The optical and electronic properties of self-organized quantum dots (QDs) are of decisive importance for the performance of laser diodes with such QDs as the active medium. Of primary interest is the gain with regard to its spectral position, width, magnitude, and temporal behavior. We survey the mutually stimulating advances in the theoretical understanding of QD gain and the performance of QD lasers. Emphasis is put on the temperature dependence of the carrier distribution function.
Spectral hole burning effects are observed as strong spectral intensity modulations in the emission spectra of broad and narrow stripe quantum-dot lasers with ridge waveguide. The modulation is attributed to lateral-cavity resonances burning holes in the inhomogeneously broadened spectral gain profile of the quantum dots. Lateral cavity engineering is expected to be crucial for optimizing quantum-dot laser performance and for potential realizing of wavelength-stabilized devices.
Ground state lasing of electrically driven vertical-cavity surface-emitting lasers with a quantum-dot (QD) gain medium grown using metal-organic vapor phase epitaxy was realized. The devices use stacked InGaAs QD layers, placed in the field intensity antinodes of the cavity formed by selectively oxidized distributed Bragg reflectors. Devices with 3×3 QD layers demonstrate at 20°C a cw output power of 1.45mW at 1.1μm emission wavelength. The peak external efficiency was 45%, limited by lateral carrier spreading within the 4λ cavity and a reduction of the internal efficiency above 60°C. A minimum threshold current of 85μA was obtained from a device with a 1μm aperture.
We investigated the impact of mesa etching profiles on the emission spectra of In(Ga)As quantum dot ridge waveguide lasers grown by metal-organic chemical vapour deposition. The mesa etching was terminated: (i) well before the waveguide, (ii) directly at the waveguide, (iii) after the waveguide forming tilted sidewalls and (iv) after the waveguide forming vertical sidewalls. We found a dramatic impact of the etching profiles on the spectral intensity modulation of the longitudinal modes. The spectral hole burning effect due to the Fabry-Pérot cavity resonances causes strong modulation of the lasing spectrum, if the etching profile is terminated at the waveguide, or when the mesa sidewalls are tilted. In addition, deep-etched-through mesas with vertical sidewalls demonstrate extra spectral features induced by the high Q-factor modes originating due to the total internal reflection at the vertical sidewalls. In contrast, no intensity modulation is found in the shallow mesa devices, due to the weak effective refractive index step. The present results indicate extended opportunities for the emission spectrum control characteristic of quantum dot lasers.
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