Nonlinear optical phenomena have been investigated over the last three decades in many different materials and a great deal of progress has been made in both the basic science and device applications.(1] Although interest in nonlinear organic materials dates back to the early days of nonlinear optics, it is only in the last five years that progress has been sustained and rapid. The purpose of this paper is to examine progress in developing new nonlinear organic materials and in their application to devices for comparison with current state-of-the-art devices using other material systems.

The exceptional electro-optic properties of poled polymer films, coupled with the power and flexibility of thin film fabrication and photolithographic processing, may make possible a new class of integrated optic systems combining the processing power of VLSI with a dense, high bandwidth, photonic interconnection and switching network. We report on the recent development and initial test results of two electro-optic polymer based integrated optic devices for optical interconnection applications. The first is an optical railtap for the distribution of many different optical signals from a single CW laser diode, and the second is a traveling wave Mach-Zehnder integrated optic modulator, which was modulated at frequencies up to 20 GHz.

The non-resonant third order nonlinearity of conjugated polymers appears to be potentially useful for all-optical devices in waveguide formats.[l,2] This nonlinearity manifests itself as an intensity-dependent refractive index which leads to a nonlinear phase shift over some propagation distance. Device research over the last few years has shown that there are certain minimum requirements for the nonlinear phase shift that need to be achieved over one absorption length of the material.[l,3] There are two principal sources of absorption, the usual linear absorption which is independent of fluence, and two photon absorption for which the absorption scales linearly with intensity. Thus the usefulness of a nonlinear material for all-optical switching devices can be evaluated from a limited number of material parameters, namely n2 (in n = n0 + n2I where I is the local intensity), α0 which is the low power absorption coefficient and β which is the two photon coefficient (in α = α0 + βI). The problem for a given material is to identify spectral regions over which the minimum required phase shift can be achieved.

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

Our approach to the problem is to develop an understanding of the molecular basis of non-linear optical activity, incorporate the most promising candidates into tractable polymers, and evaluate these materials in device format. This paper will review some of the recent developments in the NLO materials effort at Hoechst Celanese.

The explosive growth under way in the performance of digital electronic systems is directly traceable to the impact of Si MOS device scaling and its accompanying increase in functionality at the chip level. The increasing data rates and chip I/O that have accompanied this scaling have driven the evolution of both system architectures and the interconnection and packaging technologies (i.e. chip carriers, printed circuit boards, bus structures) that support this CMOS functionality and dominate overall system volume. The further scaling of CMOS technology towards 0.1 – 2.5μm minimum gate lengths coupled with modest increases in chip size (i.e. 2–4 cm), promise upwards of a 100 fold increase in chip-level complexity. The resulting emergence of Ultra Large Scale Integrated (ULSI) processor array chips and wafer-scale memory (solid state disks) hold the potential for extremely compact distributed computing systems. Through a continuation of the evolutionary trends of system scaling, application of chip level process technology to higher system levels, and mixed technology integration (e.g. BiCMOS); present advanced packaging technologies based upon Multi-Chip Modules (MCM) will mature into hybrid wafer-level three-dimensional silicon systems allowing burdensome driver and communication control functions now designed at the chip level to move off chip into the active silicon interconnection substrate where network transmission and control can both be implemented. Realization of the performance potential of these silicon ULSI systems will largely depend on the successful implementation of this wafer-level communication network linking the high-density of processing nodes. The role that any advanced technology (i.e. high-speed normal electronic, superconducting, optical) will serve within this network will be determined by the degree of connectivity it can achieve in this scaled environment and by its ability to benignly coexist with the dominant CMOS technology, the network's physical and functional foundation[l].

Recently, advanced photonic devices have been fabricated in the laboratory and are becoming commercially available. Thus, there is considerable interest in inexpensive but efficient non-linear optical (NLO) materials that are simple to make and work with. In the last three years a large number of publications and patents have appeared describing NLO properties of organic materials, usually dyes, incorporated into or synthetically attached to polymers [1]. Such materials must be oriented before they have second-order NLO activity. Two methods have been used. In one, contact poling [2–5], two electrodes are formed on or in the material and an electric field is placed between them. In the other, corona poling, a discharge deposits charge on the polymer, which creates a strong orienting field [6–8]. One could generalize that contact poling is (more) difficult to do, but the results are easy to understand, while corona poling is simple to do, but the results are (more) difficult to understand. This paper describes a set of corona poling experiments.

We have used the highly sensitive technique of Photothermal Deflection Spectroscopy (PDS) to measure changes in the infra-red absorption spectra of MEHPPV, P3HT and Polydiacetylene-4BCMU induced by pumping these polymers with light above the π - π* transition energy. In contrast to previous chopped light transmission measurements of these effects, the PDS technique can 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 that above-gap light causes the appearance of a broad infra-red peak in α, which continues to grow-in hours after the pump light is first applied. For this polymer the general shape of the absorption spectra in the unpumped state mimics the photo-induced changes, suggesting that remnant photo-induced states determine the maximum transparency observed under normal experimental conditions. For P3HT and to a lesser extent, MEHPPV, we also observe irreversible photo-induced absorption components which we tentatively identify with photo-induced oxidation of the polymer matrix.

