*Article* **Irradiation E**ff**ects of Swift Heavy Ions Detected by Refractive Index Depth Profiling**

**Hiroshi Amekura 1,\* , Rang Li <sup>2</sup> , Nariaki Okubo <sup>3</sup> , Norito Ishikawa <sup>3</sup> and Feng Chen <sup>2</sup>**


Received: 25 September 2020; Accepted: 10 November 2020; Published: 16 November 2020

**Abstract:** Evolution of depth profiles of the refractive index in Y3Al5O<sup>12</sup> (YAG) crystals were studied under 200 MeV <sup>136</sup>Xe14<sup>+</sup> ion irradiation, since the index can be related with the stress change and/or the defect formation by the irradiation. Using the prism-coupling and the end-surface coupling methods, various waveguide (WG) modes were detected. Then, the index depth profiles were determined by reproducing the observed WG modes. The index changes were observed at three different depth regions; (i) a sharp dip at 13 µm in depth, which is attributed to the nuclear stopping *S*<sup>n</sup> peak, (ii) a plateau near the surface between 0 and 3 µm in depth, which can be ascribed to the electronic stopping *S*e, since *S*<sup>e</sup> has a very broad peak at the surface, and (iii) a broad peak at 6 µm in depth. Since the last peak is ascribed to neither of *S*<sup>e</sup> nor *S*<sup>n</sup> peak, it could be attributed to the synergy effect of *S*<sup>e</sup> and *S*n.

**Keywords:** swift heavy ion; YAG (Y3Al5O12); refractive index profiling; synergy effect; optical waveguide
