2.5 Photonics and Optoelectronics of 2D Semiconductor TMDCs
39
dependent optical selection rules). These novel device schemes promise for instance
efficient transmission of information over long distances via polarised photons.
Overall, it is clear that the integration of TMDCs into photonic structures and
circuits becomes increasingly important. This is also benefiting from the generally
much-relaxed lattice matching constraints compared to other semiconductor crystalline systems regarding deposition of materials onto each other (e.g. onto silicon,
silica or polymer substrates). Besides the employment of mere monolayer and bilayer
materials as saturable absorbers [253–255], which can be used for (passive) ultrafast modulation of intensities, TMDCs have become attractive as active material
(see below) or in connection with waveguides [256–259], photonic crystals [11,
260], nanoresonators [261] and in-plane grating/cavity elements [156], promising
advances in integrated photonics [9].
Roadmap Towards 2D Photonics
In order to pave the way for these concepts to become practical for applications,
current challenges have to be addressed, where strong interaction between materials
sciences and physics will be key to a successful development of this field:
Firstly, scalable production of 2D materials with high quality and crystallinity is
still a problem, although several methods have been developed to grow, transfer, as
well as isolate monolayers and continuously improved to deliver the desired tailormade 2D crystals [5, 7].
Secondly, the improvement of PL quantum yield (QY) (cf. Fig. 2.12a, also see for
instance [142, 156, 262]) is important for optical structures that incorporate TMDCs
for light-emitting devices [14], polariton physics [248, 249] (cf. Fig.2.12b, c) and
ultralow threshold lasing [70–72, 74, 263] (cf. Fig. 2.12d).
Thirdly, the rich exciton physics of TMDCs has to be further explored [21],
understood and utilised.
Nevertheless, a plethora of studies can be enabled on the lab scale, as the following
examples of 2D–microcavity research reflect.
2D-Materials–Grating Structures for Integrated Photonics
Recently, circular grating structures with central half-wavelength cavity gap, also
referred to as circular in-plane distributed-Bragg-reflector-based optical microcavity (CIDBROM, similarly possible in linear, parallel grating configuration, as PIDBROM), have been investigated with the aim to deliver strong in-plane confinement
of light [156]. Such optical confinement was achieved by the lateral height-profile
modulation in a dielectric substrate (see Fig. 2.13). Simultaneously good out-of-plane
confinement was provided according to simple optical interference considerations
as a consequence of the air-trench depths (also see the Supporting Information of
[156]). This approach, which used air–GaP CIDBROMs in combination with monolayer WS 2 on buffering thin hBN, not only aims at the Purcell enhancement and
directionality of output offered by the radial-symmetric “bull’s-eye” Bragg grating
structure (also studied for different wavelength regions as well as applications in the
literature [264–268]). It also aims at the maximisation of light–matter interaction in
terms of optical in-coupling and out-coupling in the central region of the designed
ring pattern on top of the grating with its high refractive index contrast.
39
dependent optical selection rules). These novel device schemes promise for instance
efficient transmission of information over long distances via polarised photons.
Overall, it is clear that the integration of TMDCs into photonic structures and
circuits becomes increasingly important. This is also benefiting from the generally
much-relaxed lattice matching constraints compared to other semiconductor crystalline systems regarding deposition of materials onto each other (e.g. onto silicon,
silica or polymer substrates). Besides the employment of mere monolayer and bilayer
materials as saturable absorbers [253–255], which can be used for (passive) ultrafast modulation of intensities, TMDCs have become attractive as active material
(see below) or in connection with waveguides [256–259], photonic crystals [11,
260], nanoresonators [261] and in-plane grating/cavity elements [156], promising
advances in integrated photonics [9].
Roadmap Towards 2D Photonics
In order to pave the way for these concepts to become practical for applications,
current challenges have to be addressed, where strong interaction between materials
sciences and physics will be key to a successful development of this field:
Firstly, scalable production of 2D materials with high quality and crystallinity is
still a problem, although several methods have been developed to grow, transfer, as
well as isolate monolayers and continuously improved to deliver the desired tailormade 2D crystals [5, 7].
Secondly, the improvement of PL quantum yield (QY) (cf. Fig. 2.12a, also see for
instance [142, 156, 262]) is important for optical structures that incorporate TMDCs
for light-emitting devices [14], polariton physics [248, 249] (cf. Fig.2.12b, c) and
ultralow threshold lasing [70–72, 74, 263] (cf. Fig. 2.12d).
Thirdly, the rich exciton physics of TMDCs has to be further explored [21],
understood and utilised.
Nevertheless, a plethora of studies can be enabled on the lab scale, as the following
examples of 2D–microcavity research reflect.
2D-Materials–Grating Structures for Integrated Photonics
Recently, circular grating structures with central half-wavelength cavity gap, also
referred to as circular in-plane distributed-Bragg-reflector-based optical microcavity (CIDBROM, similarly possible in linear, parallel grating configuration, as PIDBROM), have been investigated with the aim to deliver strong in-plane confinement
of light [156]. Such optical confinement was achieved by the lateral height-profile
modulation in a dielectric substrate (see Fig. 2.13). Simultaneously good out-of-plane
confinement was provided according to simple optical interference considerations
as a consequence of the air-trench depths (also see the Supporting Information of
[156]). This approach, which used air–GaP CIDBROMs in combination with monolayer WS 2 on buffering thin hBN, not only aims at the Purcell enhancement and
directionality of output offered by the radial-symmetric “bull’s-eye” Bragg grating
structure (also studied for different wavelength regions as well as applications in the
literature [264–268]). It also aims at the maximisation of light–matter interaction in
terms of optical in-coupling and out-coupling in the central region of the designed
ring pattern on top of the grating with its high refractive index contrast.