When the pump beam power is near threshold, the symmetry
of the generated light is weakly broken resulting in a
transverse pattern containing two spots. This corresponds to
two distinct output beams.
Our switch is based on the combination of strong nonlinear interactions and transverse optical patterns. Strong nonlinear interactions allow weak beams to interact with each other thus, in principle, enabling few-photon switching. Transverse optical pattern refers to the spatial distribution of output light beams when they are projected onto a screen perpendicular to the propagation direction. These patterns (and thus the output beams) are very sensitive to weak perturbation from additional light (i.e. the switching beam). This allows a weak switching beam to control a much stronger output beam.
As discussed in the Patterns section, our experimental system consists of laser beams that counterpropagate through a rubidium vapor and induce an instability that in turn generates new beams of light. The patterns formed by projecting these beams onto a transverse plane can be controlled by perturbing the system.
An animation indicating the effect of a weak switching
beam directed along the cone. When the switching beam is on,
the spots rotate by 60° to a new orientation. The panes
in the lower right are examples of experimentally observed
patterns with and without the switch beam.
A third laser beam can be used to control the orientation of the pattern emitted from the rubidium vapor. Shown here is an animation illustrating the effect of a weak switching beam (yellow). When the switching beam is off, the pattern is a stable two-spot shape with a specific orientation, the spots shown here are at roughly 5 o'clock and 11 o'clock. We usually call this state of our switch the "off" state, although it could be called anything (0, up, A, etc.).
As illustrated, if a weak laser beam is sent through the medium, it can effect the orientation of the output pattern, thus controlling the direction the output light is travelling. We find that a switching beam is most effective at controlling the orientation when it passes along the surface of the cone (blue) that corresponds to the natural angle of emission for the generated light (see Patterns).
Examples of two sets of image data are included in the animation. These correspond to the appropriate state of the switch (on or off). As shown, the output light forms distinct spots on the measurement screen, this allows us to select individual spots in order to distinguish the two states of the switch and quantitatively measure the effect of the switching beam (see Results).