Solution-processed halide perovskite semiconductors are emerging as a promising photonic material platform with potential applications in various fields, including lasers¹⁻³, light-emitting diodes⁴⁻⁶, and solar cells⁷⁻⁹. Halide perovskite has been considered as an exceptional gain material for low-cost lasers due to its high photoluminescence quantum yield, bandgap tunability, and narrow radiation spectrum¹⁰⁻¹². Since Deschler et al. demonstrated an optically pumped lasing from halide perovskite in 2014¹³, perovskite lasers have been constructed using different kinds of external microcavities, such as Fabry–Pérot (F-P) cavities¹⁴, photonic crystal (PC)^15,16, and whispering-gallery-mode (WGM)¹⁷⁻¹⁹. Distributed feedback (DFB) and vertical cavity surface emitting perovskite lasers have been recently investigated, enabling greater control of the wavelength and beam shape of the laser²⁰⁻²³. However, they have been constrained to operate at a fixed wavelength.

In the digital field, where the lasers are widely used in integrated forms, tunable and compact lasers are highly desired. Recently, tunable compact photonics devices based on phase change materials (PCMs), such as chalcogenide glass²⁴⁻²⁸ and vanadium dioxide^29,30, have attracted much attention³¹. Nevertheless, although the PCMs have been broadly exploited in the transmissive and reflective structures, they cannot be used for realizing tunable lasers due to the absence of optical gain. Therefore, developing the PCM possessing tunable dielectric constant and optical gain is necessary, which can provide solutions for tunable light-emitting devices.

Halide perovskites are a burgeoning family of photonics materials, benefitting from their high permittivity and extraordinary luminescence performance³². They have been applied to dielectric nanophotonic structures³³ and light-radiating devices^34,35 such as photoluminescence-enhanced metamaterials^36,37, high-resolution color displays³⁸⁻⁴⁰, and low-threshold lasers⁴¹⁻⁴³. Moreover, due to the strong interference between organic ligands and inorganic framework, halide perovskites exhibit a rich variety of crystallographic states that can be controlled by pressure⁴⁴, chemical composition⁴⁵, and temperature⁴⁶. These state transitions can induce significant variation in refractive index⁴⁷. Notably, the excitons in the halide perovskites possess a small Bohr radius and efficient electrostatic Coulomb coupling, resulting in their distinct optical characteristics, including effective quantum yield, great exciton binding energy, and efficient radiative recombination⁴⁸. Because of these excellent features, halide perovskites are promising candidates for the gain medium in the lasing applications⁴⁹. Nevertheless, the usages of halide perovskites as a phase-change tunable gain medium have yet to be exploited.

Here, we present a spectrally tunable single-mode vertical-cavity near-infrared (NIR) microlaser, based on MAPbI₃ halide perovskite film, which acts as a phase-change tunable gain medium. The high-quality (Q) factor vertical cavity consists of two highly reflectors (Au mirror and distributed Bragg reflector (DBR)) parallel to each other, with the methylammonium lead iodide CH₃NH₃PbI₃ (MAPbI₃) gain medium sandwiched between them. The change in luminescence and refractive index upon structural state change leads to a broad spectral tunability in lasing frequency ranging from 790.6 nm to 799.7 nm, and with a significant tuning rate of ~0.30 nm K⁻¹. This rate is one order higher in magnitude compared to the traditional semiconductor lasers⁵⁰. The single-mode lasing emission with excellent spatial coherence can be readily obtained through this simple vertical-cavity. The constructed vertical cavity surface emitting laser (VCSEL) exhibits extraordinary performance, with a low lasing threshold of ~23.5 μJ cm⁻², and a broad tuning range of the emission frequency from 790.6 nm to 799.7 nm. In addition, the device maintains its performance over 2.4×10⁷ cycles of optical pulse excitation with a duration of ~50 min. Our work not only advances the development of tunable perovskite VCSELs but also shed light on the next generation of the integrated photonics.

