243Nanomaterials for nonlinear optical (NLO) applications: a review

© 2%12 Adva]ced Study Ce]ter Co. Ltd.

Rev. Adv. Mater. Sci. 30 (2012) 243-253

Corresponding author: Dakshanamoorthy Arivuoli, e-mail: arivuoli@annauniv.edu

NANOMATERIALS FOR NONLINEAR OPTICAL (NLO)APPLICATIONS: A REVIEW

Sagadevan Suresh and Dakshanamoorthy Arivuoli

Crystal Growth Centre, Anna University, Chennai-600 025, India

Received: December 07, 2011

Abstract. Nonlinear optics is given increasing attention due to its wide application in the area oflaser technology, optical communication and data storage technology .The growth of activities onNLO properties of nanomaterials, proposed by the agitation of understanding new science andpotential hope for applications in daily life as optical devices, photonic circuits, and environmentalsensor as well as in medical diagnostics. Intense research has been fueled by the need forpractical optical device that can deal the deficiencies of conventional technologies. The leadingmaterials could have very high bulk second order NLO values well beyond those available today,which in turn would enable optical switches and modulators of smaller dimensions than what iscurrently available, while at the same time substantially reducing the cost of fabrication of electro-optic (EO) devices.This paper mainly focus on recent advances in second and third order NLOproperties of nanomaterials and understanding new science behind the extraordinary NLO valuesof nanomaterials. It also discusses about the development of nanomaterial based opticaltechnology.

1. INTRODUCTION

Nonlinear optics, which studies the interaction ofintense light field with matter, is a relatively newfield in physics with lots of fundamental scientificand technological potential applications [1-3]. Nonlinear optical (NLO) effects are analyzed byconsidering the response of the dielectric materialat the atomic level to the electric fields of an intenselight beam. The propagation of a wave through amaterial produces changes in the spatial andtemporal distribution of electrical charges as theelectrons and atoms interact with theelectromagnetic fields of the wave. The main effectof the forces exerted by the field on the chargedparticles is displacement of the valence electronsfrom their normal orbits. This perturbation createselectric dipoles whose macroscopic manifestationis the polarization. Thus nonlinear Optics (NLO) isthe study of interaction of intense electromagnetic

field with materials to produce modified fields thatare different from the input field in phase, frequencyor amplitude. Second harmonic generation (SHG)is a nonlinear optical process that results in theconversion of an input optical wave into an outputwave of twice the input frequency. The lightpropagated through a crystalline solid, which lacksa center of symmetry, generates light at secondand higher harmonics of the applied frequency. Suchfrequency doubling processes are commonly usedto 33 produce green light (532 nm) from, for example,a Nd: YAG (yttrium-aluminium-garnet) laseroperating at 1064 nm. This important nonlinearproperty of noncentrosymmetric crystals is calledsecond harmonic generation.

Following the rapid development ofnanotechnology in the past few decades, a largenumber of nanomaterials have been shown topossess remarkable NLO properties, whichmotivates the design and fabrication of nano and

244 S. Suresh and D. Arivuoli

nano-scale photonic and photoelectronic devices[4].The most products of nanotechnology asexamples are carbon-based nanomaterials: from 3Dcarbon black or nanoparticles [5,6] to 0D fullerenes[7,8] to 1D carbo] ]a]otubes (CNTs [9–12] a]dthen to 2D graphenes [13,14] discovered mostrecently. In addition to the outstanding mechanical,electrical and thermal properties [15-18], the uniqueNLO properties of CNTs have generated muchresearch interest from both experimental andtheoretical aspects [19–21]. The Z-sca] is a simplebut powerful technique to characterize the NLOproperties of materials, including nonlinearabsorption, scattering or refraction [22]. In thismethod a laser beam is focused by a convex lensto create an intensity-spatially-varied optical field.When an optical material is moved around the focalpoint along the z-axis, one can readily obtaininformation on the variation of transmission againstincident intensity, and hence the nonlinearparameters of interest. The present paper,summarize and evaluate the achievements in thedevelopment of second and third order NLOnanomaterials.

