LINEAR OPTICAL EFFECTS IN NANOSTRUCTURED SOLIDS

Oulu University August 28, 2008

M.V. Lomonosov Moscow State University, Physics Department

Russian Research Center “Kurchatov Institute”

LINEAR OPTICAL EFFECTS IN LINEAR OPTICAL EFFECTS IN

NANOSTRUCTURED SOLIDSNANOSTRUCTURED SOLIDS

Pavel Kashkarov

Lecture 1


I. Introduction

II. Photonic crystals and anisotropic

layers based on porous silicon

III. Silicon nanocrystals in dielectric matrix

for optoelectronic applications

IV. Silicon nanocrystals for biomedical

applications

V. Conclusions

Outline


1. The idea that nanostructuring of homogenous and isotropic media results in completely new optical properties was suggested many years ago (see excellent textbook “Principles of Optics” by M. Born and E. Wolf). But at that time there was not appropriate technology to fabricate nanostructured materials.

2. There were suggested two types of structures with form anisotropy: parallel layers and periodically arranged nanocylinders. Both structures show a strong birefringence.

Photonic crystals and anisotropic layers based on porous silicon


Form Birefringence

Lord Rayleigh


3. Only recently an electrochemical technique was applied to fabricate such structures on semiconductor substrate (mainly silicon).

4. Another type of an artificial medium is a photonic crystal. It turns out that it also can be easily made by the electrochemical treatment of semiconductors.

5. The obvious advantage of porous nanostructured media is the possibility to change the optical properties of the sample by filling pores by different gaseous, liquid and solid substances.

...... Photonic crystals and anisotropic layers based on porous silicon


Electrochemical nanostucturing of Electrochemical nanostucturing of semiconductorssemiconductors

cross-section ofw ires is 1-5 nm

th ickness of porouslayer is 1-100 m

quantum w ires

metal plate

HFsolution

c-Si


Different morphologies of porous siliconDifferent morphologies of porous silicon

Preferential pore growth in <100> crystallographyc directionmicroporous

(100) surface, =10 cm

mesoporous

(110) surface, =3 mcm


Tailoring of the Refractive Index by Tailoring of the Refractive Index by NanostucturingNanostucturing

Effective media approximation(Bruggeman model )

eff of the disordered heterogeneous mixture:

0)1(22

eff

Si

eff

Sieff

d

eff

d PP

where P is porosity

refractive index: eff

n

0 50 100 150 200 250 300

40

50

60

70

80

p-Si (75 mcm)

Po

rosi

ty (

%)

Current Density (mA/cm2)

2.6

2.4

2.2

2.0

1.8

1.6

1.4

Re

fra

ctiv

e in

de

x


10000 15000 20000 25000 30000 350000.0

0.2

0.4

0.6

0.8

1.0

Ref

lect

ivity

Wavenumber (cm-1)

1200 900 600 300

photonic

bandgap

Wavelength (nm)

Ideal Bragg reflector

12 pairs n1=1.3, n2=2

Porous Silicon Based Photonic CrystalsPorous Silicon Based Photonic Crystals

c-Si

n1d1+n2d2=/2

0 10 20 30 40 50 60 70 80 90 100

0

20

40

60

80

100

Cu

rre

nt d

en

sity

(m

A/c

m2 )

time (sec)

Bragg condition:


n1

n2

1D 1D PPhotonic hotonic CCrystal rystal BBased oased onn PPorous orous SSiliconilicon

d2

d1


Linear Linear OOptical ptical PProperties of roperties of PPhotonic hotonic CCrystalrystalss BBased oased onn PPorous orous SSiliconilicon

0,0

0,5

1,00,0

0,5

1,0

8 11 14 17 20 23 260,0

0,5

1,0

(B)

Ref

lect

ivity

(A)

(C)

Wavenumber ( 103 cm-1)

1,2 1 0,8 0,6 0,4

Wavelength (m)


