NRL Memorandum Report 6482

AD-A210 599

Eye/Sensor Protection Against Laser Irradiation

Organic Nonlinear Optical Materials

0MICHAEL E. BOYLE AND ROBERT F. COZZENS

0Polymeric Materials Branch

Chemistry Division

June 12, 1989

DTICS ELECTF

JUL 28 1989 3 3

Approved for public release; distribution unlimited.

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4 PERFORMING ORGANIZATION REPORT NUMBER(S) 5 MONiTOR Ni CIR,AN ZAT.ON R[ Py,T t- ~NRL Memorandum Report 6482

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8a NAME OF FUNDING/ SPONSORING Bb OFFICE SYMBOL 9 PROCIREM0N' NSTP,,1EN 0%,iL'iORGANIZATION (if applicable)

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PROGRAM pp(i"' TA' Wi~iNITAilngtn, A 221-500 LEMNT NO NO FO AC )SIN NOAiligto, V 2217-0006223 4N

11 TITLE (Include Security Cassification)Eye/Sensor Protection Against Laser IrradiationOrganic Nonlinear Optical Materials

,2 PERSONAL AUTHOR(S)

Boyl1e. mJR_ and Cozzens. R.F.1

3a TYPE OF REPORT T3b TIME COVERED 14 DATE OF REPORT (Year, Month, Oakj) ;5 PA1-I (O,*J*

Final IFROM _4 TO _1_jB8 111989 June 12 9916 SUPPLEMENTARY NOTATION

17 COSATI CODES TB SUBJECT TERMS (Continue on reverse if necessary and iderifit by block numrber)

FIELD GROUP SUB-GROUP

T9 ABIST -CT (Continue on reverse if necessary and identify by block number)

Recent developments in organic nonlinear optical materials for application to eye and sensor protectionare reviewed. This compendium includes a brief discussion of the functioning of the eye, delineation of someof the important eye protection parameters and an introduction to the origin of nonlinear optical effects andhow they are measured. Specific examples of proposed or prototyped protection devices are also presented.A compilation of noteworthy organic third-order nonlinear optical materials is included as an appendix. _iI: /2

-Michael E. Bovle (0)7727 oe620D Form 1473, JUN 86 Previoiuis editions are obsolete Ti i 'I '1,,

S/N (1102- LI-() I 4-66()

CONTENTS

IN T R O D U C TIO N ............................................................................................................ I

THE HUMAN EYE ........................................................................................................... 5

V ision .. ............................................................................................................ ...... 5V ision D am age .......................................................................................................... . 7Protection Strategies ..................................................................................................... I I

ORGANIC NONLINEAR OPTICAL MATERIALS .................................................................... I

Theory ........................... 13Linear Refractive Index ................................................................................................. 13Molecular Polarizabilitq ............................................................................................... 13Second-Order Molecular Properties .................................................................................. 17Third-Order Molecular Properties .................................................................................... 20Material Properties ...................................................................................................... 20Material Measurements: X(21 Materials .............................................................................. 27Material Measurements: X(3) Materials .............................................................................. 27M aterials Progress ....................................................................................................... 35

DEVICE CONCEPTS ........................................................................................................ 43

M aterial C apabilities .................................................................................................... 43D evice D esigns ........................................................................................................... 44

PROPOSED FUTURE RESEARCH EFFORTS ......................................................................... 54

ACKNOWLEDGEMENTS ................................................................................................... 54

RE FE R E N C E S ................................................................................................................. 78

I Acoession For

INTIS ORA&IDTIO TAB 0

/ Unannounced 0/ Justifloation

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Availability Codes

Avail and/orDist Special

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EYE/SENSOR PROTECTION AGAINST LASER IRRADIATION

ORGANIC NONLINEAR OPTICAL MATERIALS

INTRODUCTION

-Lasers are playing an important and increasing role inmodern society. Their present uses range from compact discplayers to optical data-storage and communication systems.Because of this wide-spread use, the continuing expansion oflasers into other arenas and t"e4w _mage thresholds of humaneyes and electro-optic sensort Fig. 1ijThere is increasingconcern about eye and sensor protecti6n from laser irradiation. 5.r "

Coupling these factors with the varied frequenciesavailable using today's high-powered lasers (Table 1 and Fig.2) makes eye and sensor protection a complex and difficulttask. That is, an eye/sensor protection device must be capableof responding to a wide range of wavelengths (from the UV tothe IR), able to handle irradiances on the order of mega togigawatts/cm2 , the output currently available in commonlaboratory environments, and be transparent in the absence ofan incident laser beam. Such a protection device must alsohave a response time on the order of picoseconds (10-12 sec) orbetter to safeguard against pulsed laser irradiation.

The eye/sensor protection devices available today arenarrow band filters 3 , 4 that can only protect against a limitednumber of fixed wavelengths; they cannot protect against afrequency agile laser. Research is underway in many differentdisciplines to develop frequency agile protection devices andone area that appears particularly promising involves the useof organic polymeric nonlinear optical materials. Nonlinear

Manuscript approved April 20. 1989.

MAXIMUM PERMISSIBLE EXPOSURE OF EYES

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100 101 102 10 3 10 4

EXPOSURE (sec)

Figure 1: A plot of the maximum exposure of eyes to laser

irradiation as a function of irradiance and wavelength. For

exposure times greater than 104 seconds, there is a constant

irradiance threshold. This figure was adapted from Ref. 1.

2

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optical materials are those whose optical properties have anonlinear dependence on the intensity of the incident light.The design flexibility and fast response ti e (10-14 - 10-15sec) offered by organic polymeric nonlinear optical maLerialshas made this a very active and promising researcharea.5 - 1 2 , 15,17-2 (Nonlinear optical processes on this timescale are mainly electronic in nature.

5- 1 7 )

It is the purpose of this report to review the progressthat has been made in developing organic polymeric nonlinearoptical materials with respect to eye/sensor protectiontechnology. We begin by discussing the functioning of the eyeand defining the desired eye protection parameters. This isfollowed by a brief introduction to the origin of nonlinearoptical effects and how they are measured. Recent developmentsin nonlinear optical organic materials are then presented, withspecific examples of proposed or prototyped eye/sensorprotection devices following. Finally, future areas ofinterest are defined.

