Femtosecond Laser-Excitation-Driven High Frequency Standing Spin Waves in Nanoscale Dielectric Thin Films of Iron Garnets

Abstract

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Measurement of transient strain induced by two-photon excitation

“…Recent publications on ferrimagnetic garnets reported magnetization dynamics via thermally induced crystalline anisotropy modification [23,32,33], similar to previous work in antiferromagnets [34][35][36]. More specifically, optically excited coherent and incoherent phonons are reported to generate spin waves (SWs) [27,37,38]. A quadratic fluence dependence of the spin-wave amplitude was discovered in several garnets already with moderate fluences below 15 mJ/cm 2 [25,38].…”

“…From this, we determine the two-photon absorption coefficient β ≈ 2 cm/GW for exciting Bi 1 Y 2 Fe 5 O 12 at 800 nm. We believe that our quantitative evaluation of the strain generated by two-photon absorption is particularly important for a full understanding of lightdriven spin manipulation in magnetic insulators [23,27,[32][33][34][35][36][37][38], but it has more general relevance for ultrafast science on supposedly nonabsorbing or transparent matter. The large peak intensities around 1 TW/cm 2 necessary to drive 0.1% strain can be achieved both by high pump fluences and short pulse durations.…”

“…More specifically, optically excited coherent and incoherent phonons are reported to generate spin waves (SWs) [27,37,38]. A quadratic fluence dependence of the spin-wave amplitude was discovered in several garnets already with moderate fluences below 15 mJ/cm 2 [25,38].…”

“…Here, we present nondestructive UXRD measurements on the very same Bi:YIG film [38], to quantify the lattice dynamics upon below-band-gap femtosecond laser excitation with fluences up to 100 mJ/cm 2 . This corresponds to intensities, which are typical of experiments that manipulate magnetization in insulators by femtosecond light pulses [19,33,37,38], and we find a pronounced quadratic fluence dependence of the strain amplitude that quantifies coherent and incoherent phonons as propagating strain pulses and localized thermal expansion, respectively. The strain amplitude is inversely proportional to the pump pulse duration, which indicates a short lifetime of the intermediate state during the two-photon excitation.…”

“…Following the experiments for the excitation of spin waves [24,25,38,50], we excited Bi:YIG with NIR light with a short pulse duration (150 fs) and incident fluences up to 100 mJ/cm 2 . We used the very same x = 1 sample as Deb et al [38]. At fluences comparable to the NUV excitation no significant strain was detected.…”