  Complex optical systems such as high-quality microcavities enabled by
advanced lithography and processing techniques paved the way to various
light-matter interactions (LMI) studies. Without lattice-matching constraints
in epitaxy, coating techniques or shaky open cavity constructions,
sub-micrometer-precise lithographic development of a polymer photoresist paves
the way to polymer microcavity structures for various spectral regions based on
the material's transparency and the geometrical sizes. We introduce a new
approach based on 3D nanowriting in photoresist, which can be employed to
achieve microscopic photonic Fabry-P\'erot cavity structures with
mechanically-tunable resonator modes and polymer/air Bragg mirrors, directly on
a chip or device substrate. We demonstrate by transfer-matrix calculations and
computer-assisted modelling that open microcavities with up to two "air-Bragg"
reflectors comprising alternating polymer/air mirror-pair layers enable
compression-induced mode tuning that can benefit many LMI experiments, such as
with 2D materials, nanoparticles and molecules.
