Scientists at the Department of Energy’s Oak Ridge National Laboratory and the University of Nebraska have developed an easier way to generate electrons for nanoscale-scale imaging and sensitivity, providing a useful new tool for materials science, bioimaging and research fundamental quantum.
In a study published in New Journal of Physics, the researchers reported that firing intense laser pulses through a fiber optic nanotype caused the yeast to emit electrons, creating a fast “electronic weapon” that could be used to investigate materials. The device allows researchers to quickly examine surfaces from any angle, which offers a huge advantage over existing less mobile techniques.
“It works on the principle of activating light, so light enters and stimulates electrons in the metal just in the right way so that they gain enough energy to come out,” said Ali Passian of ORNL’s Science Quantum Information group.
Electrons are an invaluable tool for looking closely at the surface properties of materials. Subatomic particles, which have shorter wavelengths than photons – light particles – can magnify objects at nanometer resolution, or one billionth of a meter – exponentially higher than the magnification of light.
Since the mid-2000s, researchers have used sharp nanotips to release electrons into strongly concentrated beams. Nanotypes provide improved spatial and temporal resolution compared to other scanning electron microscopy techniques, helping researchers better track continuous nanoscale interactions. In these techniques, electrons are emitted when photons excite tips.
However, prior to this study, nanotype emission methods have relied on external light stimulation. To generate electrons, the researchers had to carefully line the laser beams to the top of the nanotype.
“Before, lasers had to track tips, which is technologically a much harder thing to do,” said Herman Batelaan, a co-author of the study that leads the electron control research at the University of Nebraska. The difficulty of the task limited how quickly images could be taken and from which position.
But Passian had an idea for a different approach. By shooting laser light through a flexible optical fiber to illuminate its metal-clad connected nanotype, he predicted that it would create an easier maneuverable tool.
“The idea was that because it is simple and restrained – light spreads from the inside – you can control different pieces of material at different heights and side positions,” Passian said.
To find out if his idea was possible, Passian teamed up with Batelaan and then graduate student Sam Keramati at the University of Nebraska. The Nebraska team used a femtosecond laser to shoot ultra-short, intense pulses through an optical fiber and in a vacuum chamber. In the room, light moved through a gold-plated fiber nanotype that was fabricated at ORNL.
The team actually observed controlled electron emission from the nanotype. Analyzing the data, they proposed that the mechanism that enables the emission is not simple, but rather involves a combination of factors.
One factor is that the shape and metallic coating of the nanotype generates an electric field that helps push electrons from the tip. Another factor is that this electric field at the top of the nanotype can be increased by the specific wavelength of the laser light.
“By tuning the femtosecond laser to the right wavelength, which we call the surface plasmon resonance wavelength, we found that we have above the threshold emission,” Keramati said. Surface plasmon resonance indicates a collective oscillation of electrons on the metal surface. Emission above the threshold occurs when electrons absorb enough energy from photons to be fired with an initial kinetic energy.
To verify that the electrons were emitted due to light and not heat, the team studied the nanotypes themselves. The tips suffered no damage during the experiment, indicating that the emission mechanism is easily driven by light.
An additional advantage of the new technique, they discovered, is that the fast switching ability of the laser source allows them to control electron emission at speeds faster than one nanosecond. This will give them a better way to capture images at a faster speed. Such images can then be merged almost like a film to follow complex interactions at the nanoscale.
Satisfied with these initial discoveries, the team decided to test whether they could achieve a result similar to a much less powerful continuous-wave laser, the same type found in a daily laser pointer. To compensate for the lack of laser energy, they raised the voltage at the nanotype, creating a change in the energy potential that they believed could help expel electrons. To their surprise, it worked.
“To our knowledge, this is the lowest laser intensity that has given rise to the emission of electrons from nanotips,” said Keramati, now a postdoctoral researcher, about the results published in Letters of Applied Physics.
“Now instead of having a powerful, extremely expensive laser, you can go with a $ 10 diode laser,” Batelaan noted.
Although continuous wave lasers lack the ability to quickly switch to more powerful femtosecond lasers, slow interruption offers its advantages; respectively, the ability to better control the duration and number of electrons emitted by nanotips.
The team demonstrated, in fact, that the control provided by the slow interruption enabled the emission of the electron within the limits needed for a futuristic application called the electronic ghost image. The recently demonstrated light phantom image utilizes the quantum properties of light in image-sensitive specimens, such as living biological cells, at a very low exposure.
By combining multiple fiber nanotips together, the team hopes to achieve the electronic ghost image at the nanos scale.
“Surface Plasmon Increases Rapid Electron Emission from Metallized Optical Fiber Nanotypes” by Sam Keramati, Ali Passian, Vineet Khullar, Joshua Beck, Cornelis Uiterwaal, and Herman Batelaan, August 24, 2020, New Journal of Physics.
DOI: 10.1088 / 1367-2630 / aba85b
“Photofield electron emission from a fiber optic nanotype” by S. Keramati, A. Passian, V. Khullar and H. Batelaan, 10 August 2020, Letters of Applied Physics.
DOI: 10.1063 / 5.0014873
Initial research for this work was supported by the ORNL Seed Research and Development Laboratory Program Money Seed Fund. Research at the University of Nebraska was supported by a grant from the UNL Collaborative Initiative and the National Science Foundation under award numbers EPS-1430519 and PHY-1912504.