arXiv subjects

Javier Aizpurua

Publications and source records attributed to Javier Aizpurua.

At least 37 records · Page 2Linked to original sources

arXiv2023

Identifying unbound strong bunching and the breakdown of the Rotating Wave Approximation in the quantum Rabi model

We use a recently derived gauge-invariant formulation of the problem of a two-level system coupled to an optical cavity, to explore the transition between the weak, and the ultra-strong coupling regimes of light-matter interaction. We explore this transition using the intensity correlations $g^{(2)}(τ)$ of the emitted light, and find strong, unbounded bunching of the emission from systems governed by the Rabi Hamiltonian. Surprisingly, this effect is observed not only in the ultra-strong coupling regime, but also for weakly coupled systems, where the Jaynes-Cummings Hamiltonian would predict the opposite, antibunched emission. This suggests that the higher-order correlations are a particularly sensitive probe of the divergence between the Jaynes-Cummings and Rabi Hamiltonians, and can serve as an indicator of the breakdown of the rotating wave approximation. Our findings indicate also that the boundary between the weakly, strongly, and ultra-strongly coupled dynamics, is much richer than currently accepted.

quant-ph↗

arXiv2023

Probing the electromagnetic response of dielectric antennas by vortex electron beams

Focused beams of electrons, which act as both sources, and sensors of electric fields, can be used to characterise the electric response of complex photonic systems by locally probing the induced optical near fields. This functionality can be complemented by embracing the recently developed vortex electron beams (VEBs), made up of electrons with orbital angular momentum, which could, in addition, probe induced magnetic near fields. In this work, we revisit the theoretical description of this technique, dubbed vortex Electron Energy-Loss Spectroscopy (v-EELS). We map the fundamental, quantum-mechanical picture of the scattering of the VEB electrons to the intuitive classical models, which treat the electron beams as a superposition of linear electric and magnetic currents. We then apply this formalism to characterise the optical response of dielectric nanoantennas with v-EELS. Our calculations reveal that VEB electrons probe electric or magnetic modes with different efficiency, which can be adjusted by changing either beam vorticity or acceleration voltage to determine the nature of the probed excitations. We also study a chirally-arranged nanostructure, which in the interaction with electron vortices produces dichroism in electron energy loss spectra. Our theoretical work establishes VEBs as versatile probes that could provide information on optical excitations otherwise inaccessible with conventional electron beams.

physics.optics↗

arXiv2023

Molecular-Induced Chirality Transfer to Plasmonic Lattice Modes

Molecular chirality plays fundamental roles in biology. The chiral response of a molecule occurs at a specific spectral position, determined by its molecular structure. This fingerprint can be transferred to other spectral regions via the interaction with localized surface plasmon resonances of gold nanoparticles. Here, we demonstrate that molecular chirality transfer occurs also for plasmonic lattice modes, providing a very effective and tunable means to control chirality. We use colloidal self-assembly to fabricate non-close packed, periodic arrays of gold nanoparticles, which are embedded in a polymer film containing chiral molecules. In the presence of the chiral molecules, the SLRs become optically active, i.e. showing handedness-dependent excitation. Numerical simulations with varying lattice parameters show circular dichroism peaks shifting along with the spectral positions of the lattice modes, corroborating the chirality transfer to these collective modes. A semi-analytical model based on the coupling of molecular and plasmonic resonances rationalizes this chirality transfer.

physics.optics↗

arXiv2023

Probing optical anapoles with fast electron beams

Optical anapoles are intriguing charge-current distributions characterized by a strong suppression of electromagnetic radiation. They originate from the destructive interference of the radiation produced by electric and toroidal multipoles. Although anapoles in dielectric structures have been probed and mapped with a combination of near- and far-field optical techniques, their excitation using fast electron beams has not been explored so far. Here, we theoretically and experimentally analyze the excitation of optical anapoles in tungsten disulfide (WS$_2$) nanodisks using Electron Energy Loss Spectroscopy (EELS) in Scanning Transmission Electron Microscopy (STEM). We observe prominent dips in the electron energy loss spectra and associate them with the excitation of optical anapoles and anapole-exciton hybrids. We are able to map the anapoles excited in the WS$_2$ nanodisks with subnanometer resolution and find that their excitation can be controlled by placing the electron beam at different positions on the nanodisk. Considering current research on the anapole phenomenon, we envision EELS in STEM to become a useful tool for accessing optical anapoles appearing in a variety of dielectric nanoresonators.

physics.optics↗

arXiv2022

Preservation and destruction of the purity of two-photon states in the interaction with a nanoscatterer

