| Nonlinear optics is an important branch of optics,which describes the interaction between light and medium.Since the first experimental demonstration for second harmonic generation using ruby lasers by Franken et al in 1961,the nonlinear optical processes have played an important role in many scientific and technological fields.Researchers use nonlinear process to control the radiation frequency of laser effectively,based on this principle,the wavelength range of laser can be extended to the short wavelength region of visible spectrum.At present,most of the effective frequency conversion processes are implemented in inorganic birefringent crystals at macro-scale.Oversized crystal size hinders the development of on-chip tunable nonlinear optics.Hybrid plasmon waveguides are formed by coupling surface plasmon polarion(SPP)structures with dielectric waveguides,which provides a new solution to the above problems.The waveguide also utilizes the characteristics of the surface plasmon which can break through the diffraction limit and the dielectric waveguide with high nonlinearity coefficient and low propagation loss.It can realize the nonlinearity process at the sub-wavelength scale.It opens the door for the design of integrated non-linear devices.This paper focuses on the second harmonic generation(SHG)effect of hybrid plasmon waveguides,and makes a thorough analysis and research on how to improve the conversion efficiency of SHG and how to control SHG beams.The main research contents are as follows:(1)The second harmonic generation effect of the hybrid plasmon waveguide structure based on‘semiconductor nanowires,gap insulator and ultra smooth gold film'is studied.The nonlinear conversion efficiency of the device is as high as 4 x10-5W-1,which is several orders of magnitude higher than that of surface plasmon or semiconductor structures reported previously.The strong spatial overlap between waveguide modes and nonlinear materials,as well as the momentum conservation between incident and reflection modes,are the key factors for this high efficiency.The emission angle of SH photons is linearly correlated with the excitation wavelength,which indicates that the nonlinear beam steering can be achieved in the sub-wavelength range.Our work opens up a new way to achieve efficient and tunable nonlinear coherent sources.(2)Here,we demonstrated the flexible steering of the second harmonic(SH)beams in nanophotonic waveguides using gratings.The emission angle of the SH signals can be shifted by an amount of 60°from the waveguide axis into the collection cone,allowing the detection of the otherwise undetectable SHG from co-propagating waveguide modes in the high-loss waveguides.The conversion efficiency of SH photons from co-propagating modes contributes up to 26.7%of the total SH photons(3×10-5W-1).As pump wavelength varied from 770 to 840 nm,the nonlinear beam is swept by up to 22°.This design functions as a nonlinear grating coupler for nanophotonic systems and could be used for efficient nonlinear coherent light generation and integrated nonlinear nanophotonics.(3)Second harmonics in hybrid plasmon waveguides are remotely excited by SPP excited by gratings.When surface plasmons are coupled into Cd Se nanobelts,the hybrid plasmon modes propagating in opposite directions interfere with each other and form vertical upward emission SHG.The refractive index changes on the upper and lower surfaces of Cd Se nanobelts also form Fabry-Perot cavity.Therefore,the intensity of second harmonic far-field emission is closely related to the thickness of Cd Se nanobelts.Experiments and calculations Show that when the thickness of Cd Se nanobelts is close to 240 nm(wavelength is 800 nm),a Fabry-Perot resonance is formed,which makes the SHG emission enhanced by about two orders of magnitude.Our discovery provides an efficient on-chip frequency doubling source,which provides an important idea for integrated non-linear devices. |
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