  Optical tools are of great promise for generation of spin waves due to the
possibility to manipulate on ultrashort time scales and to provide local
excitation. However, a single laser pulse can inject spin waves only with a
broad frequency spectrum, resulting in a short propagation distance and low
amplitude. Here we excite a magnetic garnet film by a train of fs-laser pulses
with 1 GHz repetition rate so that pulse separation is smaller than decay time
of the magnetic modes which allows to achieve collective photonic impact on
magnetization. It establishes a quasi-stationary source of SWs, namely a
coherent magnon accumulation ("magnon cloud"). This approach has several
appealing features: (i) the source is tunable; (ii) the SW amplitude can be
significantly enhanced; (iii) the spectrum of the generated SWs is quite narrow
that provides longer propagation distance; (iv) the periodic pumping results in
almost constant in time SW amplitude up to 100 um away from the source; and (v)
the SW emission shows a pronounced directionality. These results expand the
capabilities of ultrafast coherent optical control of magnetization and pave a
way for applications in data processing, including the quantum regime. The
quasi-stationary magnon accumulation might be also of interest for the problem
of magnon Bose-Einstein condensate.