For the demonstration of the phase-change perovskite tunable VCSEL, we have chosen a prototypical halide perovskite, MAPbI₃, a high refractive index PCM that undergoes a phase change between tetragonal and orthorhombic at 130–160 K^47,51 (Fig. 1(b)). This phase change leads to a substantial modulation of both refractive index and gain spectrum. The complex refractive index N_eff (N_eff = n_eff + i × k_eff ) of a MAPbI₃ layer in both orthorhombic and tetragonal phases is measured in the 700 nm to 850 nm spectral range using a variable angle spectroscopic ellipsometry (VASE). As shown in supplementary Fig. S1, the material possesses relatively high n_eff for both tetragonal and orthorhombic structural states across a wide temperature range, allowing efficient confinement and modulation of light at the subwavelength scale. The phase change can produce a remarkable tunability of the n_eff. The n_eff for the tetragonal state is much larger than the orthorhombic state over the visible-NIR spectrum from 700 nm to 850 nm, with Δn_eff reaching 0.19 between tetragonal phase (160 K) and orthorhombic phase (130 K) around the wavelength of 784 nm. We schematically present the structure of MAPbI₃ VCSEL fabricated on top of a quartz glass substrate in Fig. 1(a). The red square encircled by the dashed lines represents the MAPbI₃ gain film between the bottom DBR mirror and the top Au reflector. The bottom DBR reflector consists of 12 pairs of SiO₂/Ta₂O₅ stacked layers with quarter wavelength thicknesses to achieve high reflectivity. After sputtering the bottom DBR on the quartz glass substrate, we directly spin-coat MAPbI₃ on the surface of the DBR and anneal it to form the thin film. Subsequently, the Au mirror was deposited on the MAPbI₃ as the back reflector. Note that, the silver could also serve as a promising reflector due to its additional pumping efficiency. The period of the DBR and the thickness of each layer are optimized to enhance optical confinement and improve the spectral characteristics crucial to the overall device performance at the targeted lasing wavelength of 795 nm. The cross-sectional scanning electron microscope (SEM) picture of the fabricated microcavity structure is shown in Fig. 1(c), where the DBR multilayers can be clearly distinguished. Fig. 1(d) presents the SEM image (at 20000× magnification) of the MAPbI₃ film grown on the distributed Bragg reflector (DBR). The image clearly reveals the nanocrystal (NC) features in the synthesized MAPbI₃ perovskite, with the grain size measured in the range of several hundred nanometers. Meanwhile, we performed a statistical analysis of the crystal sizes in MAPbI₃ films based on SEM images. The detailed statistical data are presented in supplementary Fig. S2. Approximately 72% of the particle size distribution falls within the range of 100 nm to 200 nm. The surface roughnesses of the DBR mirror, the MAPbI₃ film, and the microcavity are measured using an atomic force microscope (AFM) as shown in supplementary Fig. S3. The root-mean-square (RMS) values of the surface roughness were 1.2 nm, 4.1 nm, and 11.5 nm, respectively, illustrating the high quality and smooth surface of the microcavity VCSEL. In supplementary Fig. S4(a–c), the microscopic morphology of the MAPbI₃ film on the quartz glass substrate was measured, manifesting the outstanding crystallinity of the nanocrystalline layer. In supplementary Fig. S4(d), the temperature-dependent XRD spectra of the MAPbI₃ film were investigated, uncovering its phase transition from tetragonal to orthorhombic with decreasing temperature.

