It has been recognized that certain media having a polarization susceptability provide sensitive ways of manipulating beams of incident electromagnetic radiation. Such media are said to possess nonlinear polarization. The size of the effects attributable to such nonlinear polarization depends on the arrangement of electrically charged particles (electrons, ions and nuclei) within the media. To obtain the highest nonlinear polarizaton property of a medium, the molecules within the medium must be arranged so that the nonlinear properties of the individual polar molecules within the medium do not cancel each other out.
On a molecular level the polarization of a medium can be described by the following expression: EQU .mu.=.mu..sub.0 +.alpha.E+.beta.EE+.gamma.EEE +. . .
where
.mu. is the induced dipole moment
.mu..sub.0 is the permanent dipole moment
E is local electric field
.alpha., .beta., and .gamma. are tensors representing the linear, second order and third order polarizabilities, respectively. .beta. and .gamma. are also referred to as the first and second hyperpolarizabilities, respectively.
On the molecular level first order or linear polarization is described by .alpha.E; second order or first nonlinear polarization by .beta.EE; and third order or second nonlinear polarization by .gamma.EEE.
The polarization of an ensemble of molecules induced by an applied electric field can be described by the following expression: EQU P=P.sub.0 +.chi..sup.(1) E+.chi..sup.(2) EE+.chi..sup.(3) EEE +. . .
where
P is the induced polarization
P.sub.o is the permanent polarization
E is the applied electric field
.chi..sup.(1), .chi..sup.(2) and .chi..sup.(3) are tensors representing the linear, second order and third order polarization susceptibilities, respectively.
.chi..sup.(2) arises from the second order molecular polarizability or first hyperpolarizability, .beta., and .chi..sup.(3) arises from further hyperpolarizabilities, etc. As tensor quantities, the susceptibilities, .chi..sup.(i), are highly symmetry dependent; odd order coefficients are nonvanishing for all materials, but even order coefficients, e.g., .chi..sup.(2), are nonvanishing only for noncentrosymmetric materials.
Franken et al., Physical Review Letters, 7, 118-119 (1961), disclose the observation of second harmonic generation (SHG) upon the projection of a pulsed ruby laser beam through crystalline quartz. The use of a laser remains the only practical way to generate an E large enough to be able to detect the SHG phenomenon.
Second order nonlinear optical phenomena, such as SHG, sum and difference frequency generation, parametric processes and electro-optical effects, all arise from the .chi..sup.(2) term. Consequently, for significant nonlinear optical phenomena it is desirable that a molecule possess a large hyperpolarizability, .beta., and that an ensemble of such molecules possess a large .chi..sup.(2).
The art has recognized that organic molecules having conjugated .pi.-electron systems or low-lying charge transfer excited states often have extremely large hyperpolarizabilities, but unfavorable alignment of the molecules in the crystalline phase, in thin films or in other forms can result in a centrosymmetric material in which the .chi..sup.(2) vanishes. This problem may be circumvented by using a chiral molecule to insure a noncentrosymmetric (i.e., symmetrical about its center) crystal, but problems associated with the creation and maintenance of a high level of optical purity limit the value of this approach. In addition, optical activity per se does not guarantee that .chi..sup.(2) will be large, only that it will not be zero.
One approach to the problem disclosed by Anderson et al. U.S. Pat. No. 4,818,898 involves the formation of inclusion complexes consisting of a crystalline lattice forming host compound with continuous channels containing a nonlinearly polarizable guest compound having a second order polarizability greater than 10.sup.-30 electrostatic units.
Ulman et al. U.S. Pat. No. 4,792,208 discloses an optical article containing a medium exhibiting a second order polarization susceptibility greater than 10.sup.-9 electrostatic units comprised of polar aligned noncentrosymmetric molecular dipoles having an electron donor moiety linked through a conjugated .pi. bonding system to an electron acceptor moiety to permit oscillation of the molecular dipole between a lower polarity ground state and a higher polarity excited state. A wide variety of donor and acceptor moieties are disclosed with a sulfonyl electron acceptor group in combination with a hydrocarbon substituted electron donor group preferred. The second order nonlinearity is achieved by the polar alignment of the molecular dipoles, in, for example, polymeric binders, to form Langmuir-Blodgett (LB) films.
While the art continues to investigate the alteration of chemical structure to continuously increase molecular nonlinearity and corresponding molecular dipole moment, this approach may not always produce the material having the best combination of overall properties. Large dipole moment, which favors strong polar alignment, can often be associated with aggregation and solubility problems, allowing only a small amount of optically active material to be contained in a binder. Furthermore alterations in chemical structure to increase nonlinearity can often adversely effect the color of the resulting material. Since the first and second harmonic wavelengths of diode lasers lie near 800 and 400 nanometers, respectively, the optimum nonlinear molecules must have high transparency, i.e., very low absorption at these wavelengths, and, in addition, possess high photochemical stability under the conditions of exposure to high optical intensities. One of the major chemical challenges in this area of technology is to discover molecules that have high nonlinearity, but absorb very little light over the visible range of wavelengths from 350 to 850 nanometers.
Accordingly, it is an object of this invention to provide a novel nonlinear optical medium which exhibits a high transparency over the visible range of wavelengths.
It is another object of this invention to provide a nonlinear optical element comprising a transparent medium selected from fluorinated sulfones and fluorinated ketones.
It is a further object of this invention to provide a novel composition which is a physical blend of fluorinated sulfones or ketones with a polymer component.
It is an additional object to provide an apparatus for second harmonic generation.
These and other objects and advantages will become apparent from the accompanying description and examples.