Study Of Properties On Optical Waveguides And Fabrication Of Photonic Crystal Slabs In LiNbO₃

Optical waveguide is the basis of the devices in integrated optics and is widely used in the optical telecommunication field. In recent years, people have continuously explored the methods for fabricating wavguide and have continuously improved the performance of the waveguide. As an effective method for fabricating waveguide, ion implantation technique attracts many people's interesting and has achieved great development. Ion exchange is another important way for fabricating optical waveguide. In this thesis, ion implantation and proton exchange have been exploited for fabricating optical waveguide in lithium niobate crystal.Ion implantation is including high-energy light ion implantaion (such as H, He ion, etc.) with large dose and high-energy heavier ion implantaion (such as O, P and Si ion, etc.) with low dose. In general, the waveguide formed by light ion implantation with MeV energy and the dose in order of 1016 ions/cm2. A buried low-index barrier inside the substrate is generated by nuclear energy deposition at the end of the ion track. So, a waveguide can be formed between the barrier and the air. By MeV heavier ion implantation at a low dose, a large increase of refractive index occurs in the near-surface region by the decrease of spontaneous polarization and a negative index barrier might exist at the end of the ion track. Usually, the dose of heavier ion implantation can be 1～3 order lower than that of light ion implantation.Being an artificially arranged periodic electromagnetic structures in optical wavelength, photonic crystals can be used to confine the light. The 3D photonic crystals owns intriguing application prospect and can confine the light wave in any directions. However, the fabrication of 3D photonic crystals is much difficult, in particular on the optical crystal material. As alternative system, the 2D photonic crystal slab has been proposed that promises easier fabrication using existing techniques. This is a dielectric structure that has only two-dimensional periodicity and uses index guiding to confine light in the third dimension. However, the photonic crystal slabs retain almost all of the desirable properties of true photonic crystals. Being the basic elements of the integrated photonic systems, optical waveguide has an important role. The photonic crystal slabs can confine the light wave with the index guiding in the third direction. However, the strong vertical confinement lends a large probability to the band gaps. In order to couple light wave into the photonic crystals, there is one in-coupling or out-coupling ridge optical waveguide on each side of the photonic crystal slabs. We have done a lot of investigations on such ridge waveguides, in particular the single-mode waveguides because of their smaller volume and lower loss, compared to the multi-mode ones. So, we reckon that the photonic crystal slabs need the supports of the traditional waveguides.The purpose of this paper is to investigate the photonic crystal slabs on basis of the ion implanted waveguide. So, we need to study the ion implanted waveguides to meet the photonic crystals. Firstly, such waveguide should be single-mode waveguide at wavelength of 1550 nm with the low propagation loss. Secondly, the depth of this waveguide should be thin enough to make the side walls of the holes vertical using existing techniques. In addition, thin waveguide can not induce high-order mode in photonic crystal and can avoid the coupling loss.In this paper, we focus on the waveguides in the congruent lithium niobate (CLN) and the stoichiometric lithium niobate (SLN). Lithium niobate called "silicon of photonics " is widely used in advanced photonics and nonlinear optics due to its intriguing combination of excellent electro-optics, acousto-optic, elastic-optic and nonlinear-optic characteristics. As the basic components of integrated photonic system, optical waveguide structures have been used to realize many functional devices in lithium niobate, including electro-optic modulators, waveguide laser, photorefractive spatial solitons. photonic bandgap crystals and so on. There are a lot of defects in CLN, because the ratio of Li/Nb in CLN is smaller than 1. Nevertheless, the ratio of Li/Nb in SLN is upgraded and closes to 1. So, SLN shows the improved performances because of the fewer defects in SLN than that in CLN. For example, the SLN crystal has a lower coercive field, but the nonlinear coefficient, electro-optics coefficient, sensitivity of photorefractive and laser threshold are much higher than those of CLN.The main results of this thesis are shown as following: 1. The single-mode waveguides formed by ion implantation combined with proton exchangeThe proton-exchanged waveguides were implanted by O ion with a dose of 1×1015 ions/cm2 by energy from 600 to 1400 keV at the energy interval of 200 keV. We have formed the single-mode waveguides at wavelength of 633 nm and 1539 nm. Rutherford backscattering/channeling measurement has been carried out in 800 keV O ion implanted and proton exchanged waveguides in order to observe the atomic displacements from the lattice sites. The results show that there are different damage profiles. The refractive index profile of sing-mode waveguide in lithium niobaie with index-raised well and low-index barrier type has been obtained based on Intensity Calculation Method. We have fabricated the ridge waveguides on the 800 keV O ion implanted planar waveguide by photolithographic technique and Ar ion beam etching. The measured near-field intensity profiles are reasonable consistent to the calculated ones. The propagation loss measured by Fabry-Perot Method is～2.2 dB/cm. The results show that ion implantation can modulate the modes in proton-exchanged waveguide.2. Helium ion implanted lithium niobate single-mode waveguidesWe have fabricated single-mode planar waveguide by 400 keV He ion with a dose of 3×1016 ions/crn2 at liquid nitrogen temperature. The shape of the measured damage profile by RBS/channeling technique is similar to that of the ordinary refractive index profile by the Intensity Calculation Method. Further, We have fabricated the ridge waveguides by photolithographic technique and Ar ion beam etching and have measured and simulated the near-field intensity profile. The propagation loss of the ridge waveguide for ne is about 1.9 dB/cm. We have also fabricated the waveguide by He ion with the same energy and dose at room temperature. In both conditions, the refractive indices of waveguides for ne are increased and for no are decreased. However, the changed ranges of ne and no at low temperature are both larger than those at room temperature. From the RBS/channeling spectra, it can be seen that more defects can retain in the waveguide formed at low temperature. In both conditions, damage can increase ne and decrease no.3. CLN and SLN single-mode waveguides formed by double-energy O ion implantationWe have fabricated single-mode waveguide only after annealing at 260℃for 30 min by double-energy O ion at room temperature. The implanted energies were 550 keV and 250 keV, at doses of 6×1014 and 3×1014 ions/cm2, respectively. We have simulated the extraordinary refractive index profile of the annealed waveguide based on the measured damage profile. The refractive index in waveguide region is increase. We have obtained a homogeneous damage profile and a homogeneous near-field intensity profile by double-energy O ion implantation. The propagation loss of the annealed waveguide is about 0.5 dB/cm. We have also fabricated planar waveguide in SLN at the same implanted conditions. Combined lithographic technique and ion implantation technique, we have formed the channel waveguide on SLN. We have measure the dark mode spectra for the planar waveguides before and after annealing. We reckon that the damage induced during ion implantation can dominate the changes of extraordinary refractive index. We have also measured the near-field intensity profile of the channel waveguides.4. Simulation and fabrication photonic crystal slabs in lithium niobateWe have simulated the projected band diagrams and the transmittance spectra of photonic crystals in lithium niobate. Based on the simulated parameters, we have fabricated the photonic crystals on proton-exchanged lithium niobate by focus ion beam (FIB) technique. We have fabricated 2D photonic crystals and 1D photonic-bandgap microcavity in lithium niobate single crystal membrane bonded on the silicon dioxide substrate by FIB technique.