Obtaining Subwavelength Optical Spots Using Nanoscale Ridge Apertures

Concentrating light into a nanometer domain is needed for optically based materials processing at the nanoscale. Conventional nanometer-sized apertures suffer from low light transmission, therefore poor near-field radiation. It has been suggested that ridge apertures in various shapes can provide enhanced transmission while maintaining the subwavelength optical resolution. In this work, the near-field radiation from an $H$-shaped ridge nanoaperture fabricated in an aluminum thin film is experimentally characterized using near-field scanning optical microscopy. With the incident light polarized along the direction across the gap in the $H$ aperture, the $H$ aperture is capable of providing an optical spot of about $106nm$ by $80nm$ in full-width half-maximum size, which is comparable to its gap size and substantially smaller than those obtained from the square and rectangular apertures of the same opening area. Finite different time domain simulations are used to explain the experimental results. Variations between the spot sizes obtained from a $3×3$ array of $H$ apertures are about 4–6%. The consistency and reliability of the near-field radiation from the $H$ apertures show their potential as an efficient near-field light source for materials processing at the nanoscale.