In Fig. 2(a), we show the measured absorptance and photoluminescence (PL) spectra of the MAPbI₃ thin film for both tetragonal (orange line) and orthorhombic (green line) states, respectively. The absorption edge of the tetragonal state appears around 740 nm, and the PL exhibits two peaks at around 740 nm and 787 nm. On the other hand, the absorption edge and PL peak for the orthorhombic state center occur around 780 nm and 790 nm. The PL spectra demonstrate sharp profiles with full width at half-maximum (FWHM) of ~25.5 nm and 24.5 nm for both tetragonal and orthorhombic states, respectively. The temperature dependence of the spontaneous PL radiation of MAPbI₃ are characterized and shown in Fig. S5(a) and S5(b). At the tetragonal state, the PL peak center corresponding to the near-band-edge transition gradually shifts from 770 nm to 790 nm by decreasing the temperature from 298 K to 160 K. By further reducing the temperature through the tetragonal-orthorhombic phase transition, the PL begins to blueshift, and the second peak PL occurs at a shorter wavelength of 743 nm at the orthorhombic state (140 K). The quantum efficiency of the MAPbI₃ thin film was measured using a Steady-State and Transient Fluorescence and Phosphorescence Spectrometer (Edinburgh, FLS 1000), with an absolute quantum efficiency of 42% at room temperature. To investigate the suitability of MAPbI₃ as a gain medium for laser, we measure the amplified spontaneous emission (ASE) spectra of the ~300 nm thick MAPbI₃ film layer with both tetragonal (orange line) and orthorhombic (green line) states, via strip excitation^52,53 (Fig. 2(b)). For the tetragonal MAPbI₃ layer, when the pump fluence is increased above 7.9 μJ cm⁻², distinct signatures of ASE with an ultranarrow emission linewidth of ~7 nm and a steep increase of ASE peak intensity can be observed. By changing the state of MAPbI₃ from tetragonal to orthorhombic, the emission band narrows to ~6.5 nm at the pump fluence of 4 μJ cm⁻², implying that the ASE threshold can be maintained during the phase transition. A red shift of ~12 nm for the ASE peak was also observed as the temperature increased from 130 K (tetragonal phase) to 160 K (orthorhombic phase) due to the variation in N_eff of MAPbI₃. The small threshold and wide tunability in ASE spectra across the phase change indicate that MAPbI₃ phase change perovskite is a promising gain dielectric for tunable lasing. To match up the peak wavelengths of ASE of MAPbI₃ film with the PL of tetragonal (λ_t = 785 nm) and orthorhombic (λ_o = 805 nm) states, both the DBR and Au mirrors should be designed to have a broad reflectance band covering the near-infrared-wavelength region. Additionally, they should remain transparent at the pumping wavelength (λ_p= 405 nm). As shown in Fig. 2(c), the normalized reflectance spectra (from 770 nm to 820 nm) of the bottom DBRs and top Au reflectors exhibit a large reflectance up to ~99.8%, while maintaining a high transparency for the wavelength less than 410 nm (R ≤ 20%). Such a broad high-reflectance band covers the whole ASE of the MAPbI₃ nanocrystals perovskite for both tetragonal and orthorhombic states and benefits the intense optical confinement. In Fig. 2(d), we show the measured PL spectra of phase-change perovskite VCSEL structure in both tetragonal and orthorhombic states pumped by the low-excitation powers of 29.5 μJ cm⁻² and 8.1 μJ cm⁻², respectively. Herein, the pumping source is a femtosecond (fs) laser (1030 nm, PHAROS PH2) with optical parametric amplifier output of 130 fs pulse width, 405 nm wavelength, and 400 kHz repetition. The PL bands can be redshifted by changing the phase from tetragonal and orthorhombic by increasing the temperature from 130 K to 160 K. The uncoupled exciton spectrum is absent, showing that the photonic modes are efficiently coupled with the perovskite excitons in the vertical cavity⁵⁴.

For the VCSEL structure, the resonant mode from the vertical cavity determines the laser emission wavelength. In order to align the resonance modes of the different structural states of tetragonal and orthorhombic to their ASE spectral spectrum, we determined the thickness of the perovskite film, which is ~300 nm, as shown in supplementary Fig. S6(a). For the tetragonal (orthorhombic) state, the lasing peak occurs at λ = 790.6 nm (λ = 799.7 nm), which is close to the peak of the ASE spectrum at λ = 791 nm (λ = 803 nm). Above the lasing threshold, the emission rate through biexciton or single-exciton recombination increases considerably, benefiting the stimulated radiation over spontaneous recombination⁵².

