A Terahertz Quantum Cascade Laser Uses Sound Pulses to Speed Data Transmission

Researchers from the UK’s University of Leeds (UoL) and the University of Nottingham (UoN) have been focusing on ways to use sound and light to generate ultra-fast data transfer by controlling terahertz quantum cascade lasers. 

Quantum cascade lasers are a technology thought by physicists to be the key to increasing data transmission rates to levels of 100 gigabits per second or more. To put this into context, such a level of speed is around one thousand times faster than is currently possible by using Ethernet, which operates at 100 megabits per second. 

 

Bypassing Physical Limitations with Acoustic Waves

Terahertz quantum cascade lasers (QCLs) are capable of emitting light in the terahertz range of the electromagnetic spectrum. They are invisible and have many promising properties, including being able to penetrate materials that are opaque to visible light and detect a variety of molecules. 

To transmit data, however, data must be encoded onto the laser beam, and the laws of physics limit how quickly electronic systems can modulate this. To bypass this limitation, the researchers used an acoustic wave to modulate light. 

 

The quantum cascade laser designed by University of Leeds and University of Nottingham researchers. Image used courtesy the University of Leeds.

 

A QCL consists of a series of quantum wells. These are small areas that hold electrons at specific energy levels. When one drops from one well to the next, it emits a photon, allowing a single electron to produce several photons.

To modulate the emission of these photons and successfully encode data onto the QCL’s beam, the research team attached a thin aluminum film to one of its contacts. This film was then hit with pulses from a different type of laser. Each pulse caused the aluminum to produce an acoustic wave that then ran through the QCL and slightly deform the structure.

“It’s as if the whole system’s being shaken really,” says John Cunningham, a professor of electronic and electrical engineering at UoL who led the research. “It changes the probability of electron transfer between the quantum wells.”

 

Achieving 100 Percent Modulation

To create the prototype system, the team used an off-the-shelf QCL. This achieved modulation of only around 6 percent. However, Cunningham said that it should be possible to achieve 100 percent modulation by redesigning the laser so that the quantum wells are specifically designed and engineered to respond to acoustic waves. 

Where electronic circuits can modulate a laser at a few tens of gigahertz, Cunningham says that an acoustic system like his could increase this to hundreds of gigahertz, potentially more. 

Cunningham has also indicated that he would like to incorporate a sonic equivalent to a laser, a saser, invented by physicists at UoN that contributed to the UoL team’s research. This, it is said, would make the system more compact and efficient. 

Further trials will take place to adjust the process and towards achieving full control over the photon emissions from the laser.