Low temperature near-field optical mapping and control of electron wavefunctions confined in nanostructures

我们建立了一套近场扫描光学显微镜(NSOM)系统，该系统在低温磁场高达5T的情况下具有高空间分辨率。高达30-50 nm的空间分辨率使我们能够看到限制在单个量子点中的激子波函数。在GaAsAlGaAs界面态量子点中，基态激子、激发态激子和双激子态的波函数被清晰地描绘出来。数值计算很好地再现了实验结果的定量性质。我们还应用这一技术可视化了电场诱导量子点系统中激子的局域态密度。我们观察到被限制在较低能态的电子局域化更强烈。我们研究了稀氮化物合金系统，如GaAs和GaInNAs，通过局域和非局域电子态的空间分辨光谱来阐明N原子的不均匀分布。通过测量功率依赖的光致发光光谱和高分辨率的光致发光图像，直观地显示了激子限制势的深度和空间分布。对于两个不同N含量的GaInNAs量子阱，我们观察到了限制在N团簇中的强局域激子和非局域激子，这表明合金形成的开始。从单个量子点的磁致发光光谱中观察到激子发射的反磁位移。参照理论值，我们从一个由限制在量子点中的激子的玻尔半径决定的抗磁系数来估计量子点的大小。计算结果与实空间NSOM映射结果吻合较好。

We have established a near-field scanning optical microscope (NSOM) system with a high spatial resolution under a magnetic field up to 5T at low temperature. The spatial resolution as high as 30-50 nm enabled us to visualize exciton wave functions confined in single quantum dots.In GaAs/AlGaAs interface quantum dots, the wavefunctions of ground-state exciton, excited-state exciton, and biexciton states are clearly mapped out. A numerical calculation reproduced the experimental results well in their quantitative nature. We also applied this technique to visualize the local density of states of excitons confined in an electric-field induced quantum dot system. We observed that electrons confined in lower energy states localized more strongly.Dilute nitride alloy systems such as GaNAs and GaInNAs were investigated to clarify nonuniform distribution of N atoms through the spatially resolved spectroscopy of localized and delocalized electron states. The depths and spatial profiles of exciton confinement potential were visualized through the measurement of power-dependent PL spectra and high-resolution PL images. For two GaInNAs quantum wells with different N contents, we observed strongly localized excitons confined in N clusters and rather delocalized excitons, which indicate the onset of alloy formation.From magneto-PL spectroscopy of a single QD, a diamagnetic shift of the exciton emission was observed. Referring to the theoretical value we estimate the size of the QD from a diamagnetic coefficient, which is determined by the Bohr radius of excitons confined in QDs. The result was in good agreement with that obtained by the real-space NSOM mapping.