Dielectric apertures for mode control in low threshold and single mode vertical-cavity surface-emitting lasers

Abstract

Oxide-confinement has changed the design approach to realize low threshold, high efficiency vertical-cavity surface-emitting lasers (VCSELs). Numerous record results have resulted including ultralow threshold currents (<50 CIA), high wall-plug efficiency (-60 %), and high speed (-20 GHz). However, the aperture approach so effectively demonstrated with selective oxidation extends beyond low threshold VCSELs and represents a new means of controlling optical modes in otherwise planar, Fabry-Perot microcavities. Besides low threshold VCSELs, semiconductor dielectric apertures formed from semi-insulating low temperature AlGaAs are useful to induce antiguiding and realize single mode operation in VCSELs, and will perhaps lead to high single mode VCSEL power. In addition, oxide-confining apertures can also be used to achieve controlled spontaneous lifetimes with quantum dot active regions. The same cavity design principles are readily extended to other solid-state and polymer light emitters and lasers.The dielectric aperture works by modifying the electromagnetic mode density in the high Q Fabry-Perot microcavity. A blue-shifted resonance in the region containing the thin dielectric layer appears as a lateral waveguide cut-off frequency that in essence confines optical modes in the apertured region. Detailed theoretical modeling shows that optical loss is sensitive to both aperture design (thickness and shape) and placement within the cavity, and also elucidates the role of mode confinement on threshold for small area gain regions. For anti-guided VCSELs, the thin dielectric layer is designed to achieve a red-shifted resonance outside the active area of the laser, using either an increased dielectric constant in the thin layer or a slight lengthening of the cavity. Low temperature growth using molecular beam epitaxy provides a convenient means for making the semiconductor aperture electrically insulating.Besides low threshold and single mode VCSELs, very small active regions based on quantum dots show significant lifetime changes in an oxide-apertured microcavity. The quantum dot emitters are needed to eliminate the spatial and spectral diffusion that limits lifetime modification with a quantum well. Recently in moderately small (1 pm square) oxide-apertured Fabry-Perot microcavities we have measured spontaneous emission rate enhancements of 40 YO.In addition, sharp resonances can be seen in the spontaneous emission rate that correspond to transverse modes of the aperture. This modification is expected to increase with fkrther size reduction in the oxide confining region, perhaps providing a path to high speed, high efficiency microcavity light emitting diodes. The mode structure of such small cavities, along with processing details and experimental results on VCSELs will be presented.