Polymeric thin films containing asymmetrical chromophores have been under investigation for their second-order nonlinear optical (NLO), piezoelectric, and/or pyroelectric properties for over ten years. This area of research has yielded several types of versatile polymers useful for applications such as optical second harmonic generation and electro-optic modulation of optical signals. The first generation organic NLO polymers (NLOPs) were comprised of small asymmetrical chromophores dissolved in glassy polymers (guest-host systems). The materials science has since evolved to sidechain and mainchain systems in which the asymmeteric chromophore is chemically attached to the polymer, as discussed in more detail below.
Optical nonlinearity
To a first approximation, it is the molecular structure of the chromophore and its orientation that dictate the nonlinear optical properties of the system, and it is the polymer structure that dictates the processability and temporal stability of the final product. Second-order nonlinear optical films do not have a center of symmetry. In order to understand the symmetry requirements, one must consider the three-dimensionality of the electric polarizability in the material, the directions of the applied electric fields, and the electric fields of light rays passing through the material. A treatment most often used to describe this interaction is the following nonlinear polarization equation (a power series in electric field strength, E: EQU P=.sub..chi..sup.(1) E+.sub..chi..sup.(2) EE+.sub..chi..sup.(3) EEE+ . . .
wherein P is the polarization in the material, .chi..sup.(1) is the coefficient for linear interaction with E, .chi..sup.(2) is the second-order nonlinear optical coefficient, and .chi..sup.(3) is the third-order nonlinear optical coefficient. The second-order nonlinear optical coefficient is a third-rank tensor whose tensor elements are all zero in materials possessing the center-of-inversion symmetry.
In order for organic films to have large second-order nonlinear optical coefficients, .chi..sup.(2), they must contain a high concentration of asymmetrical, highly hyperpolarizable chromophores, arranged in a highly polarized configuration. NLOP chromophores must have an inherently large molecular second-order nonlinearity, .beta. (also called second-order susceptibility, quadratic susceptibility, and first hyperpolarizability).
For predicting the magnitude of the first molecular hyperpolarizability coefficient beta (.beta.), to a first approximation, considering only the ground state of the chromophore and the first excited singlet state leads to a very successful two-state model. The most important requirement for a large beta is that the asymmetrical chromophore has a low energy of transition between ground and first excited singlet state. It is also necessary that there be a large change in dipole moment in going between the ground state to the first excited singlet state, and that the transition dipole moment be large. In general, large II-conjugated links (e.g., vinylenes, phenylenes, thienylenes, etc.) connecting properly balanced electron donor and acceptor groups are desirable. In this disclosure, "NLOP" is used only in reference to polymers that can be formed into films that exhibit significant second-order optical nonlinearity, that is, the NLOP .chi..sup.(2) is at least three times greater than the .chi..sup.(2) of quartz which is about 1.0 pm/V.
The following mathematical treatment can be applied to an isolated chromophore's dipole moment: .mu.=.mu..sub.o +.alpha.E+.beta.EE+ . . . , where .mu. is the dipole moment upon applying the electric field E, .mu..sub.o is the ground state dipole moment, .beta. is a third rank tensor describing the chromophore's second order susceptibility, which is also called the quadratic hyperpolarizability or first hyperpolarizability.
Guest-Host NLOP
The simplest form of nonlinear optical polymer, called guest-host (G-H) systems, are solid solutions of small chromophoric molecules and high molecular weight polymers. Generally the G-H system contains about 10 to 20% by weight of the chromophore (higher levels tend to phase separate or have lower glass transition temperatures due to plasticization). The chromophores in G-H systems are somewhat labile (they diffuse by translation and rotation) and they evaporate or sublime at elevated temperatures. Small chromophores, which are easily absorbed through the skin, can be toxic, mutagenic, teratogenic, and carcinogenic. High molecular weight polymers cannot be absorbed through the skin. Therefore, by attaching the chromophore to the polymer, the lability and toxicity problems are solved.
