Strain
engineering is an attractive method to induce and control
anisotropy for polarized optoelectronic applications with two-dimensional
(2D) materials. Herein, we have investigated the nonlinear optical
coefficient dispersion relationship and the second-harmonic generation
(SHG) pattern evolution under the uniaxial strains for graphene, WS2, GaSe, and In2Se3 monolayers. The uniaxial
strain can break the in-plane symmetry of 2D materials, leading to
both trade-off breaking of the nonlinear coefficient and new emergent
nonlinear coefficients. In such a case, a classical sixfold ϕ-dependent
SHG pattern is transformed into a distorted sixfold SHG pattern under
the strain. Due to the lattice symmetry breaking and the uneven charge
density distribution in strained 2D materials, the SHG patterns also
depend on the excitation photon energy. The results could give a guide
for the SHG pattern analysis in experiments, suggesting strain engineering
on 2D materials for the tunable anisotropy in polarized and flexible
nonlinear optical devices.
Dispersion
property and second harmonic generation (SHG) pattern
of novel two-dimensional (2D) van der Waals heterostructures (vdWHs)
is of great significance not only for the characterization of material
symmetry but also for understanding nonlinear photophysical phenomena.
Herein, we demonstrate the SHG response of 2D type-I (MoTe2/WSe2) and type-II (MoSe2/WSe2)
band alignment of vdWHs. In the dispersion relation of the second-order
nonlinear coefficient, the pronounced peaks of the d
16 element for both vdWHs are mainly contributed by resonance
in the interband transition processes, whereas other elements are
derived from the intraband transition processes because of the highly
efficient charge transfer from WSe2 to MoTe2 in type-I vdWHs and the ultrafast charge separation between WSe2 and MoSe2 in type-II vdWHs, respectively. Besides,
more nonzero nonlinear coefficient elements can participate in a nonlinear
response at the oblique incidence, to which special attention needs
paid. The polarization angle α-dependent SHG patterns display
a rotational fourfold symmetry, whereas the azimuthal angle ϕ-dependent
SHG patterns show sixfold symmetry for both type-I and type-II vdWHs
at any wavelength under normal incidence. Under oblique incidence,
the α-dependent (ϕ-dependent) SHG patterns will reduce
to twofold (threefold) symmetry for both vdWHs. The results highlight
the potential to deterministically engineer novel nonlinear optical
properties for tunable anisotropic applications of nonlinear optoelectronic
devices based on vdWHs.
Low-symmetry of ReS2 has not only in-plane but also out-of-plane anisotropic light scattering, which is complicated, yet interesting with intrinsic strong electron-phonon coupling. In such case, the Raman tensor also...
Structural symmetry-breaking plays a crucial role in
determining
the second harmonic generation (SHG) intensity and pattern for two-dimensional
(2D) materials. Herein, we report that the giant optical SHG can be
achieved by designing a Janus structure for ReS2 and ReSe2. The designing of a Janus ReSSe monolayer structure can break
both the in-plane and out-of-plane symmetry of monolayer ReS2 and ReSe2, resulting in the occurrence of larger second-order
nonlinear coefficients in the in-plane (d
11, d
16, d
21, and d
22) and out-of-plane (d
15 and d
31) components.
The second-order nonlinear coefficient dispersion properties for the
asymmetric Janus ReSSe monolayer give rise to a multifaceted dependence
of the SHG on the azimuthal and polarization angles at different incident
wavelengths including double and quadruple symmetries, rotation of
polar axis, and variations in intensity. These results highlight the
potential to deterministically engineer novel nonlinear optical properties
in the designing of the Janus structure based on layered materials.
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