The Study Of Harmonic Generation In Field-enhanced Nanostructures

The interaction between light and matter has been widely studied and applied.Frequency conversion has always been the focus in nonlinear optical materials which has been widely applied in characterization of nanostructures materials,coherent ultraviolet light generation,super-continuous light generation,spectroscopy,biological imaging and sensing,micro-nano medicine,quantum optics,and the use of existing lasers to extend spectral range.Due to the inherent weak nonlinear response of natural materials,the nonlinear optical response of the traditional bulk materials enhances by using the complex phase matching techniques.With the rapid development of nanofabrication technology,it has been found that new synthetic metamaterials,including plasmon,metal-dielectric hybrid nanostructures and all-dielectric nanostructures can strongly localize the optical field,which provides another way to study the optical nonlinear enhancement.Especially,all-dielectric nanostructures have been paid more attention recently due to their advantages of large nonlinearity and high threshold damage.This dessertation mainly focuses on the enhancement of local optical field and nonlinear high harmonic generation in all-dielectric nanostructures.The main innovative results are listed as follows:Firstly,we study the intensity of the harmonic generated by the all-dielectric Si nanostructures excited by different two-color fields.It was found that there are two Fano formants in the linear transmission spectrum of nanostructures,and then the harmonic generation under the excitation of different two-color fields were investigated,respectively.(1)When the strongest peak of the Fano resonance of the fundamental field and its third-harmonic field are combined as the two-color field,the intensity of the high harmonics are increased by one order of magnitude than that of the single-color fundamental field.Furthermore,the intensity of the third harmonic are controlled by adjusting the relative phase of the third-harmonic field.A phase delay of 0.2?occurs when the relative phase is at the range of-2??2?,which can be attributed to the time delay.(2)When the wavelengths of the two-color field are the two resonant wavelengths respectively,the mixing phenomenon of integer n(?₁-?₂)of frequency difference appears.It is proved that the method of enhancing the efficiency of high harmonic generation by field regulation in atoms can also be applied to the all-dielectric nanostructures.Secondly,we investigate the local of electromagnetic field with multi-Fano resonan nanostructures and the intensity of the harmonics excited by the resonance wavelength.When the resonance wavelength is 2090 nm,the maximum local area of the electric field in the medium is achieved,the harmonic intensity is maximally enhanced when this resonance wavelength is used as the excitation wavelength,compared with the harmonic intensity of the non-patterned film,the harmonic intensity is increased by more than two orders of magnitude.In addition,when the laser wavelength is at the resonant peak of the wavelength of 1828 nm,the harmonic intensity can also be increased by two orders of magnitude.Therefore,the harmonics enhancement can be achieved in a nanostructure by using different field excitations.Our results show that the efficiency of harmonic generation can be improved by adjusting both the light field and the nanostructures.The results of the study have important application prospects in optical switching,light capture,extreme ultraviolet pulse,biosensors and so on.