2.1.

Structure of Perovskite

As a kind of chemical material with an ${\mathrm{ABX}}_3$-type structure [Fig. 1(a)], the crystal structure of perovskite is the same as calcium titanate (${\mathrm{CaTiO}}_3$).⁴³ “A” could be an organic molecular group such as methylamino (MA) or an inorganic element such as cesium (Cs); “B” is generally a metal ion, such as lead (Pb), tin (Sn), and bismuth (Bi); “X” refers to a halide ion containing Cl, Br, and I. The tolerance factor ($t$) is generally used to evaluate the structural formability and stability, calculated as $t=(r_{\mathrm{A}}+r_{\mathrm{X}})/\sqrt{2}(r_{\mathrm{B}}+r_{\mathrm{X}})$, where $r_{\mathrm{A}}$, $r_{\mathrm{B}}$, and $r_{\mathrm{X}}$ are ionic radii of A, B, and X sites, respectively. Li et al.⁴⁵ introduced the “octahedral factor” ($\mu$) to investigate the regularities of formability for cubic perovskite ${\mathrm{ABX}}_3$, which was defined as $\mu =r_{\mathrm{B}}/r_{\mathrm{X}}$ and suggested the formation of the halide perovskites under the conditions of $0.813<t<1.107$ and $0.377<\mu <0.895$. Then, Sun et al. introduced ${(t+\mu )}^{\eta }$ to evaluate the thermodynamic stability of hailde perovskite, where $\eta$ is the atomic packing fraction in a crystal structure. By calculating decomposition energies of 138 perovskite compounds [Fig. 1(b)], they demonstrated better accuracy of ${(t+\mu )}^{\eta }$ than the evaluation of $t$ and $\mu$ alone.^44,46

Fig. 1

(a) Structural model of metal lead perovskites. Figures reproduced from Ref. 43. (b) The $(t,\mu )$ map for 138 perovskite compounds. Figures reproduced from Ref. 44. (c) Nanoscale morphologies of halide perovskites.

2.2.

Perovskite Quantum Dots

Based on the ${\mathrm{ABX}}_3$ halide perovskites, the morphologies can be controlled with different dimensional nanostructures, such as 0D QDs, 1D NWs, 2D NPs, and 3D MSs.^{37,40,47–65} Since different nanostructures will result in variable structure–property relationships at the nanoscale level, various strategies have been reported for controlling the form and size of the perovskite NCs, including changing the reaction temperature, the reaction time, and the ligand combinations during synthesis.^37–42 In addition, capping ligands with different structures and lengths can also affect the nucleation and growth rate, hence the structure of perovskite NCs can be adjusted owing to their anchoring and steric effects.^47–49

As is well known, traditional semiconductor QDs have a quantum size effect. In the case of perovskite QDs, the bandgap can also be tuned via halide component regulation.^38–42,51 Zhang et al.²⁵ developed a ligand-assisted reprecipitation (LARP) method to fabricate ${\mathrm{MAPbBr}}_3$ QDs at room temperature. Their PLQYs were up to $\sim 70\%$ [Fig. 2(a)]. By mixing ${\mathrm{PbX}}_2$ salts into the precursors, the authors tuned emission in the range of 407 to 734 nm [Figs. 2(c) and 2(d)].²⁵ In the case of all-inorganic ones, the emission spectra of ${\mathrm{CsPbX}}_3$ QDs fabricated by the high temperature method could be tunable over 410 to 700 nm with narrow half maximum of 12 to 42 nm and radiative lifetime of 1 to 29 ns [Figs. 2(e) and 2(f)].⁴² Subsequently, they proposed an anion-exchange process to tune the emission of colloidal ${\mathrm{CsPbX}}_3$ QDs via postsynthetic reactions with different compounds [Figs. 2(g) and 2(h)].⁴¹ Besides the hot-injection technique, the room-temperature synthesis for perovskite QDs was also studied.³⁹ In 2016, Zeng and coworkers developed a room-temperature method to fabricate ${\mathrm{CsPbX}}_3$ QDs via supersaturated recrystallization. In this process, the crystallization process occurred in the transform of ${\mathrm{Cs}}^{+}$, ${\mathrm{Pb}}^{2+}$, and ${\mathrm{X}}^{-}$ ions from soluble to insoluble solvents in the absence of inert gas within a few seconds, as shown in Figs. 2(i) and 2(j).³⁹ Although crystallized at room temperature, these ${\mathrm{CsPbX}}_3$ QDs held superior optical properties with PLQYs above 70%, and PLs can remain at $\sim 90\%$ after aging 30 days in the air. Except for being regulated by changing composition, the bandgap of perovskite QDs also can be tuned by the size regulation of QDs. Chen et al.⁶⁶ fabricated ${\mathrm{CsPbBr}}_3$ QDs with an average diameter from 7.1 to 12.3 nm by modifying the temperature, and the corresponding PL emission peaks could be tuned from 493 to 531 nm. Fang et al.⁶⁷ synthesized ${\mathrm{MAPbBr}}_3$ QDs with tunable average diameter from 2.82 to 5.29 nm by varying the additive amount of surfactant, and the corresponding PL emission peaks could shift from 436 to 520 nm due to the quantum confinement effect. Most recently, there have been much more researches about perovskite QDs by composition engineering for wider optoelectronic applications.^68–70

Fig. 2

(a) Schematic of LARP technique. Figures reproduced from Ref. 25. (b) Schematic of precursor and optical image of ${\mathrm{MAPbBr}}_3$ solution. Figures reproduced from Ref. 25. (c) Optical images of ${\mathrm{MAPbX}}_3$ solution under natural light and under 365 nm excitation. Figures reproduced from Ref. 25. (d) PL spectra of ${\mathrm{MAPbX}}_3$ QDs. Figures reproduced from Ref. 25. (e) PL optical images and PL spectra of ${\mathrm{CsPbX}}_3$ QDs. Figures reproduced from Ref. 42. (f) Time-resolved PL decays for ${\mathrm{CsPbX}}_3$ QDs. Figures reproduced from Ref. 42. (g) Schematic of the anion-exchange of ${\mathrm{CsPbX}}_3$. Figures reproduced from Ref. 41. (h) TEM images of ${\mathrm{CsPbX}}_3$ QDs with various PL. Figures reproduced from Ref. 41. (i) Schematic of room-temperature fabrication of ${\mathrm{CsPbX}}_3$ QDs. Figures reproduced from Ref. 39. (j) Optical images of ${\mathrm{CsPbX}}_3$ QDs after the addition of precursor ion solutions for 3 s. Figures reproduced from Ref. 39.

2.3.

Perovskite Nanowires and Nanorods

Perovskite 1D NWs and NRs are more applicable in the field of optoelectronic applications due to their special anisotropic structures. In the growth of 1D perovskite structures, reaction temperature, reaction time, and organic ligands are critical factors for crystallization.^71–74 Deng et al.⁷¹ first fabricated ${\mathrm{MAPbI}}_3$ NWs via the one-step solution method. In this process, the precursor solution containing ${\mathrm{PbI}}_2$ and ${\mathrm{CH}}_3{\mathrm{NH}}_3\mathrm{I}$ was dropped onto a substrate and then heated at different temperatures. Finally, uniform ${\mathrm{MAPbI}}_3$ NWs were obtained after heating at 80°C for 10 min. In 2017, they fabricated ${\mathrm{Cs}}_x{(\mathrm{MA})}_{1-x}{\mathrm{PbI}}_3$ NWs through a two-step solution method.⁷² As shown in Fig. 3(a), ${\mathrm{PbI}}_2$ powder was dissolved in water at 75°C initially, then ${\mathrm{PbI}}_2$ separated out when the solution cooled down to room temperature. With the addition of CsI and MAI, perovskite NWs could be formed after shaking for a few seconds. The length and diameter of obtained perovskite NWs could reach $10\mu \mathrm{m}$ and several hundred nanometers. Moreover, the amount of perovskite NWs was related to the concentration of ${\mathrm{PbI}}_2$ separated out from aqueous solution.⁷² Zhu et al. developed a direct conversion of ${\mathrm{MAPbI}}_3$ film into NWs through a recrystallization process [Fig. 3(b)]. The first step was the formation of perovskite film from a mixture of ${\mathrm{PbCl}}_2$ and ${\mathrm{CH}}_3{\mathrm{NH}}_3\mathrm{I}$.⁷³ Then, a mixture solution containing DMF and isopropyl was dropped onto the as-grown perovskite film. Along with the evaporation of the solvent, NWs could be formed [Fig. 3(b)].⁷³ Furthermore, they found the content of DMF in isopropyl, and the rotation speed could affect the sizes of prepared ${\mathrm{MAPbI}}_3$ NWs.

Fig. 3

(a) Schematic of the fabrication process for the ${\mathrm{Cs}}_x{({\mathrm{CH}}_3{\mathrm{NH}}_3)}_{1-x}{\mathrm{PbI}}_3$ NWs. Figures reproduced from Ref. 72. (b) Schematic of the formation of the ${\mathrm{MAPbI}}_3$ NWs by recrystallization process. Figures reproduced from Ref. 73. (c) TEM images of as-grown ${\mathrm{CsPbBr}}_3$ NCs with increasing times. Figures reproduced from Ref. 51. (d) Absorption and PL spectra of ${\mathrm{CsPbBr}}_3$ NWs. Figures reproduced from Ref. 56. (e) Schematic of the passivation effect by HX on the length of ${\mathrm{CsPbX}}_3$ NWs and TEM images of the synthesized ${\mathrm{CsPbX}}_3$ NWs. Figures reproduced from Ref. 58. (f) Normalized absorption, PL spectra, and photographs of ${\mathrm{CsPbX}}_3$ NWs. Figures reproduced from Ref. 58.

