Low-threshold wavelength-tunable ultraviolet vertical-cavity surface-emitting lasers from 376 to 409 nm

Optically pumped wavelength-tunable vertical-cavity surface-emitting lasers(VCSELs)operating in the ultraviolet A(UVA)spectrum were demonstrated.The VCSELs feature double dielectric distributed brag reflectors and a wedge-shaped cavity fabricated using the substrate transfer technique and laser lift off,resulting in a graded cavity length in one device.A resonant period gain structure is used in the InGaN/GaN multi-quantum well active region to enhance the coupling between the cavity mode field and the active layers.The optical field inside the cavity is modulated by the cavity length;thus,tunable lasing at different wavelengths is realized at different points of a single VCSEL chip.The lasing wavelength extends from 376 to 409 nm,covering most of the UVA band below the band gap of GaN.The threshold pumping power density of the UVA VCSELs at different wavelengths ranges from 383 to 466 kW/cm^(2),which is among the lowest values for ultraviolet(UV)VCSELs.This study is promising for the development of small-footprint,power-efficient UV light sources.