Ji Wang, Lin Li
Optical diffraction limit traditionally confines the minimum focused laser spot to approximately half the laser wavelength, restricting nanoscale fabrication capabilities. This study demonstrates far-field femtosecond laser direct writing of sub-diffraction-limit grooves on Cu in air by employing an infrared femtosecond laser with a 1035 nm wavelength. The method involves polarization modulation of the laser through a radially polarized beam, using a double-axicon lens system to obtain a thin annular profile. After focusing with a high numerical-aperture (NA=0.95) objective lens, this configuration generates a longitudinally enhanced electric field with an unprecedented 98% purity. Through far-field processing on Cu, the characteristic width of the nanogroove is 47 nm, corresponding to 1/22 of the laser wavelength. Furthermore, multiple pass etching enhanced the nanogroove depth by threefold up to 75 nm, while there is only a small increase in groove width. A comparison of material removal mechanisms between longitudinal field and transverse field is made through atomic scale modelling. This mask-free, far-field approach simplifies nanoscale patterning and holds potential for applications in nanoelectronics and nano-photonics. • Sub-diffraction-limit etching: achieved nanogrooves with a minimum width of 47 nm (λ/22) on copper plated silicon wafers using a 1035 nm femtosecond laser in the far field. • Longitudinal field enhancement: generated a 98% pure longitudinally polarized electric field via a double-axicon lens and high-NA focusing. • Etching depth enhancement: controlled depth enhancement up to 75 nm through repeated processing • Mask-free fabrication: vertically polarized electric fields eliminate the need for masks, reducing costs and complexity.