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9 Semiconductor Spintronics
9.4.5 Spin Laser
Laser is an electromagnetic radiation having some distinctive features like high
coherency, high monochromaticity, high directionality and high intensity. It is one
of the outstanding inventions of twentieth century and has brought the revolution
in diversified field of science and technology. It is said that ‘wherever you go and
whatever you do: the Laser will follow you’. Laser has also given the birth of a
specialized field in spintronics and known as opto-spintronics. In Chap. 7, we have
already discussed the manipulation of electron spin by laser light. But that was the
conventional laser light that uses the charges of current carriers only. Spintronics also
opens up the door of exclusively innovative types of device, such as spin-polarized
light source (Spin LED and Spin Laser). Interestingly, the spin lasers may offer a
corridor to several realistic room temperature spintronic devices, which will not be
limited to magnetoresistance only. In this section, we will discuss the construction
and working principle of spin laser and brief comparison between conventional and
spin laser (Zuti et al. 2020; Lee et al. 2014). As semiconductor lasers are bipolar
devices, simultaneous description of electrons and holes is crucial.
9.4.5.1 Construction
Both the conventional lasers and spin lasers have three basic elements: (i) the active
(gain) region, (ii) the resonant cavity and (iii) the pumping mechanism. The major
difference of spin lasers is the spin imbalance in the active region, which yields
critical changes in their action. This spin imbalance is accountable for emission of
circularly polarized light. This is the consequence of the conservation of the total
angular momentum during electron hole recombination.
Semiconductor-based Vertical-Cavity-Surface-Emitting-Lasers (VCSELs) and
Vertical-External-Cavity-Surface-Emitting-Lasers (VECSELs) are considered to
be the ideal choice for the realization of spin-polarized laser sources (Fig. 9.13).
The resonator (cavity) is made up of two semiconductor Distributed Bragg Reflectors (DBRs). Each DBR is constructed by multiple layers of alternating materials
exhibiting dissimilar refractive index. The gain region is based on quantum wells
(QWs) or quantum dots (QDs) and a total thickness of only a few micrometres.
For applications, the active region can be electrically pumped. The most common
emission wavelengths of VCSELs are in the range of 750–980 nm (GaAs/AlGaAs
QWs). Larger wavelengths of 1.3, 1.55 μm (for application in telecom industry)
or even beyond 2 μm (suitable for gas sensing technology) can be obtained with
dilute nitrides (GaIn-NAs/GaAs QWs) and from devices based on indium phosphide
(InAlGaAsP/InPQWs). The nature of pumping mechanism for monolithic VCSEL,
VECSEL can be optical, electrical or mixed pumping.
9 Semiconductor Spintronics
9.4.5 Spin Laser
Laser is an electromagnetic radiation having some distinctive features like high
coherency, high monochromaticity, high directionality and high intensity. It is one
of the outstanding inventions of twentieth century and has brought the revolution
in diversified field of science and technology. It is said that ‘wherever you go and
whatever you do: the Laser will follow you’. Laser has also given the birth of a
specialized field in spintronics and known as opto-spintronics. In Chap. 7, we have
already discussed the manipulation of electron spin by laser light. But that was the
conventional laser light that uses the charges of current carriers only. Spintronics also
opens up the door of exclusively innovative types of device, such as spin-polarized
light source (Spin LED and Spin Laser). Interestingly, the spin lasers may offer a
corridor to several realistic room temperature spintronic devices, which will not be
limited to magnetoresistance only. In this section, we will discuss the construction
and working principle of spin laser and brief comparison between conventional and
spin laser (Zuti et al. 2020; Lee et al. 2014). As semiconductor lasers are bipolar
devices, simultaneous description of electrons and holes is crucial.
9.4.5.1 Construction
Both the conventional lasers and spin lasers have three basic elements: (i) the active
(gain) region, (ii) the resonant cavity and (iii) the pumping mechanism. The major
difference of spin lasers is the spin imbalance in the active region, which yields
critical changes in their action. This spin imbalance is accountable for emission of
circularly polarized light. This is the consequence of the conservation of the total
angular momentum during electron hole recombination.
Semiconductor-based Vertical-Cavity-Surface-Emitting-Lasers (VCSELs) and
Vertical-External-Cavity-Surface-Emitting-Lasers (VECSELs) are considered to
be the ideal choice for the realization of spin-polarized laser sources (Fig. 9.13).
The resonator (cavity) is made up of two semiconductor Distributed Bragg Reflectors (DBRs). Each DBR is constructed by multiple layers of alternating materials
exhibiting dissimilar refractive index. The gain region is based on quantum wells
(QWs) or quantum dots (QDs) and a total thickness of only a few micrometres.
For applications, the active region can be electrically pumped. The most common
emission wavelengths of VCSELs are in the range of 750–980 nm (GaAs/AlGaAs
QWs). Larger wavelengths of 1.3, 1.55 μm (for application in telecom industry)
or even beyond 2 μm (suitable for gas sensing technology) can be obtained with
dilute nitrides (GaIn-NAs/GaAs QWs) and from devices based on indium phosphide
(InAlGaAsP/InPQWs). The nature of pumping mechanism for monolithic VCSEL,
VECSEL can be optical, electrical or mixed pumping.