The applications of topological photonics have been intensively investigated, including one-way waveguide and topological lasers. Especially, the topological lasers have attracted broad attention in recent years, which have been proposed and demonstrated in various systems, including 1D edge state in 2D systems, 0D boundary state in 1D lattice and topological bulk state around band edge.
Most of them are at microscale. The topological nanolaser with small footprint, low threshold and high energy-efficiency has yet to be explored. Recently, a new type of higher-order topological insulators which have lower dimensional boundary state has been proposed and demonstrated in many systems, including 2D photonic crystal.
In the second-order 2D topological photonic crystal slab, there exist the gapped 1D edge states and mid-gap 0D corner state. This localized corner state provides a new platform to realize topological nanolaser.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Xiulai Xu from Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, China, and collaborators have demonstrated a low-threshold topological nanolaser in 2D topological photonic crystal nanocavity.
Based on the second-order corner state, a topological nanocavity is designed and fabricated. The quality factor (Q) is further optimized with a theoretical maximum of 50,000. The corner state is demonstrated to be robust against defects in bulk photonic crystal.
A lasing behaviour with low threshold and high spontaneous emission coupling factor (β) is observed. The performance is comparable with that of conventional semiconductor lasers, indicating the great prospect in a wide range of applications for topological nanophotonic circuitry.
The topological nanocavity consists of two kinds of photonic crystal structure with the common bandstructure and different topologies which are characterized by 2D Zak phase. According to the bulk-edge-corner correspondence, the mid-gap 0D corner state can be induced by the quantized edge dipole polarization, which is highly localized at the intersection of two boundaries.
The Q is optimized with smoother spatial distribution of corner state by adjusting the gap distance (g) between the trivial and nontrival photonic crystal slabs.
The designed topological nanocavities with different parameters are fabricated into GaAs slabs with a high density of InGaAs quantum dots. The trend of Q with g agrees well with the theoretical prediction, while the values are approximately an order of magnitude lower than the theoretical prediction due to the fabrication imperfection.
Although the Q and resonance wavelength of the corner state are susceptible to disorder around the corner, the corner state is topologically protected by the nontrivial 2D Zak phases of the bulk band and robust against to the defects in bulk photonic crystal, which has been demonstrated experimentally.
A lasing behavior with high performance is observed at 4.2 K with quantum dots as the gain medium. The lasing threshold is about 1 μW and β is about 0.25. The performance is much better than that of topological edge lasers, especially the threshold which is about three orders of magnitude lower than most of the topological edge lasers. The high performance results from the strong optical confinement in the cavity due to the small mode volume and high Q.
This result downscales the applications of topological photonics into nanoscale, which will be of great significance to the development of topological nanophotonic circuitry. Furthermore, the topological nanocavity can greatly enhance light-matter interaction, therefore enabling the investigation of cavity quantum electrodynamics and the further potential applications in topological nanophotonic devices.