Origin of Periodic Modulations in the Transient Reflectivity Signal at   Cryogenic Temperatures
  Periodic modulations that appear in the low-temperature transient reflectivity signal of a GaAsP/AlGaAs single quantum well is studied. Similar anomalous oscillations are also observed in layered manganite [K. Kouyama et.al. J. Phys. Soc. Jpn. 76:123702(1-3), 2007]. We show that such periodic modulations are caused by changes in the linear reflectivity of the sample during transient reflectivity measurements. Studied carried out on reflectivity of different materials under identical conditions shows that these modulations on the true transient reflectivity signal are caused by condensation of residual gases on the surface of quantum well. Methods to obtain reliable transient reflectivity data are also described. 
Salahuddin Khan, Rama Chari, J. Jayabalan, Suparna Pal, T. K. Sharma,   A. K. Sagar, M. S. Ansari, P. K. Kush
physics.optics