Electron-hole asymmetry and two-state lasing in quantum dot lasers

Abstract

We study the decrease of the ground-state output with increasing current in two-state quantum dot lasing. We show that the asymmetry in the thermal population redistribution breaks the symmetric dynamical evolution of the electron-hole pairs. This fully explains the transition from two-state to single-state lasing observed experimentally. The model also reproduces the temperature dependence of the two-state lasing. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1995947͔Laser devices based on self-assembled quantum dots are a promising source of new physics.1 The three-dimensional ͑3D͒ confinement of the carriers induces a discrete density of states that implies, for instance, a reduction of the threshold current, 2 a low chirp, a weak temperature dependence, 3 and a reduced sensitivity to optical feedback 4 at telecom wavelengths on GaAs substrates. The recombination of groundstate ͑GS͒ electrons and holes leads to GS lasing but recombination of excited-states ͑ES͒ electrons and holes can also lead to lasing at lower wavelengths. The occurrence of a secondary threshold, involving a second electron-hole pair of levels, was predicted for a sufficiently high bias.5 Increasing the current further, the ES will eventually become the only surviving line at the expense of the GS transition.6-9 For injection rates exceeding the second threshold, theory only predicts up to now that the GS intensity becomes a constant while experiments display a reduction down to zero of the GS emission. The aim of this Letter is to analyze both experimentally and theoretically this behavior.The self-organized quantum dot ͑QD͒ active region heterostructure consisted of six InGaAs QD layers embedded in quantum well using dots in a well ͑DWELL͒ technology. 10Single transverse mode ridge waveguide lasers were fabricated with lengths of 750 m, 1 mm, 1.5 mm, and 2 mm, ridge widths ranging from 3 to 5 m, and a depth of 0.9 m. Both facets were left uncoated. These devices were mounted epitaxial side up on Peltier-controlled copper heat sinks. While long devices emitted in the GS near 1310 nm, 1-mm-long devices emitted first in the GS but then also in the ES ͑1240 nm͒. The shortest devices emitted in the first excited state for all currents. The light-current curves, characteristic of a 1-mm-device obtained in both cw and pulsed regimes, are shown in Fig. 1. In both cases, as the injection current increases, there is a current range where the power in GS decreased while the power state in ES increased as previously observed. [6][7][8][9] It is also worth to note that the differential efficiency is larger for the ES, as shown on Fig. 1, highlighting its increased density of states. Operation in the pulsed regime is necessary to remove the role of thermal effects. Figure 1 corresponds to a pulse width of 100 ns duration with a repetition rate of 100 kHz. We verified that the behavior is similar with pulse widths down to 6 ns. The qualitative invariance of the dynamical response with respect to the pulse duration indicates that Joule heating e...

Rate-equations based model for the 2D-0D direct channel in quantum dot lasers under CW and gain-switching

“…This switching from GS to ES1 emission wavelength was, at first, reported in [3] and justified later with the use of simple numerical models. In [4], on the basis of a non-excitonic 2 level (GS,ES1) rate equation system the GS switch-off was explained in terms of the asymmetry between electron and hole population redistribution in the dot states.…”

Modeling of gain and phase dynamics in quantum dot amplifiers

“…Despite the general acceptance of excitonic models to describe some of the quantum dot characteristics, electron-hole models have showed success explaining the L-I characteristics (Viktorov et al 2005) and the temperature dependence of photoluminescence (Dawson et al 2005) of QD devices. Recently, a simple model electron-hole model was used to reproduce gain measurements of two-color pump and probe experiments .…”

Impact of the carrier relaxation paths on two-state operation in quantum dot lasers

“…Normally, the long (≥1mm) devices emit exclusively in the GS, 1-mm-long devices may emit simultaneously in the GS and the ES and the short (≤1mm) devices emit exclusively in the ES for all currents. This scenario has been experimentally confirmed [6] and became a subject of the intense theoretical study [7,8]. To explain the experimental results, a rate equation model for the electron and hole populations in both the ground and excited states assumes a cascade-like (wetting layer (WL)-ES-GS) relaxation pathway for the carriers in the dot: the carriers first captured by the ES, with the followed relaxation into the GS.…”