Nonlinear Optics: An Overview

Dr. C. VijayanDept. of Physics

IIT Madras

www.physics.iitm.ac.in/~cvijayan

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The technology of generating and harnessing light

and other forms of energy whose quantum is the Photon

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From Stone Age to Photon AgeMechanical to Electrical to Electronic to Photonic

The emerging area of PhotonicsHarnessing energy and information carrying capacity of Photons

EM theory at the heartNonlinear Optics, Applied Optics, Optoelectronics,Electrooptics

The three `C’s of PhotonicsCommunication, Control and Computing

Nonlinearity is requird!

We are nonlinear!

Linear vs nonlinearStimulus vs response, Extreme stimulusLinear response theoryTransport properties, mass, charge, heat

Stress vs starin

Welcome to the nonlinear world!

Linear regime

NonLinear regime

Electric field

Polarization

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What exactly is NON LINEAR?Nonlinear dependence of polarization

on optical field

Pi=ε0 (χij(1) Ej + χijk

(2) EjEk + χijkl(3) Ej Ek El + …)

χ(n) : susceptibility of n th order

(n+1) th rank tensor with [3(n+1)] components

Linear optics : superposition

No energy transfer among waves

Nonlinearity Energy transfer; wave mixing

n th order, (n+1) wave mixing

Second Order Nonlinear Optical Processes• Second harmonic generation

• Linear electro-optic effect : Pockel’s effect

• Optical rectification

Third order Nonlinear Optical Effects

• Third harmonic generation

• Phase conjugation

• Stimulated Raman scattering

• Self focusing

• Bistability

• Quadratic electro-optic effect : Kerr Effect

Large and fast nonlinearity: A materials problem

Second order NLO effects

χ(2)

ω 2ω

SHG : Second harmonic generation

χ(2)ω1

Sum and difference frequency generation

ω2

ω1+ω2

ω1−ω2

• Optical parametric oscillation• Applications of wave mixing

today’s laser technology : Ti: Saphire

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Optical Phase Conjugation : idea

1 2

3

4

The phase of wave 4 is reversed w.r.t that of wave 3i.e. 4 is a time-reversed replica of 3

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Reflection of a distorted wave from an ordinary mirror

Plane wave distorter ord. mirror

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Reflection of a distorted wave from a PC mirror

Plane wave distorter PC mirror

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Optical Phase Conjugation: Distortion healing

e.g. fiber optic communications, adaptive optics

1 24

3

mediumOptical phase conjugation“Probe beam gets diffracted by gratings created by pump1 and pump2”

1 2

3 4

Experimental set-up

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Ordinary mirror vs. “OPC mirror”

Ordinary mirror

Phase conjugate mirror

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Optical Phase Conjugation:Applications

Apart from measurement of chi-3 to characterize the NL material, wave front reversal (time-reversal) and distortion-healing properties of OPC are used in Adaptive Optics applications such as

Precise targeting of a beam,

Precise beam control in lithography,

Aberration-correction in amplifiersin fiber-optic communications etc

Optical data and image processing

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OPC in lithography

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OPC in adaptive optics

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Distortion-correction in fiber optic communication

Self focusing

Gaussian laser beam

Nonlinear medium

Applications :•Beam shaping•Pulse shaping – soliton propagation•Optical limiting•Measurement of n2

n(I) = n0 + n2I

IIIIIIII((((((

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Closed z scan

aperture

laser detector

Transmitted intensity against distance

Self-focusing defocusing

Z scan traces of ZnTPP

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Low power optical limiting“Smart materials”

KSB, PH, CV : Materials Letters, Volume 58(7-8), 1223-1226 (2004)

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Pulse shaping, solitonsA pulse can have a frequency that varies in time.

This pulse increases its frequency linearly in time; (from red to blue).In analogy to bird sounds, "chirped" pulse.

Group velocity dispersion and self focusing effects can be combined in certain nonlinear fibers so as to achieve soliton propagation where signals propagate without distortion for a large distance.

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Mechanisms of optical nonlinearity

Electronic, ionic, molecular :Free carriers, bound electrons, excitons.

Orientational, saturable absorption’

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Conventional Materials•Inorganic salts

KDP, ADP, BaTiO3, BBO

•Semiconductors GaAs, Si, Ge, InSb

•Organic MaterialsPolymers, dyes, urea

Required:Large and Fast nonlinearityNatural materials do not satisfy this twin requirement

Novel Artificial Materials Molecular engineering, band gap engineeringQD, CNT, PBG

Porphyrin : the Wonder Molecule

•Chemistry, Structure, •Molecular Engineering, •Charge transfer, •Biochemistry;Photosynthesis,secrets of life?

“Why Blood is red and Grass is green“

Nonlinear Optical Effects in engineered porphyrins

Nonlinear refractive index, self diffraction, self-focusing, optical bistability, limiting;

The splendor and glory ofCarbon the Great!

Diamonds, Organics, Fullerenes, Nanotubes,

What next?Your research results !

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Physics of nanoclusters

Plenty of room at the Bottom –R. P. Feynman

• Low-Dimensional Structures/Nanoclusters

• Quantum Mechanics in daily life -Particle in a boxCD players, satellite receivers

• Free electron-bound electrons-

Jailed electrons

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“‘Quantum Mechanics’ is the description of the behavior of matter and light in all its details and, in particular, of the happenings on an atomic scale. Things on a very small scale behave like nothing that you have any direct experience about. They do not behave like waves, they do not behave like particles, they do not behave like clouds, or billiard balls, or weights on springs, or like anything that you have ever seen.”

--Richard P. Feynman

The concept of quantum confinement

PbS quantum Dot

KSB,PH,CV : Talanta, 66 (1), 160 (2005)

Quantum Confinement Effects

Remarkable alteration of optical and electrical properties

The Exciton : Quasiparticle in a crystal lattice

•Bandgap breaks up into discretelevels

•Band-edge depends on dot size

•Modifiable absorption/emission – white light

•Sharp exciton peaks at RT : Signature blueshift in absorption and emission

•Photonic switching, limiting, smart materials

Exciton Energy levels near the edges of bands

0

0.5

1

1.5

2

2.5

3

3.5

250 350 450 550

Wavelength (nm)

O.D

.

CB

Bulk cut-off

Exciton peak

VB

PN, CV, YVGSM :Journal of Applied Physics, 91(3), 1509 (2002)

Bandgap engineered all the way from UV to IROptical absorption spectrum of nano PbS

From 3000 nm to 300 nm

Samples ‘a’ to ‘e’ : Size 14 nm to 4 nm

KSB and CV : Invited Chapter inAdvances in Nanoscience and Nanotechnology, (book)Publ. NISCAIR, CSIR, 2004

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Multiphton absorptione.g. in nano PbS with ultrashort pulses

Linear optics

Nonlinear optics

KSB, CV, PH, JT, RP : Accepted, SPIE Int. conf, Santiago,USA, 2005

When excited with high intensity light,more than one photon can be simultaneously excited

Circles denote experimental data;Red line : fit to two photon absorptionBlue line: fit to three photon absorption

Contrast enhancement and error-correctionby nonlinear optical image processing

Before processing After processing

KS, CV, MPK Optics and Lasers in Engineering, 44 (2), 122-129 (2006)