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Quasi-BIC and Reflective Mode Coupling Drives SHG Beyond the Q Factor Limit.

Wenkai Yang1, Hao Qin1, Boqing Liu1,2

  • 1School of Engineering, College of Engineering, Computing and Cybernetics, the Australian National University, Canberra ACT, Australia.

Small (Weinheim an Der Bergstrasse, Germany)
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Summary

Researchers enhanced second harmonic generation (SHG) in metasurfaces by boosting absorptance at quasi-bound state in the continuum (quasi-BIC) resonances. This approach achieves strong SHG even with lower quality factors, offering a new design framework.

Keywords:
absorptancedual‐resonance metasurfacenonlinear opticsquasi‐BICsecond harmonic generation

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Area of Science:

  • Nonlinear Optics
  • Metasurfaces
  • Quantum Information Processing

Background:

  • Second harmonic generation (SHG) is crucial for frequency conversion and quantum information processing.
  • Enhancing SHG in metasurfaces using quasi-bound state in the continuum (quasi-BIC) resonances faces limitations due to material properties and quality (Q) factors.

Purpose of the Study:

  • To propose and validate an alternative strategy for enhancing SHG by increasing absorptance (A) at quasi-BIC resonances.
  • To establish a general design framework for absorptance-engineered metasurfaces for improved SHG.

Main Methods:

  • Utilized temporal coupled mode theory (TCMT) to derive an analytical expression linking SHG intensity with Q factor and absorptance.
  • Designed and fabricated a 3R phase molybdenum disulfide (3R-MoS2) metasurface by spectrally aligning a reflective resonance with a transmitted quasi-BIC mode.
  • Experimentally validated the enhanced absorptance and SHG efficiency.

Main Results:

  • Demonstrated that strong SHG can be achieved with optimized absorptance, even with finite Q factors.
  • The fabricated 3R-MoS2 metasurface showed an SHG conversion efficiency of approximately 3 × 10-5 at 9 GW cm-2.
  • Achieved over a 40-fold enhancement in SHG compared to an unpatterned flake.

Conclusions:

  • The proposed strategy of enhancing absorptance at quasi-BIC resonance is effective for boosting SHG.
  • The experimental results align well with the theoretical model, confirming the design framework.
  • This work provides a new avenue for designing metasurfaces for efficient nonlinear optical processes.