  Exposure of metal surfaces to multiple ultrashort laser pulses under certain
conditions leads to the formation of well-defined periodic surface structures.
We show how the period of such structures in steel can be tuned over a wide
range by controlling the complex interaction mechanisms triggered in the
material. Amongst the different irradiation parameters that influence the
properties of the induced structures, the angle of incidence of the laser beam
occupies a prominent role. We present an experimental and theoretical
investigation of this angle dependence in steel upon irradiation with laser
pulses of 120 fs duration and 800 nm wavelength, while moving the sample at
constant speed. Our findings can be grouped into two blocks. First, we observe
the spatial coexistence of two different ripple periods at the steel surface,
both featuring inverse scaling upon angle increase, which are related to
forward and backward propagation of surface plasmon polaritons. To understand
the underlying physical phenomena, we extend a recently developed model that
takes into account quantitative properties of the surface roughness to the case
of absorbing metals (large imaginary part of the dielectric function), and
obtain an excellent match with the experimentally observed angle dependence. As
second important finding, we observe a quill writing effect, also termed
non-reciprocal writing, in form of a significant change of the ripple period
upon reversing the sample movement direction. This remarkable phenomenon has
been observed so far only inside dielectric materials and our work underlines
its importance also in laser surface processing. We attribute the origin of
symmetry breaking to the non-symmetric micro- and nanoscale roughness induced
upon static multiple pulse irradiation, leading to non-symmetric modification
of the wavevector of the coupled surface plasmon polariton.
