_Applied_  
_sciences_  
_Review_  
Active and Quantum Integrated Photonic Elements by Ion  
Exchange in Glass  
[PERSON] 1,4 and [PERSON] 2,*+  
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Footnote 6: [[https://www.mdpi](https://www.mdpi)]([https://www.mdpi](https://www.mdpi)). strengthening) became a standard in the industry in the 1960s, when, for instance, Corning developed Chemcor(r) glass, a chemically strengthened glass that was intended to be used in phone booths, prison windows, eyeglasses and automobile windshields. In parallel to these events, there were interesting advances in the optical field, too: in the 1910s it was observed that the ion-exchange process was also inducing a change in the refractive index of the glass, and in the 1960s integrated optics moved its first steps, with waveguiding structures that required materials having slightly different refractive indices [3]. Since then, ion exchange has been proven to provide an excellent technological platform for optical integration. Optical waveguides in glass by ion exchange were first fabricated in 1972 using electrically induced migration of thallium ions into a borosilicate crown glass that contained K20 and Na2O [4], but soon waveguides were also produced by thermal ion exchange from thallium, potassium or silver nitrate, with Ag\({}^{+}\) giving the best results (besides also being much safer than Tl\({}^{+}\)) [5]. Quite obviously, there was also a growing industrial interest, as testified by several patents; for instance, the US Patent 3,857,689 by [PERSON] et al., filed in 1972 and published in 1974, concerned the \"Ion exchange process for manufacturing integrated optical circuits\" [6]. Figure 2 shows a schematic view of the process, where the glass plate (11) with the Ti mask (12) is immersed in a molten salt bath (13); as an example, the patent indicated that for a Schott F2 lead silicate glass the process produces a surface refractive index difference \(\Delta\)n = 0.005 when immersing the glass for about 3 h in a molten salt bath heated at 450 \({}^{\circ}\)C and containing Tl2 SO4 and ZnSO4 at an equal mole ratio [6]. Almost at the same time, the US Patent 3,836,348A by [PERSON] et al., filed again by Nippon Selfoc Co. Ltd. in 1973 and published in 1974, presented a \"Method for manufacturing optical integrated circuits utilizing an external electric field\" [7]. They claimed that, by using Tl\({}^{+}\) ions and an electrical field to drive the ion diffusion, a refractive index profile close to a rectangular one can be achieved. In the following years, the fabrication technology of glass waveguides employing several ion exchangers (K\({}^{+}\), Cs\({}^{+}\), Rb\({}^{+}\), Cu\({}^{+}\), Li\({}^{+}\) and so on) experienced a great development, and commercial devices were designed and tested (see, for instance, Reference [8]). In parallel, ion-exchange technology was also exploited to create gradient index (GRIN) glass rods [9] and rod lenses with parabolic index profile, suitable for photographic systems [10]  
Figure 1: Image of a part of a two-lancet stained window in the Florence Cathedral, representing San Miniato (St Minias) on the left and San Paolo (St Paul) on the right.