Lithium niobate (LiNbO3), also called the 'silicon of photonics', is indispensable in advanced photonics and nonlinear optics(1-10). For many applications, however, the material is too polluted by transition metals, which are unavoidable at the parts per million level. These impurities serve as sources and traps for photoelectrons, causing optical damage and hampering the usability of LiNbO3. Efforts have therefore been made to get rid of the photoexcitable electrons(11,12). Here we introduce a method termed 'optical cleaning'. We show theoretically and experimentally that, if the material is heated to moderate temperatures, allowing ions to migrate and to maintain charge neutrality, an appropriately moving light beam pushes photoexcitable electrons out of the illuminated region like a brush, and provides exponential cleaning. This promises purification levels that are beyond the reach of current technologies.
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