The lasing feature of the MAPbI₃ VCSEL is investigated by collecting the light emission under a backscattering configuration. The light emission spectra of the tetragonal VCSEL under different pumping fluences are measured at 130 K and shown in the left column of Fig. 3(a). A weak broad radiation spectrum is observed for F_pump < 48 μJ cm⁻², which is dominated by the PL spectrum³². At a larger fluence of F_pump = 55.7 μJ cm⁻², a single narrow lasing emission peak appears at 790.6 nm. Figure 3(a) shows the light-light (L-L) curve and FWHMs of the PL spectra of the 790.6 nm peak under different pumping fluences. The threshold energy density of lasing appears around 48 μJ cm⁻², where the FWHM shrinks sharply from 18 nm to 0.7 nm. When the MAPbI₃ layer undergoes a phase change from tetragonal to orthorhombic, the lasing wavelength changes from 790.6 nm to 799.7 nm, as shown in the left column of Fig. 3(b). In Fig. S7, we have measured the emission spectra of VCSEL at the temperatures of 130K, 140K, 150 K and 160 K, respectively. The tuning rate was the ratio of the shift of wavelength with respect to the change of temperature, which is approximately 0.30 nm K⁻¹ rate. Meanwhile, the state transition redshifts the absorptance edge of MAPbI₃, leading to a better spectral overlap between the gain and lasing mode of the vertical cavity. Consequently, the lasing threshold decreases to ~23.5 μJ cm⁻² at 160 K for the orthorhombic VCSEL (the right column of Fig. 3(b)). In supplementary Fig. S8, the laser characteristics of MAPbI₃ VCSEL were investigated at 135 K, 140 K, and 150 K by analyzing light emission in the backscattering configuration. The light emission spectra of the VCSEL under various pump fluences were measured at 135 K, as illustrated in the upper row of Fig. S8(a). At pump fluences below 42 μJ cm⁻², a weak, broad emission spectrum was observed. When the pump fluence increased to 42.8 μJ cm⁻², a single, narrow lasing emission peak emerged at approximately 792 nm. Similar laser performance was observed at 140 K and 150 K. At 140 K, the laser emission peak was centered around 793.5 nm, with a lasing threshold of approximately 24.9 μJ cm⁻². At 150 K, the lasing threshold decreased to approximately 22.2 μJ cm⁻², and the central wavelength of the laser emission peak shifted to about 796.5 nm.

To examine the temperature dependence of the lasing threshold, L-L curves of the VCSEL were measured across temperatures ranging from 130 K to 160 K, with the results presented in Fig. 4(a). The laser characteristics of the MAPbI₃ VCSEL at 135 K, 140 K, and 150 K are detailed in supplementary Fig. S8. Notably, the lasing threshold exhibits an unusual trend, first decreasing and then slightly increasing (Fig. 4(b)), deviating from the typically observed monotonic increase in lasing threshold with rising temperature⁵⁵. The reduction in the lasing threshold between 160 K and 150 K is attributed to a decrease in Auger recombination losses and an increase in the bimolecular recombination constant and radiative recombination rate as the temperature decreases. These factors facilitate population inversion and laser emission at lower pump energy densities⁵⁶. Interestingly, the Auger recombination rate of MAPbI₃ is highly phase-dependent, significantly decreasing in the orthorhombic phase but increasing several-fold in the tetragonal phase⁵⁷. The sudden increase in the lasing threshold at 130 K and 135 K is ascribed to a phase transition in the MAPbI₃ active layer from orthorhombic to tetragonal as the temperature decreases, causing a sharp rise in Auger recombination losses. Within the temperature range of 140 K to 150 K, the perovskite active layer exists in a mixed phase state, with the lasing threshold influenced by both the temperature and the ongoing phase transition of the perovskite.

