Polarized Emission from Coupled Quantum Dots is Strong Along Coupling Direction

Coupled quantum dots could be considered as an essential building block in the advancement of scalable quantum devices by manipulating the quantum states of two adjacent quantum dots (QDs) both optically and electrically.

For polarized excitation parallel (θex = 90°) and perpendicular (θex = 0°) to the coupled direction, different anisotropies are obtained in the photoluminescence intensity of excitons and biexcitons. Two quantum dots (QD1 and QD2) contain excitons (X1 and X2) and local biexcitons (X1X1 and X2X2), respectively. However, coupled biexcitons (X1X2) are generated only by the parallel polarization excitation (θex = 90°). Image Credit: Kwangseuk Kyhm.

Scientists have recently achieved notable advancement in manipulating the coupled states of CQDs, but separate control of vertically stacked quantum dots is still difficult. Until every dot can offer a logical bit operation, this turns out to be a limit in progress towards scalable qubit arrays.

Laterally coupled QDs can be used as an alternative system if the lateral coupling of two dots and the charge state are individually controlled. For instance, tunable vector electric fields with arbitrary angles and magnitude can be produced in laterally coupled QDs by implementing four electrodes in a mesa structure.

A new study reported in the Light: Science & Applications journal by an international research team from the United Kingdom, South Korea, and China describes that the emission from laterally coupled quantum dots is strongly polarized along the coupling direction (90°), and it is possible to shape its polarization anisotropy by altering the orientation of the polarized excitation.

When the non-resonant excitation’s linear polarization is perpendicular to the coupled direction (0°), local biexcitons and excitons from the two individual quantum dots still exhibit an emission anisotropy with minimal polarization (10%).

However, when the excitation polarization is parallel to the coupled direction, the polarization anisotropy of coupled biexcitons, local biexcitons, and excitons becomes improved with a 74% degree of polarization. Furthermore, the researchers observed a constant anisotropy in time-resolved photoluminescence.

Thus, the coupling of laterally coupled QDs can be manipulated by excitation polarization. Particularly, the researchers showed the optically controlled anisotropic wavefunctions as a factor of the anisotropy of the decay rate and emission intensity.

Such findings verify the feasibility of the optical shaping of the polarization anisotropy in laterally coupled QDs, where the spatial arrangement of biexcitons and excitons can be regulated by excitation polarization.

New information storage technologies could use this polarized light to control the optical properties of coupled quantum dots. The idea of combining quantum dots—nanoscale semiconductor crystals—into coupled pairs has attracted great attention due to the increased number of exotic quantum states that can be realized for storing data.

Study Researchers

This 'optical shaping' reflects different arrangements of excitons-bound states of electrons and holes-in the coupled dots and could open new avenues for data storage and thermoelectric energy harvesting,” added the study authors.

Journal Reference:

Kim, H., et al. (2020) Optical shaping of the polarization anisotropy in a laterally coupled quantum dot dimer. Light: Science & Applications. doi.org/10.1038/s41377-020-0339-3.

Source: http://english.ciomp.cas.cn/

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