Strong Strong OOptical ptical AAnisotropy in (110) nisotropy in (110) PPorous orous SSiliconilicon

n 0 0 1 < n 1 - 1 0

( 1 1 0 )

[ 0 1 0 ] [ 1 0 0 ]

n 0 0 1 > n 1 - 1 0

[ 1 - 1 0 ]

[ 0 0 1 ]

e

S i

d

0)()(

)1(,2

,2

,1

,1

iieffieff

iieff

iieffieff

iieff

Lp

Lp

Generalized

Bruggeman model


Effect of Effect of PorosityPorosity on on BBirefringence of irefringence of NNanostructured anostructured SSilicon ilicon FFilmsilms

Nanostructured Si films have large birefringence value δn =18%

medium Δn

Crystalline Si 5 10-6

Iceland spar 0.15Por-Si (110) 0.24

For layers prepared at j =100 mA/cm2 :

Δn=0.24<n> = (no+ne)/2 = 1.3

δn=Δn/<n> = 0.181.2

1.4

1.6

1.8

2.0

25 50 75 100

0.0

0.1

0.2

0.3

0.4

ne

no

Ref

ract

ive

inde

x

porosity

80%

60%

65%

Current density, mA/cm2

n o-n e


Polarization Polarization TTunable unable PPhotonic hotonic CCrystalsrystals

1000 1200 1400 16000.0

0.2

0.4

0.6

0.8

1.0

12 pairs

R

efle

ctan

ce

Wavelength (nm)

Reflection spectraReflection spectra


Oxidized Porous SiliconOxidized Porous Silicon

• Thermal oxidation of birefringent por-Si film results in formation of chemically stable, transparent for visible radiation medium, which possesses optical anisotropy.

Por-Si

X-ray diffraction

c-Si Oxydized por-Si


950 oC 2.5 h

Transparent birefringent film

Oxidized por-Si film on a mirror under a polarizer.

10 20 30 40 50 60 70 80 900.005

0.010

0.015

0.020

0.025

|no -

n e|

Current density (mА/сm2)

In oxidized por-Si birefringence is 2 times higher than in quartz

Birefringence of Oxidized Porous SiliconBirefringence of Oxidized Porous Silicon


1. Silicon was, is and will be the main material for microelectronics, but application of silicon in optoelectronics is limited.

2. Low probability of radiative electron transition in Si can be increased by formation of nanoparticles or/and by an introduction of activators of luminescence.

3. A Er3+ ion possesses rather promising properties as a luminescence activator in Si. The Er3+ luminescence line (λ=1.54µm) corresponds to a maximum of transmission in the quartz fiber waveguides.

4. For formation of light emitting device we combined both approaches

Silicon nanocrystals in dielectric matrix for optoelectronic applications


Light absorption in semiconductors (interband transitions)

1. Direct gap materials (GaAs, InP, CdTe, CdS )

*h

v*e

gc m

pp

m

pEp

2)(,

2)(

22

f

i0 –

0

Eg

Е

h Eg

p

<111> <000> <100>

LGaAs

Eg=1.52 эВ

Е

p

hvc

vc

h

vch

vc

pp

cmaahp

cmhp

ppp

EEh

,

700,

34

)10(/

)1010(/

0


Light absorption in semiconductors (interband transitions)

2. Indirect gap materials (Ge, Si, GaP )

phonvch

vc

pppp

EEh

3. Heisenberg uncertainty principle

,2/

2/

dp

xp

where d is a nanoparticle size


Quantum size effect

EEE

mmd

nhEEE

md

nh

m

pEdhnp

ndnph

gg

hehe

ee

eee

eee

0

**2

22

*2

22

*

2

11

4

42;2/

,...2,1,2/;/


Quantum size effect

vcEEh

2D structure

Quantum size effect enables one to modify absorption and luminescence spectra of semiconductor nanoparticles

The effect becomes appreciable when the size of a particle is less then 10 nm


Size-controlled Si nanocrystals in nc-Si/SiOSize-controlled Si nanocrystals in nc-Si/SiO22

SuperlatticesSuperlattices

Preparation Details:

1. Alternating evaporation of SiO powder in vacuum 10-7 mbar or in oxygen atmosphere under oxygen partial pressure of 10-4 mbar. This changes the stoichiometry x of SiOx alternatively between 1 and 2.

