Instabilities for a coherently driven absorber in a Fabry-Perot cavity

We consider a suitable set of equations which describe the interaction of the electric field with a system of N two-level atoms contained in a Fabry-Perot and driven by a cw laser beam (optical bistability). The stability of the stationary solutions is analyzed. Instabilities of the self-pulsing type are found only when the bistability parameter C is larger than 60. This shows that, in order to observe self-pulsing, a ring cavity is more suitable than a Fabry-Perot.     

Resonance in quantum dot fluorescence in a photonic bandgap liquid crystal host

Microcavity resonance is demonstrated in nanocrystal quantum dot fluorescence in a one-dimensional (1D) chiral photonic bandgap cholesteric-liquid crystal host under cw excitation. The resonance demonstrates coupling between quantum dot fluorescence and the cholesteric microcavity. Observed at a band edge of a photonic stop band, this resonance has circular polarization due to microcavity chirality with 4.9 times intensity enhancement in comparison with polarization of the opposite handedness. The circular-polarization dissymmetry factor g(e) of this resonance is ~1.3. We also demonstrate photon antibunching of a single quantum dot in a similar glassy cholesteric microcavity. These results are important in cholesteric-laser research, in which so far only dyes were used, as well as for room-temperature single-photon source applications.     

Photon beats from a single semiconductor quantum dot

Single-photon interference is observed on the ultranarrow long-term stable exciton resonance of an individual semiconductor quantum dot. This interference is related to the fine-structure splitting and allows direct conclusions about the coherence properties of the exciton. When selectively addressing a particular dot by quasiresonant phonon-assisted excitation, despite a rapid orientation relaxation on a 1-ps time scale, coherence is partly maintained. No significant further decoherence occurs when the ground state is reached until the exciton recombines radiatively (approximately 300 ps).     

Anomalous phonon-mediated damping of a driven quantum dot embedded in a high-Q semiconductor microcavity

We describe and employ a recently developed polaron master equation model to study the fluorescence spectra of a coherently driven quantum dot (QD) placed within a high-Q semiconductor microcavity (with Q the quality factor). We investigate phonon-induced damping in a regime where many cavity photons are required, and we also compare the resonance fluorescence spectra obtained using an effective phonon master equation in Lindblad form where simple analytical expressions are identified for various phonon-induced scattering rates. We consider two separate continuous-wave pumping scenarios, where either the system is driven through exciton pumping or the system is driven via the cavity. The cavity-QED (quantum electrodynamics) system is pumped sufficiently strongly such that the low-energy sideband of the Mollow triplet is tuned across the cavity mode resonance which is negatively detuned from the QD. For comparison, we also consider the case where the QD-cavity detuning is large enough such that the Mollow triplets do not spectrally overlap with the cavity mode. We find that the full width at half maximum (FWHM) of the high-energy Mollow sideband shows a pronounced nonlinear dependence on the pump intensity when the low-energy component of the triplet overlaps with the cavity mode (or vice versa), and can even be reduced with increased pumping. However, the FWHM depends linearly on the pump intensity when the Mollow triplets are far from the cavity resonance. We observe similar fluorescence spectra for both the exciton-driven system and the cavity-driven system.     

Overdamping of coherently driven quantum systems

The phenomenon of overdamping, when frictional forces overwhelm the restoring force of an harmonic oscillator, is well known in classical mechanics. Analogous phenomena occur in time-dependent quantum mechanics when probability loss rates (e.g. photoionization rates) become larger than the characteristic inverse times for coherent excitation (the Rabi frequencies). These results often seem surprising, even for simple two-state quantum systems. We discuss examples of this overdamping phenomenon—when an increase of the loss rate actually produces a slower rate of probability loss and even freezes population—for a number of quantum systems: steady and pulsed two-state excitation, time evolution with adiabatic passage, simple multistate chains, and branched chains of excitation linkage. In each situation there occurs for very large damping, often unexpectedly, phenomena akin to the overdamping of the classical harmonic oscillator.     

Resonance fluorescence spectrum of two atoms,coherently driven by a strong resonant laser field

In Lehmberg's approach, we consider the resonance fluorescence spectrum of two radiatively interacting atoms. In the strong field limit we have obtained analytical solutions for the spectrum of the symmetric and antisymmetric modes without decoupling approximation. Our solutions are valid for all values of the distance r₁₂ separating the atoms. The spectrum of the symmetric modes contains additional sidebands in 2Ω (Ω is the Rabi frequency) with amplitude dependent on (a/Ω)², where a is a parameter dependent on r₁₂. The antisymmetric part of the spectrum has no additional sidebands in 2Ω. For small distances r₁₂ (a=1) our results for the symmetric modes are identical with those of Agarwal et al. apart from the so-called scaling factor. For large distances r₁₂ (a=0) the spectra of the symmetric and antisymmetric modes are identical with the well-known one-atom spectrum.     

Resonance fluorescence from semiconductor quantum dots: beyond the Mollow triplet

We show that the resonance fluorescence spectrum of a quantum dot excited by a strong optical pulse contains multiple peaks beyond those of the Mollow triplet. We show that as the area of the optical pulse is increased, new side peaks split off the central peak and shift in frequency. A simple analytical theory has been derived, which quantitatively accounts for the appearance and position of the peaks. This theory explains the physics responsible for the multiple peaks. By considering the time-dependent spectrum we demonstrate a time ordering of the side peaks, which is further evidence for the suggested physical explanation.     