2. SURVEY OF NONLINEAR OPTICS

The development of nonlinear optical (NLO) materi-als has been driven by a multitude of important tech-nological applications that can be accomplished ifsuitable materials are available [23–26]. Future ge]-erations of optoelectronic devices for telecommuni-cations, information storage, optical switching, andsignal processing are predicted to a large degreeon the development of materials with exceptionalNLO responses. A large number of organic -conju-gated molecules have been investigated in the lastthirty years for suitability to function as componentsin hypothetical NLO materials [27-32]. Second-har-monic generation (SHG) was first observed in asingle crystal of quartz by Franken et al. [33]. Para-metric amplification was observed in lithium niobate(LiNbO3) by two-wave mixing in temperaturetunedsingle crystals [34]. Rentzepis and Pao [35] madethe first observation of SHG in an organic material,benzpyrene, in 1964. Heilmeir [36] examined hex-amethylenetetramine single crystal SHG in the sameyear [37]. Two other organic materials followed rap-idly: hippuric acid and benzil [38]. Benzil was thefirst material that proved relatively easy to grow intolarge single crystals. Over the last two decades thestudy of nonlinear optical process in organic andpolymer systems has enjoyed rapid and sustainedgrowth [23-38].

The rapid growth of the field is mainly due to thetechnological promise of these materials.Traditionally, the materials used to measure second-order NLO behavior were inorganic crystals, suchas lithium niobate (LiNbO3) and potassiumdihydrogen phosphate (KDP). The opticalnonlinearity in these materials is to a large extentcaused by the nuclear displacement in an appliedelectric field, and to a smaller extent by themovement of the electrons. This limits the bandwidthof the modulator. Organic materials have a numberof advantages over inorganic materials for NLOapplications [39-48]. At the molecular level, theyneed to be non-centrosymmetric. A large number oforganic -conjugated molecules have beeninvestigated [39-46] in the last twenty years. Theoutcome of the results has helped to establishcertain guidelines for molecular design to get goodsecond order NLO materials. However, roughly morethan 80% of all -conjugated organic molecules crys-tallize in centro-symmetric space groups, thereforeproducing materials with no second order bulk sus-ceptibility. To overcome this limitation, organic NLOmaterial doped or covalently attached in polymers,have been introduced by Dalton et al. [49]. A few ofthese chromophores have served as componentsof functioning polymer-based optoelectronic devices;the physical properties of all these prototype mate-rials possess one or more critical deficiencies thatrender commercialization of these systems imprac-tical [25-31]. The ability to integrate metalnanoparticles into biological systems has greatestimpact in biology and biomedicine [50-73]. Devel-opment of nanobased biosensors has increased tre-mendously over the past few years as demonstratedby the large number of scientific publications in thisarea. The emerging ability to control the patterns ofmatter on the nanometer length scale can be ex-pected to lead to entirely new types of biologicalsensors [74-84]. These new systems will be ca-pable of sensing at the single-molecule level in liv-ing cells, and capable of parallel integration for thedetection of multiple signals, enabling a diversity ofsimultaneous experiments, as well as bettercrosschecks and controls.

3. THEORY OF NONLINEAR OPTICS

When the electromagnetic field of a laser beam isacting on an atom or a molecule, it induces electricpolarization, which gives rise to many of the unusualand interesting properties that are optically nonlinear.In a dielectric material, the influence of an electricfield causes distortion in the spatial distribution

245Nanomaterials for nonlinear optical (NLO) applications: a review

between the electrons and the nucleus. These dis-tortions cause electric dipoles, which in-turn mani-fest as polarization. At very low fields, the inducedpolarization is directly proportional to the electricfield. However, at intense electric fields, polariza-tion becomes independent of the field and the sus-ceptibility becomes field dependent. The inducedpolarization is capable of multiplying the fundamen-tal frequency to second, third order and even higherharmonics. The irradiation from the oscillating di-poles differs in amplitude with respect to the inci-dent sinusoidal electric field. As a consequence,the distorted reradiated waves contain different fre-quencies from that of the incident wave. When theelectric field associated with the radiation is small,the induced polarization is given by

P E(1)

0, (1)

where P is the polarization vector, E is the electricfield vector, (1) is the linear susceptibility, and 0 isthe permittivity of free space. When the opticalelectric field strength is very high and comparableto the intra-atomic electric field, the inducedpolarization is given by

P E E E E E E(1) ( 2 ) (3 )

0. . . . . . ... , (2)

where (2), (3) .… are the ]o]li]ear susceptibilitiesof the medium.

