BEGIN:VCALENDAR
VERSION:2.0
PRODID:Icfo
X-PUBLISHED-TTL:P1W
BEGIN:VEVENT
UID:69d2560d0f32f
DTSTART:20230323T100000Z
SEQUENCE:0
TRANSP:OPAQUE
LOCATION:ICFO Auditorium and Online (Teams)
SUMMARY:ICFO | FRANCESCO ANDREOLI
CLASS:PUBLIC
DESCRIPTION:Our ability to confine\, guide\, and bend light has led to asto
 nishing technological achievements\, playing a fundamental role in diverse
  fields like microscopy\, photochemistry\, telecommunications or material 
 design. The key property of materials that allows to control light is the 
 refractive index. Notably\, regardless of very different microscopic struc
 tures\, all natural materials exhibit a modest\, near-unity index of refra
 ction\, n ~ 1. This universality suggests the existence of some simple\, u
 biquitous origin\, whose complete characterization from microscopic consid
 erations\, surprisingly\, is still missing. Moreover\, one can wonder whic
 h principles might allow to synthesize a material with an ultra-high index
 \, to boost the performance of photonic devices.\nIn this thesis\, we addr
 ess these questions from an atomic-physics standpoint\, exploring if the m
 acroscopic optical properties can be related to simple\, electrodynamical 
 processes occurring between well-separated atoms\, which only interact via
  light scattering. Standard theories neglect that light can be scattered m
 ultiple times\, and lead to unphysical predictions when strong interferenc
 e occurs between the coherent atomic emission\, such as in dense atomic en
 sembles or ordered lattices. We here develop new techniques to address the
  physics of multiple light scattering\, with the ultimate goal of understa
 nding the fundamental limits to the refractive index\, as well as proposin
 g unexpected photonic applications. Our results are divided in three parts
 .\nFirst\, we investigate an ensemble of ideal atoms with increasing atomi
 c density\, starting from the dilute gas limit\, up to dense regimes where
  a non-perturbative treatment of multiple scattering and near-field intera
 ctions is required. In this situation\, we find that these effects limit t
 he index to a maximum value of n ~ 1.7\, in contrast with standard theorie
 s. We propose an explanation based upon strong-disorder renormalization gr
 oup theory\, in which the near-field interactions combined with random ato
 mic positions result in an inhomogeneous broadening of the atomic resonanc
 e frequencies. This basic mechanism ensures that regardless of the physica
 l atomic density\, light at any given frequency only interacts with at mos
 t a few near-resonant atoms per cubic wavelength\, thus limiting the index
  attainable.\nAfterwards\, we show that a radically different behavior is 
 expected for an ideal\, atomic crystal. As long as the inter-atomic intera
 ctions are only mediated by multiple scattering\, each 2D array of the cry
 stal exhibits a lossless\, single-mode response\, which builds up a very l
 arge and purely real refractive index. To address the limits to this pictu
 re\, we extend our theoretical analysis to much higher densities\, where t
 he electronic orbitals on neighboring nuclei begin to overlap. We develop 
 a minimal model to include the onset of this regime into our non-perturbat
 ive analysis of multiple light scattering\, arguing that the emergence of 
 quantum magnetism\, density-density correlations and tunneling dynamics of
  the electrons effectively suppresses the single-mode response\, decreasin
 g the index back to unity. Nonetheless\, right before the onset of chemist
 ry\, our theory predicts that an ultra-high-index (n ~ 30) and low-loss ma
 terial could in principle be allowed by the laws of nature.\nFinally\, ins
 pired by the impressive optical response of atomic arrays\, we propose the
 ir use as a more complex optical device\, namely a thin lens. The building
  blocks of this \"atomic metalens\" are composed of three consecutive 2D a
 rrays\, whose distance and lattice constants are suitably chosen to guaran
 tee a high transmission of light\, as well as an arbitrary phase shift. To
  characterize its efficiency and prove its robustness against losses\, we 
 perform large-scale numerical simulations\, on a number of atoms between o
 ne and two orders of magnitude larger than comparable works.\n&nbsp\;
DTSTAMP:20260405T123109Z
END:VEVENT
END:VCALENDAR