2. SiO/SiO2 superlatticies are characterized by the thickness of the SiO layers varied between 1 and 3 nm and the thickness of SiO2 layers between 2 and 3 nm. The number of periods varied between 30 and 90.

3. The evaporated samples were annealed at 1100 oC under N2 atmosphere. Thus nc-Si/ SiO2 superlattices were obtained. were obtained.

4. Er doped nc-Si/SiO2 superlattice were produced by implantation with Er ions (energy 300 keV, doses 1014 – 5·1016 cm-2) followed by TA at 900 oC for 5-60 minutes.

3nm

M. Zacharias et al., APL 80, 661 (2002).

Er


Size Dependent PL of nc-Si/SiOSize Dependent PL of nc-Si/SiO22

Structures Structures

Size of Si nanocrystals is controlled by initial thickness of SiO layer in SiO/SiO2 superlattice

The peak position of PL spectrum is determined by quantum confinement and excitonic effects in Si nanocrystals in SiO2 matrix.

1.0 1.2 1.4 1.6 1.8 2.0

No

rma

lize

d P

L I

nte

ns

ity

Photon Energy (eV)

dSiO

= 6 ... 2 nm


Effect of Er doping on PL of nc-Si/SiOEffect of Er doping on PL of nc-Si/SiO22

Er doping results in both two order of magnitude quenching of the exciton PL and a strong emission line at 1.5 m

Er-doped a-SiO2 layers does not exhibit efficient Er photoluminescence

Efficient energy transfer from electronic excitation of Si nanocrystals to Er ions

0.8 1.0 1.2 1.4 1.6 1.8 2.0

10-3

10-2

10-1

100

4F9/2

-4I15/2

4I9/2

-4I15/2

4I11/2

-4I15/2

4I13/2

-4I15/2

hexc

= 3.7 eVT= 300 K

nc-Si/SiO2

dSi = 3 nm

nc-Si/SiO2

nc-Si/SiO2:Er

a-SiO2:Er

PL

Inte

nsity

(ar

b. u

n.)

Photon Energy (eV)

Er3+

1.5 m(0.8 eV)

P.K.Kashkarov et al., JETP 64,1123 (2003)


Transient PL Investigation of ErTransient PL Investigation of Er3+3+ Population Population InversionInversion

• Lifetime of Er-related PL becomes shorter at Iexc > 0.1 W/cm2 that correlates with the population inversion of Er3+

0.01 0.1 1

1

2

3

4

5

nc-Si/SiO2:Er

d=4 nm

decay

rise

Tim

e (

ms)

Pump power (W/cm2)0.01 0.1 1

0.1

1

N1/N

Er= 1 -

rise/

decay

N1/N

Er

Pump power (W/cm2)

0.5

N1

N0


Silicon nanocrystals for biomedical Silicon nanocrystals for biomedical applicationsapplications

1. Oxygen molecules exist in several forms: non-active one (ground state) and active ones (excited states)

2. In the active form oxygen is toxic and therefore fatal for any live cells

3. This property of active oxygen is used to clean water containing harmful microbes and is the base of our immunity

4. In some cases generation of active oxygen can be applied for cancer therapy

5. But how an oxygen molecule can be transferred into active form?


1

O O

1.63 eV

0.98 eV

3

1

Optical Excitation is inefficient Photosensitizer is required

Excited States:

• Spin Singlet• Energy-rich• High chemical Reactivity: S(O2) + S(Mol.) S(Mol.)

Ground State:

• Spin Triplet• Paramagnetic• Chemically inert: T(O2) + S(Mol.) S(Mol.)

