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

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

Influence of direct dipole-dipole interactions on the optical response of 2D materials in strongly inhomogeneous infrared cavity fields

A two-dimensional (2D) material, formed for example by a self-assembled molecular monolayer or by a single layer of a van der Walls material, can couple efficiently with photonic nanocavities, potentially reaching the strong coupling regime. The coupling can be modelled using classical harmonic oscillator models or cavity quantum electrodynamics Hamiltonians that often neglect the direct dipole-dipole interactions within the monolayer. Here, we diagonalize the full Hamiltonian of the system, including these direct dipole-dipole interactions. The main effect on the optical properties of a typical 2D system is simply to renormalize the effective energy of the bright collective excitation of the monolayer that couples with the nanophotonic mode. On the other hand, we show that for situations of extreme field confinement, large transition dipole moments and low losses, fully including the direct dipole-dipole interactions is critical to correctly capture the optical response, with many collective states participating in it. To quantify this result, we propose a simple equation that indicates the condition for which the direct interactions strongly modify the optical response.Comment: 16 pages, 10 figure

Optoelectronics of subnanometric metallic gaps

Trabajo presentado al 44th Winter Colloquium on the Physics of Quantum Electronics, celebrado en Utah (USA) del 5 al 9 de enero de 2014.Peer reviewe

Nonlinear Optical Response of a Plasmonic Nanoantenna to Circularly Polarized Light: Rotation of Multipolar Charge Density and Near-Field Spin Angular Momentum Inversion

The spin and orbital angular momentum carried by electromagnetic pulses open new perspectives to control nonlinear processes in light–matter interactions, with a wealth of potential applications. In this work, we use time-dependent density functional theory (TDDFT) to study the nonlinear optical response of a free-electron plasmonic nanowire to an intense, circularly polarized electromagnetic pulse. In contrast to the well-studied case of the linear polarization, we find that the nth harmonic optical response to circularly polarized light is determined by the multipole moment of order n of the induced nonlinear charge density that rotates around the nanowire axis at the fundamental frequency. As a consequence, the frequency conversion in the far field is suppressed, whereas electric near fields at all harmonic frequencies are induced in the proximity of the nanowire surface. These near fields are circularly polarized with handedness opposite to that of the incident pulse, thus producing an inversion of the spin angular momentum. An analytical approach based on general symmetry constraints nicely explains our numerical findings and allows for generalization of the TDDFT results. This work thus offers new insights into nonlinear optical processes in nanoscale plasmonic nanostructures that allow for the manipulation of the angular momentum of light at harmonic frequencies.We acknowledge financial support from project IT1526–22 of the Department of Education of the Basque Government, and projects PID2019–107432GB-I00 and PID2022–139579NB-I00, funded by MCIN/AEI/10.13039/501100011033 and “FEDER Una manera de hacer Europa”

Dispersive surface-response formalism to address nonlocality in extreme plasmonic field confinement

The surface-response formalism (SRF), where quantum surface-response corrections are incorporated into the classical electromagnetic theory via the Feibelman parameters, serves to address quantum effects in the optical response of metallic nanostructures. So far, the Feibelman parameters have been typically obtained from many-body calculations performed in the long-wavelength approximation, which neglects the nonlocality of the optical response in the direction parallel to the metal–dielectric interface, thus preventing to address the optical response of systems with extreme field confinement. To improve this approach, we introduce a dispersive SRF based on a general Feibelman parameter d ⊥(ω, k ‖), which is a function of both the excitation frequency, ω, and the wavenumber parallel to the planar metal surface, k ‖. An explicit comparison with time-dependent density functional theory (TDDFT) results shows that the dispersive SRF correctly describes the plasmonic response of planar and nonplanar systems featuring extreme field confinement. This work thus significantly extends the applicability range of the SRF, contributing to the development of computationally efficient semiclassical descriptions of light–matter interaction that capture quantum effects.MCIN/AEI/10.13039/501100011033/ (PID2019-107432GB-I00); Department of Education of the Basque Government (IT1526-22); “Investissements d’Avenir” LabEx PALM (ANR-10-LABX-0039-PALM)

