Measurement of transient strain induced by two-photon excitation

By ultrafast x-ray diffraction we quantify the strain from coherent and incoherent phonons generated by one- and two-photon absorption. We investigated the ferrimagnetic insulator bismuth-doped yttrium iron garnet, which is a workhorse for laser-induced spin dynamics that may be excited indirectly via phonons. We identify the two-photon absorption by the quadratic intensity dependence of the transient strain and confirm a short lifetime of the intermediate state via the inverse proportional dependence on the pump-pulse duration. We determine the two-photon absorption coefficient using the linear relation between strain and absorbed energy density. For large intensities of about $1\mathrm{TW}/{\mathrm{cm}}^2$ considerable strain amplitudes of 0.1% are driven exclusively by two-photon absorption.

Figure 1

(a) Sketched pump-probe geometry. (b) Sample structure and spatial intensity profiles in Bi:YIG for NUV (blue) and NIR illumination (red). For NIR light the linear absorption is negligible (dashed line) and it remains small (solid line) when including two-photon absorption, assuming an intensity of $1\mathrm{TW}/{\mathrm{cm}}^2$. (c) Scattered x-ray intensity is plotted as a function of $\vartheta$, showing the (800) Bragg reflection of ${\mathrm{Bi}}_2{\mathrm{Y}}_1{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ before $8\mathrm{mJ}/{\mathrm{cm}}^2$ NUV excitation and for a selection of pump-probe delays.

Figure 2

Transient lattice strain $\eta$ for different excitation parameters, measured with UXRD. Solid lines act as a guide to the eye. (a) Fluence series for ${\mathrm{Bi}}_2{\mathrm{Y}}_1{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ with femtosecond NUV excitation. For ${\mathrm{Bi}}_1{\mathrm{Y}}_2{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ we show (b) the fluence series with 120 fs NIR excitation and (c) the pulse duration series with $100\mathrm{mJ}/{\mathrm{cm}}^2$ NIR excitation. The gray data set has a similar pulse duration but opposite linear chirp.

Figure 3

Averaged lattice strain $\overline{\eta }$ in Bi:YIG between 75 ps and 1 ns evaluated from the data set in Fig. 2. (a) $\overline{\eta }$ as a function of the fluence at NUV excitation of ${\mathrm{Bi}}_2{\mathrm{Y}}_1{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (blue) and NIR excitation of ${\mathrm{Bi}}_1{\mathrm{Y}}_2{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (red). The NUV data are fitted with a linear function and a quadratic fluence dependence is observed for the NIR excitation as predicted by the presented modeling [see Eq. (7)]. (b) Pulse duration dependence of $\overline{\eta }$ upon NIR excitation of ${\mathrm{Bi}}_1{\mathrm{Y}}_2{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. The gray data point was obtained under different chirp conditions. The solid line results from Eq. (7) with the same parameters as in (a). The vertical error bars are the standard deviation of $\eta$ and the horizontal error bars are estimated from the pump spot size fluctuation and the GDD of the mirrors.