| Terahertz waves have unique and important value in scientific research and applications.With the development of terahertz source,detection and application technologies,higher requirements are placed on terahertz functional devices.The optical and electrical control methods in traditional devices cannot meet the non-reciprocal transmission function of terahertz waves,while the terahertz magnetic control devices based on the magneto-optical effect could achieve both non-reciprocal and magnetic tunable properties,and could realize magneto-optical isolation,modulation and magnetic field sensing,which has a special and important role in terahertz application systems.Single crystal garnet films have a special position in microwave and optical communication technology.Actively carrying out basic research on its related applications in the terahertz band is conducive to the development of new terahertz magneto-optical control devices.However,the iron-containing magnetic garnets are not uniformly melted,and large-sized crystals cannot be directly obtained by the pulling method.The single-crystal films prepared by traditional physical and chemical deposition methods are too thin to be suitable for the terahertz wavelength range,the practical application requirements of magneto-optical devices.In this dissertation,focusing on the iron-containing garnet thin or thick films,through liquid phase epitaxy method,with the goal of achieving high-efficiency magneto-optical regulation and non-reciprocal transmission of terahertz waves,the design,preparation and characterization,as well as terahertz optical and magneto-optical properties of various magnetic garnet materials are carried out.The results achieved are as follows:1)Aiming at the problem of lattice mismatch between the liquid phase epitaxy YIG film and the GGG substrate,the preparation method of the thick film of La substituted YIG was studied.Through the optimization of the formula and process,single crystal thick La:YIG film sample with a total thickness of 210 ?m on both sides of three inches was prepared.The crystallographic state of the material is good,the surface is smooth and clean,and it has basic physical properties for terahertz light and magneto-optical applications.The optical and magneto-optical properties of the prepared single-crystal La:YIG samples in the terahertz frequency range were investigated.It is found that La:YIG has high transmittance,refractive index and relatively low absorption coefficient in the terahertz band.Using the self-built terahertz magneto-optical Faraday test system,it was found that the La:YIG sample has a strong Faraday magneto-optical effect in the terahertz frequency band,and it is a good magneto-optical crystal.The amplitude of the terahertz pulse can be modulated by changing the strength of the external magnetic field,and the characteristics of the linear polarization of the terahertz are basically unchanged.The Verdet constant is maintained at 100 °/mm/T over a broad spectral range.The results show that the La:YIG has the characteristics of low loss,high dielectric constant,strong magneto-optical response,and magneto-optical tunability in the terahertz range.2)Low-temperature terahertz optical and magneto-optical tests of La:YIG crystals were carried out.The characteristics of terahertz light and magneto-optics at low temperature are quite different from those at room temperature,and the effect of terahertz magneto-optical regulation at a temperature of 80 K is much better than that at room temperature.In the temperature range from 80 K to 210 K,the transmittance of the samples decreased gradually with the increase of temperature.In addition,linearly polarized terahertz time-domain pulses exhibit not only amplitude changes but also phase shifts with temperature changes.Using the terahertz pulses as a probe,the La:YIG sample was excited by an 800 nm ultrafast laser,and a picosecond high-frequency resonance was observed.Using a laser to pattern the surface of the La:YIG film,it was found that the reflected terahertz pulses changed greatly,the reflection amplitude increased,and the phase shifted.Based on the La:YIG crystal combined with the periodic asymmetric metasurface structure,the non-reciprocal optical rotation enhancement and polarization regulation in the narrow band range of terahertz waves are realized,and the Faraday rotation angle of the device is enhanced by 3 times compared with that of the pure material.3)In order to reduce the external magnetic field required by the magneto-optical crystal and enhance the terahertz magneto-optical response,the single-crystal Bi:RIG sample with thickness of 406 ?m was designed and farbricated.The research results show that the compound substitution of Gd,Bi and Yb for the regular dodecahedral c-site in the garnet crystal is conducive to the matching of lattice constants,and a single crystal iron garnet film thicker than that of La:YIG is obtained.The saturation external magnetic field of the crystal is significantly reduced by the introduction of Gd,and the terahertz magneto-optical response is greatly improved at low frequencies due to the introduction of Bi ions.The macroscopic action mechanism of the magneto-optical non-reciprocal properties of the sample is analyzed.The Faraday rotation angle that can be achieved by a single sample is increased from 3° of La:YIG to 7° of Bi:RIG,and the required external magnetic field strength is reduced from 155 m T to 100 m T.Magneto-optic Verdet constant is enhanced from 100 °/mm/T for La:YIG to 260 °/mm/T for Bi:RIG.4)In view of the technical bottleneck encountered at present,thicker single crystal film samples cannot be obtained.YIG ferrite with a thickness of 600 ?m was prepared by the solid-phase sintering method.The absorption coefficient of this ceramic material in the terahertz band is not high.For the application of magneto-optical isolators or modulators,the terahertz optical path has been improved.However,its magneto-optical response is lower than that of the thinner single-crystal La:YIG and Bi:RIG films,the Faraday rotation angle is ?2°,and the corresponding Verdet constant is only ?48°/mm/T,which is much lower than that of Bi:RIG and La:RIG.The design and preparation of ironcontaining single crystal garnet thick films with strong magneto-optical effect are still the core of device applications. |
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