Studies On Light Field Manipulation And Nonlinear Imaging In Microstructured Materials

Nonlinear optics mainly studies the interaction between light and materials to achieve some special optical functions,such as the use of frequency conversion to achieve multi-wavelength laser tuning;using nonlinear optical effects for imaging and optical information processing and so on.With the integration of optical functions and the miniaturization of devices,it is difficult for natural materials to meet the needs of technological development.So the research on artificial microstructured materials has been rapidly developed.Optical superlattices is an artificial microstructured material that can achieve a variety of nonlinear optical effects.With the continuous improvement of microfabrication technology,the theory and applications of periodic,quasi-periodic and aperiodic superlattice structures are becoming more and more mature.On the basis of microstructured materials,studies on several important applications in nonlinear optics have been conducted in this thesis,such as phase matching,light field manipulation and imaging.The innovations and main contents are as follows:(1)Combining nonlinear optics with guided optics,phase matching is achieved bv the coupling between waveguides and a simple waveguide structure design is proposed to achieve efficient output of the third harmonic wave.The analytical solutions of the amplitude under different phase matching conditions are discussed by solving the third harmonic coupling equations in waveguide directional couplers.It is found that when the double phase-matching conditions are satisfied,the amplitude of the cascaded third harmonic generation(CTHG)process is proportional to the square of the transmission distance.There are other situations that the amplitude of the CTHG will increase only linearly proportional to the distance,for example when the second harmonic generation(SHG)process is phase-mismatched.Based on this,we propose a new design scheme to realize high-efficiency CTHG.We find that the core thickness and the internal separation of the two waveguides in the directional coupler can be adjusted to meet the required phase-matching conditions.The main advantage of this approach lies in the convenience to design the waveguide structure.(2)The applications of nonlinear beam shaping and light field manipulation are studied in one-dimensional periodical nonlinear photonic crystals with a defect embedded.The nonlinear nonreciprocal response and the time-reversed effect of the SHG process are focused.The study of these two nonlinear effects provides new ideas for the development of devices such as optical isolators,optical absorbers and optical rectifiers.The nonreciprocal response has been deduced analytically by solving the nonlinear coupled equations with the analytical solution of non-reciprocity parameters obtained.On the basis of the theoretical analysis,a new scheme is proposed to design structures where any target nonreciprocal response can be reached by adjusting the structure parameters.In the time-reversed SHG process,two kinds of structures with different symmetries are presented to realize the nonlinear time reversal effect.A completely reciprocal nonlinear response is also found in the same process.Furthermore,a multi-section-cascaded structure is proposed to realize the nonlinear time reversal at any given position.(3)A new nonlinear imaging principle is proposed.The diffraction effect is well suppressed in the imaging process.Arbitrary ferroelectric domain structures without any periodicity can be observed directly and instantly with this method.This second harmonic imaging technique has unique advantages such as simplicity,stability,nondestruction and high imaging resolution,which will lead to great potential applications.Utilizing the special diffraction properties of ferroelectric domain walls in nonlinear imaging,Fresnel diffraction process for the single pair of antiparallel ferroelectric domains is conducted by analytical solution and numerical calculation.It is found that the images of domains are bright while the regions of the domain wall are dark.The line width of the domain wall image is proportional to the square root of the propagation distance with a fixed wavelength.The diffraction effect in this process is greatly suppressed.Inspired by the imaging results of a single domain wall,nonlinear imaging for irregularly distributed ferroelectric domain structures has been further investigated.The nearly dtfrraction-free property of the domain wall in the second harmonic imaging process is further verified through simulation and experiments.(4)Through the study on the theory of computer-generated holography,a new coding method for superlattice structure design is proposed to realize high quality and high resolution nonlinear holographic imaging.We call it a quasi-multi-coding method.The principle of this coding method is explained by Fourier spectral analysis.Two classical cases where the fundamental wave propagates along the polarization direction or perpendicularly to the polarization direction are discussed by theoretical calculations.It is found that compared with the traditional binary modulation,the quasi-multi-coding method has a stronger inhibitory effect on the diffraction peak.Correspondingly,the imaging profiles are finer and the details are clearer.Afterwards,the contour image is experimentally observed to further verify the advantages of quasi-multi-coding method,which is effective to reduce diffraction effect and improve imaging quality.This method provides a possibility of achieving nonlinear high-resolution imaging.