Blue laser operation lacks a comprehensive theory

Blue lasers emit electromagnetic radiation whose wavelength falls within the blue or violet (400-480 nanometers) light spectrum. Since their discovery in the 90’s, they have been used extensively in information technology, environmental monitoring, and medical diagnostics; but their best-known application is the high-definition Blu-ray player. In 2014, the Nobel in Physics was awarded to the inventors of the first blue light-emitting diode (LED), which was a crucial step towards the blue laser. Nonetheless, the exact way blue laser light is generated has never been explained adequately. EPFL scientists have now shown that blue lasers feature unexpected event between electrons and electron holes that does not fit any known theory. Their work is published in Nature Communications.

One of the most common types of blue laser is produced using a semiconductor made of gallium-nitride (GaN). The semiconductor consists of two layers of GaN, one that is rich in electrons (n-type) and one that is rich in electron holes (p-type). Put together, the two layers create what is known as a p-n junction diode. When electrons are injected into the GaN semiconductor, they combine with the electron holes to release energy in the form of photons. The photons then interact with more incoming electrons, creating even more photons, in a self-perpetuating process called resonance. The photons are then ejected through a mirror, generating the blue-colored laser.

A team led by Benoît Deveaud and Nicolas Grandjean at EPFL has now discovered that the interaction between electrons and electron holes in the GaN semiconductor is stranger than previously theorized. When electrons interact with the positively charged electron holes, they form a quasiparticle called an exciton, which are neutral and can transfer energy without transferring electrical charge. Excitons also begin to combine together to form bi-excitons, creating an insulating gas of excitons and bi-excitons inside the GaN semiconductor.

When the density of this gas increases to a certain point, it is expected that it will eventually transition from an insulating gas of excitons to a conductive plasma of electrons and holes. This event is called the Mott transition after Nobel laureate Sir Nevill F. Mott who predicted it in 1949. The Mott transition can have significant effects on the optical and electrical properties of semiconductors, affecting the stability and quality of the lasers they generate.

Investigating the fundamental aspects of the Mott transition in blue lasers, the EPFL scientists observed that, against common beliefs, the biexcitons in a GaN semiconductor are more stable towards the Mott transition than the excitons. This is significant because current views on the Mott transition in semiconductor lasers neglect bi-excitons because they are expected to be less robust.

“The remarkable thing is that there is currently no theory to explain this observation,” says Benoît Deveaud. “It is entirely new.” The team’s next step would now be to engage the theoretical physics community to develop a new theory that explains their findings, which call for a thorough review of the theories of biexciton stability in the high-density regime of laser semiconductors.

Reference

Shahmohammadi M, Jacopin G, Rossbach G, Levrat J, Feltin E, Carlin JF, Ganière JD, Butté R, Grandjean N, Deveaud B. Biexcitonic molecules survive excitons at the Mott transition.Nat Commun. 2014 Oct 24;5:5251. doi: 10.1038/ncomms6251.