176
5 Optical Measurement Techniques
5.6.4 Magneto-Optical Studies
Magneto-optical studies are typically used to study the magnetic response of an excitonic system. For instance in 2D materials or conventional quantum wells, excitons
exhibit a magnetic momentum which aligns with the external field. This leads to a
diamagnetic energy shift and a lifted degeneracy of the energy states for different
spin orientations by a Zeeman splitting. Therefore, left- and right-circularly polarised
light address individual excitonic transitions when a strong magnetic field of a few
Tesla is applied. Currently, these magnetic-field studies are particularly of interest in
the domain of 2D valleytronics [91, 93] owing to the pseudo-spin attributed to the
distinct valleys in the most common 2D semiconductors and the high polarisation
anisotropy in these materials.
The influence of the magnetic field on excitons can be for instance used not only
to determine the electronic g-factor of individual Coulomb-bound species [172], but
also the exciton Bohr radius change as a function of the dielectric environment [173].
Recent studies with very-strong pulsed magnetic fields have also provided estimates
of the 2D exciton’s effective reduced mass in a monolayer material [174].
In the domain of polariton research using quantum-well microcavity systems,
strong magnetic fields were used to study magneto-optical effects in polariton gases.
One such experiment investigated the non-equilibrium spin-Meissner effect and
polarisation of condensed polaritons, referred to as spinor condensates [175], while
another study analysed coherence properties of (spin-resolved) polariton condensates
by second-order temporal-autocorrelation-function measurements in fields up to 5 T
[61].
However, one major motivation to apply magnetic fields was given by the need
to clearly evidence the matter component of the usually hybrid emitters in a polariton system when driven above the condensation threshold. The challenging task to
prove the existence of light–matter-coupled quasi-particles in the nonlinear emission
regime, particularly when the peculiar dispersion characteristics of cavity–polaritons
is absent, could only be addressed by Zeeman splitting investigations [14, 27]. This
magneto-optical method was previously applied and verified using polariton traps
formed by buried modulations of the cavity length in the semiconductor microcavity
structure, which gave rise to quantised polariton states due to the laterally-confining
potentials provided [136]. The coexistence of polaritonic planar-resonator modes
and photonic-quantum-box polariton modes allowed a systematic estimation of the
matter fractions for the detectable modes, both, through the Zeeman splitting and the
diamagnetic shift [136, 176].
References
1. S.W. Hell, Nobel lecture: nanoscopy with freely propagating light. Rev. Mod. Phys. 87, 1169–
1181 (2015)
2. M. Shah, L.M. Schneider, A. Rahimi-Iman, Observation of intralayer and interlayer excitons in monolayered WSe 2 /WS 2 Heterostructure. Semiconductors 53(16), 2140–2146 (2019).
5 Optical Measurement Techniques
5.6.4 Magneto-Optical Studies
Magneto-optical studies are typically used to study the magnetic response of an excitonic system. For instance in 2D materials or conventional quantum wells, excitons
exhibit a magnetic momentum which aligns with the external field. This leads to a
diamagnetic energy shift and a lifted degeneracy of the energy states for different
spin orientations by a Zeeman splitting. Therefore, left- and right-circularly polarised
light address individual excitonic transitions when a strong magnetic field of a few
Tesla is applied. Currently, these magnetic-field studies are particularly of interest in
the domain of 2D valleytronics [91, 93] owing to the pseudo-spin attributed to the
distinct valleys in the most common 2D semiconductors and the high polarisation
anisotropy in these materials.
The influence of the magnetic field on excitons can be for instance used not only
to determine the electronic g-factor of individual Coulomb-bound species [172], but
also the exciton Bohr radius change as a function of the dielectric environment [173].
Recent studies with very-strong pulsed magnetic fields have also provided estimates
of the 2D exciton’s effective reduced mass in a monolayer material [174].
In the domain of polariton research using quantum-well microcavity systems,
strong magnetic fields were used to study magneto-optical effects in polariton gases.
One such experiment investigated the non-equilibrium spin-Meissner effect and
polarisation of condensed polaritons, referred to as spinor condensates [175], while
another study analysed coherence properties of (spin-resolved) polariton condensates
by second-order temporal-autocorrelation-function measurements in fields up to 5 T
[61].
However, one major motivation to apply magnetic fields was given by the need
to clearly evidence the matter component of the usually hybrid emitters in a polariton system when driven above the condensation threshold. The challenging task to
prove the existence of light–matter-coupled quasi-particles in the nonlinear emission
regime, particularly when the peculiar dispersion characteristics of cavity–polaritons
is absent, could only be addressed by Zeeman splitting investigations [14, 27]. This
magneto-optical method was previously applied and verified using polariton traps
formed by buried modulations of the cavity length in the semiconductor microcavity
structure, which gave rise to quantised polariton states due to the laterally-confining
potentials provided [136]. The coexistence of polaritonic planar-resonator modes
and photonic-quantum-box polariton modes allowed a systematic estimation of the
matter fractions for the detectable modes, both, through the Zeeman splitting and the
diamagnetic shift [136, 176].
References
1. S.W. Hell, Nobel lecture: nanoscopy with freely propagating light. Rev. Mod. Phys. 87, 1169–
1181 (2015)
2. M. Shah, L.M. Schneider, A. Rahimi-Iman, Observation of intralayer and interlayer excitons in monolayered WSe 2 /WS 2 Heterostructure. Semiconductors 53(16), 2140–2146 (2019).