The phenomenon of chirality, a fascinating aspect of nature, is gaining increasing attention. Chiral materials exhibiting circularly polarized luminescence (CPL) are emerging in various fields, including biological probes [1–3], three-dimensional displays [4–7], optoelectronic devices [8–11], and photocatalytic asymmetric synthesis [12–15]. Chiral lasers, in particular, hold promising applications in areas such as biological imaging [16–18] and quantum information processing [19–21]. For instance, Yuan et al. fabricated chiral biological microlasers using green fluorescent proteins or chiral biomolecules as gain material [22]; Zhang et al. designed a resonant metasurface structure that utilizes the intrinsic chirality and giant field enhancement of the metasurfaces to achieve controlled emission of circularly polarized laser [23]. However, the development of chiral lasers faces challenges, including the high cost of obtaining chiral gain materials and the complexity of fabricating structures to manipulate chirality. Consequently, there is a pressing need for cost-effective optical materials and scalable manufacturing strategies for chiral microlasers/nanolasers.

Metal-organic frameworks (MOFs) are a class of porous crystal material composed of metal ions or clusters interconnected by organic ligands through coordination bonds [24–26]. Their open and versatile structure enables the synthesis of chiral MOFs (CMOFs) using chiral molecular assembly methods. Using such CMOFs as a platform to generate CPL signals is uniquely advantageous. The interconnected dimensionally tunable porous skeletons in MOFs can encapsulate dye molecules. This encapsulation minimizes intermolecular interactions, thereby reducing aggregation-caused quenching (ACQ) effects among dyes [27,28]. Additionally, the strategic integration of crystal synthesis and chiral modules in MOFs facilitates the creation of precise chiral frameworks, providing a detailed model to study the connection between structural and optical chirality [29–34]. For instance, Zhao et al. successfully constructed chiral zeolite imidazolate framework (ZIF) displaying CPL by co-blending dimethylthiol-derived chiral emitters with ZIF-8 nanoparticles [35]. The obtained chiral ZIF nanomaterials bring fluorescence intensity enhancement to the chiral emitters, and an order of magnitude significantly amplifies the luminescence asymmetry factor (glum). Zhang et al. investigated the circularly polarized luminescence properties of chiral MOFs [36]. They prepared tunable CPL-emitting crystalline composites (L/D-MOFs⊃achiralfluorophore) using enantiomeric MOFs with helical apertures. By confining achiral organic fluorophores within the nanoscale helical channels of the enantiomer L/D-CMOF, they achieved the amplification of the emission asymmetry factor and further produced a white CPL with multiple dyes.

While most research has focused on the spontaneous emission (SE) process of CMOFs⊃dye, the potential of stimulated emission-induced circularly polarized amplified spontaneous emission (ASE) and lasing phenomena remains underexplored. Chiral lasing, with its enhanced coherence and emission intensity, can unlock new possibilities for chiral materials. Here, we delved into the circular polarization properties of stimulated emission from CMOFs/achiral luminophores and developed a straightforward, cost-effective method for constructing CMOFs-based chiral lasers. Initially, we achieved the chiral ASE phenomenon using chiral ZIF-8 powder co-assembled with achiral laser dyes 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). The chiral MOF host matrix was built by transferring chiral guest L/D-histidine to the non-chiral ZIF-8 nanoparticles (NPs). After loading achiral dye as the emitter, the obtained L/D-ZIF⊃DCM NPs were confirmed to exhibit vigorous CPL activity first. Subsequently, we observed that the synthesized L/D-ZIF⊃DCM NPs could attain stable ASE when population inversion occurred. The ASE intensity not only depends on the pump energy but also is influenced by the chiral polarization of the pump light and the chiral structure of the samples. We also observed that the right-handed (left-handed) component of emitted light from L-ZIF⊃DCM (D-ZIF⊃DCM) consistently surpassed its left-handed (right-handed) counterpart in both photoluminescence and ASE signals. Finally, we explored a strategy for chiral laser construction by integrating a vertical surface-emitting resonant cavity with L/D-ZIF⊃DCM NPs. Notably, the glum has increased to four-fold (from ±0.085 to ±0.176 and to ±0.310) during the process of converting from SE to ASE and then to lasing emission. The multiple increase of glum presents a novel approach to amplify chiral signals and also proposes an effective strategy to fabricate chiral lasers.

