CAREER: Short-Wavelength Vertical-Cavity Surface-Emitting Laser Arrays Using Nonpolar and Semipolar GaN

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The research objective of this Faculty Early Career Development (CAREER) Program award is to demonstrate the first short-wavelength (blue and green) vertical-cavity surface-emitting laser (VCSEL) arrays with stable and predictable light polarization. The approach will use an innovative fabrication process to address issues with GaN-based materials that have previously limited the output power, wavelength range, and functionality of short-wavelength VCSELs. Fundamental challenges that have prevented systematic control of light polarization in VCSELs will be overcome with novel materials growth processes. The realization of short-wavelength VCSELs with stable and predictable light polarization will enable unique applications in high-density optical data storage, high-resolution printing, lighting, displays, projectors, miniature atomic clocks, and chemical/biological sensing that are not easily accomplished with conventional edge-emitting lasers. To address a variety of polarization-sensitive applications that require higher-power light emitters, the devices will be formed into polarization-pinned arrays. The educational and outreach components are aimed at promoting interest in science, technology, engineering, and mathematics (STEM) disciplines and developing scientific literacy at the undergraduate and K-12 levels. These activities are focused on working with underrepresented students, including Native Americans, Hispanics, and women. Specific educational objectives are to collaborate with the Southwestern Indian Polytechnic Institute on an education and outreach program, continue mentoring activities for underrepresented populations and local K-12 teachers, and develop a combined undergraduate/graduate course in fabrication techniques for optoelectronic devices. The unique material properties of nonpolar and semipolar orientations of Gallium nitride (GaN) will be combined with a novel flip-chip fabrication scheme to enable the first polarization-pinned GaN-based VCSEL arrays and the first semipolar GaN-based green VCSELs. The novel fabrication scheme utilizes band-gap-selective photoelectrochemical etching to remove the growth substrate, enable high-reflectance dielectric distributed Bragg reflector mirrors, and provide fine control of the cavity length. The nonpolar and semipolar GaN platform exhibits large and highly anisotropic optical gain, which provides a simple method to achieve polarization pinning in short wavelength GaN-based VCSELs. When combined, these approaches enable blue and green VCSEL arrays that are applicable to polarization-sensitive optical systems and provide orders-of-magnitude higher output powers than single VCSELs. An output power of 50 mW from a 10x10 element nonpolar blue VCSEL array is set as an initial target. The thorough study of nonpolar and semipolar VCSELs will generate new fundamental knowledge about the correlations between polarization-pinning with band structure, higher-order modes, gain-cavity resonance offset, and operating conditions. The polarization, spectral purity, and noise properties of individual devices as well as arrays will be investigated with confocal microscopy and correlated to the material and device properties. Finally, the first investigation of band-gap-selective photoelectrochemical etching on semipolar GaN may reveal new fundamental materials phenomena and be applicable to other semipolar light-emitting devices.