Researchers Demonstrate VCSEL Control Using Ultrathin Metalenses

Researchers led by Patrice Genevet of the Centre de Recherche sur l’Hétéro-Epitaxie et ses Applications (CRHEA) at the University Côte d’Azur and collaborators at the Key Laboratory of Optoelectronics Technology at Beijing University of Technology have demonstrated a way to achieve single-axis control in vertical-cavity surface-emitting lasers (VCSELs) using flat, ultrathin optical structures called metalenses.

Beam deflection performance of the MS-VCSELs. a. Optical images of the designed metasurfaces for beam steering. b. Radius of the beams deflected at different angles as a function of the propagation distance, which can be well-fitted into Gaussian functions. c. Measured beam patterns of the MS-VCSELs with different deflection angles at the distance of Z=10 cm. Courtesy of Patrice Genevet.

Since first proposed by Kenicha Iga from the Tokyo Institute of Technology in 1977, VCSELs have emerged as a versatile and efficient light source. A team of researchers in France and China has now augmented the VCSEL’s capabilities by integrating a nanopatterned beam-shaping structure into each laser during wafer-scale processing. The team believes this approach could make it possible to create light wavefronts designed to order, and thus construct devices such as ultracompact programmable laser-on-chip arrays with whatever beam profiles are required.

Like other lasers, VCSELs contain a cavity in which light is generated and emitted to produce stimulated emission. Metalenses produce similar effects, but with a different mechanism. In the CRHEA team’s work, the metalenses are made from a semiconducting gallium arsenide (GaAs) film patterned with nanometer-size circular pillars.

These “nanopillars” are separated by distances shorter than the wavelength of the light they are designed to shape, and they act like optical antennas, introducing spatially varying phase delays in the light rays that pass through them and molding the light beam according to the desired profile. The result is a metasurface that can be tuned for specific wavelengths of light simply by changing the size, diameter, and spacing between the nanoantennas.

To collimate their laser, Genevet and his colleagues used a metalens design containing antennas of different shapes and sizes. These antennas cause the phase delays to be distributed radially around the lens, such that light rays are increasingly refracted farther away from the center, thereby shaping and focusing the wavefront of the incident light.

“The approach proposed is essentially nonintrusive and enables monolithic integration of optical lenses directly at the chip level,” Genevet said.

In their experiments, detailed in Nature Nanotechnology, the researchers fabricated VCSELs in a “back-emitting” configuration. They directly integrated the metasurfaces into the lasers by sculpting the bottom (substrate) surfaces of the lasers into them. The result, which they term a metasurface integrated (MS) VSCEL, means that the metalenses serve as purely passive beam-shaping elements and therefore do not alter the laser’s properties in any way or compromise its performance.

Genevet said he is looking into several applications his team sees as potentially interesting, “starting from basic lasers for computer mouses, Blu-ray sources, all the way to the realization of compact lidar systems.”

Genevet’s technique, which enables arbitrary wavefront engineering of laser emissions at the ultracompact wafer level, would simplify what he calls “the previously tedious optical assembly process.”