Intraband Carrier Photoexcitation in Quantum Dot Lasers

Abstract

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...

Ultrafast Electron Transfer from Photoexcited CdSe Quantum Dots to Methylviologen

“…An appropriate synthesis allows for the fabrication of QDs with different diameters and high-fluorescence quantum yield. Therefore, there are many potential application fields for QDs like LEDs, [1][2][3][4][5] lasers, [6][7][8][9] photovoltaic devices, [10][11][12][13] or fluorescence markers in biological systems. [14,15] Absorption of a photon with an appropriate wavelength leads to the generation of an electron-hole pair (exciton), which is delocalized throughout the QD.…”

Modeling and analysis of distributed feedback quantum dot passively mode-locked lasers

“…Moreover, n GS is the ground-state carrier probability that will be described later. In this study, we neglect the inhomogeneous broadening due to the nonuniform distribution of quantum dots and we use a single-Lorentzian gain spectrum profile in simulations [28,[31][32][33].…”

Refractive Index Dynamics and Linewidth Enhancement Factor in $p$-Doped InAs–GaAs Quantum-Dot Amplifiers

“…Transparency of the active medium is reached when the net number of stimulated transitions is zero, and thus no pump-induced changes associated with these transitions are present after intraband thermalization. However, a pump-induced two-photon absorption (TPA) of probe photons (mainly via transitions in the wetting layer and barrier continuum) as well as a pump-induced free carrier absorption (FCA) [22] are still present at transparency and give rise to the observed transients. In particular the significantly different phase dynamics in the p-doped devices show the importance of hole FCA.…”