Over 1.65 GWcm⁻²sr⁻¹ brightness yellow VECSEL

Yellow lasers operating in the 560–600 nm wavelength range play critical roles in numerous high-demand fields, including atomic cooling and trapping, optogenetics, ophthalmic diagnosis and treatment, and sodium guide stars. For instance, in sodium guide star technology, 589 nm lasers can undergo resonant interaction with the sodium atomic layer at an altitude of 80–100 km, thereby generating high-brightness fluorescent radiation in the reverse direction and fulfilling a pivotal function in the imaging correction system of astronomical telescopes. Among various laser technologies, intracavity second harmonic generation (SHG) based on vertical external cavity surface-emitting lasers (VECSELs) emerges as a novel approach to achieving a high-brightness and high-efficiency yellow source.

 

Two unresolved core issues still hinder the commercialization of yellow VECSELs. First, the indium content in InGaAs quantum wells needs to be increased to ~40% to achieve 1.2 μm gain. This impairs lattice matching between quantum wells and the GaAs substrate, thereby inducing misfit dislocations in epitaxial growth. On the other hand, MOCVD has the advantages of a high growth rate and superior mass-production compatibility, but its gas-phase epitaxial growth mode, combined with relatively high growth temperature, poses greater challenges to strain modulation and interface quality control.

 

In a new paper published in Light: Science & Applications, a semiconductor laser team from the College of Advanced Interdisciplinary Studies, National University of Defense Technology, and Suzhou Everbright Photonics Co., Ltd has made significant progress in 1180 nm gain chip MOCVD epitaxy and high-brightness yellow second-harmonic generation.

 

To enhance the performance of 1180 nm fundamental-frequency gain chips, they optimized chip design and epitaxial growth: the flip-chip chip structure was modified to enable substrate removal and boost thermal load capacity; a tensile-strained GaAsP layer was designed to compensate for the compressive-strained InGaAs active region, with GaAs insertion layers suppressing In/P interdiffusion at quantum well interfaces. Breaking the MOCVD bottleneck for high-strain epitaxy, an innovative variable-temperature growth strategy was proposed—low-temperature growth suppressed indium segregation, while higher-temperature GaAsP growth improved phosphorus incorporation efficiency to optimize overall lattice strain. Repeated temperature ramping-induced multiple quantum well annealing effectively reduced crystal defects. Subsequent tests verified significant improvements in crystal quality and thermal stability, with the fabricated 1180 nm gain chip delivering over 45 W continuous-wave power and a slope efficiency exceeding 50%.

 

Furthermore, the technical advantages of the VECSEL in generating ultra-high-brightness yellow lasers were verified. To address the brightness degradation caused by high-order transverse mode oscillation, the resonator structure for intracavity second harmonic generation was optimized, and pump parameters were adjusted to suppress the gain of high-order modes. Test results demonstrated that the 590 nm yellow laser achieved an output power of 6.2 W with a slope efficiency exceeding 17%. The laser output maintained a fundamental mode close to the diffraction limit, with a beam quality factor (M²) < 1.1, and the calculated brightness exceeded 1.65 GW·cm⁻²·sr⁻¹, whose performance is comparable to that of solid-state lasers and fiber lasers.

 

Future outlook: Adopt novel strain engineering (e.g., strain buffer layers, patterned substrates) to improve lattice mismatch; strengthen in-situ monitoring for precise growth front control, combined with reactor optimization to reduce gas-phase memory effects; optimize doping and pumping for efficient electrical/optical pumping, enabling small-volume, lightweight applications and accelerating VECSEL’s lab-to-commercialization transition.

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