Colloidal nanocrystals are free-standing pieces of semiconductor whose dimensions are on the nanometer length scale. Due to their incredibly small size, quantum effects play a dominant role in how excitons (electron-hole pairs) behave in these nanocrystals, especially in their atom-like interactions with light. Today, these nanocrystals have found numerous commercial applications ranging from solar cells to energy-efficient televisions.

Recently, a new class of colloidal nanocrystals called perovskite nanocrystals (due to their constituent lead-halide perovskite material) was discovered that has garnered the immediate attention of researchers worldwide. Among their unique properties, there has been evidence that perovskite nanocrystals emit light with unprecedented efficiency due to the so-called “dark” exciton state, which slows light emission dramatically in other nanocrystals, above the “bright” light emitting triplet states. Conflicting evidence has also been reported, which argue alternative mechanisms for efficient light emission from a conventional, reversed energy level structure.

By applying multi-dimensional coherent spectroscopy, we have circumvented the inhomogeneous spectral broadening that usually obscures signatures of fine-structure in nanocrystals to shed light on this problem. We have found evidence of a unique energy level structure in which the dark state is interleaved between the bright triplet states. The relative position of this dark state may also be controlled by varying the nanocrystal size.

Another surprising finding involves the bright triplet states themselves. By taking advantage of the fact that the three triplet states emit light with orthogonal polarizations, we generated quantum superpositions of triplet states that exhibit a minimal increase in decoherence from 10 K to 40 K, and possibly to even higher temperatures. This result positions perovskite nanocrystals as an attractive bottom-up material platform for quantum information applications.

To read more about this work, also see the Feature on Phys.org.

Reference: A. Liu, D. B. Almeida, L. G. Bonato, G. Nagamine, L. F. Zagonel, A. F. Nogueira, L. A. Padilha, S. T. Cundiff, “Partially-Bright Triplet Excitons in Perovskite Nanocrystals,”  Science Advances 7, eabb3594 (2020).
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