Conventional semiconductor lasers, such as Fabry–Pérot (FP) cavity lasers, distributed feedback (DFB) lasers, and vertical-cavity surface-emitting lasers (VCSELs), cannot simultaneously achieve single-mode operation, high power output, and a small divergence angle. However, photonic crystal surface-emitting lasers (PCSELs) utilize Bragg diffraction in two-dimensional photonic crystals to achieve high-power, single-mode laser output with a small divergence angle (Figure 1), making them one of the hot research topics both domestically and internationally.
Gallium nitride (GaN)-based semiconductor materials have a direct bandgap, with emission wavelengths spanning the visible to deep ultraviolet spectrum, offering advantages such as high luminous efficiency and excellent chemical stability, making them suitable for PCSEL fabrication. GaN-based PCSELs hold promising applications in emerging fields such as novel displays, material processing, laser lighting, underwater communication, interstellar communication, chip atomic clocks, deep-space exploration, atomic radar, and laser medicine, attracting widespread attention.
Figure 1. Structural diagrams, typical far-field divergence angles, and output spectral characteristics of FP edge-emitting lasers, DFB edge-emitting lasers, VCSELs, and PCSELs.
Professor Noda's team at Kyoto University in Japan first proposed the concept of PCSELs in 1999 and reported the first room-temperature electro-injection lasing of GaN-based violet PCSELs in Science 319, 445 (2008). Subsequently, in collaboration with Japan's Stanley Company in 2022 and Japan's Nichia Company in 2024, they further extended the emission wavelength of GaN-based PCSELs to the blue and green light bands. Currently, only Japan has achieved electrical injection-induced emission of GaN-based PCSELs globally.
In collaboration with the Key Laboratory of Semiconductor Display Materials and Chips and the Suzhou Laboratory, both established by the Suzhou Institute of Nanotechnology and Nanoscience of the Chinese Academy of Sciences, a GaN-based photonic crystal surface-emitting laser (PCSEL) was recently developed, and room-temperature electro-injection lasing was achieved.
The research team first simulated and designed the structure of the GaN-based PCSEL device, then epitaxially grew high-quality GaN-based laser materials, and developed low-damage photonic crystal etching and passivation processes to fabricate the GaN-based PCSEL device, with a photonic crystal area size of 400×400 μm² (Figure 2). By measuring the band structure of the GaN-based PCSEL in the Γ-X direction using angle-resolved spectroscopy (Figure 3), it was observed that: at low injection currents, the band structure is clear, with mode C having the highest intensity; as the current increases, the intensity of non-radiative mode B significantly enhances until lasing occurs. By measuring the band structure, it was determined that the device lases in the fundamental mode B, with a mode half-width of approximately 0.05 nm near the threshold current.
Figure 2. (a) Schematic diagram of the GaN-based PCSEL structure, (b) photonic crystal band structure obtained from optical pumping tests, and (c) surface and (d) cross-sectional scanning electron microscope images of the photonic crystal.
Figure 3. (a–e) Band structure of the GaN-based PCSEL in the Γ–X direction measured at different injection currents, (f) Curve showing the variation of the peak wavelength and spectral half-width of the GaN-based PCSEL with injection current.
Based on the above work, the research team achieved room-temperature electrical injection lasing of a GaN-based photonic crystal surface-emitting laser (Figure 4), with a lasing wavelength of approximately 415 nm, a threshold current of 21.96 A, a corresponding threshold current density of approximately 13.7 kA/cm², and a peak output power of approximately 170 mW. In the next step, the team plans to use high-quality GaN single-crystal substrates to design a new GaN-based PCSEL structure and overcome challenges in PCSEL device fabrication and packaging/thermal management technology to achieve high-power (10–100 W) single-mode laser output.
Figure 4. GaN-based PCSEL: (a) electroluminescence spectra at different injection currents, (b) output optical power-current-voltage curves, (c) far-field spot, and (d) before and (e) after near-field images of GaN-based PCSEL lasing.
