Near-Field Scanning Magneto-Optical Spectroscopy of Wigner Molecules

We study the emission spectra of single self-organized InP/GaInP QDs (size 100-220 nm) using high-spatialresolution, low-temperature (5K) near-field scanning optical microscope (NSOM) operating at magnetic field strength B=0-10T. The dots contain up to twenty electrons and represent natural Wigner molecules (WM). We observed vibronic-type shake-up structure in single electron QDs manifesting formation of two electron (2e) WM in photo-excited state. We found that relative intensities of the shake-up components described well by vibronic Frank-Condon factors giving for dots having parabolic confinement energy h omega 0= 1.2-4 meV molecule bond lengths 40-140 nm. We used measurements of magnetic-field-induced shifts to distinguish emission of 2e-WM and singly charged exciton (trion). We also observed magnetic-field-induced molecular-droplet transition for two electron dot, emitting through doubly charge exciton (tetron) at zero magnetic field.