Along with χ(3), the magnitude of the optical absorption in the transparent window below the principal absorption edge is an important parameter which will ultimately determine the utility of conjugated polymers in active integrated optical devices. With an absorptance sensitivity of < 10-5, Photothermal Deflection Spectroscopy (PDS) is ideal for determining the absorption coefficients of thin films of “transparent” materials. We have used PDS to measure the optical absorption spectra of the conjugated polymers poly[1,4-phenylenevinylene] (and derivitives) and polydiacetylene-4BCMU in the spectral region from 0.55 eV to 3 eV. Our spectra show that the shape of the absorption edge varies considerably from polymer to polymer, with polydiacetylene-4BCMU having the steepest absorption edge. The minimum absorption coefficients measured varied somewhat 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 further improvements in transparency in this spectral region might be achieved via deuteration or fluorination.

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

With the enhanced sensitivity of Photothermal Deflection Spectroscopy (PDS), vibrational overtone spectra have been obtained in organic thin films for the first time, covering the range from the near IR to the visible (0.4 - 2 eV). The absorption spectra of polycarbonate, for instance, exhibits C-I1 stretch modes for Δn> I where n is the vibrational quantum level. Previous measurements were constrained to either long fibers, which pose the problem of various scattering losses not related to absorption, or to a restricted wavelength region in the visible accessible by dye laser sources. The unparalleled sensitivity of PDS allows precise determination of frequency, lineshape and intensity of the various modes, even for films ∼10μm thick over a broad energy range. The overtone spectra can be used as a probe of various basic molecular properties. In a manner similar to NMR spectroscopy, it is possible to study specific atomic bonds.

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

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

For potential optoelectronic applications, new polymeric thin films containing nonlinear optical chromophores aligned by electric field poling for a χ(2) nonlinear optical response have been prepared and studied. Spectroscopic and optical waveguiding techniques, as well as second harmonic generation and electrooptic measurements, are being used to characterize these thin film materials. Here we optimize the chromophores in spectral response, nonlinearity, concentration and degree of alignment. Stability in the nonlinearity has been improved considerably by attaching the chromophores to crosslinked epoxy polymers. Some applications of these materials for electrooptic phase modulators will be presented.

Planar wavcguides were made from a linear epoxy copolymer of bisphcnol-A diglycidyl ether and amino-nitro-tolanc. The refractive indices, ni-M and n-E, and the clectrooptic coeflicients, r33 and r13, of the poled nonlinear optical polymcr films were determined by measurements of the wavcguide modes of these films at a wavelength of 632.8nm, and the results arc reported for both electrode and corona poling. The ratio of the second order susceptibilities was found to exceed significantly the theoretically expected value of 3. Restricted motion of the nonlinear chromophorcs during poling provides a possible explanation of this discrepancy.

We have used a finite element method to calculate the nonlinear polarization of small nematic droplets, for both the radial and bipolar director configurations. The nonlinear polarization of droplets with a radial director is concentrated along the droplet axis, while for the bipolar droplets, the nonlinear polarization is more uniform across the droplet. The nonlinear polarization of the bipolar configuration has also been shown to be sensitive to the angle between the droplet axis and the applied field.

Photorefractive materials comprise an important category of electrooptic materials, and are important constituent elements in a wide range of devices designed specifically for use in optical information processing and computing systems. Critical issues affecting the development and applicability of photorefractive materials are examined from the perspective of photonic neural network implementations that incorporate photorefractive volume holographic interconnections.

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

Photochromism in barium titanate was found to affect its deep and shallow trap properties. Argon ion laser illumination at 514.5 nm photo-induced semi-permanent (thermally reversible) changes in the visible absorption spectrum of barium titanate that were stable at room temperature. Increases in absorption at 514.5 nm, also the operating wavelength used to determine photorefractive properties, caused a decrease in the photorefractive response time but also a decrease in the sensitivity. When the photochromism was inactive the crystal shows light-induced absorption. With the photochromism saturated the crystal showed light-induced transparency. A model with two photoactive levels is used to characterize the photorefractive properties of this photochromic crystal.

A series of cobalt-doped barium titanate crystals were grown that have high beam-coupling gain. Our photorefractive characterization shows that with increasing cobalt concentration in the crystal a deep level is formed and the relative effect of shallow levels on the space-charge field amplitude diminishes. This gives the crystal a long dark storage time. We find that cobalt-doping in barium titanate increases the two-beam coupling gain by reducing the dark conductivity and increasing the total effective trap density.

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

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