Enhanced Nonlinear Optical Response from Nano-Scale ...
Transcript of Enhanced Nonlinear Optical Response from Nano-Scale ...
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Robert W. Boyd The Institute of Optics,
University of Rochester, Rochester, NY 14627, USA
Enhanced Nonlinear Optical Response from Nano-Scale Composite Materials
Presented at the Workshop on Frontiers in Nanophotonics and Plasmonics,Guarujá, SP, Brazil, November 10-14, 2007.
with special thanks to: Nick Lepeshkin, Giovanni Piredda, Aaron Schweinsberg, John Sipe, David D. Smith, and many others.
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The Promise of Nonlinear Optics
Nonlinear optical techniques hold great promise for applications including:
• Photonic Devices
• Quantum Imaging
• Quantum Computing/Communications
• Optical Switching
• Optical Power Limiters
• All-Optical Image Processing
But the lack of high-quality photonic material is often the chief limitation in implementing these ideas.
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Composite Materials for Nonlinear OpticsWant large nonlinear response for applications in photonics
One specific goal: Composite with (3) exceeding those of constituents
Approaches:
- Nanocomposite materialsDistance scale of mixing <<Enhanced NL response by local field effects
- Microcomposite materials (photonic crystals, etc.)Distance scale of mixingConstructive interference increase E and NL response
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Material Systems for Composite NLO Materials
All-dielectric composite materialsMinimum loss, but limited NL response
Metal-dielectric composite materialsLarger loss, but larger NL response
Note that (3) of gold 106 (3) of silica glass!Also, metal-dielectric composites possess surface plasmon
resonances, which can further enhance the NL response.
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Nanocomposite Materials for Nonlinear Optics
• Maxwell Garnett • Bruggeman (interdispersed)
• Fractal Structure • Layered
λ
2a
b
εi
εh
εaεb
εa
εb
scale size of inhomogeneity << optical wavelength
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Composite Optical Materials for PhotonicsOur approach is to fabricate nanocomposite materials withspecialized optical properties.
Motivation: Composite material can possess best propertiesof each constituent. And can perhaps even exceed propertiesof each constituent.
Example: Form a composite material for which (3) exceedsthose of its constituents. This can occurs because of local-field effects.
We have significant experience in this field (Fischer, Gehr,Nelson), and have achieved a factor of 3 enhancement in (3).
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Enhancement of the NLO Response
Under very general conditions, we can express the NLresponse as
eff(3) = f L2 L 2 (3)
L =+ 23
For a homogeneous material
L =3 h
m + 2 h
For a spherical particle of dielectric constant embedded in a host of dielectic constant
m
h
where f is the volume fraction of nonlinear material and L is the local-field factor.
Under appropriate conditions, the product can exceed unity. f L2 L 2
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Gold-Doped Glass: A Maxwell-Garnett Composite
gold volume fraction approximately 10-6
gold particles approximately 10 nm diameter
• Red color is because the material absorbs very strong in the blue, at the surface plasmon frequency
• Composite materials can possess properties very dierentfrom those of their constituents.
Developmental Glass, Corning Inc.
Red Glass CaraeNurenberg, ca. 1700
Huelsmann Museum, Bielefeld
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Demonstration of Enhanced NLO Response
• Alternating layers of TiO2 and the conjugated polymer PBZT.
• Measure NL phase shift as a function of angle of incidence
Fischer, Boyd, Gehr, Jenekhe, Osaheni, Sipe, and Weller-Brophy, Phys. Rev. Lett. 74, 1871, 1995.Gehr, Fischer, Boyd, and Sipe, Phys. Rev. A 53, 2792 1996.
— =^
DE
0( )e is continuous.
implies that
Thus field is concentrated in lower index material.
.
