Parallel direct laser writing in three dimensions with spatially dependent aberration correction

Alexander Jesacher,Martin J Booth

doi:10.1364/oe.18.021090

Abstract

We propose a hologram design process which aims at reducing aberrations in parallel three-dimensional direct laser writing applications. One principle of the approach is to minimise the diffractive power of holograms while retaining the degree of parallelisation. This reduces focal distortion caused by chromatic aberration. We address associated problems such as the zero diffraction order and aberrations induced by a potential refractive index mismatch between the immersion medium of the microscope objective and the fabrication substrate. Results from fabrication in diamond, fused silica and lithium niobate are presented.

Highlights

Direct laser writing (DLW) is a promising technique for the creation of photonic devices at the micro scale
We propose a hologram design process which aims at reducing aberrations in parallel three-dimensional direct laser writing applications
We propose a practical approach for parallel direct laser writing which aims at reducing chromatic aberration by choosing a specific hologram design

Summary

Introduction

Direct laser writing (DLW) is a promising technique for the creation of photonic devices at the micro scale. We propose a practical approach for parallel direct laser writing which aims at reducing chromatic aberration by choosing a specific hologram design. A three-dimensional arrangement of spots is encoded in a single hologram and the whole crystal is built up by multiples of these elementary building blocks Both approaches require the same number of laser pulses for completing the entire crystal structure, the second one suffers far less from chromatic aberration. In almost all practical cases a refractive index mismatch is present between the immersion medium of the objective lens and the fabrication substrate This introduces a depth-dependent spherical aberration (SA), because axial rays of the focussing cone converge at a different axial position than marginal rays. We present experimental results confirming the effectiveness of our method

Hologram design

Experimental set-up and results

Findings

Discussion and summary