Carrier dynamics in a tunneling injection quantum dot semiconductor optical amplifier

The process of tunneling injection is known to improve the dynamical characteristics of quantum well and quantum dot lasers; in the latter, it also improves the temperature performance. The advantage of the tunneling injection process stems from the fact that it avoids hot carrier injection, which is a key performance-limiting factor in all semiconductor lasers. The tunneling injection process is not fully understood microscopically and therefore it is difficult to optimize those laser structures. We present here a numerical study of the broadband carrier dynamics in a tunneling injection quantum dot gain medium in the form of an optical amplifier operating at $1.55\mu \mathrm{m}$. Charge carrier tunneling occurs in a hybrid state that joins the quantum dot first excited state and the confined quantum well–injection well states. The hybrid state, which is placed energetically roughly one longitudinal optic phonon above the ground state and has a spectral extent of about $5\text{meV}$, dominates the carrier injection to the ground state. We calculate the dynamical response of the inversion across the entire gain spectrum following a short pulse perturbation at various wavelengths and for two bias currents. At a high bias of $200\text{mA}$, the entire spectrum exhibits gain; at $30\text{mA}$, the system exhibits a mixed gain-absorption spectrum. The carrier dynamics in the injection well is calculated simultaneously. We discuss the role of the pulse excitation wavelengths relative to the gain spectrum peak and demonstrate that the injection well responds to all perturbation wavelengths, even those which are far from the region where the tunneling injection process dominates.

Figure 1

Energy band structure of a TI-QD SOA for a 4.5-nm-wide IW and QDs with in-plane dimensions of $16\times 24$ nm and a height of 3 nm. The values of the energy levels are $-13.35$, $-41.15$, $-73.62$, and $-133.80\text{meV}$ below the reservoir level. The tunneling process from the IW to the QD ground state is shown schematically. The conventional cascade carrier relaxation process from the common reservoir to the QD ground state takes ${\tau }_{d_{\text{cap}}}\approx 4\text{ps}$. The tunneling injection design shortens the recovery process to ${\tau }_{\text{tun}}\approx 1\text{ps}$ and diminishes all other carrier capture processes to the ground state.

Figure 2

Spatial evolution of the population inversions across the QD spectrum after a perturbation by a 100-pJ optical pulse at $1530\text{nm}$. The amplifier is biased at $200\text{mA}$, where it exhibits high gain across its entire gain spectrum.

Figure 3

Steady state inversion spectra of the QD population for a bias of (a) 200 and (b) 30 mA.

Figure 4

Time evolution of the inversion following the perturbation at different pump wavelengths: 1480 (blue trace), 1530 (green trace), and $1590\text{nm}$ (red trace). The QD ground-state inversions are normalized to their respective values prior to the perturbation. The inversions are probed at (a) $-1465$, (b) $-1530$, and (c) $-1590$ nm. The SOA is biased at 200 mA, where it exhibits high gain across its entire gain spectrum.

Figure 5

Time-resolved spectral profiles of the ground-state recovery following the perturbation at different pulse wavelengths: 1480 (blue trace), 1530 (green trace), and $1590\text{nm}$ (red trace). The SOA is biased at 200 mA, where it exhibits high gain across its entire gain spectrum.

Figure 6

Injection well electron (solid line) and hole (dashed line) densities for the three perturbations of different spectral locations. The SOA is biased at 200 mA, where it exhibits high gain across its entire gain spectrum.

Figure 7

Time evolution of the inversion following the perturbation at different pump wavelengths: 1480 (blue trace), 1530 (green trace), and $1590\text{nm}$ (red trace). The inversions are probed at (a) $-1465$, (b)$-1530$, and (c) $-1590$ nm. The SOA is biased at 30 mA, where it exhibits a mixed gain-absorption state.

Figure 8

Time-resolved spectral profiles of the ground-state recovery following the perturbation at different pulse wavelengths: 1480 (blue trace), 1530 (green trace), and $1590\text{nm}$ (red trace). The SOA is biased at 30 mA, where it exhibits a mixed gain-absorption state.

Figure 9

Injection well electron (solid line) and hole (dashed line) densities for the three perturbations of different spectral locations. The SOA is biased at 30 mA, where it exhibits a mixed gain-absorption state.