Researchers demonstrate record speed with advanced spectroscopy technique

Researchers have developed an advanced spectrometer that can acquire data with exceptionally high speed. The new spectrometer could be useful for a variety of applications including remote sensing, real-time biological imaging and machine vision.

Spectrometers measure the color of light absorbed or emitted from a substance. However, using such systems for complex and detailed measurement typically requires long data acquisition times.

“Our new system can measure a spectrum in mere microseconds,” said research team leader Scott B. Papp from the National Institute of Standards and Technology and the University of Colorado, Boulder. “This means it could be used for chemical studies in the dynamic environment of power plants or jet engines, for quality control of pharmaceuticals or semiconductors flying by on a production line, or for video imaging of biological samples.”

In The Optical Society (OSA) journal Optics Express, lead author David R. Carlson and colleagues Daniel D. Hickstein and Papp report the first dual-comb spectrometer with a pulse repetition rate of 10 gigahertz. They demonstrate it by carrying out spectroscopy experiments on pressurized gases and semiconductor wafers.

“Frequency combs are already known to be useful for spectroscopy,” said Carlson. “Our research is focused on building new, high-speed frequency combs that can make a spectrometer that operates hundreds of times faster than current technologies.”

Getting data faster

Dual-comb spectroscopy uses two optical sources, known as optical frequency combs that emit a spectrum of colors — or frequencies — perfectly spaced like the teeth on a comb. Frequency combs are useful for spectroscopy because they provide access to a wide range of colors that can be used to distinguish various substances.

To create a dual-comb spectroscopy system with extremely fast acquisition and a wide range of colors, the researchers brought together techniques from several different disciplines, including nanofabrication, microwave electronics, spectroscopy and microscopy.

The frequency combs in the new system use an optical modulator driven by an electronic signal to carve a continuous laser beam into a sequence of very short pulses. These pulses of light pass through nanophotonic nonlinear waveguides on a microchip, which generates many colors of light simultaneously. This multi-color output, known as a supercontinuum, can then be used to make precise spectroscopy measurements of solids, liquids and gases.

The chip-based nanophotonic nonlinear waveguides were a key component in this new system. These channels confine light within structures that are a centimeter long but only nanometers wide. Their small size and low light losses combined with the properties of the material they are made from allow them to convert light from one wavelength to another very efficiently to create the supercontinuum.

“The frequency comb source itself is also unique compared to most other dual-comb systems because it is generated by carving a continuous laser beam into pulses with an electro-optic modulator,” said Carlson. “This means the reliability and tunability of the laser can be exceptionally high across a wide range of operating conditions, an important feature when looking at future applications outside of a laboratory environment.”

Analyzing gases and solids

To demonstrate the versatility of the new dual-comb spectrometer, the researchers used it to perform linear absorption spectroscopy on gases of different pressure. They also operated it in a slightly different configuration to perform the advanced analytical technique known as nonlinear Raman spectroscopy on semiconductor materials. Nonlinear Raman spectroscopy, which uses pulses of light to characterize the vibrations of molecules in a sample, has not previously been performed using an electro-optic frequency comb.

The high data acquisition speeds that are possible with electro-optic combs operating at gigahertz pulse rates are ideal for making spectroscopy measurements of fast and non-repeatable events.

“It may be possible to analyze and capture the chemical signatures during an explosion or combustion event,” said Carlson. “Similarly, in biological imaging the ability to create images in real time of living tissues without requiring chemical labeling would be immensely valuable to biological researchers.”

The researchers are now working to improve the system’s performance to make it practical for applications like real-time biological imaging and to simplify and shrink the experimental setup so that it could be operated outside of the lab.

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Single-particle spectroscopy of CsPbBr3 perovskite reveals the origin low electrolumine

Metal halide perovskites have recently emerged as an exceptionally promising alternative material for next generation optoelectronic applications. Especially, nanoscale-size perovskite structures posse remarkable photophysical properties, such as direct bandgap, color tunability, large absorption cross-section, and narrow photoluminescence linewidth. Together with their low cost, feasibility for scale-up synthesis, solution processability and compatibility with existing optoelectronic device components, these properties make metal halide perovskite nanocrystals a feasible alternative to other semiconducting materials for a range of light-emitting applications including displays, lighting, lasers, as well as memory devices.

However, while perovskite nanocrystals show very high photoluminescence yield, electroluminescence devices prepared from such nanocrystals have long suffered from low efficiency. Recent efforts have concentrated on device engineering to overcome this problem, but there has been so far no systematic study on the nanoscale physical origin of the poor efficiencies. Here, the team of prof. Martin Vacha from Tokyo Tech used single-particle microscopic detection and spectroscopy to study the electroluminescence process on the level of individual nanocrystals.

The team used nanocrystals of the perovskite CsPbBr3 surface-passivated with oleic acid ligands, dispersed in thin film of a conducting polymer which was used as an emission layer in a light-emitting device (LED). The device was constructed for use on top of an inverted fluorescence microscope which enabled comparison of electroluminescence and photoluminescence from the same nanocrystals. The CsPbBr3 nanocrystals form aggregates within the emission layer, with each aggregate containing tens to hundreds of individual nanocrystals. The researchers used an advanced microscopic technique of super-resolution imaging to find out that while in photoluminescence all the nanocrystals in the aggregate emit light, in electroluminescence only a small number (typically 3-7) of the nanocrystals are actively emitting. The electroluminescence from only a limited number of nanocrystals is a result of size distribution and the consequent energy landscape within the aggregate. Electrical charges which are injected into the device during the operation are captured on individual nanocrystals and efficiently funneled towards the largest nanocrystals. The largest nanocrystals within the aggregate have the smallest energy bandgap, and their valence and conduction bands work as traps for charges captured originally at the surrounding nanocrystals. The conductive environment present between the nanocrystals enables efficient migration of the charges to these traps from where the electroluminescence takes place.

Another important finding is that the intensity of electroluminescence from the actively emitting nanocrystals is not constant but rather shows strong fluctuations, so called blinking. Such blinking is not present in photoluminescence of the same aggregates. The researchers have previously found that the blinking can be caused by the conductive matrix as well as by externally applied electric field (ACS Nano13, 2019, 624). In the LED device, the blinking phenomenon is a crucial factor that contributes to the lower efficiency in electroluminescence. The researchers concluded that electroluminescence efficiency is only about one third of that of photoluminescence due to the presence of the blinking phenomenon.

The present work points a way towards efficient nanoscale characterization of electroluminescence of halide perovskite materials for light-emitting applications. One of the keys towards higher efficiency will be surface engineering of the nanocrystals that would suppress the intensity fluctuations.

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Materials provided by Tokyo Institute of Technology. Note: Content may be edited for style and length.

Journal Reference:

  1. Dharmendar Kumar Sharma, Shuzo Hirata, Martin Vacha. Single-particle electroluminescence of CsPbBr3 perovskite nanocrystals reveals particle-selective recombination and blinking as key efficiency factors. Nature Communications, 2019; 10 (1) DOI: 10.1038/s41467-019-12512-y

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Tokyo Institute of Technology. “Single-particle spectroscopy of CsPbBr3 perovskite reveals the origin low electrolumine.” ScienceDaily. ScienceDaily, 9 October 2019. .

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Tokyo Institute of Technology. “Single-particle spectroscopy of CsPbBr3 perovskite reveals the origin low electrolumine.” ScienceDaily. (accessed October 9, 2019).

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