200
6 Effects of Quantisation
mission schemes) and shorter pulses from mode-locked lasers. This is for instance
discussed in [77].
Moreover, inhomogeneously broadened emission spectra of semiconductor quantum dots grown in the Stranski–Krastanov epitaxy mode enable wider wavelength
tunability and promise shorter pulses as a consequence of an increased gain bandwidth. In addition, a stronger temperature dependence of emission helps variation
of (sub-ps) pulse durations, for instance in self-mode-locked (SML) semiconductor
disk lasers (SDLs) [78]. Although quantum-dot lasers cannot provide as high chargecarrier densities as in their quantum-well counterparts, record high output powers
from quantum-dot SDLs have been demonstrated at 1 and 1.1 µm in excess of 8 and
7 W, respectively, for each single-chip laser device [79, 80]. To compensate for the
reduced densities in such quantum-dots gain structures, the active region comprises
numerous quantum-dot layer stacks distributed in the field maxima positions of these
chips’ (half-)microcavity. Further details about SDL achievements can be found for
instance in [81, 82].
Besides conventional semiconductor lasers (cf. [83, 84]), nanolasers have strongly
benefited from the development of quantum-dot systems incorporated into highquality optical microresonators [85], for instance discussed in [12]. Conveniently,
their exciton resonances and the coupling situation can be tuned in external fields for
light–matter interaction studies [86, 87]. Particularly, single-quantum-dot lasers in
the regime of a high cavity-mediated Purcell effect [88] promise ultralow thresholds
(cf. [12, 66, 67]).
On the electronics side, nanotransistors based on single quantum dots,
11 such
as single-electron transistors that employ resonant-tunneling effects and Coulomb
blockade to control a one-electron flow and storage, could pave the way for higher
bit densities and lower power consumption (see for instance [89, 90]).
On the quantum optics side, the nonclassical properties of individual dots that
act as artificial atoms in solids provide a reliable source of single photons, even
electrically-driven (cf. [91–93], and reviewed for instance in [94]). Such sources
are highly attractive for quantum information and communication schemes, as their
antibunching of the photon statistics and indistinguishability can be exploited in
quantum cryptography (see for instance [73, 95–98]), as the non-cloning theorem
and the quantum nature of photons promise detectable eavesdropping efforts (by a
third party, referred to as person Eve) on the quantum channel established between
two parties (commonly referred to as Alice and Bob in the literature).
Usually, single-photon sources are excited using commercially-available modelocked Ti:sapphire lasers. However, as they have limited (upper) repetition rates
of typically 80 MHz, the achievable single-photon flux is restricted by the excitation
scheme. While electrical pumping is desired for practical applications, repetition rates
are usually well below that of their optical counterparts. Without elaborating on the
advantages and disadvantages of ultrafast Ti:sapphire lasers, it is worth noting that,
11 Semiconductor quantum dots are sometimes also referred to as nanoislands, which typically form
on a wetting layer, when epitaxially grown, or can be the result of nanopatterning into a predefined
substrate area.
6 Effects of Quantisation
mission schemes) and shorter pulses from mode-locked lasers. This is for instance
discussed in [77].
Moreover, inhomogeneously broadened emission spectra of semiconductor quantum dots grown in the Stranski–Krastanov epitaxy mode enable wider wavelength
tunability and promise shorter pulses as a consequence of an increased gain bandwidth. In addition, a stronger temperature dependence of emission helps variation
of (sub-ps) pulse durations, for instance in self-mode-locked (SML) semiconductor
disk lasers (SDLs) [78]. Although quantum-dot lasers cannot provide as high chargecarrier densities as in their quantum-well counterparts, record high output powers
from quantum-dot SDLs have been demonstrated at 1 and 1.1 µm in excess of 8 and
7 W, respectively, for each single-chip laser device [79, 80]. To compensate for the
reduced densities in such quantum-dots gain structures, the active region comprises
numerous quantum-dot layer stacks distributed in the field maxima positions of these
chips’ (half-)microcavity. Further details about SDL achievements can be found for
instance in [81, 82].
Besides conventional semiconductor lasers (cf. [83, 84]), nanolasers have strongly
benefited from the development of quantum-dot systems incorporated into highquality optical microresonators [85], for instance discussed in [12]. Conveniently,
their exciton resonances and the coupling situation can be tuned in external fields for
light–matter interaction studies [86, 87]. Particularly, single-quantum-dot lasers in
the regime of a high cavity-mediated Purcell effect [88] promise ultralow thresholds
(cf. [12, 66, 67]).
On the electronics side, nanotransistors based on single quantum dots,
11 such
as single-electron transistors that employ resonant-tunneling effects and Coulomb
blockade to control a one-electron flow and storage, could pave the way for higher
bit densities and lower power consumption (see for instance [89, 90]).
On the quantum optics side, the nonclassical properties of individual dots that
act as artificial atoms in solids provide a reliable source of single photons, even
electrically-driven (cf. [91–93], and reviewed for instance in [94]). Such sources
are highly attractive for quantum information and communication schemes, as their
antibunching of the photon statistics and indistinguishability can be exploited in
quantum cryptography (see for instance [73, 95–98]), as the non-cloning theorem
and the quantum nature of photons promise detectable eavesdropping efforts (by a
third party, referred to as person Eve) on the quantum channel established between
two parties (commonly referred to as Alice and Bob in the literature).
Usually, single-photon sources are excited using commercially-available modelocked Ti:sapphire lasers. However, as they have limited (upper) repetition rates
of typically 80 MHz, the achievable single-photon flux is restricted by the excitation
scheme. While electrical pumping is desired for practical applications, repetition rates
are usually well below that of their optical counterparts. Without elaborating on the
advantages and disadvantages of ultrafast Ti:sapphire lasers, it is worth noting that,
11 Semiconductor quantum dots are sometimes also referred to as nanoislands, which typically form
on a wetting layer, when epitaxially grown, or can be the result of nanopatterning into a predefined
substrate area.