Characterization And Application Of Second-harmonic Generation And Two-photon Fluorescence In Micro-and Nanomaterials

Second-harmonic(SH)generation(also referred to as frequency doubling)and two-photon fluorescence(TPF)are both fundamental nonlinear optical effects,which not only allows the research on nonlinear light-mater interactions,but also provides possibilities for practical applications in many fields.Besides the access of light source at more wavelengths through frequency conversion,SH offers a nonlinear optical way for the characterization of matter.For example,SH is extremely sensitive to the symmetry of matter and polarization dependent,which makes it a diagnostic of the crystal structure as well as the surface and shape properties of the optical materials;besides,the spatial interference SH signals reflect the phase information of matter.In the era of nanotechnology,SH and TPF have become the significant means for characterizing micro-and nanostructures,which enable the detection of characteristics of structures in nanoscale,and high resolution nonlinear imaging combined with modern microscopy.In this thesis,we have developed SH and TPF techniques to characterize two kinds of micro-and nanostructures systematically,which are cadmium sulfide(CdS)nanowire and periodically poled ferroelectric domain structures.We also studied the SH and TPF imaging of biological tissues,and the application of two-photon excited fluorescence resonance energy transfer on oxygen sensing.The main results we obtained are listed as follows:1.The determination method of the CdS nanowire's second order nonlinear coefficients based on the forward-irradiated SH signals was studied theoretically.We analyzed the second order nonlinear polarization of a single CdS nanowire induced by a plane wave fundamental field and the intensities of the transmitted SH signals dependent on the polarizations of the fundamental field with the basic formalism of nonlinear optics.2.The non-resonant SH from a single CdS nanowire was experimentally investigated?We observed the SH signals with imaging and spectroscopy methods,built the polarization resolved measurement system of SH signals,and detected the intensities of the polarized SH under the excitation of the fundamental waves with continuously rotating polarization angles.The theory model and the experiment results can validate each other.Under the non-resonant excitation of a laser source at the wavelength of 1050nm,the ratios among the independent elements of the second order nonlinear coefficients d33,d31,and d15 were determined.The absolute value of the coefficients was also determined by comparing the SH intensity from the CdS nanowire with that from a reference crystal.We used the second order nonlinear coefficients of bulk CdS for contrast with our experiment results of the nanowire counterpart,analyzed the origin of the difference,and discussed the effects of the nanowire's diameter on the polarization dependence of SH intensities.3.The resonant SH from a single CdS nanowire was also studied by experiment.As the wavelength of the light source was tuned into the two-photon absorption region of CdS,the excitation condition become resonant.Characterization of resonant SH from CdS nanowire was demonstrated at a excitation wavelength of 740nm.Not only the SH signals,but also the intense two-photon fluorescence emitted spontaneously from the CdS nanowire were observed,and polarization resolved measurements were conducted.The ratio of d33 and d31 was determined under such resonant condition,and compared with the result under non-resonant excitation.The effect of the polarization dependent resonance strength on SH was discussed.4.Second harmonic interference imaging of ferroelectric domain structure through scanning confocal microscopy was presented.High resolution SH intensity imaging was achieved by collecting the surface SH signals from the periodically poled LiTaO3 crystal.A SH reference field was introduced by a piece of homogeneous LiTaO3 plate,which interfered with the SH field generated from the domain structures,so that the phase shift ? between the SH signals from the antiparallel domains was visualized.The tuning of the reference field's phase by rotating the reference plate was proposed,whose effects on the interference imaging were observed experimentally.By varying the imaging plane,the evolution of interference images at different depths was demonstrated,which was attributed to the additional phase shift introduced by the imaging depth.Such imaging method is applicable to the examination of domains structures in nonlinear film materials.5.Nonlinear optics imaging of biological tissues and two-photon excited fluorescence resonance energy transfer oxygen sensing was studied experimentally.SH and TPF imaging of various animal and human tissues was obtained,and the comparisons between images of normal and cancer tissues provide information for the diagnosis of cancer.Besides,membranes consisting of semiconductor quantum dots and oxygen sensitive dyes embedded in polymer matrix were fabricated,which can be excited by two-photon absorption and fluorescence resonance energy transfer(FRET).Emission spectra of the sample under different oxygen concentration were measured,and the dye luminescence quenching is approximately linearly dependent on dissolved oxygen concentration.