Toroidal Response of Asymmetric Metasurfaces with Multiple High Q-Factor Resonances

1. Toroidal Response of Asymmetric Metasurfaces with Multiple High Q-Factor Resonances Samuel Gomez Mentor: Sirak M. Mekonen Searles Applied & Materials Physics Laboratory Howard University June 20, 2018

2. Introduction/Background: Metamaterials • Metamaterials have shown • great capability when it comes to manipulating electromagnetic waves • Composed of unit cells in periodic patterns, which are smaller than the wavelength of the waves they are influencing • Each individual structure composing the material is a “metamolecule”

3. Recent Progress • Possibilities for negative indexes of refraction • Beam-forming applications have been explored through use of gradient metasurfaces • Metasurfaces make possible a high quality factor, which would be useful for sensing and slow-light devices, as well as precise measurement of small resonance shifts • Additionally, with regards to beam control, non-uniform metasurfaces have a greater degree of freedom • THz waves, which are studied through the use of metamaterials, offer a higher spatial resolution, as well as a nonionizing effect

4. Graphene capabilities in metamaterial/THz wave research • Graphene is a very intriguing material, as its properties can be modified in order to produce certain results in research • Its fermi level can me modified through an applied gate voltage • By putting sheets of graphene in contact with metasurfaces, we can tune the resonance frequency of the composite • By stacking sheets of the them, you can modify the optical conductivity of the graphene • Graphene has optical transparency, high electron mobility, and it is flexible • Has plasmons (collective oscillations of charge carriers) • Interestingly, at plasmon resonance, THz optical conductivity of patterned graphene becomes independent of frequency

5. Split Ring Resonators (SRR) • Split rings resonators are composed of various geometries, and it is through these that you can tune responses to THz waves Ramdass, Adrian, et al. “Using a Split Ring Resonator to Harvest RF Energy from a PCB Transmission Line.” YouTube, YouTube, 19 Apr. 2016, www.youtube.com/watch?v=qVH7E6wMnAE.

6. Base Structures for each experiment Zheng, Xiaobo. “Tuning the Terahertz Trapped Modes of Conductively Couple Fano-Resonators in Reflectional and Rotational Symmetry.” Optical Materials Express, vol. 8, no. 1, 1 Jan. 2018, pp. 105–118. Chen, Xu, and Wenhui Fan. "Study of theinteractionbetweengraphene and planarterahertzmetamaterialwithtoroidal dipolar resonance." Opticsletters 42.10 (2017): 2034-2037ianyu Xiang, Tao Lei, SenHu, JiaoChen, XiaojunHuang, and Helin Yang • A toroidal dipole is an electromagnetic occurrence that has been observed through experiments using metamaterials. • It is produced by currents flowing on the surface of a torus shape along its meridians. Here, an array of magnetic dipoles are arranged in the “head-to-tail configuration along a torus”.

7. Group Comparisons (experiments) [1] Chen, Xu, and Wenhui Fan. "Study of theinteractionbetweengraphene and planarterahertzmetamaterialwithtoroidal dipolar resonance." Opticsletters 42.10 (2017): 2034-2037ianyu Xiang, Tao Lei, SenHu, JiaoChen, XiaojunHuang, and Helin Yang

8. Methodology for SAMPL • One of the main differences in design was the different gap lengths (g1,g2). • Chen et al used the same gap spacing for each gap, also for Opt. Mat. Exp. • Chen et al also used much larger dimensions for their design, in the millimeter range. The periodicity of SAMPL designs was 6 microns less than that of the Opt. Mat. Exp. Structure. • All experiments varied the distances between the gaps and the centers of the material. SAMPL symmetric design

9. Symmetries + = + = Reflectional preserved Rotational and Reflectional preserved + = + = No symmetry Reflectional preserved

10. How it works

11. Case 1 Case 2 Case 3

12. Chen et al vs. Optical Materials Express • The structure used by Chenet al is comparable to the rotational geometry from Opt. Mat. Exp, however its dimensions are larger (mm). • Closer and sharper peaks on the transmission graph than the rotationally symmetric design. • X direction polarized wave (parallel to gaps) • Only rotational symmetry + = Chen, Xu, and Wenhui Fan. "Study of theinteractionbetweengraphene and planarterahertzmetamaterialwithtoroidal dipolar resonance." Opticsletters 42.10 (2017): 2034-2037ianyu Xiang, Tao Lei, SenHu, JiaoChen, XiaojunHuang, and Helin Yang

13. Summary of program • Worked more on analysis and comparison of data from SAMPL structures and other groups’ data • Analyzed significance of symmetry and different forms in exciting various electromagnetic occurrences • Learned how to work with THz spectroscopy on the HASSP • Became familiar with finite element method through running numerical simulations • Worked with programs such as LaTeX, Igor, HFSS

14. Acknowledgements • Financial support from the REU Site in Physics at Howard University NSF Award PHY 1659224 is gratefully acknowledged