Control of Photon Polarization in GaAs/AlAs Single Quantum Dot Emission

Abstract

Polarization encoded single photons and photon pairs are expected to play a major role in future quantum communication schemes [1]. Therefore, control of polarization properties of single quantum dot emission is of a great value. We study magnetic field induced transition between anisotropy controlled and Zeeman controlled emission from individual GaAs/AlAs quantum dot (QD). We demonstrate the utility of these studies, involving polarized photon correlation measurements, for determination of the anisotropic exchange splitting of excitonic states in quantum dots. The sample was mounted directly on a microscope objective [2] and placed inside a pumped helium cryostat. Continuous wave excitation was performed with use of tunable Ti:Al2O3 laser at wavelength of 718 nm assuring excitation below the energy gap of the barrier material. The typical power of excitation beam was 100μW over the spot of a diameter of about 1μm. Temperature was kept constant at 1.8K. Photoluminescence was excited and collected through the same microscope objective. Correlation measurements were performed in a Hanbury-Brown and Twiss setup. The signal after polarization and spectral filtering was detected using two avalanche photodiodes. Polarization resolved crosscorrelations between biexciton (XX) and exciton (X) photons emitted subsequently in a radiative cascade were measured in magnetic field ranging from 0 to 0.5 T, applied in Faraday configuration. Polarization correlation of photon pairs in parallel linear polarizations appeared as a bunching peak in histograms of correlated counts in the absence of magnetic field. No polarization correlation in circular basis was observed at B = 0T. This is expected for QDs exhibiting in-plane anisotropy, since intermediate X state of the cascade is split by energy of electron-hole exchange interaction into two components emitting in orthogonal linear polarizations. Application of magnetic field transforms Anisotropic Exchange Splitting (AES) into Zeeman splitting of the X level, reducing thus the influence of the anisotropy. In a sufficiently high magnetic field pure exciton eigenstates of angular momentum M = +/1 are observed [3]. Expected conversion of linearly polarized excitonic states to circularly polarized ones is demonstrated by increase of the degree of circular polarization correlation with increasing magnetic field. We apply a simple model proposed by Besombes et al. [4] to describe the progressive change of the polarization correlation with increasing field. The model allows calculating of the degree of polarization correlation as a function of Zeeman splitting to AES ratio. By fitting the model to the experimental data, we were able to determine the anisotropic exchange splitting AES value of order of tens of μeV. This method of the AES determination can be used even in cases when direct measurement by polarization resolved microphotoluminescence is not sensitive enough.