First-principles Study Of The Nonlinear Optical Property Of Nanomaterials

Nonlinear optical?NLO?materials have the ability to extend the spectral range of lasers,and have been widely applied in electro-optic devices,optical switches and optical communication.With the rapid improvement in electro-optics,the devices become more and more integrated and miniaturized.However,it is nearly impossible to cut commonly used three-dimensional inorganic materials,organic materials and polymers into pieces with nano-sizes.Therefore,it is emergent to find nanomaterials with excellent NLO properties.Here,we employ first-principles calculations to study the NLO properties of nanotubes and two-dimensional nanomaterials.We explore the mechanism of excellent NLO properties in nanomaterials,investigate how the stacking sequences affect the NLO properties of two-dimensional materials,and try to extend the applications of NLO phenomena.Our main findings are in the following.?1?We first investigate the electronic and NLO properties for bulk GeC polytypes,g-GeC,zigzag and chiral GeC single-walled nanotubes?SWNTs?.We find that single-layer g-GeC and corresponding zigzag nanotubes display direct bandgaps,and the bandgaps of most zigzag nanotubes are in the visible wavelengths.The static second-order NLO susceptibility of bulk GeC polytyes is slightly larger than that of AgGaS₂ crystals.Remarkably,the static SHG coefficients for small diameter zigzag nanotubes are even much larger than that of an archetypical NLO semiconductor GaAs,whereas the second-order NLO coefficient for the other GeC SWNTs and g-GeC can be comparable with that of GaAs.Furthermore,to explore the mechanism why the SHG response is much stronger in g-GeC and g-GeC nanotubes,we try to decompose static NLO susceptibilities into contributions from different valence bands and quantitatively calculate the NLO contributions from the high density of states.We find that the high density of states in the top of the valence band region is the origin of strong nonlinear optical properties of g-GeC and corresponding nanotubes.Such a mechanism helps to find excellent NLO materials.?2?We have carried out a first-principles study of the energetic and mechanical stabilities,electronic and second-order NLO properties of GaX?X=S,Se,Te?monolayers and few-layers.Firstly,GaX few-layers are energetically and mechanically stable.The electronic band structures of GaX bilayers?trilayers?are very similar to that of the GaSe monolayer.The static SHG coefficient of GaS and GaSe monolayers is twice as large as that of an archetypical NLO semiconductor AgGaS₂,while the static SHG coefficient of the GaTe monolayer can be comparable of that of GaAs crystals.The magnitudes of the SHG coefficients of bilayers and trilayers exfoliated from bulk?-GaSe are very close to that of the GaSe monolayer,whereas the magnitude of the SHG coefficient of trilayers exfoliated from the bulk?-GaSe crystal is about 1/3 that of single-layer GaSe.?3?We have carried out first-principles calculations of electronic properties,SHG coefficients,THz absorption and phase-matching properties of transition-metal dilchalcogenides TMDC monolayers including MoS₂,MoSe₂ and MoS_2?1-x?Se_2x.The calculated zero-frequency?^?2??0?of monolayer TMDCs are comparable with that of GaAs crystals,and show a perfect linear dependence on E_g^-3/2.E_g is the bandgap of monolayer TMDCs.Monolayer TMCDs exhibit small absorption over broadband THz frequencies,and can achieve phase-matching conditions at broad THz frequencies for SHG.Briefly,the large zero-frequency SHG coefficient,low absorption and perfect phase-matching properties at THz frequencies enable monolayer TMDCs to be promising candidates in producing THz radiation via SHG.Moreover,our calculation result also suggests that THz radiation can be attained via DFG in monolayer TMDCs.?4?We have carried out first-principles calculations of structural,electronic,SHG and piezoelectric properties of single-and few-layer?-M₂X₃?M=Ga,In;X=S,Se?.We propose a completely new method,i.e.switching the electric dipole orientation to tune electronic structures and bandgaps,which is very advantageous to tune the sensing photon energy window and achieve the semiconducting nature.The in-plane SHG coefficients of?-M₂X₃ monolayers are comparable with that of AgGaS₂ crystals.More importantly,we find out-of-plane SHG in?-M₂X₃ monolayers and their out-of-plane SHG coefficients are comparable with that of GaAs crystals.?-M₂X₃monolayers exhibit strong in-plane and considerable out-of-plane piezoelectricity.Furthermore,out-of-plane piezoelectricity vanishes in bilayer AB?-Ga₂S₃,while it is greatly enhanced in bilayer AA?-Ga₂S₃ as the electric dipoles are aligned in parallel.