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 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 ∼400 ps period while the average low temperature phase fraction decreases monotonically for ∼2 ns reflecting MnAs heat diffusion. Equilibrium structures are restored in 100 ns-2 μs via substrate heat diffusion. Excitation of transient periodic domains from the homogeneous low temperature phase can occur for temperatures as low as 4 C but only after ∼20 ns during film cooling.