Physicists in the US have created a new kind of integrated circuit that can switch light without first converting it into an electronic signal. The light is instead transformed into a quasiparticle called an “exciton”, which consists of an electron bound to a hole. The excition can then be manipulated within a semiconductor chip before being converted back into light.
As well as boosting the performance of optical communications networks, the breakthrough could provide physicists with a new kind of quantum gas to study.
Optical telecoms networks are very efficient at transmitting vast quantities of data in the form of light pulses. However, processing all this data involves converting the light into pulses of electrons that can be manipulated using semiconductor devices — a process that involves expensive and power-hungry components.
The problem is that there is currently no practical way of performing logical operations on light itself. Some physicists believe that the solution lies in a compromise of sorts — converting light into excitons, which behave like both photons and electrons.

Electron-hole pair

An exciton is created when a photon is absorbed by a semiconductor, creating an electron-hole pair that propagates through the material until it annihilates to create another photon. While a photon cannot be easily controlled with an applied voltage, an exciton comprises two charged particles and can therefore be manipulated by applying a voltage across the semiconductor.
Now, Leonid Butov and colleagues at the University of California at San Diego and Santa Barbara, have used this property to create the first excitonic integrated circuit (EXIC).
Their device is made from three identical switches that were fabricated on a wafer of gallium arsenide (GaAs). Each switch contains two quantum wells (extremely thin layers of doped GaAs) that are separated by several nanometres. When an exciton is created in the region of the wells, the electron and hole move apart and the electron travels through one quantum well and the hole through the other (Sciencexpress).

Less likely to annihilate

Because the electron and hole are separated, these “indirect” excitons are much less likely to annihilate and can therefore travel hundreds of micrometres along the quantum wells, before annihilating at the output of the switch to create light.
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However, if a voltage is applied to the wells, the pairs face an energy barrier and cannot travel along the wells. As a result, no light reaches the output.
The switches were arranged in a “three beam star”, with one optical terminal of each switch connected at a central axis point. This circuit can be used in a number of different switch configurations — depending on the voltages applied to the individual switches and which terminals of the star are used as inputs and which are used as outputs. For example, the device was operated as a two-way switch in which an input optical signal to one switch could be routed to the output of one of two other switches.
One downside of the EXIC in the GaAs structure is that it only works at temperatures below about 40K, because at higher temperatures the electron and hole will not bind to create an exciton.
However, this binding temperature is a property of the semiconductor and higher temperatures could be achieved using a material other than GaAs.
The team have also shown that these long-lived excitons can be cooled rapidly to very low temperatures. As a result, Butov’s next project is to study the fundamental physics of an ultracold gas of excitons, which is an ultracold bosonic system.