The nonlinear susceptibilities have decreasingmagnitudes as their order increases at (1): (2): (3)

1:10-8: 10-16. The first order susceptibility which isthe linear term, (1), gives rise to refractive index,absorption, dispersion and birefringence of themedium. The second order, (2), gives rise to SecondHarmonic Generation (SHG), frequency mixing andparametric generation, while the third order nonlinearsusceptibility, (3),gives rise to third harmonicgeneration, stimulated Raman scattering, opticalbistability and conjugation. Supposing that we wantto study the interaction of two travelling waves

E z t E t k z1 1 1 1( , ) cos , (3)

E z t E t k z2 2 2 2( , ) cos . (4)

Considering the second order nonlinearity inpolarization alone

P E( 2 ) 2 , (5)

P E t k z E t

k z E E t k z t k z

( 2 ) 2 2 2 2

1 1 1 2 2

2 1 2 1 1 2 2

c o s c o s

2 c o s c o s .

( 6 )

It c a n b e fo u n d th a t th e p o la r iz a tio n c o n s is ts o fa n u m b e r o f c o m p o n e n ts w ith d iffe re n t fre q u e n c ie sv iz .,

P E t k z( 2 ) 2

1 1 1 1 1c o s 2 , ( 7 )

P E t k z( 2 ) 2

2 2 2 2 2c o s 2 , ( 8 )

P P E E

t k k z

( 2 )

1 1 2 2 1 2

1 2 1 2cos ,

(9)

P P E E

t k k z

( 2)

1 1 2 2 1 2

1 2 1 2cos ,

(10)

and a steady term

directP E E

( 2 )

2 2

1 2.

2

(11)

The different components of nonlinear polarizationgenerate electromagnetic waves having frequenciesdifferent from those of the incident waves. Fractionof the incident energy used to create nonlinearpolarization can be reradiated at one or more numberof different frequencies.

By employing proper phase matching conditionsit is possible to generate any one of thesecomponents of the polarization wave with highefficiency.

k k k

nn n

c c c

1 2 3

3 31 1 2 2(Or) . (12)

Franken et al. (1961) observed the frequencydoubling for the first time, by irradiating a quartzcrystal with a ruby laser beam that operated at 694.2nm. A very small amount of the light striking thecrystal was converted into a light with a wavelengthof 347.1 nm. This wavelength lies in the ultravioletregion of the spectrum and is exactly half thewavelength and twice the frequency of the incidentlaser light. For efficient energy transfer, the twowaves should remain in phase i.e., n1= n2. Due tonormal dispersion occurring in the materials in theoptical region, the radiation will generally lag behindthe polarization wave. The phase mismatch betweenthe polarization and electromagnetic wave is givenby

k4

. (13)

246 S. Suresh and D. Arivuoli

For improving the efficiency of the doubled fre-quency, the crystal has to be phase matched .Thedispersion in the materials can be offset by usingthe natural birefringence. There exist two indices ofrefraction for a given direction of propagation, corre-sponding to the two allocated orthogonally polar-ized modes. By an appropriate choice of polariza-tion and direction of propagation, it is often possibleto obtain phase matching or index matching, where

k = 0. To realize the nonlinear effect, a suitablemedium is required. The conversion efficiency, SHG

defined as the ratio of the emerging second har-monic power P2 to the incident power P is one ofthe most useful measures of the performance of anonlinear crystal. It is represented by

kd

P P

P Akn

2 2 23 / 2

2 0

2

30

sin2

2 ,

2

ll

l (14)

where k = k2 -2k ,k = 2 / ,

0 = permittivity of free space,

0 = permeability of free space,n = index of refraction,

= angular frequency of incident light,l = length of the crystal,A = beam area.

