Novel Subwavelength Grating Surface Emitting Laser

We present a new class of surface emitting lasers utilizing a single-layer high-index-contrast subwavelength grating, instead of conventional DBR. Single polarization mode emission with low threshold current and 1mW output power was obtained at 850nm wavelength. The realization of VCSELs has been significantly limited by the choice of material available for making highly reflective distributed Bragg reflectors (DBRs). This remains to be one major bottleneck for blue-green and mid-wavelength infrared (MWIR) regimes. Furthermore, it is challenging to make wavelength-tunable VCSELs, where the requirements on mirror bandwidth and reflectivity are stringent [1]. Recently, we reported the experimental demonstration of a novel mirror design using a high-contrast subwavelength grating (HC-SWG), which exhibited with a very broad 1.3-1.8µm (∆λ/λ>30%) reflection spectrum with high reflectivity (R>99%) [2]. The SWG consists of only ONE single layer 1D grating structure formed by periodic stripes of high and low index materials (e.g. Si and air) that is sandwiched in-between two low index cladding layers (e.g. SiO 2 or air) on the top and below. The simplicity of the design and versatility to adapt to various wavelengths make HC-SWG an excellent reflector candidate to integrate with various optoelectronic devices. However, it was uncertain whether the HC-SWG can yield high enough reflectivity and substantially large angular tolerance required for a VCSEL. Here, we demonstrate a novel surface emitting laser using integrated HC-SWG as the top mirror of VCSEL, rather than conventional DBR, for the first time. Fig. 1 shows the device structure consisting of an active region with GaAs quantum wells in-between a SWG-based top mirror and 34 pairs of bottom DBR. The top mirror of the VCSEL comprises 4 pairs of DBR layers and a highly reflective SWG to provide optical feedback, where the gratings (periodic stripes of AlGaAs and air) are freely suspended, with air as the low index cladding layers on the top and bottom. Since the SWG design contains 1D symmetry, it is polarization sensitive – TM light polarized perpendicular to the grating lines sees higher reflectivity than the TE light. The calculated reflectivity for the TM polarized light is >99.9% for 0.83-0.88µm wavelength ranges while that for TE polarized light is merely 95%, hence making the SWG an excellent candidate for 850nm VCSEL with excellent polarization control. The SWG was defined on VCSEL mesas using electron-beam lithography on PMMA resist, which was then etched by reactive ion etching. A wet chemical based selective etching …