Light-matter interactions in condensed media at room-temperature are fundamentally limited by electron-phonon coupling. For instance, while the excitation cross-section of an isolated atom, or of a single quantum emitter at cryogenic temperatures, can reach one half of the wavelength of light squared (meaning that ~50% of incoming photons will interact for a diffraction-limited excitation); this value is reduced by 6-7 orders of magnitude for a fluorescent molecule or for a colloidal quantum dot at room temperature because of homogeneous phonon broadening. In order to render the exceptional optical properties of single quantum systems (such as single-photon emission and nonlinearities) efficiently accessible at room temperature and in condensed media, it is essential to enhance and optimize these interaction cross-sections. In this presentation, I will detail some of our recent work towards this goal. In particular, I will describe how DNA-based self-assembly can be used to introduce, in a deterministic way, a controlled number of quantum emitters in the nanoscale hot-spot of a plasmonic resonator. Using this approach, we can enhance single-photon emission from fluorescent molecules by more than two orders of magnitude in a weak-coupling regime (ACS Nano 10, 4806 (2016)). Using five organic molecules, it is also possible to reach a strong-coupling regime with a single dimer of gold nanoparticles (ACS Nano 15, 14732 (2021)). An alternative platform to plasmonics, in order to enhance light-matter interactions at room temperature, is the use of nanoscale optical resonators made of high-index dielectric materials such as silicon or gallium phosphide. I will discuss some of our recent work on the use of silicon resonators to enhance or inhibit spontaneous emission from electric or magnetic optical emitters (Phys. Rev. Applied 6, 064016 (2016) & Nano Lett. 18, 3481 (2018)); as well as the development of colloidal dielectric resonators to enhance quadratic or cubic nonlinear optical properties.

Full spatiotemporal resolution of the evolution of plasmonic fields is a major goal in plasmonics in order to investigate the buildup and decay of collective electron phenomena. Here, we demonstrate few-femtosecond probing of plasmon transients uniquely combined with nm-scale sensitivity. In this work, we use broadband femtosecond pulses made up of ~1.5 eV photons to generate photoelectrons in the strong fields of the laser focus. These electrons then probe the optical near-fields in a time-resolved and spatially highly selective manner, due to the fact that photoemission is much more probable, and the electrons can gain higher energy, where the local fields are high. In contrast to previous experiment, by filtering for a certain kinetic energy range of photoemitted electrons, we can limit the measurement for rescattering electrons resulting in a sub-nm surface sensitivity and selectivity for plasmonic hot spots. To realize this novel concept, we illuminate plasmonic nanoparticles with time-delayed replicas of few-cycle pulses of a femtosecond laser oscillator, resulting in electron emission. The photoelectron spectra were recorded for a set of delays of the interferometer arms. By filtering for the highest electron kinetic energies, we could record autocorrelation functions of the hot-spot field evolution of plasmonic nanostructures. Through the analysis of the resulting energy-resolved autocorrelation traces, we could demonstrate that even the plasmon oscillation decay after the ultrafast excitation is in the sub-10-fs range under these extreme conditions. By establishing this ultrafast time-resolved characterization technique, the buildup and decay of collective electron oscillations can be investigated with unprecedented spatiotemporal resolution and plasmonic nanoparticles can be tailored for ultrafast optics applications such as near-field-enhanced high harmonic generation, near-field spectroscopy etc.

Optically-induced magnetism has drawn considerable interest in the past years for its ability to speed up magnetic processes. For example, static magnetic fields have been demonstrated to be generated in non-magnetic plasmonic (gold) nanoparticles and nano-apertures. Using a simplified hydrodynamic model of the free electron gas of metal, we theoretically investigate the IFE and resulting optomagnetism in a thin gold film as well as in axis-symmetric plasmonic nanostructures under illumination with various focused light. The resulting static magnetic field is found to be maximum and dramatically confined at the corners and edges of the plasmonic structures, which reveals the ability of metallic discontinuities to concentrate and tailor static magnetic fields on the nanoscale. Plasmonics can thus generate and tune static magnetic fields on the nanoscale, potentially impacting small-scale magnetic tweezing and sensing as well as the generation of magneto-optical effects and spin-waves.

To achieve quantum computing on chip, the chip-scale photonic quantum technologies indispensably request efficiently coupling and guiding spontaneous emission from solid-state quantum emitters (QEs) as central blocks for high-quality single-photon sources. Here, we experimentally demonstrate a deterministically positioned nano-diamond (ND) as QE which is manipulated on chip via Archimedean spiral gratings to spontaneously trigger single photons carrying spin and orbital angular momentum superposition states.

Recently, light absorption in optical nanoantennas has attracted growing interest. In particular, hot carrier generation in metallic (plasmonic) nanostructures and thermo-optical effects in dielectric Si and Ge nanoresonators offer novel opportunities for light-harnessing and energy conversion devices. Here, we report the construction, optoelectronic and photoelectrochemical characterization of plasmon-driven photodiodes and photocathodes based on a metal/p-type gallium nitride (p-GaN) heterostructure that operate within the visible regime via hot-hole injection. Next, we discuss how self-induced optical heating in Si and Ge nanoresonators affects their optical response and how it could be employed in optical devices and metasurfaces.