THE HUMAN EYE

Of all the organs of the human body, the eye is probablythe most fascinating and intricate: its sensitivity tobrightness can vary by a factor of 100 billion, the darkadapted eye is capable of detecting single photons and it workswith nearly 100% quantum efficiency.2 , -7It is these kinds ofextraordinary qualities that make it difficult to protect theeye from laser damage.

VISION

The physical process of vision begins when light enters theeye at the cornea (index of refraction = 1.376)22 which is amajor focussing element (Fig. 3a). Further focussing isprovided by the lens, the next element in the optical path,which allows for near and distant vision by changing shape.The iris, located behind the lens, is a power limiter; itvaries the pupil size, controlling the amount of light enteringthe eye. The incident light continues on through a clearjelly-like substance, the vitreous humor, onto thephotosensitive retina, the detector.

The retina is composed of photoreceptors, nerve cells andpigment layers. The photoreceptors, the light sensitivecomponents, are located in the last cellular layer and pointaway from the light source (Fig. 3b). There are of two typesof photoreceptors, rods and cones. Cones are found packed inthe fovea (Fig. 3a) and are responsible for color vision andvision in bright light while the rods are distributedthroughout the remainder of the retina and are responsible forvision in dim light. 2 ,2 2 The structure of rods and cones isdifferent (Fig. 3c), the largest difference being in the region

5

ROD PHOTORECEPTrOR

(b) CELLS(a)

OUTER SEGMENT

ROD

LIGHTDISC MEMBRANE

LIGHT

COE

Figure 3: The major components of the human eye are shown in(a), with expanded views of the structure of the retina (b) andof th- rod and cone cells (c). This figure was adapted fromRef. 22.

6

where the light absorbing pigments (called rhodopsins) areembedded in regenerating membrane discs.

22

The photosensitive rhodopsins contain the light absorbingmolecule retinal which is chemically linked to the proteinopsin. The absorption of light induces an isomerization inretinal (Fig. 4) which is thought to cause a structural changein opsin.22 The isomerization of the protein is believed toseparate charged groups and thereby effectively store energy(quantm yield of -60%) within a few picoseconds (10-12secs). 2 The transduction of this light-induced response tothe neuronal network in the retina is accomplished via theplasma membrane and chemical messengers.

2

VISION DAMAGE

Although vision is limited to the eye's response over arelatively narrow wavelength region (see Fig. 2), light fromoutside this region can have a profound effect on sight. Thecornea absorbs infrared radiation (1.4-10 mm)1-3, 23 and thecornea combined with the lens can absorb near ultravioletradiation (0.2-0.4 pm). 1-3 ,23 Therefore, ultraviolet andinfrared radiation can damage the cornea causingphotokeratitis, corneal burns and cataracts1 -3,23 and therebyimpair vision. However, the greatest danger comes fromvisible to near-infrared radiation (0.4 - 1.4 Am) which thecornea and lens transmit, focussing onto the retina (opticalgains on the order of 105).1 -3 Damage to the part of theretina providing fine detail discrimination and colorsensitivity, the fovea, drastically affects vision while damageto the retina outside the fovea does not seriously impair theability to see. 2 ,3 Unfortunately, little recovery is possiblefrom damage to either part of the retina.2 ,3

Injuries to the eye from laser irradiation are usuallygrouped into three classes: photochemical, thermal andmechanical. 1,2,23,24 Photochemical damage involves chemicalbond breaking and is associated with long exposures to shortwavelength light (blue to ultraviolet) 4Thermal damage iscaused by visible and infrared radiation for pulse lengths of 1microsecond or longer 2 ,24 and includes denaturizationprocesses, i.e., the uncoiling of protein molecules resultingfrom the breaking of weak hydrogen bonds. Such processes canlead to the rupture of cell walls and enzyme inactivations.Very short pulse lengths (less than microseconds) result inmechanical damage to the eye in the form of acoustic and shockwaves. 2 ,24 It is the latter two damage mechanisms which are ofthe greatest interest with respect to eye protection from laserirradiation.

Thermal eye damage is usually discussed in terms of theamount of energy incident on the eye: it is the cumulativeenergy that causes injury. 1-3 The following are commonly used

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eye damage thresholds, We:1

* For pulse durations of 1 - 18 ns: We = 0.5 AJ/cm2

* For longer pulse durations, up to 10 st-s:We = 1.8 t3 /4 mJ/cm 2 (t in seconds)

As an example, usini the forementioned damage thresholdsand a value of 100 mW/cmL for the output of the sun, 2 an eyeprotection device must attenuate the light by a factorequivalent to 1.5 optical density units (ODU) (a transmissionreduction of 97%) when directed at the sun.

PROTECTION STRATEGIES

Any successful eye/sensor protection device must interactwith and attenuate the laser light before it reaches thedetector system. The interaction of light with matter isusually classified in one of three categories: absorption,dispersion or scattering. Absorption can be an effectiveprotection strategy and representative examples of absorptiondevices under investigation include particle suspension,chalcogenide, V0 2 , Ge, and two-photon absorption activatedpower limiters. However, such devices often have reducedtransparency in the visible spectral region or unacceptableresponse times. Therefore, much of the recent research intoeye/sensor protection has focussed on using dispersion orscattering to redirect the light and this is where nonlinearoptical materials have the greatest potential for impact in thenear term: nonlinear optical materials can have unique indexof refraction properties and fast response times.

ORGANIC NONLINEAR OPTICAL MATERIALS

Nonlinear optical materials have been known and studied forover two decades with most research efforts being successfullydirected at inorganic materials, 5 ,2 5 ,2 6 in particular,inorganic crystals 2 7, glasses 2 8 ,2 9 and semiconductors.3 0 - 3 2

The most familiar example of inorganic nonlinear opticalmaterials are crystals such as potassium dihydrogen phosphate(KDP) and lithium niobate (LiNb03). However, more recently,interest has focussed on such inorganic materials as tungstenbronze crystals. 3 3 With the recent emphasis on opticalcomputing and communication, a need for nonlinear opticalmaterials with better mechanical processing and physicalproperties than available in typical inorganic nonlinearoptical materials has become apparent and researchers haveturned to examine organic polymericmaterials .9,14,17-21,34-39 It is now generally agreed thatorganic materials have the potential for nonlinear opticaleffects which are orders of magnitude better than currentlyused inorganic materials.5, 6 ,12, 14,17-21,34,37,38,40-45 Thisis based on the origin of the nonlinear optical effect inorganic materials: the easily polarized molecular electricfields. 7 , 1 0 , 1 4 ,1 5 Extensive research is underway on thedevelopment of nonlinear optical organic materials and a

11

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detailed theoretical understandinq and description of theorigin of these optical effects. 5-2 1 ,26-189 It is the latterthat is discussed in the following section.

