Broadband Nonlinear Optical Properties And Applications Of The Epsilon-near-zero Materials

The permittivity is closely related to the electromagnetic properties of the medium.In nature,the permittivity of most substances is non-zero.In recent years,with the development of metamaterials and metasurfaces,it has been possible to artificially fabricate materials with near zero permittivity parameters（epsilon-near-zero,ENZ）.As the permittivity approaches zero,ENZ material exhibits many novel physical properties,such as electromagnetic tunneling,high directional radiation,supercoupling,ultra-thin perfect absorption,slow light capture,subwavelength imaging and so on.Near zero refractive index materials have attracted wide range of applications in the field of the electromagnetic wave control,neotype antenna and waveguide devices,nonlinear optics,photoelectric modulator due to its unique electromagnetic/optical properties.With the increase of incident light intensity,ENZ materials show significant optical nonlinear enhancement effect.Until now,studies have found that some heavily doped-semiconductor materials and transition metal nitrides have ENZ properties in visible or near infrared bands,and have significantly enhanced photorefractive index changes and ultrafast nonlinear responses at ENZ wavelengths.In addition,ENZ materials also show the enhanced harmonic generation characteristics,optical frequency conversion,electro-optical effect and other nonlinear optical effects,and have been verified in all-optical switches,optical isolators,and optical frequency shifters.In this dissertation,aiming at the application requirements of broadband,ultrafast and high-damage threshold nonlinear optoelectronic devices,we studies the broadband and ultrafast nonlinear optical properties of the ENZ materials and their composite structures,and further explores its application potential in micro-nano optoelectronic devices,such as pulse shaping,optical modulators and photodiodes.The main contents of this dissertation are as follows:（1）The broadband and ultrafast third-order nonlinear optical response of the indium tin oxides（ITO）film was studied,and the nonlinear refractive index with a maximum enhancement of about 47 times and the nonlinear absorption coefficient with a maximum enhancement of about 28 times are obtained experimentally.The maximum third-harmonic conversion efficiency of about3.75×10^-7near the ENZ wavelength is also obtained in the near-infrared range,which is about 10 times larger than that in the non-ENZ region.Furthermore,the ultrafast carrier dynamics responses of ITO and TiN were also studied,and the ultrafast relaxation times of hundreds of femtoseconds and tens of picoseconds were obtained respectively.By using the analysis of the two-temperature model,the energy transfer during the interaction between laser and matters were revealed,including the evolution process of electron temperature,lattice temperature and non-thermalized electron energy density with interaction time.Thermalization and cooling of hot electrons are the two main processes that determine the transient response of femtosecond pulses interacting with the nonlinear materials,and the nonlinear process is primarily derived to the modification of the energy distribution in the conduction-band electrons caused by laser-induced electron heating.（2）The composite metasurface of ITO and metal nanoantenna was designed and fabricated,it is found that its nonlinear coefficient is two orders of magnitude larger than that of pure ITO substrate.The nonlinear response of the metasurface shows polarization and wavelength-dependent nonlinear,where the magnitude and sign of the nonlinear parameters can be tuned by changing the polarization state of the incident laser.In addition,the metasurface also shows significant enhancement effect in the third-harmonic generation.The change of the metasurface’s refractive index will induce a blue or red-shift in the beam frequency,which is determined by the sign and magnitude of the nonlinear refractive index.Herein,we study the frequency shift characteristics of the metasurface under the femtosecond pumping,and obtain the maximum blue-shift of 1.97 THz and red-shift of 1.12 THz.Finally,we experimentally investigated the nonlinear optical response of the composite structures,including Au/ITO in the visible region,which the enhanced are derived from the local surface plasmon resonance,and the VSe₂/ITO composite structure at its ENZ wavelength.The results show that the nonlinear refraction and absorption are enhanced due to the presence of the ENZ substrate.（3）The nonreciprocal devices and saturable absorption devices based on ENZ materials were designed and fabricated,photodiode devices with high nonreciprocal factor and broadband pulsed laser output were obtained experimentally.Based on the nonlinear absorption enhancement characteristics of the ENZ material of TiN,a tandem structure of TiN-C₆₀was constructed to study the nonreciprocal transmission characteristics of laser.A passive photondiode in the visible spectrum was designed,and a high nonreciprocal factor of about 9.5 dB was obtained at ENZ wavelength.In addition,based on the broadband saturation absorption and ultrafast nonlinear response of ITO,the optical pulse shaping ability using ITO as a nonlinear optical modulator in 1μm,1.5μm and 2.8μm were experimentally studied,respectively.In 1μm band,stable Q-switched ytterbium-doped fiber laser pulse generation was realized,and the shortest pulse width of 382 ns is obtained.At 1.5μm,a highly stable passive mode-locked Erbium-doped laser pulse was achieved with a minimum pulse width of606 fs and a signal-to-noise ratio of 80.5 dB.A stable Q-switched Erbium-doped fluoride fiber laser pulse output with a minimum pulse width of 652 ns,a signal-to-noise ratio of 43 dB,and an output efficiency of 15.8%was obtained at 2.8μm.