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Chinese Physics Letters

Optical Beams in Media with Spatial Dispersion

郭旗

Published Jan 1, 2003• 0 citations
OpenAlex

Abstract

We theoretically predict a novel phenomenon of the diffraction-free propagation of the paraxial optical beam in the isotropic or cubic linear medium with spatial dispersion.Under the weak spatial dispersion condition,the paraxial wave equation is derived,in which there is the diffraction term with the coefficient Γ that depends on the compensation between the classic diffraction and spatial dispersion.Near the exction dipole absorption line,we can have Γ=0,then the beam propagates diffraction-freely,and its shape keeps unchangeable.Discussed are also the physical mechanism and the possible experimental candidates of crystals for the phenomenon.

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We theoretically predict a novel phenomenon of the diffraction-free propagation of the paraxial optical beam in the isotropic or cubic linear medium with spatial dispersion.Under the weak spatial dispersion condition,the paraxial wave equation is derived,in which there is the diffraction term with the coefficient Γ that depends on the compensation between the classic diffraction and spatial dispersion.Near the exction dipole absorption line,we can have Γ=0,then the beam propagates diffraction-freely,and its shape keeps unchangeable.Discussed are also the physical mechanism and the possible experimental candidates of crystals for the phenomenon.
The propagation of a weak optical pulse through a resonant two-level atomic medium is discussed in the paraxial approximation. Transverse propagation effects within that approximation are treated exactly by development of the method of Fresnel transforms. In particular, for a temporally short pulse with an amplitude of arbitrary transverse spatial dependence it is shown that longitudinal propagation is not affected by diffractive spreading. Thus diffraction does not attenuate the characteristic ringing of one-dimensional pulse propagation in resonant media.
The transmission spectra of inversionless amplification are investigated. Propagation of a weak probe optical beam through a three-level medium pumped with a microwave strong beam is numerically modeled. Three various schemes of transitions under the conditions providing the amplification of the probe field without inversion are taken to calculate the atomic polarization, which enters the paraxial wave equations. Diffraction and self-action ofbeams due to nonlinear absorption and refraction are properly taken into account. These effects together with linear absorption and refraction form a complex distribution of gain across the beams. As a result the amplified beam acquires ring transverse patterns and slight asymmetry in the frequency characteristics.
Propagation of a weak probe optical beam through a three-level medium pumped with a microwave strong beam is numerically studied. P-scheme of transitions under the conditions providing the amplification of the probe field without inversion is taken to calculate the atomic polarization which enters the paraxial wave equations. Diffraction and self-action of beams due to nonlinear absorption and refraction are properly taken into account. These effects together with linear absorption and refraction form a complex distribution of gain across the beams. As a result the amplified beam acquires ring transverse patterns and slight asymmetry in the frequency characteristics.
Phase conjugated light waves reverse phase distortions in media with negligible absorption or for paraxial beam propagation [1]. We examine here a different case in which a volume conjugating medium is itself imperfect, possessing phase distortions and a conjugating efficiency dependent on grating spatial frequency. We show theoretically that for multiplexed signals, cross-talk-free phase-conjugation is obtained only for negligible spatial-frequency dispersion and a phase-conjugated reference or pump wave, and we present supporting experimental results.
We suggest a scheme to manipulate paraxial diffraction by utilizing the dependency of a four-wave mixing process on the relative angle between the light fields. A microscopic model for four-wave mixing in a Λ-type level structure is introduced and compared to recent experimental data. We show that images with feature size as low as 10 μm can propagate with very little or even negative diffraction. The mechanism is completely different from that conserving the shape of spatial solitons in nonlinear media, as here diffraction is suppressed for arbitrary spatial profiles. At the same time, the gain inherent to the nonlinear process prevents loss and allows for operating at high optical depths. Our scheme does not rely on atomic motion and is thus applicable to both gaseous and solid media.
Propagation of a weak probe optical beam through a three- level medium pumped with a microwave strong beam is numerically studied. Three various schemes of transitions under the conditions providing the amplification of the probe field without inversion are taken to calculate the atomic polarization which enters the paraxial wave equations. Diffraction and self-action of beams due to nonlinear absorption and refraction are properly taken into account. These effects together with linear absorption and refraction form a complex distribution of gain across the beams. As a result the amplified beam acquires ring transverse patterns and slight asymmetry in the frequency characteristics.