Towards Bright Single-Photon Emission in Elliptical Micropillars

Abstract

In recent years, single-photon sources (SPSs) based on the emission of a single semiconductor quantum dot (QD) have been actively developed. While the purity and indistinguishability of single photons are already close to ideal values, the high brightness of SPSs remains a challenge. The widely used resonant excitation with cross-polarization filtering usually leads to at least a two-fold reduction in the single-photon counts rate, since single-photon emission is usually unpolarized, or its polarization state is close to that of the exciting laser. One of the solutions is the use of polarization-selective microcavities, which allows one to redirect most of the QD emission to a specific polarization determined by the optical mode of the microcavity. In the present work, elliptical micropillars with distributed Bragg reflectors are investigated theoretically and experimentally as a promising design of such polarization-selective microcavities. The impact of ellipticity, ellipse area and verticality of the side walls on the splitting of the optical fundamental mode is investigated. The study of the near-field pattern allows us to detect the presence of higher-order optical modes, which are classified theoretically. The possibility of obtaining strongly polarized single-photon QD radiation associated with the short-wavelength fundamental cavity mode is shown.

Keywords: elliptical micropillars; optical modes; quantum dots; single-photon source.

Figure 1

(a) MBE-grown semiconductor heterostructure consisting of 30(18) DBR layers and a GaAs $\lambda$-cavity with embedded self-assembled InAs QDs. (b) SEM image of a single elliptical micropillar and (c) a regular array of such micropillars, taken with a scanning electron microscope at a 45 degree angle. The height of the micropillars is 7.1 $\mathsf{\mu }$m.

Figure 2

(a) Simulated polarized light emission spectrum for several high-order modes of an elliptical micropillar with a major (minor) axis of 2.6 $\mathsf{\mu }$m (1.3 $\mathsf{\mu }$m). (b) Calculated and measured near-field intensity distribution profiles of the electromagnetic modes of the elliptical microresonator. The arrows show the mode polarization. (c) The fundamental mode splitting versus the minor to major axis ratio for elliptical micropillars with a cavity area from 2.6 to 16 $\mathsf{\mu }$m${}^2$ according to experimental data (black symbols) obtained from the measured reflectance spectra and simulation of elliptical micropillars (red symbols) with a cavity anisotropic refractive index.

Figure 3

(a) Reflectance spectra of elliptical micropillars with a side-wall inclination angle of 3.5 degrees (left) and no side-wall angle (right). The insets show SEM images of microcavities. (b) Simulation of validated fundamental mode splitting curves as a function of minor to major axis ratio for elliptical micropillars with vertical side walls for cavity area from 1.2 to 16 $\mathsf{\mu }$m${}^2$, including anisotropic refractive index.

Figure 4

(a) Reflectance spectrum of an elliptical microcavity, (b) PL spectrum of a QD under resonant excitation and (c) the corresponding second-order autocorrelation histogram.

Figure 5

(a) Time resolved PL measurement of a charged QD under resonant excitation. (b) Polarization-resolved PL measurement of the QD under non-resonant excitation conditions.