Near-field imaging of femtosecond laser ablated sub-λ/4 holes in lithium niobate

We report on the direct femtosecond laser ablation of sub-lambda/4 (80â-250 nm) holes in LiNbO3 crystals and on its local near-field imaging. We show that the near-field transmission of holes can feature an attenuation of ~75% at hole central position, and a ~20% transmission enhancement at its sides. This high-contrast ring-shaped near-field distribution is found to be in agreement with simulations, suggesting the surface relief as the main contrast mechanism.