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VERSION:2.0
PRODID:Icfo
X-PUBLISHED-TTL:P1W
BEGIN:VEVENT
UID:69d4b3c64adb4
DTSTART:20251024T080000Z
SEQUENCE:0
TRANSP:OPAQUE
LOCATION:ICFO Auditorium
SUMMARY:ICFO | YINA WU
CLASS:PUBLIC
DESCRIPTION:Light has long served as a fundamental probe in scientific obse
 rvation\, yet conventional optical microscopy faces an inherent limitation
  in spatial resolution. In 1873\, Ernst Abbe formulated the diffraction li
 mit\, establishing a fundamental barrier: traditional microscopes cannot r
 esolve features smaller than approximately half the wavelength of light (o
 n the order of a few hundred nanometers in the visible)1. This restriction
  rendered viruses\, proteins\, and other nanoscale structures beyond the r
 each of optical observation.\nPushing observation to the nanoscale demande
 d transcending the limits of classical optics. This can be done by moving 
 from photons to electrons\, with a substantial reduction in the correspond
 ing wavelength. Current state-of-the-art aberration-corrected STEM-EELS in
 struments can resolve spectral features with meV-scale energy resolution w
 hile maintaining sub-˚angstr&ouml\;m spatial precision2\,3\, enabling dir
 ect visualization of electromagnetic near-fields and dark excitations inac
 cessible to conventional optical microscopy techniques4. In STEM-EELS syst
 ems\, the electron beam is used to excite localized electromagnetic modes 
 within a sample\, generating characteristic energy losses associated with 
 plasmons\, phonons\, and hybrid polariton modes.\nThis thesis mainly explo
 res electron-driven photon emission and polaritons at the nanoscale. First
 \, we discuss the basics of plasmons and phonons as polaritons that can be
  sampled through electron microscopy. The corresponding theoretical framew
 orks are presented in Chapter 2 and Chapter 3. Specifically\, in Chapter 2
 \, we study infrared plasmons in fluorine-doped indium oxide nanocube dime
 rs. By bringing two cubes close together and observing the energy loss exp
 erienced by the electrons\, we observe how their infrared plasmons change 
 when the cubes are approached and nearly touching at a point\, along an ed
 ge\, or over a face. We observe that a sharp low-energy mode appears only 
 for point or line contacts and vanishes once the cubes are pulled apart\, 
 thus establishing a nonsingular transition between the regimes of cube tou
 ching and non-touching.\nComplementing these plasmonic polaritons with an 
 electron beam\, Chapter examines the excitation of vibrational phonon pola
 ritons in a finite hexagonal boron nitride nanostructure. We build an atom
 istic framework based on first-principles calculations to investigate the 
 vibrational phonon modes\, showing that nonlocal effects are important to 
 be considered and that the simple local model fails to capture some key fe
 atures of these modes. This study bridges the gap between bulk material pr
 operties and nanoscale behavior\, demonstrating that nonlocal effects and 
 surface interactions are indispensable for understanding phonon polaritons
  in confined systems. Unlike passive probe techniques such as EELS\, integ
 rated nanophotonic devices require active\, electrically driven light sour
 ces. One promising approach utilizes light emission from inelastic electro
 n tunneling (LIET) in metal-insulator-metal (MIM) junctions\, where tunnel
 ing electrons exhibit ˚angstr&ouml\;m-scale wavelengths 5\,6. Specificall
 y\, in Chapter 4\, we move from passive probing to active emission. We des
 ign an MIM tunnel junction covered by a gold antenna metasurface. When ele
 ctrons tunnel across a thin oxide layer\, they lose energy and emit photon
 s. The system is complemented by plasmonic antennas that boost this weak p
 rocess\, producing bright and uniform light emission from a large area. Th
 eory matches the measured spectrum and shows that the resulting devices ca
 n detect changes in the refractive index of thin films placed on top of th
 e metasurface\, thus serving as sensors in which no external light is requ
 ired.\nIn summary\, this work helps us gain new insights into how electron
 s can both reveal and generate optical fields on the nanometer scale by co
 mbining advanced microscopy techniques with detailed theoretical work. The
 se findings could lead to practical applications in sensors\, light source
 s\, and photonic circuits based on plasmons and phonon polaritons in low-d
 imensional materials.\n&nbsp\;\nFriday October 24\, 10:00 h. ICFO Auditori
 um \nThesis Director: Prof. Dr. Javier Garc&iacute\;a de Abajo
DTSTAMP:20260407T073534Z
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