Creation of a second-order optical susceptibility in phosphate glasses

Second-order optical susceptibilities (χ(2)) were induced in two phosphate glass systems—lanthanum phosphate (LaP), and Ce3+-doped sodium aluminum phosphate (IOG-1)—for their potential use in nonlinear optical devices. Glass samples of 1 mm thick were subjected to high voltages at elevated temperatures for several tens of minutes before cooling down and later removing the applied voltage. This process is known as thermal poling. The induced χ(2) was studied as a function of poling conditions (voltage, temperature, and time). The spatial χ (2) profile was characterized by the Maker fringe technique, and structural changes in the glass were studied with Raman spectroscopy. Various charge migration models were investigated to explain the microscopic origin of the induced χ(2).; For thermally poled LaP, a χ(2) was induced under the following conditions: applied voltage of 3–4 kV, poling temperature of 300–350°C, and poling time of 45–60 minutes. The nonlinear region is located near the surface on the anode side and has an approximate width of 30 μm. The origin of the induced χ(2) involves migration of alkali ions during thermal poling, resulting in a built-in space charge field which in turn induces a χ(2) via the Kerr effect and/or dipole reorientation. There is no indication of structural change. The overall magnitude of the induced χ(2) is approximately 0.88 pm/V.; For thermally poled IOG-1, smaller applied voltages (1–2 kV) and lower poling temperatures (100–250°C) were used to prevent an overshoot current resulting from Na+ migration during thermal poling. The induced χ(2) profile consists of an anodic surface layer and a bulk contribution. The width of the anodic surface layer ranges from 24 to 40 μm, depending on poling conditions. Migration of Na + ions, which are present in large concentration in the glass, gives rise to the induced surface χ(2) and causes structural changes near the anodic surface. The origin of the bulk χ (2) is believed to come from dipole reorientation via the applied voltage. The anodic surface χ(2) and the bulk χ (2) are opposite in sign, giving rise to a χ(2) with an overall magnitude of 0.04 pm/V.