Oscillator strengths of dark charged excitons at low electron filling factors

By direct absorption spectroscopy and comparison to photoluminescence (PL), we investigate negatively charged excitons in dilute two-dimensional electron systems at temperatures down to $T=40\mathrm{}\hspace{0.5em}\mathrm{mK}$ in the regime of the fractional quantum Hall effect. At very low temperatures, for filling factor $\nu <1/3,$ an additional excitation appears in the PL spectrum, between the well-known singlet and triplet excitons. The observation of a similar excitation by PL was reported very recently [G. Yusa et al., Phys. Rev. Lett. 87, 216402 (2001)], and the excitation was assigned, in spite of the PL intensity similar to that of the neutral exciton, to be due to a dark triplet exciton. By comparing PL and direct absorption spectra in optically thin samples at $T<\mathrm{}100\mathrm{}\hspace{0.5em}\mathrm{mK},$ we can identify the new excitation indeed as a “dark” mode, since we find that the oscillator strength is much smaller than those of the “bright” modes.