Vertical cavity surface emitting lasers, or VCSELs, are among the most widely used laser sources today, powering applications from data communications to consumer products such as optical mice and smartphones for face recognition and ranging. Despite their success, VCSELs rely on a complex vertical epitaxial stack that includes bottom and top distributed Bragg reflectors around a gain region. These mirrors typically require many precisely grown layers, which increases growth complexity and limits practical wavelength choices, with the most common devices operating near 850 nm and 980 nm. Moving VCSELs to other wavelength bands, or integrating multiple wavelengths on the same wafer, is difficult, and monolithic integration with silicon photonics and electronics is typically not straightforward.
In a new paper published in Light: Science and Applications, a team of researchers led by Professor Dries Van Thourhout at the Photonics Research Group of Ghent university - imec, Belgium and dr. Bernardette Kunert at imec, report a new class of surface emitting lasers grown directly on standard 300 mm silicon wafers. Using aspect ratio trapping and nano-ridge engineering, the researchers grow high quality III-V active material on silicon in the form of ordered nano-ridge arrays and then engineer these arrays as a one-dimensional photonic crystal that supports a symmetry protected bound state in the continuum mode. This approach enables strong in-plane confinement together with vertical surface emission from a compact device footprint, while offering straightforward wavelength control through the nano-ridge geometry and array period.
A major advantage of the nano-ridge surface-emitting laser approach is flexibility in wavelength and scalability. Unlike conventional VCSELs, where the emission wavelength is tightly linked to a complex, many-layer mirror stack that is difficult to shift beyond the most common communication bands, the nano-ridge device derives its lasing condition from a one-dimensional photonic-crystal mode in the ridge array. As a result, the emission wavelength can be tuned at wafer scale by adjusting design parameters such as the array period and nano-ridge width, and can also be trimmed through post-processing steps like adding dielectric layers. “With VCSELs you quickly run into wavelength and growth complexity limits,” said Professor Dries Van Thourhout. “Here, the wavelength becomes a design parameter, which makes multi-wavelength arrays on the same wafer a realistic target.”
From a physics perspective, the work leverages bound states in the continuum, a counterintuitive confinement concept first proposed in quantum mechanics by von Neumann and Wigner in 1929. In the nano-ridge photonic crystal, a symmetry-protected state supports strong in-plane confinement while still enabling vertical emission, allowing a compact surface-emitting cavity on silicon. “It is exciting because this mechanism lets us confine a lasing mode inside nano-ridges that are only about 500 nanometers tall, directly above a silicon substrate, and still achieve surface emission from a very small footprint,” said PhD candidate and first author Eslam Fahmy. “The nano-ridges are not just the gain medium: they also form the cavity, confine the mode, and let the light couple out vertically.”
Finally, because the devices are epitaxially grown directly on standard 300 mm silicon wafers, the approach is well aligned with wafer-scale manufacturing and co-integration with other photonic and electronic components. While this demonstration uses optical pumping, the authors point to their previously demonstrated electrically injected in-plane nano-ridge lasers as a clear route toward electrically pumped nano-ridge surface emitters, provided the contacts are designed to avoid disturbing the optical mode and blocking the surface emission. “This is the first breakthrough toward electrically pumped surface emitters on the nano-ridge platform,” Eslam Fahmy added. “It combines compactness, tunability, and manufacturability in a way that is very hard to achieve with traditional surface-emitting lasers.”
Light Science & Applications
One-dimensional photonic crystal nano-ridge surface emitting lasers epitaxially grown on a standard 300 mm silicon wafer