Post-growth optical performance engineering of VCSELs using monolithic high-contrast grating mirrors

This dissertation explores the replacement of the majority of the top coupling distributed Bragg reflector (DBR) of a vertical-cavity surface-emitting laser (VCSEL) with a monolithic high contrast gratings (MHCGs) as an alternative VCSEL design. Only needing one fabrication layer, the MHCG is fully defined by its three geometric parameters the grating period P, the grating fill factor F, and the grating height H, which in turn define its reflective properties, such as resonance wavelength and side mode suppression. By making the MHCG the dominant factor in the reflective properties of the top mirror we want to exploit it as a flexible tool to engineer the emission characteristics of a composite MHCG DBR VCSEL. Even though the epitaxial material used in this study does not allow a large tuning of the emission wavelength, this work is indented as a proof-of-concept that multiple sets of grating geometries exist for the same epitaxial structure which can sustain lasing. Within this work we fabricate and characterize composite MHCG DBR VCSELs with a systematic variation of the grating period P, the grating fill factor F, and the oxide aperture diameter phi and investigate the influence of these design parameters on the VCSEL performance. The results of the MHCG VCSELs are compared to measurements of epitaxially identical double DBR VCSELs to directly contrast benefits and drawbacks of this alternate VCSEL design. The thesis also describes a processing flow to fabricate MHCGs with a large geometric variety by means of electron beam lithography and integrate them into a subsequent ultra-violet lithography process flow without the need of resource intensive computing for proximity corrections. Furthermore, this work suggests and demonstrates a process flow that allows the transfer of VCSEL arrays onto a Si substrate, paving the way for GaAs-based semiconductor lasers on silicon-based integrate photonic chips as well as exposing the backside of the transferred mesa, as a first step towards the fabrication of a VCSEL structure with MHCGs on both sides. During the characterization of the fabricated devices we report record static light output power-current-voltage (LIV) performance for our MHCG DBR VCSELs with threshold currents as low as 0.25 mA, wall plug efficiencies of 8%, optical output powers exceeding 1.6 mW, and stable linearly polarized emission with orthogonal polarization suppression ratios (OPSRs) > 33 dB. By varying the grating designs, record current-induced wavelength tuning ranges up to 13.4 nm (multi-mode) and 9 nm (single-mode) and single-mode emission with a side-mode suppression ratio (SMSR) of up to 46 dB and > 40 dB even beyond thermal rollover are achieved. Moreover, the MHCG DBR VCSELs feature excellent thermal properties with thermal resistance of only 2.46 K/mW, and we observe grating design-dependent side-mode suppression behavior; from a normal oxide aperture diameter dependent number of side modes up to oxide aperture diameter independent single-mode emission up to a 9 µm oxide aperture diameter. We show a grating geometry dependent emission wavelength tuning of 6 nm, which exceeds the simulations for this epitaxial structure and showcases the potential for post growth resonance wavelength tuning via the MHCG geometry. We show that the emission wavelength tuning is mainly limited by the remaining DBR pairs in the top coupling mirror and show a potential of e.g. 25 nm tuning for a MHCG-only top mirror. As such this work paves the way for multi-wavelength 2D VCSEL arrays fabricated on the same wafer piece. This work further reports the first small-signal modulation frequency response measurements of MHCG based VCSELs with record bandwidths up to 25 GHz for single-mode MHCG DBR VCSELs and up to 30 GHz for two- and three-mode MHCG DBR VCSELs, which was also the maximum achieved by the double DBR VCSELs of the same epitaxial material. This shows, that the MHCG design puts no fundamental limit to the modulation bandwidth of the epitaxial material. Additionally, we investigate the influence of the grating design on the dynamic VCSEL properties. The highest modulation bandwidths are achieved at significantly larger oxide aperture diameters compared to the double DBR VCSEL design, reducing the current densities during operation and promising improved device life times. Furthermore, by replacing the majority of the p-doped DBR the input impedance and thus the ohmic losses of the MHCG VCSEL are reduced significantly from 150 to 25 Ohms. This work demonstrates that MHCGs, as a key source of optical power reflectance, can be strategically designed to tailor a VCSEL's output characteristics. By varying the MHCG geometry, we can produce side-by-side VCSELs (on the same epitaxial wafer) with vastly different emission and performance properties.