The British team achieved 10 nanometer aperture processing on sapphire, breaking the limit of far field optical diffraction

Sapphire has the advantages of super hardness, corrosion resistance, high temperature resistance, good light transmission in the ultraviolet infrared band, etc. It has a broad application prospect in military industry, medical equipment and other fields. However, these characteristics also make it difficult to carry out mechanical processing and chemical corrosion processing.

Recently, researchers from the Laser Processing Research Center of the University of Manchester in the UK announced that they used femtosecond laser to create a hole with a diameter of 10 nanometers on sapphire. This result achieved a new breakthrough in the fine processing of sapphire crystal surface by femtosecond laser.

(Image source: Laser Processing Research Center, University of Manchester, UK)

It is reported that scientists used a newly developed method to conduct experiments, demonstrating the high purity longitudinal femtosecond laser field (that is, parallel to the optical axis) and its interaction with polished silicon, copper and sapphire. In the experiment, they first focused a non planar lens of 0.75 NA to form a beam of 800 nm femtosecond laser, so as to confirm the existence of the longitudinal field, and then used a lens of 0.95 NA to further understand the characteristics of the focused longitudinal field and its impact on laser material processing. Then, they also studied the polarization state, beam intensity distribution and wave front ablation profile, and compared the results with the theoretical model of longitudinal field.

Finally, they demonstrated the material processing effect with a resolution of up to 10 nm on the polished sapphire in the case of space separation, which is far beyond the far field diffraction limit of 800 nm wavelength, and far exceeds the previously published results based on the strong nonlinear laser material interaction process.

In order to verify the cross-section characteristics of these micro holes, researchers conducted focused ion beam cross-section. The results show that the pore diameter is 30 nm, the depth is more than 500 nm, and the taper is zero.

Dr. Olivier Allegre, co-author of the paper, added: "The extremely small characteristic size, very high aspect ratio and parallel hole wall observed in this study may indicate that the material removal mechanism induced by longitudinal field is fundamentally different from that induced by transverse linear polarization. This phenomenon is rare in laser material processing with the scale of single pulse. The laser beam with longitudinal field can behave somewhat like a particle accelerator, making electrons (negatively charged) and ions (positively charged) ejected more efficiently than a standard coulomb explosion. The very deep holes produced in our experiments indicate that there is the possibility of electron acceleration and charge polarization in the holes to remove material. "

The significance of this study is to show that the infrared laser beam is used to treat super-resolution materials and breaks the optical diffraction limit in the far field. Most of the previous methods are either based on expensive extreme ultraviolet (EUV) laser wavelength and work within the optical diffraction limit; Either near-field optics is used, which makes the working distance too close to be used in practice. This method realizes the high aspect ratio feature of laser processing, and points out a new material removal mechanism.

Source: OFweek