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Study On Waveguide Property Of Ion-implanted LiNbO3 Crystal Using Etching And Ellipsometry Techniques

Posted on:2009-08-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:H P LiuFull Text:PDF
GTID:1118360245996134Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
Optical waveguide is a guiding structure which based on the theory of light total reflection on the interface of two different-refractive-index media and limits the light to travel in the transparent material of micron scale.Waveguide is the basic unit of integration optics and is also the basic structure of passive and active devices,such as optical coupler,waveguide modular,waveguide switch,etc.At the same time, waveguide is the basic structure of the all optical network.It plays an important role in the fabrication of various optical devices because of its excellent characteristics, possibility in integration and rather low cost in manufacturing.Scientists and engineers are trying to find various ways to fabricate "high-quality" optical waveguide.The methods which are commonly used to fabricate various waveguides include diffusion,exchange,film deposition and ion implantation.Ion implantation, as a mature method to modify the surface property of materials,is an effective method for fabricating various waveguide structures.This method has been adopted for waveguide fabrication in a large number of optical materials.Ion implantation has many advantages,such as:(1)relatively flexible selection of a wide range of available ions for implantation;(2)the controllable implantation parameters,such as species, energies and doses of the implanted ions in forming optical waveguide;(3)Ion implantation can be carried out under various temperature;(4)Two or multi-storey structure,and thus of complex optical waveguide can be formed by ion implantation; (5)Ion implantation is carried out in a vacuum environment,therefore the surface pollution of material can be prevented.Since the above advantages,ion implantation becomes one of the most promising ways for optical waveguides fabrication.Up to now,many waveguide structures have been formed by ion implantation in many optical materials such as optical crystals,glass,semiconductors and polymers etc.Lithium niobate(LiNbO3 LN)is an important optical crystal,with the following characteristics:(1)large electro-optical,birefringence,nonlinear optical coefficients, intensive acoustic-optic,photoelastic,photorefraction,piezoelectric,pyroelectric, ferroelectric,photovoltaic effect and other physical effect;(2)stable mechanical properties,high-temperature and corrosion resistance;(3)easy to grow to large crystal,easy processing and low cost;(4)showing a wide range of special performance by introducing different ions into crystal.So the LiNbO3 crystal has been widely used in many fields,such as acoustic-surface-wave filter,optical waveguide, electric-optic modulator,double frequency conversion,holographic storage,etc.It's an outstanding artificial crystal,and plays the similar role as the silicon single crystal in semiconductor.It's one of the best crystals which have the most excellent optical performance and comprehensive index.Up to now,as we have known,it's the most basic and important function material which used in optoelectronics field. Especially in recent years,with the development of rare-earth doping engineering, domain engineering,growth and processing technology of near stoichiometric crystal, the study on the function and performance of LN waveguide and substance material, as well as the photoelectronic and photonics devices increased dramatically.It makes it possible to be used in the fields of optical communications,military confrontation, optical data storage,optical gyroscope,optical remote sensing and laser technology, and becomes the optical "Si" material.As the above advantages and its important value,Lithium niobate becomes the most preferred optical crystal for ion implanted waveguide.For the optical waveguide formed by ion implantation,the refractive index profile of ion-implanted layer decides the main characteristics of waveguide.The nonlinear characters in waveguides also have important relationship with the refractive index profile.Therefore,the refractive index profile in the waveguide by ion implantation is not only the basis of optical waveguides,as well as the theoretical fundamental for design and fabrication of ion implanted waveguides.Direct measurement of the refractive index in optical waveguide is difficult.In order to determine the refractive index profile of the waveguide,the scientists have put forward a number of methods, such as WKB and derived iWKB method introduced by Wentzel,Kramers and Brillouin;and PIPR(refractive index distribution parameters reconstruction)by Fluck etc.These methods are adapted to the refractive index distributions in many waveguides and have demonstrated their good applications in reconstruct the refractive index profile.All of them are used based on the measured dark-mode spectrum of waveguide.However,the dark mode spectrum in ion implanted waveguide obtained by prism coupling usually contains several unsteady modes(e.g. leakage mode)due to the existence of optical index barrier.They are not the real guiding mode in the waveguide but come from the multiple reflections between surface and the index barrier.So the above-mentioned methods are not entirely suitable for describing the index distribution in ion implanted waveguide in some cases.In order to solve this problem,Chandler and Lama proposed a new method in 1986,named reflectivity calculation method,short for RCM.It can be used to reconstruct the refractive index profile of ion-implanted waveguides successfully.In the calculation of refractive index profile of ion-implanted waveguides,RCM is the main method used by researcher presently.This method has made great achievements in analyzing ion-implanted waveguide,especially for the waveguide with many modes.But for the waveguide that have less modes(less than three),RCM may introduce large error to the reconstructed index profile.Moreover,as for the second half-Gauss-curve profile(refractive index profile behind the barrier),the result of RCM is not satisfactory.To solve this problem,we proposed the method by combining etching technique with ellipsometry method