= 7 s

= 50 min

Electronic Structure of Molecular OxygenElectronic Structure of Molecular Oxygen


Photosensitization: Basic PrinciplePhotosensitization: Basic Principle

Efficient Energy Transfer requires: Small Spatial Separation between Donor and Acceptor

• Spectral Overlap of Donor and Acceptor Energy Bands• High Quantum Efficiency of Donor Luminescence

Donor Acceptor

S0

S1

S

T

T0

Energy Transfer

h

R


Si Nanocrystal Assemblies as Photosensitizers?Si Nanocrystal Assemblies as Photosensitizers?

1.2 1.4 1.6 1.8 2.0 2.2 2.4

0.0

0.5

1.0 FE(Si)

PL

In

ten

sit

y (

arb

.un

its

)

Energy (eV)

A.G.Cullis and L.T.Canham, Nature 353, 335 (1991)

1.2 1.4 1.6 1.8 2.0 2.2 2.4

101

102

103

S

T

S

Energy (eV)

Ex

cit

on

Lif

eti

me

(µ

s)

•Simple Electrochemical Preparation •Open Nanostructure•Efficient Photolumenescence

• Broad Tunable Emission Band • Long Exciton Lifetime (µs - ms)


0.8 1.2 1.6 2.0

10 -4

10 -3

10 -2

10 -1

100 T=5 K Vacuum Physisorbed O

2

1 3

1

Energy (eV)

PL

In

ten

sit

y (

arb

. u

nit

s)

Effect of Physisorbed Oxygen Molecules on Effect of Physisorbed Oxygen Molecules on PL of Si Nanocrystal AssemblyPL of Si Nanocrystal Assembly

Evidence for Energy Transfer from Excitons to O2

• PL Quenching of Excitonic and Defect Emission Band

• 1 3 Emission Line of 1O2


Singlet Oxygen Photosensitization in WaterSinglet Oxygen Photosensitization in Water

1 mg of nano-Si (from micropor-Si) dispersed in 3 ml of

H2O

%40E

for SO ~ 1-3 s: NSO ~ 1015 – 1016 (1/cm3)

7.1)()( oxygen

PLvac

PL dttIdttI


Effect of Porous Si on cancer cellsEffect of Porous Si on cancer cells(in vitro experiments with mouse fibroblasts)(in vitro experiments with mouse fibroblasts)

Cancer cell number vs porous Si concentration in the dark (blue symbols) and after illumination (red symbols)

Cells were counted by using optical density measurements


DNA AnalysisDNA Analysis

Histogram of DNA content for the cancer cells kept in the nutrient solution with dispersed Histogram of DNA content for the cancer cells kept in the nutrient solution with dispersed porous Si (1.5 g/l) in darkness (blue curve) and after illumination (red curve). Symbols G1, S, porous Si (1.5 g/l) in darkness (blue curve) and after illumination (red curve). Symbols G1, S, and G2 mark different cycles of the cell proliferation. The apoptotic cell region is marked by and G2 mark different cycles of the cell proliferation. The apoptotic cell region is marked by A. Inset shows the relative contribution of G1, S and G2 regions A. Inset shows the relative contribution of G1, S and G2 regions vsvs nc-Si concentration in the nc-Si concentration in the dark (blue symbols) and after illumination (red symbols)dark (blue symbols) and after illumination (red symbols)


Photodynamic cancer therapyPhotodynamic cancer therapy

nc-Si

tumour

Si Si

SiSi

Si

SiSi

Si

Si

Si

Si

SiSi

SiSi

nc-Si

nc-Si

h triplet 3О2

singlet 1О2


General Conclusions • Nanostructuring of homogenous and isotropic

Si-crystals enables one to form photonic media with unique properties

• Ensembles of Si-nanocrystals in a dielectric matrix are promising base for silicon laser compatible with microelectronic technology

• Bio-compatible Si-nanoparticles are effective photosensitizers of singlet oxygen generation what can be applied for photodynamic cancer therapy


Thank you for the attention!

LINEAR OPTICAL EFFECTS IN NANOSTRUCTURED SOLIDS

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