We further measure the lifetime of VCSEL devices under continuous pulse excitation (405 nm, 130 fs, 8 kHz) at both the tetragonal and orthorhombic states. We optically pump the devices to 1.1 F_th (F_th represented the threshold pump fluence) under ambient conditions and record the integrated radiation intensities against time. An 80% lifetime (T₈₀) is defined as the time when the output intensity is reduced to 80% of the initial power. This tunable perovskite VCSEL could investigating the properties of superconducting materials, where the superconducting phenomenon typically occurs under low-temperature conditions, this laser could provide a stable, wavelength-tunable light source. Such a light source would be ideal for probing the optical response characteristics of superconducting materials across different superconducting states, such as changes in reflectivity and absorptivity. This capability would facilitate a deeper understanding of the underlying superconducting mechanisms and the material properties. As can be seen in Fig. 4(c), the laser intensity decreases relatively slowly, maintaining 80% of initial level after ~50 mins (2.4×10⁷ pumping pulses). For the orthorhombic state, the VCSEL under the fs pumping pulses possesses even better stability (T₈₀ > 2 h/5.76×10⁷ pumping pulses). It was found that this pulse count is comparable to values reported in existing studies on perovskite lasers excited by pulses of the same category⁵⁸⁻⁶¹. We also demonstrate that the device exhibits good operational stability during the transition from tetragonal to orthorhombic state, as shown in Fig. 4(d). Recent studies have explored MAPbI₃-based perovskite lasers employing various structures, including symmetric waveguides⁶², WGM⁶³, DFB⁶⁴, PC⁶⁵, and vertical cavities⁶⁶. However, these designs exhibit notable limitations. For instance, symmetric waveguide lasers display a relatively broad FWHM of approximately 3 nm, while DFB and PC perovskite lasers require significantly high lasing thresholds (~200 μJ cm⁻²). Additionally, none of these approaches offer a tunable operating wavelength. In contrast, our proposed structure demonstrates a distinct advantage with a tunable lasing wavelength range of 790.6 nm to 799.7 nm, a considerably narrower FWHM of 0.7 nm, and a significantly lower lasing threshold of approximately 22 μJ cm⁻², outperforming previously reported designs.

We have demonstrated a tunable perovskite VCSEL by integrating MAPbI₃ thin film with a Fabry-Perot cavity, composed of a top DBR and a bottom Au mirror. Our approach combines the NCs with judiciously designed resonance cavity. This was achieved via the optimization of the MAPbI₃ layer thickness and the DBR reflector geometry, which enabled a single-mode lasing emission with both high Q factors and low threshold lasing intensities for both tetragonal and orthorhombic states. Importantly, large tunability is achieved by altering the state of MAPbI₃ perovskite, resulting in a shift of lasing wavelength from 790.6 nm to 799.7 nm. Moreover, the utilization of MAPbI₃ film ensured high stability of VCSEL, with the device maintaining its performance over hours of operation at femtosecond pulsed pumping under ambient conditions. Our work may offer new opportunities for space and wavelength division multiplexing applications in optics communications, dense neuromorphic devices and optical readout detectors. Moreover, we anticipate that the realization of highly efficient and stable electrically injected perovskite lasers would pave the way for the development of miniaturized, low-power laser sources. This advancement would lay a solid foundation for their practical implementation in various applications and open up a promising avenue for future research.

We have made ultrathin MAPbI₃ film from 1.2 M predecessor solution of PbI₂ (99.99%, TCI) and CH₃NH₃I (Dyesol) (molar ratio 1:1) in anhydrous dimethylformamide (DMF, Sigma-Aldrich). The prepared solution was magnetically stirred overnight under room temperature in a glovebox filled with N₂, then filtered by a polyvinylidene fluoride (PVDF) syringe filter (0.45 μm) and left on the hot plate under 373 K for ~1 hour before spin-coating. Before the deposition of perovskite, quartz substrates were cleaned using the solution: Hellmanex II (Hellma Analytics, 2 mL) in deionized (DI) water (200 mL) under a temperature of 353 K for 10 min. Afterwards, the substrate was washed with DI water and dried with the nitrogen flow and cleaning treatment of oxygen plasma. We have spin-coated the perovskite precursor solution onto the quartz substrates at a speed of 5100 rpm for 30 s using antisolvent engineering method. It can be seen from Fig. S6(b) that the thickness of the perovskite film is about 300 nm when the spin coating speed is 5100 rpm. We drop-casted the toluene on the substrates with a spin-coating duration of 5 s. The films were thermally annealed at 373 K for 15 min.

