Enhanced second and third harmonic generations of vertical and planar spiral MoS₂ nanosheets

Abstract

Molybdenum disulfide (MoS), as one of the atomically thin two-dimensional transition metal dichalcogenides has novel layer-dependent optical and electronic properties, which make it competitive for potential applications in optoelectronics. Here, we report chemical vapor deposition growth of vertically-standing and planar spiral MoS nanosheets. These vertical spiral MoS nanosheets are formed by the compression between planar spiral MoS in a close proximity. Both structures have a polytype 3R stacking with broken inversion symmetry, exhibiting strong second and third harmonic generations.

Nonlinear optics of two‐dimensional transition metal dichalcogenides

“…Apart from the 2H phase, there coexists 3R and 1T′ (semimetal monoclinic) phase . For 3R phase, the monolayer is identical to 2H case; however, the adjacent layers of the 3R are not mirrored and the inversion symmetry is broken (Figure B) .…”

“…Apart from the 2H phase, there coexists 3R and 1T 0 (semimetal monoclinic) phase. [30][31][32]68,79,80 For 3R phase, the monolayer is identical to 2H case; however, the adjacent layers of the 3R are not mirrored and the inversion symmetry is broken ( Figure 6B). 32 SHG intensity of 3R crystal quadratically depends on the layer numbers when it is within six layers, which is distinguished from the 2H case ( Figure 6C).…”

“…For instance, the inversion symmetry of bilayer TMDs can be broken by electric field, charge injection, and strain to produce SHG. In addition, SHG intensity can be tuned and enhanced by the excitonic effects, whereas TMDs with 3R and 1T phase show different SHG behavior compared with naturally occurred 2H phase . Additionally, the artificial stacking of the heterostructure can change SHG process completely .…”

“…In addition, SHG intensity can be tuned and enhanced by the excitonic effects, whereas TMDs with 3R and 1T phase show different SHG behavior compared with naturally occurred 2H phase. 32,79,80 Additionally, the artificial stacking of the heterostructure can change SHG process completely. 63,76,81 We will discuss those extrinsic SHG in detail in the next section.…”

Dynamic theory of nanophotonic control of two-dimensional semiconductor nonlinearities

“…Note that the in-plane symmetry can be restored by adding a second TMDC layer, which leads to the second harmonic being suppressed 23,58 . Furthermore, we note that the SHG is polarization dependent, i.e., for a particular symmetry axis the second harmonic is weak 21,23 and for not perfectly aligned stacked TMDC layers strong second harmonic signals can be found 53,59 . While, for reasons of clarity, we shall in the following assume a k-independent intraband dipole moment, effects such as a polarisation dependent SHG, can in principle be taken into account via a k-dependent matrix element.…”