Chapter 1
Second-Order Nonlinear Optical Polymers An Overview
Downloaded by RADBOUD UNIV NIJMEGEN on December 1, 2014 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch001
Geoffrey A. Lindsay U.S. Navy, Code 474220D, NAWCWPNS, China Lake, CA 93555-6001
The science and technology of second-order nonlinear optical polymers (NLOP) are briefly reviewed in an attempt to explain: (1) why these materials are important; (2) the origin of optical nonlinearity; (3) the strengths and weaknesses of various polymer topologies; (4) thin film processing techniques; (5) how to create and measure polar order (noncentrosymmetry); (6) stability issues; (7) fabrication of waveguides; and (8) useful devices that can be made from NLOP.
Organic polymeric thin films for photonic applications has been an exciting, rapidly evolving area of research over the last decade (i). This book is focused on one class of materials within this field: second-order nonlinear optical polymeric (NLOP) films (2). Recent developments on NLOP films portend exciting new possibilities for low cost integrated devices for the telecommunication and datacommunication industries, although NLOP devices are not yet commercial. At the heart of the new capabilities are electro-optic (EO) waveguides made from polymer films that switch optical signals from one path to another and modulate the phase or amplitude of an optical signal at greater than 40 GHz. NLOP films can also be used for sum-difference frequency generation (e.g., frequency-doubling). Typical NLOP films are glassy polymers containing asymmetric chromophores (also called dyes), which point generally in the same direction, making the film asymmetrically polarizable. Several processes for aligning the chromophores are under development, including electric-field poling (at the softening temperature) and various self-assembly techniques. The NLOP film is dielectric and undergoes a change in index of refraction when an electric field is applied across it, which is the property of interest for optical switching and modulating.
This chapter not subject to U.S. copyright Published 1995 American Chemical Society In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by RADBOUD UNIV NIJMEGEN on December 1, 2014 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch001
2
POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS
Competing inorganic materials such as lithium niobate (3) and III-V materials, such as GaAs/AlGaAs multiple quantum wells (4), are more mature in their development but have made limited penetration in the commercial market place for high speed integrated optical devices, due in part to material property drawbacks such as fragility, high dielectric constant at microwave frequencies, and processing difficulties (5). NLOP overcome these particular material property deficiencies, but there are still many problems to solve, such as low cost ways to efficiently couple light into and out of the waveguides. Obtaining a high nonlinear optical coefficient in a thermally stable polymer film has been the major quest in NLOP materials research. Chapters in this book report good progress in that regard. NLOP are now being made that have excellent stability (this is discussed in the section on stability). Today's NLOP are superior EO materials for very high speed devices due to their low dielectric constant, e, at all frequencies (e is less than 4 for NLOP and greater than 30 for LiNb03) (la). Polymers are much easier to integrate in silicon devices and in large arrays. Today's NLOP's do not exceed LiNb03 in EO performance in low speed devices because of it's higher index of refraction, n (the figure of merits, n r and are discussed in more detail in the section on devices); however, chromophores under development will likely change that situation in the future. NLOPfilmsare being developed by many worldwide companies (Akzo-Nobel, 3M, AlliedSignal, Lockheed, Hercules, IBM, Dow, NTT, Ciba-Geigy, France Telecom, and more). Several large companies (Hoechst-Celanese, AT&T, DuPont, Kodak, Sandoz) cut back their R&D programs over the last 5 years for a variety of business reasons, e.g., NLOP may not compete with direct modulation of lasers for the large volume, low speed (MHz) applications, and the potential market for very high speed devices is rather limited in the next 5 years. However, the author believes NLOP will most definitely find commercial niches in the out years as photonic systems proliferate. Most new materials need years of laboratory incubation and feedback from potential customers in order to clear all the hurdles and find business niches. 3
Chromophores The molecular origin of optical nonlinearity is due to the electrical polarization of the chromophore as it interacts with electromagnetic radiation. This phenomenon is described by the Schrodinger wave equation (6). Second-order nonlinearity normally occurs in noncentrosymmetric materials. Therefore, asymmetric chromophores are of interest and they must be at least partially aligned in the same direction (i.e., they must have polar alignment in the NLOP film). Some symmetrical multipolar molecules can exhibit second-order nonlinearity (see reference (Id), but this class of chromophore will not be discussed here. Macroscopic optical properties of NLOP films depend on the electrical polarization in the film (these relations are in the literature (7)). The electrical polarization in a material is simply the dipole moment per unit volume. The linear and
In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
1. LINDSAY
Second-Order Nonlinear Optical Polymers: Overview
3
nonlinear optical behavior of a NLOP film can be described by a series expansion of the polarization in the material, P, in powers of the applied electric fields, E: (1)
P = Po + 3C E + X
( 2 ) E
E +X
( 3 ) E E E
+ ...
(1)
where P is the permanent (or ground state) polarization, and are the susceptibilities. (Note that subscripts are omitted here for brevity ~ %() is a third-rank tensor.) The term describes ordinary linear behavior (refraction and absorption). The %() term (the first nonlinear term) describes the optical effects resulting from the interaction of two electrical fields, e.g., laser and radio frequency fields. The x ® effects are sometimes referred to as "three-wave mixing," i.e., (u)3;a)i,G)2) where coi and C02 are frequencies of the applied fields, and ©3 is the resulting optical frequency. The %() term in equation 1 describes four-wave mixing, and is important here when electric-field-induced third-order effects exist. The same kind of mathematical treatment can be applied to an isolated chromophore's dipole moment (8): |i = | i + )2f2
(2)
