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Glass Poling


Poling of glass is the physical process through which the distribution of the electrical charges is changed. In principle the charges are randomnly distributed and no permanent electric field exists inside the glass.

When the charges are moved and fixed at a place then a permanent field will be recorded in the glass. This electric field will permit various optical functions in the glass, impossible otherwise. The resulting effect would be like having positive and negative poles as in a battery, but inside an optical fibre.

The effect will be a change of the optical fibre properties. For instance glass poling will permit to realize second-harmonic light generation which consists on converting an input light into another wavelength, twice the original radiation. For instance a near infrared radiation around 1030 nm could be converted with this process to the 515 nm wavelength, corresponding to green light.

Glass poling also allows for the creation of the linear electro-optic effect that can be used for other functions like light modulation.

So, glass poling relies on recording an electric field which breaks the original symmetry of the material. Poling of glass is done by applying high voltage to the medium, while exciting it with heat, ultraviolet light or some other source of energy. Heat will permit to the charges to move by diffusion and the high voltage permits to give a direction to the charges displacement.

Optical poling of silica fibers allows for second-harmonic generation through the creation of a self-organized periodic distribution of charges at the core-cladding interface.

UV poling received much attention because of the high non-linearity reported, but interest dwindled when various groups failed to reproduce the results.

Strong electric fields are created by thermal poling of silica, subjecting the glass simultaneously to temperatures in the range of 280 °C and a few kilovolts bias for several minutes. Cations are mobile at elevated temperature (e.g., Na+) and are displaced by the poling field from the anode side of the sample. This creates a region a few micrometers thick of high electrical resistivity depleted of positive ions near the anodic surface. The depleted region is negatively charged, and if the sample is cooled to room temperature with the poling voltage is on, the distribution of electrons becomes frozen. After poling, positive charge attracted to the anodic surface and negative charge inside the glass create a recorded field that can reach 109 V/m. More detailed studies, show that there is little or no accumulation of cations near the cathode electrode, and that the layer nearest to the anode suffers partial neutralization if poling persists for an excessively long time. The process of glass poling is very similar to the one used for Anodic bonding, where the recorded electric field bonds the glass sample to the anode.


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