Room-Temperature Distributed Feedback CsPbBr₃ Perovskite Lasers Integrated on Silicon Nitride Waveguide Platform

Lead halide perovskites are solution processable semiconductors, which have shown promise for applications in solar cells and light emitting diodes. Further, the feasibility of device fabrication at low- temperature (100 °C) opens the door to explore new opportunities on integrated lasers on-chip, waveguide lasers on the silicon nitride (Si₃N₄) platform [1] and other electro-optical modulators [2]. However, perovskites have much higher refractive indices (n = 2.3-2.6) than that of a typical Si₃N₄ waveguide (n = 2). Such a contrasting refractive index at the perovskite/Si₃N₄ interface makes it challenging to extract light from the perovskite lasers. In this work, we address this challenge by using the perovskite as the emission medium and the Si₃N₄ as the cavity medium. Upon optical pumping, photons emitted from the perovskite couple to the cavity via the evanescent field for light amplification leading towards lasing on-chip ([1], [2]).<br/><br/>In this study, the cesium lead bromide (CsPbBr₃) thin film (90 nm) is used as a laser gain medium and Si₃N₄ as a distributed feedback (DFB) medium. The DFB constitutes of a 200 nm thick Si₃N₄ rib waveguide with a quarter-wavelength shifted first-order grating structure etched directly into the Si₃N₄ waveguide. Using a finite difference time domain simulation (Lumerical software), the grating dimensions and the cross-section of the Si₃N₄ waveguide was optimised to maximize the effective refractive index of the waveguide mode. The mode overlap between the perovskite film and Si₃N₄ waveguide mode was identified to be ~24%. Using the optimised configuration obtained from the simulation studies, our experiments were designed to realise the perovskite DFB lasers on a 6” Si wafer with 2.3 μm of thermally grown SiO₂. The experimental workflow to achieve a high-quality DFB system will be presented. 90 nm thick CsPbBr₃ film was spin coated on the DFB structure followed by a planar hot process to reduce the grain boundary defects [3]. The perovskite film was patterned as reported previously ([1], [3]) and encapsulated with a 1 µm thick PMMA protective layer. The device was optically pumped using a 300 nm pulsed excitation source with 300 ps pulse width and 1 kHz repetition rate at room temperature. At low excitation fluence, the weak photoluminescence signal showed a broad linewidth (16 nm). With increasing pump fluence, a single peak with ultranarrow linewidth (0.24 nm) was observed. An S-shaped curve for the emission intensity vs pump fluence confirming a clear sign of lasing with a threshold power density of 775 μJ/cm² will be presented. Further, the change in the emission wavelength upon varying the grating periodicity and etch depth will be presented. Our work demonstrates a DFB CsPbBr₃ laser on a silicon nitride waveguide platform operating at room temperature, which opens new avenues for integrated optoelectronics.<br/><br/><b>Acknowledgments:</b> This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 956270 and NRW project PEROVSKET funding code No EFRE-0801508.<br/><br/><b>References</b><br/><br/>[1] P. J. Cegielski <i>et al.</i>, ‘Monolithically Integrated Perovskite Semiconductor Lasers on Silicon Photonic Chips by Scalable Top-Down Fabrication’, <i>Nano Lett.</i>, vol. 18, no. 11, pp. 6915–6923, Nov. 2018.<br/>[2] P. J. Cegielski <i>et al.</i>, ‘Integrated perovskite lasers on a silicon nitride waveguide platform by cost-effective high throughput fabrication’, <i>Opt. Express</i>, vol. 25, no. 12, pp. 13199–13206, Jun. 2017.<br/>[3] N. Pourdavoud <i>et al.</i>, ‘Room-Temperature Stimulated Emission and Lasing in Recrystallized Cesium Lead Bromide Perovskite Thin Films’, <i>Adv. Mater.</i>, vol. 31, no. 39, p. 1903717, 2019.