I will report on a switchable time-varying mirror, composed of an ITO-Au stack, which can be efficiently modulated in time, by a driving ultrafast laser pulse, with over a ten-fold increase in reflectivity and a bandwidth increase of the reflected pulse to 31 THz. This temporal response is unbounded by the pump pulse bandwidth and originates from the shortening of the response time of the mirror beyond saturation. Moreover, I will show the dissipative self-organisation of programmable random lasers from the reversible out-of-equilibrium self-assembly of colloids. They can thus reconfigure and cooperate by emulating the ever-evolving spatiotemporal relationship between structure and functionality typical of living matter.

The ongoing drive towards the miniaturization of nonlinear optics is motivated by the number of functionalities it could bring about in integrated devices, such as frequency conversion, information processing, and holography. The main hindrance to the technological deployment of nonlinear optics is the perturbative character of nonlinear interactions, whose intrinsic weakness is compounded at the nanoscale by the small volume of matter involved. The exploitation of the resonant electromagnetic modes supported by nanostructures in order to increase the strength of light–matter coupling is therefore a thriving area of research. In this work, we investigate gold dimers, where a V-antenna tuned to resonate at a telecom frequency ω (corresponding to a 1550 nm wavelength) is electromagnetically coupled to a rod resonating at 2ω. The structures were patterned lithographically out of a 40 nm-thick monocrystalline gold flake with a focused-ion (Ga) beam. In our experiment, a pump pulse at ω is mixed with its frequency-doubled replica, resulting in SFG at frequency ω+2ω = 3ω. A thorough experimental characterization discloses a rich phenomenology as the SFG is ruled by the resonant response. Such sensitive dependence on the geometry of the system and the excitation is unraveled through a systematic comparison to full-vectorial numerical simulations. In our dimers, on the one hand the doubly resonant design boosts both pumps of the SFG; on the other hand, the non-centrosymmetric shape allows interference of SFG with third-harmonic generation (THG), which also occurs at ω+ω+ω = 3ω. Delay traces of the signal at 3ω exhibit strong interference fringes (power modulation above 400%) with a 2ω periodicity. The small dephasing between pumps required to achieve such sizable modulation can be imparted by either mechanical or electro-optic means; along with the instantaneous (electronic) character of the materials response, this suggests enticing perspectives for ultrafast modulation and coherent control.

The study of nanoantenna arrays has been a major research subject since the improvement of nanofabrication techniques allowed the design of subwavelength antennas, which proved to have new and interesting optical properties. Negative or near zero index behavior, extraordinary optical diffraction or transmission, huge field enhancement, the paths opened by these new optical devices are very diverse. Our team specialises in the study of Infrared Metal-Insulator-Metal resonators, for which the most important applications are telecommunications and detection. In this work, we have specifically studied the effects of positional disorder on the optical behavior of such nanoresonator arrays. The general question we strive to answer is : how are the different loss channels (outgoing fields and absorption) modified between a periodic array and a disordered array? Therefore, we use numerical simulations and caracterisation techniques to quantify precisely the difference in reflectance, diffraction and scattering when periodic arrays are disordered. We also take into account the different forms that disorder can have, whether it is a perturbation of a periodic array or an independent set of random positions. So far, we have shown, both numerically and experimentally, that in the case of disordered nanoresonators, scattering is a very important loss channel. Indeed, the total scattered power in a disordered array is equivalent to the total diffracted power in a periodic array of the same resonators. Since resonator arrays can be optimised to have an extraordinary optical diffraction, i.e. a (possibly much) larger diffracted power than the specularly reflected power, our work shows that very highly scattering surfaces can be designed. We are currently working on more complex arrays, especially with more than one type of resonator, in order to study the interplay of order, disorder, and coupling between resonators.

Cold spots are dark sub-wavelength pockets which might emerge near a nanoantenna, should the components of some far-field illumination cancel out with scattered light. Similar regions of zero electric are applied to diffraction limit-beating microscopy techniques, such as MINFLUX, and commonly delivered in two-dimensional form by a doughnut beam. An illuminated nanoantenna could be the fulcrum for ultra-fast, remote, and non-mechanical control of a more confined, three-dimensional zero in the form of a cold spot. Though previously explored in the literature, the current extent of cold spot position control is an ON/OFF switching behaviour between a few well-known locations or involves a physical change to the nanoantenna shape. We show that by changing only the polarisation, amplitude, and phase of two plane waves, a unique, zero-magnitude and highly sub-wavelength cold spot can be created and moved anywhere in the space around a nanoantenna of any arbitrary shape. The technique exploits the linearity of Maxwell's equations and could be adapted to manipulate any phenomena governed by the linear wave equation, including acoustic scattering. This is a means for ultra-fast, remote, and non-mechanical sub-wavelength electric field manipulation.