As for all-inorganic perovskite, in 2015, Yang and coworkers used a solution method to synthesize single-crystalline ${\mathrm{CsPbX}}_3$ NWs first. The reaction temperature was set as 150°C to 250°C.⁵¹ They found that the reaction time was critical to the growth of NWs. As shown in Fig. 3(c), the SEM of prepared ${\mathrm{CsPbBr}}_3$ with different reaction times showed perovskite nanocubes formed initially, then NS and NW formed at 90 min [Fig. 3(c)].⁵¹ In the formation of Cs-based perovskite NWs, surface ligands could affect the width and size. Imran et al.⁵⁶ tuned the width of ${\mathrm{CsPbX}}_3$ NWs from 10 to 20 nm by regulating the ratio of octylamine to oleylamine and varying the reaction time. They found that the width of NWs can be decreased below $\sim 5\mathrm{nm}$ by introducing carboxylic acids with short aliphatic chains. Correspondingly, the emission spectra of ${\mathrm{CsPbBr}}_3$ NWs could be tuned from 524 to 473 nm [Fig. 3(d)].⁵⁶ Amgar et al.⁵⁸ found that various hydrohalic acids (HX, X = Cl, Br, and I) affect the length of ${\mathrm{CsPbBr}}_3$ NWs efficiently. With the increasing amount of HX, the length of NWs would be shortened [Fig. 3(e)]. Using this method, the emission of the ${\mathrm{CsPbBr}}_3$ NWs could be tunable in the range of 423 to 505 nm [Fig. 3(f)].⁵⁸ ${\mathrm{CsPbBr}}_3$ NWs/NRs can also be synthesized by a low-temperature method. Dong’s group⁷⁴ fabricated ${\mathrm{CsPbBr}}_3$ perovskite NRs with controllable size in a polymer matrix. Then, Liu et al. fabricated single-crystalline ${\mathrm{CsPbBr}}_3$ NWs without inert gas at room temperature. By increasing the reaction time, the length of NWs could be increased from nanometers to micrometers, and the diameter could be tuned from 2.5 to 32.0 nm. Moreover, using this method, the emission spectra of ${\mathrm{CsPbX}}_3$ NWs could be tuned from 434 to 681 nm.⁵⁷

Besides the above-mentioned solution-process, plenty of works have been reported on synthesizing perovskite NWs and NRs by vapor-phase growth.⁴⁹ More than ever, the vapor-phase process can control the morphology and crystalline phase of perovskite NCs efficiently. It has been demonstrated that the growth temperature and the substrates are critical for the orientation of perovskite NWs in vapor-growth. Xing et al.⁴⁹ first used the vapor-phase technique to fabricate perovskite NWs. First, ${\mathrm{PbI}}_2$ NWs were deposited on ${\mathrm{SiO}}_2$ substrates by the chemical vapor deposition (CVD) method [Fig. 4(a)].⁴⁹ Consequently, ${\mathrm{PbI}}_2$ was converted into ${\mathrm{MAPbX}}_3$ after the reaction with MAX through a CVD process. As shown in Fig. 4(b), the prepared ${\mathrm{MAPbI}}_3$ wires had a length about tens of micrometers and a diameter of $\sim 500\mathrm{nm}$. In Figs. 4(c) and 4(d), they indicated that ${\mathrm{MAPbI}}_3$ NW grows along the [100] direction.⁴⁹ Due to the thermal decomposition of organic hybrid perovskite occurring easily at high temperatures, direct vapor–phase growth of hybrid perovskites is more challenging. However, the vapor–phase technique is an attractive method for all-inorganic perovskites, which have better thermostability. Zhou et al.⁷⁵ prepared ${\mathrm{CsPbX}}_3$ NRs with high crystallization quality and regular triangular morphology through a vapor deposition method [Fig. 4(e)]. As shown in the SEM image [Fig. 4(f)], prepared ${\mathrm{CsPbX}}_3$ NRs had a triangular cross section, smooth surfaces, and a length of 2 to $20\mu \mathrm{m}$. They demonstrated that the reaction temperature was critical for the control of perovskite NCs during the growth of triangular ${\mathrm{CsPbBr}}_3$ NRs. Moreover, the emission of these as-grown ${\mathrm{CsPbX}}_3$ NRs also can be tuned from 415 to 673 nm by halide component regulation [Fig. 4(g)].⁷⁵

Fig. 4

(a) SEM image of ${\mathrm{PbI}}_2$ NWs. Figures reproduced from Ref. 49. (b) Optical microscopy image of ${\mathrm{MAPbI}}_3$ NWs. Figures reproduced from Ref. 49. Structure simulation images of (c) ${\mathrm{PbI}}_2$ NW and (d) ${\mathrm{MAPbI}}_3$ NW. Figures reproduced from Ref. 49. (e) Schematic of the ${\mathrm{CsPbX}}_3$ triangular micro/NRs. Figures reproduced from Ref. 75. (f) SEM image of ${\mathrm{CsPbBr}}_3$ triangular rods. Figures reproduced from Ref. 75. (g) Real-color image and PL spectra of ${\mathrm{CsPbX}}_3$ triangular rods. Figures reproduced from Ref. 75. (h), (i) SEM images of the ${\mathrm{CsPbBr}}_3$ NWs. Figures reproduced from Ref. 62.

In addition, it has been confirmed that the substrate can affect the grain orientation growth of perovskite NWs.⁷⁶ Chen et al.⁶² fabricated ${\mathrm{CsPbX}}_3$ wires on mica by the CVD method. During the growth of ${\mathrm{CsPbBr}}_3$ NWs, heteroepitaxial matching occurred in the interface between ${\mathrm{CsPbBr}}_3$ NCs and mica substrate. Then, the formation of NWs was caused by the asymmetric lattice mismatch with the mica substrate. As shown in Figs. 4(h) and 4(i), the obtained ${\mathrm{CsPbBr}}_3$ wires were well-aligned, surface-bound, and formed a network with a length about tens of $\mu \mathrm{m}$ and width of $\sim 1\mu \mathrm{m}$, respectively.⁶² Moreover, various nanostructures could be formed by controlling the deposition time, such as single NWs, Y-shaped branches, and interconnected NW or MW networks.⁶²

2.4.

2D Metal Halide Perovskite Nano/Microstructures

2.4.1.

Perovskite nano/microplates

The unique and excellent properties of 2D structured perovskite such as NSs, NPs, and microdisks (MDs) make them promising for potential optoelectronic devices.⁷⁷ Sichert et al.⁶¹ synthesized ${\mathrm{MAPbBr}}_3$ NPs and investigated the quantum size effect of NPs via the solution method. They found that the thickness of ${\mathrm{MAPbBr}}_3$ NPs was reduced with the increase of the content of OA [Figs. 5(a) and 5(b)]. Correspondingly, the PL emission was tuned from the green to violet region [Fig. 5(c)].⁶¹ Qin et al. prepared ${\mathrm{MAPbI}}_3$ NPs via a two-step solution method. First, ${\mathrm{PbI}}_2/\mathrm{DMF}$ solvent was spin-coated onto a substrate to form ${\mathrm{PbI}}_2$ thin films. Then, the formed ${\mathrm{PbI}}_2$ thin film was immersed into MAI solution, in which ${\mathrm{MAPbI}}_3$ single NCs were formed.⁸¹

Fig. 5

(a) Schematic of the synthesis of ${\mathrm{MAPbBr}}_3$ NPs. Figures reproduced from Ref. 61. (b) Quantum size effect in ${\mathrm{MAPbBr}}_3$ NPs. Figures reproduced from Ref. 61. (c) Bandgap tuning in ${\mathrm{MAPbBr}}_3$ NPs and micro/NRs via size or compositional control. Figures reproduced from Ref. 61. (d) PL spectra of the halide–anion exchanged ${\mathrm{CsPbX}}_3$ NPs. Figures reproduced from Ref. 37. (e) 2D ${\mathrm{CsPbBr}}_3$ NSs. Figures reproduced from Ref. 37. (f) SEM images of ${\mathrm{CsPb}}_2{\mathrm{Br}}_5$ MP. Figures reproduced from Ref. 78. (g) Top: Schematic of the growth of 2D ${\mathrm{CsPbX}}_3$ NPs and NSs from ${\mathrm{CsPbX}}_3$ NRs. Bottom: TEM images of ${\mathrm{CsPbBr}}_3$ NCs for different times. Figures reproduced from Ref. 79. (h) Schematic of the fabrication of ${\mathrm{MAPbI}}_3$ NCs using a vapor-transport system. Figures reproduced from Ref. 80. (i) Thickness of ${\mathrm{PbI}}_2$ platelets before and after being converted to ${\mathrm{MAPbI}}_3$. Figures reproduced from Ref. 80. (j) Optical images of as-grown ${\mathrm{MAPbI}}_3$ NCs with different temperature and pressure. Figures reproduced from Ref. 64.

