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

An
optical switch developed at the Joint Quantum Institute (JQI) spurs the
prospective integration of photonics and electronics. What, isn’t
electronics good enough? Well, nothing travels faster than light, and in
the effort to speed up the processing and transmission of information,
the combined use of light parcels (photons) along with electricity
parcels (electrons) is desirable for developing a workable
opto-electronic protocol.

   

The
JQI switch can steer a beam of light from one direction to another in
only 120 picoseconds (120 trillionths of a second), requiring very
little power, only about 90 attojoules (90 x 10^-18 joules). At the
wavelength used, in the near infrared (921 nm), this amounts to about
140 photons. These new results are being published in an upcoming issue
of the journal Physical Review Letters.

   

The
centerpiece of most electronic gear is the transistor, a solid-state
component in which a gate signal is applied to a nearby tiny conducting
pathway, thus switching on and off the passage of an information signal.
The analogous process in photonics would be a solid-state component
which acts as a gate, enabling or disabling the passage of light through
a nearby waveguide, or as a router, for switching beams in different
directions.

   

In
the JQI experiment, prepared and conducted at the University of
Maryland and at the National Institute for Standards and Technology
(NIST) by Edo Waks and his colleagues, an all-optical switch has been
created using a quantum dot (the equivalent of a gate) placed inside a
resonant cavity. The dot, consisting of a nm-sized sandwich of the
elements indium and arsenic, is so tiny that electrons moving inside can
emit light at only discrete wavelengths, as if the dot were an atom.
The quantum dot sits inside a photonic crystal, a material that has been
bored with many tiny holes. The holes preclude the passage of light
through the crystal except for a narrow wavelength range.

   

Actually,
the dot sits inside a small hole-free arcade which acts like a resonant
cavity. When light travels down the nearby waveguide some of it makes
its way into the cavity, where it interacts with the quantum dot. And it
is this interaction which can transform the waveguide’s transmission
properties. Although 140 photons are needed in the waveguide to produce
switching action, only about 6 photons actually are needed to bring
about modulation of the QD, thus throwing the switch.

   

Previous
optical switches have been able to work only by using bulky
nonlinear-crystals and high input power. The JQI switch, by contrast,
achieves high-nonlinear interactions using a single quantum dot and very
low power input. Switching required only 90 aJ of power, some five
times less than the best previous reported device made at labs in Japan,
which itself used 100 times less power than other all-optical switches.
The Japanese switch, however, has the advantage of operating at room
temperature, while the JQI switch requires a temperature of around 40 K.

   

Continuing
our analogy with electronics: light traveling down the waveguide (the
equivalent of the conducting pathway in a transistor) in the form of an
information-carrying (probe) beam can be switched from one direction to
another using the presence of a second pulse, a control (pump) beam. To
steer the probe beam out the side of the device, the slightly detuned
pump beam needs to arrive simultaneously with the probe beam, which is
on resonance with the dot. The dot lies just off the center track of the
waveguide, inside the cavity. The temperature of the quantum dot is
tuned to be resonant with the cavity, resulting in strong coupling. If
the pump beam does not arrive at the same time as the probe, the probe
beam will exit in another direction

   

So,
is this quantum-dot switch an “optical transistor”? Not quite, says JQI
scientist Ranojoy Bose. “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.

   

But
Bose says he expects an improvement (reduction) in the number of
photons needed to switch the resonant cavity on and off. In the
meantime, the JQI switch represents a great start toward creating a
usable ultrafast, low-energy on-chip signal router. “Our paper shows
that switching can be achieved physically by using only 6 photons of
energy, which is completely unprecedented. This is the achievement of
fundamental physical milestones—sub-100-aJ switching and switching near
the single photon level,” Bose says.

   

Low photon number optical switching with a single quantum dot coupled to a photonic crystal cavity

Source: Joint Quantum Institute