Telecom-frequency, silicon-based impurities embedded in dielectric Mie resonators towards directional emission Abstract: Sub-micrometric, dielectric objects featuring high permittivity and reduced absorption losses enable for efficient light management, potentially enhancing and extending the performances of opto-electronic devices [1]. The resonant scattering supported by individual dielectric antennas is generally rather broad [2] (50-100 nm for the fundamental Mie resonances at visible and near-infrared frequency). Common materials for the implementation of these devices are IV-IV-based compounds. As such, light emission in these systems has been elusive so far. Here we use low-resolution optical lithography and plasma etching joined with solid state dewetting of crystalline, ultra-thin silicon on insulator (c-UT-SOI) to form monocrystalline, atomically-smooth, silicon-based Mie resonators in well controlled large periodic arrays. By using ion implant of carbon followed by high temperature annealing and proton implant, we activate telecom frequency, light emitting G-centers (complex impurities) within the Si-based antennas. We engineer the light emission by tuning carbon dose, beam energy and islands size in order to optimize the coupling between the emitters and the Mie resonances. These results are relevant for the fabrication of quantum light emitters with high directivity exploiting individual G-centers embedded in sub-micrometric antennas. References: 1. T. Liu, R. Xu, P. Yu, Z. Wang, and J. Takahara, “Multipole and multimode engineering in mie resonance-based metastructures,”Nanophotonics9 (5), 1115–1137(2020). 2. T. Coenen, J. van de Groep, and A. Polman, “Resonant modes of single silicon Nano cavities excited by electron irradiation,”ACSNano7 (2), 1689–1698(2013).

Optical nonlinearities are known to coherently couple the amplitude and the phase of light, which can lead to the formation of perfectly periodic waveforms – known as frequency combs. Recently, self-starting frequency combs that do not rely on the emission of short pulses are appearing in numerous semiconductor laser types, among which is the quantum cascade laser. This novel type of combs is gaining vast attention from researchers due to their self-starting nature and compactness, making them an ideal platform for further development of spectroscopic applications. Their spontaneous formation was explained through an interplay of phenomenological nonlinearity and dispersion in the laser active region, although the actual physical processes remained unclear until now. Here we show that Bloch gain – a phenomenon described by Bloch and Zener in the 1930s – plays an essential role in their formation. We demonstrate that a Bloch gain contribution is present in any quantum cascade laser and becomes particularly dominant under saturation. Bloch gain in QCLs with ultrafast gain recovery induces a giant Kerr nonlinearity, which is two orders of magnitude larger than the bulk values. The resonant Kerr nonlinearity provides coherent coupling between the amplitude and the phase of the laser field, which serves as a locking mechanism for frequency comb operation. We show that in Fabry-Pérot QCLs this results in frequency-modulated combs with a linear frequency chirp. In ring cavity QCLs, the Bloch gain is able to induce a single-mode instability by tuning the laser in the phase turbulence regime. This can lead to the formation of locked spatial patterns that are related to dissipative Kerr solitons, paving the way towards electrically pumped Kerr combs.

The engineering of efficient photodetectors with a nonlinear response to light is an important task for modern optoelectronics because this allows the fabrication of fully light-controlled devices. The two-photon absorption (TPA) phenomenon allows 3D confinement of the excitation and thus, hence, the fabrication of 3D arrays of light-sensitive elements. Moreover, this type of excitation ensures the control of the response efficiency by tuning the intensity of the light. Current technologies mainly use photodiodes based on bulk semiconductors, but the use of colloidal semiconductor quantum dots (QDs) has more advantages due to ease of processing, precisely-controlled optoelectronic properties, quick response, and compatibility with flexible substrates. Moreover, QDs have very high two-photon absorption cross-sections in a wide spectral range in the infrared region. Nevertheless, to activate such photodetectors in the nonlinear optical mode, peak excitation intensities higher than GW/cm^2 are still required. To reduce the excitation intensity it is needed to further increase the TPA. To achieve it we designed plasmon–exciton hybrid material using plasmon nanoparticles (PNPs) with an absorption peak overlapped with excitation wavelength. We found that the effective TPA in QDs near PNPs increased by the order of magnitude due to the nonlinear energy transfer from plasmons in PNPs to excitons of QDs. Finally, we designed a near-infrared plasmon–exciton photodetector based on the designed plasmon-exciton hybrid material. We investigated the effect of plasmons on the photoresponse of the photodetector under near-infrared two-photon excitation of QDs. As a result, we have found that the photocurrent was up to 38 times higher in the detector filled with QD–PNP plasmon–exciton hybrid material than in the detector filled with QDs alone. This study was supported by the Russian Science Foundation, grant no. 18-72-10143-П. V.K. has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie, grant agreement no. 101025664 (QESPEM).

