Observation of Exciton Polaritons in a ZnO Microcavity with HfO₂/SiO₂ Distributed Bragg Reflectors

Abstract

We have investigated the characteristics of the exciton polaritons in a ZnO microcavity. The microcavity consists of an effective one-wavelength thick ZnO active layer and HfO 2 /SiO 2 distributed Bragg reflectors (DBRs) at the bottom and top. We adopted rf magnetron sputtering and pulsed laser deposition for the preparation of the DBR and ZnO layer, respectively. Angle-resolved reflectance and photoluminescence spectra demonstrate the formation of the cavity polaritons. The cavity polariton dispersions are analyzed using a phenomenological Hamiltonian for the coupling between the cavity photon and three kinds of fundamental excitons labeled A, B, and C. The vacuum Rabi splitting energy is estimated to be $80 meV. The giant Rabi splitting energy reflects the large exciton oscillator strength of ZnO.Semiconductor microcavities have attracted much attention from the aspect of controlling the optical responses of exciton polaritons in a strong exciton-photon coupling regime resulting in the formation of cavity polaritons. 1) The prominent applications of cavity polaritons are polariton lasing, 2,3) parametric polariton amplifiers, 4,5) Bose-Einstein condensation of polaritons, 6,7) and enhancement of entangled-photon generation, 8) which are attractive in semiconductor physics. Until now, GaAs-based microcavities have been major samples owing to the well-established method for the sample preparation using molecular beam epitaxy or metal-organic chemical vapor deposition (MOCVD). Recently, microcavities consisting of wide-gap semiconductors such as GaN 9-11) and ZnO 12,13) have been developed from the viewpoint of large exciton binding energies, 28 meV for GaN 14) and 61 meV for ZnO, 15) leading to the high stability of the excitonic system.In this letter, we report on the fabrication of a ZnO-based microcavity with distributed Bragg reflectors (DBRs) and the observation of cavity polaritons. Note that the preparation method for the ZnO-based microcavity has not yet been established, which is mainly due to the problem of the fabrication of DBRs suitable for ZnO. In ref. 12, ZrO 2 /MgO DBRs were prepared by pulsed laser deposition (PLD). On the other hand, in ref. 13, bottom and top DBRs were AlGaN/GaN and SiO 2 /Si 3 N 4 systems grown by MOCVD and remote plasma enhanced CVD, respectively. In the present work, we adopted rf magnetron sputtering for the preparation of HfO 2 /SiO 2 DBRs at the bottom and top of the microcavity because the sputtering method is convenient and powerful for the deposition of oxide materials. For the ZnO active layer, we used PLD to grow a high quality film. The cavity polaritons were observed with angleresolved reflectance spectroscopy and photoluminescence (PL) spectroscopy. The experimental results were analyzed by calculating the cavity polariton dispersions with a phenomenological Hamiltonian for the coupling between the cavity photon and excitons, where we consider the A, B, and C excitons peculiar to ZnO. In the earlier works on ZnO-based microcavities, 12,13) such a precise analysis ...

Active‐layer‐thickness dependence of Rabi splitting energies in ZnO microcavities

“…Thus, the cavity polariton consists of four branches: the lower polariton branch (LPB), first middle polariton branch (MPB1), second middle polariton branch (MPB2), and upper polariton branch (UPB). In our previous works, we re-ported the observation of the cavity polariton branches [11][12][13] and confirmed the thermal stability of cavity polations in the ZnO microcavity [12]. However, little has been known about the active-layer-thickness dependence of the exciton-photon interaction energy, the so-called Rabi splitting energy.…”

Exciton polaritons in ZnO microcavities with different active layer thicknesses

“…The best‐fitted values of E 0 , Ω A , Ω B , and Ω C are 3.275 eV, 30 meV, 71 meV, and 84 meV, respectively. In our previous work 10, we could not observe the UPB mode in a ZnO λ ‐cavity. The disappearance of the UPB mode is discussed below.…”

Temperature dependence of cavity-polariton energies in ZnO and CuCl microcavities

“…In order to analyze the incidence-angle dependence of the energies of the four reflectance dips, we calculated the cavity-polariton dispersions using a phenomenological Hamiltonian for the strong coupling between the cavity photon and the A, B, and C excitons peculiar to ZnO. The phenomenological Hamiltonian is given by the following 4 Â 4 matrix: 10,12 …”

“…1 Strong coupling between the exciton and the cavity photon results in the formation of cavity polaritons. Recently, wide-gap semiconductors such as GaN, 2-7 ZnO, [8][9][10][11][12][13] and copper halides [14][15][16][17] have been adopted as active layers of microcavities from the viewpoint of the stability of excitonic systems, which is advantageous to realizing Bose-Einstein condensation of cavity polaritons 7 and polariton lasing. 5,6,13 In our previous works, we precisely investigated the characteristics of the cavity polaritons in the ZnO (Refs.…”