Anomalous Circular Polarization of Photoluminescence Spectra of Individual CdSe Nanocrystals in an Applied Magnetic Field

We study the low-temperature magnetophotoluminescence from individual CdSe nanocrystals. Nanocrystals having a small “bright” exciton fine structure splitting (${\Delta }_{XY}<0.5\mathrm{meV}$) exhibit a conventional left and right circularly polarized Zeeman photoluminescence doublet in applied magnetic fields. In contrast, nanocrystals with large ${\Delta }_{XY}$ ($>1\mathrm{meV}$) show an anomalous magnetophotoluminescence polarization, wherein the lower-energy peak becomes circularly polarized with increasing field, while the higher-energy peak remains linearly polarized. This unusual behavior arises from strong mixing between the absorbing and emitting bright exciton levels due to strong anisotropic exchange interactions.

Figure 1

(a) Zeeman splitting of bright $|\pm 1⟩$ excitons in cylindrically symmetric QDs. (b) Zeeman splitting of a bright exciton in asymmetric QDs having an intrinsic linearly polarized $XY$ fine structure in zero $B$. (c) Circularly polarized (${\sigma }^{\pm }$) PL (top three spectra) from a nearly symmetric NQD from 1–5 T, showing clear Zeeman splitting and strong circular polarization. Linearly polarized PL at $B=0$ (bottom spectrum) shows a small intrinsic fine structure splitting (${\Delta }_{XY}=0.28\mathrm{meV}$). $T=4\mathrm{K}$. The lower-energy peaks in the 1 T and 5 T spectra likely result from spectral wandering. (d) Same, from another NQD, which shows significant fluctuations in the PL line position. By simultaneously acquiring PL spectra for both ${\sigma }^{+}$ and ${\sigma }^{-}$ polarizations, we can still reliably measure the Zeeman splitting. (e, f) The energy splitting between ${\sigma }^{\pm }$ PL peaks (open circles) and between linearly polarized PL peaks (solid squares) from (c, d), respectively.

Figure 2

(a) Linearly polarized PL at $B=0$ from a single NQD, showing a large fine structure splitting (${\Delta }_{XY}=1.1\mathrm{meV}$) of the emitting ($1^L$) bright exciton (the bottom spectrum) along with spectra of circularly polarized PL for $B$ from 1, 3, and 5 T (three upper spectra). With increasing $B$, the low-energy peak becomes ${\sigma }^{+}$ polarized, while the higher-energy peak shows no circular polarization. (b) DCP of these two peaks (circles and squares, respectively). (c) The ${\sigma }^{\pm }$ PL at 5 T and the linearly polarized PL at 0 T from another NQD that exhibits a large ${\Delta }_{XY}$ and similarly anomalous polarization behavior.

Figure 3

(a) The four linearly polarized bright exciton levels that derive from $1^L$ and $1^U$, as a function of ${\Delta }_{XY}$, which characterizes the strength of anisotropic exchange. While $1^{L,U}$ excitons are twofold degenerate for cylindrically symmetric NQDs (left), they split as the NQD becomes asymmetric in the $XY$ plane and ${\Delta }_{XY}$ increases (right). States $1^{L+}$ and $1^{U-}$ approach as ${\Delta }_{XY}$ increases. (b, c) Calculated $1^{L\pm }$ and $1^{U\pm }$ exciton energies versus $B$ ($\parallel c$) for NQDs having a small and large anisotropic exchange (${\Delta }_{XY}=0.28$ and 1.0 meV, respectively). The dashed circle shows the “anticrossing” region. (d, e) Calculated DCP for these states.