Weyl semimetals are a group of materials with a single touching point of valence and conduction bands and a linear electron dispersion in the adjacent electronic bandstructure, giving rise quasi-massless electrons and topologically protected surface states. However, previous works on Weyl semimetals were largely limited to bulk material and therefore limited in means of flexibility and on-chip device applications. Our work demonstrates first steps towards nanophotonics applications with Weyl semimetals by thoroughly characterizing the linear and nonlinear optical properties of recently realized epitaxial grown thin films of the prototypical Weyl semimetal niobium phosphide (NbP). Despite the dominant absorption over the entire spectrum, pronounced third-harmonic generation could be detected, reaching a maximum efficiency of 10-4 %, being orders of magnitude higher than for common dielectric materials. Furthermore, non-degenerate pump-probe measurements with 10 fs pulses were performed to investigate the ultrafast electronic and optical response of bulk NbP crystal and NbP thin film. For the first, absorption effects and subsequent scattering processes dominate the ultrafast response and show the characteristics of the linear dispersion regime in the electronic bandstructure. On the other hand, the NbP thin films reveal a response that is generated by the nonlinear optical Kerr effect and gives an ultrafast modulation depth of close to 1 % and with a complete relaxation within 100 fs. These findings would allow all-optical switching based on NbP with full-width half maximum bandwidth of 10-20 MHz. Altogether, our results show that the epitaxial grown NbP thin films inherit the optical properties of the bulk crystal and further boost the nonlinear optical interactions. Combined with the possibility of common top-down fabrication techniques enabled by thin films, this paves the way for future applications of nanophotonic devices based on the promising properties of Weyl semimetals.

Metal nanostructures are largely used to enhance light-matter interactions and the overarching optical response by means of plasmonic resonances and surface-enhanced spectroscopy. They can also be designed to improve efficiency of nonlinear optical processes like second and third harmonic generation, despite their high absorption in the visible and near infrared range. When the metallic layer thickness is reduced to a few nanometers in thickness, light-matter interactions can display new phenomena where conventional approximations may not always be applicable. While planar structures are generally the simplest to fabricate and easier to simulate, the efficiency of the harmonic signals are generally quite small but can be enhanced when resonant nanostructures are implemented. In this work we design a gold nanograting with resonant features around 800nm. When the fundamental beam is tuned around the resonance wavelength in the near-infrared region we obtain a second and third harmonic signal tuned in the opaque region of gold. We firstly measure TM and TE components of the SH signal at 400 nm as a function of the fundamental beam polarization, tuning the fundamental wavelength around the resonance. Then we experimentally estimate the relative enhancement induced by the grating with respect to SHG from planar gold, finding a maximum enhancement of 800 for the central resonant wavelength. We calculate the predicted harmonic conversion efficiencies of the grating employing our microscopic hydrodynamic approach to model light-matter interactions. This model relies on temporal and spatial derivatives and mere knowledge of the effective electron mass to determine the relative magnitudes of surface and volume. Our simulations predict an enhancement factor close to 1000, of the same order of magnitude as our measurements. The same model predicts a 500-fold enhancement for the THG with respect to the plain gold layer.

This Conference Presentation, “Rare-earth ions-doped ferroelectric nanocrystals for optical sensing of electric potential in biological systems,” was recorded at Photonics Europe 2022 held in Strasbourg, France.

This Conference Presentation, “Nanophosphor-based photonic architectures for efficient light conversion,” was recorded at Photonics Europe 2022 held in Strasbourg, France.

Cancer is one of the most serious threats to human health, not only because of the frequency of disease but also because of the severe side effects experienced by the patience during the chemotherapy and radiotherapy treatments, such as immunosuppression and drug resistance. Surgery, as a treatment method, does circumvent some of the side effect issues; however, it is highly invasive and not always possible. In this respect, Photodynamic Therapy (PDT), which localizes the harmful effects of the sensitizer to areas exposed to radiation, has attracted considerable interest as an alternative, minimally invasive treatment, potentially offering no long-term side effects. In search for PDT agents, conjugated polymers nanoparticles (CPNs) have proven themselves to be a versatile class of materials, with many advantageous properties for biomedical applications. Here, we report the development of CPNs with absorption and emission bands in the 1st near-infrared (NIR) biological transparency window, for combined NIR bioimaging and PDT application. We show that the synthesis procedure of the CPNs can be optimised to achieve CPNs of highest possible fluorescence quantum yield and singlet oxygen production, by varying the ratio of the conjugated polymer and stabilizing copolymer in the precursor solution, as well as changing its pH. We further demonstrate the feasibility of our CPNs for the combined NIR bioimaging/PDT applications on a range of different cancerous and normal cell lines. Furthermore, we show that by modifying our CPNs with a tumour-specific ligand, specific cancerous cell lines can be targeted.

