At the Joint Quantum Institute, a research team, comprising Edo Waks and colleagues, has developed an all-optical switch utilizing a quantum dot integrated within a tiny hole-free arcade, which behaves like a resonant cavity.
This is the setup of a waveguide made from a photonic crystal. A quantum dot (QD) is placed inside a tiny zone (cavity) clear of holes. Light is sent into and out of the waveguide via endcaps (the semi-circular structure at both ends, indicated by green arrows). If properly timed (the synchronicity time, tau, being less than about 100 ps), a pump (control) 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. (Credit: Ranojoy Bose, JQI)
The quantum dot, which comprises nano-sized sandwich of arsenic and indium, can emit only discrete wavelength light due to its small size. It is placed within a photonic crystal that has several tiny holes, which allow light to traverse the crystal only for a narrow range of wavelengths. When light traverses the waveguide adjacent to the resonant cavity, some amount of light enters into the cavity and reacts with the quantum dot. This interplay can modify the transmission properties of the waveguide. To create a switching action, 140 photons are required in the waveguide. However, to produce the quantum dot modulation, only 6 photons are required, thus flinging the switch.
Earlier versions of optical switches needed high input power and large nonlinear-crystals for operation. Conversely, the JQI switch performs high-nonlinear interactions utilizing an input power of 90 aJ and a single quantum dot. It changes the direction of light within 120 ps. The input power value of the JQI switch is five times lower than the previous record set by a device developed at labs in Japan. However, the Japanese switch can operate at room temperature, whereas the JQI switch needs a temperature of roughly 40 K.
A second pulse called a pump or control beam is able to change the direction of light traversing the waveguide as a probe beam. To make the probe beam to exit from the side of the device, the probe beam and the slightly detuned pump beam should arrive at the same time. 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 are not arrived concurrently, then the probe beam will leave in another direction.
Ranojoy Bose, one of the researchers, informed that this quantum-dot switch is not an optical transistor but a low-photon-number pulse. Bose anticipates a reduction in the photon count required for switching on and off the resonant cavity. This JQI switch embodies the possibility of realizing a usable ultrafast, low-energy on-chip signal router.
Source: http://jqi.umd.edu/