SUPPRESSION OF OPTICAL DAMAGE IN HOLMIUM DOPED LITHIUM NIOBATE AT 532 NM
Nathan O'Connell1, Eftihia Barnes1, Nicolas Balli1, Madhab Pokhrel2, Edvard Kokanyan3, Anush Movsesyan4, Dhiraj Sardar1.
1University of Texas at San Antonio, San Antonio, TX, 2The University of Texas-Pan American, Edinburg, TX, 3Institute for Physical Research, National Academy of Sciences of the Republic of Armenia, Ashtarak, AM, 4Armenian State Pedagogical University after Khachatur Abovyan, Yerevan, AM.
Lithium niobate (LN) is an important optical material used in a multitude of applications including waveguides and acousto-optic and electro-optic devices. It is especially valued for its nonlinear properties, e.g., second harmonic generation. However, when LN is irradiated by light above a certain intensity level, defects inside the crystal act as electron traps creating internal photovoltaic fields. These photovoltaic fields modulate the local refractive index through the linear electro-optic effect and severely distort the transmitted beam. This undesirable effect is known as photorefractive damage and is one of the main contributors of optical damage in LN. We have examined the effect of Ho3+ doping on the 532 nm optical damage in a series of congruent LN crystals with Ho3+ concentrations ranging from 0.01 to 2 wt. %. A continuous wave 532 nm Gaussian beam, polarized parallel to the z-axis of the crystals, was focused on the front face of each sample and, after 5 minutes of continuous illumination, an image of the transmitted beam was captured with a CCD camera. Transient optical damage measurements were also recorded using the pseudo Z scan configuration. Notably, the optical damage was almost entirely suppressed in the 2 wt. % doped crystal for the entire range of experimental intensities (up to 15 kW/cm2). This suppression is thought to originate from the reduction of the bulk photovoltaic effect caused by the modification of the defect structure of congruent LN by the incorporation of the Ho3+ cations or from higher order pyroelectric effects.