Near-field spectroscopy and microscopy of single quantum dots

1. Motivation / Overview

The conventional microscopy is excellent spectral and temporal selectivity, but its spatial resolution is limited by diffraction to roughly half of wavelength. In these days, electron-beam lithography, scanning tunneling microscopy, atomic force microscopy provide atomic-scale resolution and can be used to manipulate sub-nano structure but not suitable for observation of spectral and dynamic properties.

Resolution beyond diffraction limit is usually obtained by using near-field scanning optical microscopy (NSOM), in which a small aperture providing illumination is scanned close to object under observation.

A quantum dot like as an Artificial atom has discrete energy structure, quantum confinement effect and high quantum yield properties. So quantum dots have studied as representative applications such as laser diode, light emitting diode, photovoltaic cell, optical switch.

NSOM is powerful tools in order to investigate optical properties of single quantum dots in spatial, spectral and temporal experiments.

2. Experimental Setup
NSOM_2_Setup.jpg

3. NSOM Probe
NSOM probe fabricated by chemical methods using hydrofluoric acid(HF) dilute solution. This method has high reproducibility, making wide range of cone angle and small apex diameter.
NSOM_3_NSOM_Probe.jpg

4. Results and Discussion

1) Optical Properties of NSOM Probe

NSOM_4_Results_1.jpg
2) PL spectroscopy of bimodal-size distribution of InAs-AlGaAs QDs   NSOM_4_Results_2.jpg

This data is the PL microscopic image of selected three PL peaks (1.3762eV, 1.3854eV and 1.3884eV) whthin the same scaning area (500nm× 500nm). We have observed the PL spectra associated with the size distribution depending on different locations of the bimodal QDs and characterized the size-difference of single QDs having separate PL peaks.

3) Spectral Diffusion in InGaAs QDs

Excitation power dependence Electric field dependence
NSOM_4_Results_3.jpg NSOM_4_Results_4.jpg

We first observed the spectral diffusion in III-V structures of semiconductor quantum dot. The fluctuation amplitude depends on  the excitation density and the density of generated charge carriers at defects. The synchronous emission lines must each originate from the same siangle quantum dot.
NSOM_4_Results_5.jpg