Due to physical limits, data transmission based on direct modulation of the light intensity cannot take place much faster than with a bandwidth of 40 to 50 gigahertz without complex modulation concepts. In order to achieve such high bandwidths, high electrical currents are required; the energetic efficiency of data transmission decreases massively with increasing bandwidth. “Unless we change the technology soon, data transmission and the Internet will consume more energy than we currently produce on Earth,” predicts Prof. Dr. Martin Hofmann of the University of Bochum. Together with colleagues, Martin Hofmann is therefore researching an alternative technology.
Using lasers measuring just a few micrometers, provided by a team from the University of Ulm, the researchers generate a light wave whose oscillation direction changes periodically in a special way. This is circularly polarized light that is produced by superimposing two light waves polarized linearly perpendicular to each other.
In linearly polarised light, the vector that describes the electric field of a light wave oscillates constantly in one plane. With circularly polarized light, it rotates around the direction of propagation. The trick: If the two linearly polarized light waves differ in their frequency, an oscillating circular polarization is created in which the direction of oscillation reverses again and again – at an adjustable speed. The researchers have now experimentally shown that oscillation can take place at 200 gigahertz. How much faster it can still become is unclear. A theoretical limit has not yet been defined.
Oscillation alone does not transport information, however, and polarization must be modulated for this to happen. The researchers from Bochum have experimentally confirmed that this is possible in principle. Using numerical simulations, together with the team led by Prof. Dr. Igor Žutić and PhD student Gaofeng Xu from the University at Buffalo, they also showed that modulation of polarization and thus information transmission with more than 200 gigahertz is theoretically possible.
In order to generate a modulated circular polarization, two factors are decisive: The laser must be operated in such a way that it simultaneously emits two light waves polarized perpendicular to each other, the superposition of which produces the circular polarization. In addition, the frequency of the two emitted light waves must be sufficiently different for fast oscillation to occur.
The laser light is generated in a semiconductor crystal into which the researchers inject electrons and defect electrons. When they collide, light particles are released. The spin of the injected electrons is crucial for the light to achieve the desired polarization. Only when the spin of the electrons is aligned in a certain way does the emitted light have the appropriate polarization – a challenge because the spin alignment is quickly lost. The researchers must therefore insert the electrons as close as possible to the point in the laser where the light particle is to be produced. Hofmann’s team has already filed a patent application for an idea on how this can be achieved with the aid of a ferromagnetic material.
The frequency difference in the two emitted light waves required for oscillation is generated using a technique developed by the Ulm team led by Prof. Dr. Rainer Michalzik and PhD student Tobias Pusch. The semiconductor crystal used is birefringent. The refractive index is therefore slightly different for the two perpendicularly polarized light waves that emerge from the crystal. As a result, the waves have different frequencies. By bending the semiconductor crystal, the researchers can adjust the difference in the refractive index and thus the frequency difference. It determines the speed of oscillation, which could ultimately be the basis for accelerated data transmission.
The researchers admit that the method cannot yet be used in practice. It is said that “much more technical optimization” is required. After all, the study shows the potential of the spin laser for broadband data transmission.