We present an optical method for the study of ultrafast oscillating systems, by using a train of femtosecond laser pulses of continuously tunable repetition rate. We also use this technique to measure an upper limit for the vibration frequency of artificial cell membranes (liposomes). We insert a potential sensitive dye molecule, with strong absorption at the laser wavelength, in the liposomes bilayers and illuminate it with the femtosecond pulses. The reaction of the membrane to this excitation is then analyzed by measuring the changes in absorption of the suspension.