Zn(NO3)2·6H2O(99%) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM, 95%) were purchased from Aladdin. L/D-histidine (99%), 2-methylimidazole (98%), methanol (99.5%), and N,N-dimethylformamide (DMF, 99.5%) were purchased from Macklin. All chemicals were used as received without further purification.

First, 0.6 g of 2-methylimidazole was dissolved in 30 mL of methanol, and then 0.54 g of Zn(NO3)2·6H2O was dissolved in 30 mL of methanol. The configured Zn(NO3)2·6H2O solution was slowly added to the 2-methylimidazole solution, and the solution gradually changed to white. The reaction system was stirred at room temperature for 24 h. The solid product was collected by centrifugation and the product was washed twice by repeated centrifugation with methanol. Finally, the white powder was dried in an oven at 40°C.

0.52 g of 2-methylimidazole and 0.14 g of L/D-histidine were dissolved in 30 mL of a solution of water and methanol (volume ratio 2:3). 120 μL of triethylamine was added to the solution and stirred for 10 min at room temperature. Then, 30 mL of Zn(NO3)2·6H2O (1.8 mmol) methanol solution was slowly added to the mixed ligand solution. The reaction system was stirred at room temperature for 24 h and the crude product was obtained by centrifugation. The product was washed twice with a mixture of methanol and water and dried in an oven at 40°C.

0.52 g of 2-methylimidazole, 0.14 g of L/D-histidine, and 0.018 g of DCM were dissolved in 30 mL of a solution of water and methanol (volume ratio 2:3). 120 μL of triethylamine was added to the solution and stirred for 10 min at room temperature. Then, 30 mL of Zn(NO3)2·6H2O (1.8 mmol) methanol solution was slowly added to the mixed ligand solution. The reaction system was stirred at room temperature for 24 h and the crude product was obtained by centrifugation. The product was washed twice with N,N-dimethylformamide, and then twice with a mixture of water and methanol. Finally, the powder obtained was dried in an oven at 40°C.

First, L/D-ZIF⊃DCMDMF solution was prepared at a concentration of 300 mg/mL. Then 0.3 mL of the solution was added dropwise to the distributed Bragg reflector (DBR) mirror and slowly heated on a hot plate at 40°C. Finally, the silver-coated glass sheet was placed over the DBR and taped tightly around it.

The scanning electron microscopy (SEM) images were obtained on a Hitachi’s REGULUS8230. Transmission electron microscopy (TEM) images and energy dispersive spectrometer (EDS) mapping were obtained on the JEM-2100 (JEOL). The powder X-ray diffraction (XRD) pattern was measured on a SmartLab 9kW. The absorption spectra were measured on a Shimadzu uv3600 (Japan). CD spectra were measured on MOS-500 BIOLOGIC, France. The optical signal was collected by a spectrometer (QE65PRO, Ocean Optics, resolution of ∼0.4nm, integration time of 100 ms).

A straightforward co-assembly strategy was employed to synthesize chiral ZIF-8 loaded with the achiral laser dye [Fig. 1(a)]. In the synthesis process, 2-methylimidazole, L/D-histidine, and organic dye DCM were dissolved in a mixed solution of water and methanol, followed by the addition of triethylamine as a crystallization regulator to regulate the growth of ZIF-8 crystals. Upon adding Zn2+ to this mixed solution and waiting for the MOF growth to complete, the final product was collected and purified to eliminate the excess DCM. The literature suggests that the binding of L/D-histidine to ZIF-8 may be stochastic, with a ZIF-8 cell containing two cages with 24 methylimidazole ligands, each cage randomly binding 2–3 L/D-histidine molecules as linkers [37,38]. During the growth of chiral ZIF-8, the DCM dye is encapsulated into the nanochannels. Other common dyes were also studied, such as rhodamine and coumarin; unluckily, only the linear guest molecules can be encapsulated in the chiral ZIF-8 (Figs. 6 and 7, Appendix A) [37].

