题目：Optical beam shaping and its application in laser nanofabrication

报告人：Dr.Han Lin（Postdoctoral Research Fellow, Swinburne University of Technology）

主持人：李向平教授

时间：2016年9月30日下午4:00

地点：曾宪梓科学馆405会议室

报告摘要：

The ever increasing demand for fabrication of structures with micro/nanometer scale and feature size compels the two-photon (2P) excitation based three dimensional(3D) scanning direct laser writing (DLW) system, in which the optical sensitivematerial is manufactured by introducing a localized physical or chemical changein the ultra-small focal volume with a tightly focused laser beam. Due to the highlyconfined property DLW based on the 2P process has higher spatial resolution comparedto that based on the single-photon process. In addition, 3D structures can befabricated because no linear absorption of the material allows manufacturing deepinside the volume of the material. However, several challenges have limited the developmentof DLW. First, the axial elongation of the focal spot resulting from thelimited convergence angle of the focusing lens is a hurdle to achieve isotropic 3Dproperties of the fabricated devices. Second, due to the focusing nature of the DLWtechnique, only a small portion of the output laser power is used to induce a high intensityfocal field, leading to low power efficiency. Third, the manufacturing speedis low as a result of the limitation of the light steering devices. Therefore, the fabricationof large-area high-quality 3D micro/nanostructures using the DLWtechniqueremains a great challenge.

In this presentation, I will show in order to enhance thefocusable energy efficiency and remove the asymmetry caused by the depolarizationeffect, an amplitude-modulated radially polarized beam is used togenerate an axial-super-resolved quasi-spherical focal spot for the first time. In addition, to speed up the fabrication process and enhance the power efficiencyof the DLW technique,a new method based on the vectorialdiffraction theory is developed for diffraction-limited multifocal array generationusing high NA objectives. Further, the vectorial Debye theory and implementing a modificationfactor in the phase generation process, the intensity of each focus in the multifocalarray can be individually controlled to realize 99% intensity uniformity of thediffraction-limited multifocal spot array focused by a high NA objective. Combinedwith a DLP nanofabrication system, we demonstrate the fabrication of high-quality2D micro-void arrays with or without embedded arbitrary defects. The high qualityallows us to fabricate functional 3D photonic crystals (PhCs) with scalable multiorderstop gaps which present more than 80% suppression in transmission.The intensity distribution of each focal spot in the multifocal array can be furtherengineered to realize non-Airy patterns through the modification of the amplitude,phase or polarization distribution at the back aperture plane of an objective. It is shown for the first time that a multifocal arrayof cylindrically polarized non-Airy patterns with identical intensity distributionsand polarization states at each focus can be generated. A multifocal array of non-Airy patterns in the shape of a split-ring (SR) pattern is generated through the useof a spatially-shifted circularly polarized vortex beam to enhance the processingspeed and power efficiency of the laser fabrication of complex micro-structures.The experiments show that an array of SR micro-structures can be fabricated in thepolymer by employing the designed spatially-shifted circular polarized vortex witha single exposure of femtosecond laser beam, which makes the fabrication of largearea photonic devices possible.