Electron cotunneling in a semiconductor quantum dot

We report transport measurements on a semiconductor quantum dot with a small number of confined electrons. In the Coulomb blockade regime, conduction is dominated by cotunneling processes. These can be either elastic or inelastic, depending on whether they leave the dot in its ground state or drive it into an excited state, respectively. We are able to discriminate between these two contributions and show that inelastic events can occur only if the applied bias exceeds the lowest excitation energy. Implications to energy-level spectroscopy are discussed.     

Luminescence of a semiconductor quantum dot system

A microscopic theory is used to study photoluminescence of semiconductor quantum dots under the influence of Coulomb and carrier-photon correlation effects beyond the Hartree-Fock level. We investigate the emission spectrum and the decay properties of the time-resolved luminescence from initially excited quantum dots. The influence of the correlations is included within a cluster expansion scheme up to the singlet-doublet level.     

Dephasing processes in a single semiconductor quantum dot

We discuss the decoherence dynamics in a single semiconductor quantum dot and analyze two dephasing mechanisms. In the first part of the review, we examine the intrinsic source of dephasing provided by the coupling to acoustic phonons. We show that the non-perturbative reaction of the lattice to the interband optical transition results in a composite optical spectrum with a central zero-phonon line and lateral side-bands. In fact, these acoustic phonon side-bands completely dominate the quantum dot optical response at room temperature. In the second part of the article, we focus on the extrinsic dephasing mechanism of spectral diffusion that determines the quantum dot decoherence at low temperatures. We interpret the variations of both width and shape of the zero-phonon line as due to the fluctuating electrostatic environment. In particular, we demonstrate the existence of a motional narrowing regime in the limit of low incident power or low temperature, thus revealing an unconventional phenomenology compared to nuclear magnetic resonance. To cite this article: G. Cassabois, R. Ferreira, C. R. Physique 9 (2008).     

Control of population flow in coherently driven quantum ladders

A technique for adiabatic control of the population flow through a preselected decaying excited level in a three-level quantum ladder is presented. The population flow through the intermediate or upper level is controlled efficiently and robustly by varying the pulse delay between a pair of partly overlapping coherent laser pulses. The technique is analyzed theoretically and demonstrated in an experiment with Na2 molecules.     

Instabilities for a coherently driven Fabry-Perot cavity with a very thin absorber

We study the multi-longitudinal mode instability in purely absorptive optical bistability in the limit of nonlinear absorber much thinner than one wavelength. Under that assumption the dynamical equations are space-independent difference-differential equations which allow for an analysis as simple as for the ring cavity, with two additional parameters related to the position of the absorber in the cavity. Different instability scenarios are found as the relative position varies.     

Two-dimensional atom localization via quantum interference in a coherently driven inverted-Y system

A scheme for two-dimensional (2D) subwavelength atom localization is proposed, in which the atom is in an inverted-Y configuration and driven by two orthogonal standing-wave lasers. Due to the spatial dependence of atom-field interaction, the quantities of system, including the frequency of spontaneously emitted photon, the population in the excited state, and the probe absorption, carry information about the position of atom in standing-wave fields. We exploit this fact to 2D atom localization, and obtain a high precision and resolution in the position probability distribution.     

Spin readout and initialization in a semiconductor quantum dot

Electron spin qubits in semiconductors are attractive from the viewpoint of long coherence times. However, single spin measurement is challenging. Several promising schemes incorporate ancillary tunnel couplings that may provide unwanted channels for decoherence. Here, we propose a novel spin-charge transduction scheme, converting spin information to orbital information within a single quantum dot by microwave excitation. The same quantum dot can be used for rapid initialization, gating, and readout. We present detailed modeling of such a device in silicon to confirm its feasibility.     

Exciton-spin memory with a semiconductor quantum dot molecule

We report on a single photon and spin storage device based on a semiconductor quantum dot molecule. Optically excited single electron-hole pairs are trapped within the molecule, and their recombination rate is electrically controlled over 3 orders of magnitude. Single photons are stored up to 1 μs and read out on a subnanosecond time scale. By using resonant excitation, the circular polarization of individual photons is transferred into the spin state of electron-hole pairs with a fidelity above 80%, which does not degrade for storage times up to the 12.5 ns repetition period of the experiment.     

Self-assembled quantum dot semiconductor nanostructure modeling

The calculation of the electronic structure of a vertical multifold stack of self-assembled InAs/GaAs lens-shaped quantum dots is presented. The developed numerical method is based on a rigorous Hamiltonian formulation of an eight-band k.p perturbation accounting for the carrier kinetics and lattice-mismatch strain endured by the islands. The considered implementation is built upon a custom partition of the unit cell. The accompanying validation analysis—consisting of a comprehensive hierarchy of convergence criteria, qualitative, and quantitative test cases—unequivocally shows that the obtained results adhere to the prescribed zero dimensional physics.     

Optical solitons in semiconductor quantum dot waveguides

A theory of self-induced transparency for a TM-mode propagating in a planar semiconductor waveguide sandwiched between two dielectric media is developed. A transition layer between the waveguide and one of the connected media is described using a model of a two-dimensional sheet of quantum dots. Explicit analytical expressions for the optical soliton in the presence of single-excitonic and biexcitonic transitions are obtained with realistic parameters which can be reached in current experiments.     

Single-electron-phonon interaction in a suspended quantum dot phonon cavity

An electron-phonon cavity consisting of a quantum dot embedded in a freestanding GaAs/AlGaAs membrane is characterized using Coulomb blockade measurements at low temperatures. We find a complete suppression of single electron tunneling around zero bias leading to the formation of an energy gap in the transport spectrum. The observed effect is induced by the excitation of a localized phonon mode confined in the cavity. This phonon blockade of transport is lifted at discrete magnetic fields where higher electronic states with nonzero angular momentum are brought into resonance with the phonon energy.