Dynamics of refractive-index changes and two-beam coupling in resonant media

Ragini Saxena, Pochi Yeh

Light-induced change in the index of refraction of a Kerr medium is well known.1 Illumination with a spatially periodic light intensity leads to the formation of an index grating in the Kerr medium. We make a comparative study of the response time and power requirement for index grating formation in photorefractive and Kerr media. We examine the conditions in which the index grating in the Kerr medium will be spatially shifted from the light intensity that induces it. Two-beam coupling via Bragg diffraction from the nonlocal index grating is studied.2 The coupling constant is found to depend on the relaxation mechanism of the atoms in the Kerr media and the frequency detunings of the two beams from the line center.

Pavel A Apanasevich, Anatolii A Afanas'ev, А. И. Урбанович

Kazuo Kuroda

Nonlinear optics normally involves the change in the susceptibility of a material due to the presence of optical radiation. In many cases, the light-matter interaction produces a change in the refractive index of the medium, as occurs in photorefractive media, as well as in Kerr-like media, Brillouin-active media and absorbing media with thermallyinduced refractive index changes. Such nonlinear refractive index variation corresponds to a change in the real part of the optical susceptibility %. In resonant materials, the optical radiation induces stimulated emission (or absorption) causing transfer of population between different energy states of the medium. This can lead to saturation of the amplification (or absorption) corresponding to changes in the imaginary part of the susceptibility x= i2odk where the amplitude gain coefficient a(I) is a saturable function of the intensity /, and k is the magnitude of the wavevector of the optical radiation. The interference of two (or more) coherent beams I(x) in the laser amplifier can cause a modulation of the population inversion N(x). This modulation of the inversion is commonly known as spatial hole burning in lasers and leads to formation of a gain-grating a(x) = 1/2GN(X) where G is the stimulated emission cross-section. The gain grating can act as a very efficient diffractive optical element exhibiting high diffraction efficiency that may be even greater than unity [16] and also very high phase conjugate reflectivity using four-wave mixing (FWM) geometries [5-7].

Nicholas Proite, Daniel Sikes, Deniz D. Yavuz

Oleg L. Antipov, A.S. Kuzhelev, Dmitry V. Chausov +1 more

The mechanism for refractive-index changes that accompany changes in population during pumping and amplification of an optical beam is studied in a Nd:YAG laser crystal by both resonant four-wave mixing and interferometric techniques. It is found that these refractive-index changes (connected with the difference in polarizability of excited and unexcited Nd3+ ions) have two components: a small noninertial component that accompanies an amplified optical pulse and the major changes that are intensified with a delay time of ∼3 µs. The latter effect is explained by population (or depopulation) of a higher-lying energy level 2(F2)5/2 of the 4f electron shell of Nd3+ ions, which has large polarizability in near-IR and visible light. The real part of the nonlinear optical resonant susceptibility of the inverted Nd:YAG caused by the refractive-index changes is also discussed.

F. T. Arecchi, Vittorio Degiorgio, Carlo Giacomo Someda