THEORY

Linear Refractive Index

In order to develop a more comprehensive understanding ofnonlinear optical effects and materials, it will be useful tobriefly examine the origin of the linear refractive index.

Consider the classical model (Lorentzian) of an atom withone electron and the effect of applying a static electricfield: the electron-nucleus distance is altered - apolarization is induced. For the simple case considered here,the distance change is linearly proportional to the appliedfield. If an oscillating electric field (like in a lowintensity monochromatic light beam) is applied, the electronoscillates about its equilibrium position. This oscillatingdipole emits electromagnetic radiation (light) at the samefrequency as the incident light but with a different phase dueto the restoring forces acting on the electron.

Extending this simple example to include a row of N atoms(Fig. 7), we see that a monochromatic light wave having passedthrough the N atoms will have a different phase than if it hadnot. That is, the light wave appears to move more slowlythrough the sample than through the surrounding vacuum. Thephase difference is directly related to the number of atoms inthe row, N, and therefore to the sample length or thickness.The ratio of the speed of light in the sample, Csample, and invacuum, Cvacuum, is known as the index of refraction, n =Cvacuum/Csample.

An oscillating dipole has a toroidal shaped radiationpattern (a sin 0 dependence, where 0 is the angle between theaxis of the dipole and the direction of observation) andtherefore reradiates light in many directions. However, onlythe reradiation in the forward direction is phase-matched andtherefore additive. That is, any radiation not in the forwarddirection is out of phase with the radiation in that samedirection from other atoms and so destructively interferes.Also implicit in the simple models discussed above, is that allthe dipoles radiate the same fraction of the incident wave. Ifone group of dipoles radiates a different fraction, some of thereradiated light will be visible in other directions -scattered radiation.

Molecular Polarizability

Noncentrosymmetric molecules (those without a center ofsymmetry) have complex charge distributions and, therefore,possess an intrinsic dipole. Placing such a molecule in a

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static electric field (E) distorts the charge distributionchanging the dipole. When the electric field is small comparedto the internal fields due to the electron!, the molecularpolarizability (p), which is proportional to the dipole moment,can be described in a power series49

p = p 0 + .E + f:EE + 'EEE+ .... (1)

where a, p. y are the static molecular polarizability tensors,respectively, the linear polarizability and the second andthird-order hyperpolarizability. At lower field intensities(small E's) only the first term in equation 1 (a term) has anappreciable effect on p, and this is the case discussed in theearlier atomic example. As the field intensity increases, thesecond two terms (the second- and third-orderhyperpolarizabilities, respectively) become more important. Itis these latter two terms that are responsible for a molecule'snonlinear optical behavior. Physically, these terms representa measure of the size of the nonlinear effect. That is, howeasy it is to induce a polarization or equivalently, howtightly bound the electrons are to the nuclear framework. (Thelooser the binding, the further the electrons can be drivenaway from the nuclear framework resulting in a largerpolarization and thereby, a larger nonlinear optical effect.)The above equation is modified for centrosymmetric molecules bythe removal of the polarizability term Po (with a center ofsymmetry, there is no intrinsic polarizability) and removal ofthe second-order term (P = 0). Details are discussed below.

With the advent of the laser, large optical fields becameavailable and it was natural to consider time-dependent fields.The equation describing the time-dependent molecularpolarizability retains the notation of the static case forhistorical reasons,49 i.e.,

P = P0 + a(-w;w)eE(w) + fA(-w;w1,w 2):E(wl )E(w 2 ) +-7(-w; w,w2,w3) (w I )E(w 2 )E(w 3 ) + .... (2)

where the coefficients are complex tensors and have frequencydependence. For example, the common linear polarization terma(-w;w) is composed of a real part, corresponding to the indexof refraction, and an imaginary part, corresponding toabsorption. The frequency dependent tensor notation usesnegative signs to indicate conservation of momentum andsubscript arguments to indicate the frequencies of the electricfields. For example, in second harmonic generation, thesecond-order microscopic polarizability tensor is representedas #(-2w;w,w) or for the linear electro-optic effect (electric-field-induced-birefringence with a linear field dependence),

We can obtain some important general information about themolecular properties associated with nonlinear optical effectsby examining equation 2. First, consider an isotropic molecule

15

MOLECULAR DIPOLE MOMENT

(A) ROTATION

oz

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I II IC C C C C

S

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Figure 8: In (a) a noncentrosymmetric rigid rod molecule is usedto demonstrate the origin of the molecular dipole. The arrowsindicate the direction of the intrinsic dipole moment. In (b) theorigin of the molecular dipole is shown to be due to vibrations in

a centrosymmetric molecule, i.e., one that possesses a center ofsymmetry.

16

and the second-order hyperpolarizability term involved insecond harmonic generation: +p(2) = fl(-2w;w,w)E(w)E(w). Foran isotropic molecule, f is independent of direction andtherefore constant. Thus, if the axis direction is reversed (x*-x, y- -y, z-*-z) while leaving the electric field and the

dipole moment unchanged in direction, the second-order term ofequation 2 becomes, _p( 21 = f(-2w;w,w).-E(w))(-E(w)) = +PThis can only be true if +p(2) = -p( ), i.e., R = 0. In otherwords, centrosymmetric molecules only have contributions fromodd-order terms in equation 2; they cannot exhibit even-ordereffects such as second harmonicgeneration.5,8,12,17-21,35,41,49-51,53-56

Second, it is clear that molecules with easily polarizedelectron clouds have the greatest potential for large nonlinearoptical coefficients. Thus, we do not expect saturated organicmolecules (i.e., those without r bonds) to exhibit much of anonlinear optical effect: the bonding electrons are welllocalized so only small changes in charge distribution withchanges in local field environments are expected. 35,41,48-52However, unsaturated, conjugated molecules with their large Welectron delocalization should and do exhibit large nonlinearoptical responses. 17-21 ,35,41,48- 51 Amore detailed look atthe appropriate molecular properties for specific nonlinearresponses is presented below.

