Photonic Crystal Surface Emitting Lasers

Abstract

Since the initial proposal in 1999 by Noda et al., photonic crystal surface emitting lasers (PCSELs) have proven to achieve large area, coherent lasing with a narrow, single mode beam. Owing to their unique orthogonal electrical/optical cavity scheme, PCSELs have emerged as one of the most promising platforms for high power diode lasers. The evanescent coupling of the optical mode in the active region with the photonic crystal layer enables large area two dimensional in-plane lasing mode in the photonic crystal cavity due to bandedge effect. Bragg’s multi-order diffraction provides the necessary feedback for the lasing and enhances surface normal laser emission. Such in-plane lasing mode can be scaled to hundreds of micrometers or even millimeters to achieve high power laser emission while maintain the single mode characteristics. In this thesis, monolithic high power PCSEL is reported on GaAs substrate with Watt class high output power from a single 200 µm x 200 µm emitter at 1040 nm using InGaAs MQWs active region. Right angled isosceles triangular airholes square lattice photonic crystal is fabricated using reactive ion etching process. Metalorganic chemical vapor deposition assisted regrowth process has been employed to preserve airholes (voids) in the photonic crystal cavity for electrical fabrication process. COMSOL Multiphysics and S4 based on rigorous coupled wave analysis software packages are explored to investigate the modal characteristics of triangular airholes photonic crystal structure. Innovative indium bump flip-chip bonding process has been optimized to enable uniform charge injection into large PCSEL emitter and surface emission is collected from the backside through substrate. Thermal issues triggered due to high current operation are mitigated by taking advantage of flip-chip bonded Si with comparatively high thermal conductivity of 150 W/m-K better than that of GaAs 45 W/m-K. The PCSEL/Si flip-chip bonding also demonstrates high yield 2D array (8x8 devices) scaling of PCSEL emitters with various PC lattice designs enabling beam steering on a single chip, coherent PCSEL array and lateral confinement PCSELs. The measured spectral characteristics show a dominant mode with narrow full width half maximum of 0.05 nm and excellent side mode suppression ratio of greater than 40 dB is achieved. The L-I-V and spectral characteristics of the PCSEL device is measured in both pulsed operation and continuous wave (c.w.) operation. The experiment results are also correlated to theoretical and numerical analysis to understand the mode properties and lasing conditions. The cavity scaling challenges on PCSELs while maintaining the single mode emission are also investigated. Innovative approach to estimate the maximum single mode emission size of the PCSEL is reported using quality factor and wave vector relationship. The gain-threshold discrimination between fundamental mode and higher order modes is calculated for various photonic crystal cavity designs and formulated the feasibility of scaling PCSELs to millimeters scale. The temporal changes in the photonic crystal cavity at higher current injection levels that trigger multi-mode emission are summarized and lateral heterostructure photonic crystal cavities are recommended for optical confinement to achieve single mode emission and compensate the electrically induced changes.