The right panel of Fig. 1(a) shows the stimulated emission process of DCM-loaded CMOFs. Excitation by external light causes electrons to jump from the ground state (S0) to a higher energy state (lowest excited state, S1) with different magnetic quantum numbers. Following the establishment of population inversion between S0 and S1, chiral MOFs break the equilibrium between left- and right-handed circular polarizations in stimulated emission. Hence, by introducing guest achiral luminophores into the chiral host frame, effective transmission and expression of chirality from host to guest can be achieved. After the dye molecules were encapsulated within the chiral ZIF-8 framework, the nanoporous skeleton in the MOF effectively inhibited the ACQ of the dye. Therefore, we can judge the successful encapsulation of dyes by observing the emission behavior of the chiral ZIF-8 powders [Fig. 7(a)].

Figures 1(b)–1(e) show the SEM images of the synthesized L/D-ZIF⊃DCM NPs. The noticeable increase in cell volume and alteration in morphology of L/D-ZIF⊃DCM compared to pure ZIF-8 (Fig. 8, Appendix B) can be attributed to the structural chirality imparted to ZIF-8 by the introduction of L/D-histidine. The structural similarities between L/D-ZIF⊃DCM and L-ZIF (shown in Fig. 9, Appendix B) confirm that unlike L/D-histidine, the dye molecules are encapsulated inside ZIF-8 rather than being assembled onto its framework. TEM images in Figs. 1(f)–1(i) validate the structural chirality of L/D-ZIF⊃DCM NPs. EDS mapping (Fig. 10, Appendix B) reveals a uniform distribution of C, N, O, and Zn in L/D-ZIF⊃DCM NPs.

As shown in Fig. 2(a), the XRD peaks of L/D-ZIF⊃DCM align with those of achiral ZIF-8 as well as the simulated data, confirming that L/D-ZIF⊃DCM maintains the same crystal structure as ZIF-8. Figure 2(b) displays the absorption and fluorescence spectra of the organic dye DCM in methanol. Figure 2(b) presents the absorption and fluorescence spectra of DCM in methanol, highlighting a broad absorption band between 400 and 550 nm and a fluorescence range from 550 nm to 700 nm. Additionally, we analyzed the absorption spectra of L-ZIF encapsulating various dyes (L-ZIF⊃dye) as shown in Fig. 11, Appendix B. The results revealed that alongside the intrinsic absorption peaks of L-ZIF (200–300 nm), new absorption peaks emerged corresponding to the absorption bands of the encapsulated dyes. L/D-ZIF⊃DCM displays a stable circular dichroic (CD) signal in the DCM gain range (Fig. 12, Appendix B), with L-ZIF⊃DCM showing a negative CD signal and D-ZIF⊃DCM the opposite, as depicted in Fig. 2(c). This difference highlights the distinct responses of the two chiral structures to circularly polarized light. Interestingly, L/D-ZIF⊃DCM exhibits CPL signals similar to its CD properties [Fig. 2(d)], emphasizing its effectiveness in displaying chiral properties.

To investigate the chiral characteristics of L/D-ZIF⊃DCM, the emission spectra of the film samples under circularly polarized laser excitation were explored using the experimental setup depicted in Fig. 3(a). The film samples were prepared by compressing equal amounts of L/D-ZIF⊃DCM powder between two glass sheets. The wavelength of the pump light was set at 532 nm, coinciding with the wavelengths where the CD spectra of L-ZIF⊃DCM and D-ZIF⊃DCM show significant negative and positive signals, respectively.

The pump laser beam passed through a quarter-wave plate, converting its vertical polarization into either left- or right-handed circular polarization. Subsequently, the laser beam, after being focused through a lens, was directed vertically onto the sample. Emissions from the sample were captured by a horizontally positioned objective lens and analyzed with an optical spectrometer. Initially, the emission spectrum of L-ZIF⊃DCM under a left-handed circularly polarized laser excitation was measured, as illustrated in Fig. 3(b). At the pump energy density of 1.27mJcm−2, a broad fluorescence peak centered at 610 nm with a full width at half-maximum (FWHM) of about 55 nm was observed. As the pump energy density gradually increases, the emission intensity increases and the spectrum narrows. When the pump energy density exceeds 5.09mJcm−2, the FWHM reduces to ∼20nm, and the emission intensity increases abruptly, indicating a transition from broad SE to ASE. The integrated emission intensity and FWHM versus pump energy density are shown in the illustration, indicating a threshold of ∼5.7mJcm−2 for ASE.

