ta_winter_college_ on_optics_and_energy_ico-ictp_award _2010.pdf

Optical p nonlinearities in organic g materials
Prof. Cleber R. Mendonca
http://www.photonics.ifsc.usp.br

Sao Carlos
University of Sao Paulo - Brazil
students 77.000
52.000 undergrad.
25.000 grad.
employers 15.000
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• Sao Paulo
• Sao Carlos (9.000)
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University of Sao Paulo – in Sao Carlos

University of Sao Paulo – in Sao Carlos

Professors: 80
Instituto de Física de São Carlos
Employers: 180
(technical and administration)
Students: 450 (undergrad)
( g )
100 (master)
140 (phD)
Several research areas in Physics
and Material Sciences

Grupo de Fotonica
Photonics Groups
The purpose of the Photonics Group is to
develop p fundamen<strong>ta</strong>l science and applied pp
technology in Optics and Photonics
Some of the research areas
• Nonlinear optics
• Coherent control of light matter interaction
• fs-laser microfabrication and micromachining
• Optical spectroscopy
•Optical p storage g

Optical p nonlinearities in organic g materials
Prof. Cleber R. Mendonca
http://www.photonics.ifsc.usp.br

Outline
introduction to nonlinear optics
nonlinear optics in organic materials
experimen<strong>ta</strong>l p methods
examples of some results

Nonlinear optics
Nonlinear Optics
Nonlinear Optics
The branch of optics that describes optical phenomena that occur when very
intense light is used

harmonic oscillator
Linear optics
linear response
P =
χE
E rad.

anharmonic oscillator
P
=
χ
Nonlinear optics
nonlinear polarization response
( 1
)
E
+
χ
( 2 )
E
2
+
high light intensity
E rad. ~ E inter.
χ
( 3 )
E
3
+
...

P
Nonlinear optics
nonlinear li expansion i of f th the polarization l i ti
r
( 1 ) ( 2 )
( 3 )
= χ . E + χ : EE
+ χ M
r
r
r r r
EEE
+ ...
linear SHG THG
processes Kerr effect

Second order processes
Nonlinear Optics
( 2
χ
Second Harmonic Generation
ω 22ω
)

(
χ
2 )
=
0
Nonlinear polarization
Nonlinear Optics
χ
( 3)
Third order processes

Third order processes
Nonlinear Optics
χ
( 3)
ω
3ω
NNonlinear li media
di
ωω

Third order processes
Kerr media:
Nonlinear Optics
( 3
χ
)
n = n0
+ n2I
Index of refraction depends on the light intensity
n
2
≈
χ
( 3 )

Kerr media:
n n0
n2I
+ =
Self phase modulation
centre symmetric:
P
n 2>0 Material behaves as a convergent lens
y
2
x
d
Sample
f
= χ
( 3) E
3
NL
χ () 2 = 0
z

ω
Nonlinear optics
Nonlinear
material
• Intense light induced nonlinear response
in the material
• Material change the light in a nonlinear
way
2ω
3ω
ω ?
Self action
effect

χ (3) is a complex quantity
Nonlinear Optics

Third order processes: χ (3)
Refractive process:
Absorptive process:
n n0
n2I
+ = 0 2
• self-phase modulation
• lens-like effect
α α + ββ
I
= 0
• nonlinear absorption
•two-photon p absorption p

Two-photon absorption (2PA) process
Phenomenon does not described for the Classical Physics and does not
observed until the development of the Laser.
2ω
Theoretical model: Maria Göppert-Mayer, 1931
Two photons from an intense laser light beam are simul<strong>ta</strong>neously absorbed
p g y
in the same “quantum act”, leading the molecule to some excited s<strong>ta</strong>te with
energy equivalent to the absorbed two photons.
ω
ω

applications of 2PA - optical limiting
low intensity high intensity
To protect eye and sensors from intense laser pulses

applications of 2PA - two-photon fluorescence
α α + βI
= 0
ω
ω
light emission
fl fluorescence

2PA
localization of the exci<strong>ta</strong>tion with 2PA
dilute solution of fluorescent dye
1PA
spatial confinement of exci<strong>ta</strong>tion

two-photon fluorescence microscopy
microscopy by two-photon fluorescence
3D image of a cell
Human chromosome
Laboratory for Optics and
Biosciences
Ecole polytechnique Fluorescent marker ⇒ fluorophores

applications of 2PA - microfabrication
two-photon polymerization
Venus s<strong>ta</strong>tue
Two-photon Two photon polymerization
Opt. Exp. 12, 5521-5528 (2004)
oscillator
NNature t 412, 412 697-698 697 698 (2001)
Bull

