QDs in microcavities advance blue diode lasers

Dr. Jörg Schwartz, joerg.schwartz@photonics.com

Newly developed monolithically integrated nitride laser microcavities that incorporate quantum dots (QDs) could be an important step in making better blue and green diode lasers as well as single-photon sources.

A team of solid-state physicists at the University of Bremen were able to introduce InGaN quantum dots into fully epitaxial monolithic microcavities that they grew by metallorganic vapor phase epitaxy. They then added pillar-shaped microcavities by focused ion-beam etching. Their impact was measurable via microphotoluminescence measurements, which showed discrete resonator modes of the microcavities, as well as emission lines of single quantum dots. Pillar-structured microcavities exhibit single-mode emission, which permits control of the direction and polarization of the emission.

Confining the light in a very small optically active material is a well known way to improve the efficiency of laser devices – and to change their material properties. This is where microscopic cavities and quantum dots come in. In recent years, new ways to manipulate the light-matter interaction have been found by modifying the geometry of photonic components at nanometer scales. Such photonic structuring, applied in photonic crystals, can modify the optical density of states, and it even allows emission and absorption rates to be enhanced or suppressed.

This is known as the Purcell effect and can be used to control spontaneous emission – an interesting option for those who want to study quantum effects or get single photons. On the other hand, semiconductor microcavities offer potential for new optoelectronic devices and optical switches, but also for more exotic things like spin-memory elements and polariton devices.

Monolithic pillars etched into the gallium nitride laser material act as microcavities, exhibiting characteristic modes in the emission spectrum. Courtesy of the University of Bremen.

“To date, blue diode lasers’ microcavities are made using hybrid technologies,” said Dr. Kathrin Sebald, a member of the research team. “This means that the laser mirrors have been added using a different material platform, such as SiO₂/Si₃N₄.” Besides the production advantages of using just a single material platform, this also offers better support for including quantum dots, she explained.

Quantum dots in a laser medium strongly confine charge carriers, which leads the quantum dots to exhibit an electronic structure similar to that of atoms. As a result, the laser’s performance is closer to a gas laser’s and avoids some of the disadvantages of traditional semiconductor lasers, with improvements in modulation bandwidth, lasing threshold, relative intensity noise, linewidth enhancement factor and temperature insensitivity.

The quantum dot active region may also be engineered to operate at different wavelengths by varying quantum dot size and composition. This allows quantum dot lasers to be fabricated to operate at wavelengths previously not possible using semiconductor laser technology.

This is expected to be a benefit for LED and vertical-cavity surface-emitting laser applications. “One issue to be addressed still is the Q-factor of the device,” Sebald said, referring to the ability of the cavity to confine the light. “In this respect, the monolithic solution does not yet match the hybrid approach.”