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Enhanced EO Response of LayeredComposite Materials
Diode Laser(1.37 um)
Polarizer
Photodiode Detector
Gold Electrode
ITO
BaTiO3
Ω =1kHz
Lockin Amplifier
Signal Generator
dc Voltmeter
Glass Substrate
AF-30/Polycarbonate
χ ω ωε ωε ω
ε ωε ω
εε
εε
χ ω ωijkleff
aeff
a
eff
a
eff
a
eff
aijklaf( ) ( )( ; , , )
( )
( )
( )
( )
( )
( )
( )
( )( ; , , )′ =
′′
′Ω Ω
ΩΩ
ΩΩ
Ω Ω1 21
1
2
21 2
• AF-30 (10%) in polycarbonate (spin coated)n=1.58 ε(dc) = 2.9
• barium titante (rf sputtered)n=1.98 ε(dc) = 15
χ
χzzzz
zzzz
i m V( )
( )
( . . ) ( / ) %
. ( )
3 21 2
3
3 2 0 2 10 25
3 2
= + × ±
≈
−
AF-30 / polycarbonate
3.2 times enhancement in agreement with theory
R. L. Nelson, R. W. Boyd, Appl. Phys. Lett. 74, 2417, 1999.
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Role of Metals in Composite NLO MaterialsAll-dielectric composite materials
Minimum loss, but limited NL response
Metal-dielectric composite materialsLarger loss, but larger NL response
Note that (3) of gold 106 (3) of silica glass!Also, metal-dielectric composites possess surface plasmon
resonances, which can further enhance the NL response.
How to minimize lossminimize attenuation by dilution (in liquid colloids)minimimze attenuation through metal-dielectric PBG structures
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8
6
9
4
1
7
2
5
3
-10 0 10 20 30z (cm)
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
r
D.D. Smith, G. Fischer, R.W. Boyd, D.A.Gregory, JOSA B 14, 1625, 1997.
Increasing GoldContent
2(3) (3) (3)eff i hf L= +L2
norm
aliz
ed tr
ansm
ittan
ce
Counterintuitive Consequence of Local Field Effects
Cancellation of two contributions that have the same signGold nanoparticles in a saturable absorber dye solution (13 μM HITCI)
(3)Im < 0
(3)Im > 0
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Comparison of Bulk and Colloidal Gold
0.90
0.92
0.94
0.96
0.98
1.00
1.02
1.04
-100 -50 0
50
100 150 200
z (m
m
)-20 -15 -10 -5 0 5 10 15 20
Z (cm)
1.0
1.1
1.2
1.3
1.4
1.5
1.6
Open Aperture Z-Scans of Gold Colloid and Au film at 532nm
Nonlinearities possess opposite sign!
Au Colloid
5 nm Au Film
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Nonlinear Optical Response of Semicontinuous Metal Films
Measure nonlinear response as function of gold fill fraction
0.0 0.2 0.4 0.6 0.8 1.0-4.0x10-3
-2.0x10-3
0.0
2.0x10-3
4.0x10-3
6.0x10-3
fill fraction
nonl
inea
r abs
orpt
ion β
(cm
/W)
percolation threshold (?)MG theory (D=2.38)
(with D. D. Smith and G. Piredda)
Note: Maxwell Garnett theory not valid at high fill fractions!
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E
z
Cu
L0
E
z
Cu
R.S. Bennink, Y.K. Yoon, R.W. Boyd, and J. E. Sipe, Opt. Lett. 24, 1416, 1999.
• Metals have very large optical nonlinearities but low transmission
• Solution: construct metal-dielectric photonic crystal structure(linear properties studied earlier by Bloemer and Scalora)
80 nm of copperT = 0.3%
• Low transmission is because metals are highly reflecting(not because they are absorbing!)
Accessing the Optical Nonlinearity of Metals with Metal-Dielectric Photonic Crystal Structures
SiO2 PC structure
bulk metal80 nm of copper (total)
T = 10%
Greater than 10 times enhancement of NLO response is predicted!
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Intraband (d-p) absorption
Drude reflection region
“Low ” loss
“Loss” mechanisms in copper
2
4
6
8
200 400 600 800 1000
, nm
k , ''
Cu
'
''
We work at 650 nm.