SECOND-ORDER MOLECULAR PROPERTIES

The majority of work reported on organic nonlinear opticalmaterials has focussed on second-ordereffects8 ,12 ,14 ,17- 21 ,40 -75 (molecules with large # coefficientsin equation 2) for the obvious reason that the effect is largerthan the third order response and therefore easier to measure.There exists a good understanding of the origin of this effectand how to optimize molecular and bulk material properties toenhance it. ,12,17-20,35,37,38,40-75 For organic molecules thegeneral molecular properties that are required for good second-order nonlinear response are: noncentrosymmetry, planarity anddelocalized electronsystems.8 ,12,1720,35,37,38,40,41,44,45,48,54-68 Additionally,substituent groups that enhance the charge asymmetry of themolecule, i.e., strong electron donor and electron acceptorgroups, lead to low-lyinq charqe-transfer resonance states andthereby enhanced l's.8,12,17-20,40,41,54,55,57 -59,63,69,72 -75The charge asymmetry inducing substituents make it easier topolarize the molecule: the flow of charge is enhanced in onedirection - like an "optical diode". (See Fig. 9) .49-51

In the following table, examples showing the importance ofthe above molecularro erties in enhancing second-ordereffects are given.

17

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.;he theory and understanding of third-order processes:n equation 2 is significant) and their origin in

o~ganc icl-ecules is still in its infancy but has seenremarkable proqress over the ast f wyears.6-1 ,1 48-20,34,37,38,48,51,76-89 The molecular originof this effect is believed to be related to the correlatedmotion of electrons and to hiqhly charje correlated vir ualexcitations (Fig.10).58,1 0 , r8 ,20, 37, ,48,74,76,78-8h,84The correlated motions are believed to arise from the combinedaction of electron-phonon and coulombicinteractions.6 ,8 ,10 ,37,38,48,74,76,78-80,85 However, themagnitude of the electron-phonon coupling contribution is notclear.10,76

Identification and characterization of the molecularproperties which lead to enhanced third-order effects is understudy. The most important molecular property appears to beplanar conjugation. There is some evidence that ladderpolymers (polymers formed using fused aromatic rings) may besuperior to open chain polymers because of improved r orbitaloverlap 13' 37 ,38,76,80-83 and it is predicted by someresearchers that I will not increase beyond that observed for20-25 repeat units (- 60 A).6 ,37 ,38 ,7 6 , 8 ,79 There is alsoexperimental evidence suggesting that the incorporation ofcertain heteroatoms into the conjugation lenqth candramatically increase the third-order effect 0,37,38,76,81,82,again through improved 7 orbital overlaps. Another propertythat may be important and that is under investigation is theeffect of intermolecular bonding, e.g., hydrogen bonding.90

Charge-induced asymmetry as a means for enhancing third-order effects remains a controversial issue: a great deal ofelectron delocalization may arise from charge-inducedseparations. Further experimental investigations are requiredin order to assess its importance.86-8 8

MATERIAL PROPERTIES

The polarization induced in a material by the applicationof an electric field, E, is described by

P = P0 + X(1 )E + X(2 )EE + X(3 )EEE + ... (3)

where P is the material. polarizability, P0 is the intrinsicpolarizability and x ' are the macroscopic materialcoefficients known as the material susceptibilities. Themacroscopic susceptibilities are related to the microscopichyperpolarizabilities as shown below.

X( = NcF(w) 2 (4)X(2 ) = NfF(w 1 )F(w2 )F(w3 ) (5)X ) = N1F(w1 )F(w2 )F(w3 )F(w4 ) (6)

20

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N is the number density of molecules and F is the local fieldfactor at the specified frequency, i.e., the value of theelectric field at the -ite of the molecule. F is a difficultquantity to estimate because of the mutual polarization ofmolecules in dense phases. 49,50,57,61,93 Therefore, great caremust be taken in relating microscopic and macroscopicquantities. This is especially true for equations relatingX(2 to the second-order polarizability, fl, because a detailedknowledge of the projection of the tensor onto the orientedmolecule is required.1 7 ,4 1 49' 50 ,57 ,9 3 Equation 5 is for thesimplest case of a "rigid lattice-oriented gas" where all themolecules oint in the same direction and are fixed inspace. 17' ,50

Molecular oriertation within a medium is an importantaspect of both X(2) and X(3 ) nonlinear opticalmaterials. 12,17-21,43,53,56,63-66,72-74,-9 1-94 The more orderedthe molecules within the material, the larger the materialresponse (see Fig. 11). The two most popular methods fororderinq molecules in materials are, electric fieldpoling,53 ,56 ,6 4 ,92-9 4 primarily used for ( 2) materials, andLangmuii-Blodgett film deposition, 43 ,66,9 used for both X(2 )

and X( 3 ) materials.

Electric field poling works well for X(2 ) materials becauseof the required molecular charge asymmetry and its applicationto guest/host systems is diagrammed in Fig. 12. Initially, theguests are frozen in a random orientation in the polymerichost. The material is then heated to a temperature greaterthan the glass transition temperature of the host. This allowsthe guest molecules to rotate within the host. An electricfield is then applied, causing the guest molecules topreferentially orient along the field direction. The materialis cooled below the host's glass transition temperature, whilemaintaining the orienting electric field, thus freezing theguest molecules in an ordered, noncentr9s mmetric orientation.The use of this technique to orient X( 2 ) active side chains ofmain chain polymer backbones (Fig. 13) is also obvious and anarea of active research. 41 ,72 ,73-96 -100

While high degrees of order can be initially achieved usingthis technique, the molecular order does decay with time due tothermal relaxation and the inevitable increase in entropy ofthe system. Half-life for randomization can vary from a fewdays to a few years depending on the polin9 conditions and theparticular molecules involved. 53 , " 0 ,10i A concentratedresearch effort is underway to increase half-lives. 5 3 , 100, 101For example, adding functional groups to the pendant sidechains 9 to serve as order preserving 'Hooks and Eyes' (Fig.14), or dispersing the nonlinear optically active molecules inpiezoelectric hosts where the intrinsic electric field9 4 aremethods being examined to help maintain the preferredintermolecular orientation.

22

Types of Molecular Packing

Packing Type Order Magnitude

Non-centric Second Moderate

IiNon-centric Second Strong

1111 Centric Third Strong

Centric Third Weak

4-

Direction of dipole moment

Figure 11: The effects of molecular packing on the magnitudeof second and third order non-linear optical materialproperties are indicated above. The arrows indicate thedirection of the dipole moment.

