## 3 Results and Discussion  
Figure 1 is the resulting spectrum of the prism coupling measurement with a wavelength of 632.8 nm for an oxygen-irradiated yttria-stabilized zirconia crystal with an ion energy of 4.5 MeV and fluence 1.5 \(\times\) 10\({}^{15}\) ions/cm\({}^{2}\) at room temperature. Here, the refractive index of the pure crystal (n\({}_{\text{sub}}\)) is also marked for comparison. According to the experimental principle of mode-line measurement, when the waveguide condition is satisfied, the laser beam will couple into the waveguide, resulting in a lack of reflected light (the intensity of the mode-line spectrum decreases). Generally, each intensity decrease corresponds to one waveguide mode. It can be observed that only one sharp drop is  
detected, indicating that a monomode waveguide is achieved in the crystal by oxygen irradiation. It should be noted that there is no drop detected in the sample with ion fluence of \(5\times 10^{14}\) ions/cm\({}^{2}\). Based on the data of the prism coupling measurement, the refractive index near the crystal surface is reconstructed using a reflectivity calculation method [19], which is testified to be particularly suitable for ion-irradiated materials. The refractive index of the oxygen-irradiated waveguide is depicted in Figure 2. As is shown, the 'barrier wall' and 'enhanced well' construct the waveguide configuration with a thickness of 2.2 \(\upmu\)m. The decrease in refractive index at a depth of 2.2 \(\upmu\)m is related to the degree of damage induced from energetic oxygen irradiation. The enhancement of the refractive index in the waveguide region helps to confine the light transmission to some extent. The larger the refractive index contrast between the barrier and the waveguide layer, the stronger the capability of the waveguide to confine the light transmission. The near-field optical intensity image of the basic TE\({}_{0}\) mode in the crystal waveguide is depicted in Figure 3a, which is obtained from the end-face coupling measurement with a wavelength of 632.8 nm. Based on the refractive index distribution in Figure 2, the propagation of light in the crystal waveguide is simulated by applying the beam propagation method [20; 21; 22], which is famous for its numerical calculation in photonics. Figure 1: Mode-line spectrum of crystal waveguide irradiated by energetic oxygen ions. Figure 2: (**a**) 1D and (**b**) 2D refractive index profiles near the crystal surface after oxygen ion irradiation. The mode intensity distribution is shown in Figure 3b. By comparing the two near-field results, it can be concluded that there is a reasonable consistency between the experimental image and the calculated data. That also means the possibility of designing optical devices, because the refractive index near the surface of the crystal after ion irradiation can be well customized by nondestructive methods. Lattice modification in the irradiated crystal is investigated by using the high-resolution X-ray diffraction technique. Figure 4 shows the experimental high-resolution X-ray diffraction curves from the crystals before and after irradiation. All three curves show a main diffraction peak at 20 = 73.5, created by diffraction of the unperturbed region of the crystal. The peak from the irradiated crystals is much broader than that of the pristine crystal, indicating that un-irradiated crystals have better single crystalline properties. The diffraction spectra of irradiated crystals show the asymmetric broadened peak at 20 < 73.5, which is caused by diffraction of the damaged lattice. It should be noted that there are some satellite peaks in the diffraction spectrum of the irradiated crystal, and its number increases with the increase in the irradiation fluence. These oscillating fringes are on the left side of the main peak, which is characteristic of damage of tensile strain in the oxygen irradiated crystals. The Rutherford backscattering/channeling spectra of the oxygen ion-irradiated crystals are measured, and the results are shown in Figure 5a.