. Copyright 2018, John Wiley and Sons. (b) Lasing spectra from homologous 2D perovskite single crystals with different n values and bandgaps. Adapted with permission from [57]. Copyright 2018, American Chemical Society. (c) Lasing of exfoliated 2D perovskite microflakes. Top, lasing spectra (left) and integrated intensity (right) of n=3 under different pump fluences and corresponding optical images; bottom, lasing (n = 3–5) and ASE (n=2) spectra of 2D perovskite with different n values and corresponding lasing thresholds at different temperatures. Adapted with permission from [18]. Copyright 2019, John Wiley and Sons.

Fig. 11. Lasing behaviors of 2D perovskite microcavity array. (a) SEM image and schematics show the microring and nanowire structures consisiting of multi-QW structure; (b) top, light harvesting of the nominal n=2 perovskite thin film for ASE; bottom, gain coefficient and PLQY for perovskite thin film with different nominal n values; (c) emission images and lasing spectra of microring (upper) and nanowire (lower) arrays at different pump fluences; (d) relation of lasing mode spacing versus ring diameter or nanowire length indicating the WGM lasing mode of microring array (left) and F–P lasing mode of nanowire array (right). Adapted with permission from [61,77]. Copyright 2018, John Wiley and Sons.

Fig. 12. Lasing behaviors of 2D perovskite embedded vertical F–P cavity. (a) Lasing of segregated quasi-2D perovskite microcrystals in vertical cavity: top, schematic of cavity structure (left) and PL images (right) of segregated patterns; bottom, pump fluence-dependent emission spectra, integrated intensity, and FWHM, indicating the lasing threshold of ∼500  μJ/cm2. Adapted from [17] with permission of AIP Publishing. (b) Vertical cavity biexciton lasing of 2D perovskite: top left, schematic of DBR cavity consisting of Bragg mirror, spin-coated DA2PbI4, PMMA, and top silver mirror; top right, pump fluence-dependent cavity emission at 2.28 and 2.41 eV, indicating the lasing mode and nonlasing mode; bottom, emission spectra of cavity at different optical pump powers with and without silver mirror, and perovskite thin film on glass. Adapted from [134]. Copyright 2018, John Wiley and Sons.

Fig. 13. Enhanced TPL emission from a 2D perovskite-microsphere cavity structure. (a) Emission images of pure perovskite flake and with SiO2 microsphere under the pump power of 0.1 mW; (b) TPL spectra of bare perovskite flake and perovskite-microsphere cavity structures with different sizes of microspheres. Adapted with permission from [136]. Copyright 2018, John Wiley and Sons.

Fig. 14. Ultrasensitive and polarized light detection of 2D perovskite nanowires. (a) Schematics of 2D perovskite nanowire photodetectors. Top, photodetectors based on polycrystalline thin films and nanowire array; bottom, carrier dynamics in the photodetector of single crystalline nanowires indicating the organic barriers for suppressing the dark current and conductive channels at crystalline edges for excitonic dissociation and free-carrier conduction. Adapted from [143] with permission of Springer Nature, Nature Electronics, Copyright 2018. (b) 2D perovskite nanowires for polarized light detection: top, PL spectra excited by light with polarization parallel (red) and perpendicular (black) to the nanowire orientation (left) and polar plot of PL intensity with different excitation polarizations (right); bottom, schematic of the perovskite/graphene hybrid device (left), photocurrent as a function of excitation energy at fixed power and source-drain voltage (middle), and polarization dependent photocurrent under pulsed laser at 2.52 eV and source-drain bias of 30 mV (right). Adapted with permission from [53]. Copyright 2019, John Wiley and Sons.