23

ELECTRIC FIELD POLING

Iheat heat

E-field on/ __

GLASSY, RANDOM ORIENTATION RUBBERY, RANDOM, FREE TO ROTATE

coo E-field off

RUBBERY, ALIGNED GLASSY, ALIGNED

Figure 12: A graphic depiction of the electric field polingtechnique. In the first step, the composite material is heatedabove the glass transition temperature of the polymer host. Anelectric field is then applied to orient the molecules and thematerial is allowed to cool, freezing the guest molecules in aparticular orientation. Adapted from Ref. 185.

24

ELECTRIC FIELD POLING OFLIQUID CRYSTAL SIDE CHAIFT POLYMERS

E-E

Figure 13: The electric field poling technique applied toliquid crystal side chain polymers. The rectangles representthe nonlinear mesogens; the hatched boxes are to assist inindicating orientation.

25

CHEMICAL "tHOOKS AND EYES"

Figure 14: Chemical "Hooks and Eyes" could be used to assistin maintaining the orientation of the non-linear mesogenicpendant groups (the rectangular boxes as in Fig. 13) of liquidcrystal side chain polymers after poling. Such a concept could bebased on strong photo-induced covalent bonds (crosslinking) orweaker hydrogen bonding.

26

The Langmuir-Blodgett deposition technique allows formonomolecular control of orientation andcomposition. 43 ,66 ,9 5 ,102-107 This technique requires surfaceactive molecules i.e., molecules with hydrophilic andhydrophobic ends, and is useful in preparing both x(2 ) and X(3 )

materials. The technique is diagrammed in Fig. 15. Thesurface active molecules are spread onto the surface of thewater and compressed to form a monomolecular film. Then, byrepeated dipping, monomolecular film layers can be depositedonto a substrate. Unfortunately, Langmuir-Blodgett films areoften contaminated with crystallite regions causingunacceptable amounts of light scattering.9 5,102-1 T4

MATERIAL MEASUREMENTS: X(2 ) MATERIALS

There are several experimental methods available formeasuring/characterizing second-order nonlinear opticalmaterials.1 08 ,109 Several of these methods are listed belowand diagrams (Figs. 15-20) describing the techniques are shownon the following pages along with some of their advantages anddisadvantages. The choice of characterizing method depends onthe type of descriptive information desired and the physicalform of the material.

(2) CHARACTERIZATION METHODSbZ(UJNJ HARMO o4, t7ENERATION

SINGLE CRYSTAL 3ZCOND HARMONIC GENERATIONEFISH

POLYMER POLINGELECTRO-OPTIC EFFECT

MATERIAL MEASUREMENTS: X(3 ) MATERIALS

Experiment complexity dramatically increases when makingmeasurements on '(j) materials as compared to X(2 )

materials:78 ,79 09 all materials, ranging from air to thesample holder, can exhibit third-order effects (there are nosymmetry restrictions) 17 ,18 ,20 ,34 ,35 ,41 and the effect is smallwhile the sample materials are often of poor opticalquality. 14 ,85,46,81,82 In addition, the experimenter must beconcerned with resonant vs. nonresonant effects and, in somemeasurement methods, with the oriq in of the intensity dependentindex of refraction, n2.9 ,11 ,29 ,81,82,110,111 Beforedescribing a few of the methods available for measuring Xmaterials, resonant enhancement and intensity dependentrefractive indices will be briefly discussed.

X(3 ) may be expressed as the sum of two types ofsusceptibilities due to resonant (R) and nonresonant (NR)contributions1 0,17,18,20,110

X(3 ) = X(3)R + X(3)NR (7)

27

LANGMUIR-BL ODGETT TECHNIQUE

SAMPLESOLUTION MOLECULES

WATER-WATER

SPREADING

A-t A A AAAA A A

FILM COMPRESSION

DEPOSITION (st LAYER)

( 2n d LAYER)

(3 rd LAYER)

Figure 15: A descriptive outline of the production of a thin-film using the Langmuir-Blodgett technique.

28

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The nonresonant susceptibility contributions occur in spectralregions far from any opt i.cal absorption of the material, whileresonant contributions occur at frequencies near or resonantwith the material's optical absorptions. Resonance effects areusually measured at absorption band edges and can causeconsiderable enhancement of the nonlinear opticalsusceptibility through admixtures of excited state propertiesand contributions from excited state dynamics. For materialsof interest in eye/sensor protection, the nonresonant effectsare of greatest interest because they are responsible forbroadband response with good transparency at ambient lightlevels.

Another important aspect of X(3 ) materials is the intensitydependent refractive index, n2 . At high irradiance levels, amaterial's index of refraction can be described by

n = no + n2(I) (8)

where no is the linear refractive index and n2 is the intensitydependent refractive index.9 ,11,29,110,111 At nonresonantoptical frequencies, n2 can arise from any of the mechanismslisted in the following table.9, 29 ,81 ,82,110

TABLE 4MECHANISMS OF NONRESONANT SELF-INDUCED INDEX CHANGES

MECHANISM n2(esu) (sec)

MOLECULAR-ORIENTATION KERR EFFECT 10-11 - 10-12 10-11 - io-12MOLECULAR-REDISTRIBUTION (LIBRATIONS) 10-12 - 10-13 - 2 x 10-13NONLINEAR ELECTRONIC POLARIZABILITY 10-8 - 10-14 10-14 - 10-15ELECTROSTRICTION 10-11 - 10-12 10-8 _ 10-9THERMAL EFFECTS 10-4 - 10-

5 10 -

1 - I

Of these mechanisms, the nonlinear electronic polarizability isthe most important for eye/sensor protection due to its fastresponse time. While the molecular-orientation Kerr effect(quadratic electric-f ield-induced-birefringence) is also fast,it suffers from a saturation effect at high light intensitiesdue to the complete orientation of molecules. T10 , 112

There can also be a resonant contribution to n2 when near astrong absorption frequency. At resonant or near resonantfrequencies, some of the light is absorbed which causes aredistribution of the material's electronic energylevels.8 ,10 ,29 ,110 This results in a change in the dispersionassociated with the absorption band, a resonant enhancedintensity dependent index of refraction. Such an effect is ingeneral not compatible with the broadband response required for

34

an eye/sensor protection device.

