Nanophotonics in organics

Fundamental processes and applications Alain Fort

Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS) CNRS UMR 7504

Département d’Optique ultrarapide et Nanophotonique (DON), 23 rue du Lœss, BP 43, 67034 Strasbourg Cedex 2, France

e-mail :alain.fort@ipcms.u-strasbg.fr

Nanophotonics in organics

Nanophotonics:

Science of light-matter interactions taking place on wavelength and sub-wavelength

scales

Huge interest

Study of novel fundamental properties of the matter at the nanoscale

Fabrication of new efficient electrooptical devices for engineering applications

such as:

- waveguides

- couplers

- switches

- modulators

- wavelength division multiplexers,

- amplifiers

- detectors

- diodes

- lasers

……

Usual technologies for imaging nano-objects:

- Near-field Scanning Optical Microscopy (NSOM or SNOM)

- Scanning Probe Microscopy (SPM)

- photoassisted Scanning Tunneling Microscopy (photoSTM)

- Surface plasmon optics

In Organics, especially well adapted processes for nano-imaging

and fabrication of nano-devices:

- Two-photon absorption (TPA)

- Second harmonic generation (SHG)

- Photopolymerization

OUTLINE

Nanophotonics in organics

Fundamental processes and applications

I. Introduction: the diffraction limit

II. Far-field and near-field microscopy

III. Quadratic processes (TPA, SHG)

IV. Photopolymerization

V. Examples of optical devices

- optical memories

- artificial muscles

- light induced self-written wave guides

VI. Concluding remarks

The diffraction limit

Airy disc

Diffraction pattern for a circular aperture

How to distinguish two point sources of equal brightness?

The diffraction limit

Ernest ABBE

Dd minimum lateral resolution (cf Rayleigh criterion)

n refractive index

nsin(q ) Numerical Aperture (N.A.)

Rayleigh criterion

Δθ in radians

D diameter of the aperture

Image of two points separated by 1000 nm

l = 550 nm

Dd = 320 nm

l = 10 µm (IR)

Dd = 9 µm

Image of one point

l = 10 µm (IR) l = 550 nm

Propagating waves

Evanescent waves

Low spatial frequencies

High spatial frequencies

Helmholtz equation

Propagating waves = low-pass filter : no sub l/2 spatial informations

Far Field / Near Field

Sample

Illumination in transmission

Illumination in reflection Far-Field domain

Propagative waves

Low spatial resolution

Near-Field domain

Evanescent waves

High spatial resolution

Far Field / Near Field

Far Far- field detector Far Illumination in reflection

Sample

Illumination in transmission

Subwavelength probe

Nano-probe to change evanescent waves into propagating waves

Scanning Near-field Microscope (SNOM or NSOM)

Far-field excitation and detection R. Bachelot, P. Gleyzes, and A. C. Boccara, Opt. Lett.,20,1924 1995)

Conversion Near-field Far-field

Piezoelectric scanning unit

Role of the tip

tip

PSTM Collection Reflexion

Illumination Illumination / collection

Ebbesen et al., Appl. Phys. A, 89, 225 (2007)

Surface plasmons

V-groove S-bend guide

STEM image SNOM image Channel Plasmon Polariton (CPP)

in a Waveguide Ring Resonator (WRR)

Collection

Our goal : Study of the local optical properties (refractive index, Second Harmonic Generation)

of organic integrated circuits using a home-made SNOM

Radko et al., Laser Phys. Lett., 2005

SNOM in illumination/collection mode :

reflectivity strongly depends

on the refractive index

Other imaging approaches

Nanoimaging using confocal microscopy

Conventional wide-field fluorescence

microscope (~1 mm)

CCD

Dichroic mirror

objective

Focal plane (sample)

Excitation spot (20 to 200 mm)

Notch filter

Disadvantage: high background signal

Limited spatial selectivity

Advantage : observation of many particles

camera

Confocal microscope: role of the pinhole

Allow the 3D imaging Fluorescence from the focal volume is passing

All other light is locked :

A

A

B

B B

PM

x

y

Laser beam direction

pinhole

Marvin Minsky 1957

1. Light rays from outside the focal plane not recorded.

2. No blurring with defocusing = optical sectioning.

3. True, three-dimensional data sets .

4. Scanning in x/y-direction as well as in z-direction : objects

viewed from all sides.

5. Stray light and photo bleaching are minimized.

6. Image processing allows extended focus image with the

same resolution (in conventional microscopy reduction of the

aperture sacrifices resolution.

Advantages of confocal microscopy

Optical quadratic processes for spatially confined

light-matter interactions

“Physics would be dull and life most unfulfilling if all physical phenomena

around us were linear. Fortunately, we are living in a nonlinear world.

