Strong second-harmonic generation by sublattice polarization in non-uniformly strained monolayer graphene

Abstract

Despite the potential of graphene for building a variety of quantum photonic devices, its centrosymmetric nature forbids the observation of second harmonic generation (SHG) for developing second-order nonlinear devices. To activate SHG in graphene, extensive research efforts have been directed towards disrupting graphene’s inversion symmetry using external stimuli like electric fields. However, these methods fail to engineer graphene’s lattice symmetry, which is the root cause of the forbidden SHG. Here, we harness strain engineering to directly manipulate graphene’s lattice arrangement and induce sublattice polarization to activate SHG. Surprisingly, the SHG signal is boosted 50-fold at low temperatures, which can be explained by resonant transitions between strain-induced pseudo-Landau levels. The second-order susceptibility of strained graphene is found to be larger than that of hexagonal boron nitride with intrinsic broken inversion symmetry. Our demonstration of strong SHG in strained graphene offers promising possibilities for developing high-efficiency nonlinear devices for integrated quantum circuits.

Raman Investigation, Second Harmonic Generation, and Ultrasensitive Photoresponse of Low Symmetry Transition Metal Halide PdBr2

“…The power exponent between the SHG intensity and excitation power was fitted as 1.97, nearly a quadratic dependence (Figure d). These results confirm the second-order NLO process and are in line with the prediction of the electric-dipole theory. − As discussed above, PdBr 2 belongs to the centrosymmetric point group C 2 h . Due to the presence of central inversion symmetry between adjacent layers, it is expected that PdBr 2 would exhibit a negligible second-order susceptibility, resulting in the inhibition of SHG signals.…”

Optical Second Harmonic Generation of Low-Dimensional Semiconductor Materials

“…Second harmonic generation (SHG) provides a non-invasive, straightforward, and efficient characterization method for determining the properties of low-dimensional materials. It can be employed to identify characteristics [ 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 ], such as the layer number, crystal lattice symmetry and orientation, twist angle, strain direction, and intensity.…”

Mechanical, electronic, optical, piezoelectric and ferroic properties of strained graphene and other strained monolayers and multilayers: an update

“…Sublattice charge imbalance has also been verified through optical second-harmonics: the schematics on figure 10(a) show graphene on nanopillars again (see figures 8 and 9), and this sample is irradiated with a pump laser at 1035 nm. Figure 10(b) is a measure of strain through Raman spectra: according to [129], the sample is non-uniformly strained up to ∼2.4% by the nanopillar array. The temperature-dependent peak at half wavelength (1035/2 nm) displayed in figure 10(c) confirms the creation of a second harmonic on non-uniformly-strained graphene.…”