Characterisation of optical waveguides in BGO crystals fabricated by N⁺ ion implantation

Abstract

HungaryBismuth germanate crystal (BOO) has gained considerable attention due to its very attractive physical and optical properties. Thanks to its very good radiation length, short decay time, emission efficiency and response linearity, it is widely used as scintillator material for radiation detectors in high-energy physics and in medical diagnostic systems such as PET (positron emission tomography) [1]. It also has a relatively high large electro optic coefficient (3 .3 pm/V) and second and third order non linear properties have been reported [2], which make this material also suitable for many applications in the field of opto-electronic devices.The possibility of fabricating guided-wave structures in this material is therefore of great interest, since it can allow the development of miniaturised radiation detectors for PET or, exploiting the optical confinement and long interaction length offered by integrated optics, optical devices based on nonlinear effects.Among the available waveguide fabrication processes, ion implantation has been proved as a universal technique in order to produce integrated optical guiding structures in many glassy [3] and crystalline materials thanks to its high versatility and good controllability, so that the fabrication of optical waveguides in BOO crystals by He+ ion implantation has indeed already been reported [4].In this paper we describe the fabrication and characterisation of slab optical waveguides in eulytine BOO crystal obtained by N+ high energy ion implantation. Utilising heavier ions for the implantation process has been proved to produce the same refractive index change at smaller doses than when light ions such as He + are used [5] and we therefore intended to assess the effects of N+ ions implantation in order to demonstrate the feasibility of this process and optimise it with the future aim of developing waveguide based optical devices.SRlM calculations of the stopping range were carried out in order to choose the appropriate energy of the N+ ions to obtain an irradiated thickness of the sample sufficient to support guided modes in the visible and NIR wavelength range.Square areas of eulytine BOO samples have therefore been irradiated at 3.5 MeV energy with doses of 2·10 15 , 4 . 10 15 ,8 . 10 15 , 16 . 10 15 ions/cm2 on an area of6x6 mm2.Propagation modes have thereafter been observed in all samples by prism coupling in the visible-NIR spectrum and their effective indexes measured by dark line spectroscopy. The processing of these experimental data has allowed the characterisation of the refractive index change induced by the implantation in the crystal as a function of the dose.They have also allowed confirming the presence of a barrier waveguide structure in all samples, where an increase of the refractive index in the volume of crystal crossed by the ions is accompanied by a lower refractive index layer at the end of the stopping range of the ions. References