Quantum State Control And Detection In Single Quantum Dots

Due to the three-dimensional confinements,semiconductor quantum dots(QDs)have atom-like discrete energy levels.As a solid-state quantum system,semiconductor QDs have important applications in quantum information processing and optoelectric devices.Here,we fabricated the n-i-Schottky device with self-assemble InAs/GaAs QDs embedded,and investigated the exciton states and carriers tunneling properties with low-temperature photoluminescence(PL)and photocurrent(PC)spectroscopies.The main results are as follows:1.We investigated high-resolution PL and photocurrent PC spectroscopies of a single self-assembled InAs/GaAs QD embedded in an n-i-Schottky device with an applied magnetic field in Faraday and Voigt geometries.The single-QD PC spectrum of neutral exciton(X~0)is obtained by sweeping the bias-dependent X~0 transition energy to achieve resonance with a fixed narrow bandwidth laser through the quantum-confined Stark effect.With a magnetic field applied in Faraday geometry,the diamagnetic effect and the Zeeman splitting of X~0 are observed both in PL and PC spectra.When the magnetic field is applied in Voigt geometry,the mixture of bright and dark states results in an observation of dark-exciton states,which are confirmed by the polarization-resolved PL and PC spectra.2.We observed an increase in the carrier tunneling time in a single quantum dot(QD)with a magnetic field in Faraday geometry using PC spectroscopy.A nearly 60%increase in hole tunneling time is observed with an applied magnetic field equal to 9 T due to the modulation of the hole wavefunction.For a truncated pyramid QD,the hole tunnels out faster at the lateral edge of the QD due to the reduced barrier height.The magnetic field in Faraday geometry shrinks the hole wave function at the center of the QD plane,which weakens the tunneling at the lateral edge and increases the average tunneling time.This mechanism also works for the electron,but the effect is smaller.The electron wave function is more localized at the center of the QD due to the uniform confining potential;therefore,the relatively weak shrinkage caused by the magnetic field does not reduce the tunneling rate significantly.3.Understanding the carrier excitation and transport processes at the single-charge level plays a key role in QD-based solar cells and photodetectors.Here we investigated Coulomb-induced giant PC enhancement of positively charged trions(X~+)in a single InAs/GaAs QD by high-resolution two-color photocurrent spectroscopy.The Coulomb repulsion between the two holes in X~+increases the tunneling rate of one hole,and the remaining hole can be reused as the initial state to regenerate X~+.This process results in the photocurrent amplitude of X~+being up to 30 times larger than that of the neutral exciton.The analysis of the hole tunneling time through power-dependent saturation PC measurement and four-level-rate-equation model gives the equivalent change of hole tunnel barriers caused by Coulomb interaction between two holes as 8.05 meV during the tunneling process.Our work provides a fundamental understanding of energy conversion for solar cells on the nanoscale to increase internal quantum efficiency for energy harvesting.