Optical path modulation in transient photoreflectance of CdMnTe layers W. Farah and D. Scalbert Groupe d’Etude des Semiconducteurs, UMR5650 CNRS, Universite ´ Montpellier 2, Place Euge `ne Bataillon, 34095 Montpellier Cedex, France M. Nawrocki and J. A. Gaj Institute of Experimental Physics, Warsaw University, Hoza 69, 00-681 Warszawa, Poland E. Janik, G. Karczewski, and T. Wojtowicz Institute of Physics, Polish Academy of Sciences, al. Lotnikow 32, 02-668 Warszawa, Poland Received 1 December 1997 Optical path modulation—an interference-based photoreflectance method—is used to study the influence of magnetic properties on the band structure of Cd 1-x Mn x Te. The results obtained due to the extreme sensitivity of this interferometric method indicate a renormalization of the effective mass under the influence of magnetic fluctuations, not observed so far. Time-dependent measurements supply additional arguments for the thermo- magnetic character of the modulation and allow one to study magnetic relaxation phenomena. S0163-18299801012-1 Photoreflectance PR is a powerful tool for the investiga- tion of optical properties of semiconductors. 1 Indeed the ap- plication of external perturbation to the sample provides of- ten a reliable identification of the observed optical transitions, and the derivative nature of the spectra obtained allows a precise determination of their energy. Among vari- ous possible modulations mechanisms in PR, surface electric field effects, 2 thermal effects, 3 band filling effects, 4 screening of excitons by free carriers, 5 and so on, surface electric field modulation by light is generally the most efficient one. In diluted magnetic semiconductors DMS the exchange interaction between free carriers and magnetic ions is known to influence strongly the optical properties of the host semiconductor. 6 As a result new magnetic modulation mechanisms of photoreflectance are possible. 7 In this paper we introduce optical path modulation OPM based on a variation of the refractive index of a DMS layer forming a Fabry-Pe ´ rot resonator. The refractive index is in- fluenced by magnetic fluctuations of the layer and its mag- netization induced by an external magnetic field. Interference phenomena are known to change the line shape of PR, however, OPM produces PR structures de- noted hereafter as OPM structures in the gap of the semi- conductor where no optical transitions exist. We shall dem- onstrate that both energy and oscillator strength modulations, attributable to magnetic effects, contribute to OPM. Energy modulation can be suppressed by tuning the magnetic field, revealing a contribution of oscillator strength modulation. Moreover, OPM structures turn out to be much more sensi- tive to oscillator strength modulation than the usual exciton line. We studied two 3-m-thick Cd 1 -x Mn x Te epilayers, with compositions x =0.02 and 0.032, grown on a GaAs substrate by molecular beam epitaxy MBE. The epilayers were glued on a glass plate and the substrates were removed by selective etching. This operation opens up the possibility to pump and probe both faces of the layer four configurations are al- lowed. X-ray diffraction and luminescence spectra revealed good structural and optical properties of the layers. A pump- probe setup was used to record transient photoreflectance TPR. The pump and probe beams were provided respec- tively by the visible lines of a cw-argon laser and an Al 2 O 3 :Ti tunable laser. Both beams were pulsed outside the laser cavity by synchronized acousto-optic modulators with a repetition rate of 20 kHz. The relative delay  t between pump and probe pulses was electronically controlled. The pumped beam had a maximum peak intensity of about 15 W/cm 2 and was unfocused. The probe beam was weakly focused at the center of the pump spot and had a mean power of about 25 W. The pump beam was modulated at a fre- quency in the range of 130–360 Hz allowing phase-sensitive detection of TPR. A positive TPR signal corresponds to an increase of reflectivity under illumination. The experiments were done at the temperature of superfluid helium, in Fara- day geometry. Figure 1 shows typical reflectivity and TPR FIG. 1. Reflectivity and transient photoreflectance TPR spectra of a 3-m-thick layer of Cd 0.97 Mn 0.03 Te glued on a glass plate and immersed in superfluid helium ( T =2 K), at zero magnetic field. The TPR spectrum is measured for probe pulses set at the end of the pump pulses inset shows the pump-probe configuration. PHYSICAL REVIEW B 15 APRIL 1998-I VOLUME 57, NUMBER 15 57 0163-1829/98/5715/87704/$15.00 8770 © 1998 The American Physical Society