The design of dielectric environment for ultra long lifetime of graphene plasmon Dr. Qing Dai 22/10/2015

Surface Plasmons (SPs) Surface free charges oscillations - E(t) + Manipulation of light at nanoscale Spatial confinement Local field enhancement Science, 2010, 328, 440

Metamaterials map  Ultra-high mobility  Low carrier density  Low interband losses Graphene is an ideal material for plasmon! Atwater, Science, 2011 Plasmon lifetime Graphene Calculated lifetime : ~10 ps in mid-IR ~10 ns in THz A. Principi, et al. PRB, 2013, 88,

Properties of graphene plasmon ： Electrical tunability Low intrinsic losses Broad band operation Suitability of on-chip integration High field enhancement for strong light-matter interaction T. Low, et al. ACS Nano, 2014, 8, 1086 J.N. Chen, et al. Nature, 2012, 487, 77 Graphene Plasmons

Motivation & Objectives Substrate All atoms are exposed to the substrate A small portion of atoms are exposed to substrate Au Graphene Substrate design for high performance GP ？ Surface phonon mode hybridization ？ Electron Scattering (dangling bond, impurities) ？ Radiation loss (surface roughness)

Near-field (s-SNOM) Far-field extinction spectrum （ FTIR ） Nanoribbon arrays to excite local plasmon; Extinction spectra obtained ( cm -1 ); Multi-modes characterization. Experimental measurement of graphene plasmon Metal-coated AFM tips to excite propagating plasmon (~15 nm); High-order demodulated harmonics of the near-field signals to obtain weak signals;

Hybridization of GP and surface phonons  Free standing --- one peak; On silica substrate --- three peaks

Graphene Plasmon coplanar coupling Decrease of inter-ribbon spacing caused red shifts of plasmonic resonances f is ribbon to period ratio Coplanar coupling of GPs is verified by resonance red shift

Plasmon resonance varied as inter-ribbon spacing Ribbon to period ratio Plasmon coupling strength on various Substrates Ribbon to period ratio Non-polar substrate results in stronger coplanar coupling X. Yang, et al. Small,

Van der Waals heterostructures: for long lifetime Plasmon-phonon hybrid modes:  Long lifetime of phonons in monolayer crystals;  High field enhancement. Design of ultra-thin functional device:  Combine functions of varied 2D crystals;  Excellent electrical properties. G/BN heterostructure Geim, et al. Nature, 2014

Graphene/h-BN heterostructure Graphene/monolayer h-BN/SiO 2 Graphene/SiO 2 1 、 Peak 4 origins from the coupling with LO phonon of h-BN （ 1370 cm -1 ）； 2 、 the positions of Peaks 1 and 2 move slightly. (806, 820 cm -1 ) 。 The linewidths become narrower.

Out-of-plane TO phonon （ 820 cm -1 ） of BN interact with graphene plasmon. Graphene/h-BN heterostructure

For monolayer h-BN: 3 optical phonons Remote phonon scattering mechanism ： effective electrical potential 1, in-plane LO phonon (1370 cm -1 ) 2, in-plane TO phonon (820 cm -1 ) 3, out-of-plane TO phonon (820 cm -1 ) A new coupling mode: the o-TO phonon interact with plasmon.

Fano resonance EIT Graphene/h-BN heterostructure

Lifetime of the hybrid modes lifetime of Fano peaks:  =2h/  Lifetimes: Near BN phonon: ~1.6 ps; Far from phonons: ~100 fs; Near SiO 2 phonon: ~180 fs; Without coupling: ~40 fs; For Ag LSP: ~10 fs.

900 cm cm cm cm cm cm cm cm cm -1 G/BN heterostructure G/SiO 2 device Near-field images

900 cm cm cm -1 Plasmon wavelength: 260 nm Incident wavelength: 900 cm -1 Confinement: 43 Lifetime: 73 fs Plasmon wavelength: 180 nm Incident wavelength: 950 cm -1 Confinement: 59 Lifetime: 66 fs Plasmon wavelength: 140 nm Incident wavelength:1000 cm -1 Confinement: 72 Lifetime: 54 fs Near-field images

Dispersion

 Graphene plasmon: electrical tunability & dielectric environment effect;  Substrate surface phonon effects on GP: Tradeoff between modulation bandwidth\coupling efficiency ;  Long lifetime hybrid modes of plasmon and phonon: Van der Waals heterostructures can improve plasmon lifetime significantly by combine the low loss and highly field enhancement. Summary

Acknowledgements Thank you for your attention! National Natural Science Foundation of China (NSFC) The Recruitment Program of Global Experts