Of the characterization methods listed .elow and diagrammedin the following pages (Figs. 21-25) onl third harmonicgeneration does not depend on n2 .8 1 ,82 ,109 ,112- 1 16 As before,the sample form and the type of information desired dictate theappropriate characterization method.

X(3 ) CHARACTERIZATION METHODSDEGENERATE/NONDEGENERATE FOUR WAVE MIXING

M-LINE TECHNIQUESURFACE PLASMON TECHNIQUE

OPTICAL KERR EFFECTTHIRD HARMONIC GENERATION

MATERIALS PROGRESS

X(2 ) Materials

Most of the organic material development work has involvedX systems: organic crystals, guest/host type structures,liquid crystal side chain polymers and 1i*quid crystalpolymers. 14 ,17 ,19 ,2 1 ,37, 38, 40-75,10117 Of these generaltypes, the latter three are of greatest interest because of thedifficulties associated with growing reproducible, high opticalquality crystals and the accompanying processingproblems.1-2,118 Van der Waals crystals are formed by dipolarforces between molecules and so, good second-order moleculeswith their large dipole moments are difficult to assemble intoa noncentrosymmetric bulk crystal. 12 Researchers are examiningdifferent approaches to crystal growth using forces strongerthan van der Waals attractions such as hydrogen bonding andmolecular salts.17 ,4 1

Non-cryst1l ine organic materials appear to be promising atthis time: X 2j values on the order of 10-7 esu have beenreported with response times on the order ofmicroseconds.1 4 ,7,38,96,98,100 The majority of work in thisarena has focussed on guest/host type structures where thesecond-order molecule is a guest in a polymer host e.g., anazo-dye in poly(methyl methacrylate) .12,64,93,17t Oneparticularly interesting example of this type of system is theuse of a crystalline copolymer of vinylidene fluoride andtrifluoroethylene as the host and an aminoazo compound as theguest. 9 4 In this case, the host is a ferroelectric compositewith electric fields of 106 V/cm permanently induced in theamorphous regions containing the guest molecules .94 Thisshould lead to saturation orientation f the guestmolecules 57 ,9 4 and thereby a large X

A significant amount of work has also been reportedinvolving Langmuir-Blodqett films of highly polarizablemolecules.40,43,66,95,1O -0307

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However, probably the most promising area of organicmaterial development involves liquid crystal side chain andliquid crystal main chainpolymers. ,14,37,38,57,99,100,11 7,119- 121 Liquid crystal sidechain polymers are formed by attaching mesogenic units topolymer backbones using spacer groups to decourle mWion of theside chain from the backbone (See Fig. 26). 41 ,19 - Aelectric field is used to orient the side chains. x(2) valueson the order of 10-9 esu have been reported with response timeson the order of microseconds fo.L these kinds ofmaterials.14 ,37 ,38 ,96 ,98 Preliminary results showing GHzmodulation (nanosecond response times) have also recently beenreported, but not published.

14 ,96 ,9 8

On the other hand, main chain liquid crystal polymers1 19

(also known as polar polymers) have demonstrated substantiallyenhanced second-order effects as compared to their monomers.

57

This enhancement is theorized to arise from the structuralordering of the monomers. Further enhancement may be possibleby orienting the polymer chains themselves (see the discussionin Proposed Future Research Efforts).

A major focus of future X(2 ) research will be to developmaterials with X(2) oriented parallel to the material surfacerather than perpendicular. 14,37,38,98 Such a development wouldallow for greater usable surface areas and possibly thinnerfilms than are currently available. Both are advances thatwould make meeting the optical requirements for eye/sensorprotection devices more realistically attainable.

An extensive compilation of X(2 ) materials and their

properties can be found in reference 19.

X(3 ) Materials

Although the origin of third order effects is not fullyunderstood progress has been made in developin materials withuseful X(3) properties. 9- 11 ,16 ,18 ,20 ,37 ,39 ,81 89 ,122-141Organic X(3) materials can be somewhat arbitrarily divided intofour main classes: fused ring polymers, long chain unsaturatedpolymers, organometallic polymers and miscellapeous. AppendixA contains a brief compilation of reported Xt3 ) materials foreach of these areas and an additional area, inorganics,included for comparison.

The largest reported X(3 ) values are for long chainunsaturated polymers, specifically the poly(diacetylene) (PDA)materials, which have received extensiveattention. 9 ,16 ,34 ,111 ,122- 139 Awide variety of substituentgroups to the main chain have been studied, but overall, PDAmaterials are highly absorbing in the visible region of thespectrum and the reported values of X(3 ) are resonant-enhanced;undesirable qualities for eye/sensor protection. However,steady advances in the magnitude of the third-order effect have

41

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been reported.9 ,34 ,1 11 ,123 ,131 ,13 2

Recent investigations of ladder poymers and rigid-rodpolymers have reported nonresonant xtJ3 values on the order of10- esu and are a promising new area ofresearch. 9 ,10 ,13 , 7 ,38 ,44 ,78-80 ,140-14 2 These large nonlineareffects are believed to arise from incre sed w orbital overlap,as compared to open chain polymers. 13 ,37 ,3 ,7 678-80 Currentresearch efforts are focussing on the effects of substituentgroups and heteroatom substitution, which preliminary resultssuggest may dramatically increase the opticalnonlinearity.9,37,38,78,80,81,82,89

An exciting new area in organic X(3 ) material developmentis organometallics.143-145 The incorporation of inorganiccomponents into organic olymer systems has produced x(3)values on the order of 10 -Tesu. 14 "- 45 Investigations intothe effects of different inorganic components in thesematerials are currently underway. 1 43- 145 Inorganic/organicguest/host systems have also shown third-order effects but themagnitude is not clear.

Among the miscellaneous X(3 ) mat rials, small metalparticles in colloidal suspensions, 146 azo dye attachedcopolymers 47 and dye doped glasses1 48,149 have all exhibitedrather large x(3 ) and n2 values.

Future research efforts on organic X(3 ) materials are beingdirected at developing a fuller understanding of the origin ofthe third-order effect via small systematic changes on themolecular level, i.e., trial and error. 37 ,38 ,81 ,82Concomitantly, material development will occur. The othermajor research thrust will be in developing materials withbetter optical properties as well as improved mechanical andprocessing properties. 14,37,38 These latter properties arecritical to the development of useful protection devices.

