We report optical experiments of a charge tunable, single nanowire quantum dot subject to an electric field tuned by two independent voltages. First, we control tunneling events through an applied electric field along the nanowire growth direction. Second, we modify the chemical potential in the nanowire with a back-gate. We combine these two field-effects to isolate a single electron and independently tune the tunnel coupling of the quantum dot with the contacts. Such charge control is a first requirement for opto-electrical single electron spin experiments on a nanowire quantum dot.KEYWORDS Nanowires, optically-active quantum dots, single electron charging, opto-electronics S ingle, optically active quantum dots are widely investigated due to the ability to combine both single electron charging 1,2 and single 3 or entangled 4 photonemission, which are all key requirements for quantum information processing applications. 5 Nanowire quantum dots (NW-QDs) offer additional functionalities over selfassembled quantum dots since they are embedded in a onedimensional system instead of a three-dimensional host matrix. Therefore, the single electron (hole) transport channel is naturally aligned to the optically active quantum dot in the nanowire, advantageous for combining both quantum optics 6 and transport. [7][8][9] In addition, due to the small radial dimensions of the nanowires, electrostatic gate geometries are highly versatile 7,8 and axial heterostructure design is not limited by strain. As an example, the combination of Si sections, which are free of nuclear spins, with optically addressable electronic levels in III-V materials is promising for extending electron spin storage times. Prior to the work presented here, we have shown that a single InAsP quantum dot grown in an InP nanowire geometry is optically active, exhibits narrow emission lines, spin polarization memory effects, 10 and can be embedded in a LED device geometry. 11 Furthermore, it is predicted that NW-QDs are ideal sources of entangled photons due to the nanowire symmetry. 12 Recently, an electron spin-to-charge conversion read-out scheme has been proposed, 13 which is compatible with controlled storage of carriers up to microseconds. 14 Such storage times are promising since single spins in selfassembled quantum dots (SA-QDs) have been initialized, coherently manipulated, and read-out within picosecond time scales. 15,16 The proposed spin read-out scheme 13 highly depends on the overall tunnel coupling between the SA-QD energy levels and the continuum, determined by the quantum dot-to-contact spatial separation, which is fixed during growth. 17 In this letter, we present electrical control and optical read-out of the number of electrons residing in a single InAs 0.25 P 0.75 quantum dot embedded in an InP nanowire. We first identify the neutral exciton by photocurrent spectroscopy. Second, we demonstrate that the electron number can be controlled by an electric field applied along the nanowire growth direction or by an electrostatic bac...
We control the electrostatic environment of a single InAsP quantum dot in an InP nanowire with two contacts and two lateral gates positioned to an individual nanowire. We empty the quantum dot of excess charges and apply an electric field across its radial dimension. A large tuning range for the biexciton binding energy of 3 meV is obtained in a lateral electric field. At finite lateral electric field the exciton and biexciton emission overlap within their optical line width resulting in an enhancement of the observed photoluminescence intensity. The electric field dependence of the exciton and biexciton is compared to theoretical predictions and found to be in good qualitative agreement. This result is promising toward generating entangled photon pairs on demand without the requirement to remove the anisotropic exchange splitting from asymmetric quantum dots.
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