Two-photon 3D lithography: A Versatile Fabrication Method for Complex 3D Shapes and Optical Interconnects within the Scope of Innovative Industrial Applications

Abstract

Laser material processing using intense laser pulses, which are provided from ultrafast laser systems, enable sophisticated structuring methods such as the two-photon 3D lithography. This method is based on the simultaneous absorption of two photons in a photosensitive material that is transparent for the laser wavelength. The non linear laser-matter interaction induces material changes that are tightly confined around the laser focus and build up a structure in the volume of the material. These intrinsic 3D capabilities allow easy fabrication of a physical structure from a CAD model. In combination with suitable materials, it is possible to realize complex 3D structures and photonic microand nano-systems. We introduce our experimental setup comprising laser source, three axes sample stage and 3D registration of the sample. We present two distinct applications of the versatile two-photon 3D lithography on organic photosensitive materials: first, the fabrication of complex arbitrarily shaped 3D micro-structures and second, the fabrication of direct laser-written, embedded multimode waveguides that are aligned relative to preconfigured printed wiring boards (PWBs). In the first case, the high resolution aspect of the method is addressed. We could fabricate small structures with feature sizes less than 1μm in a material with a very efficient photo initiator at a low concentration of only 0.025 wt%. In the case of waveguide fabrication, the lithographic method is combined with astigmatic beam focusing for a larger interaction volume as required for writing multimode waveguides. Due to the sample registration prior to the waveguide fabrication, the alignment of the waveguides becomes an intrinsic part of the fabrication process itself. A single organic-inorganic hybrid material is used for both, the waveguide core and its cladding, because the material exhibits a sufficiently large increase of the refractive index upon laser irradiation. The function of such waveguides is demonstrated by monitoring transmitted light at the waveguide exit and the optical loss of the fabricated waveguides is found to be approx. 0.64 dB/cm from cut back measurements. The total optical loss of a 12 cm optical interconnect on the PWB (including coupling losses at both waveguide caps and propagation loss) is in the range 11-20 dB. The ultimate goal of this approach is the large scale fabrication of leading-edge PWBs with an integrated optical communication layer.