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Nonlinear Optical Materials

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NLO materials for light conversion
18
Nonlinear optics Prof. V. Krishnakumar Professor and Head Department of Physics Periyar University Salem – 636 011, India
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Page 1: Nonlinear Optical Materials

Nonlinear optics

Prof. V. KrishnakumarProfessor and Head

Department of PhysicsPeriyar University

Salem – 636 011, India

Page 2: Nonlinear Optical Materials

TOPICS • Linear optics vs. Non-linear optics• Importance of Non-linear optics• Linear & Non-linear polarization.• Phenomenon associated with NLO• Materials applied in NLO

• Applications• Future

Page 3: Nonlinear Optical Materials

Linear Optics vs Non Linear Optics

• Linear optics- ‘Optics of weak light’:

Light is deflected or delayed but its frequency is unchanged.

• Non-Linear optics-‘Optics of intense light’:

We are concerned with the effects that light itself induces as it propagates through the medium.

Page 4: Nonlinear Optical Materials

Non-Linear optics produces many exotic events

•Nonlinear optics allows us to change the color of a light beam, to change its shape in space and time, to switch telecommunica-tions systems, and to create the shortest events ever made by Man

Ex: Sending infrared light into a crystal yielded this display of green light

Page 5: Nonlinear Optical Materials

Introduction

• What does the index of refraction mean?

• Linear Region : Efield << Intra-Atomic field. “n” is independent from the light intensity, “I”.

• Nonlinear Region: Efield ~ Intra-Atomic field. Modified electron distribution, “n” depends on “I”.

Page 6: Nonlinear Optical Materials

In Non-Linear Optics

If irradiance is high enough vibrations at all frequencies corresponding to all energy differences between populated states are produced.

Page 7: Nonlinear Optical Materials

Introduction

• Nonlinear Optics: Study of interaction of light in matter

• We can control “n” by the light itself or manipulate one beam with the other.

• Leads to a Great variety of technical innovations.

1961, Peter Franken, Ruby Laser

Page 8: Nonlinear Optical Materials

Importance of ‘NLO’• Optical wave manipulation is one of the future

technologies for optical processing.• It has various applications in fiber-optic

communications and optoelectronics which makes it an increasingly important topic among electrical engineers.

Page 9: Nonlinear Optical Materials

Nonlinear polarization• Linear medium: low field intensity

• Nonlinear medium: high field intensity

PED += 0ε EED r 0εεε =⋅=

Linear polarization

PED += 0ε

Nonlinear polarization

EP ⋅= χε0

NLL PPEEEP +=+⋅+⋅+⋅= ...3)3(2)2(0 χχχε

NLLlkjijklkjijkjiji PPEEEEEDEP +=++⋅+⋅= ...420 χχε

Linear susceptibility

tensor

2nd order nonlinear

susceptibility tensor

3rd order nonlinear

susceptibility tensor

Summation over repeated indices

χε += 1r

i, j, k = x, y, z

Page 10: Nonlinear Optical Materials

Sum frequency generation (SFG)Difference frequency generation (DFG)

• 2nd order optical nonlinearity• Start with two beams ω = ω1

and ω = ω2– SFG: ω3 = ω1 + ω2 , k3 = k1 + k2– DFG: ω3 = ω1 - ω2 , k3 = k1 - k2

• SFG/DFG for photodetection– Use a 1060 nm laser to convert 10

μm mid-infrared radiation to 960 nm near-infrared radiation that can be handled by low-cost detectors

Sum frequency Pump laser

Laser emissionSHG

Image courtesy of Institut für Angewandte Physik

Nonlinear optics is a colorful discipline!

Page 11: Nonlinear Optical Materials

Introduction to nonlinear optics…Sum frequency generation

Example of second order nonlinear optical effects

1ω3ω2ω = ω1+ω2

SHG, THG and higher harmonic generation

Page 12: Nonlinear Optical Materials

Second harmonic generation (SHG): two photons of frequency ω yield one of frequency 2ω.

∑=γβ

γβαβγα ωωωωωχω,

)2(0 )()(),;2(ε)2( EEP

),;2()2( ωωωχαβγ : symmetric under interchange of β and γ.

A Chemist view of nonlinear optics

Chemist

Page 13: Nonlinear Optical Materials

Criteria: Absence of centrosymmetry for χ(2) materials; absence of absorptions at inconvenient frequency: P= εo{ χ(1).E +χ(2)E.E + χ(3)E.E.E+….. }

Page 14: Nonlinear Optical Materials

Light polarized normal to c-axis: high refractive index

⇒ can choose any angle θ , still same index

k

c-axis

Phase matching

Page 15: Nonlinear Optical Materials

Light polarized along c-axis: low refractive index

⇒ different index for different angles θ

k

c-axis

Page 16: Nonlinear Optical Materials

Suppose n2ω > nω

Field normal to c-axis

θ

k

c-axis

n2ω

θ

k

c-axis

n2ω θ

k

c-axis

Field partially parallel to c-axis

If 2ω light hascomponent // c-axis

⇒ phase matching possible

Page 17: Nonlinear Optical Materials

Phase matching condition

• Only when 2k1 = k2 will SHG be efficient– n(λ1) = n(λ2)

• General rule for parametric processes– SHG, SFG/DFG, THG, FWM– momentum conservation

2k1 = k2

2k1 ≠ k2

~ 100% SHG conversion efficiency is possible by optimizing phase matching!

Page 18: Nonlinear Optical Materials

Applications:

• Optical phase conjugation

• Optical parametric oscillators

• Optical computing

• Optical switching

• Optical data storage


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