  We introduce a generalized theoretical approach to study action signals
induced by the absorption of two-photons from two phase modulated laser beams
and subject it to experimental testing for two types of photoactive samples,
solution of rhodamine 6G and GaP photodiode. In our experiment, the phases of
the laser beams are modulated at the frequencies f1 and f2, respectively. The
action signals, such as photoluminescence and photocurrent, which result from
the absorption of two photons, are isolated at frequencies m f (f=|f1-f2|,
m=0,1,2...). We demonstrate that the ratio of the amplitudes of the secondary
(m=2) and the primary (m=1) signals is sensitive to the type of relaxation
process taken place in the system and thus can be used for its identification.
Such sensitivity originates from cumulative effects of non-equilibrated state
of the system between the light pulses. When the cumulative effects are small,
i.e. the relaxation time is much shorter then the laser repetition rate or the
laser intensity is high enough to dominate the system behavior, the ratio
achieves its reference value 1:4 (the signature of two-photon absorption). In
the intermediate regimes the ratio changes rapidly with the growth of intensity
from zero value in case of second order relaxation process, while it
demonstrates slow monotonic decrease for linear relaxation. In the article we
also determine the value of the recombination rate in a GaP photodiode by using
the above approach.