Sidechain NLOP
Sidechain (SC) polymers are comprised of assymetric chromophores which are chemically linked or tethered to the backbone at one end of the chromophore, e.g., at the electron accepting end or at the electron donating end, and the majority of the chromophore is pendent to the backbone. Most of the early reported SC polymers were derived from the free radical copolymerization of (meth) acrylic functional chromophores and methyl methacrylate. Some SC polymers have been designed for Langmuir-Blodgett deposition, e.g., the polymers described in U.S. Pat. No. 5,162,453, issued Nov. 10, 1992 to Hall et al, and U.S. Pat. No. 5,225,285, issued Jul. 6, 1993 to the same inventors, which are described below and incorporated herein by reference.
Mainchain NLOP
Mainchain (MC) polymers are comprised of asymmetrical chromophores which are linked in the backbone at both ends of the chromophore, and the majority of the chromophore forms part of the backbone, the rest of the backbone being connecting groups, bridging one chromophore to the next. The asymmetrical chromophores can be linked in a head-to-tail configuration (isoregic), in a head-to-head configuration (syndioregic), or randomly head-to-tail and head-to-head (aregic). The mainchain chromophoric topology is inherently more stable than the sidechain topology because the chromophore has one less degree of freedom of motion.
Stability
Geometric considerations, such as how to orient the asymmetrical chromophore in a polar alignment, how to increase the glass transition temperature of the NLOP, and how to prevent relaxation of the chromophore in NLOPs are of utmost importance for improving long term optical stability. There are two types of stability. Physical stability refers to stability of alignment of the chromophore. Chemical stability refers to the integrity of the original chemical structure of the chromophore. Chemical changes can be brought about by thermal energy, electromagnetic energy (photons), reactions with oxygen or other chemicals, or any combination of the above. Chemical changes can range from isomerization (cis--trans), to cyclization, to oxidation, to bond cleavage, or to free radical additions, all of which change the refractive index and .chi..sup.(2) properties. The temporal stability of a .chi..sup.(2) NLOP polymer film refers to how well the physical stability(alignment) and chemical stability of the chromophores are maintained at a given temperature. The processing stability refers to how well the polymer holds up under film processing and packaging conditions.
In order to mass produce low cost integrated devices, reliable thin film processing techniques are necessary for integration with silicon chip manufacturing. If these polymers can be easily processed, their uniquely low dielectric constant and large second-order nonlinear optical coefficient (.chi..sup.(2)) will ensure their strong competition for high speed optical signal processing applications. Several processes for giving polar order to the chromophores in a polymer film exist. These include electric-field poling and self-assembly techniques, such as Langmuir-Blodgett (LB) processing.
Electric-Field Poling
Thin films of the polymer are prepared for poling by spin-coating a liquid solution containing about 10% of the polymer onto a solid substrate. The solvent is removed by baking the film just above the glass transition temperature (T.sub.g). An electric field is applied across the film by corona poling the film as it sits on a grounded conductor plane, or by charging two electrodes contacting the film. There may also be thin cladding or buffer layers between the electrodes and the nonlinear optical polymer. The films are typically poled at a field strength of about 50 to several hundred volts per micron. The electric field is applied at or above T.sub.g for 10 to 60 minutes, then the film is cooled with the field on. After the external field is removed, a net alignment of dipole moments can remain essentially locked in the film for years as long as the temperature of the film remains well below any solid state transition, such as the T.sub.g. This imparts noncentrosymmetry to the film which is necessary for its second-order nonlinear optical properties.
There are major problems with electric-field poling. The polymer must be heated to quite high temperatures at which the Brownian motion and rotation of the molecules works against the torque exerted by the electric field on the dipole moment of the chromophores. Hence, at higher temperatures, the chromophores will be less well ordered (i.e., the degree of order .apprxeq..mu.E/kT, where .mu. is the ground state dipole moment of the chromophore, E is the applied electric field, k is Boltzmann's constant, and T is the temperature). Polymers containing formal charges are difficult to pole with an electric field because the charges tend to migrate through the polymer and short-out the electrode. It is difficult to have layers with opposed orientations of the chromophores within the same polymer film as is sometimes desirable for phase matching in frequency doubling applications.
