2.46 MHz Ultranarrow\nLine Width Vertical-Cavity Surface-Emitting\nLasers with Cholesteric Liquid Crystals Coupled Cavity

Due to their high level of integration, vertical-cavity\nsurface-emitting\nlasers (VCSELs) are promising candidates for applications in quantum\ncommunication and sub-Doppler atomic microsystems. However, there\nis an urgent need to develop narrow line width, high-quality VCSELs\nthat provide stable and reliable emission to meet commercial demands.\nHere, we demonstrate a novel coupled-cavity VCSEL integrated with\na cholesteric liquid crystal (CLC) film to develop high-performance\nultranarrow line width VCSELs. The CLC film can be flexibly integrated\nonto the surface of the VCSEL and applies weak light feedback which\nenhances the coherent superposition and compresses the line width\nof the VCSEL. It has been shown that when the length of the coupled\ncavity is less than 150 μm, it can suppress the higher-order\nmodes and significantly reduce the threshold current of VCSEL. Furthermore,\nthe CLC-VCSEL exhibits circular polarization output and an impressive\nline width of 2.46 MHz, which is the narrowest micron-scale integration\nVCSEL as reported, comparable to the line width of distributed Bragg\nreflection and distributed feedback semiconductor lasers. Compared\nwith recently reported high-performance VCSELs and semiconductor lasers,\nthe CLC-VCSELs achieve a lower threshold current and an enhanced beam\nquality. This micrometer-scale coupled cavity VCSEL will provide more\npossibilities for the development of next-generation VCSELs and integrated\npackages for CLC modulation. Moreover, the CLC optical feedback technique\nis not exclusive to VCSEL and can also be applied to other optoelectronic\ndevices, such as low-power edge-emitting lasers, quantum dot lasers,\nand so forth.