The factor k represents the amount of phasemismatch between the second harmonic wave frontsgenerated at different points in the crystal. If k iszero (proper phase matching) then the interferenceterm,

k

k

2

2

sin2

1

2

l

l (15)

reaches a maximum value that in-turn maximizesthe efficiency of the SHG process. The efficiency ofsecond harmonic generation is directly proportionalto the intensity I( )=P /A of the incidentfundamental radiation.

4. NON-LINEAR OPTICALGENERATION

The nanomaterials with particle size or film thick-ness much small than the coherent length, the phasematching condition is usually neglected and thesurface nonlinearity makes a apparent contributiondue to the enhanced surface to volume ratio. Surface

second harmonic generation (SHG) from metals wasestablished on the existence of the nonlinearE( E)source term that has a large contribution atthe boundary due to the discontinuity of the latticestructure and the presence of the bulk magneticdipole term E ×H/ /t arising from Lorentz force ofelectrons. Accordingly the theory of SHG from metalsurface was built up and modified by thephenomenological parameters (a,b)which,respectively, express the components of currentdensity that are normal and parallel to the surfaceas proposed by Rudnick and Stern [85]. However,the discussion of azimuthally scanned SHGdepending on the interface relation of metal filmsand the formation of nanoparticles on siliconsubstrate is still rarely discussed [86].For metalparticles with structure of inversion symmetry, theelectric quadruple field within the selvedge region isthe dominant source for the generation of secondharmonic light [87-88]. The excitation of surfaceplasmon (SP), which couples the incident field topropagate along the surface, is thus a main strategyfor the enhancement of second harmonic generation(SHG). The efficiency of generating surface plasmondepends on the momentum conservation of theelectromagnetic waves, which has a rather narrowbandwidth of wave-vectors. The random orientationof the scattered light of the innumerablenanoparticles pursues the phase matching condition.Recently, a significant growth of the intensity of thesecond harmonic generation (SHG) reflected frommetallic island films [89-91] have been reported. Theenhancement of the SHG of small metallic particlescan be clarified by evaluating, quantum mechanically,the quadrupole susceptibility with the exploiting ofquantum sphere model [92]. The current sourceJ(2 ) for the SHG in the S direction is related to thepolarization P(2 ) by

PJ i P

t

(2 )(2 ) 2 (2 ). (16)

We can write the second harmonic polarization by

Q

P E Ei

E E

1(2 )

2

,

(17)

where Q

= (1/(2i ) ) is the electric quadrupole,

which can be evaluated quantum mechanically. Onlythe electrons at states near the Fermi energy cancontribute to quadruple transition. Atediouscalculation with isotropic average of the polarization

247Nanomaterials for nonlinear optical (NLO) applications: a review

Fig. 1. Schematic experimental setup for SHG efficiency measurement.

directions implies the values of electric quadrupoleas given by

kFQ

k

kF

k

F

A e E

km

A e E

R m k

A e E

R m

3 5 / 2 max

254

3 2 max

5 23

3 3 / 2

52 3

16 1

2 12

241 1

2 1

161... ,

(18)

whereis A = P nks

nk ks sn

the total average

value of the angular distribution in the tensorcomponents, nk and nk are values of the angulardistribution of pnk in the a direction and Qnk in theabdirection respectively and P is the permutationoperator of . The Q

contains three terms

which are independent, inverse linear dependent,and inverse quadratic dependent on the particleradius, respectively. The second and third terms ofabove equation clearly adduces the enhancementof the quadrupole susceptibilityas the particle sizeR reduces.