Firstly, the DBR mirror with ~99.8% reflectance for 795 nm were fabricated on a quartz glass substrate using Magnetron Sputter Deposition system (AJA Orion5). The DBR (12 pairs of Ta₂O₅/SiO₂ alternating layers) is directly sputtered onto quartz. Secondly, we have cleaned out the DBR mirror with acetone, 2-propanol, and deionized water under sonication for 15 min. Thirdly, the cleaned DBR mirrors were processed with oxygen plasma for 10 min before a spin-coating process. After that, MAPbI₃ film was grown on the DBR substrate using the method above. And after perovskite film deposition, a high-quality Au film (100 nm) was deposited on the MAPbI₃ films to act as the back reflector by magnetron sputtering.

The surface morphology of MAPbI₃ films was measured by optical microscope (Olympus BX53M). We have measured the surface roughness of MAPbI₃ film grown on the quartz substrate by the atomic force microscope (AFM, Park NX10). High-resolution SEM was performed using a Field emission scanning electron microscope (JSM-7900F) at 5 kV. We have investigated the temperature-dependent XRD spectra of the MAPbI₃ film. We have employed a Bruker F8 Focus power XRD with monochromatized Cu Kα radiation (λ = 1.5418 Å) to record powder XRD patterns of MAPbI₃ film residing on the quartz glass substrate. We measured UV−visible absorptance spectra of the MAPbI₃ using SHIMADZU UV-3600Plus spectrophotometer. Both the steady-state photoluminescent emission spectrum and the amplified spontaneous emission (ASE) spectrum were measured using a home-built micro-photoluminescence (μ-PL) system. Light from a 405 nm laser was focused onto the MAPbI₃ film using a 4 ×, 0.1 NA objective lens. The PL signal was collected by the same objective lens and directed to a spectrometer integrated with a CCD detector cooled by liquid nitrogen (Princeton HRS-500) with a maximum grating density of 1800 g mm⁻¹ grating. For the measurement of photoluminescence spectrum, a 405 nm continuous wavelength laser (PicoQuant) was used as the pumping source. For the ASE spectrum, a 405 nm pulsed laser (pulse width: 130 fs, repetition rate: 80 kHz) was used as the pumping source. To minimize the influence of ambient light, the entire experimental system was covered with a black cloth.

The lasing characterizations were carried out on a home-built micro-photoluminescence (μ-PL) system at low temperature in a vacuumed atmosphere. A femtosecond pulsed laser (wavelength: 405 nm, pulse width: 130 fs, repetition rate: 8 kHz) was used as the excitation source, which was focused by a microscopy objective (50 ×), and this was used for the excitation of the samples. The excitation laser spot was around 30 μm in diameter. The resulting signal was collected by the same objective lens and directed to a spectrometer with a grating density of 1200 g mm⁻¹. Low-temperature measurement was performed in a cryostat (Montana Instruments S50). The entire experimental system was still covered with a black cloth.

Simulations

The simulated reflectance spectra in Fig. 2(c) were performed by the Ansys Optics software relied on the 2D Finite-Different-Time-Domain (FDTD) method. The periodic boundary conditions were used in the x-y plane and the perfectly matched layer (PML) condition was defined along vertical direction (z-axis). A plane wave source located near the perovskite layer was used to excite the system, and the reflectance was numerically achieved via a power monitor located in front of the device. A uniform FDTD square mesh (5 nm ×5 nm × 5 nm) was employed to diminish the numerical error during the FDTD calculation. Our numerical models defined the Au with the losses using Palik permittivity data. The permittivity of the perovskite MAPbI₃ film under both structural phases of tetragonal and orthorhombic were defined from the VASE experimentally measured ones in Fig. S1.