${\mathrm{CsPbBr}}_3$ NPs were prepared by Bekenstein et al.³⁷ through a hot-injection method. They demonstrated that the reaction temperature is critical for the shape and thickness of ${\mathrm{CsPbBr}}_3$ NPs. As the temperature decreased from 150°C to 130°C, the shape of ${\mathrm{CsPbBr}}_3$ NCs evolved from nanocubes to NPs. Correspondingly, the PL emission was shifted from 512 to 405 nm [Figs. 5(d) and 5(e)].³⁷ When the temperature decreased to 90°C and 100°C, the thin ${\mathrm{CsPbBr}}_3$ NPs were obtained with lengths of about 200 to 300 nm [Fig. 5(d)].³⁷ Except for the reaction temperature, surface ligands also affect the formation of ${\mathrm{CsPbX}}_3$ NPs, which was demonstrated by Pan et al. in 2016. During the growth of NPs, ${\mathrm{CsPbX}}_3$ NPs were obtained at a relatively lower reaction temperature (120°C to 140°C). They obtained thinner ${\mathrm{CsPbX}}_3$ NPs with shorter chain amines.⁸² In addition, the reaction time was also found to be critical for the formation of perovskite NPs.^78,79 By adding the ${\mathrm{PbBr}}_2$ concentration and increasing the reaction time above 1 h (135°C), a ${\mathrm{CsPb}}_2{\mathrm{Br}}_5$ microplate (MP) with a micrometer order size and regular end faces could be obtained [Fig. 5(f)].⁷⁸ In 2018, Li et al. demonstrated that 2D ${\mathrm{CsPbX}}_3$ NPs and NSs can be obtained by varying the reaction time [Fig. 5(g)]. The thickness can be controlled in the range of 3 to 6 nm and the width in the range of 0.1 to $1\mu \mathrm{m}$.⁷⁹ Huang et al. reported a method for spontaneous crystallization of perovskite NCs in nonpolar organic solvent by mixing precursor ligand complexes without any heat treatment. By varying the ratio of monovalent to ${\mathrm{Pb}}^{2+}$ cation–ligand complexes, the shape of the NCs can be controlled from 3D nanocubes to 2D nanoplatelets.⁸³

Similar to the NWs, perovskite NPs can also be formed by the vapor synthesis method. Xiong and coworkers⁸⁰ reported the CVD growth of ${\mathrm{MAPbI}}_3$ NPs. These NPs exhibited triangular or hexagonal platelet shapes, with thickness of 10 to 300 nm and lateral dimensions of 5 to $30\mu \mathrm{m}$ [Figs. 5(h) and 5(i)]. ${\mathrm{PbX}}_2$ platelets were first grown on mica via van der Waals epitaxy and then converted to ${\mathrm{MAPbX}}_3$ NPs with the existence of MAX. In 2016, Bao and coworkers developed a combined method containing a solution process and a vapor-phase conversion process to prepare ${\mathrm{MAPbI}}_3$ NSs. First, ${\mathrm{PbI}}_2$ flakes were dropped on a silica substrate and then heated. In this process, the temperature plays a crucial role in the nucleation and growth of 2D ${\mathrm{PbI}}_2$ NSs, since the amount of nucleation sites is controlled by temperature. Subsequently, ${\mathrm{MAPbI}}_3$ NSs were formed after the conversion reaction with MAI.⁸⁴ During the vapor-phase growth, the growth pressure and temperature both could affect the formation of perovskite NCs. Liu et al.⁶⁴ fabricated 2D ${\mathrm{MAPbBr}}_3$ platelets (001) via the CVD method. As shown in Fig. 5(j), the square-shaped platelets could not form, as the growth pressure and temperature were low. By increasing the pressure, 2D platelets and 3D spheres could be observed. The average thickness of ${\mathrm{MAPbBr}}_3$ platelets increased from 29 to 73 nm, and the lateral size increased from 6 to $10\mu \mathrm{m}$ with the pressure increasing from 140 to 200 Torr.⁶⁴

As for all-inorganic 2D perovskite NCs, Zeng and coworkers⁸⁵ synthesized ultrathin ${\mathrm{CsPbBr}}_3$ NPs (thickness $\sim 148.8\mathrm{nm}$) on a mica substrate by van der Waals epitaxy through heating the ${\mathrm{PbBr}}_2$ and CsBr mixture. Zheng et al.⁸⁶ synthesized 2D ${\mathrm{CsPbI}}_3$ perovskite NSs with high quality, controllable morphology, and ultrathin thickness ($\sim 6.0\mathrm{nm}$) via a space-confined vapor-phase epitaxial growth. In 2020, Yang and coworkers developed a facile method to pattern ${\mathrm{CsPbX}}_3$ plate arrays with crystal size (200 nm to $1\mu \mathrm{m}$) and spacing (2 to $20\mu \mathrm{m}$). These plate arrays were confined by prepatterned hydrophobic/hydrophilic surfaces.⁶⁵ The method can evade the restriction of lattice matching between perovskite and substrates, enabling a large-area growth of 2D perovskite NCs with excellent crystalline quality.⁸⁵

2.4.2.

Metasurface

A metasurface is a type of 2D optical element composed of units with subwavelength scale size, producing resonant coupling between electric and magnetic components of the incident electromagnetic fields.^87–91 Several functionalities were demonstrated on all-dielectric metasurfaces, such as optical encoding, optical wavefront molding, polarization beam splitter, and enhanced PL.^92–94 Perovskite-based metasurfaces demonstrated potential for nonlinear absorption and optical encoding.⁹⁵ Metasurfce structures can be realized by nanopatterning thin film. Many conventional nanofabrication techniques have been used for the fabrication of perovskite metasurfaces, such as nanoimprinting, electron beam lithography (EBL), focused ion beam milling (FIB), and inductively coupled plasma etching (ICP).^{90,94,96–98}

Gholipour et al.⁹⁷ first used the FIB technique to fabricate ${\mathrm{MAPbI}}_3$ metasurfaces (thickness $\sim 200\mathrm{nm}$), which consisted of nanogratings and nanoslit metamolecules. Moreover, they demonstrated that the emission and quality factor of the reflection resonances can be tuned by varying the grating period.⁹⁷ Makarov et al.⁹⁸ developed nanoimprinting technology for patterning ${\mathrm{Cs}}_{\alpha }{\mathrm{FA}}_{\beta }{\mathrm{MA}}_{\gamma }\mathrm{Pb}{({\mathrm{I}}_x{\mathrm{Br}}_y)}_3$ metasurfaces, enabling them to enhance their linear and nonlinear PL [Figs. 6(a)–6(d)]. After the spin-coating of the perovskite film with thickness of $\sim 200\mathrm{nm}$, nanoimprinting with nanopillar and nanostripe molds was performed on perovskite thin film to form metasurfaces. They demonstrated that these metasurfaces can enhance linear PL eight times and nonlinear PL 70 times.⁹⁸ Jeong et al.⁹⁴ presented a polymer-assisted nanoimprinting method for fabricating large-area ${\mathrm{CsPbX}}_3$ nanopatterns. As shown in Fig. 6(e), during their nanoimprinting process, a precursor solution was spin-coated on a substrate initially, and then the nanoimprinting mold was pressed on the precursor film with thermal treatment subsequently. Thus, ${\mathrm{CsPbX}}_3$ was crystallized within the confines of molds [Fig. 6(f)]. This method could be easily extended to large-area perovskite patterns on different substrates.⁹⁴ In addition, Fan et al.⁹⁰ used the EBL flowed ICP technique to prepare near-infrared ${\mathrm{MAPbBr}}_3$ perovskite metasurfaces [Fig. 6(g)]. Based on these metasurfaces, many types of nonlinear processes and enhanced PL could be observed [Fig. 6(h)].⁹⁰ The authors presented the application of perovskite metasurfaces on optical encryption.⁹⁰ The perovskite metasurface also can be used in optical phase control, which was confirmed by Zhang et al. in 2019. They also used the EBL flowed ICP technique to prepare ${\mathrm{MAPbX}}_3$ cut-wire metasurfaces on metal substrates [Figs. 6(i) and 6(j)].⁹⁶ They found that these ${\mathrm{MAPbX}}_3$ metasurfaces can generate a full phase control from 0 to $2\pi$ and high-efficiency and broadband polarization. Finally, they proved the potential application in holographic images based on the unique property of perovskite metasurfaces.⁹⁶

Fig. 6

(a) Perovskite metasurfaces with enhanced emission. Figures reproduced from Ref. 98. SEM images of perovskite with (b) nanostripe and (c) nanohole structures. Figures reproduced from Ref. 98. (d) Enhanced PL spectra from perovskite metasurfaces with different structures. Figures reproduced from Ref. 98. (e) Schematics of the polymer-assisted nanoimprinting process for perovskite nanopatterns. Figures reproduced from Ref. 94. (f) SEM images of various perovskite nanopatterns. Figures reproduced from Ref. 94. (g) SEM images of ${\mathrm{MAPbBr}}_3$ metasurface for nonlinear imaging. Figures reproduced from Ref. 90. (h) The nonlinear PL and linear PL images of ${\mathrm{MAPbBr}}_3$ metasurfaces. Figures reproduced from Ref. 90. (i) SEM image of ${\mathrm{MAPbBr}}_3$ metasurface. Figures reproduced from Ref. 96. (j) The field distributions of ${\mathrm{MAPbBr}}_3$ perovskite metasurface. Figures reproduced from Ref. 96.

2.5.

