Excitation of magnon accumulation by laser clocking as a source of long-range spin waves in transparent magnetic films

摘要

Optical tools are of great promise for generation of spin waves due to thepossibility to manipulate on ultrashort time scales and to provide localexcitation. However, a single laser pulse can inject spin waves only with abroad frequency spectrum, resulting in a short propagation distance and lowamplitude. Here we excite a magnetic garnet film by a train of fs-laser pulseswith 1 GHz repetition rate so that pulse separation is smaller than decay timeof the magnetic modes which allows to achieve collective photonic impact onmagnetization. It establishes a quasi-stationary source of SWs, namely acoherent magnon accumulation ("magnon cloud"). This approach has severalappealing features: (i) the source is tunable; (ii) the SW amplitude can besignificantly enhanced; (iii) the spectrum of the generated SWs is quite narrowthat provides longer propagation distance; (iv) the periodic pumping results inalmost 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 thecapabilities of ultrafast coherent optical control of magnetization and pave away for applications in data processing, including the quantum regime. Thequasi-stationary magnon accumulation might be also of interest for the problemof magnon Bose-Einstein condensate.