The current state of materials development in nonlinear optics is summarized, and the promise of these materials is critically evaluated. Properties and important materials constants of current commercial materials and of new, promising, inorganic and organic molecular and polymeric materials with potential in second- and third-order nonlinear optical applications are presented.

https://science.sciencemag.org/content/sci/253/5017/281.full.pdf

Nonlinear optical materials.

Nonlinear optics of liquid crystalline materials

We report the observation of nonlinear photorefraction in pure aligned nematic liquid crystals in the presence of an applied dc electric field The effect is attributed to nematic axis reorientation created by the photoinduced space-charge field in combination with the applied dc field Strong optical nonlinearity, self-diffraction, and beam-coupling effects are observed

Observation of orientational photorefractive effects in nematic liquid crystals

We provide an in-depth treatment of the various mechanisms by which an incident light beam can produce an intensity- or flux-dependent change in the refractive index and absorption coefficient of different materials. Whenever possible, the mechanisms are initially traced to single-atom and -molecule effects in order to provide physical understanding. Representative values are given for the various mechanisms. Nine different mechanisms are discussed, starting with the Kerr effect due to atoms and/or molecules with discrete states, including organic materials such as molecules and conjugated polymers. Simplified two and/or three-level models provide useful information, and these are summarized. The nonlinear optics of semiconductors is reviewed for both bulk and quantum-confined semiconductors, focusing on the most common types II–VI and III–V. Also discussed in some detail are the different nonlinear mechanisms that occur in liquid crystals and photorefractive media. Additional nonlinear material systems and mechanisms such as glasses, molecular reorientation of single molecules, the electrostrictive effect, the nuclear effect (vibrational contributions), cascading, and the ever-present thermal effects are quantified, and representative tables of values are given.

Nonlinear refraction and absorption: mechanisms and magnitudes

In recent years ferroelectric domain patterning has become a popular topic of physical research because it enables photonic applications as well as data storage. For generation of tailored domain structures and for further understanding of ferroelectricity, a visualization of the domain patterns is required. A large number of imaging techniques have therefore been developed. This review summarizes these techniques and highlights systematically their strengths and weaknesses.

Visualization of ferroelectric domains in bulk single crystals

We present concrete evidence for the formation of ferroelectric domain gratings induced by photorefractive space‐charge electric fields in top‐seeded solution‐grown barium titanate crystals. These domain gratings are not destroyed by light and, by applying a field, can be reconverted into photorefractive gratings that diffract much more light than the photorefractive gratings that create them, as much as 67% of an incident beam.

Photorefractive and domain gratings in barium titanate

We describe a microscope that preferentially displays moving objects. This all-optical device operates in real-time using the nonlinear coupling of optical beams in a photorefractive crystal of barium titanate. A holographic image is continuously written and compared with the real image, and any discrepancy between the two images is immediately visible. The device can be used to highlight moving objects, such as protozoans, while suppressing the image of any stationary objects in the background. The device needs only milliwatts of laser power, and can detect objects moving at velocities of a few micrometres per second.

The transient detection microscope

Two coherent light beams can couple in any absorbing material owing to a light-induced modulation of the material’s dielectric constant. In photorefractive crystals the coupling caused by these absorption gratings appears in addition to any electro-optic coupling, complicating the interpretation of data. However, in contrast with the electro-optic gratings formed by charge diffusion, absorption gratings do not necessarily vanish as the beam-crossing angle approaches zero if there is more than one absorbing level. We show that a plot of the coupling strength of the absorption gratings versus the beam-crossing angle is characterized by only three parameters, independent of the number of absorbing levels. We use absorption gratings with a two-level model to determine experimentally some important crystal parameters, including the relative density of donors and acceptors in a barium titanate crystal. Our values agree with those obtained from measurements of the bulk light-induced absorption of the crystal.

Absorption gratings in photorefractive crystals with multiple levels

Two optical beams can couple in a photorefractive crystal without using the electro-optic effect. Beam coupling is due to a spatially modulated absorption caused by the rearrangement of trapped charges. We use these gratings to determine the effective photorefractive trap density for several barium titanate crystals.

Measuring photorefractive trap density without the electro-optic effect.

We present a method to separate the effects of trap gratings and electro-optic gratings in BaTiO3 crystals, and we determine the true spatial shift between the electro-optic grating and the optical intensity pattern. At small beam-crossing angles this spatial shift is strongly affected by a photogalvanic current in the crystal.

Roger S. Cudney

Papers

Photorefractive and domain gratings in barium titanate

The transient detection microscope

Absorption gratings in photorefractive crystals with multiple levels

Measuring photorefractive trap density without the electro-optic effect.

Measurement of the photorefractive phase shift