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Applied Physics Letters

Saturation of absorption and gain in a quantum dot diode with continuous-wave driving

A. Tierno, T. Ackemann, Christopher G. Leburn, Christian T. A. Brown

Published Dec 6, 2010• 9 citations
Publisher

Abstract

We have observed saturation of absorption and gain in ensembles of InAs/GaAs self-assembled quantum dots at room temperature with continuous-wave driving in the 1.24–1.28 μm wavelength range. The saturation intensity was found to be Isat=(1.4–4.5)×109 W/m2 in absorption and Isat=(0.2–3.8)×109 W/m2 in gain, depending on the detuning.

DOI
10.1063/1.3524824

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We have observed saturation of absorption and gain in ensembles of InAs/GaAs self-assembled quantum dots at room temperature with continuous-wave driving in the 1.24–1.28 μm wavelength range. The saturation intensity was found to be Isat=(1.4–4.5)×109 W/m2 in absorption and Isat=(0.2–3.8)×109 W/m2 in gain, depending on the detuning.
Spectral broadening and cross relaxation of an aggregate of highly carrier-injected quantum dots have been studied experimentally by evaluating the gain saturation and optical-wave mixing characteristics of traveling-wave laser amplifiers that include self-assembled 0.98-µm InGaAs dots. Measured signal-gain spectra at room temperature showed that the gain decreases over the entire gain spectra when an intense and narrow-linewidth saturating signal is injected at a higher-order subband transition of the quantum dot. Differential-gain spectrum was calculated from the saturated and unsaturated signal gain spectra, and revealed asymmetric gain saturation with respect to the wavelength of the saturating signal. These results suggest that cross relaxation is so fast that the dot aggregate has a homogeneously broadened gain spectrum. Self-saturation and cross-saturation characteristics, and the detuning characteristics of highly nondegenerate four-wave mixing were also investigated. The latter revealed that the characteristic times of two cross-relaxation processes are about 1 ps and 100 fs.
The nonlinear optical response of self-assembled quantum dots to cw driving is analyzed via numerical simulations of a spatially resolved rate equation model. The saturation is shown to follow a behavior in between the one for a dominantly, homogeneously, and inhomogeneously broadened medium. Self-lensing is suggested to probe the refractive-index nonlinearities and to open a complementary way of characterizing phase-amplitude coupling ($\ensuremath{\alpha}$ factor) in quantum-dots samples. For conservative assumptions on current samples the minimum focal length is predicted to be $\ifmmode\pm\else\textpm\fi{}1.7\text{ }\text{mm}$ for an input beam with $15\text{ }\ensuremath{\mu}\text{m}$ radius at a detuning of 1.1 inhomogeneous linewidths from gain center.
We analyze the strength of the nonlinear refractive index change associated with the saturation of the ground state absorption or gain of self-assembled quantum dot samples under cw optical injection. The model is based on a two-level framework but takes into account the strong coupling of quantum dots to the wetting layer by thermal excitations and Auger processes. Due to the inhomogeneous broadening and limited density the effect turns out to be weak though it should be experimentally detectable using self-lensing, the shift of Fabry-Perot resonances or interferometric schemes.