Creating a blue nanolaser

Blue lasers emit light in a wavelength that our eyes see as blue or purple (400-480 nm). In Blu-ray discs, this short wavelength (405 nm) makes the laser’s “point” extremely tiny, allowing it to write and read information far more densely than the red laser of DVDs (650 nm). Researchers at EPFL have now developed a high-quality, room-temperature blue laser nanobeam that could even enter inside a cell. The system is deposited on silicon, making it both biologically compatible and cost-effective. The work is published in Nano Letters.

The group of Nicolas Grandjean at EPFL developed a blue laser using a method based on light-controlling devices called “photonic crystal nanocavities”. These are tiny devices that can trap light between mirrors and hold it in a small space for a fraction of a second. The researchers built nanocavities – which look like a series of tiny holes – on gallium nitride, which was chemically “grown” onto a silicon wafer. Gallium nitride is contains two energy bands: one that is rich in electrons and one that is rich in electron holes. Notably, gallium nitride is used in blue LEDs, which won the Physics Nobel prize in 2014.

Working at room temperature, electrons are injected into the semiconductor and combine with electron holes to release energy in the form of photons, which are trapped inside the nanocavity. The photons then interact with more incoming electrons, creating even more photons in a self-sustaining manner. The photons are then ejected like a narrow beam of blue-colored laser.

Lasers produced in this way are referred to as a “one-dimensional photonic crystal nanobeams”. One way they can be evaluated is by measuring the sharpness of their resonance. Referred to as “Q-factor”, it describes how much light the nanocavity can store for a prolonged period of time; thus, a high Q-factor means very little energy loss. The system that Grandjean’s team produced registered one of the highest Q-factors for short-wavelength lasers, meaning that it is outstandingly energy-efficient.

Because the laser is made with biologically-friendly materials, it could also be used in a cutting-edge field called “optogenetics”. Here, light is used to turn genes inside cells on and off, leading to global biological changes, such as brain function. The short wavelength of Grandjean’s blue laser, combined with its energy efficiency at room temperature, paves the way for an entirely new generation of biological tools. Additionally, it provides an invaluable new tool for fundamental nano-scale experiments for studying the interactions between light and matter, such as explored by the field of optomechanics.

“This is both a breakthrough in terms of basic science and applied technology,” says Grandjean. “The design of our nanocavity is simple and cheap to manufacture, making the laser very attractive to optomechanics, high-integration density, low-power consumption photonic circuits, and needle-like biosensors. But here, lasing is obtained through ‘optical pumping’ by creating of electron-hole pairs that recombine to eventually give photons. The next step would be to make an actual device that where such electron-hole pairs are created by current flowing through the laser.”

Reference

Vico Triviño N, Butté R, Carlin J-F, Grandjean N. Continuous wave blue lasing in III-nitride nanobeam cavity on silicon.Nano Letters 13 Jan 2015. DOI: 10.1021/nl504432d