Tunability Of Epsilon And Nonlinear Optical Effects In Epsilon-Near-Zero Media

Micro-nano integrated optoelectronic devices and systems,due to their distinctive features of miniaturization,mass production,and low cost,provide important opportunities for the upgrading of related traditional industries and achieve leapfrog development,and have become one of the fastest growing industries in the world."China's Advanced Manufacturing Technology 2030 Roadmap" lists the multi-material,cross-scale and integrated design of micro nano manufacturing equipment as one of the ten key technology fields.Among them,integrated optoelectronic devices can realize multi-functional composite,which is expected to solve the problems of optical system integration and miniaturization,and is the current development frontier of optoelectronic field.As an important part of the future integrated photonic chip,nonlinear optical materials have important application prospects in the fields of electro-optic modulators,optical frequency conversion,holographic storage,and microwave photonics.Therefore,the development of low-dimensional materials and microstructures with large nonlinear optical response is great significance to the research and development of low energy consumption,miniaturization,and high-efficiency integrated optoelectronic devices.In recent years,a variety of methods have been developed to enhance the intrinsic nonlinear optical response of low dimensional materials,such as built-in electric field,building metamaterial structures and so on.However,the enhancement of nonlinear optical response intensity by these technologies is still very limited(for example,the second harmonic conversion efficiency of aluminum nano antenna array with plasma resonance characteristics is only 70 times higher),so it is urgent to find new technologies and materials to enhance nonlinear optical effect and finally use them in the development and application of integrated photonic chips.According to the boundary conditions of Maxwell equations and material equations,when the real and imaginary parts of the epsilon approach zero,the electric field will increase greatly at the interface.According to the theory of nonlinear optics,large electric field can not only improve the nonlinear conversion efficiency,but also stimulate many kinds of nonlinear optical response.Therefore,studying the nonlinear optical effect of Epsilon-Near-Zero(ENZ)materials,exploring its preparation process,adjusting its ENZ wavelength,and enhancing its nonlinear optical response have important scientific significance and technical value.In this paper,transparent conductive oxide with near zero epsilon is the main research object,a controllable preparation method of metal oxide ITO(In2O3:Sn)is established,and the influence of sputtering power and annealing conditions on the epsilon of ITO is explored.The new properties of ITO film at ENZ wavelength are found,the high-order harmonics generation is realized and the shortest output wavelength is 206 nm.It was discovered for the first time that AZO(ZnO:Al)films generate supercontinuum spectrum at the ENZ wavelength.It significantly improves the energy conversion efficiency in the process of surface nonlinear optics,opens up a new direction of low dimensional nonlinear optical materials based on transparent metal oxide films,and lays the core material foundation for future miniaturized integrated optoelectronic devices.The specific research works are as follows:1.ITO films were prepared by magnetron sputtering.The crystal structure,surface morphology,thickness,optical and electrical properties of these films were characterized.We have successfully prepared dense,uniform,well adhered,good adhesion,rough and granular ITO films.2.In the process of preparing ITO thin films,we found that different preparation conditions have obvious effects on the crystal structure,carrier concentration and electron mobility of the samples.By changing the sputtering power and annealing temperature,the influence of the preparation conditions on the structure,surface morphology and epsilon of the ITO film was studied.With the change of sputtering power and annealing temperature,the carrier concentration and electron mobility in the film will change,which leads to a change in the dispersion relationship of epsilon.The study found that as the sputtering power increases,the wavelength of the ITO film near zero epsilon wavelength will blue shift,which can be adjusted in the mid-infrared region of 3121-4826 nm.The ENZ wavelength of ITO films changed with the annealing temperature,and the change region was 1051-1820 nm.With the increase of annealing temperature,the ENZ wavelength of ITO film decreases gradually.With this mechanism,we have successfully adjusted the ENZ wavelength of ITO films to 1030 nm,which matches the wavelength of commercial Yb fiber laser.3.Using Yb femtosecond fiber laser as the pump source,the process of high-order nonlinear optical response on the surface of ITO film was studied.Based on the ITO film exhibiting a large electric field enhancement effect at 1030 nm,the second,third,fourth,fifth harmonic and supercontinuum generation of the ITO film from visible light to short wave ultraviolet region is realized and the shortest output wavelength is 206 nm.Through comparative experiments,the second-order and third-order nonlinear polarizabilities of the ITO film at 1030 nm are calculated to be 0.273 pm/V and 2.48×10-20 m2/V2,respectively.It is concluded that the conversion efficiencies of second,third,fourth and fifth harmonics generation are 3.2×10-3,7.05×10-4,1.59×10-4 and 7.08×10-6,respectively.As the incident power increases,the spectrum gradually broadens,forming a supercontinuum(303-945 nm)covering ultraviolet-visible-near infrared.4.With optical parametric oscillator(OPO)laser as the pump source,the process of supercontinuum generation on the surface of the AZO film was studied.The AZO film was prepared by pulsed laser deposition technology,and its ENZ wavelength is at 1501 nm.The supercontinuum spectrum(406-1100 nm)is obtained on the AZO thin film with the thickness of 140 nm.The energy conversion efficiency is 3.94%W-1,which is 16 orders of magnitude higher than the conventional surface nonlinear process.Through theoretical analysis,the generation mechanism of the wide-band supercontinuum spectrum comes from the superposition of enhanced second harmonic,third harmonic,four-wave mixing and stimulated Raman scattering.It provides a new idea for the design of low-cost,miniaturized and high-efficiency supercontinuum light source in the future.