Quantum dots ͑QDs͒ of high symmetry ͑e.g., C 3v ͒ have degenerate bright exciton states, unlike QDs of C 2v symmetry, making them intrinsically suitable for the generation of entangled photon pairs. Deviations from C 3v symmetry are detected in real QDs by polarization-resolved photoluminescence spectroscopy in side-view geometry of InGaAs/AlGaAs dots formed in tetrahedral pyramids. The theoretical analysis reveals both an additional symmetry plane and weak symmetry breaking, as well as the interplay with electron-hole and hole-hole exchange interactions manifested by the excitonic fine structure. DOI: 10.1103/PhysRevB.81.161307 PACS number͑s͒: 78.67.Hc, 71.70.Gm, 73.21.La, 78.55.Ϫm Semiconductor quantum dots ͑QDs͒ exhibit atomiclike energy spectra potentially useful in the area of quantuminformation processing. The indistinguishable radiation paths of the biexciton cascade decay have been proposed as the source of polarization-entangled photons.1 In the conventional QD fabrication process the nucleation of strained InAs QDs occurs spontaneously on the ͑001͒ plane of Zincblende crystals. The symmetry of these QDs is thus limited by the crystal to C 2v .2 The resulting anisotropy of the confined exciton breaks the degeneracy of its bright states, which prohibits entanglement and produces a fine structure splitting ͑FSS͒ characterized by the emission of two linearly polarized photons of unequal energies. Nevertheless, entangled photon pairs from such QDs have been detected by means of careful preselection of particular QDs, 3,4 by spectral postselection, 5 at the price of losing photons, or by the heavy use of external magnetic fields to restore the intermediate level degeneracy. 6 In the quest of more efficient QD sources of entangled photons, it was recently predicted that replacing the conventional GaAs barriers by InP significantly reduces the exciton FSS in such InAs self-assembled QDs. 7 Until now, however, studies of the FSS of neutral and charged exciton complexes have been limited to QDs of C 2v or lower symmetry. [2][3][4][5][6][7][8][9][10][11] In this Rapid Communication, we experimentally and theoretically investigate the FSS in QDs with high symmetry. Zincblende QDs of C 3v symmetry can ideally be achieved by choosing ͓111͔ as the crystallographic direction of crystal growth instead of the conventional ͓001͔ direction. For this growth geometry, including the lack of inversion symmetry in the crystal and the effects of strain and piezoelectric fields, the minimal symmetry is C 3v as long as the QD heterostructure has symmetrical shape. Here we utilize InGaAs/AlGaAs QDs that allow the simultaneous study of the FSS of dominating heavy-hole ͑hh͒ and light-hole ͑lh͒ excitons as well as a hybrid hh-lh trion by side-view polarization-resolved photoluminescence ͑PL͒ spectroscopy. We show how these trion states can probe a small symmetry breaking in otherwise ideal C 3v QDs due to exchange interactions.The polarization properties of the exciton fine structure depend on the symmetries of the initial and fi...

A systematic and simple theoretical approach is proposed to analyze true degeneracies and polarized decay patterns of exciton complexes in semiconductor quantum dots. The results provide reliable spectral signatures for efficient symmetry characterization, and predict original features for low C(2 nu) and high C(3 nu) symmetries. Excellent agreement with single quantum dot spectroscopy of real pyramidal InGaAs/AlGaAs quantum dots grown along [111] is demonstrated. The high sensitivity of biexciton quantum states to exact high symmetry can be turned into an efficient uninvasive postgrowth selection procedure for quantum entanglement applications

Coupling between photonic-crystal defect microcavities is observed to result in a splitting not only of the mode wavelength but also of the modal loss. It is discussed that the characteristics of the loss splitting may have an important impact on the optical energy transfer between the coupled resonators. The loss splitting--given by the imaginary part of the coupling strength--is found to arise from the difference in diffractive out-of-plane radiation losses of the symmetric and the antisymmetric modes of the coupled system. An approach to control the splitting via coupling barrier engineering is presented.