Real applications in nonlinear optics
Very intense light: femtosecond pulses
Ti:Sapphire lasers
100 fs 50 fs 20 fs 1fs= 10 -15 s
Laser intensities ~ 100 GW/cm 2
1 x 10 11 W/cm 2
Laser pointer: 1 mW/cm 2 (1 x10 -3 W/ cm 2 )

Organic materials
• Flexibility to tune the nonlinear optical response by manipulating the
molecular structure
• π-conjugated structures
σ
C C Conjugation: j g alternation of single g and doubles bonds
π
between carbon atoms

Conjugation π-conjugation
σ bond: forms a strong chemical bond; localized
π bond: weaker bond; out of the C atoms axis
π bond in conjugated system: delocalized electrons
high optical nonlinearities
χ (3)

Research
• study of optical nonlinearities in organic materials
• fs-laser microfabrication
• optical storage and surface relief gratings in azopolymers
• coherent control of light matter interaction

Research
• study of optical nonlinearities in organic materials
• fs-laser microfabrication
• optical storage and surface relief gratings in azopolymers
• coherent control of light matter interaction

Optical nonlinearities in organic materials
• Unders<strong>ta</strong>nding the physical principles behind two-photon absorption
• Unders<strong>ta</strong>nding the relationship between molecular structure and
two-photon absorption
• Developing molecules with high optical nonlinearities that can be
used for application pp

norrmalized
transmmit<strong>ta</strong>nce
Normalized
Transmit<strong>ta</strong>ce
1.04 1.04
1.00 1.00
0.96 0.96
Z-scan (nonlinear absorption)
open aperture Z-scan
z0
-10 -5 0 5 10
Z
α ( I ) = α0
+ βI
T ( z)
Δ T ∝ ββ
I
∑ ∞
=
m=
0
[ − q0
( z,
0 ) ]
3 / 2 ( m + 1)
q +
0 m
2 2 ( z / )
0( z,
t ) = βI0L
/ 1 z0

150 fs laser system
Ti:Sapphire amplifier
775 nm
150 fs
800 μJ

nonlinear absorption
α α + ββ
I
= 0
nonlinear refraction
n n0
n2I
+ =
Nonlinear spectrum
intense laser (ultra short pulses)
discrete λ s
δ(λ) ( ) n2(λ) 2( )
nonlinear spectrum p ???

Nonlinear absorption spectrum
Optical parametric amplifier
460 - 2600 nm
≈ 120 ffs
20-60 μJ

Azoaromatic samples
HO H2C H2C HO H2C H2C HO H2C H2C HO H2C H2C HO H2C H2C H 3C H 2C
HO H2C H2C H 3CH 2C
N N
AZO
H2N N N NH2 DIAMINO
H 2N
H 2N
N
N
N N
N N NO 2
Cl
N N NO 2
N N NO 2
Cl
p-AMINO
DO3
DR19Cl
DR19
N N N NO 2 DR13
N
N N NO 2
DR1

Linear absorption of azoaromatic compounds
absorbance
0.9
0.6
0.3
00 0.0
AZOB
PAMINO
DO3
DR13
DR1
DR19
DR19Cl
DIAMINO
300 400 500 600
wavelength (nm)

Normalizzed
Transmitt<strong>ta</strong>nce
1.00
0.95
0.90
0.85
α α = αα
+ ββ
I 0
Two-photon absorption
700 nm
750 nm
860 nm
930 nm
1010 nm
O 2N
Cl
N
DR13
[ ( ) ]
∑
( )
∞ − q0
z,
0
T ( z)
= 3 / 2
Z / cm m=
0 ( m + 1)
-0.5 0.0 0.5
β: two-photon absorption coefficient
N
N
CH 2CH 3
CH 2CH 2OH
m

Absorbance
0.9
0.6
03 0.3
0.0
0.6
0.3
0.0
0.9
0.6
0.3
0.0
0.6
0.3
00 0.0
0.6
0.3
0.0
Pseudostilbenes
DO3
DR1
DR19
DR13
DR19-Cl
400 600 800 1000
λ (nm)
600
300
0
400
200
0
600
300
0
600
300
0
600
300
0
δ (GM)
δ
Absorbbance
0.9
0.6
0.3
0.0
0.6
03 0.3
0.0
AAminoazobenzenes b
400 600 800 1000
λ (nm)
PAMINO 600
300
M)
0
(GM
DIAMINO δ
( ) ⎥ ⎤
2
νν
⎡ A A1
A A2
( ν ) ∝
⎢
+
600
300
( ) ( ) ( ) ⎥ ⎥
2 2
2 2
2 2
ν −ν
+ Γ ν − 2ν
+ Γ ν − 2ν
+ Γ
⎢ ⎢
⎣
i0 i0
f10
f10
f2
0
f2
0
0
⎦

