Fano interference with the alternating asymmetry parameter in time-domain experiments

It has been found that the time-domain reflection signal arising after the excitation of a bismuth single crystal at helium temperature by an ultrashort laser pulse exhibits Fano interference in the presence of the interaction between the lattice and charge degrees of freedom. This property is manifested in the characteristic spectral lineshape of the Fourier-transformed signal, whose asymmetry parameter is a periodic function of the delay time τ between the pumping and probing pulses. The most interesting property is the change of the sign of the asymmetry parameter when τ varies, which indicates that time reversal symmetry is broken. Various possible causes of the effect, observed for the first time, are discussed.