Nonlinear optical phenomena have been investigated over the last threedecades in many different materials and a great deal of progress has beenmade in both the basic science and device applications.(1] Although interestin nonlinear organic materials dates back to the early days of nonlinearoptics, it is only in the last five years that progress has been sustainedand rapid. The purpose of this paper is to examine progress in developingnew nonlinear organic materials and in their application to devices forcomparison with current state-of-the-art devices using other materialsystems.

The exceptional electro-optic properties of poled polymer films, coupledwith the power and flexibility of thin film fabrication andphotolithographic processing, may make possible a new class of integratedoptic systems combining the processing power of VLSI with a dense, highbandwidth, photonic interconnection and switching network. We report on therecent development and initial test results of two electro-optic polymerbased integrated optic devices for optical interconnection applications. Thefirst is an optical railtap for the distribution of many different opticalsignals from a single CW laser diode, and the second is a traveling waveMach-Zehnder integrated optic modulator, which was modulated at frequenciesup to 20 GHz.

The non-resonant third order nonlinearity of conjugated polymers appears tobe potentially useful for all-optical devices in waveguide formats.[l,2]This nonlinearity manifests itself as an intensity-dependent refractiveindex which leads to a nonlinear phase shift over some propagation distance.Device research over the last few years has shown that there are certainminimum requirements for the nonlinear phase shift that need to be achievedover one absorption length of the material.[l,3] There are two principalsources of absorption, the usual linear absorption which is independent offluence, and two photon absorption for which the absorption scales linearlywith intensity. Thus the usefulness of a nonlinear material for all-opticalswitching devices can be evaluated from a limited number of materialparameters, namely n2 (in n = n0 + n2Iwhere I is the local intensity), α0 which is the low powerabsorption coefficient and β which is the two photoncoefficient (in α = α0 + βI). The problem for agiven material is to identify spectral regions over which the minimumrequired phase shift can be achieved.

Organic and polymeric materials have emerged in recent years as candidatesfor advanced device and systems applications. This interest has arisen fromthe promise of extraordinary optical, structural, and mechanical propertiesof certain organic materials, and from the fundamental success of moleculardesign performed to create new materials.

Our approach to the problem is to develop an understanding of the molecularbasis of non-linear optical activity, incorporate the most promisingcandidates into tractable polymers, and evaluate these materials in deviceformat. This paper will review some of the recent developments in the NLOmaterials effort at Hoechst Celanese.

The explosive growth under way in the performance of digital electronicsystems is directly traceable to the impact of Si MOS device scaling and itsaccompanying increase in functionality at the chip level. The increasingdata rates and chip I/O that have accompanied this scaling have driven theevolution of both system architectures and the interconnection and packagingtechnologies (i.e. chip carriers, printed circuit boards, bus structures)that support this CMOS functionality and dominate overall system volume. Thefurther scaling of CMOS technology towards 0.1 – 2.5μmminimum gate lengths coupled with modest increases in chip size (i.e. 2–4cm), promise upwards of a 100 fold increase in chip-level complexity. Theresulting emergence of Ultra Large Scale Integrated (ULSI) processor arraychips and wafer-scale memory (solid state disks) hold the potential forextremely compact distributed computing systems. Through a continuation ofthe evolutionary trends of system scaling, application of chip level processtechnology to higher system levels, and mixed technology integration (e.g.BiCMOS); present advanced packaging technologies based upon Multi-ChipModules (MCM) will mature into hybrid wafer-level three-dimensional siliconsystems allowing burdensome driver and communication control functions nowdesigned at the chip level to move off chip into the active siliconinterconnection substrate where network transmission and control can both beimplemented. Realization of the performance potential of these silicon ULSIsystems will largely depend on the successful implementation of thiswafer-level communication network linking the high-density of processingnodes. The role that any advanced technology (i.e. high-speed normalelectronic, superconducting, optical) will serve within this network will bedetermined by the degree of connectivity it can achieve in this scaledenvironment and by its ability to benignly coexist with the dominant CMOStechnology, the network's physical and functional foundation[l].

