Voltage-assisted cooling: a new route to enhance χ(2) during thermal poling

Modifying the standard constant-voltage poling procedure significantly enhances the in-built electric field strength (and consequently χ (2) ) and leads to control of the nonlinear region evolution, both crucial parameters to integrate nonlinearity in waveguiding regions. Second-order susceptibility induced by the formation of a space-charge region is widely accepted as the macroscopic effect of standard thermal poling, which consists of inducing ionic migration by high static electric field applied across a pre-heated amorphous material and subsequently freezing it with the same voltage still applied (1). While several studies have investigated how the dynamic SHG behaves in respect to different poling parameters (such as time (2), voltage (3), atmosphere (4)), none to date has been performed when the voltage is increased during the cooling phase. By adopting such a voltage-assisted treatment, we dramatically improve the second harmonic (SH) signal compared to the standard poling procedure, for identical conditions at the start of it; most importantly, the second order susceptibility χ(2) is doubled when the initially constant voltage is raised up to higher voltage during cooling, as a combined result of both a dampen increase in the depth of the nonlinear region and enhanced frozen-in electric field. In this work, we report a comparative study between voltage-assisted and standard thermal poling carried out in air on two sets of Herasil1 samples, with different thickness of S=0.2mm and 1.0mm. In order to isolate how applying the voltage differently affects second-order nonlinearities (SONs), both poling time and temperature are set at tpol=10min and T=280°C, respectively. Two pressed-contact silicon plates are used to apply an initial constant voltage (in the range between 2 and 5kV) across the samples. After tpol, either the same or higher voltages are delivered to the electrodes during the cooling time, until the temperature contribution to poling can be considered significant (T>200°C). However, regardless of which mechanism is responsible for creation of the nonlinearity (nonlinear dipole orientation and/or permanent space charge field (5)), increasing voltage during standard poling has led to increase the width (depth) of the nonlinear region though (3) and the nonlinear efficiency could be improved only for optimum poling temperature, as high as 400°C (6). Firstly, we investigate the spatially resolved evolution of the depletion region when both standard constant- voltage (e.g., 3kV) and voltage-assisted cooling (e.g., from 3kV (Vinitial) up to Vcooling=4kV) are performed. Table 1. Nonlinear depth evolution under constant voltage (e.g., 3kV) and voltage-assisted cooling (e.g., 3-4kV).