arXiv subjects

R. J. Warburton

Publications and source records attributed to R. J. Warburton.

30 records · Page 2Linked to original sources

arXiv2007

Contrast in transmission spectroscopy of a single quantum dot

We perform transmission spectroscopy on single quantum dots and examine the effects of a resident carriers spin, the incident laser spot size, polarization, and power on the experimental contrast. We demonstrate a factor of 4 improvement in the maximum contrast by using a solid immersion lens to decrease the spot area. This increase yields a maximum signal to noise ratio of 2000 Hz-1/2, which will allow for MHz detection frequencies. We anticipate that this improvement will allow further investigation of spectral fluctuation and open up the feasibility for an all-optical read-out of an electron spin in a quantum dot.

cond-mat.mes-hall↗

arXiv2006

Manipulating exciton fine-structure in quantum dots with a lateral electric field

The fine structure of the neutral exciton in a single self assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5 meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. We show that the lateral electric field strongly affects the exciton fine structure splitting due to active manipulation of the single particle wave-functions. Remarkably, the splitting can be tuned over large values and through zero.

cond-mat.mes-hall↗

arXiv2006

A stable fiber-based Fabry-Perot cavity

We report the development of a fiber-based, tunable optical cavity with open access. The cavity is of the Fabry-Perot type and is formed with miniature spherical mirrors positioned on the end of single- or multi-mode optical fibers by a transfer technique which involves lifting a high-quality mirror from a smooth convex substrate, either a ball lens or micro-lens. The cavities typically have a finesse of $\sim 1,000$ and a mode volume of 600 $μ$m$^3$. We demonstrate the detection of small ensembles of cold Rb atoms guided through such a cavity on an atom chip.

physics.atom-ph↗

arXiv2005

Absorption and photoluminescence spectroscopy on a single self-assembled charge-tunable quantum dot

We have performed detailed photoluminescence (PL) and absorption spectroscopy on the same single self-assembled quantum dot in a charge-tunable device. The transition from neutral to charged exciton in the PL occurs at a more negative voltage than the corresponding transition in absorption. We have developed a model of the Coulomb blockade to account for this observation. At large negative bias, the absorption broadens as a result of electron and hole tunneling. We observe resonant features in this regime whenever the quantum dot hole level is resonant with two-dimensional hole states located at the capping layer-blocking barrier interface in our structure.

cond-mat.mes-hall↗

arXiv2005

Voltage-controlled electron-hole interaction in a single quantum dot

The ground state of neutral and negatively charged excitons confined to a single self-assembled InGaAs quantum dot is probed in a direct absorption experiment by high resolution laser spectroscopy. We show how the anisotropic electron-hole exchange interaction depends on the exciton charge and demonstrate how the interaction can be switched on and off with a small dc voltage. Furthermore, we report polarization sensitive analysis of the excitonic interband transition in a single quantum dot as a function of charge with and without magnetic field.

cond-mat.mes-hall↗

arXiv2005

Spin-selective optical absorption of singly charged excitons in a quantum dot

We investigate high resolution laser absorption spectroscopy of a single InGaAs/GaAs self-assembled quantum dot embedded in a field-effect structure. We show experimentally that the interband optical absorption to the lower Zeeman branch of the singly charged exciton is strongly inhibited due to spin (Pauli) blockade of the optical transition. At high magnetic fields the optical absorption to the upper Zeeman branch dominates the absorption spectrum. We find however that the spin blockade is not complete and a 10% leakage remains at high magnetic fields. Applying a gate voltage to empty the dot of its resident electron turns off the spin blockade. This effect is observed at 1.5 K and up to 9 Tesla.

cond-mat.mes-hall↗

arXiv2004

Voltage-Controlled Optics of a Quantum Dot

We show how the optical properties of a single semiconductor quantum dot can be controlled with a small dc voltage applied to a gate electrode. We find that the transmission spectrum of the neutral exciton exhibits two narrow lines with $\sim 2$ $μ$eV linewidth. The splitting into two linearly polarized components arises through an exchange interaction within the exciton. The exchange interaction can be turned off by choosing a gate voltage where the dot is occupied with an additional electron. Saturation spectroscopy demonstrates that the neutral exciton behaves as a two-level system. Our experiments show that the remaining problem for manipulating excitonic quantum states in this system is spectral fluctuation on a $μ$eV energy scale.

cond-mat.mes-hall↗

arXiv2003

Kondo-excitons and Auger processes in self-assembled quantum dots

We describe theoretically novel excitons in self-assembled quantum dots interacting with a two-dimensional electron gas in the wetting layer. In the presence of the Fermi sea, the optical lines become strongly voltage-dependent. If the electron spin is nonzero, the width of optical lines is given by k_BT_K, where T_K is Kondo temperature. If the spin is zero, the exciton couples with the continuum due to Auger-like processes. This leads to anticrossings in a magnetic field. Some of the described phenomena are observed in recent experiments.

cond-mat.mes-hall↗

arXiv2002

Kondo excitons in self-assembled quantum dots

We describe novel excitons in quantum dots by allowing for an interaction with a Fermi sea of electrons. We argue that these excitons can be realized very simply with self-assembled quantum dots, using the wetting layer as host for the Fermi sea. We show that a tunnel hybridization of a charged exciton with the Fermi sea leads to two striking effects in the optical spectra. First, the photoluminescence lines become strongly dependent on the vertical bias. Secondly, if the exciton spin is nonzero, the Kondo effect leads to peculiar photoluminescence line shapes with a line width determined by the Kondo temperature.

cond-mat.mes-hall↗

arXiv2002

Polarized excitons in nanorings and the `optical' Aharonov-Bohm effect

The quantum nature of matter lies in the wave function phases that accumulate while particles move along their trajectories. A prominent example is the Aharonov-Bohm phase, which has been studied in connection with the conductance of nanostructures. However, optical response in solids is determined by neutral excitations, for which no sensitivity to magnetic flux would be expected. We propose a new mechanism for the topological phase of a neutral particle, a polarized exciton confined to a semiconductor quantum ring. We predict that this magnetic-field induced phase may strongly affect excitons in a system with cylindrical symmetry, resulting in switching between `bright' exciton ground states and novel `dark' states with nearly infinite lifetimes. Since excitons determine the optical response of semiconductors, the predicted phase can be used to tailor photon emission from quantum nanostructures.

cond-mat.mes-hall↗

arXiv2002

Magneto-optical properties of charged excitons in quantum dots

We present both experimental and theoretical results on the influence of a magnetic field on excitons in semiconductor quantum dots. We find a pronounced difference between the strong and weak confinement regimes. For weak confinement, the excitonic diamagnetic shift is strongly dependent on surplus charge, corresponding to a reversal in sign of the conventional diamagnetic shift for neutral excitons. In this limit, we argue that the optical properties of excitons with two or more extra electrons are fundamentally different to those of the neutral exciton and trion.

cond-mat.str-el↗

arXiv1999

Zero-field spin splitting in InAs-AlSb quantum wells revisited

We present magnetotransport experiments on high-quality InAs-AlSb quantum wells that show a perfectly clean single-period Shubnikov-de Haas oscillation down to very low magnetic fields. In contrast to theoretical expectations based on an asymmetry induced zero-field spin splitting, no beating effect is observed. The carrier density has been changed by the persistent photo conductivity effect as well as via the application of hydrostatic pressure in order to influence the electric field at the interface of the electron gas. Still no indication of spin splitting at zero magnetic field was observed in spite of highly resolved Shubnikov- de Haas oscillations up to filling factors of 200. This surprising and unexpected result is discussed in view of other recently published data.

cond-mat.mes-hall↗