When chromophores in a molten polymer film are exposed to an electric field, the torque of the applied field on the dipoles of the chromophores aligns the average direction of their ground state dipole moments parallel to the direction of the applied field. The largest component of the .beta. tensor of a chromophore is usually at some angle away from the direction of the ground state dipole moment of the chromophore, but "goes along for the ride" and becomes aligned (on the average) as the dipoles are aligning with the electric field. .chi..sup.(2) is a tensor quantity describing the second-order nonlinear optical properties of the film, and is a result of the sum of the .beta. tensors of the individual chromophores. Hence, for electric-field-poled films, the .chi..sup.(2) tensor has conical symmetry around the direction of the applied electric field. Thus, the largest component of the .chi..sup.(2) tensor (e.g., d.sub.33 for corona poling) results from those parts of the largest components of the .beta. tensors that are projected onto the direction of the applied electric field. Estimates of .beta. from molecular orbital calculations, combined with estimates of the conical angle about the poling direction from birefringence measurements, have resulted in reasonably accurate d.sub.33 predictions.
Langmuir-Blodgett (LB) Processing
To make films by LB processing, an organic compound is floated on a liquid, usually water, in a trough which holds the water. A solid substrate is dipped through the air-water interface depositing on the substrate a single molecular layer. Thicker films comprised of multilayers of polymer are built up by dipping the substrate repeatedly into and/or out of the LB trough, depositing one molecular layer per stroke. Turn-key, computer-automated, multi-compartment troughs are available from many commercial suppliers, such as NIMA (Coventry CV4 7EZ, England), NLE (Nagoya, 468, Japan), and KSV (SF-00380 Helsinki, Finland).
In LB processing, the polymer molecules are designed to have hydrophilic and hydrophobic groups which cause the polymer to float in one conformation on the water. These hydrophilic/hydrophobic forces are useful for removing the centrosymmetry by orienting the asymmetrical chromophores with respect to the plane of the film.
Multilayer LB films can be formed in three different configurations. Historically these are called "X"-, "Y"- and "Z"-type films, where X is made by depositing always on the down-stroke, Z is made by depositing always on the up-stroke, and Y is made by alternating up- and down-strokes. For the case in which the large dipole moment of the chromophore is normal to the plane of the substrate, all-up or all-down films will be polarized, and the up-down films will not be polarized (dipoles in adjacent layers cancel out). However, in Y-type deposition, sometimes the chromophores will align in a herring-bone pattern due to shear forces in the dipping direction caused by pulling and pushing the substrate through the film floating on the water or by local packing considerations. The herring-bone pattern results in a net polarization in the plane of the film, and can be detected by the generation of second harmonic light. The Y configuration is thermodynamically more stable (e.g., sometimes X and Z configurations spontaneously rearrange in the solid state to the Y configuration). Another way to make polar films by Y-type deposition is to alternately interleave asymmetrical chromophoric polymer layers with optically inert polymer layers. Or one may use a two-compartment LB trough, building up bilayers in which the two polymers are configured with the charge transfer axis of their chromophores respectively pointing in the opposite sense with regard to the hydrophilic and hydrophobic parts of the polymer to which they are attached.
Molecular weight (and its distribution) is one of several key parameters which affect the physical behavior of polymer molecules in monolayer films at the air-water interface (Langmuir films). The modulus and viscosity of Langmuir films under surface compression vary greatly with molecular weight.
