2.5 Photonics and Optoelectronics of 2D Semiconductor TMDCs
43
polarisation of 2D polaritons up to room temperature and explored optical control of
polaritons in their microcavities [272, 277–280]. Still, further optimisation of these
systems is needed to obtain polariton condensates, the demonstration of which is
basically imminent given all the recent achievements in 2D-materials-based microcavities.
Ultralow-Threshold Lasing
An equally exciting research direction arose from the efforts to achieve ultralowthreshold lasing with 2D semiconductors. Here, the Purcell effect for a gain medium
coupled to an optical micro- or nanocavity can significantly enhance the spontaneous emission rate, which has important consequences for the lasing threshold
[281]. Ultralow-threshold optically-pumped lasing has been reported recently in a
coupled monolayer-WSe 2 –photonic-crystal high-quality
10 (high-Q) nanocavity system under a continuous-wave (CW) pumping for a wide temperature range below
250 K [70] (see Fig.2.15 (top left box)). Also, room-temperature CW lasing from
oxygen-plasma-treated few-layer MoS 2 coupled to an optical ring resonator was
reported, making use of high-Q whispering-gallery modes of the microcavity [71]
(see Fig. 2.15 (bottom box)). Similarly, lasing in a high-Q optical ring resonator
coupled to monolayer WS 2 was reported under pulsed optical excitation at low temperature [72] (see Fig. 2.15 (top right box)). However, the lasing thresholds in these
studies drastically differ and the unambiguous demonstration of a transition to lasing
is still pending. A comprehensive nanolaser demonstration with profound threshold characterisation can be for instance achieved with the help of photon statistics
measurements and coherence studies.
Rich Photonics—from Optical Gain to Quantum Emitters
Although gain has been predicted by the study of population inversion in WS 2 [81]
(that is in the regime of an uncorrelated electron–hole plasma), ultralow-threshold
lasers usually exploit the Purcell effect [282] with excitonic species in quantum
structures [283, 284]. These species can range from excitons, trions and biexcitons
to bound excitons and exciton–polaritons in TMDCs depending on the temperature
range, sample properties and environment (cf. [40, 47, 52, 87]). Even defect states can
play a significant role with regard to TMDC emission, as their use for single-photon
emission with 2D materials reveals [285–289]. These examples strongly motivate to
explore the rich application potentials in the field of (nano)photonics.
Optomechanical Experiments Hardly Touched
Ultimately, beyond the employment of 2D–cavity systems for optoelectronic coupling, 2D-membrane mechanical resonators and miniature high-finesse optical cavities can enable sophisticated (quantum-)optomechanical experiments—a hardly
explored domain with huge demand for fundamental research [290].
10 The quality of a resonator or microcavity is expressed by Q = E/γ , the quality factor, with mode
energy E and mode linewidth γ in full-width at half-maximum (FWHM).
43
polarisation of 2D polaritons up to room temperature and explored optical control of
polaritons in their microcavities [272, 277–280]. Still, further optimisation of these
systems is needed to obtain polariton condensates, the demonstration of which is
basically imminent given all the recent achievements in 2D-materials-based microcavities.
Ultralow-Threshold Lasing
An equally exciting research direction arose from the efforts to achieve ultralowthreshold lasing with 2D semiconductors. Here, the Purcell effect for a gain medium
coupled to an optical micro- or nanocavity can significantly enhance the spontaneous emission rate, which has important consequences for the lasing threshold
[281]. Ultralow-threshold optically-pumped lasing has been reported recently in a
coupled monolayer-WSe 2 –photonic-crystal high-quality
10 (high-Q) nanocavity system under a continuous-wave (CW) pumping for a wide temperature range below
250 K [70] (see Fig.2.15 (top left box)). Also, room-temperature CW lasing from
oxygen-plasma-treated few-layer MoS 2 coupled to an optical ring resonator was
reported, making use of high-Q whispering-gallery modes of the microcavity [71]
(see Fig. 2.15 (bottom box)). Similarly, lasing in a high-Q optical ring resonator
coupled to monolayer WS 2 was reported under pulsed optical excitation at low temperature [72] (see Fig. 2.15 (top right box)). However, the lasing thresholds in these
studies drastically differ and the unambiguous demonstration of a transition to lasing
is still pending. A comprehensive nanolaser demonstration with profound threshold characterisation can be for instance achieved with the help of photon statistics
measurements and coherence studies.
Rich Photonics—from Optical Gain to Quantum Emitters
Although gain has been predicted by the study of population inversion in WS 2 [81]
(that is in the regime of an uncorrelated electron–hole plasma), ultralow-threshold
lasers usually exploit the Purcell effect [282] with excitonic species in quantum
structures [283, 284]. These species can range from excitons, trions and biexcitons
to bound excitons and exciton–polaritons in TMDCs depending on the temperature
range, sample properties and environment (cf. [40, 47, 52, 87]). Even defect states can
play a significant role with regard to TMDC emission, as their use for single-photon
emission with 2D materials reveals [285–289]. These examples strongly motivate to
explore the rich application potentials in the field of (nano)photonics.
Optomechanical Experiments Hardly Touched
Ultimately, beyond the employment of 2D–cavity systems for optoelectronic coupling, 2D-membrane mechanical resonators and miniature high-finesse optical cavities can enable sophisticated (quantum-)optomechanical experiments—a hardly
explored domain with huge demand for fundamental research [290].
10 The quality of a resonator or microcavity is expressed by Q = E/γ , the quality factor, with mode
energy E and mode linewidth γ in full-width at half-maximum (FWHM).