Lattice Damage And Frequency Doubling In Lithium Niobate Waveguide Structures Formed By Ion Irradiation

Optical waveguides are the key element of the integrated photonic devices. Waveguide lasers, waveguide amplifiers, SHG and OPO in nonlinear waveguides and optical solitons in photorefractive waveguides have attracted lots of attensions in recent years. Waveguides have a dimension similar to the operating wavelengths. Light's behavior in waveguides is different with that in bulk materials. Waveguide structures are usually formed by using micro-nanofabrication technology, such as film growth, etching, and lithography. In waveguide, the light is confined in the high-refractive region and propagates as a stable mode. In addition, using the electro-optic, acousto-optic, and nonlinear properties, functional photonics devices can be achieved based on waveguide structures. Research on waveguides involves a wide range of subjects, such as optics, lasers, crystals and materials science, solid state physics, nonlinear optics, and microelectronics.As a material modification technology, ion implantation has been widely used in metals, insulators, magnetic materials, amorphous and surface physics, chemistry, medicine, and metallurgy. Ion implantation has also been developed as a mature method to fabricate waveguide structures in many optical crystals, ceramics, polymers, and glasses because it offers accurate control of the depth and fluence of the ions, as well as a universal mechanism of waveguide formation. Combined with lithography and etching technique, ion implantation can fabricate channel and ridge waveguides and functional optical elements. Up to now, as a waveguide formation method, ion implantation can be devided into light-ion (H and He) implantation and heavy ion (such as C, O, Si, Ar, Cu etc.) implantation. Ion energy usually is between0.2MeV-6MeV. The fluence of light ions and heavy ions needed for waveguide formation is about1016ions/cm2and1013ions/cm2-1014ions/cm2, respectively. In general, heavy ion implantation is a relatively more effective method. Now, swift heavy ion (heavy ions with energy usually between20MeV-several GeV) irradiation, as a new way to fabricate waveguides, attracts more and more attention. Compared to low-energy implantation, it needs much lower fluence (about1012ions/cm2) and offers a new mechanism for waveguide formation.Ion implantation is usually simulated by using SRIM (The Stopping and Range of ions in matter) software which is based on a Monte-Karlo method and can obtain ions'range, straggle, profile, electronic energy loss and nuclear energy loss. After implantation, lattice damage, waveguide properties, change of the optical properties, of the samples have to be investigated, and laser or second harmonic generation would be performed. Waveguide properties include modes'indices, modes'profile, refractive index profile, and loss, which usually could be obtained in prism-coupling experiments and end-face coupling experiments and could be improved by thermal annealing. RSoft is a photonics design software, includes BeamPROP, FullWAVE, and BandSOLVE etc, and is used on calculation of waveguides'modes and simulation of the optical structures here.Lithium niobate (LN) is used in this dissertation. LN has been one of the most attractive materials due to its outstanding acousto-optic, electro-optic, and nonlinear properties. In congtuent lithium niobate (CLN), the Li/Nb concentration ratio is about48.4/51.6. This nonstoichiometry introduces anti-site defects which result in high internal electric fields and strong photorefractive damage in laser applications. Compared to CLN, near-stoichiometric lithium niobate (SLN) crystals have much improved properties.The work in this dissertation is based on LN waveguides formed by ion implantation and irradiation, mainly contains the following three aspects:lattice damage in implanted LN, waveguide formation in LN and SLN by ion implantation and irradiation, and waveguide-based applications (SHG and photonic crystals).LN crystals were implanted with4MeV O ions. Lattice damage in LN implanted at low fluence (6×1014ions/cm2) coincides with the vacancy profile simulated by SRIM. LNs implanted at high fluences (2×1015ions/cm2and4×1015ions/cm2) have a amorphous surface layer which expands with the fluence. This effect is attributed to electronic energy loss of4-MeV O ions and is explaned by the thermal spike model. Ion-implanted LN waveguide is covered by the amorphous surface layer which has a lower refractive index than the LN crystal. At a fluence of4×1015ions/cm2, the amorphous layer is about900nm thick. Therefore,633nm light can not couple into the buried waveguide but the1539nm light can. The refractive index profiles were calculated from the damage profiles which were measured by Rutherford backscattering and channeling technique (RBS&C).A proton-exchanged LN was subjected to70-MeV argon-ion irradiation. Lattice damage was investigated by RBS&C technique. It was found that lattice disorder induced by the proton exchange was partially recovered and the proton-exchanged layer was broadened. It indicated that the lithium ions underneath the initial proton-exchanged layer migrated to the surface during swift argon-ion irradiation and supplemented the lack of lithium ions in the initial proton-exchanged layer. This recrystallization is similar to the effect of thermal annealing and is ascribed to the massive electronic energy loss of argon ions.6-MeV O-ion implantation did not induce an effect like this in proton-exchanged LN at several fluencies (2×1012ions/cm2,2×1013ions/cm2, and6×1014ions/cm2). On the contrary,6-MeV O-ion implantation at low fluence makes the proton-exchanged laye totally amorphous. Therefore, combined proton exchange and ion implantion can produce a buried waveguide with a thickness-tunable amorphous surface layer.Waveguide was fabricated in SLN by200-MeV argon-ion irradiation at a fluence of2×1012ions/cm2. Guided modes were detected in the visible and near-infrared wavelength regions. There are nine modes at1539nm wavelength, which indicates SLN waveguide's ability to support mode at longer wavelengths. Micro-Raman spectra at156cm-1recorded in the SLN waveguide layer and the SLN substrate have narrower peaks than the spectrum of the CLN crystal, which indicates that the waveguide layer is still a near-stoichiometric layer. This demonstrates that swift argon-ion irradiation is an appropriate method of fabricating waveguide structures in SLN crystals.Planar and channel waveguides were fabricated in PPLN crystals with a period of5μm by6-MeV O-ion implantation at a fluence of6×1014ions/cm2. One mode was detected at1539nm wavelength, which confirms that the PPLN waveguides support modes at wavelengths near980nm. The single-pass second harmonic generations were carried out in these waveguides. The temperature-tuning curve in the waveguide is higher than that in the bulk PPLN crystal. O-ion implantation at low fluence did not affect the periodically poled domains. For the channel waveguide, a conversion efficiency of34.5%/(Wcm2) was achieved, and1.11mW SH light at492.5nm was generated.A photonic crystal slab that was constructed as a2D hexagonal photonic crystal with a finite depth and was etched into a proton-exchanged lithium niobate waveguide was simulated by using the fimite-difference time-domian method. Both the ΓK-oriented and ΓM-oriented propagations were simulated with the fundamental TE and TM mode of LN waveguide as the incident beam. There are opaque wavelength gaps in the transmittance spectra which are correspond to the band gaps of2D photonic crystal.1D and2D photonic crystals were fabricated in proton-exchanged LN ridge waveguides by using focused ion beams. For the filter with a period of450nm, the deepest grooves have a depth of2.5μm. For the2D photonic crystal with a period of540nm, the deepest holes have a depth of1.8μm.