3D Metal Halide Perovskite Nano/Microstructures

Besides 1D and 2D structured-perovskite, perovskite-based 3D structures have also been investigated. In 2017, a two-step method for the fabrication of ${\mathrm{CsPbX}}_3$ microcubes with subwavelength size was developed by Hu et al.⁹⁹ These ${\mathrm{CsPbX}}_3$ microcubes had a regular cube shape and smooth end faces, displaying tunable emission and excellent structure stability for several months under ambient conditions. In the same year, Zhang and coworkers used the CVD method on the prepared ${\mathrm{CsPbX}}_3$ MSs with controlled diameter of $\sim 1\mu \mathrm{m}$ and tunable PL ranging from 425 to 715 nm [Figs. 7(a) and 7(b)].¹⁰⁰ Wei et al.¹⁰¹ developed an automated microreactor system to fabricate an inorganic perovskite NCs sphere by UV photoinitiated polymerization in flow-focusing microfluidics [Figs. 7(c) and 7(d)]. These obtained ${\mathrm{CsPbBr}}_3$ spheres had a large diameter around $100\mu \mathrm{m}$, and the diameter could be influenced by flow rates.¹⁰¹ Mi et al.¹⁰² used the CVD method to fabricate high-quality single ${\mathrm{MAPbBr}}_3$ crystals with a cube-corner pyramids shape and lateral dimension in the range of 2 to $10\mu \mathrm{m}$ on mica substrates [Fig. 7(e)]. Then, Yang et al.¹⁰³ also used the CVD method to fabricate ${\mathrm{CsPbI}}_3$ triangular pyramids with a spontaneous emission of $\sim 719\mathrm{nm}$ at room temperature on a $\mathrm{Si}/{\mathrm{SiO}}_2$ substrate [Fig. 7(f)].

Fig. 7

(a) SEM image of the ${\mathrm{CsPbI}}_3$ MSs. Figures reproduced from Ref. 100. (b) PL spectra of ${\mathrm{CsPbCl}}_3$, ${\mathrm{CsPbBr}}_3$, and ${\mathrm{CsPbI}}_3$ MSs. Figures reproduced from Ref. 100. (c) Monodispersed ${\mathrm{CsPbBr}}_3$ spheres under the excitation of UV light. Figures reproduced from Ref. 101. (d) SEM image of the monodispersed ${\mathrm{CsPbBr}}_3$ spheres. Figures reproduced from Ref. 101. (e) SEM image of the ${\mathrm{MAPbBr}}_3$ triangular pyramids. Figures reproduced from Ref. 102. (f) SEM image of the ${\mathrm{CsPbI}}_3$ triangular pyramids on a $\mathrm{Si}/{\mathrm{SiO}}_2$ substrate. Figures reproduced from Ref. 103. (g) SEM image and (h) schematic of the formation of ${\mathrm{CsPbX}}_3$ nanoflowers. Figures reproduced from Ref. 104. (i) Photograph (upper) and PL emission spectra (bottom) of ${\mathrm{CsPbX}}_3$ nanoflowers. Figures reproduced from Ref. 104. (j) Crystal growth of ${\mathrm{MAPbBr}}_3$ cuboids (top) and SEM images of ${\mathrm{MAPbBr}}_3$ perovskite under different reaction time (bottom). Figures reproduced from Ref. 105.

Except for the regular morphologies, complex perovskite structures have also been investigated. Chen et al.¹⁰⁴ used a seed-mediated solvothermal method to fabricate monodisperse ${\mathrm{CsPbX}}_3$ NCs with nanoflower morphology [in Figs. 7(g)–7(i)]. Figure 7(h) shows the growth process of ${\mathrm{CsPbBr}}_3$ nanoflowers, which is formed by the structure transformation from ${\mathrm{Cs}}_4{\mathrm{PbBr}}_6$ to ${\mathrm{CsPbBr}}_3$. It is obtained that ${\mathrm{CsPbX}}_3$ dodecapods contained 12 well-defined branches, with a PLQY of about $\sim 50\%$. Moreover, the PL emission could be tuned from 415 to 685 nm. They prepared a white LED device based on using ${\mathrm{CsPbBr}}_3$ nanoflowers, exhibiting the 135% National Television System Committee (NTSC) standard.¹⁰⁴ In 2019, Li et al. fabricated single crystal microcuboid-${\mathrm{MAPbBr}}_3$ and multistep-${\mathrm{MAPbBr}}_3$ NCs via the solvothermal method at 120°C. In this process, microcuboid-${\mathrm{MAPbBr}}_3$ was formed initially, and then the center of the surface was etched after long-time reaction, inducing the formation of multisteps. By adjusting the reaction temperature and time, the morphology and size of microcuboid-${\mathrm{MAPbBr}}_3$ [Fig. 7(j)] could be adjustable, with performing potential in perovskite nanolaser and other optoelectronic devices.¹⁰⁵

3.1.

Nonlinear Optical Properties

Nonlinear optics describes the nonlinear state of the interaction between light and matter.^106–111 The researches on optical nonlinear materials are fundamental to nonlinear optics devices such as optical storage, optical switches, optical amplifiers, and lasers.^112–114 Due to the multiformity of physical and chemical properties, halide perovskites have been demonstrated as promising materials as nonlinear optics materials, which are related to the component and crystal structure of perovskite NCs.¹⁰⁶

In 2015, Sargent and coworkers investigated two-photon absorption in ${\mathrm{MAPbBr}}_3$ single crystals, under ultrashort pulses 800 nm excitation [Figs. 8(a)–8(d)]. They observed two-photon PL around $\sim 572\mathrm{nm}$ with an absorption coefficient of $8.6\pm 0.5\mathrm{cm}{\mathrm{GW}}^{-1}$ at 800 nm.¹¹⁷ Later, Heiko et al. performed temperature-dependent PL measurements on ${\mathrm{MAPbBr}}_3$ single crystals under 810 nm excitation. They observed obvious wavelength shifts of PLs with variable temperatures, which was attributed to discrete transitions between several stable crystalline phases of ${\mathrm{MAPbBr}}_3$ single crystals.¹¹⁵ In 2016, Kalanoor et al. studied the nonlinear optical responses of ${\mathrm{MAPbI}}_3$ films by the Z-scan technique, under nanosecond and femtosecond pulsed lasers. The nonlinear refractive index under femtosecond excitation was $\sim 69\times {10}^{-12}$ and $\sim 34.4\times {10}^{-9}{\mathrm{cm}}^2/\mathrm{W}$ for resonant nanosecond excitation, which was equivalent to conventional semiconductors.¹¹⁸ The Z-scan study of ${\mathrm{MAPbX}}_3$ (X = Cl, Br, I) perovskite film under the 800 nm, 40 fs pulse indicated that ${\mathrm{MAPbI}}_3$ films have a relatively large nonlinear optical coefficient compared with the ${\mathrm{MAPbCl}}_3$ and ${\mathrm{MAPbBr}}_3$ films.¹¹⁹ In the case of inorganic perovskites, Sun and coworkers discovered nonlinear optical properties of ${\mathrm{CsPbX}}_3$ NCs for the first time [Figs. 8(e)–8(g)]. They observed strong two-photon absorption from 9-nm-sized ${\mathrm{CsPbBr}}_3$ NCs, with a large absorption cross-section of $\sim 1.2\times {10}^5\mathrm{GM}$.¹¹⁶ The nonlinear optical properties of ${\mathrm{CsPbX}}_3$ perovskite are highly correlated with their morphology. Jiang and coworkers¹²⁰ investigated nonlinear optical properties of ${\mathrm{CsPbBr}}_3$ NSs with a dependence on their thickness. When the thickness of ${\mathrm{CsPbBr}}_3$ NS was adjusted from $\sim 104.6$ to $\sim 195.4\mathrm{nm}$, PL intensity increased nearly three times. They demonstrated that the two-photon absorption coefficient is inversely proportional to the thickness of ${\mathrm{CsPbBr}}_3$ NSs.¹²⁰ Krishnakanth et al. investigated nonlinear optical properties from nanocubes and NRs by Z-scan technology, under femtosecond 600, 700, and 800 nm lasers. They obtained large two-photon absorption cross sections of $\sim {10}^5$ GM and strong nonlinear optical susceptibility of $\sim {10}^{-10}\mathrm{esu}$ in these films.¹²¹

Fig. 8

(a) Absorption spectrum and normalized two-photon PL spectra of single ${\mathrm{MAPbBr}}_3$ NCs. Figures reproduced from Ref. 115. (b) Schematic of two-photon absorption at 800 nm in perovskite. Figures reproduced from Ref. 115. (c) Two-photon absorption coefficient. (d) Inverse transmission versus peak intensity for typical single ${\mathrm{MAPbBr}}_3$ NCs. Figures reproduced from Ref. 115. (e)–(g) Nonlinear optics of ${\mathrm{CsPbX}}_3$ NCs: (e) linear absorption spectrum and normalized PL spectra from ${\mathrm{CsPbBr}}_3$ NCs, (f) PL decay of ${\mathrm{CsPbBr}}_3$ NCs, and (g) Z-scan responses of the ${\mathrm{CsPbBr}}_3$ NC solution and the pure solvent. Figures reproduced from Ref. 116.

