According to a study published in Science Advances, researchers from Madrid explain why the angular profile of light emission depends on the step’s orientation: in cavities as small as those between the tip and the STM sample, an atomic size defect is sufficient to trigger a significant redistribution of the electric field, which becomes quite distinct on both sides of the step.
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Scientists described a phenomenon that makes controlling light emission at the atomic level possible. The study thoroughly describes how a single atom can alter the directional profile of light emitted in scanning tunneling microscope (STM) experiments.
This institute, situated on the Autonomous University of Madrid campus, has four tunneling microscopes, including the 'Photon STM' laboratory. What's so unusual about this instrument? Because this STM microscope has an extension that lets researchers collect the light emitted during the experiments, it can measure the optical properties of samples.
The manipulation of light at the nanometer scale, below its wavelength, is intriguing because the properties of light picked up in the far field are determined by what occurs in the near field. This manipulation is possible in STM microscopes because the electromagnetic field is extremely confined between two metal nanostructures, the microscope's tip and the sample, separated by a typical distance of 1 nanometer.
This configuration is known as a nanocavity. When an element, such as an atomic defect, is introduced into this nanocavity, the system becomes a picocavity, with its own properties. The experiments discovered that introducing atomic steps into the nanocavities allows one to change the direction of light emission. Until now, researchers had no scientific explanation for the phenomenon they had observed.
The ‘Photon STM’ research group at IMDEA Nanociencia, led by Alberto Martín Jiménez and Roberto Otero, measured radiated light using a picoantenna made of a gold STM tip and a smooth surface of silver atoms with an atomic step. During a typical STM measurement, the tip sweeps across the sample back and forth to pick up the signal.
The researchers discovered that the light emitted by each electron tunneling with the appropriate energy on a monatomic step can be greater or lesser than that collected when the electron is injected into the atomically flat part of the surface.
The researchers found that the relative orientation between the step directions and the direction of light collection is the parameter that controls the intensity of light per electron by thoroughly characterizing the light emitted by numerous steps. This shows that the emission of light With a directional profile similar to a cardioid, some are preferred over others, but it is not equally distributed in all directions of space.
This phenomenon can be used to create a nanoscale device called a picoantenna, which controls the directionality of light emissions.
In conclusion, the configuration and defects of the sample being swept at the atomic scale must be considered in addition to the point-sample structure of the microscope in order to determine the electromagnetic field (light) emitted in the near field. A single atomic defect can alter the direction in which this radiation is emitted.
According to the authors, this technique could eventually be used to adjust the direction of light emission from molecules, quantum dots, or other quantum emitters. Examining the optical characteristics of atomic objects is essential for both knowledge advancement and developing systems that can be used, such as quantum computing.
This study was conducted at the Madrid Institute for Advanced Studies (IMDEA Nanociencia) and the Center for Condensed Matter Physics (IFIMAC-UAM). It was co-funded by the MSCA-PF STED grant (101108851), the MAD2D regional project of Comunidad de Madrid, the Severo Ochoa Excellence accreditation to IMDEA Nanociencia (CEX2020-001039-S), and the María de Maeztu Excellence accreditation to IFIMAC (CEX2020-000805-M).
Journal Reference:
Mateos, D. et. al. (2024) Directional picoantenna behavior of tunnel junctions formed by an atomic-scale surface defect. Science Advances. doi.org/10.1126/sciadv.adn2295