4.1. Second harmonicgeneration (SHG)

The Second harmonic generation (SHG) is a specialcase of sum frequency generation, in which photons( 1) interact with the NLO media and are convertedinto new photonswith twice the energy (2 1). As asecond order NLO process, onlynon-centrosymmetric structures are able to emit SHGsignals. Strong SHG signals have been found inmany types of single inorganic nanostructures,

including ZnO nanowires [93], GaN nanowires [94],KNbO3 nanowires [95], noble metallic nanoparticles[96-98], nanocrystals (e.g., Fe(IO3)3, KTiOPO4,BiTiO3) [99-102], as well as core/shell CdTe/CdSqua]tum dots as small as 1% – 15 ]m i] diameter[103]. The intense SHG signals were used fornanoscale imaging and to track nanoparticles incells, tissues, and organisms [104]. Pantazis et al.[102] and Hsieh et al. [101] characterized SHGsignals from BaTiO3 nanocrystals and developedbioconjugated nanocrystals as probes for cell andin vivo imaging without photobleaching or blinking.After an injection of BaTiO3 nanocrystals (~30 nm)into the zebrafish embryo, bright SHG signals couldbe detected in either superficial tissue or deep withinorgans in the zebrafish during development. Unlikefluorescent dyes, the SHG nanoprobes exhibit anarrow emission profile, resulting in a high signal-to-noise ratio (SNR) in the tissues with little autofluorescence background [102]. Based on theintense SHG signals from BaTiO3 nanocrystals, aharmonic holographic microscope has beendeveloped for 3D imaging of nanocrystals in cellswithout scanning [105].The schematic of theexperimental setup used for SHG studies is shownin the Fig. 1.

4.2. Theory of third harmonicgeneration (THG)

In nonlinear optics one normally starts with the ex-pansion of the polarization P as a function of theapplied field E. The third term with the third ordersusceptibility (3) can be written as:

i i j k

j k

P

E E E

(3 )

1 0 , , , 1 2 3 4

2 3 4

( ) ... ( ; ; ; )

( ) ( ) ( ) ... ,

l

l

(19)

248 S. Suresh and D. Arivuoli

Fig. 2. Schematic representation of the THGprocess.

where a sum over all frequencies x has to be per-formed and the Einstein convention of summing overdouble indices has been used. For most all opticalapplications the intensity dependent index of refrac-tion (IDRI) n2,I is the relevant parameter which canbe written as

In n c(3 ) 2

2, 1,1,1,1 0 03 ( ; ; ; ) / . (20)

With n0 the linear index of refraction and c the speedof light. The nonlinear susceptibility for THG (3 )

1,1,1,1

(- ; ; ;- ) is a good estimate for the parameter forIDRI. The THG can be described by solving theMaxwell’s equatio] a]d i]serti]g a ]o]li]ear sourceterm in the resulting wave equation. Similar to SHGalso in the case of THG phase matching betweenall generated harmonic waves can influence greatlythe harmonic intensity due to the dispersion of theNLO material. The phase match and thus the totalharmonic intensity can be varied by changing thepropagation length L inside the material. When wefocus only upon the most important parameters, theharmonic intensity I3 for a material is given by

I

Ln n I

c

2(3)

3 1,1,1,1

2 3

3

( ; ; ; )

3sin ,

(21)

where I is the intensity of the fundamental waveand nx and n3 are the index of the refraction of thefundamental and harmonic wave, respectively.Inpractice the thickness variation is simplyaccomplished by rotating a solid material (powder,sheet or thin film) or translating a dissolved materialinside a wedge shaped cuvet. The nonlinearsusceptibility measured by THG is completelyoriginated by electronic effects, as the opticalfrequencies are far above the natural frequencies ofall other contributions. It gives therefore in general alower limit for the nonlinear coefficients as for manyother NLO effects like the IDRI also the nuclei andeven complete molecules can have a contribution.When a single photon, or two or three photons ofthe fundamental beam arein resonance with excitedreal states of the nonlinear material (Fig. 2), strongfrequency dependent enhancement of the harmonicintensity is observed that can be absent in, e.g. theIDRI. In these cases THG seems to give an upperbound forthe nonlinear coefficient.

