Mu-Tian Cheng

Coherent exciton-plasmon interaction in the hybrid semiconductor quantum dot and metal nanoparticle complex

We studied theoretically the exciton coherent dynamics in the hybrid complex composed of CdTe quantum dot (QDs) and rodlike Au nanoparticles (NPs) by the self-consistent approach. Through adjusting the aspect ratio of the rodlike Au NPs, the radiative rate of the exciton and the nonradiative energy transfer rate from the QD to the Au NP are tunable in the wide range 0.05-4 ns(-1) and 4.4 x 10(-4) to 2.6 ns(-1), respectively; consequently, the period of Rabi oscillations of exciton population is tunable in the range 0.6 pi-9 pi.

Coherent Control of a V-Type Three-Level System in a Single Quantum Dot

In a semiconductor quantum dot, the IIx and IIy transitions to the polarization eigenstates, |x> and |y>, naturally form a three-level V-type system. Using low-temperature polarized photoluminescence spectroscopy, we have investigated the exciton dynamics arising under strong laser excitation. We also explicitly solved the density matrix equations for comparison with the experimental data. The polarization of the exciting field controls the coupling between the otherwise orthogonal states. In particular, when the system is initialized into \Y>, a polarization-tailored pulse can swap the population into |x>, and vice versa, effectively operating on the exciton spin.

Modulating emission polarization of semiconductor quantum dots through surface plasmon of metal nanorod

We theoretically investigated the dynamics of exciton populations [ρyy(t) and ρxx(t)] on two orthogonal polarization eigenstates (∣x⟩ and ∣y⟩) and the polarization ratio P(t)=[ρyy(t)−ρxx(t)]∕[ρyy(t)+ρxx(t)] of an anisotropic InGaAs quantum dot modulated by the surface plasmon of an Au nanorod (NR). In the resonance of longitudinal surface plasmon of AuNR, the polarization ratio P(t) increases from 0.22 to 0.99 during the excitation due to the efficient enhancement of Rabi frequency of the transition between the ∣y⟩ and vacuum states, and decreases from 0.02 to −0.92 after the excitation pulse due to the enhancement of decay rate of the ∣y⟩ state. This offers an approach to modulate the dynamic polarization ratio of radiative emissions.

Surface plasmon propagation in a pair of metal nanowires coupled to a nanosized optical emitter

The coupling, propagations, and far-field emissions of surface plasmons in a pair of Au nanowires with a dipole emitter have been investigated using the finite-difference time domain method. The surface plasmon wavelength is tunable from 650 to 380 nm by adjusting the distance between the two wires, which leads to an enhancement of coupling constant and density of states of the surface plasmon. The converted energy from the dipole emitter to the propagating surface plasmon as well as the far-field emission intensity of a pair of Au nanowires increase to approximately four times as large as those of a single nanowire.

Surface plasmons amplifications in single Ag nanoring.

The stimulated amplifications of surface plasmons (SPs) propagating along a single silver nanoring is theoretically investigated by considering the interactions between SPs and activated semiconductor quantum dots (SQDs). Threshold condition for the stimulated amplifications, the SP density as a function of propagation length and the maximum SP density are obtained. The SPs can be nonlinearly amplified when the pumping rate of SQDs is larger than the threshold, and the maximum value of SP density increases linearly with the pumping rate of SQDs.

Internal and external polarization memory loss in single semiconductor quantum dots

By using nonlinear resonant optical control and polarized photoluminescence, the authors studied the intensity dependence of exciton spin relaxation and photoluminescence polarization in single self-assembled InGaAs quantum dots. Data from dots with different dipole moments reveal two distinctive channels for polarization memory loss: (i) an external pathway due to carrier escape and capture to and from the wetting layer that is responsible for memory loss increasing with intensity and (ii) an internal loss channel due to intrinsic spin relaxation. The values obtained rule out a universal freezing of exciton spin relaxation in single self-assembled quantum dots related to the wetting layer.

Rabi oscillation damped by exciton leakage and Auger capture in quantum dots

The decoherence of Rabi oscillation (RO) caused by biexciton, population leakage to the wetting layer (WL), and Auger capture in semiconductor quantum dots is theoretically analyzed with multilevel optical Bloch equations. The corresponding effects on the quality factor of RO are also discussed. We have found that the biexciton effect is relatively trifling, as the pulse duration is longer than 5 ps. The population leakage to the WL leads to a decrease of the RO average even though the damping rate is similar to that observed in the experiment. Auger capture in quantum dots results in RO damping that is consistent with the experimental data, which implies that Auger capture is an important decoherence process in quantum dots.

Dynamics and the statistics of three-photon cascade emissions from single semiconductor quantum dots with pulse excitation

We analyse the population dynamics and the photon emission statistics of a single semiconductor quantum dot with an exciton–biexciton–triexciton multilevel driven by a pulse field, in which the quantum regression theorem and optical Bloch equations are employed. The populations of the three states are comparable under resonant excitation with the input pulse area around 1.5π. Three second-order cross-correlation functions demonstrate the correlated three-photon emissions at a given order from triexciton, biexciton and exciton states driven by single pulses, while the probability of the bunched three-photon cascaded emissions decreases to the minimum as the input pulse area increases to 2π in every period.

ANALYSIS OF CORRELATION FUNCTION AND POLARIZATION ENTANGLEMENT OF PHOTON PAIRS GENERATED FROM ANISOTROPIC SEMICONDUCTOR QUANTUM DOT

The dynamics of the population density matrix and polarization density matrix of the biexciton system of single semiconductor quantum dots with pulse excitation were analyzed by using master equations and quantum regression theorem. A model based on the spectral distributions of the cascade photon emissions was proposed to calculate the polarization entanglement of the photon pairs emitted from the non-degenerate biexciton system with spectral filters. The influences of the spectral filter width and the exciton energy splitting between the two orthogonally polarized eigenstates on the entanglement were theoretically analyzed. The dependence of the polarization entanglement on the input pulse area was also discussed.

Internal and external polarization memory loss in single quantum dots

We study exciton spin relaxation in single semiconductor quantum dots using non-linear resonant optical control. Experiments reveal two distinct channels for spin relaxation, external and internal, ruling out universal freezing of exciton spin relaxation.