Directional and Fast Photoluminescence from CsPbI₃ Nanocrystals Coupled to Dielectric Circular Bragg Gratings

Abstract

Lead halide perovskite nanocrystals (NCs), especially the all-inorganic perovskite NCs, have drawn substantial attention for both fundamental research and device applications in recent years due to their unique optoelectronic properties. To build high-performance nanophotonic devices based on perovskite NCs, it is highly desirable to couple the NCs to photonic nanostructures for enhancing the radiative emission rate and improving the emission directionality of the NCs. In this work, we synthesized high-quality CsPbI₃ NCs and further coupled them to dielectric circular Bragg gratings (CBGs). The efficient couplings between the perovskite NCs and the CBGs resulted in a 45.9-fold enhancement of the photoluminescence (PL) intensity and 3.2-fold acceleration of the radiative emission rate. Our work serves as an important step for building high-performance nanophotonic light emitting devices by integrating perovskite NCs with photonic nanostructures.

1. Introduction

All-inorganic perovskite nanocrystals (NCs) are an emerging class of material for lighting and display technologies due to their excellent optoelectronic properties. Compared with organic–inorganic hybrid perovskite, all-inorganic perovskite NCs exhibit several advantages, including wider wavelength tenability [1], higher quantum yield at ambient temperature [2], and lower cost. They are easy to synthesize and can be integrated with various photonic nanostructures [3,4,5]. The all-inorganic perovskite have found wide applications in optoelectronic devices, such as nanolasers [6,7,8,9,10,11], LED [12,13], solar cells [14,15,16,17], and photodetectors [18,19]. Notably, recent studies also suggest that the perovskite NCs are excellent emitters of non-classic light at cryogenic temperatures [20,21,22]. To further improve the brightness and the coherence properties of the single-photon sources based on perovskite NCs, it is highly desirable to couple the perovskite NCs to photonic nanostructures. In this work, we synthesize high-quality CsPbI₃ NCs emitting at visible light (See Appendix A) and further efficiently couple the NCs to the SiN circular Bragg gratings (CBGs) to simultaneously enhance their radiative rate and improve their emission directionality. Further developments in this direction may result in high-performance integrated devices with applications in nanophotonics and quantum optics.

2. Experimental Section

Figure 1a is an artistic sketch of the proposed device in which the CsPbI₃ NCs are located on top of the CBGs. The inset is the atomic structure of the NCs, showing the perovskite structure. The absorption and the photoluminescence (PL) spectra of the CsPbI₃ perovskite NCs at room temperature is shown in Figure 1b. Our perovskite NCs feature a wide absorption range above 700 nm. The inhomogeneous spectral broadening of NCs is around 50 nm, which is caused by size difference of the individual NCs. We characterized the morphology and size distribution of CsPbI₃ NCs by using transmission electron microscopy (TEM), as shown in the inset of Figure 1b. An average size of 20 nm was obtained, which led to a PL peak of around 700 nm at room temperature.

Targeting an emission wavelength of 700 nm, we designed and optimized the CBGs via the finite difference time domain (FDTD) method (See Appendix A for details). The simulated device consists of a silicon substrate, a 710 nm thick SiO₂ layer (n = 1.45) and a 196 nm thick Si₃N₄ (n = 2.02) CBG. The center disk radius, grating period and trench width of the CBGs are 300, 423 and 101 nm, respectively. As presented in Figure 2a, a Purcell factor of ~5.5 was obtained around 707 nm and the resonance wavelength could be fine-tuned in our experiment by varying the size of the central disk of the CBG. The inset is the far-field polar-plot at the cavity resonance, showing that most of the emitted photons were highly directional, i.e., within a divergent angle of 10 degrees. Figure 2b,c shows the electric field intensity distributions of the cavity mode in the X–Y plane and the X–Z plane, respectively. The intensity distribution in the X-Y plane is confined in the center of CBG, suggesting the lateral dissipation is largely suppressed. The numerical simulation of the intensity in the X–Z plane shows clearly that a substantial amount of photons were emitted upwards with a narrow divergent angle. The scanning electron microscope (SEM) image of the fabricated CBGs is presented in Figure 2d. The sizes of critical features in the fabricated devices are very close to our designed parameters.