The resonances of (3 )

1,1,1,1(- ; ; ;- ) can be

written as a complicated function of the energy andthe transition matrix elements of the electronic statesinvolved. In the case of three-photon resonance one

may simplify this expression by neglecting theimaginary part of (3 )

1,1,1,1(- ; ; ;- ) and other absorp-

tion terms. If one assumes that the main contribu-tion to the three-photon resonance term also involvestwo-photon resonant terms one gets:

n g n g n ga E E E

(3 )

1,1,1,1

, , ,

( 3 ; ; ; )

1/ 3 2 . (22)

If one identifies the position of the firstexcitedstate with the wavelength max of the linearabsorption spectrum one is able to give an estimateof the enhancement due to this factor. In thewavelength region of 0:5 < /(3 max) < 0.9 theenhancement factor increases with a factor five. Fora first assessment of the quality of NLO materials avalue of the NLO coefficients within an order ofmagnitude is already sufficient. We thereforeapproximate the resonance function with a two-stepfunction. This function will be considered to be aconstant if k is outside resonance.If k is within 5%at resonance we set the enhancement factor to 10.

4.3. Third harmonic generation (THG)

The Third harmonic generation (THG) is a third-orderNLO process that requires three photons ( 1) in orderto generate one photon at the tripled frequency(3 1). Unlike SHG, THG is not restricted tononcentrosymmetric structures. The unique opticalproperties of nanostructures render them ideal THGenhancing agents. For example, the surfaceplasmo] reso]a]ce from silver (e.g., 5 – 7 ]mnanoparticle array [106], 30 nm nanoparticles [107])and gold nanostructures (40 nm nanoparticles [108],gold nanorods with a length of 25 nm and aspectratio of 3.727) enhances the THG process.

249Nanomaterials for nonlinear optical (NLO) applications: a review

Fig. 3. Schematic diagram of the THG experimental setup.

Semiconducting nanomaterials also exhibit THGsignals. Chang et al. demonstrated backward THGsignals from ZnO thin films, CdSe quantum dots,and Fe3O4 nanoparticles [109]. Using a fs pulsedlaser at 1290 nm, Jung et al. [110] discovered astrong THG signal from Si nanowires as small as 5nmin diameter due to the large (3) from crystallineSi, which is 1 – 2 orders of mag]itude higher tha]materials such as crystalline CsS, TiO2, and gold[111]. The intense THG signal has been used forlabel-free imaging of nanostructures in cells andtissues [112].The schematic of the experimentalsetup used for THG studies is shown in the Fig. 3.

5. NLO PROPERTIES OFNANOMATERIALS

The nanoparticles and nanostructured materials haveattracted great interest in recent years because theirproperties such as quantum confinement of electronsand holes, surface effects, and geometricalconfinement of phonons, are markedly different fromthose of bulk materials [113]. Nanoparticle has arather large number of atoms, but itssize iscomparable with characteristic dimensionsdescribing the behavior of electrons and holes, thuscreating an intermediate regime between moleculesand bulk crystals [113]. A great deal of the recentinterest in the optical responses of metalnanoparticles,nanoapertures in metal films, andmetamaterials are focuses on enhancing localelectromag]etic fields to e]ormous facilitate light–matter interactions [114]. The Enormous enhance-ment factors of 103-106 compared to the fundamen-tal electric field at a flat metal surface has beenpredicted [114] and these strong local fields areparticularly important for nonlinear optical pro-cesses, such as SHG and THG.

6. NLO PROPERTIES OF A SINGLENANOPARTICLE

The optical nonlinearities of semiconductornanoparticles are of great interest recently. So fartheir third-order nonlinear optical (NLO) propertieshave been widely studied. However, there are onlyfew studies on second-order NLO properties,because it is believed that the centrosymmetry ornear-centrosymmetry of spherical nanoparticleseliminates their first-order hyperpolarizability val-ues to zero or near zero. And for a long time it re-mains a problem to directly study the second-orderNLO properties of such nanoscale particles by con-ventional NLO technique such as interfacial secondharmonic generation (SHG) and electric-field-inducedSHG techniques which are constrained by the ori-entational, size, and or charge restrictions [115].Fortunately, the newly developed hyper-Rayleighscattering (HRS) technique overcomes the aboverestrictions; hence second-order NLO properties ofnanoparticles can be studied. Recently, a few stud-ies were reported about HRS for the colloidal goldand insulator nanoparticle SiO2 [116]. The HRS tech-nique is used to measure second-order NLO re-sponse of a series of semiconductor nanoparticleswith different surfaces prepared by different meth-ods.