We also observed ASE behavior in L/D-ZIF⊃DCM under right-handed circularly polarized laser excitation [Fig. 3(c)]. The FWHM of L-ZIF-DCM initially at about 55 nm, narrowed to approximately 12 nm once ASE occurred. Notably, L-ZIF⊃DCM exhibited a stronger response to right-handed circularly polarized light compared to left-handed circularly polarized light. This disparity was manifested in a more intense fluorescence response under right-handed excitation at the same pump energy density and a lower ASE threshold for right-handed excitation than for left-handed excitation, as illustrated in Fig. 3(c). For the case of D-ZIF⊃DCM’s stimulated emission pumped by a chiral laser [Figs. 3(d) and 3(e)], we observed an opposite phenomenon. The emission signal of D-ZIF⊃DCM exhibited opposite intensity and ASE threshold contrast by different chiral laser pumping compared to L-ZIF⊃DCM. This can be attributed to the fact that L/D-ZIF⊃DCM with contrasting chiral structures responds differently to opposite circularly polarized optical absorption [22,39]. These results demonstrate that L/D-ZIF⊃DCM exhibits distinct ASE behavior under chiral excitation, allowing for the selection of an appropriate circular polarization laser to reduce the ASE threshold.

To further explore the chiral nature of the emission signal of the L/D-ZIF⊃DCM without optical spin injection, the optical test path was adjusted as follows [Fig. 4(a)]. The sample was excited using a 532 nm linearly polarized continuous-wave laser beam, and the emission of the sample was separated into left- and right-handed circularly polarized light (LCP and RCP) components using a combination of a quarter-wave plate and a polarizer. As for the reliability analysis of the collection optical path (combination of quarter-wave plate and polarizer), we observed optical path errors by decomposing circularly polarized light (Fig. 13, Appendix C). To quantify the asymmetric luminescence signal intensity, we introduced the glum factor to characterize the polarization, defined as glum=2(IL−IR)/(IL+IR), where I is the emission intensity, and L/R represents the left- and right-handed circular polarization components. The glum obtained is approximately equal to 0 (Fig. 14, Appendix C). According to the PL signal in Figs. 4(b) and 4(c), it can be found that different chiral structures have different circularly polarized component-dominated SE signals, with L-ZIF⊃DCM producing RCP-dominated SE and D-ZIF⊃DCM producing LCP-dominated SE. This phenomenon may be due to the fact that the L-ZIF⊃DCM (D-ZIF-DCM) absorbs more right-handed (left-handed) circularly polarized photons, leading to a larger number of photons being excited to produce more intense spontaneous emission. A glum value of ±0.085 was calculated for the SE from L/D-ZIF⊃DCM. This indicates that the larger glum obtained from SE of L/D-ZIF⊃DCM samples originates from its chiral structure and not from errors in the collection optical paths.

We subsequently measured the ASE’s glum by switching the excitation source to a 532 nm nanosecond pulsed laser, thus providing a higher pump energy to achieve population inversion. As depicted in Figs. 4(d) and 4(e), the samples inherit the chiral properties of its PL signal in the ASE state. Interestingly, for both L-ZIF⊃DCM and D-ZIF⊃DCM, we observed a significantly larger glum of ±0.176. We suggest that the marked discrepancy between the left- and right-handed circularly polarized components of the stimulated emission arises due to the sharp increase in emission intensity beyond the ASE threshold. This increase in emission intensity consequently leads to a rise in the glum value, indicating a stronger chiral response in the ASE regime compared to SE. L/D-ZIF⊃DCM NPs exhibit low thresholds with stable ASE signals, qualifying them as excellent materials for lasing applications.

