294 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 2, FEBRUARY 2005 Quantum-Dot-Based Saturable Absorber With p-n Junction for Mode-Locking of Solid-State Lasers A. A. Lagatsky, E. U. Rafailov, Member, IEEE, W. Sibbett, D. A. Livshits, A. E. Zhukov, and V. M. Ustinov Abstract—We demonstrate stable mode-locking in a Yb : KYW laser by using a quantum-dot (QD) saturable absorber structure that incorporates a p-n junction. A reduction in the output pulse duration was measured when a reverse bias was applied to the QD saturable absorber mirror. Index Terms—Active mirror, mode-lock laser, quantum dot (QD), saturable absorber, short pulse. S IGNIFICANT progress in the fabrication of quantum-dot (QD) lasers using molecular beam epitaxy (MBE) growth [1] has enabled a wide range of developments and assessments of QD-based devices [2], [3]. Semiconductor saturable absorber mirrors (SESAM) have been utilized in the mode-locking of solid-state lasers [4]. The bandwidth of such SESAM compo- nents is defined by the characteristics of the quantum wells (usually low-temperature grown or ion-implanted) that are responsible for the saturable absorption. By contrast, SESAMs based on QD structures have a broad-band absorption spectrum, due to the inhomogeneous broadening associated with a varia- tion of dot sizes, and hence, have potentially attractive features in respect of the generation of shorter pulses. In our previous studies of InGaAs QD saturable absorbers, we demonstrated mode-locked operation in a Yb : KY(WO ) (Yb : KYW) laser [5]. We also demonstrated the fast recovery time in this QD waveguided structure by using a pump–probe technique [6]. In this letter, we report on investigations of QD structures that incorporated a p-n junction within a distributed Bragg reflector (DBR) as a saturable absorber mirror. Our experimental data show that a more stable mode-locked operation of a Yb : KYW laser and that a reduction in the output pulse duration could be achieved when a reverse bias was applied to this QD-SESAM. The SESAM structure having a p-n junction permits the con- trol of the saturable absorption characteristics through the ap- plication of a voltage bias [7]. This type of SESAM structure, thus, behaves as an actively controlled saturable absorber mirror (ASAM) [8]. For instance, by supplying a suitable bias to the ASAM, it is possible to switch between continuous-wave (CW) and mode-locked regimes of laser operation. The QD structure studied in this work was grown by standard solid source MBE on an n-type GaAs (100) substrate. The DBR was composed of a sequence of the 25 pair Si-doped GaAs Manuscript received July 6, 2004; revised August 4, 2004. A. A. Lagatsky, E. U. Rafailov, and W. Sibbett are with the School of Physics and Astronomy, University of St. Andrews, Fife KY16 9SS, U.K. (e-mail: er8@st-andrews.ac.uk). D. A. Livshits, A. E. Zhukov, and V. M. Ustinov are with the Ioffe Physico- Technical Institute, St. Petersburg 194021, Russia. Digital Object Identifier 10.1109/LPT.2004.839387 Fig. 1. (a) Simplified structure scheme of the QD device and (b) representative photoluminescence characteristics from the QD-ASAM. and Al Ga As layers chosen to have a stopband centered at 1050 nm. A simplified schematic of this QD-ASAM structure together with its representative photoluminescence profile are illustrated in Fig. 1(a) and (b). The active region contains three subgroups each comprising three self-organized QD layers. The structure was capped with 0.3- m-thickness Be-doped GaAs- layer. The wafer was thinned to 100 m and the full area of the n-side was metallized with GeAu–Ni–Au. On the p-side, a ring- contact extending from 160- to 760- m diameter was formed with ZnAu–Au. Typical voltage–current characteristics for this QD-ASAM structure are shown in Fig. 2 for different window diameter devices. 1041-1135/$20.00 © 2005 IEEE