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All-optical devices allow improvements in the speed of optical communication and computation
systems by avoiding the conversion between the optical and electronic domains. The
focus of this thesis is the experimental investigation of a new type of all-optical
switch that is based on the control of optical patterns formed by nonlinear interactions
between light and matter.
The all-optical switch consists of a pair of light beams that counterpropagate through
warm rubidium vapor. These beams induce a nonlinear optical instability that gives
rise to mirrorless parametric self-oscillation and generates light in the state of
polarization that is orthogonal to that of the pump beams. In the far-field, the generated
light forms patterns consisting of two or more spots. To characterize this instability,
I observe experimentally the amount of generated power and the properties of the generated
patterns as a function of pump beam intensity, frequency, and size. Near an atomic
resonance, the instability has a very low threshold: with less than 1~mW of total
pump power, >3~$\mu$W of power is generated.
To apply this system to all-optical switching, I observe that the orientation of the
generated patterns can be controlled by introducing a symmetry-breaking perturbation
to the system. A perturbation in the form of a weak switch beam injected into the
nonlinear medium is suitable for controlling the orientation of the generated patterns.
The device operates as a switch where each state of the pattern orientation corresponds
to a state of the switch, and spatial filtering of the generated pattern defines the
output ports of the device. Measurements of the switch response show that it can be
actuated by as few as 600~photons. For a switch beam with 1/e field radius $w_0=185\,\mu$m,
600 photons correspond to $5.4\times10^{-4}$ photons/\lambdasquared which is comparable
to the best reported results from all-optical switches based on electromagnetically-induced
transparentcy (EIT). This approach to all-optical switching operates at very low light
levels and exhibits cascadability and transistorlike response. Furthermore, the sensitivity
is comparable to switches using cold-atom EIT or cavity quantum-electrodynamics techniques
but is achieved with a simpler system, requiring only one optical frequency and occurring
in warm atomic vapor.
I develop a numerical model for the switch that exhibits patterns that rotate in the
presence of a weak applied optical field. Results from this model, and from my experiment,
show that the switch response time increases as the input power decreases. I propose
that this increase is due to critical slowing down (CSD). Mapping the pattern orientation
to a simple one-dimensional system shows that CSD can account for the observed increase
in response time at low input power. The ultimate performance of the device is likely
limited by CSD and I conclude that the minimum number of photons capable of actuating
the switch is between 400 and 600 photons.
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