| Optical waveguide is one of the basic components in integrated optics and optoelectronics. Such structures allow the confinement of light to regions of the order of light wavelength by means of total reflection occuring at the junctions between the boundaries of guides and claddings. The small size of waveguide structures offers high light intensities produced by even very low powers; consequently, the nonlinearities or laser actions in waveguides may be more efficient than those in bulk materials.At present, several techniques have been developed to fabricate waveguides in optical materials, such as ion implantation, ion exchange, diffusion, ion beam etching, femtosecond laser ablation. As one of the most efficient techniques for material-prpperty modification, ion implantation has shown its unique ability for alteration of surface refractive index of large number of optical materials, forming waveguide structures. Compared with other techniques, ion implantation possesses one of the most advantageous characteristics, that is, the wide applicability of materials. Since the first proton-implanted waveguide in fused silica was reported in1968, waveguides have been so far fabricated in more than100optical materials by implantation of various ions at the energies of kilo-electron-volt (keV) up to tens of mega-electron-volt (MeV). Moreover, after annealing, the propogation losses of waveguides are reduced to be very low.High dose light ions, typically referring to H or He, are usually used to be implanted into optical materials, forming optical waveguides. After implantation, the ions will induce nuclear damage and electronic damage by the interaction with the materials, leading the change of the refractive index of ion implanted region. A buried low-index barrier inside the substrate is generated by nuclear energy deposition. A large increase of refractive index occurs in the near-surface region by the electronic damage. The refractive index of optical waveguide is very important.Since the invention of femtosecond laser from1980s, femtosecond laser technique has achieved great development. Compared with previous continuous laser and long pulse laser, femtosecond laser has several advantages, such as.very high peak power, short pulse, has been commonly used in various fields, for example, physics, chemical, biology, optoelectronics. The interaction of femtosecond laser and matrials is more and more intriguing after the invention of femtosecond Ti:sapphire laser since1990s. As the pulse of femtosecond laser is very short and the light intensity of the focus can be as high as1014W/cm2at the len focus, with a regional focus of ultra-high electric field, laser-induced multi-photon absorption, multi-ionization, and other non-linear effect are produced. Nevertheless, these nonlinear effects usually induce the loss of material even the change of structure. These ultra-high, ultra-condesed and ultra-high time recognization characteristics provide a new method for the research of matter world. Recently, the formation of waveguides by femtosecond laser ablation has been attacting many people's interest.Titanium dioxide (TiO2) has attracted much attention over last three decades and been widely used in applications such as photocatalysts, optical active cladding, dye-sensitized solar cells, due to their exellent optical properties and high chemical stability. In addition to those applications, TiO2's high refractive index and transparency at visible and near-infrared wavelengths make it a promising medium for integrated optical devices in recent years. Titanium dioxide (TiO2) rutile single crystal irradiated by infrared femtosecond (fs) laser pulses was abserved. Furthermore, the exceptionally high nonlinearity of TiO2, could lead to diverse nonlinear nanophotonic devices, such as supercontinuum sources or ultrafast all-optical switches.Recently, ring resonator geometries have become an attractive venue for realizing efficient and device-integrated SHG. Ring resonators, like waveguides, offer an advantage over other geometries in that they readily yield modes satisfying the frequency-matching requirement ω2=ω1. To date, however, most works have focused on large (millimeter) ring resonators with long modal lifetime Q>>103,(and hence narrow bandwidths1/Q) in which the long lifetimes (and hence narrow bandwidths1/Q) compensate for the relatively large modal volumes, or on smaller-scale (micron) resonators with moderate bandwidths (Q=104) that operate at low (1%) efficiencies where down-conversion effects can be neglected.Recent years, with the development of modern grown technology in superthin layer material (e.g. MBE and MOVCD) and various super-elaborate artifactitious technology, the study in small scale semiconductor microcavity arouses people's interests. Especially, after finding the Rabi splitting phenomena for strong coupling in quantum microcavity (QMC) structures originated from the pioneering work of Weisbuch and his coworkers. Semiconductor microcavity has attracted great attention due to their unique properties, and they are suitable for the fabrication of optical communication devices, such as lasers, optical filters, optical demultiplexers, optical switches, optical modulaters and nonlinear optical frequency converters, etc.This dissertation includes two parts:(1) first, to explore the possibility of planar waveguide formed in rutile by He+ion implantation; second, to use Rutherford backscattering (RBS)/channeling measurement for studying the damage in ion implanted waveguides; third, to demonstrate the properties of the ridge waveguides fabricated by the femtosecond laser ablation and the Ar ion beam etching on the basis of the planar waveguides formed above.