102 Handbook of Self Assembled Semiconductor Nanostructures for Novel Devices in Photonics and Electronics
dephasing time over a range of temperatures [135, 136] , which show signifi cantly increased
dephasing times at low (5–7 K) temperatures of several hundred picoseconds, corresponding to a
homogeneous broadening of as low as 2 μ eV [135] , which is limited by the exciton lifetime. Such
long dephasing times indicate that QD exciton coherence is maintained over suffi ciently long
timescales, which is essential for quantum information applications.
Although their emission is usually dominated by inhomogeneous broadening, the homogene-
ous linewidth can infl uence the operation of devices such as QD lasers, for example in the mode
profi le of laser emission spectra [137] . During operation of such devices, the carrier density in
the active region of the device will be greater than that seen in single QD spectroscopy and the
operating temperatures are typically around room temperature or above. Under these condi-
tions, there may be a large increase in the homogeneous broadening, for example due to carrier–
carrier interactions. As can be seen in Fig. 3.21 later in this chapter, at low temperatures wide
lasing spectra involving many longitudinal modes may be observed from QD lasers, whereas
at higher temperatures narrow lasing lines of mode groups separated by several meV are seen.
Sugawara et al. [137] have attributed this to a collective lasing action as QDs are coupled due to
homogeneous broadening, and from laser spectra obtained from InGaAs/GaAs QD lasers have
determined a value for the homogeneous broadening of 16–19 meV.
As has been outlined so far, aspects of the growth and resulting structure of In(Ga)As/GaAs
QDs signifi cantly infl uence their optical properties. The remaining sections of the chapter use
examples of QD devices to illustrate how the optical properties impact on device performance.
Other types of QD devices, including photodetectors and superluminescent LEDs, are described
elsewhere in this book but we shall concentrate on conventional devices: QD lasers, VCSELs and
semiconductor optical amplifi ers (SOAs) and on the development of novel devices (single photon
sources and spin-LEDs) that exploit some of the unique properties of QDs.
3.6 Quantum dot lasers
One of the main driving forces in QD research has been the promise of improved optoelectronic
devices. Early predictions have largely been successfully demonstrated and it is likely that com-
mercial products will appear within the next few years. However, as discussed in the previous
sections, the growth of QD devices is more complicated than QWs and questions remain con-
cerning repeatability for large-scale production. It should also be remembered that QW laser
diodes, present in CD, DVD players, printers and many sensing applications, required a decade of
research and development before they became commercially viable.
The majority of research has concentrated on development of QD lasers, for which perform-
ance improvements including low and temperature-insensitive threshold current density [30, 47] ,
zero linewidth enhancement factor [138, 139] and increased modulation speed [140] had been
predicted. For QD-based SOAs, fast gain recovery [141, 142] and pattern-effect-free amplifi cation
[143, 144] is expected and has since been demonstrated. The broad emission and gain that can
be obtained from QDs due to the variation in size and composition of the many QDs within an
ensemble has also been exploited for high-power, broadband emission from superluminescent
light-emitting diodes (SLEDs), which may be used for eye-safe biomedical imaging applications.
Output powers of several hundred mW have been demonstrated from QD-based SLEDs [145] and
by including multiple layers of QDs within an SLED structure (for which the optical properties of
each layer have been controllably varied), emission can be further broadened, to 120 nm for a
1300 nm emitting device [146, 147] . The inhomogeneously broadened optical response of a QD
ensemble can also be exploited in saturable absorber structures, either as semiconductor satura-
ble absorber mirrors (SESAMs), used to mode lock solid state lasers for ultrashort (fs) pulse gen-
eration [148] , or as absorber sections integrated into QD lasers for direct modulation [149] . The
broad absorption spectrum, ultrafast absorption recovery, long-wavelength operation and easy
incorporation into structures containing high-quality distributed Bragg refl ector (DBR) mirrors
all make In(Ga)As/GaAs QDs particularly attractive for SESAM structures, and mode-locking
of solid state lasers with QD-SESAMs has been demonstrated [150, 151] . Mode locking of QD
lasers using saturable absorber sections has realized transform-limited 7 ps pulse generation with
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