Studies of optical spatiotemporal solitons





When a beam of light propagates through a linear medium, it spreads in space due to diffraction. Likewise, a pulse of light spreads in time due to group-velocity dispersion in the linear regime. In a nonlinear medium pulse propagation can be quite different. In particular, under appropriate conditions self-focusing and self-phase modulation can counteract diffraction and dispersion in space and time, respectively. The stably confined (nonspreading) beam that results when diffraction and self-focusing balance each other is called a spatial soliton. Similarly, a temporal soliton is formed by the balance of dispersion and self-phase modulation.

Spatial and temporal solitons have been extensively studied (temporal solitons are of particular interest in optical fiber for telecommunications). However, spatio-temporal solitons (STS), simultaneously confined packets of radiation in space and time (see Figure 1), were observed for the first time only recently by our group. Due to their confinement in space and time STS are of great interest because they provide a natural system in which to observe new effects like interactions and collisions between solitons, as well as fundamental properties of soliton propagation, all of which have direct application to optical signal transfer and processing.
 

 
Figure 1: Spatial (left) and temporal (right) intensity profiles of electromagnetic radiation: (a) at the start of the nonlinear optical medium, (b) after propagation through the medium at low intensity (in the linear regime), and (c) soliton formation in time and one transverse spatial dimension at higher intensity.

In these experiments the STS observed are 2-dimensional in nature: they are confined in time and one transverse spatial dimension, but undergo linear diffraction in the second dimension (they have the shape of a large aspect ratio ellipse as seen in Fig. 1). Ultimately however these solitons are unstable at high intensity due to their lack of confinement along the 2nd spatial dimension, and they collapse into a number of filaments (Figure 2) which propagate for a short while as fully confined STS (packets of radiation confined in time and all spatial dimensions), and then collapse .
 

Figure 2: Three dimensional filaments (white spots) resulting from collapse of two dimensional STS (red and blue stripes).

To date, stable fully confined STS or "light bullets" have not been observed. The production and study of fully confined STS or "light bullets" is an area of current research in our group.

Due to the inherently nonlinear nature of their propagation, the interactions and collisions of STS provide a system in which optical signal processing can be realized and studied. Figure 3 shows simulation results and preliminary experiments demonstrating use of 2D STS as an optical AND logic gate. The fields are launched in a Y-junction geometry: with only one field present, a single STS forms propagating in the launched direction. With two fields present, a STS forms along the bisector of the launched fields. A detector placed at the center of the frame sees a signal only if both fields are launched -- a logical AND operation.
 

Figure 3: Simulation (left) and observation (right) of STS formation in a Y-junction geometry. (a) and (b) show individual STS formation with only the right- and left-going fields, respectively. (c) shows STS formation along the center with both fields present.





  • See a review article on spatiotemporal solitons entitled "The Hunt for Light Bullets- Spatiotemporal Solitons" by Frank Wise and Paolo Di Trapani appearing in the February, 2002 issue of Optics & Photonics News.

    • This paper was published in Optics & Photonics News, February, 2002 and is made available as an electronic reprint with the permission of OSA. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited. Copyright OSA (www.osa.org/pubs/osajournals).



    Project funded by the National Science Foundation, Physics Division.


    For more information, please contact Kale.