Enhanced Determination of Emission Fine Structure and Orientation of Individual Quantum Dots Based on Correction Algorithm for Spectral Diffusion

Abstract

A robust algorithm based on cross-correlations and lucky imaging reliably allows the correction of spectrally diffused datasets. This step enables the resolution-limited analysis of the emission fine structure of semiconductor quantum dots (QDs). Bright and dark excitonic transitions are resolved with optimum signal-to-noise ratio, allowing for a precise determination of the angular direction of linear polarization of the different lines. The angular phases between polarization directions are intrinsically connected to the orientations of emission dipoles. This fact provides a tool for accurate numerical computation of the azimuth ϕ and polar angle θ of the QD with respect to the optical axis. Our in-situ characterization of QD fine structure and orientation represents a precise and non-invasive method without requiring specialized equipment beyond a standard luminescence setup. In this way, important information is provided whenever efficient coupling of a quantum emitter to the electromagnetic field is targeted by various nano-and micro-optic strategies.

Electric-Field Fluctuations as the Cause of Spectral Instabilities in Colloidal Quantum Dots

“…To this end, a cross-correlation-based algorithm, robust to the naturally low signal-to-noise ratio (SNR) of the spectra, is applied. 44 Subsequently, the three emission lines are clearly aligned in energy. In the integrated spectrum, the linewidth of each feature is similar to that of individual measurements, i.e., less than 0.3 meV.…”

“…45,46 The two additional spectral peaks (A 1 and A 2 ) result from a lift of the degenerate m f = ±1, yielding two dipole-allowed transitions with orthogonal transition-dipole moments. 44,47 Within the 90 s acquisition time, the three spectral lines fluctuate synchronously within a range of approximately 1.5 meV, indicating that they originate from a single quantum emitter. Integrating over the time axis in Figure 2a (top inset), all spectral features are significantly blurred, in contrast to the individual 1 s exposure measurements.…”

“…Alternatively, spectral shifts can be evaluated in postprocessing in order to realign the spectra with one another (Figure b). To this end, a cross-correlation-based algorithm, robust to the naturally low signal-to-noise ratio (SNR) of the spectra, is applied . Subsequently, the three emission lines are clearly aligned in energy.…”

“…Following previous reports, the three narrow emission lines can be attributed to the F, A 1 , and A 2 transitions in a neutral QD. The various peaks arise from the fine-structure splitting of the exciton ground state according to the discrete projections of the angular momentum on the main axis of the nanorod ( m f ). , The weakest and lowest-energy peak (F, around −1 meV) originates from the dipole-forbidden recombination of the m f = 0 exciton state. , The two additional spectral peaks (A 1 and A 2 ) result from a lift of the degenerate m f = ±1, yielding two dipole-allowed transitions with orthogonal transition-dipole moments. , Within the 90 s acquisition time, the three spectral lines fluctuate synchronously within a range of approximately 1.5 meV, indicating that they originate from a single quantum emitter. Integrating over the time axis in Figure a (top inset), all spectral features are significantly blurred, in contrast to the individual 1 s exposure measurements.…”

“… 43 The laser is input into a confocal microscope setup constructed around a bath cryostat (Scientific Magnetics) within which the beam is focused through a 0.9 numerical aperture objective lens (Olympus, M PLAN N, ×100 magnification). 44 A sample containing the hybrid nanoparticles embedded into microcapacitor structures with a 2 μm gap on a SiO 2 substrate (see further details in Supplementary Notes 1–3 ) is placed in the focal plane of the microscope and held at a temperature of 8 K. The PL collected through the same objective lens is spectrally resolved with a grating spectrometer (PI Acton, SP2300, 2400 lines/mm grating) and detected by an electron-multiplying charge-coupled device (EMCCD) camera (Andor, Newton 970). A typical time trace of PL spectra is presented in Figure 2 a with the energy axis centered around the brightest emission line at 2.125 eV.…”