Nonlinear Optical Response of Nanocavities in Thin Metal Films Yehiam Prior Department of Chemical Physics Weizmann Institute of Science With Adi Salomon - Weizmann Institute, Rehovot Joseph Zyss, Marcin Zielinsky - ENS Cachan, France Maxim Sukharev - Arizona State University, Arizona Tamar Seideman - Northwestern University, Illinois Robert Gordon - University of Illinois, Illinois Israel Chemical Society Annual Meeting February 2012

Nano Particles Notre Dame, Paris Quantum size effect Gold nanoparticles Semiconductor nanoparticles

Nano “structures”

Transmission with d<<λ When arrays are used – sharp interference peaks are observed The transmitted intensity is much larger than classical [~(d/λ) 4 ] Explained in terms of Plasmonic excitations in the metal Nano Holes

The periodicity determines the color Transmission through an array of nano holes p SEM

We understand the linear optical properties of these structures fairly well. What about their NONLINEAR optical properties?

OUTLINE SHG from Individual cavities From isolated to coupled cavities Plasmon-molecule interactions Conclusions and future directions

OUTLINE SHG from Individual cavities From isolated to coupled cavities Plasmon-molecule interactions Conclusions and future directions

Focused Ion Beam (FIB) fabricated shapes

What is the SHG response of a nano-hole ? Ag film ~ 200nm, evaporated on glass

SHG from non interacting triangles ω 2ω2ω Experimental set-up

150nm SHG from Individual triangles with different side length 170nm 190nm 220nm 245nm 285nm

SHG from Individual triangles with different side length Experimental condition: 200nm Ag film evaporated on glass (n = 1.5) FW=940nm thus SHG at 470nm

SHG from Individual holes - size dependence

SHG from Individual holes – wavelength dependence a=210nm

An oversimplified model For equilateral triangular cavities: For square cavities: Fabry-Pérot “bouncing ball” : “diamond-like” :

An oversimplified model

SHG Polarization properties Photo diode/x Photo Diode/y

Polarization properties for an individual cavity

OUTLINE SHG from Individual cavities From isolated to coupled cavities Plasmon-molecule interactions Conclusions and future directions

What happens when the holes are closer to each other, and are allowed to interact? We observe a gradual transition from isolated holes to coupled ones (similar to the assembly of a crystal from individual molecules) The intensity, as well as the polarization properties change From individual to coupled cavities

Individual hole Coupled holes From individual to coupled cavities

(a) (b)(c) From individual to coupled cavities: Polarization

c b a d

From individual to coupled cavities: Intensity Individual (650nm) coupled (450nm) Silver Gold

From individual to coupled cavities: λ dependence Smaller signal for shorter wavelengths

From individual to coupled cavities: observations 1.Individual holes give rise to SHG 2.Two types of coupling: a.“Light only” coupling – the plasmons generated in different holes do not interact directly (i.e. the gold sample), the dependence on the number of holes is quadratic b.Plasmon coupling - the plasmons interact directly, the dependence on the number of holes is more than quadratic 3.In both cases, the coupling is characterized by different polarization properties 4.For direct plasmonic coupling, metal must support plasmonic propagation at both the fundamental and the second harmonic frequencies

Hot Spots

1 Giant SHG signals at the hot spots (almost 1000 times bigger)

OUTLINE SHG from Individual cavities From isolated to coupled cavities Plasmon-molecule interactions Conclusions and future directions

Plasmon-molecule interaction – the system

Energy[eV] Slit array periodicity [nm] Transmission[a.u.] WaveVector[m -1 ] Energy [ev] Molecular state Upper polariton Lower polariton Plasmon-molecule interaction – avoided crossing

WaveVector [m -1 ] Energy [eV] (a) (b) o Collective mode Molecular state Upper polariton Lower polariton Energy [eV] Transmission [a.u.] Plasmon-molecule interaction – strong coupling

Energy [eV] Spacer thickness[nm] Plasmon-molecule interaction – strong coupling, with a spacer layer

OUTLINE SHG from Individual cavities From isolated to coupled cavities Plasmon-molecule interactions Conclusions and future directions

1.We observed coherent SHG from individual nanocavities 2.Size and shape matter - resonances are observed 3.Two types of coupling: light mediated and plasmon mediated, giving rise to a gradual transition to a “crystal” 4.Polarization properties provide excellent characterization 5.Additional experiments and theory are needed to fully and quantitatively understand the results 6.Calculations for strong coupling with molecules 7.Engineered (nonlinear) optical properties are possible 8.Hot spots are observed, with a potential for high sensitivity spectroscopy (to the single molecule level ?)

Thank you

Hot Spots Giant SHG signals at the hot spots (almost 1000 times bigger)

Hot Spots

10D 20D 30D 40D 1e 24 5e 24 1e 25 5e 26 3e 25 Figure 3: (a) (b) Energy [eV] Transmission [a.u.] Dipole moment [Debye] RS (meV) M density [m -3 ] RS (meV)