6.3 Benefits and Applications
199
carriers and provide excitons with even higher oscillator strength and an atomistic
energy spectrum. These artificial atoms with high quantum yield and very narrow
emission lines provide a further improvement of the lasing process in optical microcavities.
Also, superlattices, i.e. artificial electronic crystals, have had their impact on laser
technology. In quantum-cascade lasers (e.g. [4]), minibands formed by periodicallyarranged multiple quantum wells allow for efficient and well-defined intersubband
transitions of charge carriers. Thereby, long-wavelength regions between the IR and
THz can be addressed with laser transitions else not available (not efficient) using
direct gap transitions. Sequences of barriers and wells not only act as the active
region, but are also needed for the design of particle drainage and injector regions,
which can be arranged around the active region for the recycling of transmitted and
relaxed carriers to form a cascaded device. However, new challenges arise when
aiming at room temperature operation due to the similarity of transition energies to
k B T (thermal energy).
Quantisation effects can also be used for band-gap engineering, as the width of a
quantum well directly influences its energy levels. Similarly, a combination of materials can be used to obtain a W-like band alignment, in which a well for holes is
sandwiched by a material posing a well for electrons within the outer barriers. Since
the double-well’s non-zero wave-function in the conduction band significantly overlaps with the hole wave-function in the sandwiched material sheet, optical transitions
from such a type-II heterostructure can act as laser transitions. Type-II active media
harness the unique tunability of the optical energy gaps by the quantum-structure’s
design parameters, which give wider access to difficult-to-reach wavelength ranges in
the infrared [74, 75] (under investigation for instance at the author’s host department
in Marburg for VECSEL chips operating close to telecom wavelengths), whereas
transitions in type-I heterostructures heavily rely on the employed material system
and its direct gap transition. Indeed, the gain dynamics of these different types of
gain media are different, as a comparison between type-II and type-I VECSEL chips
shows [76]. Moreover, type-II structures are designed to reduce parasitic chargecarrier loss channels in the gain region such as imposed by Auger processes, in
order to improve the laser efficiency in those wavelength ranges compared to type-I
structures based on suitable material systems.
6.3.2 Impact on Optoelectronics and Nanophotonics
A good example how photonic devices have improved due to the properties of quantum structures in their active region is given by the use of semiconductor quantum
dots. For short pulse generation, the fact that charge-carrier lifetimes are generally
lower than in quantum-well structures motivated quantum-dot devices development.
This is because the shorter lifetimes, both in gain structures as well as in semiconductor saturable-absorber mirrors, promise higher repetition rates (for optical trans-
199
carriers and provide excitons with even higher oscillator strength and an atomistic
energy spectrum. These artificial atoms with high quantum yield and very narrow
emission lines provide a further improvement of the lasing process in optical microcavities.
Also, superlattices, i.e. artificial electronic crystals, have had their impact on laser
technology. In quantum-cascade lasers (e.g. [4]), minibands formed by periodicallyarranged multiple quantum wells allow for efficient and well-defined intersubband
transitions of charge carriers. Thereby, long-wavelength regions between the IR and
THz can be addressed with laser transitions else not available (not efficient) using
direct gap transitions. Sequences of barriers and wells not only act as the active
region, but are also needed for the design of particle drainage and injector regions,
which can be arranged around the active region for the recycling of transmitted and
relaxed carriers to form a cascaded device. However, new challenges arise when
aiming at room temperature operation due to the similarity of transition energies to
k B T (thermal energy).
Quantisation effects can also be used for band-gap engineering, as the width of a
quantum well directly influences its energy levels. Similarly, a combination of materials can be used to obtain a W-like band alignment, in which a well for holes is
sandwiched by a material posing a well for electrons within the outer barriers. Since
the double-well’s non-zero wave-function in the conduction band significantly overlaps with the hole wave-function in the sandwiched material sheet, optical transitions
from such a type-II heterostructure can act as laser transitions. Type-II active media
harness the unique tunability of the optical energy gaps by the quantum-structure’s
design parameters, which give wider access to difficult-to-reach wavelength ranges in
the infrared [74, 75] (under investigation for instance at the author’s host department
in Marburg for VECSEL chips operating close to telecom wavelengths), whereas
transitions in type-I heterostructures heavily rely on the employed material system
and its direct gap transition. Indeed, the gain dynamics of these different types of
gain media are different, as a comparison between type-II and type-I VECSEL chips
shows [76]. Moreover, type-II structures are designed to reduce parasitic chargecarrier loss channels in the gain region such as imposed by Auger processes, in
order to improve the laser efficiency in those wavelength ranges compared to type-I
structures based on suitable material systems.
6.3.2 Impact on Optoelectronics and Nanophotonics
A good example how photonic devices have improved due to the properties of quantum structures in their active region is given by the use of semiconductor quantum
dots. For short pulse generation, the fact that charge-carrier lifetimes are generally
lower than in quantum-well structures motivated quantum-dot devices development.
This is because the shorter lifetimes, both in gain structures as well as in semiconductor saturable-absorber mirrors, promise higher repetition rates (for optical trans-