Ion Irradiated Cladding-like Waveguide In Dielectric Crystals And Sufrace Modification Of Graphene

An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum.It is a special structure which is a high refractive index core surrounded by low refractive index substrate in a dielectric crystal.As a basic component for integrated photonic systems,the optical waveguide has attracted more and more interests and studies.With the development of integrated optics,various optical waveguide devices,such as waveguide lasers,waveguide splitter,waveguide couplers,etc.,have sprung up in many kinds of fields including communication,sensing technique,materials technology,etc.In this trend,the fabrication of high-quality waveguide is a requirement for the development of various fields.Although many methods have been employed to fabricate waveguides in crystals,including femtosecond(fs)laser micromachining,focus proton beam writing,metal ion diffusion,ion implantation/irradiation,etc.,an efficient fabrication of a high coupling efficiency,low transmission loss optical waveguide is still an intriguing research topic.Graphene,since its discovery in 2004,has attracted great interest as a monolayer graphite sheet for a broad spectrum of multidisciplinary applications in electronics,optics,biology,and material sciences Graphene has saturable absorption due to the Pauli blocking effect and hot luminescence resulting from nonequilibrium carriers.These unique photonic and electronic properties make graphene an intriguing candidate for various electronic and optoelectronic devices.In optics,owing to nonlinear absorption,graphene has been applied for ultrafast laser generation in fibers and bulks as saturable absorbers(SAs)based on the energy-band structure.Moreover,graphene nanosheets exhibit strong polarization-dependent absorption of the evanescent field from photonic devices such as waveguides and fibers.This feature enables graphene-coated optical fibers to act as broadband polarizers.Ion implantation is a materials engineering process by which ions of a material are ionized and accelerated in an electrical field and impacted into a solid.In this process,the physical and chemical properties of the implanted materials would be improved.For example,it can improve the surface hardness,wear and corrosion resistance of the materials by ion beam irradiation.The ion implantation in semiconductors,such as silicon,germanium,and the compounds(e.g.,SiC,GaN and ZnO)opens new way for new device applications in photovoltaics and quantum electronics.In integrated photonic applications,ion beam technology is a well-developed way to fabricate many kinds of optical waveguides.In this thesis,I applied the ion beam technology into two works.One work is the fabrication of a cladding-like waveguide structure in Nd:YAG crystal by the multiple carbon ion beam irradiation.The other work is the first experimental study of the layer-to-layer compression and enhanced optical properties of few-layer grapheme nanosheet by applying ion irradiation.One part of the main task in this work,the fabrication of a cladding like waveguide structure,is to explore a new fabrication method of a multi-layer refractive index optical waveguide.This kind of optical waveguide composed by an outer cladding layer(air and substrate),an inner layer cladding(width of 9 ?m)and a core layer(width of 3.5?m).Compared with the monolayer waveguide,the light can be much easier coupled into the waveguide due to the confinement of the inner and the outer claddings.In this structure the waveguide laser at wavelength of 1064nm was achieved with enhanced performance in the cladding-like structure with both planer and ridge configurations by the optical pump at 810 nm.In the other part of this thesis,I demonstrate the irradiation of energetic ions(e.g.,carbon or oxygen)as an efficient method to modify the optical properties of transferred few-layer grapheme nanosheets with an area of 1 cm2.The ion irradiation not only reduces the space between the graphene layers,leaving the number of layers unchanged,it enables an enhanced contact between few-layer graphene nanosheets and the substrate.Owing to this feature,the polarization dependent linear absorption of few-layer graphene nanosheets considerably increases.In addition,the ion irradiation produces a planar optical waveguide below the grapheme layer.With this one-step irradiation processing,a platform with graphene-coated planar waveguide structure has been manufactured.In the case of the substrate of laser crystals,such as the Nd:YAG crystal investigated in this work,it is possible to achieve graphene Q-switched waveguide lasing through the interaction between the evanescent field of waveguide modes and ion-modified few-layer graphene as SA.Excellent performance such as a stable emission,shortened pulse duration(101ns),and increased repetition rate(2.3MHz)are obtained in the passively Q-switched waveguide laser systems.This work also offers a new solution to modify the optical properties of transferred few-layer graphene nanosheets in optical devices by ion irradiation.