  Energy harvesting is a modern concept which makes dissipated heat useful by
transferring thermal energy to other excitations. Most of the existing
principles for energy harvesting are realized in systems which are heated
continuously, for example generating DC voltage in thermoelectric devices. Here
we present the concept of high-frequency energy harvesting where the dissipated
heat in a sample excites resonant magnons in a 5-nm thick ferromagnetic metal
layer. The sample is excited by femtosecond laser pulses with a repetition rate
of 10 GHz which results in temperature modulation at the same frequency with
amplitude ~0.1 K. The alternating temperature excites magnons in the
ferromagnetic nanolayer which are detected by measuring the net magnetization
precession. When the magnon frequency is brought onto resonance with the
optical excitation, a 12-fold increase of the amplitude of precession indicates
efficient resonant heat transfer from the lattice to coherent magnons. The
demonstrated principle may be used for energy harvesting in various nanodevices
operating at GHz and sub-THz frequency ranges.
