Solitons

The first soliton ?

The story begins in 1834, when John Scott Russell observed a solitary wave on the canal Edinburgh-Glasgow. His story was as following:

I believe I shall best introduce the phenomenon by describing the circumstances of my own acquaintance with it. I was observing the motion of a boat which was rapidly drawn along a narrow channel by a pair of horses, when the boat suddenly stopped - not so the mass of water which it had put in motion; it accumulated around the prow of the vessel in a state of violent agitation, then suddenly leaving it behind, rolled forward with great velocity, assuming the form of a large solitary elevation, a rounded, smooth and well-defined heap of water, which continued its course along the channel without change of form or diminution of speed. I followed it on horseback, and overtook it still rolling on at a rate of some eight or nine miles an hour, preserving its original figure some thirty feet long and a foot to a foot and a half in height. Its height gradually diminished, and after a chase of one or two miles I lost it in the windings of the channel.

Recently an international gathering of scientists witnessed a re-creation of the famous 1834 'first' sighting of a soliton or solitary wave on the Union Canal near Edinburgh. A number of pictures and further info about this experience can be found here.

Two different types of solitons

Temporal solitons

When a light pulse propagates in an optical fiber, the pulse shape can change because of fiber dispersion and non-linearities in the fiber. Both effects can cancel each other for a proper choice of pulse width and pulse amplitude. Such pulses have a soliton-like nature and this phenomenon has two major advantages. First, the fiber bandwidth is not dispersion limited and second, very long propagation distances are possible, especially in combination with optical amplifiers.

Spatial solitons

The solitons we investigate are spatial solitons. When a light beam propagates in a medium, then the shape of the beam will change because of diffraction, which is a defocusing effect. When light propagates in a waveguide, then it is possible that the beam shape will not vary. Typically, a beam will be captured in a waveguide when the medium has a higher index of refraction than the surrounding medium.

In the case of a spatial soliton, there has to be a self-focusing effect that cancels out the diffraction. This can occur when the index of refraction increases due to the intensity of the light (a "bright" soliton). In the case the index of refraction decreases we speak of a "dark" soliton.

Spatial solitons in liquid crystals

Optical spatial solitons have been investigated in different types of materials characterized by various kinds of nonlinearity, like Kerr and photorefractive nonlinear responses.

It has also been demonstrated that spatial solitons can be efficiently generated in nematic liquid crystals and propagate for distances of a few millimeters. In such media, soliton formation occurs due to a large non-resonant, saturable and non-local nonlinearity arising from molecular reorientation.

Molecule reorientation

The electric field of the optical wave tends to realign the molecules so that they are parallel to the electric field. This causes an increase in the index of refraction. This nonlinearity is several magnitudes bigger than nonlinearity in other materials.

Not only molecular reorientation gives rise to nonlinearity in liquid crystals. Other mechanisms are thermal and density effects, electronic effects, - For an overview of these phenomena, see for example "Liquid crystals, physical properties and nonlinear optical phenomena", Iam-Choon Khoo, John Wiley & Sons, 1995

Our research on solitons

Our group is currently investigating soliton propagation in dye-doped liquid crystal (E7) for different geometries and different wavelengths varying from 633 nm to the near infrared.

Test configuration

Simulation programs are written for a 1D and 2D model based on the beam propagation method (BPM).

More detailed information about the results we have obtained in our research can be found in our publications.

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