Detection of deep-subwavelength dielectric layers at terahertz frequencies using semiconductor plasmonic resonators

Plasmonic bowtie antennas made of doped silicon can operate as plasmonic resonators at terahertz (THz) frequencies and provide large field enhancement close to their gap. We demonstrate both experimentally and theoretically that the field confinement close to the surface of the antenna enables the detection of ultrathin (100 nm) inorganic films, about 3750 times thinner than the free space wavelength. Based on model calculations, we conclude that the detection sensitivity and its variation with the thickness of the deposited layer are related to both the decay of the local THz field profile around the antenna and the local field enhancement in the gap of the bowtie antenna. This large field enhancement has the potential to improve the detection limits of plasmon-based biological and chemical sensors

Bridging quantum and classical plasmonics with a quantum-corrected model

Electromagnetic coupling between plasmonic resonances in metallic nanoparticles allows for engineering of the optical response and generation of strong localized near-fields. Classical electrodynamics fails to describe this coupling across sub-nanometer gaps, where quantum effects become important owing to non-local screening and the spill-out of electrons. However, full quantum simulations are not presently feasible for realistically sized systems. Here we present a novel approach, the quantum-corrected model (QCM), that incorporates quantum-mechanical effects within a classical electrodynamic framework. The QCM approach models the junction between adjacent nanoparticles by means of a local dielectric response that includes electron tunnelling and tunnelling resistivity at the gap and can be integrated within a classical electrodynamical description of large and complex structures. The QCM predicts optical properties in excellent agreement with fully quantum mechanical calculations for small interacting systems, opening a new venue for addressing quantum effects in realistic plasmonic systems

Anomalous spectral shift of near- and far-field plasmonic resonances in nanogaps

This is an open access article published under a Creative Commons Attribution (CC-BY) License.-- et al.The near-field and far-field spectral response of plasmonic systems are often assumed to be identical, due to the lack of methods that can directly compare and correlate both responses under similar environmental conditions. We develop a widely tunable optical technique to probe the near-field resonances within individual plasmonic nanostructures that can be directly compared to the corresponding far-field response. In tightly coupled nanoparticle-on-mirror constructs with nanometer-sized gaps we find >40 meV blue-shifts of the near-field compared to the dark-field scattering peak, which agrees with full electromagnetic simulations. Using a transformation optics approach, we show such shifts arise from the different spectral interference between different gap modes in the near- and far-field. The control and tuning of near-field and far-field responses demonstrated here is of paramount importance in the design of optical nanostructures for field-enhanced spectroscopy, as well as to control near-field activity monitored through the far-field of nano-optical devices.We acknowledge financial support from EPSRC grants EP/G060649/1, EP/L027151/1, EP/G037221/1, EPSRC NanoDTC, and ERC grant LINASS 320503. J.A. acknowledges support from project FIS2013-41184-P from Spanish MINECO and project NANOGUNE’14 from the Department of Industry of the Basque Country. F.B. acknowledges support from the Winton Programme for the Physics of Sustainability. R.C. acknowledges financial support from St. John’s College, Cambridge, for a Dr. Manmohan Singh Scholarship. P.A. acknowledges funding from the Helmholtz Association for the Young Investigator group VH-NG-928 within the Initiative and Networking Fund.Peer Reviewe

Nanoóptica: controlando la luz en la nanoescala

La localización de la luz en diversas nanoestructuras ha permitido superar los límites de la óptica convencional, posibilitando un control sin precedentes de diversos procesos optoelectrónicos. La comprensión de la interacción entre la luz y la materia en la nanoescala sienta las bases de la generación, control, y manipulación de haces de luz en espacios ínfimos, abriendo un nuevo abanico de posibilidades tecnológicas. Entre otras, la nanoóptica permite obtener imagenes nanoscópicas de nanopartículas y sustancias biológicas, transmitir señal óptica de alta densidad en dispositivos, realizar termoterapia contra células cancerígenas o mejorar las prestaciones de células solares.Peer Reviewe