While linearization beautifies physics, nonlinearity provides excitement in

physics.”

Y. R. Shen in The Principles of Nonlinear Optics, New York,

Wiley-Interscience, 1984.

Molecular response to a weak E.M. field

excitation

E

molecule

p a

response Weak field linear response:

Induced dipole

jijgiiep am

Permanent dipole

Microscopic polarization

a linear polarizability tensor

e local electric field related to external field for a spherical cavity through field factors:

Lorentz expression (”The theory of electrons,” B.G. Teubner ed., Leipzig,

1909)

Onsager expression (JACS, 58, 1486, 1936)

3

2)²()(

nf

)2²(

)2²()0(

n

nf

Preliminary reminders

Molecular response to a strong E.M. field

excitation

E

molecule

response p

jijgiiep am

... lkjijkkjijk

eeeee

Linear response

Non linear response

i, j, k, … refer to components expressed in a molecular frame

: 2nd order polarizability tensor or

1st order hyperpolarizability tensor or

quadratic hyperpolarizability tensor

: 3rd order polarizability tensor or

2nd order hyperpolarizability tensor or

cubic hyperpolarizability tensor

Preliminary reminders

Macroscopic response

I, J, K.. components expressed in the laboratory frame

) ) LKJIJKLKJIJKJIJII EEEEEEPEP )3()2(1

Dipoles oscillating at 2: Second Harmonic Generation (SHG)

E2 = E20cos2t = ½(E2

0 + E2

0 cos2t)

)(i ith order susceptibility

P = molecules

p je

ijgi am ...

le

ke

je

ijkke

je

ijk=

E = E0cost ,

Requirement in the dipolar approximation for quadratic optics:

non-centrosymmetric structures (molecules, functionalized polymers, crystals)

f(-x) = - f(x)

For centrosymmetric systems:

Even order polarizabilities , d, …and

even susceptibilities )2( n = 0

Preliminary reminders

NON-centrosymmetry requirement for molecules incorporated in materials:

Need of a molecular alignment

E

I, l I2, l/2 E0

I, l I, l Without poling

N

H 3 C

C H 3

O N

H 3 C NO2

=

Polymer functionalized with NLO

chromophores

NO SHG

SHG with

molecular alignment

SHG

) ) LKJIJKLKJIJKJIJII EEEEEEPEP )3()2(1

Electro-optic effect in organics

: SHG, Sum- and difference- frequency, Pockels effect,

related to molecular parameters by:

)(2

m dipole moment of the molecular ground state

L3 third-order Langevin function

E poling field felt by the chromophores

T temperature

Gas phase approximation (non-interacting dipolar chromophores)

)(cos),()( 32 q fN

4

)2(

33

)(2)(

nr zzz

and for g <<1,

kTEg /m

N number of molecules/cm3

f Lorentz factor

Dominant linear Pockels EO effect tensor

kTE

gL

5/cos

)(cos

3

3

3

mq

q

q average molecular orientation relative to the electric field

(in pm/V)

....)3()2()1( EEEEEEP

Fluorescence induced

by 2-photon absorption One-photon absorption

EP

E

E

P

)E

1 ) EE

2 ) EEE

3

) EEn

...

For intense fields

For weak fields

Macroscopic response

Frequency Frequency of the

polarisation

Processes

p, q p + q Sum of frequencies

p, q p q Difference of

frequencies

p, p 2 p Second Harmonic

Generation (SHG)

p, 0 p Pockels effect

p, p 0 Optical correction

From K.D. Dorkenoo and A. Fort , in “Multiphoton processes in organic materials

and their applications”, I. RAU and F. KAJZAR ed., Old City Publishing, 2012.

Two-photon absorption (TPA)

(a) Single photon absorption

(b) Two-photon absorption (TPA)

(c) Second harmonic generation (SHG)

Virtual state

Theoretical prediction of TPA:

M. Göppert-Mayer, Naturwissenschaften 1929, 17, 932 (1929),

Ann. Phys. 9,273 (1931)

Experimental demonstration:

W. Kaiser, C.G. Garret , Phys. Rev. Lett. , 7, 229 (1961).

M. Göppert-Mayer

Nobel prize 1961

TPA at the microscopic scale

SOS perturbative approach, 3 level contributions

Need of a high spatial density of photons during a short time

2

2

r

2photons

2photonsτ.S

P

f

σproba

: TPA cross section

: impulsion duration

P : laser power

: laser frequency 2photons

σ

S : surface

fr

femtosecond laser beams

t

(For theoretical considerations, see for example:

- Y.R. Shen, The principles of nonlinear optics, New York, Wiley-Interscience, 1984.