By embedding quantum emitters, such as quantum dots, within nanophotonic structures we have gained unprecendented control over their properties and over the photons with which they interact. Yet this control comes at a price, namely the unwanted interaction of the emitters with the noisy solid-state environment of the photonic circuits. In this talk, I will address different ways in which we attempt to address this challenge, focusing on the resultant increased coherence of quantum light-matter interactions. I will show how these improvements allow us to route photons on integrated chips and begin to use nonlinear scattering events to reshape photon wavefunctions. In this discussion, the way that these capabilities may lead to novel quantum technologies will be highlighted.

Planar nanostructures, both dielectric and plasmonic, provide exceptional light manipulation at the subwavelength scale. Here, we introduce an out-of-plane nanohole-based nanostructure design with the implementation of a unique self-rolling technique.

Acoustic vibrations of nanoparticles have raised a significant interest due to the potential to study mechanical phenomena at nanometer length scales. We investigated the transient optical response of multilayer spherical plasmonic nanoparticles formed by alternating layers of gold and silica. The nanoparticles provide a platform for a broadband nonlinear optical response from visible to near-IR spectral ranges as well as a tunable optomechanical system in which different acoustic modes can be selectively switched on/off tuning the excitation wavelength. The nonlinear response monitored through the transient absorption spectra, follows the linear optical properties of the nanoparticles with the transient signal decay relatively independent of the pump wavelength in the visible (514 nm) and infrared (1028 nm). At both pump wavelengths, more than one vibration mode is excited on the nanoparticles. The numerical modelling allows differentiating the core and shell vibrations excited at different pump wavelengths. In addition to two vibration modes which can be excited at both pump wavelengths, and related to the shell vibrations, the acoustic vibration existed by the 514 nm pump illumination can be assigned to the nanoparticle core. The observed behaviour shows that different vibrational modes of the nanoparticle can be independently optically excited by tuning the wavelength of the excitation. This observation not only expands the knowledge about the internal structure of composite plasmonic nanoparticles but also allows for the additional degree of freedom to control their optical and mechanical properties.

The integration of nano-emitters into plasmonic devices with spatial control and nanometer precision has become a great challenge. In this paper, we report on the use of a smart polymer for selectively immobilizing nano-emitters on specific preselected sites of gold nanocubes (GNC). The cunning use of the polymer is twofold. First, it records both the selected site and the future emitters-GNC distance through plasmon-assisted photopolymerization. Second, because the polymer is chemically functionalized, it makes it possible to attach the nano-emitters right at the preselected polymerized sites which subsequently “recognize” the nano-emitters to get attached. Since the resulting active medium is a spatial memory of specific plasmonic modes, it is anisotropic, making the hybrid nanosources sensitive to light polarization. The ability to adjust their statistical average lifetime by controlling the thickness of the nanopolymer is demonstrated on two kinds of nano-emitters coupled to GNC: doped polystyrene nanospheres and semiconductor colloidal quantum dots.

This Conference Presentation, “Guiding light with surface exciton-polaritons in atomically thin superlattices,” was recorded at Photonics Europe 2022 held in Strasbourg, France.

We demonstrate a non-Hermitian system by introducing topological dynamics in coherent laser arrays. It consists of a large set of mutually interacting optical vortices subject to arbitrary topological transformations realized by metasurfaces in a closed environment. Like atoms in a crystal, the vortices are arranged in a regular lattice producing Bragg diffraction peaks. Prior art focussed on demonstrations of single vortex sources employing concepts from non-Hermitian and topological photonics to add charge reconfigurability and to mitigate manufacturing fault effects. We extrapolate these concepts to vortex arrays that demonstrate tunability and robustness akin to those of topical single vortex sources.

Material surfaces irradiated with laser radiation under specific conditions can lead to laser-induced periodic surface structure (LIPSS) formation. The formation of these surface structures is a robust one-step method that allows for high throughput and cheap surface nano-structuring with spatial periods that can be greatly lower than the irradiating laser wavelength. Typically, a structural size ranges from several micrometers down to less than 100 nm and the structures strongly correlate with the polarization direction of the light used for their formation. The regularity of LIPSS formation depends on various laser and material parameters. Some metals can exhibit highly regular LIPSS (HR-LIPSS) when the correct conditions are met. One of the main drawbacks of precise LIPSS texturization is the difficulty in controlling the ripple quality. Thus, understanding the processes which lead to HR-LIPSS formation is one step in making laser-based material texturization more prevalent. The aim of this research was to create highly regular surface structures and investigate how the ripple formation depends on various laser and material properties. The experiment was carried out using femtosecond laser systems. Different laser harmonics modules were used for the LIPSS formation to cover both the IR and UV wavelengths. Structures were formed on various steel, copper, aluminum, and titanium samples employing different laser wavelengths, pulse energies, pulse overlaps, and polarizations. The resulting surface structures were then examined both visually and with the help of a scanning electron microscope (SEM). LIPSS images taken by SEM were then digitally processed to calculate the distribution of LIPSS orientation angle (DLOA) which characterizes the regularity of the surface structures. We show that by using ultraviolet femtosecond laser sources, choosing the right pulse energy, pulse overlap values, and sufficiently sharp focusing lens, HR-LIPSS can be formed on metals for which highly regular ripples don’t form using infrared laser radiation.

