LO-phonon-enhanced microcavity polariton emission S. Pau, G. Bjo ¨ rk, H. Cao, F. Tassone, R. Huang, and Y. Yamamoto Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305 R. P. Stanley IMO-DP, Ecole Polytechnique Fe ´de ´rale de Lausanne (EPFL), Lausanne CH1015, Switzerland Received 11 November 1996 We observe resonant enhancement of the microcavity exciton-polariton emission by more than three orders of magnitude when the excitation laser is tuned to a single longitudinal-optical-phonon energy above the polariton energies. Our finding provides a scheme to efficiently populate k  =0 polaritons which bypasses the slow-acoustic-phonon thermalization process. S0163-18299751104-0 Microcavity light emitting devices based on quantum well excitons as a gain medium have the potential of being very fast because of the fast ( 10 ps exciton radiative lifetime 1 and of having high quantum efficiency because of microcav- ity confinement. 2 Nevertheless, in practice the speed of such devices is slow because of slow carrier thermalization time of the order of 200 to 500 ps. Since only the k  0 in plane k vector exciton is optically active, hot excitons with large k  must first thermalize before emission of radiation. For any current injection device which initially creates hot excitons, the slow thermalization time severely limits the performance of the device. The problem is how to efficiently and rapidly populate k  0 optically active excitons. One possible solu- tion is to inject carriers at one longitudinal-optical LO- phonon energy above the exciton resonance since the electron-LO phonon scattering time is subpicosecond. 3 Recently enhanced Raman scattering in a planar semicon- ductor microcavity has been observed where the excitation laser is tuned below the lowest-order heavy-hole exciton transition virtual excitation Raman scattering. 4 The en- hancement of the light-matter interaction by the presence of the microcavity should lead not only to an enhanced Raman cross section for the virtual excitation but also should lead to enhanced Raman transition for real excitation at above the resonance. The real excitation with LO-phonon emission is particularly interesting for the applications to enhanced spon- taneous emission coupling efficiency 2  and exciton- polariton boser 5 because of its high-energy selectivity and large cross section. The normal modes of the microcavity system in a strong coupling regime are the microcavity ex- citon polaritons. 6,7 In this paper we report the first experi- mental result showing enhanced polariton emission where the excitation laser is tuned one LO-phonon energy above the k  =0 microcavity polariton energy. Aside from the in- crease in emission intensity caused by the LO-phonon reso- nance condition of the microcavity, we find that i selective excitation of upper- and lower-energy polariton branches can be achieved by tuning the laser wavelength, ii unlike the bulk, the LO-phonon line is strongly angular dependent due to the microcavity polariton dispersion effect, and iii the competition of acoustic and LO-phonon relaxation processes is evident in the polariton PL spectra at low temperature. The microcavity sample is grown by molecular-beam epitaxy and has 19 30 pairs of Bragg reflectors made of Al 0.15 Ga 0.85 As and AlAs on top bottom and a single 20 nm GaAs quantum well QW at the center of the  cavity. The Al 0.3 Ga 0.7 As cavity buffer layer is tapered in one direction so that the cavity resonance energy varies with sample posi- tion, while the QW exciton energy is constant. Reflectivity and photoluminescence PL measurements are made at 4.5 K with a tungsten lamp and a CW tunable single mode Ti:sapphire ring laser, respectively. Both the laser and the white light passed through a 10 m diameter pinhole that is imaged 1:2 onto the sample at an angle  from the normal direction. Reflectivity is measured at normal incidence (  =0). Emission is collected at an angle  from the cavity normal direction using a fiber bundle radius=100  m coupled to a spectrometer situated 2 cm away from the sample Fig. 1. All measurements are taken at an excitation density of 10 9 cm -2 well below the exciton saturation density of 10 11 cm -2 . 8 We have verified that the emission spectra is unchanged and the emission intensity is linear over two orders of magnitude of excitation power so that the en- hanced emission is neither a consequence of nonlinear satu- ration effect nor a final-state stimulation effect. Figure 2a shows the microcavity exciton polariton reso- FIG. 1. Schematic diagram showing the resonance enhanced polariton emission by emission of a single LO phonon. The polar- iton can also thermalize by multiple emissions of acoustic phonons. Also shown is the experimental configuration. PHYSICAL REVIEW B 15 JANUARY 1997-II VOLUME 55, NUMBER 4 55 0163-1829/97/554/19424/$10.00 R1942 © 1997 The American Physical Society