Simultaneous two-state lasing in quantum-dot lasers

Markus, A.; Chen, JX; Paranthoën, Cyril; Fiore, Andrea; Platz, Charly; Gauthier-Lafaye, Olivier

doi:10.1063/1.1563742

Cited by 278 publications

(182 citation statements)

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“…First, we notice an increase in threshold current (due to the increased optical loss) and a reduction in differential efficiency, as noted above. Additionally, when including photoexcitation, simultaneous lasing on ES and GS, due to the limited intraband relaxation rate, 12 is more easily observed, as shown in Figure 4. This is related to the additional loss of GS carriers due to photoexcitation, implying a faster increase of ES population.…”

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“…Indeed, carrier relaxation in QDs was already observed to take place in a few ps time scale, both by direct gain recovery experiments 11 and from the analysis of lasing spectra. 12 Additional insight is obtained from the relaxation oscillation dynamics. Indeed, if we consider the envelope of the laser response trace of the dark pulses, we observe an initial depletion followed by highly damped relaxation oscillations of the QD laser emission.…”

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“…At this voltage, the population of the lasing level is clamped. 12 Under a pulsed injection of 1 nJ, we measured the pump power detected at the output facet, after transmission through the laser. The results, as a function of the voltage, are shown in Figure 2c.…”

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“…19 Both these effects, in principle present in any diode laser, are particularly relevant in QD lasers due on one hand to the larger photoexcitation cross-section and on the other hand to the larger saturation of the lasing state due to the limited density of states, which slows down the intraband relaxation process and consequently the laser dynamics. 20 As an illustration of these effects, we have calculated the static characteristics of a two-state laser 12 with and without the presence of carrier photoexcitation (Figure 4). We assumed that the cross-section for 1.5 µm photons in our experiments is also valid for 1.3 µm cavity photons.…”

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“…Finally, the GS emission is seen ( Figure 4) to decrease above ES threshold, as observed experimentally, 21,22 while the model not including photoexcitation was not able to predict this decrease. 12 Indeed, photoexcitation of GS carriers due to ES photons plays a major role in this situation, as the rate of photoexcitation from the GS cannot be compensated by an increase of the clamped ES population. The same effect can account for the surprising observation of negative differential gain observed in the dynamics of twostate lasers.…”

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Intraband Carrier Photoexcitation in Quantum Dot Lasers

et al. 2008

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We unveil the role of bound-to-continuum photoexcitation of carriers as a relevant process that affects the performance of quantum dot (QD) lasers. We present the response of an InAs/InGaAs QD laser to a sub-band gap pump, showing an unexpected depletion of the emitted photons. We relate this observation with carrier photoexcitation through additional transmission and photocurrent measurements. We provide a theoretical support to the experimental data and highlight the important role of this process in the laser characteristics.The electronic structure of semiconductor quantum dots (QDs) presents dramatic differences from the electronic structure of the bulk material from which they are derived, exhibiting unique properties for their use in lasers and other photonic devices. Besides interband optical transitions commonly used for laser operation, intraband transitions can also occur in QDs and have been applied to QD infrared photodetectors. [1][2][3] For photon energies in the near-infrared, these transitions involve final states high in the continuum and produce an optical loss (corresponding to free-carrier absorption in bulk materials) for the wave propagating in the laser cavity. While in bulk, intraband transitions are forbidden in the first order as they do not conserve momentum, in QDs, the localized nature of the initial bound state makes the transition allowed for any photon energy, similarly to photoionisation in atoms. 4 The absorption cross-section for this process has been calculated 5 to be several orders of magnitude larger than in bulk. It can affect the performance of a QD laser both through the increased optical loss and through the gain reduction resulting from the removal of carriers from the lasing state. Despite its potential significance, to the best of our knowledge, no experimental evidence of this process and of its effect on laser characteristics has been provided. In this paper, we investigate the emission of an InAs/GaAs 1.3 µm QD laser under injection of sub-band gap 1.5 µm, 100 fs pulses, and we find clear evidence of carrier photoexcitation in the strong depletion of laser output. We further confirm this interpretation by directly measuring the pump transmission and induced photocurrent as a function of carrier population in the QDs. The experimental dynamics is well reproduced by a rate equation model, which takes into account the experimental photoexcitation cross-section. This process is then shown to play a role in the static and dynamic characteristics of QD lasers.The QD lasers analyzed were grown by molecular beam epitaxy in the Stranski-Krastanov mode, which has been proved to be a reliable method to provide semiconductor embedded dots. The active layer consists of five layers of self-assembled InAs QDs, covered by 5 nm thick InGaAs capping layers and separated from each other by a 40 nm GaAs active layer. The areal dot density of our lens-shaped QDs is 3 × 10 10 cm -2 . The waveguide was clad by 1.5 µm of AlGaAs, n-doped on the substrate side and p-doped on the t...

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