nature photonics | VOL 1 | JULY 2007 | www.nature.com/naturephotonics 395 REVIEW ARTICLE E. U. RAFAILOV* 1 , M. A. CATALUNA 1 AND W. SIBBETT 2 1 Carnegie Laboratory of Physics, School of Engineering and Physics, University of Dundee, Dundee DD1 4HN, UK 2 School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK *e-mail: e.u.rafailov@dundee.ac.uk he generation of femtosecond optical pulses 1 has opened up a range of applications from real-time monitoring of chemical reactions through to ultrahigh-bit-rate optical communications 2 , and has led to new concepts in femtosecond optical networking, signal processing and transmission. In fact, lasers operating at multigigahertz repetition rates are now becoming key components for high-capacity telecommunication systems, photonic switching devices, optical interconnects, clocks for very-large-scale integrated microprocessors and high-speed electro–optic sampling systems. To increase the applicability of these ultrafast systems, several research groups have been examining alternatives to the Ti:sapphire and other vibronic lasers, which tend to be rather bulky and ineicient. Semiconductor lasers represent one option, ofering compactness and integrability along with direct electrical control. he mode-locking of laser diodes (until now mainly based on bulk and quantum-well heterostructures) is an established technique for the generation of picosecond and high-repetition-rate optical pulses in the near-infrared spectral range 3,4 . Semiconductor lasers, although they do not yet produce the short pulses that are routinely available from the diode-pumped crystal-based lasers, are showing real promise as eicient and simple electrically pumped devices. In this context, semiconductor quantum dots (QD), which demonstrate ultrabroadband gain and absorption and ultrafast carrier dynamics, have become one of the most promising materials systems for the generation of ultrashort pulses. In this review we describe how quantum-dot structures have opened up new avenues in ultrafast science and technology, enabling eicient and compact devices for the generation and ampliication of ultrashort optical pulses at high repetition rates. UNIQUE MATERIALS FOR ULTRAFAST APPLICATIONS Quantum dots — tiny clusters of semiconductor material with dimensions of only a few nanometres — are sometimes referred to as ‘artiicial atoms’ because the charge carriers occupy only a restricted Mode-locked quantum-dot lasers Semiconductor lasers are convenient and compact sources of light, offering highly efficient operation, direct electrical control and integration opportunities. In this review we describe how semiconductor quantum-dot structures can provide an efficient means of amplifying and generating ultrafast (of the order of 100 fs), high-power and low-noise optical pulses, with the potential to boost the repetition rate of the pulses to beyond 1 THz. Such device designs are opening up new possibilities in ultrafast science and technology. set of energy levels rather like the electrons in an atom. A QD laser was proposed in 1976 (ref. 5) and the irst theoretical treatment was published in 1982 (ref. 6). he main motivation was to design a low-threshold, single-frequency and temperature-insensitive laser, taking advantage of the quantum nature of the density of states (Fig. 1a). In fact, practical devices exhibit the predicted outstandingly low thresholds 7,8 , but the spectral bandwidths of such lasers are signiicantly broader than those of conventional quantum- well (QW) lasers, owing to inhomogeneous broadening (Fig. 1b). At the moment, there are two distinct QD systems that have shown particular promise. One group of materials is based on III–V QDs epitaxially grown on a semiconductor substrate. For instance, InGaAs/InAs QDs on a GaAs substrate emit in the 1.0–1.3 μm wavelength range, which could be extended to 1.55 μm. Alternatively, InGaAs/InAs QDs can be grown on an InP substrate, which covers the 1.4–1.9 μm wavelength range. So far, the most encouraging results have been achieved using the spontaneous formation of three-dimensional islands during strained-layer epitaxial growth in a process known as the Stranski–Krastanow mechanism 9,10 . he remarkable achievements in QD epitaxial growth have enabled the fabrication of QD lasers, ampliiers and saturable absorbers ofering excellent performance. he second group of QD materials consists of semiconductor nanoparticles (for example, PbS, PbSe and CdTe) incorporated into a variety of transparent dielectric matrices. Such systems have excitonic absorption peaks in the spectral range of about 0.5–2.5 μm. Quantum-dot-doped glasses are also of interest in the context of ultrafast physics 11–13 , and they are much cheaper to produce than their epitaxial counterparts. However, these materials have not yet realized their full potential as gain media in that eicient laser emission still has not been observed. Nevertheless, they have been used successfully as saturable absorbers 14,15 . BROADBAND HIGH GAIN AMPLIFICATION Quantum-dot semiconductors are particularly exciting materials for the ampliication of femtosecond pulses because of their extremely broad gain bandwidth. hey have the potential for generating sub-100-fs pulses, if all the bandwidth is coherently engaged and dispersion efects are minimized. he unique properties of the inhomogenously broadened gain of QDs have prompted several groups to investigate QD ampliiers for continuous-wave (c.w.) and ultrashort pulse signals. Semiconductor optical ampliiers (SOAs) are of considerable interest in telecommunications and data communications owing to their