Presentation on theme: "A.V. Koudinov, Yu. G. Kusrayev A.F. Ioffe Physico-Technical Institute 194021 St.-Petersburg, Russia L. C. Smith, J. J. Davies, D. Wolverson Department."— Presentation transcript:
A.V. Koudinov, Yu. G. Kusrayev A.F. Ioffe Physico-Technical Institute 194021 St.-Petersburg, Russia L. C. Smith, J. J. Davies, D. Wolverson Department of Physics, University of Bath, Bath, UK G. Karczewski, T. Wojtowicz Institute of Physics, Polish Academy of Sciences, Warsaw, Poland Modulation of quantum well optical properties by illumination above the barrier bandgap 2. Experimental system Red illumination from Ti- sapphire laser, resonant with QW excitons, can be CW or chopped for lock-in detection. Additional blue (above-barrier gap) illumination (Ar + ion; blue arrow, left) coincident with Ti- sapphire laser spot; can also be chopped or CW. Magnetic field 0-6T Continuous rotation of sample about axis normal to diagram; Resolution limited either by spectrometer or laser; gives ±0.01 in the g-factor. Changes in the relative populations of X and T might be sufficient to explain the changes in the PL; However, we observe changes in the resonant Raman scattering and in the PLE spectra (see panel 4); These point to modifications of the exciton parameters (energy, radiative strength or linewidth) by the blue light; Such modifications are known to be the origin of photomodulated reflectivity effects (PR, panel 1); However, the changes we observe in the spin flip Raman spectra are very large; Typical PR only gives a modulation amplitude of R/R~10 -3 to 10 -4 (some of the largest PR signals we have ever seen are of size R/R~0.02 and are shown in panel 1); Spin-flip Raman scattering of Mn 2+ 3d 5 electrons (Raman paramagnetic resonance, PMR) can be enhanced by S/S ~ 5 (panel 4, #1) even for weak blue light; but can also be quenched fully (panel 4, #3) – the sign of the modulation is sample-dependent. Dramatic enhancement of PMR signal when this sample is exposed to blue light (whilst the red beam is tuned to PMR resonance near the X energy). However, the T PL is strongly quenched by the blue light. See enhancement of acoustic phonon Raman (or X PL) when red is near X Another sample shows opposite effects: a hole spin flip Raman signal (H) resonant with the T exciton is enhanced by blue light whilst the X PL is now quenched. 1. What happens if CdTe-based QWs are exposed to blue light? Most obvious change; blue light leads to strong increase in PL associated with charged excitons (trions, T) relative to exciton band (X) in this sample (non-magnetic). Often seen and attributed to a change in carrier concentration in QW; potentially useful effect, e.g, for tuning carrier concentration in CdTe-based 2DEG structures. Another well-known effect: Photomodulation of the QW Reflectivity (PR; illustrated here by PR from below-barrier- gap excitation in ZnTe QWs) PJ Klar, D Wolverson et al., Semicond. Sci. Technol. 11 1863 (1996) 4. Results on dilute magnetic quantum well structures 3. What optical phenomena are affected apart from PL? 5. Can we probe directly the resonant Raman intermediate state? 6. Spectral dependence of the Raman modulation? Perform conventional PLE (top) by scanning energy of chopped red beam and detecting PL (in this example, using a non-magnetic QW sample) New PLE variant: scan red beam but chop blue beam; detect changes in PLE and thus in the resonant intermediate state for the Raman process. Here, HH PL enhanced by blue light. Measure spin flip Raman intensity for excitation over the HH energy range; In this sample, see quenching in X range and enhancement in T range; Opposite effects to first two samples of panel 4; see X, T are again complementary T=trion X=exciton Th-P-151 detection window Summary: Large changes seen in PL, resonant Raman and PLE; QW exciton very sensitive to above-barrier excitation.