Planarity of the π-bridge
Thermally induced torsion in the molecular structure

Molecular design strategy
• Increasing the molecular conjugation
• Adding charged groups to the molecule
• Keep molecular planarity

a
intennsity
(arb. unit) u
White light continuum Z-scan
4
3
2
1
b
450 550 650 750
0
wavelength (nm)

Transm mitância No ormalizada a
1.0
0.9 RB
0.8
0.7
(d)
White light continuum Z-scan
Δ T
-0 -0.4 4 -0 -0.2 2 00 0.0 02 0.2 04 0.4
z (cm)
P (mw)
0.04
0.03
0.02
0.01
000 0.00
Espectro de Luz branca
Potência To<strong>ta</strong>l = 5 mW
500 600 700
λ (nm)
(a)

α (cm -1 )
20
15
10
5
MeH-PPV
0
400 500 600 700 800 900
Wavelength (nm)
White light continuum Z-scan
Non resonant effects
4.0
3.0
2.0
1.0
0.0
δ(10 3 GM)
Perylenes derivatives
M)
δ (10 3 GM
8
6
4
2
0
600 650 700 750 800 850 900 950
Wavelength (nm)
each measurement <strong>ta</strong>kes only a few minutes

fs-laser microfabrication
Novel concept:
build a microstructure using fs-laser and nonlinear optical processes

two-photon polymerization
applications
• micromechanics
• waveguides
• microfluidics
• biology
• optical devices

Two-photon absorption
Nonlinear interaction provides spatial confinement of the exci<strong>ta</strong>tion
α0
fs-microfabrication
α = α α + βI
= 0

Two-photon polymerization
Monomer + Photoinitiator → Polymer
light
Photoinitiator is excited by two-photon absorption
R PA ∝
2
The polymerization is confined
to the focal volume.
I
2
High spatial resolution

Two-photon polymerization
bellow the diffraction limit

Two-photon polymerization
even higher spatial resolution

Two-photon polymerization setup
CCD
camera
Beam
Expansion
Objective
z
Illumination
y
x
spacer
Scanning
Mirror
glass (150 micron)
glass (150 micron)
Laser
resin
Ti:sapphire laser oscillator
• 130 fs
• 800 nm
• 76 MHz
• 20 mW
40 x
0.65 NA
Objective

Two-photon polymerization
After the fabrication, the sample is
immersed in ethanol to wash away any
unsolidified resin and then dried

Microstructures con<strong>ta</strong>ining MEH-PPV
MEH-PPV MEH PPV
Fluorescence
Electro Luminescent
Conductive

Microstructures con<strong>ta</strong>ining MEH-PPV
waveguiding of the microstructure fabricated
on porous silica substrate (n= (n 11.185) 185)
Applications: micro-laser; fluorescent microstructures; conductive microstructures

Other studies
• microstructures for optical p storage g – birefringence g
φφ
NN
NN
E r
E r
r dye
p dye
Light Relax
NN NN NN
NN
trans cis trans
J. Appl. Phys., 102, 13109-1-13109-4 (2007)
NN
N
NN
N
φφ
p r
p r
E r
E r

Other studies
• microstructures for optical p storage g – birefringence g
Ar + ion laser irradiation
• 514.5 nm
• one minute
• intensity of 600 mW/cm2

Other studies
• microstructures for optical storage – birefringence
The sample was placed under an optical microscope between crossed
polarizers and its angle was varied with respect to the polarizer angle

Other studies
• microstructures for optical storage – birefringence
J. Appl. Phys., 102, 13109-1-13109-4 (2007)

Other studies
• 3D cell migration studies in micro-scaffolds SEM of the scaffolds
110 µm pore size
52 µm pore size
Top view
110, 52, 25, 12 µm
pore size
Side view
25, 52 µm
pore size

Other studies
• 3D cell migration studies in micro-scaffolds
Advanced Materials, 20, 4494-4498 (2008)

Thank you !
www.photonics.ifsc.usp.br

for a copy of this presen<strong>ta</strong>tion
www.photonics.ifsc.usp.br