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Accessing the Optical Nonlinearity of Metals with Metal-Dielectric Photonic Crystal Structures
50 100 150 2000
0.1
0.2
0.3
0.4
0.5
0.6
Silica layer thickness (nm)
T simple modelexact solution
50 100 150 2000
10
20
30
40
50
Silica layer thickness (nm)
|Δφ p
bg/ Δ
φ bul
k|
• Metal-dielectric structures can have high transmission.
Enhancement of NLphase shift over bulk metal
Linear transmission of PBG sample at λ = 650 nm.
Copper layers 16 nm thick
• And produce enhanced nonlinear phase shifts!
• The imaginary part of χ(3) produces a nonlinear phase shift! (And the real part of χ(3) leads to nonlinear transmission!)
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Linear Transmittance of Samples
Cu: 40 nm film
M/D PC: Cu / silica
Material (interband) feature
Structural (M/D PC) feature
5x16/98 nm (80 nm total Cu)
450 550 650 7500
0.1
0.2
wavelength, nm
bulk, 80 nm
MDPC
trans
mis
sion
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< 1 ps ~5 ps
h T~1000 K
Mechanism of nonlinear response:“Fermi smearing”
2.15 eV
valence
conduction
propertiesopticalinchange)(T IBE
G. L. Eesley, Phys. Rev. B33, 2144 (1986)H. E. Elsayed-Ali et al. Phys. Rev. Lett. 58, 1212 (1987)
Near the interband absorption edge, “Fermi smearing” is the dominantnonlinear process
d-band
p-band
χ(3) is largely imaginary
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Z-Scan Comparison of M/D PC and Bulk Sample
λ = 650 nm
I = 500 MW/cm2
Lepeshkin, Schweinsberg, Piredda, Bennink, Boyd, Phys. Rev. Lett. 93 123902 (2004).
100 0 1000.4
0.6
0.8
1
z, mm
norm
aliz
ed tr
ansm
issi
on
M/D PC
bulk Cu
(response twelve-times larger)
• We observe a large NL change in transmission
• But there is no measurable NL phase shift for either sample
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Current Project: Composite Materials for Laser Systems
Specific Goals:
(1) Design a laser host material with a very small n2 to prevent laser beam filamentation
Motivation: Design lasers with superior performance based on the use of composite materials.
(2) Control key laser parameters by means of local field effects - Einstein A coefficient - laser gain coefficient - gain saturation intensity
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Example: Control laser properties through Einstein A coefficient
A = Avac neff |L|2
Why? - long lifetime gives good energy storage - short lifetime produces high gain
How to modify the Einstein A coefficient
volume averagedlocal-field factor of immediate vicinity of emitter
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A Avac neff L 2
g0L 2
neffg0, vac
Isat neff2 Isat, vac
Dependence of Laser Parameters on Properties of Host Material
where Lneff
3+ 22
In simplest approximation, laser parameters depend only on effective refractive index of material.
0
10
20
30
40
50
1 2 3 4
enha
ncem
ent
neff
A
g0
Isat
Note that great control of laser properties is possible by this approach
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nn2 23
2 diel vac
n3n2
2n2 1
2
AA
vacA dielA
Lorentz (Virtual Cavity) ModelReal Cavity Model
Dependence of Radiative Lifetime on Refractive Index of Host
Dolgaleva, Boyd, Milonni, JOSA B 211 516 (2007).
Nd:YAG nanoparticles(20 nm) suspended in a variety of liquids
250
300
350
400
450
500
1.3 1.4 1.5 1.6 1.7
lifet
ime
(μs)
effective refractive index
no local field effects
real cavity
virtual cavity
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Conclusions• Both nano-scale and microscale structuring can lead to enhanced nonlinear optical effects
• Influence of nano-scale structuring can be understood in terms of local field effects
• Nano-scale structuring can lead to enhancement (layered results) or cancellation (dye/colloid) of NLO response
• Influence of microscale structuring can be understood in terms of properties of photonic crystals
• We hope that nanocomposite materials will lead to new opportunities in the engineering of laser systems
• Metal / dielectric photonic crystals can be designed to allow access to the large nonlinearity of metals
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Aloha!
And thank you for your attention.
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Thank you for your attention!