Toshiki Kawase

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

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.

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

We have investigated the temperature dependence of the cavity-polariton energies in ZnO and CuCl microcavities with HfO2/SiO2 distributed Bragg reflectors. From angle-resolved reflectance spectra at 10 K, we experimentally confirmed the cavity-polariton dispersions. The experimental dispersions were analyzed with a phenomenological Hamiltonian for the exciton-photon strong coupling. Furthermore, the temperature dependence of the angle-resolved reflectance spectra was measured. We analyzed the experiment results of the temperature dependence of the energies of the cavity polaritons on the basis of the phenomenological Hamiltonian, taking account of Varshnis law to treat the temperature dependence of the band-gap energy in a bulk crystal. Note that the temperature dependence of the exciton energies in a CuCl bulk crystal is extraordinary, which is in reverse to that in a ZnO bulk crystal. We have revealed that the temperature dependence of the energies of the cavity polaritons, which deviates from that of ...

Effects of Distributed Bragg Reflectors on Temporal Stability of CuCl Microcavities

We have investigated the characteristics of exciton polaritons in a CuCl microcavity with distributed Bragg reflectors (DBRs). Two sets of multilayers, PbBr2/PbF2 and HfO2/SiO2, were adopted as the DBRs in order to study the temporal stability of the CuCl microcavity. The thickness of the CuCl active layer was fixed to an effective 3λ/2 length. Angle-resolved reflectance spectra clearly demonstrate the formation of the cavity polaritons. From the phenomenological analysis with a 3×3 Hamiltonian for the cavity-polariton modes originating from the Z3 exciton, Z1,2 exciton, and cavity photon, the Rabi splitting energies are evaluated to be 97 and 162 meV for the Z3 and Z1,2 excitons, respectively, in the fresh CuCl microcavity with the PbBr2/PbF2 DBR. However, the Rabi splitting energies remarkably decrease within 6 days from the sample preparation, which is due to the degradation of the DBR resulting from alloying of PbBr2 and PbF2. On the other hand, in the CuCl microcavity with the HfO2/SiO2 DBR, the Rabi splitting energies of 105 and 168 meV for the Z3 and Z1,2 excitons, respectively, hardly change during 360 days from the sample preparation. This indicates that the stability of the oxide materials of HfO2 and SiO2 prevents the degradation of the DBR and CuCl active layer. Thus, a stable CuCl microcavity can be prepared by adopting the multilayer of HfO2/SiO2 as the DBR, which is a merit in applications.

High-sensitivity polarization modulation reflectance spectroscopy of cavity polaritons in a ZnO microcavity

We report that polarization modulation reflectance (PMR) spectroscopy is highly sensitive to the cavity polaritons in a ZnO microcavity with HfO2/SiO2 distributed Bragg reflectors. We demonstrate that the cavity-polariton dispersion, even in the energy region of strong absorption by exciton continuum states, is clearly observed by PMR spectroscopy. The PMR spectra were quantitatively analyzed by a transfer-matrix method taking into account three types of excitons labeled A, B, and C. Line-shape analysis of the PMR spectra indicates that the anisotropy of the excitonic transitions is considerable in treating the cavity polariton in the ZnO microcavity.