To generate stimulated emission oscillation within an optical cavity, we employed a Fabry–Perot (F-P) cavity design [Fig. 5(a)]. This design consisted of a parallel high-reflectivity DBR mirror and a silver film, with L/D-ZIF⊃DCM NPs serving as the optical gain medium. In the fabrication process, we first deposited a layer of L/D-ZIF⊃DCM NPs on the DBR mirror using a drop-casting method. It was crucial to achieve a gain-mediated film with a uniform texture to prevent strong scattering interactions between the high-power pump laser and the L/D-ZIF⊃DCM NPs, which could potentially damage the DBR mirror. Herein, we prepared a 300 mg/mL solution of L/D-ZIF⊃DCM NPs using DMF as a solvent. As a polar solvent, DMF exhibits a high intermolecular gravitational force, leading to a low surface tension [40]. The lower surface tension, combined with a slower evaporation rate during droplet evaporation, enables the droplet to flow from areas of high NP concentration at the edge to lower concentration at the center. This flow pattern effectively prevents the accumulation of NPs at the droplet’s edge, resulting in a uniformly distributed and completely solid L/D-ZIF⊃DCM NPs film on the DBR mirror (Fig. 15, Appendix D). Finally, we covered the glass substrate coated with silver film and sealed the edges with tape to complete the construction of the optical resonant cavity. The reflectance of DBR and Ag films was measured using a UV spectrophotometer. DBR allows the pump laser beam (532 nm) to pass through, while the reflectivity around 610 nm at the emission center of L/D-ZIF⊃DCM NPs is greater than 99.6%. The reflectivity of Ag film (98.2%) is slightly lower than that of DBR, thus guiding the lasing emission [Fig. 5(b)].

We initially examined the lasing properties of the device pumped by a nanosecond laser, capturing the transmitted emission signal across the Ag film in a vertical orientation. The emission spectra of a vertical-cavity surface-emitting laser (VCSEL) based on L-ZIF⊃DCM is shown in Fig. 5(c). Notably, the broad PL peak at 613 nm rapidly changes to a distinct lasing peak as the pump power increases. The pump power density-dependent emission intensity and FWHM of the device are shown in Fig. 5(d), where the threshold behavior of the emission intensity and the marked reduction in FWHM substantiate the generation of lasing action. The L-ZIF⊃DCM-based VCSEL exhibits an exceptionally low laser threshold of 2.3mJcm−2 (significantly smaller than the ASE threshold), which can be attributed to the increased propagation distance and the accumulation of photons inside the F-P cavity. Compared to the ASE signals observed in bare L/D-ZIF⊃DCM film, the laser linewidth decreases significantly from 13–15 nm to about 7 nm. The optical path for measuring the VCSEL circular polarization is the same as that shown in Fig. 4(a). The optical signal emitted by the VCSEL is decomposed by a quarter-wave plate and a polarizer and is also collected along the central normal direction. At the pump energy density of 38.2mJcm−2, the RCP lasing emitted by the device is 36% higher than the LCP lasing, with a glum of −0.310, indicating a significant enhancement of the excited chiral light within the F-P cavity.

The D-ZIF⊃DCM-based laser devices have similar emission spectra and lasing thresholds to the L-ZIF⊃DCM, while the intensity of LCP lasing is significantly higher than that of RCP lasing at sample pumping energy (with a glum value of approximately 0.308). Contrary to the intuitive thought that a planar F-P cavity might invert the chirality of light [41–43], our experimental results demonstrate that the transmitted light, after interaction with an active resonant cavity, retains the same chirality as the ASE signal. However, experimental results show that the transmitted light after the action of an active resonant cavity retains the same chirality as the ASE signal. The glum value of VCSEL is 1.8 times that of ASE and nearly 4 times that of PL. Such significant glum enhancement of the chiral lasing within an F-P cavity is similar to previous reports [22].

In conclusion, we have successfully achieved chiral stimulated emission in chiral MOF NPs embedded with achiral dyes. The CPL and ASE signals of the L/D-ZIF⊃DCM are manipulated by their structural configurations. Notably, the ASE intensity and threshold are influenced by the circular polarization of the pump light in relation to the intrinsic chirality of ZIF-8. Furthermore, we have developed a fabrication method for L/D-ZIF⊃DCM-based VCSELs and achieved low-threshold lasing emission. Our investigation into the circular polarization characteristics of stimulated emission process revealed a higher asymmetry between the emitted left- and right-handed polarized light compared to the standard SE and ASE signal, with the glum increasing from ±0.08 to ±0.17 and to ±0.31. These findings indicate that circularly polarized stimulated emission offers significant advantages over ordinary circularly polarized SE. Our study provides a simple and cost-effective pathway for creating chiral ASE and laser devices, poised to enhance the fabrication and widen the application scope of chiral lasers.

Acknowledgment. The authors thank the Modern Experimental Technology Center of Anhui University for the SEM and TEM measurements.