- P.N. Butcher, D. Cotter, The elements of nonlinear optics, Cambridge University Press, 1991.

- R. Boyd, Nonlinear optics, Academic Press, 2003.)

Macroscopic approach

(3) related to two-photon absorption and to the optical Kerr effect

From K.D. Dorkenoo and A. Fort, in

“Multiphoton processes in organic

materials and their applications”, I. RAU

and F. KAJZAR ed., Old City Publishing,

2012

Gaussian beam in a nonlinear media:

(a) self-focusing, (b) self-defocusing

The propagating solution can be expressed as follow:

Voxel size with TPA

Warren R. Zipfel et al. Nature biotechnology, 21, 1369 (2003)

0.3 1.3 0.6 0.85

Influence of the numerical aperture

( NA=nsinq)

N.A

N.A :

excitation

photonun

cefluorescenII 2

excitation

photons2

cefluorescenII

Excited volume < 1µm3

Two-photon induced fluorescence

Ar laser

(514,5 nm)

Ti:sapphire laser

(800 nm)

Applications of quadratic processes in organics

- Imaging

- High density optical storage

- Photopolymerization

- Optical circuits

3D Micro-fabrication and lithography

- High frequency modulators

- Organic lasers

- Photodynamic therapy

……

Laser 120fs

PM

Caméra

Spectrograph

Spectrograph

Acquisition System

z

x y

Dichroic

mirror

Beam spliter

X40 0.6

HV

sample

Filters

Filters

Pin hole

Set-up for 3D controlled excitation and detection

(TPA and SHG)

Example of application: High storage capacity disks

CD : l = 780 nm

track spacing 1.6 mm. 700 Mo

DVD : l = 650 nm

track spacing 740 nm.

5 Go

Blu-Ray : l= 405 nm

track spacing 300 nm

25 Go (1 layer) or

50 Go (2 layers)

Commercially available optical disks

Need to increase the storage capacity

TPA photopolymerization for bulk writing

NLO chromophore (DR1)

E0

1st step:poling = encoding a uniform ensemble of bits “1”

High density optical storage based on

submicron control of the molecular order

Starting material:

co-polymer PMMA - DR1

CH2 C

CH3

CH2 C

CH3

CO2 CH2 CH2

N C2H5

N

N

NO2

CO2CH3

x y

PMMA - DR1

Objective X40, X60

ON = 0.8, 1.3

polymer

substrate

Laser

80 MHz

Dt = 120 fs

l = 800 nm

1 mm 1 mm

400 nm

oriented non oriented

1 m

m

Nonlinar Orientational Hole Burning (OHB)

2nd Step: writing “0” bits by local disorientation via TPA

High density data storage

Photo-isomerization cis-trans PMMA DR1 sample

Step 1: Obtention of an aligned sample by corona

poling

Step 2: Local disorientation of DR1 molecules by

two photon absorption (photoisomerisation cycles)

Step 3: Reading of the data by SHG

3D SHG grating. Smallest distance between two

adjacent points (column on the left hand side) is about

2 μm (writing power 23 mW, exposure time 200 ms

with objectif of NA: 0.85, reading power 1.8 mW,

scanning speed 10 μm/s). 1012 dots/cm2.

4.5 µm

Data inscription

1 2 3

Bit “1” Bit “0”

Gray Scale Imaging

Gra

y s

cale

(a) Initial image of M. Göppert-Mayer, (b) Optical microscopy image, (c) SHG image

25 µm

-b-

Applied physics Letters, 90, 094103 (2007)

Adv. In Opto Electronics, D967613,(2009)

International patent (2010)

Rewritable Data Storage

(a) SHG image taken at 3 mW, 785 nm, 10

μm/s, of a grating (50 lines of 50 μm

with a pitch of 4μm) written at P = 10 mW,

800 nm (a border burned in at 105 mW serves

as a marker); (b) microscope image of the same

area; (c) SHG scan after repoling; (d)

SHG scan after rewriting 20 lines of

50 μm with a pitch of 10 μm.

Optical and AFM images of printed dots. (a) Optical

microscopy of an array of 20 lines of dots written with illumination power

increasing from 1.3 (top left) to 48.1 mW (bottom right). (b) SHG imaging

scan of the same area of the sample with a power of 2 mW. (c)–(e) AFM

scans of three areas depicted by white squares in hole depths of (c) 4,

(d) 7, and (e) 20 nm. Scale bars in all pictures are 7.5 µm.

