Near-field imaging of resonating hyperbolic polaritons in nanorod antennas made of boron nitride

Transcription

1 Near-field imaging of resonating hyperbolic polaritons in nanorod antennas made of boron nitride NanoSpain 17, San Sebastián, España F. J. Alfaro-Mozaz, P. Alonso-González, S. Vélez, I. Dolado, M. Autore, S. Mastel, F. Casanova, L. E. Hueso, P. Li, A. Y. Nikitin and R. Hillenbrand

2 Plasmons concentrate electromagnetic fields into a nanoscale spot Surface Plasmon Polaritons Localized Plasmon Resonances Gold Nanoparticle Plasmonic nanorod antenna resonances Application of plasmonic nanorod antennas 1.5 mm Topography Near-Field Intensity Nature Photonics 3, p.287 (2009) Mid-IR molecular spectroscopy Neubrech PRL 101, (2008) 2

3 Plasmon and Phonon-Polaritons Metals / doped semiconductors collective free electron oscillations (plasmons) Dielectric Function Polar crystals (SiC, h-bn, quartz...) strong lattice vibrations (phonons) Dielectric Function Reststrahlen band Preliminar Simulation of the Dipolar Resonances of representative Plasmonic and Phononic Antennas Q 10 Q > 100 Hillenbrand et al., Nature (2002) Q = ω Δω Caldwell et al., Nano Letters (2012) 3

4 h-bn is an Anisotropic Phononic Material Atomic Structure h-bn properties 2D van-der-waals (layered) material Easy to prepare by exfoliation In contrast with other phononic material such as SiC Due to its layered structure, its permittivity is highly anisotropic Optical microscope image Dielectric Function h-bn Flake Lower ε z < 0 Upper ε t < 0 Thickness ~ 40 nm Reststrahlen bands 4 J. D. Caldwell et al., Nat. Comm. 5, 5221 (2014)

5 Hyperbolic Volume Polaritons in h-bn Isofrequency Surface of h-bn in momentum space Simulation: Point source over h-bn Air θ Point Source h-bn θ Simulation: Point Source over h-bn slab Air h-bn Air θ Point Source ω = const λ 0 2 5

6 Hyperbolic Volume Polaritons in an h-bn Slab Point Source λ M0 First Volume Mode, M0 λ M1 + Second Volume Mode, M1 λ M2 + Third Volume Mode, M2 + Higher order Modes 6

7 Imaging of Hyperbolic Phonon Polaritons in h-bn Imaging of Hyperbolic Volume Polaritons in h-bn Flakes Imaging of Hyperbolic Volume and Surface Polaritons in h-bn Flakes h-bn flake M0 M0 M cm -1 SM0 Visualization of the M0 Mode S. Dai (D. Basov Laboratory), Science (2014) Visualization of the M0 Mode and the SM0 Mode P. Li, I. Dolado, Nano Letters (2016) 7

8 Hyperbolic Surface Polaritons (Dyakonov Polaritons) at the edges of h-bn flakes h-bn flake 0,25 mm h-bn flake P. Li, I. Dolado, Nano Letters (2016) 8

9 Scanning Electron Microscope Image of a batch of h-bn Antennas h-bn antenna 2 mm SiO2 Substrate 9

10 Nanoimaging of h-bn Nanorod Antennas: Longitudinal modes as a function of Length and Frequency Schematics of the experiment Near-field images as a function of frequency ω Near-field images as a function of length L 10

11 Point Spectra reveal resonances with high Q Factors SEM Image h-bn Antenna Experimental Point Spectra.72 mm 11

12 Spectral line-scan allows to connect peak position with resonant wavelength. SEM Image h-bn Antenna Experimental Line-Scan 1.72 mm d λ HP 2 12

13 Hybridized Hyperbolic Surface Modes gives rise to the Fabry- Perot resonances Dispersion of the Modes q 0 : Momentum of light in free space 13

14 Hybridized Hyperbolic Surface Modes gives rise to the Fabry- Perot resonances Dispersion of the Modes q 0 : Momentum of light in free space 14

15 Hybridized Hyperbolic Surface Modes gives rise to the Fabry- Perot resonances Dispersion of the Modes Volume Mode M0 Surface Mode SM0 q 0 : Momentum of light in free space 15

16 Hybridized Hyperbolic Surface Modes gives rise to the Fabry- Perot resonances Dispersion of the Modes Field Distribution of the Surface Modes Volume Mode M0 Surface Mode SM0 q 0 : Momentum of light in free space 16

17 Anomalous transverse Structure of the Hyperbolic Modes Plasmonic Antenna Third Order Resonance Not To Scale h-bn Antenna Third Order Resonance Neaspec 17

18 Anomalous transverse Structure of the Hyperbolic Modes h-bn Antenna Third Order Resonance E z h-bn Antenna Cross Section of the Mode Profile E z 18

19 Summary & Outlook We have fabricated and studied nanorod antennas made of h-bn Through real-space imaging and nanospectroscopy we demonstrated antenna resonances with very large Q factors (Q ~ 100) and deeply subdiffractional confinement. We showed that the resonances are due to a hybridized Hyperbolic Surface Mode (SM0-S) We expect that these antennas can be used as a building block for metasurfaces and for ultrasensitive molecule detection in the mid-ir P. Alonso-González S. Vélez, I. Dolado, F. Casanova, L. E. Hueso, M. Autore, S. Mastel P. Li, A. Y. Nikitin and R. Hillenbrand 19

20 Thanks for your attention! J. Alfaro et al, in progress Images by Pablo Alonso, Nanogune (2015)

21 Near-Field profile of the Waveguide Mode (SM0-S) changes with frequency 21

22 Propagation of the SM0-S mode 22

23 Experimental Setup 23

24 Profiles of Modes SM0-S Surface Modes SM0-A Volume Modes M0-W SM1-S SM1-A M1-2 M1-3 M4-11 M3-8 24

25 SM0-S Mode propagates even for very small antenna cross-sections 25

26 Dispersion of the Volume Modes Mn. Dispersion of the M0 mode as a function of Frequency Alexey Nikitin et al. 26