2021 Fiscal Year Annual Research Report
Study on unidirectional-emitting photonic-crystal lasers based on topological photonics
| Project/Area Number |
21F20356
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| Research Institution | Kyoto University |
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Principal Investigator |
野田 進 京都大学, 工学研究科, 教授 (10208358)
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| Co-Investigator(Kenkyū-buntansha) |
YIN XUEFAN 京都大学, 工学研究科, 外国人特別研究員
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| Project Period (FY) |
2021-04-28 – 2023-03-31
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| Keywords | Photonic crystal / Topology / Resonance |
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| Outline of Annual Research Achievements |
In 2021, we focused on the theoretical investigation upon unidirectional guided resonances (UGRs). UGRs are a type of optical resonances in Photonic-crystal (PhC) slab which only radiate towards one side of PhC slab without the need for mirrors on the other. Conventional ways to realize UGRs ask for the bound states in continuum (BICs) in 1D PhC slab, and their realization in a generic PhC structure is a formidable engineering challenge, especially when no appropriate BICs can be found in laser structure. To solve this problem, we proposed a systematic way to realize UGRs in 2D PhC slab without the premise of BICs, by utilizing the interband coupling induced from vertical symmetry breaking. We also discussed the properties of 2D UGRs, including their dependence on the in-plane symmetry, the degrees of freedom needed to realize them with respect to different in-plane symmetries, and the parameter tolerance about the realistic fabrication and realization.
Our investigation shows that the UGRs are even more ubiquitous than expected, because band-crossing and corresponding interband coupling are quite common in PhC slabs regardless of their specific material and geometry. This method broadens the scope of applications that UGRs can be applied in. With this method, one can realize UGRs in varieties of application scene, such as laser structures asking for relative week index confinement, and grating couplers with strong index contrast.
We have finished a paper based on these research achievements, which is now under review and can be previewed at: https://arxiv.org/abs/2203.02223
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
Our research progress is going as planned. The purpose of our research is to explore the potential mechanisms to further improve the performance of PCSELs, including basically two aspects: one is to improve the power extraction efficiency, another is to improve single-mode stability. Our work in year 2021 concentrated on the first target. Specifically, as for the first goal, one approach is to close the unwanted radiation channel on the bottom side of PhC slab to get rid of the useless downward radiation. To solve this problem, we found out that UGRs are good candidates as the lasing modes to realize the on-chip single-sided emission. Therefore, we implemented a theoretical study upon UGRs, figuring out the properties of UGRs in 2D PhC slab and proposing a comprehensive method to realize it. Our study has proved the possibilities that UGRs can be applied in laser structure, paving the way to realize the on-chip unidirectional laser. Now we are concentrating on the second goal, to find out a mechanism to improve the single-mode stability in PCSELs.
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| Strategy for Future Research Activity |
There are many ways to enhance the single-mode stability for PCSELs. Here we plan to investigate a mechanism to improve the single-mode stability from the concept of topology. From the viewpoint of topology, we will design the cavity modes in laser structure to connect to some bulk band topological invariants, which are robust conserved quantities against parameter disorders and imperfections. By selecting specific topological invariant, corresponding cavity resonance becomes topologically distinguishable.
As stated above, we plan to utilize the UGRs as the lasing modes. UGRs have already been proved to connect to far-field topological charges -- the winding numbers of polarization vector field around polarization singularities. Therefore, the key point of topology-enabled single-mode stability is finding out the correspondence between far-field topological charges and bulk band topological invariants such as Chern numbers.
To achieve this, first, we plan to establish a universal picture of topological charges in PhC slab. Specifically, we plan to use coupled-wave theory framework to build a radiation model, and investigate the correspondence between the bulk modes topology and far-field topological charges. Then, with this correspondence, the band topological invariants of the UGRs in laser structure with different topological charges will be calculated and designed. Further, appropriate gain profile or optical incidence will be designed to excite the UGRs with specific topological invariant.
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Research Products
(4 results)