7.5 µm

7.5 µm

Optics express,14, 9896 (2006)

JOSA B, 24,532 (2007)

Micro- and nano-fabrication through two photon

absorption polymerization: Pioneering works

2 μm

J. Serbin et al., Opt. Lett. 28, p. 301, 2003. S. Kawata, H. B. Sun, T. Tanaka, K. Takada, Nature, 412, 698 (2001).

P. Galajda and P. Ormos, Applied Physics Letters, 78, 249, 2001

S. Marder, J. Perry et al, 296, Nature, 1106 (2002)

P. N. Prasad et al. , Applied Physics Letters, 74, 170, 1999

3D micro-structures

K. -S. Lee et al., Appl. Phys. Lett., 2005, 87, 154108-1; Polym. Adv. Technol., 2006, 17, 72-82;

M. Straub, L.H. Nguyen, A. Fazlic, M. Gu, Optical Materials, 2004, 27, 359.

Nowadays, spatial resolution less than 60 nm!

TPA polymerization: Artificial Muscles

C O

O

(CH2)4

O

CO

O C

O

OC4H9OC

O

C4H9O

monomer acrylate

+

Crosslinker 1,6-hexanediol diacrylate

+

UV photoinitiator 2-benzyl-2-(dimethylamine)-4’-

morpholinobutyrophenone (Irgacure 369)

Polymer backbones Liquid - crystal unit

Cross - linker

Temperature

Polymer backbones Liquid - crystal unit

Cross - linker

Polymer backbones Liquid - crystal unit

Cross - linker

Temperature

Aligned sample

Photopolymerisation and temperature induced

reversible contraction : Liquid crystal elastomer

hn

TPA groove vs one photon groove

TPA voxel

One photon absorption

Two-photon absorption

(dimension of letter ‘‘E’’ : 800 µm)

q

l

sin

52.0xy

q

l

cos1

76.0

zzxyV

22

3

2

Température (oC) A

ng

le b

etw

ee

n 0

an

d 1

st o

rde

r (d

eg

.)

Thermoactive microsystems for diffractive optics

30 µm period grating on 30 µm thick sample

Figure of diffraction

T=80°C T=96°C

heating

T=99°C

a

b

T=99°C T=80°C T=96°C

cooling

Optics Express, 15, 6784 (2007)

Applied Physics A, 98, 119 (2010)

Applications: - Variable gratings

- Tunable DFB lasers

- Active optical dispersive elements

- Creation of permanent guides by local mastering of the refractive index

distribution through the control of the spatial photopolymerization

- Monomode and multimode light-induced self-written waveguides

- Waveguides functionalized with doping chromophores

- Active E.O. devices

Integrated optical circuits

Diffraction

Index of refraction guiding

Index manipulation :

Possibility of guided propagation

(waveguide modes)

Fundamental mode (one maximum)

ith –order mode (i = number of zeroes)

Monomode propagation

A monomode behaviour for a step-index guide requires a V-parameter less than 2.4

2a diameter of the guide

Optical Kerr effect

Nonlinear effect: change of n under application of an electric field (fast effect, fs-scale)

Non permanent guide!

n = n0 + n2I

How to create permanent waveguides?

Kerr effect disappears when I is too low need of a permanent modification

of the optical properties of the material for creating a permanent waveguide

Interest of photopolymers

n0

n1 > n0

n0

n1

Polymerization increases the refractive index

Photopolymerized waveguides

Laser beam

R. Bachelot, C. Ecoffet, D. Deloeil, P. Royer, D. J. Lougnot, Appl. Optics, 32, 5860 (2001). Integration of micrometer-sized polymer elements at the end of optical fibers by free-radical

photopolymerization

Polymerized waveguide Polymerized waveguides

Optical monomode fiber

Resin

Resin

Optical fiber

S. Kewitsch, A. Yariv, Opt. Lett., 21, 24 (1996). Self-focusing and self-trapping of optical beams upon photopolymerization

Simulation Experiment

Fiber Fiber

0.8 mm

HeNe laser beam

TPA polymerization: fibres connection

Guide created by TPA polymerization

Cell

Fiber

f core = 3 μm

f cladding 125 μm

Fiber

Fibres connection trough a curved guide

1.3

mm

0.15 mm

1.5 mm

Passive Mach-Zehnder

Appl. Phys. Lett.,

86, 211118 (2005)

EO polymer

optical waveguide

Traveling-wave

electrodes

Substrate

Active E.O. modulator

Electrodes

Light-Induced Self-Written (LISW) monomode waveguides

Monomode optical fiber Cladding: 125mm, Core 3 mm

P = 5 mW

Single waveguide

l = 514 nm

resin

Laser beam Photopolymerizable resin

Chaotic behaviour

P = 16 mW resin

Propagation instability

Light induced self written waveguides (LISW)

