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Register a new userImpact of heavy hole-light hole coupling on optical selection rules in GaAs quantum dots
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We report strong heavy hole-light mixing in GaAs quantum dots grown by droplet epitaxy. Using the neutral and charged exciton emission as a monitor we observe the direct consequence of quantum dot symmetry reduction in this strain free system. By fitting the polar diagram of the emission with simple analytical expressions obtained from k$cdot$p theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot.
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We demonstrate the real-time detection of single photogenerated electrons in two different lateral double quantum dots made in AlGaAs/GaAs/AlGaAs quantum wells having a thin or a thick AlGaAs barrier layer. The observed incident laser power and photon energy dependences of the photoelectron detection efficiency both indicate that the trapped photoelectrons are, for the thin barrier sample, predominantly photogenerated in the buffer layer followed by tunneling into one of the two dots, whereas for the thick barrier sample they are directly photogenerated in the well. For the latter, single photoelectron detection after selective excitation of the heavy and light hole state in the dot is well resolved. This ensures the applicability of our quantum well-based quantum dot systems for the coherent transfer from single photon polarization to single electron spin states.
Strong spin-orbit interaction characteristic for p-type GaAs systems, makes such systems promising for the realization of spintronic devices. Here we report on transport measurements in nanostructures fabricated on p-type, C-doped GaAs heterostructures by scanning probe oxidation lithography. We observe conductance quantization in a quantum point contact, as well as pronounced Coulomb resonances in two quantum dots with different geometries. Charging energies for both dots, extracted from Coulomb diamond measurements are in agreement with the lithographic dimensions of the dots. The absence of excited states in Coulomb diamond measurements indicates that the dots are in the multi-level transport regime.
A whole series of complementary studies have been performed on the same, single nanowire containing a quantum dot: cathodoluminescence spectroscopy and imaging, micro-photoluminescence spectroscopy under magnetic field and as a function of temperature, and energy-dispersive X-ray spectrometry and imaging. The ZnTe nanowire was deposited on a Si 3 N 4 membrane with Ti/Al patterns. The complete set of data shows that the CdTe quantum dot features the heavy-hole state as a ground state, although the compressive mismatch strain promotes a light-hole ground state as soon as the aspect ratio is larger than unity (elongated dot). A numerical calculation of the whole structure shows that the transition from the heavy-hole to the light-hole configuration is pushed toward values of the aspect ratio much larger than unity by the presence of a (Zn,Mg)Te shell, and that the effect is further enhanced by a small valence band offset between the semiconductors in the dot and around it.
We measure the spin dephasing of holes localized in self-assembled (InGa)As quantum dots by spin noise spectroscopy. The localized holes show a distinct hyperfine interaction with the nuclear spin bath despite the p-type symmetry of the valence band states. The experiments reveal a short spin relaxation time {tau}_{fast}^{hh} of 27 ns and a second, long spin relaxation time {tau}_{slow}^{hh} which exceeds the latter by more than one order of magnitude. The two times are attributed to heavy hole spins aligned perpendicular and parallel to the stochastic nuclear magnetic field. Intensity dependent measurements and numerical simulations reveal that the long relaxation time is still obscured by light absorption, despite low laser intensity and large detuning. Off-resonant light absorption causes a suppression of the spin noise signal due to the creation of a second hole entailing a vanishing hole spin polarization.
An optical-vortex is an inhomogeneous light beam having a phase singularity at its axis, where the intensity of the electric and/or magnetic field may vanish. Already well studied are the paraxial beams, which are known to carry well defined values of spin (polarization $sigma$) and orbital angular momenta; the orbital angular momentum per photon is given by the topological charge $ell$ times the Planck constant. Here we study the light-hole--to--conduction band transitions in a semiconductor quantum dot induced by a highly-focused beam originating from a $ell=1$ paraxial optical vortex. We find that at normal incidence the pulse will produce two distinct types of electron--hole pairs, depending on the relative signs of $sigma$ and $ell$. When sign($sigma$)$=$sign($ell$), the pulse will create electron--hole pairs with band+spin and envelope angular momenta both equal to one. In contrast, for sign($sigma$)$ eq$sign($ell$), the electron-hole pairs will have neither band+spin nor envelope angular momenta. A tightly-focused optical-vortex beam thus makes possible the creation of pairs that cannot be produced with plane waves at normal incidence. With the addition of co-propagating plane waves or switching techniques to change the charge $ell$ both the band+spin and the envelope angular momenta of the pair wave-function can be precisely controlled. We discuss possible applications in the field of spintronics that open up.
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