Modern optical technologies require the miniaturization and planarization of the optical devices that leads to an urgent need for efficient control of localized light. In this aspect, surface electromagnetic waves on two-dimensional structures and materials attract a lot of attention, and can potentially become the main information carriers in planar data processing optical systems. In this work, for the first time, we investigate theoretically, numerically and experimentally the properties of surface waves at the self-complementary metasurfaces obeying Babinet’s duality principle. The main features demonstrated in this manuscript are: (1) All-frequency hyperbolicity of the self-complementary metasurface. This feature is in sharp contrast to the artificial resonant two-dimensional hyperbolic structures where the hyperbolic regime is defined by the spectral width between the resonances. (2) Extremely canalized propagation of surface waves that can be switched between orthogonal directions by small frequency detuning. First, we demonstrate the plasmon canalization with a record minimum divergence. Second, we show the routing of canalized surface wave. (3) The polarization degeneracy of the surface waves supported by a self-complementary metasurface. The spectrum of any isotropic medium is always double-degenerate with respect to polarization, but for surface waves the polarization degeneracy is not fulfilled. In this work, we explicitly demonstrate the polarization degeneracy of TE- and TM-polarized surface waves paving a way to a number of applications from planar polarizer of surface waves to polarization demultiplexer. The results obtained open new opportunities for the planar optical and photonic devices, optical data transfer and processing systems, and antennas applications.

Traditional design methods (those in which a designer proposes and refines a solution) are rapidly being superseded by algorithmic alternatives. There, a computer is given only a design goal and a set of manufacturing constraints and is allowed to create any permissible solution that does the job. In nanophotonics, this is now routinely applied to photonic structures such as waveguides and demultiplexers, where the design goal could be, for example, maximised transmission at a particular wavelength. A step towards extending this to the quantum realm of single-photon interactions with atomic systems was recently taken, in which the ideas of macroscopic quantum electrodynamics are mixed with those of adjoint optimisation. In this talk, I will introduce the formalism of quantum electrodynamical inverse design, before showing a few recent proof-of-principle applications to processes such as resonant energy transfer.

From financial markets and neuronal activity in the brain to the way forest fires and diseases spread, the dynamics of complex systems are governed by critical events and emergent phenomena that are exceedingly difficult to understand or predict from underlying principles. We recently discovered that a gas of ultracold atoms continuously driven to strongly-interacting Rydberg states by an off-resonant laser field displays all the hallmarks of complex systems dynamics in a highly-controllable experimental system: (i) At early times we observe rapid growth of Rydberg excitations that has a striking correspondence with the spreading of diseases empirically observed in epidemics [1]; (ii) At later times we find that the system evolves toward a self-organised critical (SOC) state [2], a phenomenon that has been conjectured to explain the abundance of scale-invariant systems found in nature. I will discuss how these experiments can be understood in terms of an emergent atomic network that bridges the gap between mathematical models and empirical observations. This provides the opportunity to identify general principles governing non-equilibrium dynamics and to learn how seemingly universal properties emerge from microscopic physical details. References: [1] T. M. Wintermantel et al., Epidemic growth and Griffiths effects on an emergent network of excited atoms. Nature Communications 12, 103 (2021) [2] S. Helmrich et al., Signatures of self-organized criticality in an ultracold atomic gas. Nature 577, 481-486 (2020)

Controlled manipulation and trapping of submicron-size particles has many applications in different research fields, including those in the general areas of biology and soft-condensed matter physics. Optical tweezers that make use of strongly focused laser beams are widely used for this purpose. However, their trapping abilities are substantially limited by diffraction and a lack of scalability. To overcome this, nanostructures made of plasmonic materials have attracted significant attention, as their ability to concentrate energy to very small dimensions can be exploited to generate optical traps capable of acting on nanometer-size objects. Furthermore, and when compared to conventional systems, these plasmonic traps also provide large field enhancements that allow for lower input powers. Despite such advantages, these techniques still lack the ability to provide the controlled manipulation of the trapped objects over long distances. In this work we present a Brownian ratchet, based on plasmonic interactions, which can optically trap and manipulate dielectric nanometer-sized beads over long distances. For this purpose, the geometries of the plasmonic ratchets and the respective electric fields were modelled with COMSOL Multiphysics, and the optical forces experienced by the beads were calculated with COMSOL Multiphysics and processed with MATLAB®. Additionally, we experimentally demonstrate the rectification of the random thermal motion of subwavelength dielectric beads into one specific direction by periodically turning on and off a laser beam that illuminates the plasmonic nanostructure array and exploiting the asymmetries in the system.

