This invention relates to the field of optics and particularly to materials for use as a nonlinear element in nonlinear optical devices.
Recent advances in nonlinear optics (NLO) have created a new frontier for applied optics. This technology requires nonlinear optical materials, i.e., materials which alter the frequency of laser light, materials which have an index of refraction that varies with light intensity or with applied electrical field, or (in the case of photorefractive nonlinear optical materials) materials which have a local index of refraction that is changed by the spatial variation of light intensity.
Devices which have been conceived utilizing NLO materials include parametric amplifiers, oscillators, second harmonic generators, and modulators. Such devices provide, inter alia, second harmonic generation, phase-conjugate navigation, and laser beam combining which employs optical phase conjugation, as well as laser beam spatial (and spectral) mode cleanup, wavelength agile rejection filtering, laser radar, image correlation and enhancement, communications, optical data storage, and optical computing. In particular, there are readily identifiable systems applications which need lower-cost, higher-response, higher-average-power materials for optical parametric amplifier operation and second harmonic generation throughout the blue/near-ultraviolet spectral region. Bierlein, et al., U.S. Pat. No. 3,949,323, for example, depicts, in FIG. 1, the use of a nonlinear optical material to generate second harmonic radiation and, in FIG. 2, the use of such a material to modulate the intensity of a beam of polarized light. Many of these concepts rely on real-time holographic effects which can be induced in nonlinear materials. Although several of these innovations are at preliminary stages of design, such concepts reveal the power of nonlinear optics in terms of performance breakthroughs in electro-optic device technology that a decade ago would have been thought impossible.
The manufacturability of nonlinear optical devices, however, is limited by the performance of currently available NLO materials, which are primarily inorganic oxide crystals such as potassium niobate, potassium dihydrogen phosphate, lithium niobate, strontium barium niobate, and barium titanate. These inorganic oxide crystals exhibit shortcomings as nonlinear materials due to difficulty of synthesis, lack of optical quality, and slow electro-optic response times.
Although there are many organic materials (for example, urea and 2-methyl-4-nitroaniline) which exhibit very high second harmonic generation responses in powder form, there have been no practical device-quality organic NLO crystals available in the art. Typical organic crystals lack the mechanical robustness, dimensional and thermal properties, and optical quality required for fabricating optical devices.
A category of "semi-organic materials", or metal-organic coordination complexes, has been reported to show promise in the development of nonlinear optical materials. See, for example, Wenbo et al., A new organometallic nonlinear optical material-triallylthiourea mercury bromide (ATMB) crystal: growth and structure, Journal of Crystal Growth, Volume 133, Page 71 (1993) and Dong, et al., A New Aromatic Organometallic Nonlinear Optical Crystal: [Bis-4-nitropyridine-N-oxide Cadmium Chloride], Materials Research Bulletin, Volume 29, Page 73 (1994). This materials classification encompasses a very large number of semi-organic ionic crystals in which relatively large, polarizable organic molecules are incorporated into a host inorganic ionic lattice.
Advanced nonlinear optical (NLO) applications such as high speed optical phase conjugation, parametric amplification, and laser hardening require NLO crystals having high speed and high efficiency. Devices for these applications require new and improved NLO crystals in order to achieve their potential.