Conference Agenda

While mankind has successfully mapped the surface of Mars and landed a rover on a comet (ESA’s Rosetta mission), we still struggle to land a helicopter in a snowstorm, or to explore the seabed with optical means. This contrast reveals a major scientific limitation: our inability to control light propagation in dynamic scattering environments such as fog, turbid water or dense aerosols.

In this talk, I first delineate the operational domain of ultrafast wavefront shaping — the regime where ballistic filtering becomes insufficient, yet enough coherent flux remains to allow real-time correction. I then show that this regime, long considered inaccessible, can now be addressed with existing technologies. Specifically, I demonstrate that current modulators, detectors and control architectures are capable of tackling the fundamental constraints of coherence time and photon-per-mode budget. This opens a new window for imaging and focusing through rapidly evolving complex environments.

In the geometric optics approximation typical of far fields (vanishing wavelength), wavefronts emerge as constant eikonal surfaces.

The electric (E) and magnetic (H) field vectors are mutually perpendicular and tangent to the wavefront. They follow planar elliptical trajectories that reach their elliptical vertices simultaneously, "in phase".

Near a dipole, scatterer, or edge, however, the vanishing wavelength assumption fails.

Here, E and H behave differently.

They still follow planar elliptical trajectories, albeit with different phases, eccentricities, and planes.

We define (i) "vertex time" as the position dependent time when E or H reach their respective ellipse vertex,

and (ii) "phasefronts" as surfaces of constant electric or magnetic vertex time.

The spatial gradients of the vertex time are perpendicular to the E and H phasefronts, defining separate E and H phase velocity vector fields.

As we move into the far field, the E and H phasefronts converge to each other and to the wavefront, providing a quantitative measure of how much the local disturbance deviates from "far field".

Applying this concept to the field of a monochromatic point dipole, we find that, contrary to common assumptions, the dipole has no far field near its axis, no matter how far away.

Metasurfaces enable precise light manipulation, like fostering reflections close to unity, through resonance mechanisms. While traditional Bragg mirrors enable very high reflectivity they limit the achievable thermal noise. Meta-material-mirrors (MMM) can overcome the noise limitations but suffer from limited reflectivity. This trade-off is crucial for next-generation cryogenic gravitational wave detectors, such as the Einstein Telescope, which need high reflectivity and low thermal noise test mass coatings to achieve dramatic sensitivity. Hence, we are proposing a new combined design unifying the advantages from both approaches - composed of an MMM, a Fabry–Pérot spacer, and a Bragg mirror – achieving extremely high reflectance and low thermal noise. We are evaluating different 1D and 2D design approaches to achieve MMM robust to fabrication tolerances while offering broad, high reflection at 1550 nm. A key focus is on bandwidth, manufacturability, and thermal noise. This systematic analysis provides a pathway to promising MMM for production via e.g. character projection electron beam lithography, paving the way for high-performance mirrors in gravitational wave astronomy and beyond.

Metallic nano-objects play crucial roles in diverse fields, including biomedical imaging, nanomedicine, spectroscopy, and photocatalysis. Nano-objects smaller than 15 nm exhibit extremely low scattering cross-sections, posing a significant challenge for optical detection. An approach to enhance optical detection is to exploit nonlinearity of strong coupling regime, especially for elastic scattering, which is universal to all objects. However, there is still no observation of the strong coupling of elastic light scattering from nano-objects. Here, we demonstrate the strong coupling of elastic light scattering in self-assembled plasmonic nanocavities formed between a gold nanoprobe and a gold film. We employ this technique to detect individual objects with diameters down to 1.8 nm. The resonant mode of the nano-object in the nanocavity environment strongly couples with the nanocavity mode, revealing anti-crossing scattering modes under dark-field spectroscopy. The experimental result agrees with numerical calculations, which we use to extend this technique to other metals. Furthermore, our results show that scattering cross-section ratio of the nano-object scales with the electric field to fourth power, similar to surface-enhanced Raman spectroscopy. This work establishes a new possibility of elastic strong coupling and demonstrates its applicability for observing small, non-fluorescent, Raman inactive sub-15 nm objects, complementary to existing microscopes.

Phonon Polaritonic resonances, which arise from the coherent oscillations of atoms or molecules within materials, play a pivotal role in understanding and designing advanced materials with unique optical properties. In hexagonal boron nitride (hBN), these resonances are particularly prominent within the Reststrahlen bands—a frequency range where the material exhibits strong optical phonon modes. One of the most fascinating phenomena within these bands is the existence of Bound States in the Continuum (BICs). These states, despite lying within the continuum of radiative modes, remain perfectly confined without radiating energy, a property that has garnered significant interest for its potential in nanophotonic applications. Our investigation in the lower restrahlen band (LRB) of hBN ranges from 755 cm-1 to 814 cm-1 and focuses on deeply subwavelength polaritonic resonators. We exploit the fact that the polaritons in the LRB are strictly out of the plane, and therefore the fundamental radiative mode of the structure will be z-polarized and give the high-quality factored topologically protected BICs.