Plasmon Resonance Dynamics of Silver Nanoprisms by Ultrafast PumpProbe Spectroscopy

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Plasmon Resonance Dynamics of Silver Nanoprisms
by Ultrafast Pump-Probe Spectroscopy
Lille 5/12/2005
Camilla Bonati
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Outlook
Introduction and Motivations The
System Experimental Results Transient
Absorption of the plasmon resonance Interpretatio
n The Model The numerical simulations
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Interest
Introduction The system The Experiment Interpreta
tion

• Nanofabrication
• Bio/Chemical sensors

• Electrons in confined systems
• Non-linear optical properties

Size/Shape Dependent Optical Properties
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Imaging Applications
Introduction The system The Experiment Interpreta
tion
s-SNOM
SNOM
Optical microscope
5 -10 nm
50 nm
250 nm
Nanoprobe
Far-field
Near-field
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Ideal Probe
Introduction The system The Experiment Interpreta
tion
Large Cross Section
High Field Enhancement
Large Field Gradients
High Field Confinement
Kottmann et al., J. Microscopy, 202, 60, 2001
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Synthesis
Introduction The system The Experiment Interpreta
tion
Introduction The system The Experiment Interpreta
tion
Callegari A. et al., Nano Letters 3, 1565 (2003)
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Static Optical Response
Introduction The system The Experiment Interpreta
tion
Surface plasmon resonance
Triangular plates
Two different characteristic length scales!
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The Setup
Introduction The system The Experiment Interpreta
tion
Pump Probe Configuration
Amplified Ti Sa 70 fs, 1kHz
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Photoinduced dynamics
Introduction The system The Experiment Interpreta
tion
Transient absorption
Periodic oscillations of the surface plasmon band
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Dynamics and Interactions
Introduction The system The Experiment Interpreta
tion
860
Hot electrons


Wavelength /nm
Electrons cooling
740
0
120
t /ps
Particle cooling
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Dynamics and Interactions
Introduction The system The Experiment Interpreta
tion
860
Residuals
21 ps
Wavelength /nm
740
0
120
t /ps

• Which are the mechanisms driving the two
  oscillations?
• Can we describe the relevant spectral/temporal
  behavior of the particles?

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840 nm
760 nm
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Phase Behavior
Introduction The system The Experiment Interpreta
tion
Fit over the whole spectral range


Smooth Phase Change
Model the optical response of the
system Homogeneus Inhomogeneous contributions
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Analysis modeling
Introduction The system The Experiment Interpreta
tion

• from literature

• from fitting procedure

?

• from TEM images

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Analysis modeling
Introduction The system The Experiment Interpreta
tion

• Capture dynamics
• Gain information

Cosine-type oscillations ? indirect excitation
mechanism
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Simulation of Mechanical Oscillations
Introduction The system The Experiment Interpreta
tion

• Approximate numerical solution developed from an
  existing model (1)
• Macroscopic constants, homogeneous body
• Dimensions lateral size 70 nm, thickness 8 nm

(1) Cheung Y.K., Zhou D., Int. J. Solids
Struct., 39, 673 (2002)
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Simulation of Mechanical Oscillations
Introduction The system The Experiment Interpreta
tion

• Approximate numerical solution developed from an
  existing model (1)
• Macroscopic constants, homogeneous body
• Dimensions lateral size 70 nm, thickness 8 nm

(1) Cheung Y.K., Zhou D., Int. J. Solids
Struct., 39, 673 (2002)
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Mechanical Oscillations Main Features
Introduction The system The Experiment Interpreta
tion
Predominant Modulation of
Thickness
Lateral Size
2A1
1A1
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More on the Phase (1)
Introduction The system The Experiment Interpreta
tion
Smooth Phase Change Inhomogeneity
Two different excitation mechanisms Step-like Im
pulsive (35 ps) (21 ps)
thermal expansion
electron pressure
Main Oscillations (35 ps) Weak Oscillations (21
ps)
shifting band
Thickness
Lateral Size
Band Position Band Width (35 ps) (21 ps) Two
different modulation mechanisms
Band broadening
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Different Origin of the oscillations
Introduction The system The Experiment Interpreta
tion
Two different excitation mechanisms Step-like Im
pulsive (35 ps) (21 ps)
thermal expansion
electron pressure
Thickness
Lateral Size
Band Position Band Width (35 ps) (21 ps) Two
different modulation mechanisms
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More on the Phase (2)
Introduction The system The Experiment Interpreta
tion
Main Oscillations (35 ps) Weak Oscillations (21
ps)
Homogeneous Linewidth
shifting band
Band broadening
L. Bonacina et al., Nano Letters, in press (Jan.
2006)
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Conclusions

• Phase of the oscillations related to the
  inhomogeneous size distribution (Approach which
  is valid for all non-spherical particles)
• First observation of two distinct vibrational
  modes
• 1A1 Step-like excitation mechanism thermal
  expansion
• 2A1 Impulsive excitation mechanism electron
  pressure
• Estimation of the homogeneous bandwidth
• Excellent agreement between numerical simulations
  and experiment using macroscopic elastic
  properties.

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People
Prof. Majed Chergui
Andrea Callegari
Luigi Bonacina, Frank van Mourik
Dino Tonti
NCCR-Quantum photonics
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Statistics

• Average size 68.9 nm

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2
1
20 W Coolwhite
20 W Coolwhite
Induction
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