Poled Glasses
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an electric field does little to the optical wave traveling in the material. In 1991 Myers, Mukherjee, and Brueck submitted a polymer film on a silica substrate to a poling process, which consists of the application of high voltages at a temperature of ~300°C. After cooling to room temperature with the voltage still applied, the poled sample was illuminated with a strong infrared (ir) laser beam. As expected they found that material frequency-doubled the incoming radiation and some green light could be measured at the output. When a test was carried out to discard any contribution from the substrate to the frequencydoubled light, they discovered that it was the silica glass that was generating the green light and that they had induced a strong optical nonlinearity in silica by thermal poling.1 Since then poling of glasses in the context of nonlinear optics has been the subject of many publications, including the report that ultraviolet (uv) radiation can advantageously replace thermal excitation.2 Poled glass exhibits the linear electrooptic effect that can be used for the modulation and switching of light. The possibility of making fiber-compatible MRS BULLETIN/NOVEMBER 1998
devices that are orders of magnitude cheaper than their LiNbO3-based counterpart and of comparable or higher speeds is extremely attractive. Such devices will find their way into everyone's homes in fiber-to-home systems in the future—controlling the input and output of television, video, computer, telephone, and other services. Another potential application of poled-glass systems is the conversion of ir to visible or near uv radiation. Shorter wavelength radiation can be focused on smaller spot sizes, of great importance in opticalmemory systems such as compact disks, as well as in other applications in which
visible radiation is desired such as in displays. Holographic storage can also potentially make use of second-harmonic generation in glasses. Thermal poling of dielectrics (in particular waxes) is a 250-year-old research field and is the basis of electrets.3 The permanent or semipermanent polarization of dielectrics assisted by light or uv radiation has also attracted attention for many years, leading to the development of photocopying. Thermal poling of silicate glass has been studied for decades both theoretically and experimentally,4-6 and the subject has several counterparts outside nonlinear optics. The same procedure (subjecting the sample to ~300°C and hundreds of volts) is used in anodic bonding of borosilicate glass to metal. Ionic migration at temperatures reaching ~80°C in glass isolation banks of highvoltage transmission systems leads to dielectric weakening and demands costly maintenance. Therefore it is of great interest to understand the physical mechanisms behind poling, not only from a nonlinear-optics perspective but also based on interest in other areas of physics. Figure 1 illustrates some of the basic methods used to pole and characterize
Optical Poling
Probing
Optical Poling 1.06 Mm
Ge-doped silica
0.53 ^m
1.0
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