Photonic-crystal surface-emitting laser (PCSEL)
A semiconductor laser whose 2D photonic crystal provides in-plane feedback over a large area and emits vertically: watt-class single-mode power with sub-degree divergence, without facets.
A PCSEL replaces both the facet mirrors of an edge emitter and the DBR stack of a VCSEL with a two-dimensional photonic crystal layer next to the active region. The lattice couples counter-propagating in-plane waves into one coherent standing pattern across the whole crystal and diffracts a fraction vertically as the output. Because the resonator is the entire patterned area, the emitting aperture can be hundreds of microns to millimeters across while remaining single-mode, which breaks the oldest trade in diode lasers, power against beam quality.
The reported numbers explain the attention: continuous-wave single-mode operation at watt level from sub-millimeter apertures, tens of watts from scaled devices in the Kyoto lineage, divergence well under a degree straight out of the chip, and brightness rivaling fiber-coupled diode stacks with no external optics. Double-lattice and backside-reflector designs suppressed the competing band-edge modes that once capped size. Frequency-modulated PCSELs demonstrated for free-space links in 2025 extended the story from raw power toward coherent function, and the first dedicated commercial suppliers and defense programs moved the device from conference highlight to procurement item, with lidar illumination, free-space optical links, and direct-diode materials processing the lead applications.
Characterization is classic laser-diode work at unusual operating points, which is what earns the entry here. LIV sweeps run at high current density over large areas, so junction temperature extraction and thermal resistance measurement, of the kind covered by this site's junction temperature methods, dominate reliability qualification. Beam quality is certified by M-squared measurement rather than assumed from the aperture, and spectral single-modedness under current and temperature excursions is the acceptance test that separates a true PCSEL from a large LED-like emitter.
References: K. Hirose et al., Nat. Photonics 8, 406 (2014); M. Yoshida et al., Nat. Mater. 18, 121 (2019); frequency-modulated PCSEL free-space results, Nat. Photonics (2025).