Exciton polaritons in a CuBr microcavity with HfO${}_2$/SiO${}_2$ distributed Bragg reflectors

We have investigated the characteristics of exciton-photon strong coupling in a CuBr bulk microcavity that consists of a CuBr active layer with an effective thickness of λ/2 and HfO${}_2$/SiO${}_2$ distributed Bragg reflectors: λ corresponds to an effective resonant wavelength of the lowest-lying exciton. The CuBr crystal has three excitons labeled Z${}_{\mathrm{f}}$, Z${}_{1,2}$, and Z${}_3$ at the Γ point, where the Z${}_{\mathrm{f}}$ exciton originates from a triplet state, which is peculiar to CuBr. Angle-resolved reflectance spectra measured at 10 K demonstrate the strong coupling behavior of the Z${}_{\mathrm{f}}$, Z${}_{1,2}$, and Z${}_3$ excitons and cavity photon, resulting in the formation of four cavity-polariton branches. Analyzing the cavity-polariton dispersion relations based on a phenomenological Hamiltonian for the strong coupling, we evaluated the vacuum Rabi-splitting energies of the Z${}_{\mathrm{f}}$, Z${}_{1,2}$, and Z${}_3$ excitons to be 31, 108, and 84 meV, respectively. These Rabi-splitting energies reflect the magnitudes of the oscillator strengths of the relevant excitons. Furthermore, we precisely measured angle-resolved photoluminescence (PL) spectra of the lower polariton branch under a weak excitation condition. In the bottleneck region, the population of the cavity polaritons is negligible, and the PL intensity at $k$ = 0 is the highest. These facts suggest that the relaxation process of the cavity polaritons is not affected by a bottleneck effect.

Figure 1

Angle-resolved reflectance spectra of the λ/2-thick CuBr microcavity. The solid circles, open circles, solid triangles, and open triangles indicate the cavity-polariton modes.

Figure 2

Absorption spectrum of the CuBr thin film, where thin solid curve indicates the experimental result. For the line-shape analysis, Gaussian functions are used for fitting the exciton absorption bands (dashed curves). For the continuum transitions, Eq.(3) is used. The thick solid curve indicates the total fitted results.

Figure 3

Incidence-angle dependence of the energies of the four cavity-polariton modes in the λ/2-thick CuBr microcavity, where the solid circles, open circles, solid triangles, and open triangles, which are the same symbols in Fig. 1, indicate the experimental results. The solid curves depict the fitted results with Eq. (1). The dashed horizontal lines indicate the exciton energies, and the dashed curve shows the cavity-photon dispersion.

Figure 4

Relative fractions of the Z${}_{\mathrm{f}}$, Z${}_{1,2}$, and Z${}_3$ excitons and cavity photon in the LPB, MPB1, MPB2, and UPB modes as a function of incidence angle. The relative fractions correspond to the eigenvectors of Eq. (1).

Figure 5

Angle-resolved PL spectra summarized as a color image map in the λ/2-thick CuBr microcavity, where the color scale intensity is depicted on the top. The dashed curve indicates the dispersion of the LPB.