The laser is a process of amplifying optical signals and generating high-intensity coherent light through stimulated radiation and is usually composed of three parts: energy pumping source, gain medium, and optical resonator. The amplification of the laser can be quantified as the resonance ability of gain media.^122,123 For gain media, the optical gain of the semiconductor is similar to the optical absorption, which is suitable for perovskite.^124,125 At the same time, optical losses are generated in optical cavities, which mainly come from nonradiative recombination, phonon scattering, edge scattering, and field leakage in the interface of cavities.¹²⁶ Perovskite materials have ultralow density, inducing high optical gain and low optical losses for resonance in perovskite, enabling promising potential in perovskite lasers with low threshold. The optical gain of semiconductors can be calculated by the variable-stripe-length measurement, which is related to the dependence of the amplified luminescence on the length of the slit width of the excitation. Xing et al.^23,124 performed variable stripe length measurements on ${\mathrm{MAPbI}}_3$ with a gain coefficient of $\sim 250{\mathrm{cm}}^{-1}$, which was close to that of conventional semiconductor materials. The obtained optical gain coefficients of ${\mathrm{MAPbBr}}_3$ and ${\mathrm{MAPbCl}}_3$ were $\sim 300$ and $\sim 110{\mathrm{cm}}^{-1}$, respectively.^127,128 Liu et al.¹²⁹ demonstrated that the optical gain coefficient of ${\mathrm{CsPbBr}}_3$ nanocuboids can be calculated to be $\sim 502{\mathrm{cm}}^{-1}$ under the 800 nm laser. Then, Zhao et al. reported efficient two-photon ASE from ${\mathrm{CsPbBr}}_3$ single crystals with a millimeter size and an optical gain of $38{\mathrm{cm}}^{-1}$.¹³⁰

3.2.

Perovskite QDs Laser

In the case of perovskite QDs without an external cavity, the amplification was generated from multiple scattering between QDs, enabling random fluctuations of lasing modes.¹²⁷ In 2015, Kovalenko and coworkers reported low-threshold ASE from colloidal ${\mathrm{CsPbX}}_3$ NCs with an optical gain coefficient of $\sim 450{\mathrm{cm}}^{-1}$ and threshold of $\sim 5\mu \mathrm{J}/{\mathrm{cm}}^2$.¹²⁷ In Figs. 9(a)–9(c), the ASE from ${\mathrm{CsPbX}}_3$ NCs could be tuned from 440 to 700 nm. Finally, they obtained random lasing from ${\mathrm{CsPbX}}_3$ films without the resonant cavity and whispering gallery mode (WGM) lasing using a silica sphere as the resonant cavity [Fig. 9(c)].¹²⁷ Besides, coating perovskite QDs onto an external cavity, Zeng and coworkers developed another method to form resonant cavities for perovskite QDs. They obtained enhanced random lasing from strong scattering in the $\text{perovskite}/{\mathrm{SiO}}_2$ composite with low threshold of $\sim 40\mu \mathrm{J}/{\mathrm{cm}}^2$ [Figs. 9(d)–9(f)].¹³¹ Similarly, Yang et al.¹³⁶ realized upconversion random lasing from ${\mathrm{FAPbBr}}_3/\mathrm{A}\text{-}{\mathrm{SiO}}_2$ composites with a threshold of $\sim 413.7\mu \mathrm{J}/{\mathrm{cm}}^2$. Liu et al.¹³⁷ obtained WGM and random lasing with a threshold of $\sim 430\mu \mathrm{J}/{\mathrm{cm}}^2$ under 800 nm excitation by embedding ${\mathrm{CsPbBr}}_3$ QDs into a single silica sphere. In addition, microcapillary tubes can also be used to build WGM cavities for perovskite QDs. In 2015, Zeng and coworkers observed lasing emission from ${\mathrm{CsPbBr}}_3$ QDs by filling the ${\mathrm{CsPbBr}}_3$ QDs into a capillary tube, which acted as a WGM cavity for perovskite QDs film around the inner wall.¹³⁸ Later, stable two-photon pumped WGM lasing was realized by coupling ${\mathrm{CsPbBr}}_3$ and ${\mathrm{FAPbBr}}_3$ perovskite QDs into microtubules with thresholds of $\sim 0.8$ and $\sim 0.31\mathrm{mJ}/{\mathrm{cm}}^2$ [Figs. 9(g)–9(j)], respectively.^132,139

Fig. 9

(a) TEM images of ${\mathrm{CsPbBr}}_3$ QDs. Figures reproduced from Ref. 127. (b) Spectral tunability of ASE of ${\mathrm{CsPbX}}_3$ via compositional modulation. Figures reproduced from Ref. 127. (c) Evolution from PL to lasing in an MS resonator with increasing pump intensity. Figures reproduced from Ref. 127. (d) SEM image and (e) isolation effect of ${\mathrm{CsPbBr}}_3$ $\mathrm{QDs}/\mathrm{A}\text{-}{\mathrm{SiO}}_2$ composites. Figures reproduced from Ref. 131. (f) PL spectra from ${\mathrm{CsPbBr}}_3$ $\mathrm{QDs}/\mathrm{A}\text{-}{\mathrm{SiO}}_2$ composite with increasing pump intensity. Figures reproduced from Ref. 131. (g) TEM image of ${\mathrm{FAPbBr}}_3$ QDs. (h) Two-photon PL spectra from ${\mathrm{FAPbBr}}_3$ NCs in a microcapillary tube. (i) Optical image and (j) lasing emission spectra from ${\mathrm{FAPbBr}}_3$ NCs in a microcapillary tube. Figures reproduced from Ref. 132. (k) Left: PL spectra from ${\mathrm{CsPbBr}}_3$ film within/without microcavity. Right: Schematic of the ${\mathrm{CsPbBr}}_3$ VCSEL. Figures reproduced from Ref. 133. (l) Schematic of the ${\mathrm{CsPbBr}}_3$ VCSEL. Figures reproduced from Ref. 134. (m) Photograph and PL stability of flexible ${\mathrm{FAPbBr}}_3$ VCSEL. Figures reproduced from Ref. 135.

Besides the realization of perovskite QDs lasing-based silica sphere and microcapillary tube, the well-designed distributed Bragg reflector (DBR) can also be used to achieve a vertical-cavity surface-emitting laser (VCSEL).^133–135 In 2017, Zeng and coworkers first fabricated VCSELs with a sandwiched structure of $\mathrm{DBR}/{\mathrm{CsPbBr}}_3$ QDs/DBR, which exhibited a low threshold $\sim 9\mu \mathrm{J}/{\mathrm{cm}}^2$ directional output and favorable stability [Fig. 9(k)].¹³³ The lasing emission of ${\mathrm{CsPbX}}_3$-based VCSELs can be tuned in the visible light range.¹³³ In the same year, Huang et al.¹³⁴ fabricated ${\mathrm{CsPbBr}}_3$ QDs VECSLs with ultralow threshold of $\sim 0.39\mu \mathrm{J}/{\mathrm{cm}}^2$ [Fig. 9(l)]. Organic hybrid perovskites-based VCSELs have also been performed. Chen and Nurmikko¹³⁵ developed ${\mathrm{FAPbBr}}_3$-based VCSELs by embedding ${\mathrm{FAPbBr}}_3$ solid thin films in two DBRs [Fig. 9(m)] with a threshold of $\sim 18.3\mu \mathrm{J}/{\mathrm{cm}}^2$ under subnanosecond pulse excitations. They also demonstrated that the VCSEL device fabrication process can be applicable to flexile substrates, as shown in Fig. 9(m), which extended further practical applications for perovskite-based laser devices.¹³⁵ Most recently, Li et al.¹⁴⁰ fabricated a two-photon-pumped ${\mathrm{MAPbBr}}_3$ VCSEL by intergrading ${\mathrm{MAPbBr}}_3$ with DBR and Ag mirrors with a threshold of $\sim 421\mu \mathrm{J}/{\mathrm{cm}}^2$, a $Q$ factor of $\sim 1286$, and a small divergence of $\sim 0.5\mathrm{deg}$.

3.3.

Perovskite Nanowire/Nanorod Laser

Owing to the difference between the refractive index of perovskite material and air, the reflection can occur at the output interface easily, acting as optical reflector.^141,142 Hence, different from QDs, single perovskite crystals structures such as rods, wires, plates, cubes, and spheres can act as Fabry–Pérot (F-P) or WGM cavities by themselves, since the light can be confined in the resonant cavity with regular morphology and smooth end faces.¹²⁵ For the 1D NWs structure, light will propagate along 1D and form resonance between two end-facets.¹⁴³ Hence, perovskite NWs and NRs have been confirmed as potential structures in optoelectronic devices and nanoscale-integrated photonics due to their unique optical properties, such as highly coherent output and efficient waveguide effect.¹⁴³

Zhu et al.¹⁴⁴ demonstrated perovskite NW lasers using high-quality ${\mathrm{MAPbX}}_3$ NWs, which had a regular shape with rectangular cross section [Fig. 10(a)]. Tunable F-P lasing could be observed from single ${\mathrm{MAPbX}}_3$ NWs with low threshold of $\sim 0.22\mu \mathrm{J}/{\mathrm{cm}}^2$ and $Q$ factor of $\sim 3600$ at room temperature [Figs. 10(b) and 10(c)]. In the same year, Xing et al.⁴⁹ realized F-P lasing from ${\mathrm{MAPbI}}_3$ NWs with rectangular morphology and length of $\sim 20\mu \mathrm{m}$. The obtained NW laser exhibited low threshold of $\sim 11\mu \mathrm{J}/{\mathrm{cm}}^2$ and $Q$ factor of $\sim 405$, and the lasing wavelength could be tuned in the range of 551 to 777 nm.⁴⁹ In case of lasing from all-inorganic perovskite NWs, Yang and coworkers realized F-P lasing from ${\mathrm{CsPbBr}}_3$ NWs with a threshold of $\sim 5\mu \mathrm{J}/{\mathrm{cm}}^2$ and a $Q$ factor of $\sim 1009$.⁵⁵ Fu et al. realized wavelength widely tunable F-P lasing from ${\mathrm{CsPbX}}_3$ NWs. The lasing wavelength could be tuned in the visible spectral region from 420 to 710 nm [Figs. 10(e)–10(g)].¹⁴⁵ In 2017, lasing emission from triangular ${\mathrm{CsPbX}}_3$ micro/NRs with an ultrasmooth surface by the vapor-phase approach was reported. The obtained lasing could be tuned in the range from 428 to 628 nm, with low threshold of $\sim 14.1\mu \mathrm{J}/{\mathrm{cm}}^2$ and high $Q$ factor of $\sim 3500$.⁷⁵ Efficient multiphoton pumped lasing in a wide excitation wavelength range (700 to 1400 nm) was realized.¹⁴⁷ Most of the single perovskite NWs mainly exhibited single-band lasing emission. In 2020, Tang et al. fabricated a single ${\mathrm{CsPbCl}}_{3-3x}{\mathrm{Br}}_{3x}$ alloy NW via a solid–solid anion-diffusion process. They realized continuous F-P lasing in single as-prepared NWs, which could be tuned from 480 to 525 nm [Figs. 10(i) and 10(j)].¹⁴⁶