7. NONLINEAR OPTICALPROPERTIES OFNANOCOMPOSITES

The polymer nanocomposites, consisting of semi-conducting colloidal nanocrystals (NCs) embeddedin polymer matrices, are original materials able tojoin the structural flexibility and convenient process-ing of the polymers with high carrier mobility,

250 S. Suresh and D. Arivuoli

bandgap tunability, and thermal and mechanicalstability of the inorganic components. Such novelmaterials have been explored in many applicationfields, including linear and nonlinear optical devices,light-emitting diodes, optical switches, waveguides,sensors, and hard transparent coatings as protectivelayers [117 –122]. One of the most crucial pointsfor the fabrication of such a class of nanocompositesrelies on the ability to control the dispersion of thenanoparticles in the host matrix. In fact, nanoscaleparticles typically possess a strong tendency toaggregate, which might be detrimental for retainingtheir size- dependent properties. In nanocomposites,nanofillers must be finely dispersed in polymers sothat the heterogeneous nature of the material shouldbe evident only for sampling on a nanometric scale.

8. NLO PROPERTIES OF QUANTUMDOTS

Semiconductor nanostructures are considered aspromising materials for multicolor single excitationbiological labelling and dynamic three-dimensionalnanoscale optical imaging. Large nonlinear opticalcoefficients in these materials may lead to photonicsapplications, such as ultrafast optoelectronicswitches [122-128]. Moreover, nonlinear opticalprocesses may provide valuable information forproper understanding of quantum confinement andsurface effects in low-dimensional structures. TheNLO properties of semiconductor nanocrystals havebeen investigated [122-135]. Recently beamdistortion method, which is popularly called as thez-scan technique, has been widely employed tocharacterize the NLO properties of semiconductors.In the method of the z-scan, the transmittance of anonlinear medium is measured as a function of thesample position in the z-direction. The transmittancefunction not only gives immediate information aboutthe sign of the nonlinearity, but it also allows one toeasily estimate the magnitude of the nonlinearity.The technique enables one to determine both thesign and the magnitude of the nonlinear refraction(NLR) coefficient of a NLO material.

9. NLO APPLICATION IN BIOLOGICAL

Metal nanoparticles have attracted great scientificand technological interest in biomolecular detectionand clinical diagnostic application due to theirspecific physical and chemical characteristics.Several recently reported experimental resultsillustrated that nanomaterial based NLO assay canbe used for monitoring chemical processes, biologi-

cal and chemical toxins with excellent sensitivityand selectivity [50-74]. The Dendrimers are mono-disperse hyper-branched molecules comprised ofdendrons (tree-shaped units) attached to a centralcore; their well-defined and controllable structureshave made them of great interest for a vast array ofapplications including drug transportation, light har-vesting and optics [136]. Dendrimers containingmetal centers have been synthesized for applica-tions in (among others) catalysis, luminescence,sensing, and magnetic resonance imaging (MRI).Dendritic materials with enhanced nonlinear optical(NLO) properties have also attracted significant re-cent attention, because of the interest in modifyingthe propagation characteristics of intense lightbeams, of crucial importance to the emergingphotonics industries. While the focus of most stud-ies in the NLO properties of dendrimers has beenwith purely organic dendrimers [137], metal-contain-ing dendrimers for nonlinear optics is a field of in-creasing interest.

10. CONCLUSION

In conclusion, in this review, an overview of theemergence of second and third order NLOnanomaterials for the development of nanomaterialbased optical technology. Second order NLOmaterials are used in optical switching (modulation),frequency conversion (SHG, wave mixing), andelectro-optic applications, especially in EOmodulators. All of these applications rely on themanifestation of the molecular hyperpolarizability ofthe materials. It is requirement for nanomaterialsfor applications in second and third order harmonicgeneration. Thispaper also summarizes recentprogress on the development of nanomaterials basedNLO assay for chemical processes and sensingbiomolecules and toxic metals.

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