In our experiment, we fabricated a set of CBGs with the central disk radius varying from 280 to 300 nm at steps of 5 nm (See Appendix A for fabrication details). The cavity modes of the CBGs can be probed by the intrinsic PL from the Si₃N₄ under the excitation of a 532 nm laser. Figure 3a shows the cavity modes of CBGs with different radiuses through FDTD simulation before coating the polymethyl methacrylate (PMMA) layer, and Figure 3b shows the measured spectra of the cavity modes. By comparing Figure 3a,b, we can see that our simulations and experimental results match well. The cavity modes shifted from 698 to 708 nm with the increase of the size of the central disk, which is indicated by the dashed line. Figure 3c presents the simulation of the cavity modes with the encapsulation of a PMMA layer, which shows a 3–4 nm red-shift of the cavity modes. In the experiment, the NCs containing a solvent consisting of a mixture of 0.5% PMMA and CsPbI₃ solution (with a volume ratio of 1:2) were subsequently spin-coated onto the CBGs (See Appendix A). More details of the solvent are discussed in the experimental section. As shown in Figure 3d, the experimental results were in very good agreement with the simulation, showing a 3–4 nm red shift of the cavity modes by the PMMA layer. The spectra in Figure 3d were collected by a single mode fiber as a spatial filter, which gave rise to a better signal-to-noise ratio of the cavity modes (details provided in Appendix A).

To investigate the coupling between the perovskite NCs and the CBGs, we compared the PL intensities from the perovskite quantum dots (PQDs) spatially outside and inside the CBG. As shown in Figure 4a, the PL intensity of the NCs in the CBG denoted by the red curve was 45.9 times stronger than that of NCs outside the CBG denoted by the blue curve. Such a PL intensity enhancement is mostly due the coupling the NCs to the CBG cavity modes, which is efficiently collected by the optical objective. To quantitatively study the coupling strength, the lifetimes of the spontaneous emission from the CsPbI₃ PQDs were measured, as shown in Figure 4b. A 532 nm pulsed laser with an intensity of 0.6 μJ/cm² was used to excite the NCs (See Appendix A for Optical Measurement details). For the NCs without the CBG, a lifetime of 3.98 ns was extracted from the decay curve, shown in blue. Once coupled to the CBG, a significantly shortened lifetime of 1.24 ns was obtained, as shown by the red decay curve, which resulted in a Purcell factor of 3.2. Both the intensity enhancement and the lifetime reduction in our experiment strongly suggest that efficient couplings between the NCs and the CBGs were achieved.

We further studied the power-dependent PL for the coupled PQDs. Figure 4c shows representative spectra of the NCs coupled to the CBG under different excitation powers. When increasing the pump intensity from 0.2 to 1.0 μW, the PL intensity increased accordingly but no lasing signature was observed, as shown in the inset of Figure 4c. To further confirm that the enhanced PL was caused by the coupling between the NCs and CBGs, the CBG modes were deliberately detuned from NCs emission. The PL enhancement for the resonant case was 26, which is significantly higher than that of 4.1 for the off-resonant case, as presented in Figure 4d.

3. Conclusions

In conclusion, we have synthesized CsPbI₃ NCs emitting at 700 nm. By depositing NCs encapsulated in a PMMA layer on top of CBGs, a 45.9-fold enhancement of PL intensity and a 3.2-fold reduction of the lifetime were obtained, suggesting that efficient coupling between the NCs and the cavity modes of the CBGs was achieved. Further developments in this direction, e.g., coupling single NCs to the CBG, may result in coherent single-photon sources with low fabrication costs. Our work presents a viable path for building active nanophotonic devices based on perovskite NCs.