We have studied the effect of valence band mixing on the optical properties of semiconductor quantum wires by analyzing the luminescence polarization. Large polarization anisotropy is observed and directly compared to the effects predicted by a k ? p model calculation of the valence band structure. [S0031-9007(97) PACS numbers: 78.55. Cr, 71.35.Cc, 73.20.Dx, 78.66.Fd The electronic structure of spatially confined electrons in low-dimensional systems has been attracting considerable interest. In particular, the electronic and optical properties of semiconductors can be tailored in artificial nanostructures due to quantization of the electronic energy and emergence of strong excitonic features [1]. Enhancement of excitonic effects may be achieved in semiconductor quantum wires (QWRs) due to modifications of electron-hole Coulomb correlations and divergence of the one-dimensional (1D) joint density of states [2,3]. The band structure of 1D semiconductor QWRs has been studied theoretically and the prominent role of valence band mixing was described in model systems [4,5]. Unlike the case of quantum wells, the admixture of heavy hole (hh) and light hole (lh) states leads to modified energies of the optical interband transitions, which are tunable by the lateral confinement potential, and a redistribution of the oscillator strength. Moreover, it gives rise to intrinsic polarization anisotropy of the optical absorption spectra providing a unique way to gain insight into the nature of 1D valence band structures.Despite the interest spurred on by these theoretical expectations the observation of the predicted features, i.e., optical anisotropy and sharp optical resonances, have been hampered by the technological challenge in producing QWR structures with two-dimensional (2D) confinement for both the ground state and excited states and with sufficiently small level broadening. The effect of 2D quantum confinement has been evidenced in the luminescence of QWR structures prepared by different approaches [6][7][8][9]. Study of the valence band mixing requires, however, the resolution and identification of several 1D valence subbands, imposing stronger constraints on the optical quality. Furthermore, anisotropy of optical interband transitions can also occur in the presence of strain, in samples with a strong surface corrugation, and in (110)-oriented quantum wells [10]. Initial observation of optical anisotropy in the absorption or emission of most types of QWR structures have been in fact affected by some of these invasive effects and, thus, cannot be directly compared to the predicted effects of 2D confinement on valence-band mixing.In this Letter we report the observation of valence band mixing effects in the optical spectra of high quality 1D semiconductor QWRs. The origin of the optical transitions and their hh and lh character have been identified by performing photoluminescence experiments with circularly polarized light. The observed anisotropy in the linearly polarized excitation spectra of these wires is shown t...

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