Saturable absorption and optical limiting

1
Saturable absorption and optical limiting in
semicontinuous gold films.
Giovanni Piredda1, David D. Smith2, Mark Nelson3,
Youngkwon Yoon1, Robert W. Boyd1, Rongfu Xiao4
and Bettina Wendling5 1The Institute of Optics,
University of Rochester 2NASA Marshall Space
Flight Center 3New Mexico State
University 4Department of Physics, Hong Kong
University of Science and Technology, Hong
Kong 5Max Planck Institute for Polymer Science
Symposium On Materials Research 2006
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• The nonlinear optical properties of gold-silica
  composite materials vary greatly as a function of
  composition and wavelength.
• These materials can be both saturable absorbers
  and optical limiters.
• We present our experimental study
• measurement of the imaginary part of the optical
  Kerr coefficient
• connection between the linear and nonlinear
  properties
• connection between the nonlinear properties and
  the percolation threshold

3
One minute course on metal-dielectric
composites We study composite materials that
can be described by an effective dielectric
constant the size of each grain of material and
the distances between them are much smaller than
the wavelength of light a plane wave in these
kinds of media is not destroyed by
scattering. In certain metal-dielectric
composites a resonance appears in the effective
dielectric constant the plasmon resonance. At
the plasmon resonance, the nonlinear
susceptibility of the material is enhanced.
4
Isolated nanoparticles in a dielectric matrix
Maxwell Garnett theory
ei
eh
(for spherical inclusions)
resonance close to
High metal concentrations percolation
Interactions between nanoparticles Electrodynamic
retardation Effective medium theories Exact
(quasi-static) results at percolation Brouers
Shalaev theory Exact calculations
F. Brouers et al., PRB 55, 13234 (1997)
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Nonlinear optics of gold
Three contributions to the nonlinear refractive
index of gold nanoparticles with response time lt
1 ps. Intraband contribution (it does not
exist in bulk) confinement of the conduction
electrons inside the nanoparticles. Contributes a
small induced absorption Interband
contribution at 530 nm, saturation of the
interband absorption Hot electron contribution
the optical pulse modifies the equilibrium
distribution of electrons (Fermi
smearing) Largest contribution Balance gold
is an induced absorber for ps pulses at 530 nm
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Nonlinear optics of metal-dielectric composites
Nanoparticles at the plasmon resonance the field
in the metal is enhanced the nonlinearity of the
composite is consequently enhanced. In the
Maxwell Garnett regime the third order Kerr
susceptibility can be calculated assuming that
the field distribution is determined by the
linear dielectric constants.
Maxwell Garnett
with
Letting
with
(nonlinear inclusions)
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Nonlinear optics of metal-dielectric composites
Absorption in the nanoparticles grows
the resonance is smaller
less intensity in the nanoparticles
less overall absorption
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Prediction of Maxwell Garnett theory for the
nonlinearity of spherical gold nanoparticles in
silica fgold 0.2, ? 532 nm
Both the real and the imaginary part of the
susceptibility depend strongly on frequency.
eSiO22 and egold from AIP handbook c(3) of gold
from Smith D. D. et al., J. Appl. Phys. 86, 6200
(1999), taken as independent from l (?)
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Maxwell Garnett model predictions beyond its
validity limits
No complete theory exists for semicontinuous
films the Maxwell Garnett theory gives a good
hint to what we can expect.
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Samples and experimental technique

• Two series of samples
• Gold-silica cosputtered on quartz, f 0.04 to
  0.56 continuously variable across the sample (D.
  D. Smith, NASA Marshall flight center)
• Gold deposited on glass by laser ablation a
  discrete series of samples, f 0.2 to 0.8 (M.
  Nelson, New Mexico State University)

The measurement method we choose is the z-scan,
which is easy and distinguishes real and
imaginary parts of the nonlinearity. We use
pulses of around 20 ps duration.
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Our results so far
Nonlinear absorption coefficient b in the
cosputtered sample at ?  532 nm.
The nonlinearity first grows nonlinearly in the
fill fraction then decreases. The sign is never
positive.
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Our results so far
Nonlinear absorption in one of the samples
deposited by laser ablation at ? 532 nm (second
harmonic of NdYAG) f 0.42 sample not
percolated
z-scan
linear transmittance
There is no sign change in this case the
measurement is taken at a wavelength off the
plasmon resonance, which in this sample is
red-shifted.
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Conclusions and future work
We confirm that the sign of the nonlinear
absorption in a Maxwell Garnett composite is
opposite to the sign in the bulk material. We
observed both signs of nonlinearity in
gold-silica composites, and there is evidence of
a link to of the sign change to the plasmon
resonance (also outside the regime of validity of
the Maxwell Garnett theory). We must now
bracket as well as possible the region in which
the sign change happens, both in fill fraction
and in wavelength.