Laser pulse turns optical fiber nanotip into versatile electron gun

16 Sep 2020

Oak Ridge project offers easier generation of electrons for nanoscale imaging and sensing.

A project from Oak Ridge National Laboratory (ORNL) and the University of Nebraska-Lincoln has demonstrated how emission of electrons from the tip of an optical fiber can be stimulated by a laser pulse passing through that fiber.

Earlier studies have shown that sharp nanotips of fibers can act as electron sources when stimulated by light, but until now that light has had to be from an external source, carefully aligned with the apex of the nanotip.

The project's findings, published in New Journal of Physics, indicate that electron emission can instead be achieved from a pulse of laser light traveling along the optical fiber itself. This could allow more efficient nanoscale imaging and sensing applications, by allowing electrons to be delivered from previously difficult angles or locations.

"Previously, lasers had to track the fiber tips, which is technologically a much harder thing to do," said Herman Batelaan of the University of Nebraska-Lincoln. "The difficulty of that task limited how quickly images could be taken and from what position."

The breakthrough hinged on finding electron emission mechanisms that lowered the required laser power, in particular through careful design of the nano-scale tip of the optical fiber. If the fiber terminates in a tapered gold-coated nanotip of the correct dimensions, then the team's calculations indicated that the field intensity for a pulse of laser light traveling through the fiber would show a distinct hot-spot at the tip, sufficient to stimulate electron ejection.

Plasmons hold the key

To test the theory, the Nebraska team used a femtosecond laser to shoot ultrashort laser pulses through an optical fiber whose 50-nanometer-radius nanotip was coated with a thin film of gold, fabricated at ORNL.

Tests employing a range of laser wavelengths between 500 and 740 nanometers confirmed that controlled electron emission was indeed stimulated from the gold-coated nanotip. They also showed that the electric field at the nanotip's apex was enhanced by specific wavelengths of laser light, behavior thought to be related to surface plasmons.

"By tuning the femtosecond laser to the correct wavelength, which we call the surface plasmon resonance wavelength, we found that we got above threshold emission," said Nebraska's Sam Keramati. "Surface plasmon resonance signifies a collective oscillation of the electrons at the surface of the metal. Above threshold emission occurs when electrons absorb enough energy from photons to be shot out with an initial kinetic energy."

Having proven the principle, the project also found that a similar outcome was possible with a less powerful continuous wave laser, as long as the voltage at the nanotip was increased to compensate. This is thought to represent the smallest laser intensity that has given rise to electron emission from nanotips, according to the team.

"Now instead of having a powerful, extremely expensive laser, you could go with a $10 diode laser," Batelaan noted.

Squeezed light as a practical resource for microscopy

A second recent breakthrough at ORNL demonstrated what the Lab believes to be the first practical application of nonlinear interferometry, when it used squeezed light to measure the displacement of an atomic force microscope's microcantilever.

Squeezed light is a particular state that can be generated in non-linear optical processes, known to be useful in quantum metrology because the inherent noise of the electric field falls below that of the vacuum state.

Using a squeezed light source instead of a laser, the project measured the displacement of the microcantilever of an atomic force microscope with a sensitivity said to show 50-percent improvement over that possible using classical photonics, according to ORNL.

"We showed how to use squeezed light – a workhorse of quantum information science – as a practical resource for microscopy," said Ben Lawrie of ORNL's Materials Science and Technology Division. "We measured the displacement of an atomic force microscope microcantilever with sensitivity better than the standard quantum limit."