non linear optics
-
Upload
sandeep-k-nair -
Category
Education
-
view
1.638 -
download
1
Transcript of non linear optics
Linear Optics vs Non Linear Optics
Linear optics- ‘Optics of weak light’: Light is deflected or delayed but its frequency is
unchanged. Superposition principle holds
Non-Linear optics-‘Optics of intense light’: We are concerned with the effects that light itself induces
as it propagates through the medium.
Superposition principle not valid
In Linear optics
A light wave acts on a molecule, which vibrates and then emits its own light wave that interferes with the original light wave.
In Non-Linear Optics
If irradiance is high enough vibrations at all frequencies corresponding to all energy differences between populated states are produced.
OPTICS – A LIGHT MATTER INTERACTION
Nonlinear optics (NLO) is the study of interaction of intense laser light with matter.
output
NLO
samp
le
input
νh
E1
E2
Absorption
E1
E2νh
Spontaneous Emission
Light Matter interaction
E1
E2νhνh νh
Stimulated Emission
Stimulated Emission
m
Properties of Laser Beam
A laser beam •Is intense•Is Coherent•Has a very low divergence
Optical power density
Focused laser beam E ~ 1010 V/m
How does optical nonlinearity arise
The strength of the electric
field of the light wave should be in the range of atomic fields N
a0
e
νh
20/ aeEat =
220 / mea =
V/m102 10×≈atE
Applied Electric field distorts the cloud and displace the electron
Compare this with mass on a spring Separation of charges gives rise to a dipole moment Dipole moment per unit volume is called the
polarisation
++=+ --
E
Optical Nonlinearity
F= - kx
When electromagnetic waves propagate in a material, the atoms and molecules oscillate at the frequencies of the electric field associated with waves. The field associated with these EM waves polarizes the molecules in the medium, displacing them from their equilibrium positions and induces a dipole moment, p, given by
p=qd q is the electric charge and d is the field induced displacement. The polarization P, i.e. the dipole moment per unit volume, resulting from this induced dipole is given by
P=Nqd N is the electron density in the medium.
The polarizing effect of the field on the molecular dipoles depends both on the properties of the medium and on the field strength E.
P= ϵ0 χ E
When the intensity of the incident light to a material system increases the response of medium is no longer linear
Input intensity
Output
P=Nqd
?
Like loaded spring
m
F= -kx
P= ϵ0 χ E
E = Eo cosωt
(1) (2) 2 (3) 30 ...P E E Eε χ χ χ = + + +
1.Permanent Polarization
2.First order Polarization
3.Second order Polarization
4.Third Order Polarization
Second Harmonic Generation
ωω
ω2)2(χ
Third Harmonic generation
ω )3(χω3
ω
E = E1 cosω1t +E2 cosω2t
OPTICAL MIXING
22211
)2()2( )coscos( tEtEP ωωχ +=
)cos(&)cos( 2121 ωωωω −+
1ω
2ω)2(χ
1ω
2ω213 ωωω +=
Sum Frequency Generation
1ω3ω2ω
1ω
2ω)2(χ
2ω
1ω213 ωωω −=
Difference Frequency GenerationDifference Frequency Generation
1ω3ω2ω
FOCUSING OF LIGHT BY LENS
focus
Refractive Index with Intensity 3 3
0 0LP E E=εχ +εχ
0
nε
=ε
0D E E P= ε = ε +
3 20 01 L ( )( ( ) E )E= ε +χ +ε χ
3 20 01 L( ) Eε = ε + χ + ε χ
31 2 2 1 21 1
1L / /
Ln ( ) ( E )χ= +χ ++χ
20 2 0n n E≈ +
Nonlinear Refractive Index
Innn 20 +=
where 20 |)(|2
ωπ
Ecn
I = )3(20
2
2
12 χπcn
n =
Self focusing and self defocusing
The laser beam has Gaussian intensity profile. It can induce a Gaussian refractive index profile
inside the NLO sample.
)3(χ
Nd:YAGLaser
DetectorD2
Computer
Lens Aperture
+Z
Sample Cell
-ZDetector
D1 Power Meter
Z-scan Set-upZ-scan Set-up
z.swf
Z- Scan Signature
0.4
0.6
0.8
1
1.2
-20 -10 0 10 20
Z mm
No
rma
lise
d T
ran
smitt
an
ce
0.2
0.4
0.6
0.8
1
1.2
-20 -10 0 10 20
Z (mm)
Nor
mal
ised
Tra
nsm
ittan
ce
Different Class of NLO Materials
Inorganic Lithium niobium oxide, Potassium titanile Phosphate Ammonium dihydrogen phosphate, Potassium dihydrogen phosphate, Barium Metaborate
Organics Materials
Low cost Ease of fabrication Integrating in to a single devicesEasy to do fine tuning of its NLO properties by
turning its Chemical structure Low dielectric constant Inherent synthetic flexibilityHigh optical damage threshold
Enhancement of NLO response Science 281 (1998) 1653
TNLO enhancement by
Increase conjugation length Create D-π-A- π -D structure A- π -D- π –A structure
•New Frequency Generations
•Different Wavelength lasers from the same Laser Source
•Optical limiting
Input fluence
Output fluence
Applications:
Waveguide Inscriptions Lab on a Chip
Optical Signal Processing
1. Switching and routing are carried out electronically and processing in the electronic domain
2. Optical transmission in optical domain 3. Detection and processing in the electronic domain
again 1 2 3 to a single Unit