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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


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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 NAmax = nD = 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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Te nanoparticles improve solar energy conversion

Keywords: Energy research, Nanotechnology, Nanophotonics, Darkfield microspectroscopy

The conversion of solar energy is an area of large impact for Nanotechnologies. Several spectroscopic techniques are used to probe the properties and applications of nanoparticles . Researchers from China recently published an article about Tellurium nanoparticles for conversion of solar energy. They employ Raman spectroscopy, UV-VIS absorption spectroscopy and dark field scattering spectroscopy to understand the particles properties.The research is featured on Laser Focus World with the article below:
For at least the last decade, “solar thermal” technologies, in which sunlight is used to convert water into steam that runs electric turbines or performs desalination, has been a kind of darling of the investment community. About six years ago, nanoparticles started to get into this solar-thermal game when Rice University researchers added some nanoparticles to cold water and were able to make steam when they exposed the combination to sunlight.Read more
Since then, a lot of work in what is now termed photothermal conversionhas turned to the field of plasmonics, which exploits the wave of electrons that is produced when photons strike a metallic surface. However, producing plasmonic nanostructures is certainly not as straightforward as just adding some nanoparticles to water.
Now, researchers in China have combined the ease of adding nanoparticles to water with plasmonics to create a photothermal conversion process that exceeds all plasmonic or all-dielectric nanoparticles previously reported. Researchers at Sun Yat-sen University (Guangzhou, China) demonstrated in the journal Science Advances what they claim is the first material that simultaneously has both plasmonic-like and all-dielectric properties when exposed to sunlight.
The key to achieving this combination is the use of tellurium (Te) nanoparticles, which have unique optical duality, according to G. W. Yang, professor at Sun Yat-sen University and coauthor of the research.
By dispersing these nanoparticles into water, the water evaporation rate is improved by a factor of three under solar radiation. This makes it possible to increase the water temperature from 29 degrees to 85 degrees Celsius within 100 seconds.
"The Te nanoparticles perform like a plasmonic nanoparticle when it is smaller than 120 nanometers [nm] and then as a high-index all-dielectric nanoparticle when those nanoparticles are larger than 120 nm," said Yang. The Te nanoparticles are able to achieve this duality because they have a wide size distribution (from 10 to 300 nm). This enhanced absorption can cover the whole solar radiation spectrum.
Another property of the Te nanoparticle is that when it is excited by sunlight, the excitation energy is transferred entirely to the carriers (electrons and holes). This pushes the carriers out of equilibrium and into special states of momentum with higher temperatures.
Yang explains that as the system evolves toward equilibrium, these carriers relax. As the carriers scatter, it leads to a phenomenon known as Coulomb thermalization, which forms a hot gas of thermalized carriers that couple with phonons and transfer their excess energy to the lattice. This results in the efficient heating of the Te nanoparticles.
For this approach to work for commercial desalination
, Yang acknowledges that the current method of producing the Te nanoparticles with nanosecond laser ablation in liquid is limited. "Now, we are trying to prepare the Te nanoparticles by other methods," he added.
But because the Te nanoparticles have a unique optical duality, Yang envisions other applications for the technology. "We want to apply them in sensors or nanoantennas," he said.
The abstract for the paper published in Science Advances details the research findings:
Nanophotonic materials for solar energy harvesting and photothermal conversion are urgently needed to alleviate the global energy crisis. We demonstrate that a broadband absorber made of tellurium (Te) nanoparticles with a wide size distribution can absorb more than 85% solar radiation in the entire spectrum. Temperature of the absorber irradiated by sunlight can increase from 29° to 85°C within 100 s. By dispersing Te nanoparticles into water, the water evaporation rate is improved by three times under solar radiation of 78.9 mW/cm2. This photothermal conversion surpasses that of plasmonic or all-dielectric nanoparticles reported before. We also establish that the unique permittivity of Te is responsible for the high performance. The real part of permittivity experiences a transition from negative to positive in the ultraviolet-visible–near-infrared region, which endows Te nanoparticles with the plasmonic-like and all-dielectric duality. The total absorption covers the entire spectrum of solar radiation due to the enhancement by both plasmonic-like and Mie-type resonances. It is the first reported material that simultaneously has plasmonic-like and all-dielectric properties in the solar radiation region. These findings suggest that the Te nanoparticle can be expected to be an advanced photothermal conversion material for solar-enabled water evaporation.
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New, infra-red, confocal microscopy setup enables faster and higher resolution bioimaging.

