Polarization dependence of photoinduced birefringence in chalcogenide thinfilm

Polarization dependence of photoinduced birefringence has been investigated in an amorphous As-Ge-Se-S chalcogenide thin film using two 632.8 nm HeNe lasers (inducing and probing beams), which have a lower energy than the opt ical energy gap (E-OP) of the film, that is, an exposure to sub-band gap li ght (hv < E-OP). The polarization states of the inducing beam are controlle d using a pair of phase retarder and linear polarizer to obtain linearly, c ircularly and elliptically polarized light beams. In addition, the kinetics of photoinduced dichroism has also been studied. The amplitudes of dichroi sm (D) and birefringence (Deltan) increase with increasing induction time a nd eventually both are saturated even if their saturation times are not the same. The scalar absorption coefficient (alpha) of the film before photoin duction is about 6.02 x 10(3) cm(-1) at 632.8 nm, that is, it is independen t of polarization. but the alpha after photoinduction depends strongly on t he polarization of the probing beam. That is, for a 0.5-mum-thick-film indu ced for 25 s by a laser with a power of 5.48 mW/cm(2), the difference of tw o polarization absorption coefficients (Delta alpha = alpha (perpendicular to) - alpha (parallel to)) for the scalar alpha is estimated to be about 13 .8%, which corresponds to D similar to 4.2%. The photoinduced birefringence also depends strongly on the polarization states of the inducing beam. For example, for 0.9-mum-thick As-Ge-Se-S film the saturation values of Deltan are about 0.69 x 10(-2), 0.81 x 10(-2), and 1.33 x 10(-2) for the circular ly, elliptically, and linearly polarized beams, respectively. In particular , the saturation values of birefringence of the film are not constant for a n inducing history but vary significantly.