S. Follonier

Electro‐optic properties of the organic salt 4‐N,N‐dimethylamino‐4′‐N′‐methyl‐stilbazolium tosylate

We show that the organic salt 4‐N,N‐dimethylamino‐4′‐N′‐methyl‐stilbazolium tosylate (DAST) is a very interesting material for electro‐optic applications in the near infrared. The electro‐optic coefficients, namely r111, r221, r331, r113, r223, and r333, were determined in the spectral range of 700 up to 1535 nm using optical samples cut from large high quality bulk crystals. DAST combines large electro‐optic coefficients, e.g., r111=77±8 pm/V at λ=800 nm and r111=47±8 pm/V at λ=1535 nm, with low dielectric constants, e.g., e1=5.2±0.4. This combination leads to large polarization‐optical coefficients.

New nonlinear-optical organic crystal: 4-dimethyl-aminobenzaldehyde-4-nitrophenyl-hydrazone

Single crystals of 4-dimethylaminobenzaldehyde-4-nitrophenylhydrazone were grown from solution. We determined their linear-optical (absorption and refractive indices) as well as their nonlinear-optical properties. From the highest nonlinear-optical coefficient d12=270 pm/V at λ=1.542µm we determine an effective nonlinear-optical coefficient deff⩾150 pm/V for phase-matched frequency doubling that is, to the best of our knowledge, the largest reported phase-matchable coefficient. The nonlinear-optical properties are discussed in terms of the crystal structure and the molecular hyperpolarizabilities. In addition, the phase-matching configurations for second-harmonic generation and optic parametric oscillation are derived.

Polymorphism, growth and characterization of a new organic nonlinear optical crystal: 4-dimethylaminobenzaldehyde-4-nitrophenylhydrazone (DANPH)

A new organic nonlinear optical crystal, 4-dimethylaminobenzaldehyde-4-nitrophenylhydrazone (DANPH), has been prepared and investigated with regard to polymorphism, growth, and characterization of the structural and physical properties. Recrystallization of DANPH in various solvents under different conditions showed the existence of three crystalline phases, a co-crystal (form I) of DANPH with benzene in a ratio 4 to 1 in space group P21212, red greenish prisms (form II) in space group Cc, and red orange plates (form III) in space group P21c. The growth of crystals with interesting linear and nonlinear optical properties (form II) was investigated by using different growth techniques: solution growth, gel growth and physical vapor transport growth. With these methods bulk crystals, very thin plates, and films were grown, respectively. By the analysis of crystal structures and phase stabilities of the forms II and III, the mechanism of polymorphism was investigated.

Cascaded contributions to degenerate four-wave mixing in an acentric organic crystal

We show both theoretically and experimentally by degenerate four-wave mixing that the combined processes of optical rectification and the linear electro-optic effect contribute to the large effective third-order susceptibilities [14chi((3))(CS(2))] of the organic salt 4-N, N^- dimethylamino-4(?) -N(?) -methylstilbazolium toluene-p- sulfonate (DAST). Furthermore, the knowledge of the cascaded contributions was used to determine absolute values of the third-order susceptibilities as well as their sign.

Organic Materials for Second-Order Nonlinear Optics

Organic materials have become of great importance with regard to their nonlinear optical and electro-optic properties. They have attracted attention for applications in areas such as efficient frequency-conversion and high-speed light modulation. As a special feature their macroscopic properties arise from the properties of the constituent molecules. There are several possibilities to arrange these molecules macroscopically. The most common forms are single crystals [1-5], polymers [2, 6], and Langmuir-Blodgett films [7]. Besides these cases there exist other interesting concepts such as e.g. molecular beam deposition for the preparation of thin films. Since the macroscopic nonlinearity arises from the constituent molecules, the optimization of a material for a certain application consists of two steps. On the one hand, the nonlinear optical properties of the molecules have to be maximized and, on the other hand, the arrangement of these molecules in crystals or thin films has to be optimized.

Novel Organic Crystals for Nonlinear and Electro-Optics

There have been significant advances in understanding and optimizing classical π-conjugated donor-acceptor chromophores with large second-order molecular hyper-polarizabilities in the area of organic nonlinear optics in the last few years [1, 2, 3, 4, 5, 6]. However, there are only few chromophores with very large molecular hyper-polarizabilities such as donor-acceptor stilbenes and tolanes that form potentially useful crystalline materials. Our interest in molecular crystals stems from the fact that the potential upper limits of macroscopic nonlinearities and long-term orientational stability of molecular crystals are significantly superior to those of polymers [1]. In addition to a large molecular hyperpolarizability, the second-order macroscopic nonlinearities are strongly dependent of the relative arrangement and orientation of the π-conjugation chromophores in the crystalline solid [1, 3]. Unlike the third-order nonlinear optical effects, chromophores are required to arrange noncentrosymmetrically in the crystalline state so as to exhibit non-vanishing macroscopic second-order nonlinear optical responses such as second harmonic generation, frequency mixing, the linear electro-optic effect, and photorefractive effects. To be an efficient as well as useful second-order nonlinear optical crystalline material, the orientation of the chromophores in the bulk also needs to be optimized.

Novel High Nonlinearity Molecular Crystals

Absorption Crystal Normarlzed loss length SHG efficiency (dB/mm) (mm) ( /W) ples with 40% chromophore loading show excellent stability and did not show any crystallization several months after fabrication. A new sensitizer enabled the fabrication of photorefractive polymers for infrared applications. Maximum diffraction could be observed at 830 nm in 105 pm-thick samples of HR-2-54:PVK: EC2:TNFDM at an applied field of 50 V/ pm. Examples of applications with these new materials including imaging through scattering media will be presented. This work was supported by ONR through the Muri Center Camp, by AFOSR, by NSF, and by an NSF/CNRS international program. *Beckman Institute, California Institute of Technology, Pasadena, CA 91 125; Jet Propulsion Laboratory, California lnstitute of Technology, Pasadena, CA 91 109