Molecular cavity optomechanics as a theory of plasmon-enhanced Raman scattering

The exceptional enhancement of Raman scattering by localized plasmonic resonances in the near field of metallic nanoparticles, surfaces or tips (SERS, TERS) has enabled spectroscopic fingerprinting down to the single molecule level. The conventional explanation attributes the enhancement to the subwavelength confinement of the electromagnetic field near nanoantennas. Here, we introduce a new model that also accounts for the dynamical nature of the plasmon-molecule interaction. We thereby reveal an enhancement mechanism not considered before: dynamical backaction amplification of molecular vibrations. We first map the system onto the canonical Hamiltonian of cavity optomechanics, in which the molecular vibration and the plasmon are parametrically coupled. We express the vacuum optomechanical coupling rate for individual molecules in plasmonic 'hot -spots' in terms of the vibrational mode's Raman activity and find it to be orders of magnitude larger than for microfabricated optomechanical systems. Remarkably, the frequency of commonly studied molecular vibrations can be comparable to or larger than the plasmon's decay rate. Together, these considerations predict that an excitation laser blue-detuned from the plasmon resonance can parametrically amplify the molecular vibration, leading to a nonlinear enhancement of Raman emission that is not predicted by the conventional theory. Our optomechanical approach recovers known results, provides a quantitative framework for the calculation of cross -sections, and enables the design of novel systems that leverage dynamical backaction to achieve additional, mode -selective enhancements. It also provides a quantum mechanical framework to analyse plasmon-vibrational interactions in terms of molecular quantum optomechanics.

Non-classical photon-phonon correlations at room temperature

Santiago Tarrago Velez

With the development of quantum optics, photon correlations acquired a prominent role as a tool to test our understanding of physics, and played a key role in verifying the validity of quantum mechanics. The spatial and temporal correlations in a light field also reveal information about its origin, and allow us to probe the nature of the physical systems interacting with it. Additionally, with the advent of quantum technologies, they have acquired technological relevance, as they are expected to play an important role in quantum communication and quantum information processing.This thesis develops techniques that combine spontaneous Raman scattering with Time Correlated Single Photon Counting, and uses them to study the quantum mechanical nature of high frequency vibrations in crystals and molecules. We demonstrate photon bunching in the Stokes and anti-Stokes fields scattered from two ultrafast laser pulses, and use their cross-correlation to measure the 3.9 ps decay time of the optical phonon in diamond. We then employ this method to measure molecular vibrations in CS2, where we are able to excite the respective vibrational modes of the two isotopic species present in the sample in a coherent superposition, and observe quantum beating between the two signals. Stokes scattering, when combined with a projective measurement, leads to a well defined quantum state. We demonstrate this by measuring the second order correlation function of the anti-Stokes field conditional on detecting one or more photons in the Stokes field, which allows us to observe a phonon modeâs transition form a thermal state into the first excited Fock state, and measure its decay over the characteristic phonon lifetime. Finally, we use this technique to prepare a highly entangled photon-phonon state, which violates a Bell-type inequality. We measure S = 2.360 Â± 0.025, violating the CHSH inequality, compatible with the non-locality of the state.The techniques we developed open the door to the study of a broad range of physical systems, where spectroscopic information is obtained with the preparation of specific quantum states. They also hold potential for future technological use, and promote vibrational Raman scattering to a resource in nonlinear quantum optics -- where it used to be considered as a source of noise instead.

