Researchers have created 10nm-diameter holes in sapphire using a femtosecond laser.
The work, conducted at the Laser Processing Research Centre at The University of Manchester, has been published in Light: Science & Applications.
The scientists were developing a novel method to experimentally demonstrate a high-purity longitudinal femtosecond laser field (i.e., parallel to the optical axis) and its interactions with polished silicon, copper, and sapphire.
In their experiments, they focus the beam of an 800nm femtosecond laser first using an aplanatic 0.75 NA lens to confirm the presence of the longitudinal fields, and then using a 0.95 NA lens to further understand the characteristics of the focused longitudinal fields and their effects on laser materials processing.
A number of polarisation states, beam intensity distribution and wavefront ablation profiles were investigated, the results of which were compared with theoretical models of the longitudinal field.
Material processing with a resolution of 10nm – well beyond the far-field diffraction limit at 800nm wavelength – was demonstrated on polished sapphire in air.
“By focusing the longitudinal field on sapphire, holes with a diameter as small as 10nm could be created, which is much smaller than the previously published results based on strong nonlinear laser materials interaction processes,” confirmed Dr Zhaoqing Li, whose PhD project comprised the work. “To verify the cross-section characteristics of these tiny holes, focused ion beam cross-sectioning was conducted. For a 30nm diameter hole, the depth was over 500nm with a zero taper.”
Dr Olivier Allegre, who co-authored the paper added: “The extremely small feature size and very high depth to width aspect ratio with parallel hole walls observed in this study could indicate that the material removal mechanism induced with the longitudinal field is fundamentally different from those induced with a transverse linear polarisation. This phenomenon is rarely seen in laser materials processing at this scale with a single pulse. The laser beam with the longitudinal field could behave somewhat like a particle accelerator and enable the electrons (with negative charge) and ions (positive charge) to be ejected more effectively than in a standard Coulombic explosion. The very deep holes produced in our experiments show the possibility of electron acceleration and charge polarisation in the holes for material removal.”
Professor Lin Li, another co-author of the paper and a past president of AILU, commented: “The significance of this research is the demonstration of super-resolution materials processing with an infrared laser beam, which breaks the optical diffraction limit in the far field, while most previous approaches are either based on costly extreme ultraviolet (EUV) laser wavelengths that operate within the optical diffraction limit, or on the use of near field optics that make the working distance too close to be practically useful. The very high aspect ratio of the laser processed features points out a new material removal mechanism.”