Fig. 10

(a) SEM of ${\mathrm{MAPbI}}_3$ nanostructures. Figures reproduced from Ref. 144. (b) Optical image of single ${\mathrm{MAPbI}}_3$ NW. Figures reproduced from Ref. 144. (c) PL spectra of ${\mathrm{MAPbI}}_3$ NW around the lasing threshold. Figures reproduced from Ref. 144. (d) Broad tunable lasing from single-crystal ${\mathrm{MAPbX}}_3$ NW. Figures reproduced from Ref. 144. (e) SEM image of ${\mathrm{CsPbBr}}_3$ nanostructures. Figures reproduced from Ref. 145. (f) Fluorescence images of red/green/blue ${\mathrm{CsPbX}}_3$ NWs above lasing threshold. Figures reproduced from Ref. 145. (g) Broad tunable lasing from single-crystal ${\mathrm{CsPbX}}_3$ NWs. Figures reproduced from Ref. 145. (h) The photograph and PL spectra of a single ${\mathrm{CsPbCl}}_{3-3x}{\mathrm{Br}}_{3x}$ NW. Figures reproduced from Ref. 146. (i) The schematic of optically pumping lasing from a single ${\mathrm{CsPbCl}}_{3-3x}{\mathrm{Br}}_{3x}$ NW. Figures reproduced from Ref. 146. (j) Typical lasing spectra from a single ${\mathrm{CsPbCl}}_{3-3x}{\mathrm{Br}}_{3x}$ NW. Figures reproduced from Ref. 146.

3.4.

Perovskite Nano/Microplate Laser

Different from the F-P cavity formed by NWs/NRs, the perovskite 2D structure such as NPs will result in the WGM optical resonant cavity, which has a higher $Q$ factor than the F-P cavity. In 2014, Zhang et al. first realized WGM lasing from ${\mathrm{MAPbI}}_3$ NPs with well-defined hexagonal and triangular shapes under femtosecond-pulsed laser excitation. The lasing wavelength was located at $\sim 780\mathrm{nm}$ with a threshold of $\sim 37\mu \mathrm{J}/{\mathrm{cm}}^2$ [Figs. 11(a)–11(d)].⁸⁰ Liao et al.¹⁵⁰ obtained single-mode WGM lasing from single ${\mathrm{MAPbBr}}_3$ MDs peaked at $\sim 557.5\mathrm{nm}$ with a threshold of $\sim 3.6\mu \mathrm{J}/{\mathrm{cm}}^2$ and $Q$ factor of $\sim 430$. Liu et al. realized WGM lasing from ${\mathrm{MAPbI}}_3$ MP arrays with low threshold of $\sim 11\mu \mathrm{J}/{\mathrm{cm}}^2$ and $Q$ factor of $\sim 1210$.¹⁵¹ Moreover, they observed single mode lasing by shortening the size of MPs.¹⁵¹ Qi et al.¹⁵² demonstrated that the threshold of the MPs laser decreases linearly depending on the later size, and the cavity mode density increases with the size. In 2019, WGM lasing from a triangular ${\mathrm{MAPbI}}_3$ perovskite NP with a lateral length of $27\mu \mathrm{m}$ and thickness of 80 nm was realized at room temperature. The threshold of the WGM laser was $\sim 18.7\mu \mathrm{J}/{\mathrm{cm}}^2$ and $Q$ factor was $\sim 2600$ [Figs. 11(e)–11(i)].¹⁴⁸

Fig. 11

(a) Schematic of an ${\mathrm{MAPbX}}_3$ NP pumped by a pulsed laser. Figures reproduced from Ref. 80. (b) Optical image of ${\mathrm{MAPbI}}_3$ NPs under white light and laser excitation. Figures reproduced from Ref. 80. (c) Lasing spectra of hexagonal ${\mathrm{MAPbI}}_3$ NPs (upper) and the lasing mode evaluation with pumping fluence (bottom). Figures reproduced from Ref. 80. (d) Upper: Lasing spectra of triangular ${\mathrm{MAPbI}}_3$ NPs with different edge length. Bottom: The wavelength of lasing modes and $Q$-factor as a function of the triangular cavity edge length. Figures reproduced from Ref. 80. (e) Schematic of triangular ${\mathrm{MAPbI}}_3$ NPs pumped by a 343 nm laser. Figures reproduced from Ref. 148. (f) Optical image of triangular ${\mathrm{MAPbI}}_3$ NPs. Figures reproduced from Ref. 148. (g) 2D plot of a triangular ${\mathrm{MAPbI}}_3$ NP emission under different pump densities. Figures reproduced from Ref. 148. (h) The emission spectra from ${\mathrm{MAPbI}}_3$ NPs around the lasing threshold. Figures reproduced from Ref. 148. (i) Output emission intensity as a function of pump densities. Figures reproduced from Ref. 148. (j) Schematic of a ${\mathrm{CsPbX}}_3$ plate under a 400 nm laser. Figures reproduced from Ref. 149. (k) Emission spectra at different pump intensities. Figures reproduced from Ref. 149. (l) Tunable lasing spectra and images of individual ${\mathrm{CsPbX}}_3$ perovskite NPs. Figures reproduced from Ref. 149. (m) Single-mode lasing of ${\mathrm{CsPbBr}}_x{\mathrm{I}}_{3-x}$. Figures reproduced from Ref. 149. (n) Schematic of a ${\mathrm{CsPbI}}_3$ NS on mica substrate. Figures reproduced from Ref. 86. Excitation intensity-dependent emission spectra under (o) 470 nm and (p) 1200 nm excitation. Figures reproduced from Ref. 86. (q) Gaussian fitting of a lasing mode under 470 and 1200 nm laser. Figures reproduced from Ref. 86.

As for the 2D all-inorganic perovskite-based laser, Zhang et al.¹⁴⁹ obtained WGM excitonic lasing from single-crystalline ${\mathrm{CsPbX}}_3$ NPs with micron-size length and subwavelength thickness [Figs. 11(j)–11(m)]. Multicolor lasing from 410 to 700 nm was realized in these NPs at room temperature [Fig. 11(l)]. The lasing threshold of the ${\mathrm{CsPbX}}_3$ NP was as low as $\sim 2.0\mu \mathrm{J}/{\mathrm{cm}}^2$, and the linewidth of the WGM modes was $\sim 0.14$ to 0.15 nm [Fig. 11(m)].¹⁴⁹ Zheng et al.⁸⁶ demonstrated that ${\mathrm{CsPbI}}_3$ perovskite NSs possess WGM lasing under both one- and two-photon pumps with low-threshold-pumped excitation [Figs. 11(n)–11(q)]. The thresholds of lasing were $\sim 0.30$ and $\sim 2.6\mathrm{mJ}/{\mathrm{cm}}^2$ under one- (470 nm) and two-photon (1200 nm) excitation, and the $Q$ factors were $\sim 1489$ and $\sim 1179$, respectively, which is three times higher than the reported values of organic–inorganic lead halide perovskite NS. Most recently, Liu et al.¹⁵³ realized two lasing modes (F-P and WGM) in the all-inorganic perovskite ${\mathrm{CsPb}}_2{\mathrm{Br}}_5$ MPs with subwavelength thickness and uniform square shape under two-photon pump. Remarkably, low-threshold F-P multimode lasing with $Q$ factor of $\sim 3551$ and single-mode WGM lasing with $Q$ factor of $\sim 3374$ from the same MP at room temperature have been achieved successfully.

3.5.