EPFL2021

Investigation of photoinduced effects in plasmonic nanocavities

Aqeel Ahmed

Light matter interaction can be boosted by several orders of magnitude by tailoring the photonic environment, thus enabling a wide range of applications. One particular example are plasmonic nanostructures that support localized surface plasmon polariton (LSPP) leading to enhancement and confinement of incident electromagnetic fields. These novel properties provide access to a large number of optically driven effects within matter placed in the vicinity of such nanostructures. In this thesis, we investigate the effects of optical excitation on gold nanostructures in the presence of Raman active molecules. This is accomplished by fabricating nanoparticle on mirror (NPoM) plasmonic nanostructures where a gold nanoparticle is placed at a fixed distance from a smooth gold film using molecular monolayers. As the nanoparticle and mirror are placed only a few nanometers apart, optical excitation leads to formation of hybridized LSPP modes due to coupling between the plasmons within the nanoparticle and the mirror. This plasmonic coupling also results in confinement and subsequent enhancement of the incident light within the gap and is extremely sensitive to structural parameters of the NPoM. The creation of a homogeneous gap between the nanoparticle and mirror is quite challenging due the roughness of the mirror. Moreover, due to the large number of NPoM nanostructures present on the sample it is quite difficult to locate a specific NPoM. To bypass these difficulties atomically smooth and patterned substrates were developed using template stripping combined with conventional photolithography techniques. Next, in order to characterize the optical response of the NPoMs, a custom optical setup was built. The setup was primarily designed to investigate the plasmonic properties of the NPoM using elastic scattering spectroscopy along with surface enhanced Raman scattering (SERS) from the molecules within the gap. The combination of these two techniques was later used to investigate the effect of laser power on the NPoMs. It was discovered that the scattering spectra of the NPoMs changed irreversibly after laser exposure even with a few ÎŒW/ÎŒm2 incident intensities. Moreover, the plasmonic response of the NPoMs fabricated with well organized molecular monolayer changed much faster than the NPoMs prepared with disorganized ones. This phenomenon was also investigated in NPoMs prepared using a dielectric gap layer, by changing the gap conductivity, and by changing the nanoparticle shape. The experimental results combined with simulations suggest a decrease in gap height due to the reorientation of the molecular monolayer under optical excitation.

EPFL2021

Emergence and Detection of Exotic Phases in Spin-1 Systems

Frédéric Alexis Michaud

This work is devoted to the study of spin S = 1 systems, and more precisely to the emergence of exotic quantum phases in such systems, and to the establishment of tools to observe such phases. It is split in four main chapters. In the first chapter, we show how spin S = 1 systems can emerge from microscopic models, and which kinds of interaction might appear in the effective spin model. We start from a two-orbital Hubbard model, and by a strong coupling development to fourth order, we derive an effective model. We will see that three types of interaction appear beyond the Heisenberg interaction : a plaquette interaction, a biquadratic interaction and a three-spin interaction. In the second chapter, we study Raman scattering on systems with quadrupolar order to show that it can be used to probe such order. We first start by deriving an effective light scattering operator following Shastry and Shraiman calculation on spin S = 1/2 systems. Using this effective operator, we compute the Raman spectra with exact diagonalization and linear flavor-wave theory. We show that two different regimes appear depending on the incoming photon energy, and that combining this to different polarizations accessible with Raman scattering, the presence of quadrupolar order can be established with this probe. The third chapter is devoted to the study of the three-spin interaction that appeared in the first chapter on a chain. We start by establishing the classical and the mean field phase diagram of this system. We then turn to the quantum case. We show that, whatever the value of the spin is, the ground state is perfectly dimerized for a particular value of the three-spin interaction. The presence of such a point in the phase diagram implies the existence of a quantum phase transition when increasing the three-spin interaction. By an intensive numerical study, we show that this transition is continuous, and that its critical behavior is the one of a SU(2)k=2S Wess-Zumino-Witten model, at least for spins S = 1/2,1,3/2,2. In the last section of this chapter, we study the phase diagram of the chain for spin S = 1 under a magnetic field. We conclude this work with a study of the three-spin interaction on a square lattice. The classical and mean field phase diagram are established. It is shown that for a large three-spin interaction, the classical ground state is highly degenerate. This degeneracy is lifted in the quantum case by a process of order by disorder. We compute the quantum fluctuations with linear spin-wave theory, and show that some phases are selected over others. We confirm these results by an exact diagonalization study of the system.

EPFL2013

Investigation of photoinduced effects in plasmonic nanocavities

Aqeel Ahmed

EPFL2021

Emergence and Detection of Exotic Phases in Spin-1 Systems

Frédéric Alexis Michaud

EPFL2013

Non-classical photon-phonon correlations at room temperature

Santiago Tarrago Velez

EPFL2021