(2) By directly simulating Maxwell's equations via the finite-difference time-domain (FDTD) method, we numerically demonstrate the possibility of achieving highly-efficient second harmonic generation in a geometry consisting of doubly-resonant ring-resonator microcavities side-coupled to two adjacent waveguides. We find that>80%conversion efficiency can be attained at telecom wavelengths, for incident powers in the milliwatts, and for reasonably large bandwidths (Q～1000). In addition to exhibiting highly efficient frequency conversion, our geometry also leads to a host of limit-cycle behaviors for light incident above a threshold power. Our numerical results are also shown to agree to within a few percent with the predictions of a simple but rigorous coupled-mode theory framework.The main results of this dissertation are shown as following:(1) The fabrication of optical waveguides in rutile formed by MeV He ion implantationWe report on the formation and the optical properties of the planar and ridge optical waveguides in rutile TiO2crystal by He+ion implantation combined with femtosecond laser ablation technologies. Planar optical waveguides in TiO2are fabricated by high-energy (2.8MeV) He+-ion implantation with a dose of3×1016ions/cm2at room temperature. The guided modes were measured by a modal2010prism coupler at wavelength of1539nm. There are damage profiles in ion-implanted waveguides by Rutherford backscattering (RBS)/channeling measurements. The refractive-index profile of the waveguide was analyzed based on Reflected Calculation Method (RCM). The damage depth distribution is simulated by SRIM2010, and the barrier depth is6.3μmwhich is in agreement with RCM. Also ridge waveguides were fabricated by femtosecond laser ablation on2.8MeV ion implanted planar waveguide. The loss of the ridge waveguide was estimated. The measured near-field intensity distributions of the planar and ridge modes are all shown.(2) The characteristics of optical waveguides in rutile formed by triple keV He ion implantationWe report on the formation and the optical properties of the planar and ridge optical waveguides in rutile TiO2crystal by triple keV ion implantation combined with Ar ion beam etching technologies. Planar optical waveguides in TiO2are fabricated by triple low energies (450,500,550) keV He+-ion implantation with all fluences of2×1016ions/cm2at room temperature. There are damage profiles in ion-implanted waveguides by Rutherford backscattering (RBS)/channeling measurements. The damage depth distribution is simulated by SRIM2010, and the barrier depth is1.25μm. Also ridge waveguides were fabricated by Ar ion beam etching on the basis of triple keV ion implanted planar waveguide. The measured near-field intensity distributions of the planar and ridge modes are all shown.(3) High-efficiency second-harmonic generation in2D doubly-resonant χ(2) microring resonantorsBy temporal coupled mode theory and directly simulating Maxwell's equations via the finite-difference time-domain (FDTD) method, we numerically demonstrate the possibility of achieving high-efficiency second harmonic generation (SHG) in a structure consisting of a microscale doubly-resonant ring resonator side-coupled to two adjacent waveguides. For simplicity of computation, we utilized LiNbO3in2D model. In our design. there are two waveguides coupled to the ring:one waveguide for the ω1input and another for the ω2output. We place the output waveguide next to a PEC to induce a cutoff>ω1, so that ω1can not couple out via that channel. Neglecting the radiation loss,100%efficiency is achieved. In our model, we find that90%conversion efficiency can be attained by coupled mode theory, and85%conversion efficiency can be attained by FDTD. The error is acceptable. We demonstrate that in this high efficiency regime, the system also exhibits limit-cycle or bistable behavior for light incident above a threshold power.(4) High-efficiency second-harmonic generation in3D doubly-resonant χ(2) microring resonantorsWe apply the same basic principles that we validated in the2d example to a more realistic3d design. We found that we need bigger size ring resonator to meet the requirement due to the small contrast of LiNbO3and substrate. So we consider SHG from λ1=3μm in a GaAs film bonded to a SiO? substate and SHG from λ1=1.55μm in AlGaAs/SiO2material. The main criteria those used in2D model work in3D, but rather the selective rule. The substrat causes a cutoff in both waveguides. We find that88%and94%conversion efficiency can be achived in GaAs/SiO2and AlGaAs/SiO2, separately.
|
PDF Full Text Request