Perovskite Laser with 3D Structure

A single perovskite spherical 3D structure has also usually been demonstrated as a WGM cavity. In comparison with other nano/microstructure resonant cavities, the coupling between the sphere cavity and substrate was relatively weak, which resulted in less optical losses. Zhang and coworkers realized single-mode lasing in ${\mathrm{CsPbX}}_3$ MSs with regular sphere shape and submicron size at room temperature [Figs. 12(a)–12(d)].¹⁰⁰ The line width of WGM lasing was $\sim 0.09\mathrm{nm}$, the threshold was $\sim 0.42\mu \mathrm{J}/{\mathrm{cm}}^2$, and $Q$ factor was $\sim 6100$ [Fig. 12(c)]. In addition, the single-mode lasing can be tuned in the whole visible region through element modulation and size control of perovskite MSs [Fig. 12(d)].¹⁰⁰ Furthermore, they achieved two-photon single-mode lasing with linewidth of $\sim 0.037\mathrm{nm}$ and $Q$ factor of $\sim 1.5\times {10}^4$ from a single ${\mathrm{CsPbBr}}_3$ MS at room temperature, which are the best values obtained in perovskite-based micro/nanocavities until now.¹⁵⁴ Moreover, these perovskite MS lasers showed uniform lasing emission, which could be observed in the range from $-30\mathrm{deg}$ to 30 deg.^154,155

Fig. 12

(a) Schematic of a single ${\mathrm{CsPbBr}}_3$ MS under 400 nm laser. Figures reproduced from Ref. 100. (b) Lasing PL spectra from a single ${\mathrm{CsPbBr}}_3$ MS under different pump intensities. Figures reproduced from Ref. 100. (c) Lorentzian fitting of a lasing mode. Figures reproduced from Ref. 100. (d) Photograph and lasing emission of multicolor ${\mathrm{CsPbX}}_3$ MS lasers. Figures reproduced from Ref. 100. (e) SEM image and (f) schematics of F-P cavity of ${\mathrm{CsPbBr}}_3$ nanocuboids. Figures reproduced from Ref. 129. (g) Single-mode lasing spectra and (h) TA spectroscopic data of ${\mathrm{CsPbBr}}_3$ nanocuboids under two-photon excitation. Figures reproduced from Ref. 129. (i) Schematic of a cube-corner ${\mathrm{MAPbBr}}_3$ pyramid under 405 nm laser. Figures reproduced from Ref. 102. (j) PL spectra of a cube-corner ${\mathrm{MAPbBr}}_3$ and (k) output intensity as a function of excitation power. Figures reproduced from Ref. 102. (l), (m) Multimode lasing spectra of a cube-corner pyramid of ${\mathrm{MAPbBr}}_3$ on (l) mica and (m) mica/Ag. Figures reproduced from Ref. 102.

Another 3D structure generally used for perovskite lasing is the nano/microcube. Liu et al.¹²⁹ obtained F-P lasers from an individual ${\mathrm{CsPbBr}}_3$ nanocuboid with subwavelength scale for the first time [Figs. 12(e)–12(h)]. They realized single-mode F-P lasing from a ${\mathrm{CsPbBr}}_3$ nanocuboid with low thresholds of $\sim 40.2$ and $\sim 374\mu \mathrm{J}/{\mathrm{cm}}^2$ and $Q$ factors of $\sim 2075$ and $\sim 1859$ under one- and two-photon pumps, respectively.¹²⁹ The physical volume of the obtained laser is $\sim 0.49{\lambda }^3$. Moreover, the pulse duration is only $\sim 22\mathrm{ps}$, which is consistent with the resulting fast decay of SE observed by fs transient absorption spectroscopy [Fig. 12(h)].¹²⁹ Cube-corner pyramid cavities could also act as microretroreflectors. In 2018, Mi et al. realized F-P lasing in cube-corner ${\mathrm{MAPbBr}}_3$ pyramids at room temperature [Figs. 12(i)–12(m)].¹⁰² Furthermore, the threshold of lasing could be reduced from $\sim 92$ to $26\mu \mathrm{J}/{\mathrm{cm}}^2$ by coating a thin layer of Ag film on a mica substrate [Figs. 12(l) and 12(m)].¹⁰² Most recently, Yang et al.¹⁰³ also realized F-P lasing from a single ${\mathrm{CsPbI}}_3$ triangular pyramid with a microsize at low temperature. They demonstrated that the temperature-dependent lasing threshold can be reduced from $\sim 53.15$ to $21.56\mu \mathrm{J}/{\mathrm{cm}}^2$ with corresponding temperature from 223 to 148 K.¹⁰³

3.6.

Perovskite Nanolaser Array

In comparison with single perovskite lasers, laser arrays with high-density patterns and high-precision arrangements are more necessary for mass-produced, compact on-chip optoelectronic circuit integration. In 2016, Wang et al. fabricated ${\mathrm{MAPbBr}}_3$ microwire arrays and realized high density perovskite lasers from these microwire arrays [Figs. 13(a) and 13(b)], in which all of the subunits generated the same lasing emission.¹⁵⁶ The minimum unit period was 800 nm, presenting an integration density of nanolasers as high as $1250{\mathrm{mm}}^{-1}$. In 2017, Fu and coworkers prepared ${\mathrm{MAPbBr}}_3$ NW arrays with the width from 460 to 2500 nm, height from 80 to 1000 nm, and length from 10 to $50\mu \mathrm{m}$. These perovskite NW arrays were demonstrated as almost identical optical resonance cavities with a low threshold of $\sim 10.2\mu \mathrm{J}/{\mathrm{cm}}^2$ [Figs. 13(c) and 13(d)].¹⁵⁷ In 2016, Liu et al. realized WGM lasing from patterned ${\mathrm{MAPBI}}_3$ microplatelets arrays with a threshold of $\sim 11\mu \mathrm{J}/{\mathrm{cm}}^2$ and $Q$ factor up to $\sim 1210$. The wavelength tunability and single mode lasing could be selected by changing platelet sizes or breaking the symmetry of the designed laser pattern.¹⁵¹ In the same year, Feng et al.¹⁵⁸ demonstrated that the ${\mathrm{MAPbBr}}_3$ square MP array shows high-performance WGM lasing with tunable mode and low lasing threshold [Figs. 13(e) and 13(f)]. Single mode lasing was obtained from a $2.1\text{-}\mu \mathrm{m}$ ${\mathrm{MAPbBr}}_3$ square. Lin et al.¹⁵⁹ fabricated a large-area ${\mathrm{CsPbX}}_3$ QDs array by a photolithographical approach, which could be used as efficient lasing structures and emitting pixel arrays [Figs. 13(g)–13(i)]. They realized WGM lasing from the QD arrays with high $Q$ factor and demonstrated that this patterning technique can be used in large-area perovskite laser arrays with multicolor pixels [Fig. 13(g)].¹⁵⁹ Most recently, Wang et al.¹⁶⁰ fabricated large-area ${\mathrm{MAPbX}}_3$ MD arrays via a screen-printing technique [Figs. 13(j)–13(m)]. They obtained tunable WGM lasing from these ${\mathrm{MAPbX}}_3$ MD arrays with a threshold of $\sim 21.3\mu \mathrm{J}/{\mathrm{cm}}^2$ and a $Q$ factor of $\sim 1570$ successfully. Multicolor WGM lasing emission could be tuned from $\sim 510$ to 650 nm [Fig. 13(l)].¹⁶⁰ In 2020, Song and coworkers employed the topologically protected optical bounded states in the continuum (BICs) and demonstrated the ultrafast control of perovskite-based vortex microlasers at room temperature. They proved that vortex beam lasing based on perovskite metasurfaces could be switched to linearly polarized beam lasing with switching time of 1 to 1.5 ps. The energy consumption was several orders of magnitude lower than that of previously reported all-optical switching.¹⁶¹

Fig. 13

(a) SEM image of the ${\mathrm{MAPbBr}}_3$ microwire on silicon grating. Figures reproduced from Ref. 156. (b) Laser spectrum of ${\mathrm{MAPbBr}}_3$ microwire. Figures reproduced from Ref. 156. (c) Optical image of ${\mathrm{MAPbBr}}_3$ NW arrays. Figures reproduced from Ref. 157. (d) PL spectra of a single ${\mathrm{MAPbBr}}_3$ NW under 400 nm laser. Figures reproduced from Ref. 157. (e) “LASER” patterned perovskite square MPs. Figures reproduced from Ref. 158. (f) Lasing spectra from perovskite MPs with different sizes. Figures reproduced from Ref. 158. (g) PL image of green and red QD arrays. Figures reproduced from Ref. 159. (h) Emission intensity versus excitation fluence measured from a ${\mathrm{CsPbBr}}_3$ MD. Figures reproduced from Ref. 159. (i) Lasing spectra from ${\mathrm{CsPbBr}}_3$ MDs with different diameters. Figures reproduced from Ref. 159. (j) PL spectra from a typical ${\mathrm{MAPbBr}}_3$ MD with different power energies and (k) output intensity and FWHM as a function of pump intensity. Figures reproduced from Ref. 160. (l) Widely tunable lasing from ${\mathrm{MAPbX}}_3$ arrays. Figures reproduced from Ref. 160. (m) Left: SEM image of the fabricated perovskite metasurface; right: ultrafast control of the quasi-BIC microlasers. Figures reproduced from Ref. 161.

3.7.

