Quantum beats in the polarization of the spin-dependent photon echo from donor-bound excitons in CdTe/(Cd,Mg)Te quantum wells

Abstract

We study the quantum beats in the polarization of the photon echo from donor-bound exciton ensembles in semiconductor quantum wells. To induce these quantum beats, a sequence composed of a circularly polarized and a linearly polarized picosecond laser pulse in combination with an external transverse magnetic field is used. This results in an oscillatory behavior of the photon echo amplitude, detected in the σ + and σ − circular polarizations, occurring with opposite phases relative to each other. The beating frequency is the sum of the Larmor frequencies of the resident electron and the heavy hole when the second pulse is polarized along the magnetic field. The beating frequency is, on the other hand, the difference of these Larmor frequencies when the second pulse is polarized orthogonal to the magnetic field. The measurement of both beating frequencies serves as a method to determine precisely the in-plane hole g factor, including its sign. We apply this technique to observe the quantum beats in the polarization of the photon echo from the donor-bound excitons in a 20-nm-thick CdTe/Cd0.76Mg0.24Te quantum well. From these quantum beats we obtain the in-plane heavy hole g factor g h = −0.143 ± 0.005. PACS numbers:Quantum beats are a phenomenon due to resonant coherent excitation of (at least) two discrete quantum mechanical states with different energies, leading to a superposition state. Quantum beats can be manifested by oscillations in the coherent optical response of the system due to interference of the excited polarizations, where the oscillation frequency corresponds to the energy difference between the levels [1]. A typical example are the oscillations observed in resonance fluorescence or other coherent spectroscopy techniques from excitons in semiconductors, which can be represented by V-type energy level arrangements with a common crystal ground state that is optically coupled to two split excited exciton states [2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Quantum beats on excitons in semiconductors and their nanostructures have been observed for a large variety of excited states corresponding to the beating between, e.g., heavy-and light-hole excitons [2][3][4], the exiton states of a fine structure doublet with different spin configurations [5][6][7][8], as well as neutral and charged excitons [9][10][11][12]. Application of a magnetic field can be used to split (quasidegenerate) excitonic states by the Zeeman effect and to observe the corresponding quantum beats in the polarized optical response [5,13,14]. In this case, the splitting of the optically active exciton states, having opposite angular momentum projections ±1 onto the quantization axis along which also the magnetic field is applied, leads to quantum beats in the polarization rather than the intensity of the emitted light. The period of the oscilla-tions corresponds to the Zeeman splitting between the spin levels and can be used for evaluation of the g factors of the excitons. Another advantage of the Zeeman effect induced quantum bea...

Long-living dark coherence brought to light by magnetic-field controlled photon echo

“…For the PE decay one can get the following simple expression: P P E ∼ e −2γτ cos 2 Ω 0 τ . Such oscillations between two optical states was observed in the co-circular excitation and detection geometry for the spontaneous PE from trions in CdTe/CdMgTe quantum well subject to the transverse magnetic field [15,25] where both involved states had equal dephasing rates.…”

“…It allows the periodical optical coherence shuffling between optically accessible and inaccessible states leading to the oscillations in spontaneous PE signal. Such magnetic-field PE control was demonstrated in the trion system in a CdTe/CdMgTe quantum well (QW) [13][14][15]25]. It is important to note that in these works the two states involved in the shuffling are equivalent trion states with the same phase relaxation (dephasing) rates, and the accessibility of states is determined by the polarization of the corresponding optical transitions.…”

“…Full width at half maximum of the laser pulse spectra was around 1 meV, which makes possible to resonantly excite the excitonic transition without involving other exciton complexes. More details on the experimental technique could be found in [9,25].…”

Magnetophotoluminescence of Modulation-Doped CdTe Multiple Quantum Wells

“…Advanced experimental methods, including pulsed techniques, have also been applied to describe spin-dependent phenomena in CdTe/(Cd, Mg)Te structures. − …”