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New method for studying the efficiency of quantum dot luminescence

Keywords: Nanotechnology, Nanophotonics, Quantum Dots, Silicon Nanocrystals, Microspectroscopy

“Our optimized method to study internal quantum yield of thin-layer materials at variable temperatures… is broadly applicable to various light-emitting nanostructured materials.”

Jan Valenta et al.

Advances in silicon-based optoelectronics rely on the optimization of quantum dot luminescence. To this end, silicon nanocrystals (SiNC) embedded in silicon dioxide (SiO₂) provide high photoluminescence (PL) quantum yield (QY), on the order of 20%, which is size tunable in the orange to near-infrared (NIR) spectral regions. Such quality can potentially be leveraged to provide photon conversion in lighting and photovoltaic devices.

A multidisciplinary scientific team comprising researchers from the Czech Republic, Germany, Australia, and Russia has now performed a comprehensive study of both the external and the internal luminescence quantum yield (EQY and IQY, respectively) of SiNC/SiO₂ multilayers. The team’s recent work appears in Scientific Reports 9, Article 11214 (2019) and was supported by the bilateral Czech-German DFG-GACR project 16-09745 J and ZA 191/36-1. Read more

Spearheaded by scientists at Charles University (Prague, Czech Republic) and the University of Freiburg (Freiburg im Breisgau, Germany), the study reveals high efficiency of luminescence from SiNC in oxide matrix in the NIR spectral region. A cryogenically cooled back-thinned CCD camera from Teledyne Princeton Instruments afforded sensitivity from ~350 nm to ~1100 nm, whereas the use of a NIRvana InGaAs camera extended detection capabilities well into the NIR / NIR-II range (~950 nm to ~1640 nm).

The researchers investigated thin layers of SiNC in oxide matrix with optimized parameters (SiNC sizes: ~4.5 nm; SiO₂ barrier thickness: 3 nm). These materials, which were fabricated via plasma-enhanced chemical vapor deposition, revealed EQY close to 50% — near the best chemically synthetized colloidal SiNC. The IQY was determined utilizing the Purcell effect (i.e., modifying radiative decay rate by the proximity of a high-index medium in a special wedge-shape sample).

For the first time, the team notes, these experiments have been performed at variable temperatures. They go on to explain that the complete optical characterization and knowledge of both the IQY and the EQY allowed them to estimate the spectral distribution of the dark and bright nanocrystal populations within the SiNC ensemble.

Their work shows that silicon nanocrystals emitting at ~1.2 eV to ~1.3 eV are mostly bright, with IQY reaching 80% at room temperature and being reduced by thermally activated non-radiative processes. Below 100 K, the IQY approaches 100%.

The researchers posit that thin silicon nanocrystal multilayers may find application as stable and efficient NIR-luminescing layers with large Stokes shift, citing as an example the recent application of SiNC ML for advanced calibration of a two-detector microspectroscopy setup. They add that their optimized method for studying the internal quantum yield of thin-layer materials at variable temperatures employing the Purcell effect is not restricted to the SiNC ML. Read less

Reference:

Nearly perfect near-infrared luminescence efficiency of Si nanocrystals: A comprehensive quantum yield study employing the Purcell effect

Jan Valenta, Charles University, Czech Republic

Scientific Reports, 2019

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New immersion metalens for imaging quantum emitters

Keywords: Quantum Research, NV centers in Diamond, Material Science, Quantum Emitters

“This marks the first step in designing and fabricating metasurfaces for controlling photons from quantum emitters using only top-down fabrication techniques and provides a clear pathway to packaging quantum devices by eliminating the need for an objective… The immersion metalens promises major advances in performance and scalability of quantum devices.”

Tzu-Yung Huang et al.

Nitrogen-vacancy (NV) centers in diamond are single-photon emitters that hold significant promise for myriad quantum technologies and applications. Barriers to realizing this potential, however, include the refraction and reflections that occur at material interfaces, which hinder photon collection, as well as the emitters’ atomic scale, which necessitates the use of free-space optical measurement setups that prevent packaging of quantum devices.

Recently, an international research team from the United States and The Netherlands investigated an efficient way to overcome these limitations. Led by members of the Quantum Engineering Laboratory at the University of Pennsylvania (Philadelphia, USA), the team successfully designed, fabricated, and characterized a metasurface intended to collect the photoluminescence (PL) emission of a diamond NV center.Read more

The metasurface designed and fabricated by the researchers comprises nanoscale diamond pillars and functions as an immersion lens to collect and collimate the PL emission of an individual NV center. An IsoPlane 160 imaging spectrometer coupled to a PIXIS:100BX CCD camera was utilized in the metalens characterization and NV center imaging setup.

“The metalens exhibits a numerical aperture greater than 1.0, enabling efficient fiber-coupling of quantum emitters,” note the researchers in their abstract for Nature Communications 10, Article 2392 (2019). “This flexible design will lead to the miniaturization of quantum devices in a wide range of host materials and the development of metasurfaces that shape single-photon emission for coupling to optical cavities or route photons based on their quantum state.”

The researchers assert that optimized design strategies may yield a diamond metalens with a substantially larger numerical aperture (potentially approaching NA_max = n_D = 2.4). “Beyond lenses,” the team expounds, “the expanding body of research on metasurface design can be leveraged to explore phase profiles that shape emission from quantum emitter ensembles, compensate for an emitter’s dipole orientation, control coupling to orbital-angular-momentum modes, and enable chiral quantum photonics.”

This work is expected to have far-reaching implications for nanophotonics, quantum optics, and quantum nanotechnology. The top-down fabrication processes of the immersion metalens are readily compatible with those already employed to fabricate on-chip microwave antennas and electric-field gates required for dynamic spin control and Stark shifting in quantum optics applications, the researchers explain.

The metalens design can also be applied directly to many other quantum-emitter systems (e.g., spin defects in silicon carbide, quantum dots in III-V compound semiconductors, and rare-earth ions in laser crystals), the team concludes, whereas more generalized metasurface designs can mediate quantum entanglement and interference of quantum emitters.

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