Others

Surface-plasmon (SP) is an excited state with large enhancement of the electromagnetic field localized at the metal–dielectric interface, which provides confinement on the subwavelength scale, overcoming the diffraction limit of light.¹⁶² In perovskite micro/nanolasers, SPs have been demonstrated as an effective method to tailor the properties of lasers. In general, SPs could be generated by the metal layer, such as Au or Ag, and transfer along the semiconductor–metal interface. Kao et al.¹⁶³ reduced the lasing threshold of perovskite by strong exciton–plasmon coupling between the Ag and perovskite films [Fig. 14(a)],¹⁶³ in which the confined optical fields between Ag and perovskite films could be enhanced about 19.3 and 7.7 times in comparison with bare perovskites and perovskites coated by Ag thin film, respectively. In 2017, Wang et al. deposited Al nanoparticles onto the surface of ${\mathrm{CsPbBr}}_3$ perovskites. The lasing thresholds of ${\mathrm{CsPbBr}}_3$ perovskite microrods were significantly reduced by $>20\%$, and the output intensities were significantly enhanced via the plasmonic resonances.¹⁷² In 2019, Wu et al. reported a method to enhance ASE performance of ${\mathrm{MAPbI}}_3$ films by adding Au NRs-doped PMMA on ${\mathrm{MA}}_3{\mathrm{PbI}}_3$ perovskite films. The ASE threshold was significantly reduced by $\sim 36\%$, and the output intensity increased by 13.9-fold with the plasmon resonance enhancement of Au NRs.¹⁷³ Yang et al.¹⁷⁴ also reduced the lasing threshold of ${\mathrm{CsPbBr}}_3$ perovskite nanocubes significantly by $\sim 33\%$ via the surface plasmonic effect of Au nanoparticles. In 2021, single-mode upconversion plasmonic lasing from ${\mathrm{MAPbBr}}_3$ perovskite NCs was realized by Lu et al.,¹⁶⁴ exhibiting low threshold $\sim 10\mu \mathrm{J}/{\mathrm{cm}}^2$ and small mode volume $\sim 0.06{\lambda }^3$ at 6 K, where TiN was used as a promising resonance adjustable plasmonic platform [Fig. 14(b)]. Hsieh et al.¹⁶⁵ realized continuous-wave (CW) lasing from a single ${\mathrm{CsPbBr}}_3$ QD in a plasmonic gap-mode nanocavity with low threshold of $\sim 1.9\mathrm{W}/{\mathrm{cm}}^2$ and small mode volume of $\sim 0.002{\lambda }^3$ [Fig. 14(c)]. Most recently, Li et al.¹⁶⁶ proposed a hybrid nanocavity composed of ${\mathrm{CsPbBr}}_3$ nanoparticles and a thin Au film, which could realize optically controlled quantum size effect by the reversible phase transition from polycrystalline to monocrystalline [Fig. 14(d)]. These results demonstrated that SPs could not only modulate the performance of perovskite lasers but also can realize deep subdiffraction plasmonic lasers.

Fig. 14

(a) Field intensity distributions and schematic structure of Ag/PMMA/perovskite. Figures reproduced from Ref. 163. (b) Schematic and working process of plasmonic nanolaser of ${\mathrm{MAPbBr}}_3/{\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{TiN}$. Figures reproduced from Ref. 164. (c) Schematic and calculated electric field distribution of plasmonic nanolaser based on ${\mathrm{CsPbBr}}_3$ QDs. Figures reproduced from Ref. 165. (d) Schematic of phase transition from polycrystalline to monocrystalline ${\mathrm{CsPbBr}}_3$ nanoparticles by adjusting the laser power and the PL spectrum under different laser power. Figures reproduced from Ref. 166. (e) Schematic polaritons in a micro/NW cavity and lasing spectrum of ${\mathrm{CsPbBr}}_3$ NWs. Figures reproduced from Ref. 167. (f) Schematic of CW lasing of ${\mathrm{CsPbBr}}_3$ nanoribbons. Figures reproduced from Ref. 168. (g) Schematic structure of ${\mathrm{CsPbBr}}_3$ flakes/DBR microcavity and SEM image of ${\mathrm{CsPbBr}}_3$ flakes. Figures reproduced from Ref. 169. (h) Cascade energy transfer in quasi-2D perovskite and tunable ASE from solution-processed ${(\mathrm{NMA})}_2(\mathrm{FA}){\mathrm{Pb}}_2{\mathrm{Br}}_y{\mathrm{I}}_{7-y}$ films. Figures reproduced from Ref. 170. (i) Chemical structures of quasi-2D perovskite with different organic cations and CW lasing characteristics of quasi-2D perovskite films. Figures reproduced from Ref. 171.

Perovskite is also an ideal candidate to realize a room temperature exciton polariton laser, which mainly results from the strong exciton–photon coupling between the gain media and nanocavity. In perovskite laser researches, room temperature exciton polaritons have been realized with various nanostructures.^{167,168,175,176} Perovskite NCs with self-assembled morphology can provide optical resonators due to the confinement of exciton–photon coupling. On the other hand, a planar optical cavity composed of two mirrors can be used as an F-P cavity conventionally. In 2018, Liu et al.¹⁶⁷ observed strong exciton–photon coupling in single ${\mathrm{CsPbBr}}_3$ micro/NWs and ${\mathrm{MAPbBr}}_3$ micro/NWs, respectively [Fig. 14(e)]. Moreover, polariton lasing was realized at room temperature with exceptionally large vacuum Rabi splitting of $\sim 656$ and 390 meV.^167,175 Shang et al.¹⁷⁶ proved light could propagate as an exciton–photon in ${\mathrm{CsPbBr}}_3$ NWs at room temperature, increasing optical absorption and emission in comparison with bulk crystals. They demonstrated that the decrease of ${\mathrm{CsPbBr}}_3$ dimensions could enhance the exciton–photon coupling strength, which increased the exciton fraction. Furthermore, they found that the increase of temperature could significantly decrease the exciton fraction of exciton–photons, causing high thresholds and restraining CW lasing above 100 K. They successfully realized CW-pumped lasing from ${\mathrm{CsPbBr}}_3$ nanoribbons by reducing the height to $\sim 120\mathrm{nm}$ on sapphires with low threshold of $\sim 0.13\mathrm{kW}/{\mathrm{cm}}^2$ [Fig. 14(f)].¹⁶⁸ Then, they coupled ${\mathrm{MAPbBr}}_3$ NWs with a hybrid plasmonic microcavity to enhance exciton–photon interaction.¹⁷⁷ They observed a Rabi-splitting up to $\sim 564\mathrm{meV}$ in a hybrid $\text{perovskite}/{\mathrm{SiO}}_2/\mathrm{Ag}$ waveguide microcavity at room temperature. In 2017, Su et al.¹⁷⁸ reported room-temperature polariton lasing based on an epitaxy-free all-inorganic ${\mathrm{CsPbCl}}_3$ nanoplatelet embedded in DBRs, supporting F-P oscillations. The polariton lasing exhibited a threshold of $\sim 12\mu \mathrm{J}/{\mathrm{cm}}^2$. Zhang et al.¹⁶⁹ investigated the trapping of polaritons in micron-sized ${\mathrm{CsPbBr}}_3$ flakes embedded in DBRs as a microcavity [Fig. 14(g)]. They demonstrated quantized polariton states arising from the optical confinement of flakes.

In comparison with perovskite NCs with 3D structure, quasi-2D perovskites have a quantum well (QW) structure with the advantages of large exciton binding energy and low nonradiation loss and are more easily coupled with a resonant cavity. In 2018, Fieramosca et al. observed strong exciton–photon coupling from hybrid 2D perovskite flakes. The organic ligands efficiently affected the out-of-plane exciton–photon coupling, suggesting that the organic interlayer plays a significant role in the anisotropy of the exciton and exciton polariton.¹⁷⁹ Then, they observed highly interacting polaritons in ${(\mathrm{PEA})}_2{\mathrm{PbI}}_4$ with an excitonic interaction constant as $\sim 3\mu \mathrm{eV}\mu {\mathrm{m}}^2$, which was two orders higher than that of organic excitons.¹⁸⁰ Zhang et al.¹⁸¹ investigated cavity polariton modes in 2D perovskite ${(\mathrm{PEA})}_2{\mathrm{PbBr}}_4$ sheets. The perovskite layer naturally could act as an F-P cavity and exhibited evident cavity polariton modes with Rabi splitting energy of $\sim 259\mathrm{meV}$. Li et al.¹⁷⁰ first reported room temperature optical gain from 2D perovskite ${(\mathrm{NMA})}_2{\mathrm{FA}}_{n-1}{\mathrm{Pb}}_n{\mathrm{X}}_{3n+1}$ ($\mathrm{NMA}={\mathrm{C}}_{10}{\mathrm{H}}_7{\mathrm{CH}}_2{{\mathrm{NH}}_3}^{+}$). In these layered perovskite nanostructures, multiple QW phases naturally form an energy cascade, enabling an ultrafast energy transfer process from higher energy bandgap QWs ($n<5$) to lower energy bandgap QWs ($n>5$). They obtained tunable ASE ranging from 530 to 810 nm with low ASE threshold ($<20.0\mu \mathrm{J}/{\mathrm{cm}}^2$) [Fig. 14(h)]. Later, lasing based on these quasi-2D perovskite nanostructure has also been realized by researchers, e.g., Liang et al. investigated multicolor lasing from ${(\mathrm{BA})}_2{(\mathrm{MA})}_{n-1}{\mathrm{Pb}}_n{\mathrm{I}}_{3n+1}$ ($\mathrm{BA}={\mathrm{C}}_4{\mathrm{H}}_9{{\mathrm{NH}}_3}^{+}$) in 2019.¹⁸² Most recently, Liu et al. shrank the quasi-2D perovskites laser to the deep-subwavelength scale with 50 nm, which was the smallest room temperature all-dielectric laser.¹⁸³ They revealed the contribution from excitons and polarons to the high optical gain, which provided an insight into the design of next-generation integrated laser sources. Qin et al.^171,184 found that the triplet excitons in hybrid quasi-2D perovskite have a lifetime up to $1\mu \mathrm{s}$, which might cause the disappearance of the laser. Then, using a distributed-feedback (DFB) cavity with a high $Q$ and triplet management strategies, they realized stable room-temperature CW lasing in quasi-2D perovskite films [Fig. 14(i)]. The representative works about perovskite lasers in recent years are summarized in Table 1. All of these progresses prove the potential of perovskite materials in micro/nanolasers.

Table 1

Lasing performance of perovskite.