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Researchers analyze the directional peak-antenna-like behavior of tunnel junctions formed by surface defects at the atomic scale
The profile of light collected with tunneling microscopes changes when the tip is placed on an atomic step. This phenomenon can be exploited to build picoantennas, nanoscale elements that direct light.
Researchers from Madrid explain a phenomenon that makes it possible to control the direction of light emission at the atomic scale. The work offers a detailed explanation of how a single atom can change the directional profile of the light emitted in experiments with tunneling microscopes (STM, for its acronym in English Scanning Tunnelling Microscope).
The ’Photon STM’ laboratory at IMDEA Nanoscience is one of four tunneling microscopes at this institute, located on the campus of the Autonomous University of Madrid. The peculiarity of this instrument is that it can measure the optical properties of different samples, since it has an extension that allows it to collect the light emitted in the experiments.
Manipulation of light at the nanometer scale, below its wavelength, is interesting because the properties of the light collected in the far field are determined by what happens in the near field. This manipulation can be accomplished in STM microscopes because the electromagnetic field is extremely confined between two metal nanostructures, the microscope tip and the sample, which are separated by a typical distance of 1 nanometer. This configuration is called a nanocavity. If an element, such as an atomic defect, is introduced into this nanocavity, the system is called a picocavity and exhibits unique properties. It has been observed that by introducing atomic steps into the nanocavities, it is possible to modify the direction of light emission in experiments. This phenomenon, which researchers had observed previously, had no scientific explanation until now.
The ’Photon STM’ research group at IMDEA Nanoscience, led by Alberto Martín Jíménez and Roberto Otero, has made measurements of the radiated light in an experiment with a picoantenna composed of a gold STM tip and a smooth surface of silver atoms with an atomic step. During a typical measurement with an STM microscope, the tip travels across the sample, sweeping the surface back and forth as it collects the signal. The researchers observed that the light emitted by each electron that tùnels with the appropriate energy over a monoatomic step can be greater or less than that collected when the electron is injected into the atomically flat part of the surface.
Through extensive characterization of the light emitted by many steps, the researchers discovered that the parameter governing the light intensity per electron is the relative orientation between the step and light collection directions, thus demonstrating that light emission is not equally distributed in all directions in space, but that some are preferred over others with a cardioid-like directional profile.
In collaboration with Antonio Fernández, researcher at IFIMAC-UAM, the authors elucidated the mechanism by which the light emission is modified. In their work, recently published in Science Advances, they explain that in cavities as small as those between the tip and the STM sample, a defect in atomic size is sufficient to cause a significant redistribution of the electric field, which becomes very different on both sides of the step, thus explaining that the angular profile of light emission depends on the orientation of the step. This phenomenon can be exploited to fabricate a picoantenna, a nanoscale element with which to control the directionality of the emitted light.
In summary, to determine the electromagnetic field - light - emitted in the near field it is not only necessary to take into account the tip-sample structure of the microscope, but also the configuration and defects of the sample being scanned, at the atomic scale, since a single atomic defect can modify the direction in which this radiation is emitted.
The authors see potential in this method to eventually tune the direction of light emission from molecules, quantum dots or other quantum emitters. Investigating the optical properties of atomic objects is crucial not only to advance our knowledge but also to be able to design systems that have applications, for example, in quantum computing.
This work has been carried out at the Instituto Madrileño de Estudios Avanzados (IMDEA Nanociencia) and the Centro de Física de la Física de la Materia Condensada (IFIMAC-UAM), and has been co-funded with the Severo Ochoa Excellence accreditation to IMDEA Nanociencia (CEX2020-001039-S), the María de Maeztu Excellence accreditation to IFIMAC (CEX202020-000805-M), the MSCA-PF STED grant (101108851) and the MAD2D regional project of Comunidad de Madrid.
Glossary:
Nanocavity: gap formed, in the case of this article, between the tip of an STM microscope and the sample, which are approximately 1 nanometer apart.
Picoantenna: term adopted by the authors to designate the system formed by an atomic defect (such as a step of atoms) within a nanocavity.
Tunneling microscope (STM): an instrument for imaging surfaces at the atomic level, based on the concept of the tunneling effect. The tip is placed close to the surface, and electrons can "jump" from the tip to the sample thanks to the quantum tunnel effect, creating a current that can be measured, depending on the distance at which the tip is placed. If the tip sweeps the surface, a relief map, or image, is created.
Bibliographic reference
David Mateos et al. Directional picoantenna behavior of tunnel junctions formed by an atomic-scale surface defect. Sci. Adv.10, eadn2295(2024). DOI: 10.1126/sciadv.adn2295.