Keywords: Confocal Microscopy, Life Science, SWIR/NIR-II Bioimaging

Confocal microscopes are widely used in physical and in particular life sciences as they offer higher resolution as standard widefield imaging. If high acquisition speeds and frame rates need to be achieved typically scanning confocal systems are used that permit to scan a field of view with an excitiation light source with kHz rates.
The research group of Ardemis Boghossian from Lausanne, Switzerland has just published a report about their research in collaboration with Nikon and Crest Optics to transfer scanning disc confocal microscopy to the SWIR/NIR-II wavelength range using a Princeton Instruments NIRvana-ST camera for detection. SWIR imaging is increasingly established in life science due to low absorption and scattering of tissue in this wavelength band.

Read more

The researchers show with several application examples that the new setup increases the spatial resolution while still allowing for image acquisition at high frame rates. For example carbon nanotubes can be precisely localized in chloroplasts, monitoring nanoparticle movement in solvents allows to measure diffusivities and using the high vertical resolution allows for glucose concentration easurements using nanosensors at different depth of a sample.This research lays the ground work for establishing and expanding this technique to monitor the location and movement of nanoparticles and sensors.
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Spinning-disc confocal microscopy in the second near-infrared window (NIR-II)

Ardemis Boghossian, Ecole Polytechnique Federale de Lausanne, Switzerland

Scientific Reports, 2018

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New X-Ray sources for ultrafast absorption spectroscopy

Keywords: X-Ray Spectroscopy, Ultrafast Spectroscopy, Material Science

Intense laser pulses make it possible to create high quality X-Ray radiation sources in a laboratory setting where typically large synchrotron sources are needed. A research team from France has created such a source for ultrafast X-Ray absorption experiments on the femtosecond timescale. The researchers study what happens in the first few moments when a material is heated with a short, intense laser beam. The setup includes an X-Ray spectrometre where the radiation is detected with an in vacuum MTE camera.

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Improving plasmas for biomedicine with ultrafast imaging

Keywords: Life Science, Medicine, Plasmas, Ultrafast Imaging

In the field of Plasma physics ICCD cameras are often used for fast time resolved imaging due to their ability to take quick snapshots of dynamic reactions. In this paper researchers use a PI-MAX camera for teim resolved imaging of plasmas from different electrode structures. Their goal is to characterize different plasma producing electrodes for applications in life science and biomedicine. 


Plasma Jets With Needle–Ring Electrodes: The Insulated Sealing of the Needle and its Effect on the Plasma Characteristics

Michael Kong, Jiatong University, China

IEEE Transactions on Plasma Science, 2018

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Making chemical processes more efficient

Keywords: Chemistry, Nanotechnology, Plasmonics, SERS, Microspectroscopy

The use of catalysts in the chemical industry is common in the production process. However how these catalysts work exactly is not always known. The research group of Prof. Prashant Jain is using surface enhanced micro Raman spectroscopy to examine the exact mechanisms of a Ag nano particle catalyst for chemical formation of ethylene.

SERS gives the researchers a detailed look what chemical processes happen during the catalytic reaction. In the experiment a single Ag nanoparticle is observed spectroscopically. A 300m focal length SpectraPro spectrometer with a Pylon camera are recording the Raman spectra. The SERS observations show that the formation of graphene happens as a first step during the chemical process. Knowledge of the precise reaction steps then allowed the team to design a new combined graphene/Ag nanoparticle catalyst which enables the chemical process under ambient conditions using photoexcitation whereas the typical reaction scheme requires cost intensive high pressure and high temperature environments.

The team hopes to that their newly designed reaction will be used to optimize other catalytic reactions commonly used.


In situ formation of catalytically active graphene in ethylene photo-epoxidation

Prashant Jain, University of Illinois, Urbana Champaign

Nature Communications, 2018

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Real time bio particle anlaysis with plasmonic devices

Keywords: Life Science, Biosensing, Nanophotonics, Plasmonics, SERS, Microspectroscopy, Advanced Spectroscopy

At University of Minnesota the lab of Sang-Hyun Oh is working on using nanotechnology and optical methods to advance analytical and sensing method in particular for bio and life science applications. They developed a high resolution micro Raman spectroscopy system for fast measurements (the researchers talk about a factor 100x advantage in speed over similar systems) where the biological nano particles do not have to be labelled.

The team build microscopic traps for the nanoparticles using an effect called dielectrophoresis on plasmonic nano-structures. The traps capture the bio structures along a small gap that is imaged to a spectrograph so Raman spectra of all captured particles can be collected simultaneously. In addition to this multiplexing advantage gold nano-particles are used to create a SERS surface enhanced Raman effect to obtain even stronger signals

The fast system allows for real time monitoring of bio particles in cells. Having such a system will improve capabilities for cell and biological analysis and could lead to development of more capable biosensors.