Recently, advanced photonic devices have been fabricated in the laboratoryand are becoming commercially available. Thus, there is considerableinterest in inexpensive but efficient non-linear optical (NLO) materialsthat are simple to make and work with. In the last three years a largenumber of publications and patents have appeared describing NLO propertiesof organic materials, usually dyes, incorporated into or syntheticallyattached to polymers [1]. Such materials must be oriented before they havesecond-order NLO activity. Two methods have been used. In one, contactpoling [2–5], two electrodes are formed on or in the material and anelectric field is placed between them. In the other, corona poling, adischarge deposits charge on the polymer, which creates a strong orientingfield [6–8]. One could generalize that contact poling is (more) difficult todo, but the results are easy to understand, while corona poling is simple todo, but the results are (more) difficult to understand. This paper describesa set of corona poling experiments.

We have used the highly sensitive technique of Photothermal DeflectionSpectroscopy (PDS) to measure changes in the infra-red absorption spectra ofMEHPPV, P3HT and Polydiacetylene-4BCMU induced by pumping these polymerswith light above the π - π* transition energy. In contrast to previouschopped light transmission measurements of these effects, the PDS techniquecan directly measure the buildup or decay of the absorption coefficient, m,on the time scale of seconds to days. In the case of MEHPPV we observe thatabove-gap light causes the appearance of a broad infra-red peak in α, whichcontinues to grow-in hours after the pump light is first applied. For thispolymer the general shape of the absorption spectra in the unpumped statemimics the photo-induced changes, suggesting that remnant photo-inducedstates determine the maximum transparency observed under normal experimentalconditions. For P3HT and to a lesser extent, MEHPPV, we also observeirreversible photo-induced absorption components which we tentativelyidentify with photo-induced oxidation of the polymer matrix.

Along with χ(3), the magnitude of the optical absorption in thetransparent window below the principal absorption edge is an importantparameter which will ultimately determine the utility of conjugated polymersin active integrated optical devices. With an absorptance sensitivity of< 10-5, Photothermal Deflection Spectroscopy (PDS) is idealfor determining the absorption coefficients of thin films of “transparent”materials. We have used PDS to measure the optical absorption spectra of theconjugated polymers poly[1,4-phenylenevinylene] (and derivitives) andpolydiacetylene-4BCMU in the spectral region from 0.55 eV to 3 eV. Ourspectra show that the shape of the absorption edge varies considerably frompolymer to polymer, with polydiacetylene-4BCMU having the steepestabsorption edge. The minimum absorption coefficients measured variedsomewhat with sample age and quality, but were typically in the range 1 cm-1 to 10 cm-1. In the region below 1 eV,overtones of C-H stretching modes were observed, indicating that furtherimprovements in transparency in this spectral region might be achieved viadeuteration or fluorination.

Using the sensitive absorption technique of Photothermal DeflectionSpectroscopy (PI1S) it is possible to detect subtle spectral features in theabsorption spectra of polymeric thin films that cannot be detected by othermeans. This paper describes observations of weak charge-transfer complex(CIC) formation and also light-induced changes in various organic films. Thefilns studied were those commonly used as charge transport materials inorganic photoconductors, comprised of a small transport molecule and apolymer binder. The optically measured CTC bands can be used to establishsingle electron energy levels of the constituents. Reproduciblephoto-induced changes are manifest both in the strong UV transitions as wellas in the “sub-gap” absorption and appear to be correlated with thephoto-chemical stability of the constituents. The light-fatigue has both areversible as well as irreversible component.

With the enhanced sensitivity of Photothermal Deflection Spectroscopy (PDS),vibrational overtone spectra have been obtained in organic thin films forthe first time, covering the range from the near IR to the visible (0.4 - 2eV). The absorption spectra of polycarbonate, for instance, exhibits C-I1stretch modes for Δn> I where n is thevibrational quantum level. Previous measurements were constrained to eitherlong fibers, which pose the problem of various scattering losses not relatedto absorption, or to a restricted wavelength region in the visibleaccessible by dye laser sources. The unparalleled sensitivity of PDS allowsprecise determination of frequency, lineshape and intensity of the variousmodes, even for films ∼10μm thick over a broad energy range. The overtonespectra can be used as a probe of various basic molecular properties. In amanner similar to NMR spectroscopy, it is possible to study specific atomicbonds.

We have fabricated acrylic based optical channel waveguides using proximityphotolithography as well as laser direct writing. The cladding layer is aphotosensitive aliphatic urethane dimethacrylate and the guiding layer is aphotosensitive aromatic acrylated epoxy. This material system provides goodadhesion to a variety of substrate materials. Since both the guiding andcladding layers are applied, these materials can be employed in severalelectrical/optical applications including multi-chip modules using Si, SiO2, and polyimide as well as high speed electronic boardtechnologies using teflon based substrates.

