A simple solar-blind photodiode for light near 200 nm useful for angle-locking nonlinear optical crystals

We describe a simple solar-blind split-cathode photodiode for use in controlling the orientation of a nonlinear optical cyrstal such as β-barium borate (BBO) to maintain optimum conversion of pump light to tunable ultraviolet radiation in the 200-nm region. This device avoids the need for any wavelength filtering and is rugged, inexpensive and easy to build.