Spin lasers could push data into the fast lane

Ashley N. Paddock, ashley.paddock@photonics.com

A new concept for ultrafast semiconductor lasers uses the intrinsic angular momentum of electrons – also known as spin – to break previous speed barriers, with the potential to achieve modulation frequencies of well above 100 GHz. The results could be a significant step toward high-speed data transmission for the Internet of the future.

Relaxation oscillations (a) mark the maximum speed achievable in conventional semiconductor lasers. By injecting spin-polarized electrons in a micro-cavity laser, oscillations that can be much faster than the relaxation oscillations are generated in the polarization of the light field (b). Because the oscillation lifetime can easily be tuned via the current (c), such spin lasers are suited for optical data transmission. CPD = Circular polarization degree.

Researchers at Ruhr University Bochum were inspired by spintronics – combining quantum mechanical spin with semiconductor-based electronics – to develop the next generation of electronic and optoelectronic devices.

Optical data transmission by semiconductor lasers is a prerequisite for the globally networked information technology world. The maximum speed of conventional semiconductor lasers has been a limiting factor; typical modulation frequencies are at levels well below 50 GHz.

By using spin lasers, the researchers overcame the previous limits for the modulation speed. In conventional lasers, the spin of the electrons injected is entirely arbitrary, while in spin lasers, only electrons with a previously determined spin state are used. Injecting these spin-polarized electrons forces the laser to work simultaneously on two laser modes with different frequencies.

Injecting spin-polarized electrons into semiconductor-based microlasers results in modulation speeds that are superior to those of conventional lasers. Courtesy of Dr. Nils Gerhardt et al, Ruhr University Bochum.

The frequency difference can be tuned easily using birefringence in the resonator, according to researcher Dr. Nils Gerhardt. This is done simply by bending the microlaser. By coupling the two laser modes in the microresonator, oscillation with a new frequency occurs, which theoretically can reach well over 100 GHz.

The results were published online in Applied Physics Letters (doi: 10.1063/1.3651339).

“The next step will be to develop a spin laser with a modulation bandwidth significantly higher than 100 GHz in order to demonstrate the full potential of our concept,” Gerhardt said. “Furthermore, we want to investigate the polarization dynamics in detail to find the fundamental limitation for the maximum modulation speed.”

The team must address electrical spin injection at room temperature to make such devices commercially viable, Gerhardt added.