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New tunable filter for advanced Raman spectroscopy

Keywords: Advanced Spectroscopy, Nanotechnology, Material Science, Raman spectroscopy

Raman spectroscopy typically requires precise, stable and narrow band light sources to produce well defined and sharp spectral features. Therefore emission lines from gas lasers are often used as excitation sources. However it can be important to continuously tune the excitation wavelength. For example some materials like carbon nanotubes, quantum dots or nanowires show resonant Raman features where the scattering intensity is strongly enhanced when exciting at exactly the right wavelength. Also some Raman features can show energy shifts when the excitation source energy changes like the so called D band lines in graphene materials. While typically the energy shfit in Raman scattering is roughly independent of excitation energy the D bands involve influence of defects which allows for the shift of the Raman energy. 

Researchers from the lab of Tom Vosch have developed a continuous filter system based using monochromators as tunable filters and to detect the Raman scattered light. They describe how they implemented the system and synchronized movement of several system components to create a continuously tunable source using a supercontinuum laser for light input. They demonstrate the validity of their system by observing the behavior of the well understood Raman lines of graphene.

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Ultrafast spectroscopy improves understanding of silica based materials

Keywords: Material Science, Nanophotonics, time resolved PL spectroscopy, Lifetime measurements

Silica based materials that emit light under excitation have huge applications in creating novel sensors, laser sources and optical devices in general. Silica can be doped with different elements to tailor its emission properties for different applications, however the interactions and exact mechanisms determining the emission are often complex and not easy to understand. So various techniques are used for characterization of these materials.

Researchers around Prof. Ouerdane from University Lyon in France are probing microstructured silica fibers doped with Bi ions using various forms of photoluminescence spectroscopy. The PL emission changes when the fibers are exposed to gamma rays, with laser excitation energy and with time after a laser excitation pulse. The results give the researchers new insights into the structure of the Bi color centers in the silica fibers.

The team performed the time resolved PL and PL excitation measurements using a spectroscopy setup and a PI-MAX4 ICCD camera as detector for experiments with high sensitivity and time resolution from nanoseconds to microseconds


Structured blue emission in Bismuth doped fibers

Prof. Ouerdane, Lyon Univ, France

Optical Materials, 2018

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Ultrafast optical measurements to understand flames mixed with water.

Keywords: Combustion, PLIF, Chemiluminescence Imaging

Researchers Rose Padilla, Derek Dunn-Rankin and their team/collaborators report on experiments where they measure methane/air combustion flames mixed with water. The goal is to understand the effects of having water on the fuel side where in real world situations the water can be introduced arbitrarily or occurs naturally as in methane hydrates.

The researchers measure the PLIF of OH radicals to characterize the flame reaction zone using a PI-MAX4 ICCD camera. The same camera is used for chemiluminescence CH molecules as well.


Structure and behavior of water-laden CH4/air counterflow diffusion flames

D. Dunn-Rankin, UC Irvine

Combustion and Flame, 2018

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Developing cleaner combustion processes with simulated turbulence

Keywords: Combustion, PLIF

Combustion processes in real world applications, motors and engines typically involve turbulent flow patterns and complex reaction chemistries in high and lower temperature flames. The lab of Prof. Yiguang Ju from Princeton University just reported on experiments measuring the behavior of flames interacting with a vortex (simulating turbulence). One of the research goals of the Ju lab is the exploration and development of new knowledge and techniques to advance new and cleaner combustion techniques. Moreover the lab has been experts in creating cool diffusion flames and investigating turbulent combustion processes.

The lab created double - cool and hot - flame and observed as the flame interacts with a flow vortex. The researchers use a planar laser induced fluorescence technique (PLIF). A pulsed laser beam is expanded into a two dimensional sheet of light that is send through the flame. The fluorescence signal is measured with a gated PI-MAX4 ICCD camera. PLIF allows to target specific molecule species and results in a two dimensional fluorescence image. Besides high temporal and spatial resolution PLIF allows researchers to measure molecule concentrations, distributions and velocities as well.


Transient interactions between a premixed double flame and a vortex

Prof. Yiguang Yu, Princeton University

Proceedings of the Combustion Institute, 2018

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Keywords: Material Science, Advanced Spectroscopy, Hyperspectral, Surface science, Interference

Researchers around Anja Royne from Norway recently published an article about their experiments investigating surfaces of calcites. This mineral has multiple applications as building and industrial material  and it is often important to characterize the strength of the materials involved. For example depending of the surrounding medium (could be water) the formation of cracks and weakening of the material structure can be induced.

In their experiments they use rough and reactive calcite surfaces and perform measurements in a device for surface force measurements. The device uses an interferometric readout using a high resolution imaging spectrometer (Isoplane 320, PIXIS 2048B). The interference effect allows for very precise measurements of the distance between the sample surfaces.



Surface Forces Apparatus measurements of interactions between rough and reactive calcite surfaces

Anja Røyne, University of Oslo, Norway

Langmuir, 2018

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