Far-field femtosecond laser etching of sub-diffraction-limit nanogrooves on copper using high purity longitudinal field enhancement

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.