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  • All-Optical Switch Uses a Single Quantum Dot
May 2012
COLLEGE PARK, Md., May 8, 2012 — An all-optical switch created using a quantum dot placed inside a resonant cavity can switch a beam of light from one direction to another in 120 ps with very little power.

Developed at the Joint Quantum Institute, it consists of a nanosize sandwich of arsenic and indium. It is so tiny that it can emit only discrete wavelengths, as if it were an atom. It was placed inside a photonic crystal that had been bored with several tiny holes, enabling light to pass only through the crystal, for a narrow range of wavelengths.

When light traveled through the waveguide adjacent to the resonant cavity, some amount of light entered into the cavity and reacted with the quantum dot. This interaction can modify the waveguide’s transmission properties. To create a switching action, 140 photons are required in the waveguide. In the JQI experiment, however, only six photons are required to throw the switch.

The setup of a waveguide made from a photonic crystal. A quantum dot is placed inside a tiny zone clear of holes. Light is sent into and out of the waveguide via end caps (the semicircular structure at both ends, indicated by green arrows). If properly timed, a pump laser pulse will allow an accompanying probe pulse to exit out the side. If the probe and pump beams are not aligned, the probe beam will exit out the far end of the waveguide. (Image: Ranojoy Bose, JQI)

“Switching can be achieved physically by using only six photons of energy, which is completely unprecedented,” said Ranojoy Bose, a JQI scientist. “This is the achievement of fundamental physical milestones — sub-100-aJ switching and switching near the single-photon level.”

Previous optical switches needed high input power and bulky nonlinear crystals to operate. The JQI switch, by contrast, achieves high nonlinear interactions using a single quantum dot and an input power of 90 aJ. The input power value of the switch is five times lower than the record set by a device developed at labs in Japan. However, the Japanese switch can operate at room temperature, whereas the JQI switch requires a temperature of about 40 K.

Light traveling down the waveguide in the form of an information-carrying probe beam can be switched from one direction to another using a second pulse. To steer the probe beam out the side of the device, the probe beam and the slightly detuned pump beam must arrive simultaneously. Here, the probe beam is resonant with the quantum dot, which sits just off the waveguide’s center track inside the cavity. Strong coupling can be achieved by tuning the quantum dot’s temperature on resonance with the cavity. If the beams do not arrive at the same time, the probe beam will leave in another direction.

“Our waveguide-dot setup can’t yet be used to modulate a beam of light using only a weak control pulse of light — what we would call a low-photon-number pulse,” Bose said.

Bose anticipates a reduction in the photon count required for switching on and off the resonant cavity. In the meantime, the JQI switch represents a start toward creating a usable ultrafast, low-energy on-chip signal router.

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photonic crystal
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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