DEVICE CONCEPTS

Material Capabilities

In this section generic eye/sensor protection devices thatmake use of nonlinear optical materials are described. It isgenerally agreed that X(2 ) materials have now developed to thepoint where they can compete with their inorganic analogs inthe areas of optical communication and computing devices (X(2 )- 10- 7 esu).37 ,3 8,45,88 However with respect to eye/sensorprotection, devices based on X ) materials generally involvethe linear electro-optic effect or frustrated internalreflection and are relatively slow to respond (msec tonanosecond time scale). 14 ,6 8 8 ,93 ,117 , 50 While such aresponse time may be adequate for protection against CW laserirradiation, it is not adequate for pulsed laser irradiationprotection. In addition, electro-optic devices usually require

43

high voltage power sources; they are not passive devices.Therefore, most recent - .oposals for eye/sensor protectiondevices employ designs based on electronic polarizable Xmaterials (see Table 4).

State-of-the-art third-order electronic materials haveX(3)s that are on the order of 10-9esu. 14 ,3 7 ,38 ,78 ,79 ,8 1,8 2 ,160 If improvements in the opticalquality (reduced scattering and absorption) of these materialscan be made, they may find application in optical wave guidedevices. However, before they find broad device applications,X(3 must be increased by at least two orders ofmagnitude.14,37,38

Device Designs

nthe following section and figures, device designs basedon x 2) and X(3 ) materials are described.

The first concept (Fig. 27) involves the linear electro-optic effect. This effect is associated with X(2 ) materialsthat are placed in an electric field. The applied field causesthe material to become anisotropic and birefringent. Theproposed eye/sensor protection device places the materialbetween crossed polarizers. (A nonlinear opticalsolution/liquid would be housed in a material cell locatedbetween the crossed polarizers.) In the absence of an appliedfield, the crossed polarizers completely attenuate any incidentlight. Applying an electric field causes the nonlinearmaterial to rotate the plane polarized light coming through theincident polarizer allowing some light to pass through theexiting polarizer. The maximum throughput is limited to 50%because the incident light must first be polarized and thisattenuation is too great for eye protection systems. Inaddition, an adequate response time has not been demonstratedfor these materials. For guest/host materials, the responsetime is on the order of microseconds (nanosecond response timeshave been reported but not published) .98,194 For liquid orsolution samples, the effect can be fast for small molecules,on the order of tens of nanoseconds, but as the size of theactive molecule increases so does the response time.Additionally, the response time for these types of devices willdepend on overcoming the difficulties associated withgenerating fast electronic pulses in the kilovolt range.

Liquid crystals and ferroelectric liquid crystals haveshown promising results when incorporated into linear electro-optic devicesf 51-158 but suffer from the same disadvantageslisted above.

44

LINEAR EiLBCTRO OPTI DEVICE

4.HV

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igt

plane of polarization Of the niettgt

45

As diagrammed in Fig. 28, a device similar to the linearelectro-optic one described above can be designed making use ofthe optical Kerr effect. For the system diagrammed, again theresponse time depends on the rate of reorientation of theactive molecules.

A slight variation in this theme is the liquid crystalcomposite. In this device, small droplets of liquid crystalsare dispersed in a polymeric host. The application of anelectric field causes the liquid crystals in the droplets toalign making the material transparent. This device requiresthe refractive indices of the polymeric host and the orientedliquid crystal to match. As the electric field is reduced,the liquid crystals assume a more random orientation and arefractive index mismatch occurs resulting in increasedscattering - the material becomes a milky white color. Theprocess is reversible and easily adapted to device design.However, the response time of the device is on the order ofmilliseconds, too slow to protect against pulsed lasers. 19 5 (Asimilar device using microdroplets of liquid crystals anddepending on the optical Kerr effect has also been reported.19 6

The response time is on the order of 50 microseconds for thissystem.196)

A device similar to the liquid crystal composite has beenproposed that would use a dispersion of X( 3) microparticles ina polymeric host (Fig. 29). The design principle is to makeuse of the intensity dependent refractive index, n2 . At lowlight irradiances, the index matched particles and host willlet the light pass through unaffected. However, at highirradiances, the index of refraction of the nonlinear opticalmaterial changes, creating an index mismatch and therebyscattering the incoming light. The advantages to this type ofdevice are that is passive, ie., powered by the light itself,it is fast enough to proteyt against pulsed laser threats ifelectronic polarizable x 3) materials (see Table 4) are usedand that the device is normally optically transparent.

Another X(3 ) material device concept, based on four wavemixing, is diagrammed in Fig. 30. In this type of device, twocounterpropagating lasers (beams 1 and 2) setup a phase gratingin the nonlinear optical material that rejects a high intensityinput laser beam (beam 3) while letting a low intensity inputbeam pass through unaffected. These counterpropagating beamscan be at the same frequency as the input beam (degenerate fourwave mixing) or at a different frequency (nondegenerate fourwave mixing). Most device designs employ a permanent set ofcrossed laser beams, but using a beam splitter as diagrammed,the incident beam can be used to form these beams. Therejected beam (beam 4) is conjugate to the input beam which iswhy this method is sometimes referred to as phase conjugation.(By phase conjugation, we mean that the rejected beam has thesame phase as the input beam but travels in exactly the

46

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49

opposite direction, a sort of time reversal symmetry. See Fig31.) 28 ,161 This device requires the use of electronic xmaterials (Table 4) to pzotect against pulsed lasers.

Another grating type device suggested for use in eye/sensorprotection uses photorefractive materials. 162 In thesematerials, a weak probe beam and a strong incident beam arecrossed setting up an intensity distribution which in turnresults in a space charge density distribution as diagrammed inFig. 32. The resulting space charge electric field and indexmodulation are out of phase with respect to the intensitydistribution resulting in a phase grating. This type of effectis limited by the ability of charges to migrate and define thespace charge density. 1b J ,164 Literature reports suggest afundamental r p nse time limit on the order of picoseconds(10- 12 sec) .1 ,I64 The only materials observed to exhibitthis effect thus far are certain inorganic crystals. Thepossibility of such effects arising in organometallic and metaldoped polymeric materials has been proposed.

