Second-order polarization techniques for the characterization of thin films

Martti KauranenInstitute of Physics

Tampere University of TechnologyFinland

COST MP0604, Ancona, October 26, 2007 2

Where is Tampere?

TAMPERE

COST MP0604, Ancona, October 26, 2007 3

Second-order nonlinear optics

• Second-harmonic response

• Spatial symmetry– symmetry group of material

• Centrosymmetry– second-order processes electric-

dipole-forbidden– probes of surfaces and thin films

( ) ( ) i tE t E e (2)

,(2 ) ( ) ( )i j kijk

j kP E E

)2()2()2( 0 lmnijkijk C

surfacenormal

surfacenormal

mirrorplane

z

2

2

thin film

substrate

COST MP0604, Ancona, October 26, 2007 4

Multipole interactions

• Hamiltonian

• Second-order

• Magnetic and quadrupole tensors– symmetry properties are different from those of the electric-

dipole tensor

EQBmE :H

EEχM :2mee

EEχBEχEEχP eeQeemeee ::2

EEχQ :2Qee

2

e

e

e

2

e

e

m

(m)

2

m

e

e(e)

eee eem mee

weak

axial

4th rankaxial

4th rank

electric-dipole-forbidden effects can occur

COST MP0604, Ancona, October 26, 2007 5

Chirality and optical activity

• Chiral molecules– two mirror-image forms (enantiomers)– noncentrosymmetric– biological material– pharmacological molecules

• Optical activity– optical effects due to chirality– optical rotation– circular dichroism– interference between electric

and magnetic contributions– reverse sign between

the enantiomers

helicalpath

ee

kE

chiralmedium

COST MP0604, Ancona, October 26, 2007 6

Chirality and nonlinear optics

• Second-order materials– low molecular symmetry

– electric-dipole response in isotropic materials– magnetic response in centrosymmetric materials

• New probes of chirality– optical activity in the nonlinear response– sensitivity to surfaces and thin films– small amount of material– electric-dipole-allowed probes– relative magnitude ~1

high macroscopic symmetry

(a) (b)

s s

sp

pp

2

2

<<

waveplate

thin film

substrate

COST MP0604, Ancona, October 26, 2007 7

Interesting quantities

• Susceptibility tensor– symmetry group– structural features– strength of nonlinear

response

• Molecular ordering– degree of orientation

cossin221)2(

ZZZzxx

3)2( cosZZZzzz

ijk for C2 C2 symmetry featureszzzzxxzyy

xxz=xzxyyz=yzyxyz=xzyyxz=yzxzxy=zyx

-----

chiralitychirality

chirality and anisotropy

COST MP0604, Ancona, October 26, 2007 8

Thin-film characterization

• Fundamental field

• Second-harmonic field

• In-plane isotropy

ps AA psE ˆˆ)(

spsp AhAgAfAE 22)2(

1 1 ...ff a 1 1 ...gg a 1 1 ...hh a

achiral

chiralpf sh

sf sg no EDpg

ph

s s

sp

pp

2

2

<<

waveplate

thin film

substrate

COST MP0604, Ancona, October 26, 2007 9

Chiral film with in-plane isotropy

• Polyisocyanide

N N NCO

OCH2

CH3

CHN *Cn

NO2

OCOCH2CH2 CH3

COCH2CH2 CH3

O

N N NCO

OCH2

CH3

CHN *Cn

NO2

OCOCH2CH2 CH3

COCH2CH2 CH3

O

rotation angle-180 -90 0 90 180

SH in

tens

ity

0.0

0.5

1.0

rotation angle-180 -90 0 90 180

SH in

tens

ity

0.0

0.5

1.0

rotation angle of quarter waveplate (degrees)

SH in

tens

ity

COST MP0604, Ancona, October 26, 2007 10

Magnetic contributions?

• Polyisocyanide– evidence through complete tensor analysis

• Polythiophene – direct evidence

SCH3

O

( )n SCH3

O

( )n

0)(~ meezxx

eemxxzsg

rotation angle of WP (degrees)

SH in

tens

ity

rotation angle-180 -90 0 90 180

SH in

tens

ity0.0

0.5

1.0 0g0g

COST MP0604, Ancona, October 26, 2007 11

Anisotropic achiral film

• 2-docosylamine-5-nitropyridine (DCANP)

O2NC22H45

N NH

substrate

dipping

O2NC22H45

N NH

substrate

dipping

rotation angle-180 -90 0 90 180

SH in

tens

ity

0.0

0.5

1.0

rotation angle-180 -90 0 90 180

SH in

tens

ity

0.0

0.5

1.0

SH in

tens

ity

rotation angle of quarter waveplate (degrees)

COST MP0604, Ancona, October 26, 2007 12

Chiral and anisotropic film

• Thiohelicene– chiral

• Langmuir-Blodgett film– columnar aggregates– in-plane anisotropy

• Symmetry group– in-plane anisotropy– chiral– eight susceptibility components– arbitrary in-plane axes

OC12H25OC12H25

OC12H25

O

O

SC12H25O

O

O

Ssurfacenormal

z x

y

zy x

x'

y'2C

OC12H25OC12H25

OC12H25

O

O

SC12H25O

O

O

S

COST MP0604, Ancona, October 26, 2007 13

QWP

2

X

z

Y

x

s

p

s

p

Measurements

• Independent measurements– vary azimuth – detect s- and p-polarized

signals

• Recovery of susceptibility tensor– regression based data analysis– comparison of theoretical models– statistical indicators

f, g, h for each signal222)2( spsp AhAgAfAI

1 1 ...ff a

1 1 ...gg a

1 1 ...hh a

COST MP0604, Ancona, October 26, 2007 14

Experimental results

tensor components residuals

symmetry groupis actually 2D

COST MP0604, Ancona, October 26, 2007 15

Absolute probes of chirality?

• Normal incidence required– no difference between s and p

• Circular polarizations required– insensitivity to anisotropy

• Problem– even tight focusing does not

produce strong longitudinal polarization components

XY

XY

E

Epoor coupling with surface nonlinearity

COST MP0604, Ancona, October 26, 2007 16

Radial and azimuthal polarization

• Cylindrical symmetry– insensitive to anisotropy

• Focusing– longitudinal components in focus– coupling to surface nonlinearity

• Beams with handedness– sensitive to chirality?

• Combine different types of beams– radial and circular polarization

XY

E

E

E

E

EB

XY

E

E

E

E

EB

XY

E

E

E

E

EB

COST MP0604, Ancona, October 26, 2007 17

NLO microscopy of nanodots*

• Cylindrical gold nanodots**– SHG forbidden for ideal samples

at normal incidence– THG allowed

SHG THG

bright particles depend on polarization *Jeff Squier, CSM

**Tapio Niemi, ORC, TUT