| Laser manufacturing is one of the most important precision processing methods today,playing a crucial role in cutting,welding,integrated optics,additive manufacturing,and surface functionalization.However,traditional laser processing techniques are limited by diffraction barrier and suffer from severe thermal damage,which cannot meet the requirements for processing accuracy in today’s rapidly developing micro-nano science and technology.Therefore,exploring the principles and applications of laser super-diffraction limit nano-manufacturing technology has great scientific and engineering significance.Traditional optical super-diffraction-limit nanomanufacturing methods mainly include near-field optical manufacturing and nonlinear optical manufacturing.The former uses localized optical fields trapped near the nanoprobes to achieve fabrication resolution beyond the diffraction limit.However,due to the evanescent nature of the near-field waves,the working distance of near-field manufacturing is extremely short(< 50 nm),and therefore expensive and complex motion control equipment is required to manipulate the nanoprobes,which limits its practicality.The latter uses ultrafast lasers to trigger the nonlinear threshold effect,which can significantly improve the processing resolution while reducing the thermal damage.At present,nonlinear optical manufacturing has achieved feature sizes even below tens of nanometers on various materials,including polymers,metals,and dielectrics.However,due to the instability of nonlinear effect at near-threshold region,it is difficult to realize stable and controllable sub-hundred nanometer precision.In this thesis,we review the principles and current challenges of the mainstream optical super-diffraction nano-manufacturing technologies and propose the concept of optical far-field induced near-field breakdown to solve the above problems(Chapter 1).Specifically,we found that laser nano-ablation exhibits inherent anisotropy and proximity effect: the optical near-field enhancement guided by far-field polarization determines the evolution of laser ablating from discrete points to a continuous line(Chapter 2).Based on this,we achieved super-diffraction laser free writing of twodimensional nanogrooves by dynamically controlling the far-field polarization,pulse energy and scanning trajectory(processing accuracy < 20 nm,1/40 of the laser wavelength),which validates the technical credibility of the optical far-field-induced near-field breakdown(O-FIB,Chapter 3).Further,we extend the concept of optical farfield-induced near-field breakdown to three dimensions using the back-scattering interference crawling effect along the laser propagation direction,and in doing so we bypass the limitation of the Heisenberg uncertainty principle on the beam waist and divergence angle of the focal spot.Based on this,we proposed the super stealth dicing technique which permits high-precision(≤ 50 nm)and high aspect ratio(> 2000)nanomanufacturing on various semiconductors,transparent dielectrics,laser crystals and etc(Chapter 4).Finally,we summarize the main work of this thesis(Chapter 5).The main results are listed as followed:1.The anisotropy and proximity effect of laser nano-ablation.The coupling between light and structure,as well as the redistribution of the optical field at the nanoscale,is the origin of the anisotropy and proximity effect in femtosecond laser nano-ablation.Using the Born approximation,we solve the scattering wave emitted from a small nano-hole.We found that the rapidly decayed optical near fields excited at the boundary interfere with the incident beam,leading to anisotropic light intensity re-distribution and significant proximity effect during subsequent laser processing.We theoretically and experimentally demonstrated that when the laser is scanned along the polarization,a minimum non-ablative spacing arises naturally between two separate damaged structures due to destructive interference of light intensity.However,continuous damages can be formed when the laser is scanned perpendicular to the polarization direction.Therefore,to achieve high-quality precision fabrication at the nanoscale,the coupling between light and structure,as well as the redistribution of the intensity at the nanoscale,must be fully considered,which forms the basis of our subsequent work.2.The principle of optical far-field-induced near-field breakdown.The directional enhancement of the optical field guided by laser polarization is one of the fundamental principles for inducing near-field damage from far-field irradiation.By controlling the far-field parameters of the laer beam,such as polarization,pulse energy and scan trajectory,we achieve precise control of the near-field enhancement near a 20 nm nanohole.By strictly controlling the polarization to be perpendicular to the scan trajectory,we demonstrate sub-20 nm(/40, = 800 nm)free-form laser writing on titanium dioxide film.Furthermore,we extend the optical far-field-induced near-field breakdown to the internal modification of transparent dielectrics and demonstrate the universality of polarization-controlled near-field enhanced for nano-fabrication by taking the fused silica as an example.3.Back-scattering interference crawling effect and super stealth dicing technology.Limited by the Heisenberg’s uncertainty principle imposed on the waist-divergence relation,a trade-off must be made between the transverse resolution and the aspect ratio in conventional laser processing.When attempting to obtain higher transverse processing resolution by tightly focusing the laser,the divergence angle of the laser spot will increase,thereby reducing the depth and longitudinal uniformity of the process.To this end,we extend the principle of O-FIB to three dimensions and investigate the optical near-field enhancement along the optical axis.Specifically,we have identified a nanostructure elongation mechanism named back-scattering interference crawling effect,which is used to homogenize the longitudinal energy deposition during laser fabrication.Through rigorous analytical modelling and numerical simulations,we propose a super stealth dicing technology(SSD).In combination with wet etching,we have achieved sub-10 nm transverse processing widths and aspect ratios close to 10000 on fused silica,which has further enabled the realization of nanograting nanopillar and special-shaped through-holes with a sidewall roughness of only 7 nm.The SSD technology is universal and can be further applied to the nano-fabrication of a wide range of materials with different crytal symmetries,band gaps,hardness and optical properties,from laser crystals,piezoelectric materials to semiconductors.The SSD technology is expected to advance the laser cutting,grooving and drilling into the nanometer era.This thesis proposes a new approach for ultrafast laser nanomanufacturing based on the principle of optical far-field-induced near-field breakdown,with a rigorous theoretical framework as a systematic methodology.The study provides a new perspective to address the challenges of nanoscale high-precision,high-aspect-ratio structures in traditional laser processing.Thus,it holds significant importance for future exploration of ultrafast laser-based precision manufacturing technologies. |