One final device concept involves the intensity dependentrefractive index, n2. In these beam bending devices,diagrammed in Fig. 33, high intensity light alters therefractive index of the nonlinear optical material, deflectingthe beam out of the normal optical path. While such devicesare simple in concept, they are difficult to put into practice.This is because a high intensity symmetric beam will experiencea symmetric phase shift and so will not deflect out of theoptic path. 165 A variety of methods for imparting an asymmetricprofile to the input beam have been suggested, many making useof grating type structures as diagrammed in Fig. 33.

50

OPTICAL PHASE CONJUGATION

CO"IJGATE

"C UNJUATt WAV."

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TRANSFER TO THE PCM

=> tq RADIATION PRESSURE

=> NO RECOIL

~RHCPe INCIDENT WAVE

%V

ORDINARY MIRRORREFLECTED WAVE '

LHCP

RADIATION PRESSURECAUSES MIRROR TORECOIL

Figure 31: The concept of phase conjugation is illustratedusing left and right handed circularly polarized light (LHCPand RHCP, respectively) and a phase conjugate (top of figure)and ordinary mirror (bottom of figure). This figure was adaptedfrom Ref. 161.

51

PHO TOREFRA C TIVE DEVICES

• / \ \LIGf T 13EAMS

PHOTOREFnACTIVEMEDIUM

,\ LIGHTINTENSITY

-- SPACECHAnGE

E2

Figure 32: The upper half of the figure diagrams the evolutionof the photorefractive material response. Diagrammed in thelower half of the figure is a configuration which could be usedfor eye/sensor protection. At low irradiance levels, E wouldcontinue unhindered through the crystal. However, at highirradiances, E1 would be deflected out of the optical path.

52

eye damage thresholds, We:1* For pulse durations of 1 - 18 ns: We = 0.5 pJ/cm2For longer pulse durations up to 10 se_.s:

We = 1.8 t / 4 mJ/cm 2 (t in seconds)As an example, usin the forementioned damage thresholds

and a value of 100 mW/cm for the output of the sun,2 an eyeprotection device must attenuate the light by a factorequivalent to 1 .5 optical density units (ODU) (a transmissionreduction of 97%) when directed at the sun.

PROTECTION STRATEGIES

Any successful eye/sensor protection device must interactwith and attenuate the laser light before it reaches thedetector system. The interaction of light with matter isusually classified in one of three categories: absorption,dispersion or scattering. Absorption can be an effectiveprotection strategy and representative examples of absorptiondevices under investigation include particle suspension,chalcogenide, V0 2 , Ge, and two-photon absorption activatedpower limiters. However, such devices often have reducedtransparency in the visible spectral region or unacceptableresponse times. Therefore, much of the recent research intoeye/sensor protection has focussed on using dispersion orscattering to redirect the light and this is where nonlinearoptical materials have the greatest potential for impact in thenear term: nonlinear optical materials can have unique indexof refraction properties and fast response times.

ORGANIC NONLINEAR OPTICAL MATERIALS

Nonlinear optical materials have been known and studied forover two decades with most research efforts being successfullydirected at inorganic materials, 5 , 2 5 , 2 6 in particular,inorganic crystals 2 7, glasses2 8 , 2 9 and semiconductors. 3 0 - 3 2

The most familiar example of inorganic nonlinear opticalmaterials are crystals such as potassium dihydrogen phosphate(KDP) and lithium niobate (LiNbO3 ). However, more recently,interest has focussed on such inorganic materials as tungstenbronze crystals. 3 3 With the recent emphasis on opticalcomputing and communication, a need for nonlinear opticalmaterials with better mechanical processing and physicalproperties than available in typical inorganic nonlinearoptical materials has become apparent and researchers haveturned to examine orqanic polymericmaterials .9,14,17-21 34-39 It is now generally agreed thatorganic materials have the potential for nonlinear opticaleffects which are orders of maqnitude better than currentlyused inorganic materials. 5 ,6 ,12,14,17 - 21,34,37,38, 4 - 45 Thisis based on the origin of the nonlinear optical effect inorganic materials: the easily polarized molecular electricfields. 7 , 10 ,1 4 , 1 5 Extensive research is underway on thedevelopment of nonlinear optical organic materials and a

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Figure 33: An illustration of the beam bending device concept.

53

PROPOSED FUTURE RESEARCH EFFORTS

In reviewing research efforts to develop organic nonlinearoptical materials for eye/sensor protection, several novelideas have surfaced which the authors would like to propose.

The first idea involves using stretched copolymer films toassist in enhancing nonlinear optical effects in materials.Although extensive work has been reported on copolymer systems,polymer film alignment by stretching 3 , 36 has not received muchattention. The technology for stretching films is welldeveloped; this is the way Polaroid filters and lenses areprepared. The following figure (Fig. 34) is a schematicdiagram of how the process would help to increase molecularorder in the material and thereby increase the magnitude of thenonlinear optical effect.

Another idea that has been proposed by other researchersand which may have great potential, is organic superlattices.In general, organic superlattices can be envisioned asstructures with an alternating composition on a molecularscale. This alternating composition is repeated in only onedirection so that the electrons are confined along a chain.^The following diagrams (Fig. 35 and 36) indicate how theseorganic systems would imitate the more familiar semiconductorsuperlattices, and give an idea of the type of control that maybe available via substituent groups.

Production of these types of materials via copolymer

synthesis and Langmuir-Blodgett film deposition has beensuggested. These are both areas of established expertise inthe Polymeric Materials Branch at NRL.

Finally, little information was uncovered concerninginvestigations into the poitential of inorganic polymer systemsfor use in eye and sensor pzotection. Inorganic polymers, suchas the linear polyphosphazenes and polythiazyl (Fig. 37), canexhibit extensive conjugation, a necessary condition forenhanced nonlinear optical behavior, and enhanced ancillaryproperties. For example, silicon based polymer systems shouldhave enhanced thermal damage thresholds. Additional

theoretical and experimental investigations into the potentialof polymer systems like those described above should beconsidered.

ACKNOWLEDGMENTS

One of us (MEB) was an ONT Postdoctoral Fellow during partof this work and would like to thank both the Office of NavalTechnology and the American Society for Engineering Education.We would also like to thank Dr. Filbert J. Bartoli Jr., OpticalSciences Division, Naval Research Laboratory for his helpfuldiscussions and comments.

54

STRETCH

Figure 34: The enhancement in the molecular packing, and thusthe nonlinear optical effect, of a copolymer film viastretching is diagramed. The rectangles represent thenonlinear optical units or blocks in the copolymer.

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94