  We study experimentally and by micromagnetic simulations the propagation of
spin waves in 100-nm thick YIG waveguides, where the width linearly decreases
from 2 to 0.5 micrometers over a transition region with varying length between
2.5 and 10 micrometers. We show that this geometry results in a down-conversion
of the wavelength, enabling efficient generation of waves with wavelengths down
to 350 nm. We also find that this geometry leads to a modification of the group
velocity, allowing for almost-dispersionless propagation of spin-wave pulses.
Moreover, we demonstrate that the influence of energy concentration outweighs
that of damping in these YIG waveguides, resulting in an overall increase of
the spin-wave intensity during propagation in the transition region. These
findings can be utilized to improve the efficiency and functionality of
magnonic devices which use spin waves as an information carrier.
