## 3 Results and Discussion
The aligned (magenta line) and random (dark yellow line) spectra of the pristine crystal are also given for comparison. By comparing the aligned spectral heights of the pristine crystal and the irradiated samples, the lattice damage degree of the waveguide region can be obtained. As is shown, the backscattering yield of the irradiated crystal is higher than that of the pristine crystal, and the backscattering yield of the irradiated crystal increases with the increase in oxygen fluence. The aligned spectra of the crystals indicate that the irradiation of energetic oxygen ions causes a certain lattice distortion near the surface region. The depth profiles of the lattice damage near the surface region were extracted from the Rutherford backscattering/channeling spectrum, and the obtained disorder profiles of the sample irradiated with fluence of 1.5 \(\times\) 10\({}^{15}\) ions/cm\({}^{2}\) are shown in Figure 5b. This is achieved by applying a model of multiple scattering dechanneling, which is based on formulas by [PERSON] and [PERSON] [23; 24]. As is shown, the lattice damage ratio at the surface of the sample is 20%, and the crystal lattice at the end of the oxygen ion range is seriously damaged due to irradiation, which means that the crystal lattice is completely amorphous. Figure 3: The near-field optical intensity profile of the waveguide irradiated with fluence of \(1.5\times 10^{15}\) ions/cm\({}^{2}\): (**a**) experimental result collected by CCD camera; (**b**) simulation results by applying the beam propagation method. Figure 4: High-resolution X-ray diffraction curves of (**a**) pristine crystal, (**b**) crystal irradiated with fluence of \(5\times 10^{14}\) ions/cm\({}^{2}\), (**c**) crystal with fluence of \(1.5\times 10^{15}\) ions/cm\({}^{2}\). Figure 5: (**a**) Aligned and random spectra for yttria-stabilized zirconia crystals before and after oxygen irradiation. (**b**) The disorder profile near surface region were extracted from the Rutherford backscattering/channeling spectrum. In order to better understand the interaction between oxygen ions and crystals, as well as the formation mechanism of waveguides, the stopping and range of ions in the matter code [25; 26] is applied to simulate the process of oxygen irradiation into yttria-stabilized zirconia crystals. Figure 6 exhibits the energy loss of the 4.5 MeV oxygen ions into yttria-stabilized zirconia. As indicated, during the path inside the crystal, most of the energy of incident ions is lost due to electronic ionization, usually accompanied by the formation of color centers. Although it is reported in some reports that ion irradiation in the keV low-energy region usually contributes to lattice disorders caused by elastic collision between energetic ions and the target, electronic ionization may also lead to defects in the irradiation process, especially when energetic ions (close to 5 MeV) are applied [27; 28]. While at the end of the ions' trajectory, the peak value of the nuclear energy loss curve lies at a depth of 2.2 \(\upmu\)m below the crystal surface. Nuclear collisions can cause displacement damage, break the crystalline structure and reduce the physical density, resulting in a decrease in the refractive index in this region. The profile of the refractive index is consistent with the simulation results; the consistency indicates the rationality of the waveguide explanation.