Magnetic field enhanced detection of coherent phonons in a GaMnAs/GaAs film

Using a two-color time-resolved pump-probe spectroscopy scheme, we studied the generation and detection of longitudinal coherent acoustic phonons, generated by ultrafast laser pulses in a semiconductor heterostructure. Our structure was a $p$-doped, 100-nm-thick ferromagnetic GaMnAs layer grown on a GaAs substrate. By probing the transient reflectivity in the time domain, we observed a strong dependence of the coherent phonon's amplitude on the external magnetic field. Our theoretical model relates this dependence to the increase in the detectability of coherent phonons in the presence of external magnetic fields. This enhancement comes from the formation of Landau levels in the absorption spectrum and leads to large changes in the real and imaginary parts of the dielectric function with strain. When the probe laser energy is close to an allowed Landau-level transition, the detectability of the coherent phonons can be significantly enhanced. Our results suggest that not only can one increase the detection of coherent phonons with magnetic fields, one can also enhance the generation by tuning the wavelength of the pump laser pulse to coincide with a Landau-level resonance.