The optical resonances supported by nanostructures offer the possibility to enhance the interaction between matter and the quantum states of light. In this work, we provide a framework to study the scattering of quantum states of light with information encoded in their helicity by a nanostructure. We analyze the purity of the scattered output quantum state, and we find that the purity of the incident state can be lost, when it interacts with the optical resonances of the nanostructure. To explain the loss of quantum purity, we develop a physical picture based on time delays and frequency shifts between the output two-photon modes. The framework and analysis proposed in this work establishes a tool to address the interaction between quantum light and nanoenvironments.

quant-ph↗

arXiv2022

Optomechanical Effects in Nanocavity-enhanced Resonant Raman Scattering of a Single Molecule

In this article, we address the optomechanical effects in surface-enhanced resonant Raman scattering (SERRS) from a single molecule in a nano-particle on mirror (NPoM) nanocavity by developing a quantum master equation theory, which combines macroscopic quantum electrodynamics and electron-vibration interaction within the framework of open quantum system theory. We supplement the theory with electromagnetic simulations and time-dependent density functional theory calculations in order to study the SERRS of a methylene blue molecule in a realistic NPoM nanocavity. The simulations allow us not only to identify the conditions to achieve conventional optomechanical effects, such as vibrational pumping, non-linear scaling of Stokes and anti-Stokes scattering, but also to discovery distinct behaviors, such as the saturation of exciton population, the emergence of Mollow triplet side-bands, and higher-order Raman scattering. All in all, our study might guide further investigations of optomechanical effects in resonant Raman scattering.

physics.optics↗

arXiv2022

Hybrid photonic-plasmonic cavity design for very large Purcell factors at telecom wavelengths

Hybrid photonic-plasmonic cavities can be tailored to display high Q-factors and extremely small mode volumes simultaneously, which results in large values of the Purcell factor, FP. Amongst the different hybrid configurations, those based on a nanoparticle-on-a-mirror (NPoM) plasmonic cavity provide one of the lowest mode volumes, though so far their operation has been constrained to wavelengths below 1 μm. Here, we propose a hybrid configuration consisting of a silicon photonic crystal cavity with a slot at its center in which a gold nanoparticle is introduced. This hybrid system operates at telecom wavelengths and provides high Q-factor values (Q ${\approx} 10^{5}$) and small normalized mode volumes (Vm ${\approx} 10^{-4}$), leading to extremely large Purcell factor values, FP ${\approx} 10^{7}$ - ${10^{8}}$. The proposed cavity could be used in different applications such as molecular optomechanics, bio- and chemo-sensing, all-optical signal processing or enhanced Raman spectroscopy in the relevant telecom wavelength regime.

physics.optics↗

arXiv2021

Mapping Lamb, Stark and Purcell effects at a chromophore-picocavity junction with hyper-resolved fluorescence microscopy

The interactions between the excited states of a single chromophore with static and dynamic electric fields confined to a plasmonic cavity of picometer dimensions are investigated in a joint experimental and theoretical effort. In this configuration, the spatial extensions of the confined fields are smaller than the one of the molecular exciton, a property that is used to generate fluorescence maps of the chromophores with intra-molecular resolution. Theoretical simulations of the electrostatic and electrodynamic interactions occurring at the chromophore-picocavity junction are able to reproduce and interpret these hyper-resolved fluorescence maps, and reveal the key role played by subtle variations of Purcell, Lamb and Stark effects at the chromophore-picocavity junction.

cond-mat.mes-hall↗

arXiv2021

Many-Body Physics in Small Systems: Observing the Onset and Saturation of Correlation in Linear Atomic Chains

The exact study of small systems can guide us toward measures for extracting information about many-body physics as we move to more complex systems capable of quantum information processing or quantum analog simulation. We use exact diagonalization to study many electrons in short 1-D atom chains represented by long-range extended Hubbard-like models. We introduce a novel measure, the Single-Particle Excitation Content (SPEC) of an eigenstate and show that the dependence of SPEC on state number reveals the nature of the ground state, and the onset and saturation of correlation between the electrons as Coulomb interaction strength increases. We use this SPEC behavior to identify five regimes as interaction is increased: a non-interacting single-particle regime, a regime of perturbative Coulomb interaction in which the SPEC is a nearly universal function of state number, the onset and saturation of correlation, a regime of fully correlated states in which hopping is a perturbation and SPEC is a different universal function of state number, and the regime of no hopping. In particular, the behavior of the SPEC shows that when electron-electron correlation plays a minor role, all of the lowest energy states are made up primarily of single-particle excitations of the ground state, and as the Coulomb interaction increases, the lowest energy states increasingly contain many-particle excitations. In addition, the SPEC highlights a fundamental, distinct difference between a non-interacting system and one with minute, very weak interactions. While SPEC is a quantity that can be calculated for small exactly diagonalizable systems, it guides our intuition for larger systems, suggesting the nature of excitations and their distribution in the spectrum. Thus, this function, like correlation functions or order parameters, provides us with a window of intuition about the behavior of a physical system.

