1. Third harmonic imaging of plasmonic nanoantennas
Andreas Trügler, Ulrich Hohenester
Karl-Franzens-Universität Graz, Austria
Work performed together with:
T. Hanke, J. Cesar, R. Bratschitsch, A. Leitenstorfer
Lehrstuhl für Moderne Optik und Quantenelektronik, Univ. Konstanz

2. Goal of this work Agenda Tailoring spatiotemporal light confinement
in single nanoantennas…
200 nm
Agenda
- Optical antennas, experiments
Simulation of metallic nanoparticles
THG mapping of particle plasmons
200 nm

3. Nanoantennas as nonlinear emitters
Hyper-Rayleigh scattering at surface imperfections,
see M. Stockman et al., PRL 92, (2004)
Linear optics: Resolution given by wavelength l
Nonlinear optics (THG): Resolution given by l / 3
Excitation vs. Detection: Wavelength difference
Strong χ(3) nonlinearity for gold,
see T. Hanke et al., PRL 103, (2009)

4. Imaging with optical antennas
Substrate
THG intensity ~ | E |6 E
THG
Pump laser pulse
0.97 eV, 24 fs
Array of
nanoantennas
By scanning the excitation spot over the sample and observing the THG signal (in the farfield), we obtain a map of the electric fields of the particle plasmons.
See S. Kim et al., High-harmonic generation by resonant plasmon field enhancement, Nat. Lett. (2008);
T. Hanke, R. Bratschitsch, A. Univ. Konstanz, Germany (2011).

5. THG mapping of particle plasmons
Third harmonic generation (THG) map (left) and sample (right)
Excitation with fs – pulses and with a bandpass filter for wavelengths 1100 – 1500 nm
T. Hanke, R. Bratschitsch, A. Univ. Konstanz, Germany (2011).

6. THG mapping of particle plasmons
T. Hanke, R. Bratschitsch, A. Univ. Konstanz, Germany (2011).

7. THG intensity for particle plasmons
Lowest antenna volume
gives highest THG intensity !?

8. Boundary element method (BEM)
Discretization of surface integral into „boundary elements“
Collocation method … surface charges located at centers of boundary elements
from boundary conditions…
F. J. García de Abajo et al., PRB 65, (2002); U. Hohenester et al., PRB 72, (2005).

9. THG mapping of particle plasmons
Simulation of antenna structures
Result from experiment
Size of each triangle ca. 300 nm, discretisation with surface elements

10. THG intensity for particle plasmons
Lowest antenna volume gives highest THG intensity !?
Scattering intensity generated by electromagnetic fields at the surface…
Incoherent optics:
Biggest volume gives highest intensity…
Coherent optics:
Lowest volumes gives highest intensity…

11. THG autocorrelation
Autocorrelation allows to measure dephasing time of particle plasmons
THG autocorrelation intensity depends on time delay between femtosecond pulses

12. THG autocorrelation
Autocorrelation allows to measure dephasing time of particle plasmons
harmonic fields
Insert harmonic fields together with plasmon damping time:
two interacting pulses
damping
in / out of phase ratio gives 32:1
THG autocorrelation intensity depends on time delay between femtosecond pulses

13. THG autocorrelation
Autocorrelation allows to measure dephasing time of particle plasmons
Dephasing times:
rod 5.5 fs
ellipse 3.5 fs
disc 2.0 fs
THG autocorrelation intensity depends on time delay between femtosecond pulses
Weak plasmon damping effective build-up of the plasmon oscillation
Knowledge of the plasmon damping time alone suffices to predict the nonlinear intensity !

14. THG intensity vs. plasmon dephasing
THG intensity directly scales with plasmon dephasing ! Long dephasing times correspond to large THG intensities
high nonlinear emission connected to small antenna volumes
radiative damping!
rod length: nm
gap: nm

15. THG intensity vs. plasmon dephasing
THG intensity directly scales with plasmon dephasing ! Long dephasing times correspond to large THG intensities

16. High intensity linked to smallest antenna volume!
Summary & Acknowledgement
Theoretical Nanoscience
Ulrich Hohenester
Jürgen Waxenegger
KFU Graz, Austria
Temporal scale: Measuring few-fs plasmon damping times
Spatial scale: Mapping of third-harmonic emission
Radiative damping: Lowest volumes generates strongest third-harmonic emission
Moderne Optik und Quantenelektronik
Alfred Leitenstorfer
Rudolf Bratschitsch
Tobias Hanke
Vanessa Knittel
Julijan Cesar
High intensity linked to smallest antenna volume!