Time [mn] 0 5 10 15 20 25 30 35 40

1.48

1.485

1.49

1.495

1.5

1.505

1.51

Ref

ract

ive

ind

ex

White lamp

l=514 nm

chaotic zone

two

modes

single mode

V < 2.4

cladding

core

222claddingcore nnaV

l

V = 5

V = 1.5

Quasi-solitonic behavior

Simulations Experiments

V = 20 cladding

core

Optics Letters, 27, 20 (2002)

Appl. Phys. Lett., 83, 2474 (2003)

Phys. Rev. Lett., 93, 143905(2004)

Simulation of LISW propagation Simulation of LISW index profile

1P process

2P process

2P, 1 µm

2P, 2 µm

APL, 100, 221102 (2012)

2P LISW

Images of a 2P LISW waveguide

2P Fluorescence microscopy

Differential interferometric

contrast microscopy

APL, 100, 221102 (2012)

Exit mode from 2P LISW

(horizontal) (vertical)

2P LISW

Multimodes guides

Injection with mode mixing by

mechanical vibrations on the input fiber Usual injection

Excitation l = 514 nm

Multimode waveguides

Optical multimode fiber

Cladding: 250 mm

Core 62.5 mm

Self-written waveguide created using a white light from a 500W halogen lamp

(length ≈ 2 cm)

white light

Resin:

PETA /Amine/Eosine

Flow

direction

Photopolymerizable

mixture

Multimode

optical

waveguide

LISW

AIR

Building of the LISW waveguided “on the flow”

Self-writing “on the flow”

Self-writing “on the flow”

SPIE, San Diego 2010

Self-writing “on the flow”

Connecting fibers

Waveguides grown from both sides (S.N. Mensov, Yu Plushaytsev, Laser Physics, 18, 424, 2008)

Critical alignment using V-groove

AIR

AIR

Multimode fiber connections

0 50 100 150 200 250 300 350 400

-30

-25

-20

-15

-10

-5

0

Lo

sses (

dB

)

Distance (meters)

1.9 dB Applied Optics, 49, 2095, (2010)

Guides with defined shapes

Use of

- reflection (optical element inside the material or edge) and

- refraction (pre-existing refractive index gradient)

Total internal reflection (Polymer cut after pre-polymerization)

Curved guides by pre-control of the refractive index

LISW functionalized with dye

for laser operation

K. Yamashita et al.,

J. of Lightwave Techn. , 27, 4570, (2009)

O. Sugihara et al., Opt. Lett., 33, 294, (2008)

LISW functionalized with photochromic molecule

Functionalized LISW

Cell for EO modulation based on LISW doped with 5CB

LISW

ITO glass plates

Electrical

wires

Monomode

polarization

maintaining

optical fiber

Spacers

Photopolymer + 5CB

Silver

paste

Exit window

0.0

0.5

1.0

0

90

180

270

0.0

0.5

1.0

Polarization of

exiting mode

fiber in fiber out

= 0

= 0 + D

V 0

V = 0

Principle Experimental

TEM00 exit mode

Oscilloscope

High voltage

amplifier

Signal

generator

Photodiode

Cell

1

Cell

2

Ar+

laser

Polarization maintaining fibers

Experimental

Phase modulation

n5CB ≈ nfinal

Segregation of 5CB

Mode broadening

Lost of light confinement

Drawbacks with 5CB as doping chromophore:

Next step: LISW functionalized with efficient NLO chromophores for E.O modulation (Mach-Zehnder)

Optics Express, 21, 1541 (2013)

Remarks on optical integrated circuits created by

photopolymerization

Work in progress:

- Use of efficient NLO chromophores

- Thin film devices

• Relative ease of fabrication

• Various available functionalized photopolymers

• Control of LISW construction

• Demonstration of EO modulation in LISW (phase modulation)

• Interconnection (for fully integrated device)

Thanks to

K. D. Dorkenoo (Data storage, multiphoton microscopy, fs chirality)

L. Mager (Light induced self-written waveguides, optical circuits)

A. Barsella (Organic lasers, NLO organic structures)

B. Boeglin (Semi empirical calculations on molecular compounds)

G. Taupier (Two photon absorption experiments, optical activity)

IPCMS permanent members of the team

“Nonlinear Microscopy and Patterning of Organic Materials”

Thanks to

I like wrong calculations because they give more accurate results

(Hans-Jean ARP)

Hans-Jean ARP

Painter, sculptor, poet

Strasbourg 1886 – Bâle 1966

Strasbourg, Modern Museum of Art

As a conclusion… I invite you to meditate this provocative remark…..