  Direct-write multi-photon laser lithography (MPL) combines highest resolution
on the nanoscale with essentially unlimited 3D design freedom. Over the
previous years, the groundbreaking potential of this technique has been
demonstrated in various application fields, including micromechanics, material
sciences, microfluidics, life sciences as well as photonics, where in-situ
printed optical coupling elements offer new perspectives for package-level
system integration. However, millimeter-wave (mmW) and terahertz (THz) devices
could not yet leverage the unique strengths of MPL, even though the underlying
devices and structures could also greatly benefit from 3D freeform
microfabrication. One of the key challenges in this context is the fact that
functional mmW and THz structures require materials with high electrical
conductivity and low dielectric losses, which are not amenable to structuring
by multi-photon polymerization. In this work, we introduce and experimentally
demonstrate a novel approach that allows to leverage MPL for fabricating
high-performance mmW and THz structures with hitherto unachieved
functionalities. Our concept exploits in-situ printed polymer templates that
are selectively coated through highly directive metal deposition techniques in
combination with precisely aligned 3D-printed shadowing structures. The
resulting metal-coated freeform structures offer high surface quality in
combination with low dielectric losses and conductivities comparable to bulk
material values, while lending themselves to fabrication on planar mmW/THz
circuits. We experimentally show the viability of our concept by demonstrating
a series of functional THz structures such as THz interconnects, probe tips,
and suspended antennas. We believe that our approach offers disruptive
potential in the field of mmW and THz technology and may unlock an entirely new
realm of laser-based 3D manufacturing.