LB processing has the advantage over electric-field poling in that it can be done at room temperature (or lower); hence, the kT Brownian motion is much less. Furthermore, formal ionic charges on the polymer will not hinder the ordering process; in fact, they can be taken advantage of in designing hydrophilicity into the polymer. This film processing procedure has been reduced to practice for sidechain polymers, but not for mainchain chromophoric polymers. However, mainchain polymers are inherently more stable than sidechain polymers because both ends of the chromophore are attached to the backbone of the polymer, whereas only one end of the chromophore is attached to the backbone in a sidechain polymer, and the other end is dangling free at one of the interfaces of each monolayer. Furthermore, only mainchain polymers that contain the asymmetrical chromophore in a head-to-head (syndioregic) configuration can have the chromophore disposed in a polar alignment normal to the plane of the film by hydrophilic/hydrophobic forces. This is because a mainchain polymer containing chromophores in a head-to-tail configuration will have its chromophores lying on the water, in the plane of the film, and the chains can be pointing in opposite directions. The utility of the syndoregic configuration for L.B. processing will become clear in the detailed description of the invention below.
U.S. Pat. No. 5,162,453, issued Nov. 10, 1992 to Hall et al, and U.S. Pat. No. 5,225,285, issued Jul. 6, 1993 to the same inventors, describe a multilayered polarized film (AB).sub.n, fabricated by the Langmuir-Blodgett technique, in the Y-type configuration, comprising n number of bilayers AB. Each bilayer AB consists of a first layer of a sidechain chromophoric polymer A having the electron accepting end of the sidechain chromophore attached to the polymer backbone, and a second layer of another sidechain chromophoric polymer B having the electron donating end of the chromophore attached to the polymer backbone.
U.S. Pat. No. 5,247,055, issued Sep. 21, 1993 to Stenger-Smith et al, and incorporated herein by reference, describes new syndioregic chromophoric polymers, their self assembly on a surface, and electric field poling to achieve polar films exhibiting second-order nonlinear optical properties. U.S. Pat. No. 5,247,055 teaches the use of a single polymer for all layers. This patent also teaches that when monolayers are built up, one upon the next, by the X or Z types of LB deposition processes, one monolayer may be chemically cross-linked to the next monolayer. For example, carboxyl groups in one monolayer may react with epoxy groups in the next monolayer. Although not discussed in the patent, such chemical cross-linking of adjacent monolayers may override the thermal instability of identical polymer layers due to adjacent hydrophobic/hydrophilic domains and keep their dipole moments pointing in the same direction.
For films that are processed by LB or other techniques that align the chromophores by hydrophobic-hydrophilic interactions, the dipole moment of the chromophore has little influence on the actual aligning process (admittedly, dipole-dipole repulsion may have a secondary effect on alignment if the polymer is improperly designed). The largest component of the .beta. tensor generally lies along the long axis of the chromophore, which is also the charge transfer axis of the chromophore. The hydrophilic and hydrophobic groups that force alignment of the asymmetrical chromophore at an interface are positioned at the extreme ends of the long axis of the chromophore. The average direction of alignment of the long axes of the chromophores can be assumed to lie nearly perpendicular to the film surface if hydrophilic-hydrophobic forces are the only forces important to the alignment of the chromophores. However, in LB processing of polymers, there is usually an additional ordering force due to the shearing of the polymer chains as they are pulled from the air-water interface--the polymer backbones tend to align in the dipping direction. In this case, there may also be a preferred azimuthal angle of alignment of the chromophores. The average alignment of the chromophores can be estimated from 3-dimensional birefringence measurements (e.g., from polarized UV-VIS absorption due to the charge transfer oscillation in the chromophore, or from polarized second-harmonic generation measurements).
The dipoles of the chromophores do become aligned during LB processing (i.e., they "go along for the ride"). Therefore, one can still refer to these films as having "polar order." For the sake of brevity and convenience, in this patent application, the alignment of chromophores in the films will be referred to as imparting "polar order" to the films, terminology often found in the literature. As we are interested in the second-order nonlinear optical properties of the films, it will be understood that referring to their "polar order" also implies (because of the design of the accordion polymer) that the .chi..sup.(2) tensor of the film is relatively large.