Photonic cavities that efficiently confine light within small volumes for extended durations play a pivotal role in modern photonics with wide applications ranging from low-threshold lasers, ultra-small filters, photonic chips, and quantum information processing to optical communications. These photonic cavities typically include Fabry-Pérot, whispering-gallery-mode, and photonic crystal cavities.
Topological photonic cavities with robust topological protection have recently attracted great attention and shown promising applications in topological lasers, integrated photonics, robust optical delay lines, and quantum optics. Among these, a novel type of topology-controlled photonic cavities based on the near-conservation of photonic valley degree of freedom (DOF) was theoretically proposed by placing a judiciously oriented mirror at the termination of a valley photonic crystal (VPC) waveguide to localize the valley-polarized edge states [Phys. Rev. Lett. 125, 213902 (2020)]. These robust valley edge states, require flipping their valley-locking momentum for reflection.
This momentum flip involves an extended time delay controlled by the geometry of the terminating mirror. Therefore, when the effective reflection time is sufficiently long, the electromagnetic energy becomes tightly confined at the mirror’s surface, forming a subwavelength topology-controlled photonic cavity. However, due to the challenges associated with measuring the electromagnetic field distributions and the extended time delay, experimental realization of this novel topology-controlled photonic cavity remains elusive.
Associate Professor Zhen Gao’s group from the Department of Electronic and Electrical Engineering at the Southern University of Science and Technology (SUSTech) has recently reported the first experimental realization of a new type of topology-controlled photonic cavities in valley photonic crystals, which extend and complement the current design paradigm of topological photonic cavities.
Their paper, entitled “Realization of topology-controlled photonic cavities in a valley photonic crystal”, has been published in Physical Review Letters, one of the most prestigious journals in physics.
Professor Gao’s group report the first experimental realization of a topology-controlled photonic cavity. By employing microwave near-field mapping and pulse transmission measurements, they directly observe the enhanced electric field distributions and extended time delay of the topology-controlled photonic cavity. These results demonstrate that different orientations and shapes of the mirrors result in varying time delays required for the valley index flipping, leading to distinct levels of energy confinement and quality factors. These results expand the research scope of topological photonics and enrich the fundamental physical principles of photonic cavities.
The researchers terminated the VPC waveguide with a perfect electric conductor (PEC) mirror to realize a topology-controlled photonic cavity, as schematically shown in Figure 1(a) and the left panels of Figures 1(f)-1(g). When a rightward-propagating valley edge state encounters the metallic mirror, it has three different scattering channels, the upper VPC1-mirror interface, the lower VPC2-mirror interface, and the VPC waveguide. Depending on the orientation of the metallic mirror, they considered two types of terminations: zigzag [Figure 1(f)] and armchair [Figure 1(g)]. For the zigzag (armchair) termination, they examined two supercells and analyzed their eigenfrequency spectra, as shown in Figure 1(f) [Figure 1(g)], the green (blue) line representing the dispersion of the trivial interface states supported by the upper VPC1-mirror interface (lower VPC2-mirror interface).
The gapped trivial interface modes B and C (D and E) were localized near the mirror surface with a complete no-interface-mode bandgap [orange regions in Figures 1(f)-1(g)], indicating that when a valley-polarized edge state within the no-interface-mode bandgap encounters the mirror termination, it cannot leak through the upper and lower VPC-mirror interface channels. The last scattering channel was backscattering, which required the valley index of the topological edge states to reverse from K to K’. Consequently, the valley-index-flipping rate was equivalent to the leaky rate of the topology-controlled photonic cavity.
Interestingly, previous studies have revealed that the valley-index-flipping rate depends on the topology of the perturbations and the zigzag termination to significantly suppress the inter-valley scattering because of the valley conservation. In contrast, the situation differed for the armchair termination, which produced a much higher valley-index-flipping rate because of the valley conservation breaking. Therefore, if the VPC is terminated with a zigzag-oriented PEC mirror, the valley-flipping rate should be minimized. The valley edge states will be tightly localized near the mirror surface, forming a subwavelength topology-controlled photonic cavity.
Figure 1. Design of a topology-controlled photonic cavity by terminating a VPC waveguide with a metallic mirror
For the topology-controlled photonic cavity with zigzag termination that preserves the valley conservation, as shown in Figure 2(a-c), the simulated and measured Ez field distributions exhibited large electric field enhancement near the right termination, indicating the zigzag topology-controlled photonic cavity has good field confinement and high-quality factor. In contrast, for the topology-controlled photonic cavity with armchair termination [Figures 2(e)-2(f)], the simulated and measured Ez fields were almost uniformly distributed in the VPC waveguide and had no obvious field localization and enhancement near the mirror surface. This indicates that the armchair topology-controlled photonic cavity with valley breaking has an ultralow quality factor because of its high valley-index-flipping rate.
Figure 2. Observation of the frequency-domain electric field distributions of the topology-controlled photonic cavity
Lastly, the group performed time-domain pulse measurements to directly extract the reflection delay time at the mirror surface. Figure 3 shows that the zigzag mirror exhibits a longer valley-index-flipping time than the armchair mirror, which experimentally verifies the underlying physical mechanism of the topology-controlled photonic cavity.
Figure 3. Observation of the time-domain pulse transmission in the zigzag and armchair topology-controlled photonic cavities
Postdoctoral fellow Bei Yan from the Department of Electronic and Electrical Engineering at SUSTech, Ph.D. candidate Baoliang Liao from Jinan University, Ph.D. candidate Fulong Shi from Sun Yat-sen University, and Associate Professor Xiang Xi from Dongguan University of Technology are the co-first authors of this paper. Associate Professor Zhen Gao, Professor Baile Zhang from Nanyang Technological University, Assistant Professor Gui-Geng Liu from Westlake University, and Associate Professor Xiao-Dong Chen from Sun Yat-sen University are the co-corresponding authors. SUSTech is the first affiliated institution.
Paper link: https://doi.org/10.1103/PhysRevLett.134.033803
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