to the analysis of refractive index distribution in ion implantation waveguide.Etching is mainly used in micro-machining technology,including microelectronics,optoelectronics,and the production of micro-optical elements. Etching includes dry etching and wet etching.The ellipsometric measurement originated more than 100 years ago,and is one of promising optical method to determine the optical property and thickness of film based on the measurement of a change of the polarization state of light after reflection on the material surface under investigated.The method has the advantage of the following aspects:(1) non-destructive(2)measurement of high sensitivity and accuracy,(3)several parameters of material such as the index and thickness,n,k,d values can be measured simultaneously.To analyze the refractive index profile of ion-implanted waveguides by combining etching and ellipsometry technique,we firstly strip the implanted waveguide region layer by layer from the crystal surface using etching method,then measure the ellipsometric parameters from the corresponding surface after each etching.We can get the refractive index profile of waveguide area by analyzing the ellipsometric parameters finally.The advantage of this method is:(1)The result of the refractive index distribution is more reliable.Currently,the analysis of refractive index distribution in implanted waveguide is mainly based on RCM,it is to adjust the simulated parameters to reconstruct the refractive index profile which approach the measured effective index of dark-mode in waveguide. Ellipsometry is based on the measured experimental results to calculate the refractive index distribution.(2)The appliation area is wider.The waveguide with only fewer(less than 3) modes,particularly for the most value and applicable single-mode waveguides,RCM encounters difficulty.The application of ellipsometry is without restriction from the number of patterns,it can even measure the refractive index beneath the index barrier.(3)The Measurement of the refractive index distribution is more precise.The etching rate can be controlled artificially,the thickness of each etching can be very small,that is,the waveguide can be divided into many layers arbitrarily.As a consequence,we can approach the real index more reliably.The work we have done in this dissertation is focused on two aspects:(1)to analyze the light propagation property of crystal and obtain the relation between the ellipsometric parameters of divided-layer structure and the refractive index profile. It's the theoretic foundation of the paper;(2)to etch and measure the ellipsometric parameters of waveguide formed by ion implantation,analyze the refractive index profile of different waveguide structure,and study the characteristic of damage profile based on the etching behavier.There are two methods to discuss the light traveling in isotropic absorption media,complex refractive index method and vector propagation constant method.We compare one method with the other.Based on the analyzing method of light propagation in the isotropic absorption media,wave normal vector propagation constant was introduced,the propagation laws of light in single axis and biaxis absorption crystal were discussed,the representations of the some physical parameters, which are used to describe the medium properties and light propagation properties, such as wave normal refractive index,light refractive index,absorption coefficient, were derived.The relative formula of transparent crystal can be deduced from these parameters.We emphasize to give the reflection coefficients and transmission coefficients of parallel polarization light and vertical polarization light on the crystal interface.As for the divided-layer-uniform isotropic film,the ellipsometric parameters can be derived from scattering matrix methods.We regard the waveguide region formed by ion-implanted crystal as index-divided-layer-uniform film structure,then we calculated the ellipsometric parameters based on the scattering matrix methods.O2+ion is the common ion used to form the optical waveguide by ion implantation in LN crystal.We formed the waveguide with single energy,double energy and multi-energy 02+ ion in LN crystal.We studied the change of effective refractive index of dark modes in waveguide before and after 250℃annealing for half an hour.Then we used the etching and ellipsometry technology to analyze the refractive index profile of single energy and multi-energy ion-implanted waveguide.MgO-doped LN crystal has many advantages,such as the strong optical damage resistance,better photorefractive resistance and big birefringence,etc.It is widely used in nonlinear optics,laser media and waveguide fields.We made the waveguide by ion-implanted into MgO-doped LN crystal using 3 MeV O2+ions.We studied the change of effective refractive of dark modes in waveguide before and after 250℃annealing for half an hour.Then we used the etching and ellipsometry technology to analyze the refractive index profile of ion-implanted waveguide.C2+ion is another common ion used to form the optical waveguide by ion implantation methods in LN crystal.We formed the waveguides with 2MeV energy, two different doses C2+ions in LN crystal.We studied the change of effective refractive index of dark mode in waveguide before and after 250℃annealing for half an hour.Then we used the etching and ellipsometry technology to analyze the refractive index profile of ion-implanted waveguide,which is formed by little dose C2+ions.The crystal will be damaged after ion implantation,and the damage profiles affect the waveguide performance strongly.We used the etching methods to study the etching behavier of LN crystal after O2+,C2+,Si+,Ni+ ions with different energy and different doses implanted into +z and -z surface of LN crystal.Based on these behavier,we analyze the damage profile of ion-implanted region.The LN damage is very little after H+ ion implantation.We studied the change of LN crystal waveguide formed by H+ ion implantation after several times annealing under different temperature condition,and the etching behavier of +z surface and -z surface of LN after implantation and post-implant annealing.
Keywords/Search Tags:LiNbO3 crystal, ion implantation, waveguide, refractive index profile, etching, ellipsometry
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