cond-mat.mes-hall↗

arXiv2021

Hybrid Photonic-Plasmonic Cavities based on the Nanoparticle-on-a-Mirror Configuration

Hybrid photonic-plasmonic cavities have emerged as a new platform to increase light-matter interaction capable to enhance the Purcell factor in a singular way not attainable with either photonic or plasmonic cavities separately. In the hybrid cavities proposed so far, mainly consisting of metallic bow-tie antennas, the plasmonic gap sizes defined by lithography in a repeatable way are limited to minimum values \approx 10 nm. Nanoparticle-on-a-mirror (NPoM) cavities are far superior to achieve the smallest possible mode volumes, as gaps smaller than 1 nm can be created. Here, we design a hybrid cavity that combines a NPoM plasmonic cavity and a dielectric-nanobeam photonic crystal cavity operating at transverse-magnetic (TM) polarization. The metallic nanoparticle can be placed very close (< 1 nm) to the upper surface of the dielectric cavity, which acts as a low-reflectivity mirror. We demonstrate through numerical calculations that this kind of hybrid plasmonic-photonic cavity architecture exhibits quality factors, Q, above 10^{3} and normalized mode volumes, V , down to 10^{\num{-3}}, thus resulting in high Purcell factors (FP \approx 10^5), whilst being experimentally feasible with current technology. Our results suggest that hybrid cavities with sub-nm gaps should open new avenues for boosting light-matter interaction in nanophotonic systems.

physics.optics↗

arXiv2021

Complex plasmon-exciton dynamics revealed through quantum dot light emission in a nanocavity

Plasmonic cavities can confine electromagnetic radiation to deep sub-wavelength regimes. This facilitates strong coupling phenomena to be observed at the limit of individual quantum emitters. Here we report an extensive set of measurements of plasmonic cavities hosting one to a few semiconductor quantum dots. Scattering spectra show Rabi splitting, demonstrating that these devices are close to the strong coupling regime. Using Hanbury Brown and Twiss interferometry, we observe non-classical emission, allowing us to directly determine the number of emitters in each device. Surprising features in photoluminescence spectra point to the contribution of multiple excited states. Using model simulations based on an extended Jaynes Cummings Hamiltonian, we find that the involvement of a dark state of the quantum dots explains the experimental findings. The coupling of quantum emitters to plasmonic cavities thus exposes complex relaxation pathways and emerges as an unconventional means to control dynamics of quantum states.

cond-mat.mes-hall↗

arXiv2021

Microcavity phonon polaritons from weak to ultrastrong phonon-photon coupling

Strong coupling between molecular vibrations and microcavity modes has been demonstrated to modify physical and chemical properties of the molecular material. Here, we study the much less explored coupling between lattice vibrations (phonons) and microcavity modes. Embedding thin layers of hexagonal boron nitride (hBN) into classical microcavities, we demonstrate the evolution from weak to ultrastrong phonon-photon coupling when the hBN thickness is increased from a few nanometers to a fully filled cavity. Remarkably, strong coupling is achieved for hBN layers as thin as 10 nm. Further, the ultrastrong coupling in fully filled cavities yields a cavity polariton dispersion matching that of phonon polaritons in bulk hBN, highlighting that the maximum light-matter coupling in microcavities is limited to the coupling strength between photons and the bulk material. The tunable cavity phonon polaritons could become a versatile platform for studying how the coupling strength between photons and phonons may modify the properties of polar crystals.

physics.optics↗

arXiv2020

Frequency-resolved photon correlations in cavity optomechanics

Frequency-resolved photon correlations have proven to be a useful resource to unveil nonlinearities hidden in standard observables such as the spectrum or the standard (color-blind) photon correlations. In this manuscript, we analyze the frequency-resolved correlations of the photons being emitted from an optomechanical system where light is nonlinearly coupled to the quantized motion of a mechanical mode of a resonator, but where the quantum nonlinear response is typically hard to evidence. We present and unravel a rich landscape of frequency-resolved correlations, and discuss how the time-delayed correlations can reveal information about the dynamics of the system. We also study the dependence of correlations on relevant parameters such as the single-photon coupling strength, the filtering linewidth, or the thermal noise in the environment. This enriched understanding of the system can trigger new experiments to probe nonlinear phenomena in optomechanics, and provide insights into dynamics of generic nonlinear systems.

quant-ph↗

arXiv2020

Probing and steering bulk and surface phonon polaritons in uniaxial materials using fast electrons: hexagonal boron nitride