Loss measurements show a guide loss of less than 0.08 dB/cm for multi-modewaveguides fabricated using the direct write laser technique.Lithographically defined guides have a loss of 0.3 dB/cm for similar sizewaveguides.

For potential optoelectronic applications, new polymeric thin filmscontaining nonlinear optical chromophores aligned by electric field polingfor a χ(2) nonlinear optical response have been prepared andstudied. Spectroscopic and optical waveguiding techniques, as well as secondharmonic generation and electrooptic measurements, are being used tocharacterize these thin film materials. Here we optimize the chromophores inspectral response, nonlinearity, concentration and degree of alignment.Stability in the nonlinearity has been improved considerably by attachingthe chromophores to crosslinked epoxy polymers. Some applications of thesematerials for electrooptic phase modulators will be presented.

Planar wavcguides were made from a linear epoxy copolymer of bisphcnol-Adiglycidyl ether and amino-nitro-tolanc. The refractive indices, ni-M andn-E, and the clectrooptic coeflicients, r33 and r13,of the poled nonlinear optical polymcr films were determined by measurementsof the wavcguide modes of these films at a wavelength of 632.8nm, and theresults arc reported for both electrode and corona poling. The ratio of thesecond order susceptibilities was found to exceed significantly thetheoretically expected value of 3. Restricted motion of the nonlinearchromophorcs during poling provides a possible explanation of thisdiscrepancy.

We have used a finite element method to calculate the nonlinear polarizationof small nematic droplets, for both the radial and bipolar directorconfigurations. The nonlinear polarization of droplets with a radialdirector is concentrated along the droplet axis, while for the bipolardroplets, the nonlinear polarization is more uniform across the droplet. Thenonlinear polarization of the bipolar configuration has also been shown tobe sensitive to the angle between the droplet axis and the appliedfield.

Photorefractive materials comprise an important category of electroopticmaterials, and are important constituent elements in a wide range of devicesdesigned specifically for use in optical information processing andcomputing systems. Critical issues affecting the development andapplicability of photorefractive materials are examined from the perspectiveof photonic neural network implementations that incorporate photorefractivevolume holographic interconnections.

Photorefractive crystals are great for image processing, beam amplification,and phase conjugation. I will describe these and a number of other newapplications of photorefractive crystals. I will also show how light beamsin these crystals are like politicians: if given the chance they will alwayschoose the path that maximizes their own gain.

Photochromism in barium titanate was found to affect its deep and shallowtrap properties. Argon ion laser illumination at 514.5 nm photo-inducedsemi-permanent (thermally reversible) changes in the visible absorptionspectrum of barium titanate that were stable at room temperature. Increasesin absorption at 514.5 nm, also the operating wavelength used to determinephotorefractive properties, caused a decrease in the photorefractiveresponse time but also a decrease in the sensitivity. When the photochromismwas inactive the crystal shows light-induced absorption. With thephotochromism saturated the crystal showed light-induced transparency. Amodel with two photoactive levels is used to characterize thephotorefractive properties of this photochromic crystal.

A series of cobalt-doped barium titanate crystals were grown that have highbeam-coupling gain. Our photorefractive characterization shows that withincreasing cobalt concentration in the crystal a deep level is formed andthe relative effect of shallow levels on the space-charge field amplitudediminishes. This gives the crystal a long dark storage time. We find thatcobalt-doping in barium titanate increases the two-beam coupling gain byreducing the dark conductivity and increasing the total effective trapdensity.

This is a review of how and why modem epilayer growth techniques have led toa novel class of optical devices, the Quantum Well SEED's, and of how andwhy these devices, in turn, have enabled experiments on novel systemarchitectures in which digital information is processed or routed inparallel optically, to take advantage of the large space-bandwidth productof free-space optical interconnects.

The basic issues involved in the growth by molecular beam epitaxy (MBE) ofvertical-cavity surface-emitting lasers (VCSELs) are discussed. SuccessfulVCSEL optical information processing applications demonstrated to date arediscussed, including two-dimensional arrays, holographic memory retrieval,optical addressing, wavelength division multiplexing, and tunable VCSELoperation.