Ultrahigh electric and magnetic field enhancement in subwavelength all-dielectric resonators

Significance 

All-dielectric nanostructures characterized with high refractive indices have continued to attract growing research attention owing to their remarkably strong far-field optical responses attributed to resonant magnetic and electric modes. As a result, these nanostructures are potential candidates for various nanophotonic applications, including metamaterials, optical nanoantennas and enhanced nonlinear optics. It is well-known that multipolar electric and magnetic resonances of subwavelength dielectric resonators (DRs) exhibit magnetic and/or electric field enhancements. This phenomenon is significant in causing near-field couplings with other emitters, dielectric resonators and plasmonic nanostructures. However, DRs produce relatively small field enhancements that compromise the coupling efficiencies when coupled with other objects by near-field interactions. Thus, developing effective strategies to achieve larger field enhancement is highly desirable.

Lately, a significant amount of research efforts has been devoted towards obtaining larger magnetic and electric field enhancements in individual dielectric resonators. One of the simplest and most realistic ways of achieving this is by exciting a mode with high quality (Q) factor. This method is, however, limited by the low Q-factors of the common multipolar modes of individual DRs. Consequently, new concepts involving electric anapole and quasi-BIC modes have been introduced to enhance the Q-factors. In particular, whispering gallery mode (WGM) is another type of resonant mode that has drawn research attention recently due to its high Q-factors. Despite its potential applications, WGM is rarely used because most DRs have relatively larger working lengths. Moreover, although their distribution patterns are well understood, their field enhancement capabilities remain largely underexplored.

On this account, Ms. Xiao-Jing Du, Professor Zhong-Jian Yang, Mr. Ma-Long Hu, Ms. Lin Ma, and Professor Jun He from Central South University studied the electric and magnetic field enhancement capabilities of WGM of subwavelengths dielectric resonators due to their high-Q factors. The resonators used in this study were individual dielectric disks with a high refractive index of approximately n = 3.5. The authors investigated the various factors influencing the field enhancement value of WGMs. Their work is currently published in the research journal, Applied Physics Express.

The authors reported high magnetic and electric field enhancement under both TE and TM excitations. The field enhancement and Q-factor exhibited exponential scaling law with the mode index and refractive index. The WGM of the disk dielectric resonant showed the largest response when the disk radius and height were equal. For a disk dimer with subwavelength mode m = 7 and refractive index n = 3.5, a high electric field enhancement up to ~1180 was achieved in a small gap of 5 nm, which is generally higher than that produced by most reported subwavelength nanostructures such as the plasmonic ones. The larger electric field enhancement suggested the possibility of the structure having high Purcell factor. This was confirmed because the Purcell factors for the magnetic and electric dipole emitters reached as high as ~104 and ~105 respectively.

In summary, the research team successfully demonstrated that the WGMs of subwavelength DR could achieve significantly ultrahigh electric and magnetic field enhancement attributed to their high refractive index and Q-factors. The response of the WGM was mainly influenced by the height of the disk, the azimuthal mode index and the refractive index. By maintaining the strong response of the WGM, many other experimentally feasible conditions could be considered. In a statement to Advances in Engineering, the authors explained that the ultra-high magnetic and electric field enhancement reported in the study provide many opportunities such as enhancing nonlinear optics in subwavelength nanophotonic devices.

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About the author

Zhong-Jian Yang is an associate professor in Central South University, China. He received his B.S. degree and Ph.D. degree from Wuhan University in 2008 and 2013, respectively. Then he carried his post-doctoral research at Chalmers University of Technology in Sweden during 2013-2015. He joined Central South University in 2016. His current research interests include Nanophotonics, Plasmonics, Mietronics, and enhanced light–matter interactions based on optical nanoantennas.

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Reference

Du, X. J., Yang, Z. J., Hu, M. L., Ma, L., & He, J. (2021). Ultrahigh electric and magnetic near field enhancement based on high-Q whispering gallery modes in subwavelength all-dielectric resonatorsApplied Physics Express, 14(8), 082004.

Go To Applied Physics Express

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