Energy transfer between atoms and molecules mediates numerous elementary processes. During the talk, I will present a recently discovered long range high energy transfer mechanism, namely the Interatomic Coulombic Electron Capture (ICEC). ICEC is an environment assisted process in which a free electron can efficiently attach to an ion, atom or molecule. In ICEC, the excess energy of the electron attachment is transferred to a neighbour which is thus ionised. ICEC is an ultrafast process driven by the electronic correlation between two quantum objects. I will first give an overview on our current knowledge on this relevant process. I will then summarise the theoretical development and the studies our group has recently made to push forward our understanding of ICEC.

The large electric dipole moments associated with transitions between Rydberg states of atoms or molecules, make them ideal model systems with which to study Förster resonance energy transfer in collisions with other Rydberg atoms or molecules, or with polar ground-state molecules [1,2]. The sensitivity of high Rydberg states to electric fields allows these energy transfer processes to be tuned thorough resonance by exploiting the Stark effect. In this talk, I will describe recent experiments in which we have observed the resonant transfer of energy from the ground-state inversion sublevels in ammonia, to helium atoms in triplet Rydberg states with principal quantum numbers between 38 and 43. The experiments were performed at translational temperatures below 1 K in seeded, pulsed supersonic beams [3]. Electric fields up to ~10 V/cm were used to tune transitions between selected Rydberg states into resonance with the transitions between the inversion sublevels. Resonant energy transfer was identified by Rydberg-state-selective electric-field ionisation. The experimental data exhibit quantum-state-dependent resonance widths that range from 200 to 700 MHz and, in general, reduce as the value of the principal quantum number is increased. Comparison with the results of numerical calculations of the Rydberg energy-level structure and transition moments, shows a dependence of the measured widths on the electric dipole transition moments of the Rydberg-Rydberg transitions. These results are of particular interest, e.g., for applications in coherent control and quantum sensing [4-6], and in the study of low-temperature ion-molecule reactions performed within the orbit of a Rydberg electron [7]. [1] T. F. Gallagher, Phys. Rep. 210, 319 (1992) [2] F. B. Dunning, S. Buathong, Int. Rev. Phys. Chem. 37, 287 (2018) [3] K. Gawlas and S. D. Hogan, J. Phys. Chem. Lett. 11, 83 (2020) [4] M. Zeppenfeld, Euro. Phys. Lett. 118, 13002 (2017). [5] E. Kuznetsova, S. T. Rittenhouse, H. R. Sadeghpour, and S. F. Yelin, Phys. Rev. A 94, 032325 (2016). [6] E. Kuznetsova, S. T. Rittenhouse, H. R. Sadeghpour, and S. F. Yelin, Phys. Chem. Chem. Phys. 13, 17115 (2011). [7] K. Höveler, J. Deiglmayr, J. A. Agner, H. Schmutz and F. Merkt, Phys. Chem. Chem. Phys. 23, 2676 (2021).

Advances in nanoscience and material science point towards a new era when materials with unprecedented properties will be tailor-made for target applications. For example, metal-organic frameworks (MOF) are a notable class of molecular materials made by growing scaffold-like crystalline structures containing metallic ions and organic molecules. Some envisioned optical applications of MOFs are enhancement of luminescence, second harmonic generation, Raman scattering, and infrared sensing. For the full realization of their potential, the spectacular advances in fabrication techniques need to be matched by accurate and computationally efficient theoretical models. Such models should, on the one hand, underpin the computer-aided design of new materials aiming at substituting costly experimental comparisons of different alternatives. On the other hand, the theoretical tools should support the analysis and interpretation of experimental data. The multi-scale and multi-disciplinary characters of the problem makes the theory challenging. Physics and chemistry are intertwined together across spatial scales ranging from nanometers to centimeters. For example, there is a huge scale gap between the molecular unit cells of a MOF scaffold (~ 1 nanometer), and macroscopic systems such as MOF films that are hundreds of nanometers in each dimension. Additional complexity is added when the materials are placed inside an optical cavity for enhancing a particular light-matter interaction effect. The cavity modes are yet another degree of freedom amenable to optimization beyond Fabry-Perot configurations, and, when the interaction is strong enough, hybrid light-matter modes known as polaritons will form inside the cavity: The optical properties of the joint system can differ significantly from the separate responses of cavity and materials. In my talk, I will explain a new approach [1] to rigorously compute the joint optical response of molecular materials inside cavities. Using a recent connection between quantum-chemistry and Maxwell scattering theory [2], the output of ab initio DFT and TD-DFT calculations can be fueled into Maxwell solvers. When the molecules are either spatially disordered or forming a periodic arrangement, the response and modes of the joint molecules+cavity system can be very efficiently computed using a T-matrix based algorithm [3]. I will show how this approach allows a refined interpretation of recent experimental measurements involving strong light-matter coupling of a MOF inside a Fabry-Perot cavity [4]. We are convinced that our approach will be a very useful tool for the design and verification of the optical properties of new molecular materials. [1] B. Zerulla, M. Krstic, D. Beutel, C. Holzer, C. Wöll, C. Rockstuhl, and I. Fernandez-Corbaton. "Computing the joint optical response of molecular materials inside optical cavities", In preparation. [2] I. Fernandez-Corbaton, D. Beutel, C. Rockstuhl, A. Pausch, and W. Klopper. "Computation of Electromagnetic Properties of Molecular Ensembles", ChemPhysChem, 21, 9, 878 (2020) [3] D. Beutel, A. Groner, C. Rockstuhl, and I. Fernandez-Corbaton, "Efficient simulation of biperiodic, layered structures based on the T-matrix method", J. Opt. Soc. Am. B, 38, 6, 1782-1791 (2021) [4] R. Haldar, Z. Fu, R. Joseph, D. Herrero,L. Martı́n-Gomis, B.S. Richards, I. A. Howard, A. Sastre-Santos and C. Wöll. "Guest-responsive polaritons in a porous framework: chromophoric sponges in optical QED cavities", Chem. Sci., 2020, 11, 7972.

