Domain Dynamics in Thin Solid Films Following Ultrashort Pulse Excitation

MnAs epilayers grown on GaAs are used as a model system to study the effects of strain and epitaxial constraints on the dynamics of structural domains following 150 fs pulse pumping. Optical diffraction over 7 orders of magnitude of time is used to probe the evolution of the domains that are spatially periodic between 10 and $42°\mathrm{C}$ because of misfit strain and substrate mediated periodic elastic strain. Following excitation of 150 and 190 nm thick films, the domain fractions and the elastic strain oscillate with an $\sim 400\mathrm{ps}$ period while the average low temperature phase fraction decreases monotonically for $\sim 2\mathrm{ns}$ reflecting MnAs heat diffusion. Equilibrium structures are restored in $100\mathrm{ns}–2\mu \mathrm{s}$ via substrate heat diffusion. Excitation of transient periodic domains from the homogeneous low temperature phase can occur for temperatures as low as $4°\mathrm{C}$ but only after $\sim 20\mathrm{ns}$ during film cooling.

Figure 1

(a) AFM image of 190 nm thick MnAs on (001) GaAs at $T_0=22°\mathrm{C}$; light regions indicate $\alpha$-MnAs where the surface is higher. (b) Temperature dependent diffraction ($\delta$) at 387 nm; the solid curve represents a fitted ${sin}^2(\pi \xi )$.

Figure 2

(a) Time dependent $\delta$ of a 190 nm thick film at different temperatures for a $2\Phi$ pump pulse. (b) Time dependent $\delta$ for 150 and 190 nm films at $20°\mathrm{C}$ for a $2\Phi$ pump pulse. (c) Time-dependent $\delta$ for the 150 nm film at $T_0=28°\mathrm{C}$ for two different pump fluences; (d) same with $T_0=36°\mathrm{C}$. Note that $\delta$ may fall slightly below zero at certain delays due to changes in the background scattered light level under film excitation.

Figure 3

(a) Time-dependent $\delta$ of a 409 nm probe for 150 nm thick MnAs at different $T_0$; a $2\Phi$ pump pulse is used. (b) Same for $T_0=16$ and $36°\mathrm{C}$ but for longer times.