We theoretically describe how fast electrons couple to polaritonic modes in uniaxial materials by analyzing the electron energy loss (EEL) spectra. We show that in the case of an uniaxial medium with hyperbolic dispersion, bulk and surface modes can be excited by a fast electron traveling through the volume or along an infinite interface between the material and vacuum. Interestingly, and in contrast to the excitations in isotropic materials, bulk modes can be excited by fast electrons traveling outside the uniaxial medium. We demonstrate our findings with the representative uniaxial material hexagonal boron nitride. We show that the excitation of bulk and surface phonon polariton modes is strongly related to the electron velocity and highly dependent on the angle between the electron beam trajectory and the optical axis of the material. Our work provides a systematic study for understanding bulk and surface polaritons excited by a fast electron beam in hyperbolic materials and sets a way to steer and control the propagation of the polaritonic waves by changing the electron velocity and its direction.

cond-mat.mtrl-sci↗

arXiv2020

Surface-Enhanced Circular Dichroism spectroscopy on periodic dual nanostructures

Increasing the sensitivity of chiral spectroscopic techniques such as circular dichroism (CD) spectroscopy is a current aspiration in the research field of nanophotonics. Enhancing CD spectroscopy depends upon of two complementary requirements: the enhancement of the electromagnetic fields perceived by the molecules under study and the conservation of the helicity of those fields, guaranteed by duality symmetry. In this work, we introduce a systematic method to design nanostructured dual periodic photonic systems capable of enhancing molecular CD spectroscopy resonantly. As an illustration, we engineer a dual 1D silicon nanoparticle array and show that its collective optical modes can be efficiently employed to resonantly enhance by two orders of magnitude the local density of optical chirality and, thus, the CD signal obtained from a given molecular sample on its vicinity.

physics.optics↗

arXiv2020

Optomechanical Collective Effects in Surface-Enhanced Raman Scattering from Many Molecules

The interaction between molecules is commonly ignored in surface-enhanced Raman scattering (SERS). Under this assumption, the total SERS signal is described as the sum of the individual contributions of each molecule treated independently. We adopt here an optomechanical description of SERS within a cavity quantum electrodynamics framework to study how collective effects emerge from the quantum correlations of distinct molecules. We derive analytical expressions for identical molecules and implement numerical simulations to analyze two types of collective phenomena: (i) a decrease of the laser intensity threshold to observe strong non-linearities as the number of molecules increases, within intense illumination, and (ii) identification of superradiance in the SERS signal, namely a quadratic scaling with the number of molecules. The laser intensity required to observe the latter in the anti-Stokes scattering is relatively moderate, which makes it particularly accessible to experiments. Our results also show that collective phenomena can survive in the presence of moderate homogeneous and inhomogeneous broadening.

physics.optics↗

arXiv2019

Quantum description of surface-enhanced resonant Raman scattering within a hybrid-optomechanical model

Surface-Enhanced Raman Scattering (SERS) allows for detection and identification of molecular vibrational fingerprints in minute sample quantities. The SERS process can be also exploited for optical manipulation of molecular vibrations. We present a quantum description of Surface-Enhanced Resonant Raman scattering (SERRS), in analogy to hybrid cavity optomechanics, and compare the resonant situation with the off-resonant SERS. Our model predicts the existence of a regime of coherent interaction between electronic and vibrational degrees of freedom of a molecule, mediated by a plasmonic nanocavity. This coherent mechanism can be achieved by parametrically tuning the frequency and intensity of the incident pumping laser and is related to the optomechanical pumping of molecular vibrations. We find that vibrational pumping is able to selectively activate a particular vibrational mode, thus providing a mechanism to control its population and drive plasmon-assisted chemistry.

physics.optics↗

arXiv2018

Origin of the asymmetric light emission from molecular exciton-polaritons

We apply the theory of open-quantum systems to describe light emission from coherently driven molecular polaritons. Based on the microscopic Hamiltonian that commonly describes the pure dephasing of isolated molecules, we show that under strong-coupling conditions dephasing leads to a transfer of energy between the constituted polariton branches $|\pm\rangle$. When the polariton dephasing is properly accounted for, the transition from the upper to the lower polariton branch is favored and leads to a dominant population of the lower polariton branch under coherent pumping conditions. As a result, the inelastic light emission originates mainly from the lower polariton state regardless of the pumping laser frequency thus producing an asymmetric emission of light. Furthermore, we show that, when several molecules are considered, inter-molecular coupling breaks the symmetry of the system, making the originally dark polaritons to effectively interact with light. This effect is revealed in the fluorescence spectrum as new emission peaks.

cond-mat.mes-hall↗