We predict the existence of a new type of in-plane propagating exciton-polaritons that are supported by monolayer transition-metal-dichalcogenides (TMDs), which can carry large momentum in the visible spectrum. This 2D exciton-polariton (2DEP) is a result of the coupling between the electromagnetic light field and collective oscillations of excitons in TMDs. We experimentally show that the conditions required for the excitation of the 2DEP are attainable if the TMD is encapsulated with hexagonal-boron-nitride (hBN) and is cryogenically cooled. In addition, a comparison of the properties of the 2DEP with those of surface-plasmon-polaritons at the same spectral range reveals that the 2DEP exhibits two orders-of-magnitude larger wavelength confinement.

Modern techniques for studying and controlling light scattering, such as wavefront shaping (WFS), rely heavily in the interference of scattered waves. In WFS, the intensity of light is optimized, which is proportional to the absolute square of the scattering amplitude. Recently, we introduced a new type of wavefront called Mutual Extinction (ME), where the light extinction is optimized [1]. In contrast with the intensity, the light extinction is directly proportional to the scattering amplitude. Therefore, ME gives information about complex media that WFS cannot access. In our experiments, we study various strongly scattering samples such as a silicon bar and a human hair. We illuminate the sample using two crossing beams. With ME we control how much light is extinguished inside the sample, making it more opaque or more transparent [2]. For small angles, this allows us to almost double the total extinction (object twice as opaque) or nearly inhibit it (object fully transparent). The modulation range of the total extinction depends on the relative angle and phase between the incident beams, which yields information about the amplitude and phase of the scattering amplitude, respectively. Furthermore, the angle dependency of the total extinction give us information about the overall shape of the object. In this work, we show how to extract this new information from our experiments and study the application of this new technique. [1] A. Lagendijk, A. P. Mosk and W. L. Vos, "Mutual extinction and transparency of multiple incident light waves," EPL 130 34002 (2020) [2] A. Rates, A. Lagendijk, O. Akdemir, A. P. Mosk, and W. L. Vos, “Observation of mutual extinction and transparency in light scattering,” Phys. Rev. A (2021), in press.

We present a method to determine the complex coupling parameter of a two-coupled-modes system by directly measuring the coupled eigenmodes rather than their eigenvalues. This method is useful because mode-mixing can be observed even if frequency shifts can not be measured. It also allows to determine the complex coupling parameter, from which we conclude that the observed coupling is mainly conservative. We observe mode-mixing in an optical microcavity, where the modes couple primarily at the mirror surface, as confirmed by AFM measurements. The presented method is general and can be applied to other systems to measure mode coupling more accurately and to determine the nature of the coupling.

Our study focuses on numerical simulations of the coupling between a photonic crystal cavity (PCC) and spherical dielectric nanoparticle (NP) presenting an electric dipole (ED). The PCC exhibits a confined mode at a resonance wavelength of about 1600nm. Using Mie theory, the optical properties of the nanoparticle are designed (radius and optical index) so that it exhibits an ED at the same wavelength ensuring efficient coupling with the PCC. 3D-FDTD homemade codes are used to simulate this coupling by quantifying the resonance properties (wavelength and Q factor) as a function of the PCC-NP distance. The obtained behaviors are completely different for the ED case revealing the possibility to control the value of the resonance wavelength by keeping a rather large value of the Q factor. Another original study has been performed to show the influence of the lateral position of the NP on the light extraction and emission direction of the PCC in terms of 3D far-field radiation pattern.

We introduce a nano-optical platform based on Bloch surface waves (BSWs) capable of exploiting the entire cleaved end facet of a multicore optical fiber. Interconnecting various fiber cores with BSWs directly at the end of a multicore fiber opens the perspective of highly compact complex optical functionalities for the design of “lab on fiber” devices. In counterpart, optical fibers provide a unique opportunity to obtain turnkey nano-optical functions addressing a vast application domain ranging from telecommunications to medical sensing. To show the full potential of our platform, we demonstrate a multiplexing function between three fiber cores.