Fig. 4: Directionality control of dark excitons.
a PL emission momentum distribution mapping with \({{{{{{\bf{k}}}}}}}_{{{{{{\bf{x}}}}}}}\) and \({{{{{{\bf{k}}}}}}}_{{{{{{\bf{y}}}}}}}\) axis. Four shining emission spots, located at the Γ-X line and \({{{{{{\bf{k}}}}}}}_{{{{{{\bf{x}}}}}}}\) or \({{{{{{\bf{k}}}}}}}_{{{{{{\bf{y}}}}}}}\)= 0.74 with C4 symmetry, are clearly visible. b Polarization analysis of the PL emission momentum distribution in a. The four shining emission spots show radial polarization indicating they are from dark excitons. c Angle-resolved PL emission spectra mapping extracted from y-polarized PL emission momentum distribution mapping in b. It is clear to see the dark exciton directional emissions have small divergence angles at wavelengths of around 772 nm and towards oblique emission angles of around 48°. d The measured (red solid line) and FDTD simulated (dark green dashed line) PL intensity as a function of the in-plane momentum (\({{{{{{\bf{k}}}}}}}_{{{{{{\bf{y}}}}}}}/{{{{{\bf{k}}}}}}\)) along the y-direction. The full-width-half-maximum (FWHM) of the measured XD emission lobes is 7° indicating the ultra-low divergence angle of the directional emission. e Spectra extracted from oblique angles of 48° and 23° for the dark exciton emission and the bright exciton emission, respectively. f The simulated PL emission momentum distribution by the Lumerical FDTD. Four shining emission spots show high correspondence to the measured PL emission pattern in a.
