  We report non-invasive measurements of the complex field of elastic
quasimodes of a silicon wafer with chaotic shape. The amplitude and phase
spatial distribution of the flexural modes are directly obtained by Fourier
transform of time measurements. We investigate the crossover from real mode to
complex-valued quasimode, when absorption is progressively increased on one
edge of the wafer. The complexness parameter, which characterizes the degree to
which a resonance state is complex-valued, is measured for non-overlapping
resonances and is found to be proportional to the non-homogeneous contribution
to the line broadening of the resonance. A simple two-level model based on the
effective Hamiltonian formalism supports our experimental results.
