1,721,052 research outputs found
Electro-optical sampling of quantum vacuum fluctuations in dispersive dielectrics
No full textElectro-optical sampling has been recently used to perform spectrally-resolved measurements of electromagnetic vacuum fluctuations and it has been predicted it could be used to probe the population of virtual photons predicted to exist in the ground state of an ultrastrongly light-matter coupled system. In order to understand which information on the ground state of an interacting system can be acquired thanks to this technique, in this paper we will develop the quantum theory of electro-optical sampling in arbitrary dispersive dielectrics. Our theory shows that a measure of the time correlations of the vacuum fluctuations effectively implements an ellipsometry measurement on the quantum vacuum, allowing to access the frequency-dependent dielectric function without the need of any resonant incoming photon. We discuss consequences of these results on the possibility to use electro-optical sampling to access the virtual photon population
Open quantum systems with local and collective incoherent processes: Efficient numerical simulations using permutational invariance
No full textThe permutational invariance of identical two-level systems allows for an exponential reduction in the computational resources required to study the Lindblad dynamics of coupled spin-boson ensembles evolvingunder the effect of both local and collective noise. Here we take advantage of this speedup to study several important physical phenomena in the presence of local incoherent processes, in which each degree of freedomcouples to its own reservoir. Assessing the robustness of collective effects against local dissipation is paramount to predict their presence in different physical implementations. We have developed an open-source library inPYTHON, the Permutational-Invariant Quantum Solver (PIQS), which we use to study a variety of phenomena in driven-dissipative open quantum systems. We consider both local and collective incoherent processes in theweak-, strong-, and ultrastrong-coupling regimes. Using PIQS, we reproduce a series of known physical results concerning collective quantum effects and extend their study to the local driven-dissipative scenario. Our workaddresses the robustness of various collective phenomena, e.g., spin squeezing, superradiance, and quantum phase transitions, against local dissipation processes
Light-matter decoupling in the deep strong coupling regime: the breakdown of the Purcell Effect
No full textImprovements in both the photonic confinement and the emitter design have led to a steady increase in the strength of the light-matter coupling in cavity quantum electrodynamics experiments. This has allowed us to access interaction-dominated regimes in which the state of the system can only be described in terms of mixed light-matter excitations. Here we show that, when the coupling between light and matter becomes strong enough, this picture breaks down, and light and matter degrees of freedom totally decouple. A striking consequence of such a counterintuitive phenomenon is that the Purcell effect is reversed and the spontaneous emission rate, usually thought to increase with the light-matter coupling strength, plummets instead for large enough couplings
Virtual photons in the ground state of a dissipative system
No full textMuch of the novel physics predicted to be observable in the ultrastrong light-matter coupling regime rests on the hybridisation between states with different numbers of excitations, leading to a population of virtual photons in the system’s ground state. In this article, exploiting an exact diagonalization approach, we derive both analytical and numerical results for the population of virtual photons in presence of arbitrary losses. Specialising our results to the case of Lorentzian resonances we then show that the virtual photon population is only quantitatively affected by losses, even when those become the dominant energy scale. Our results demonstrate most of the ultrastrong-coupling phenomenology can be observed in loss-dominated systems which are not even in the standard strong coupling regime. We thus open the possibility to investigate ultrastrong-coupling physics to platforms that were previously considered unsuitable due to their large losses.<br/
Laser‑stabilised ionising transitions
No full textStabilising atoms exposed to intense laser fields has been a central topic in atomic physics for decades, inspiring several theoretical frameworks that describe the phenomenon under different resonance conditions and coupling regimes. Here we theoretically investigate an ionising electronic transition driven by a resonant pump field, in contrast to Kramers–Henneberger atoms, which are stabilised by non-perturbative, non-resonant laser pulses. We show that, above a critical pump intensity, the drive creates a novel metastable electronic bound state embedded in the continuum, which decays via two-photon ionisation. We calculate the resulting resonant fluorescence spectrum and find a qualitatively different structure from the familiar Mollow triplet associated with bound-to-bound transitions. This fluorescence provides a time-resolved probe of the population of the metastable state. In analogy to how the AC Stark shift is a semiclassical counterpart of the single-photon Rabi splitting observed in optical cavities, the phenomenon we describe is best understood as a semiclassical analogue of recently observed excitons bound by a single cavity photon
Polaritonics: From microcavities to sub-wavelength confinement
No full textFollowing the initial success of cavity quantum electrodynamics in atomic systems, strong coupling between light and matter excitations is now achieved in several solid-state set-ups. In those systems, the possibility to engineer quantum emitters and resonators with very different characteristics has allowed access to novel nonlinear and non-perturbative phenomena of both fundamental and applied interest. In this article, we will review some advances in the field of solid-state cavity quantum electrodynamics, focussing on the scaling of the relevant figures of merit in the transition from microcavities to sub-wavelength confinement.</p
Strong light-matter coupling in microcavities characterised by Rabi-splittings comparable to the Bragg stop-band widths
No full textThe vacuum Rabi splitting of polaritonic eigenmodes in semiconductor microcavities scales with the square root of the oscillator strength, as predicted by the coupled oscillator model and confirmed in many experiments. We show here that the square root law is no more applicable if the Rabi splitting becomes comparable or larger than the stop-band width of the Bragg mirrors forming the cavity. Once the oscillator strength becomes large enough, the material hosting excitons hybridises with the quasi-continuum microcavity Bragg modes lying outside of the stop-band, thus forming a novel kind of polaritonic resonance. We study this physics considering both two- and three-dimensional excitonic materials embedded in the microcavity. We highlight the varied phenomenology of those polaritons and develop a theoretical understanding of their most peculiar features
Optical nonlocality in polar dielectrics
No full textPhonon polaritons localised in polar nanoresonators and superlattices are being actively investigated as promising platforms for mid-infrared nanophotonics. Here we show that the nonlocal nature of the phonon response can strongly modify their nanoscale physics. Using a nonlocal dielectric approach we study dielectric nanospheres and thin dielectric films taking into account optical phonons dispersion. We discover a rich nonlocal phenomenology, qualitatively different from the one of plasmonic systems. Our theory allows us to explain the recently reported discrepancy between theory and experiments in atomic-scale superlattices, and it provides a practical tool for the design of phonon-polariton nanodevices
Theory of nonlinear polaritonics: χ<sup>(2)</sup> scattering on a β-SiC surface
No full textIn this article we provide a practical prescription to harness the rigorous microscopic, quantum-level descriptions of sub-wavelength light-matter systems provided by real-space Hopfield diagonalisation for quantum description of nonlinear scattering. A general frame to describe the practically important second-order optical nonlinearities which underpin sum and difference frequency generation is developed for arbitrarily inhomogeneous dielectric environments. Specific attention is then focussed on planar systems with optical nonlinearity mediated by a polar dielectric β-SiC halfspace. In this system we calculate the rate of second harmonic generation and the result is compared to recent experimental measurements. Furthermore the rate of difference frequency generation of subdiffraction surface phonon polaritons on the β-SiC halfspace by two plane waves is calculated. The developed theory is easily integrated with commercial finite element solvers, opening the way for calculation of second-order nonlinear scattering coefficients in complex geometries which lack analytical linear solutions
Impact of phonon nonlocality on nanogap and nanolayer polar resonators
No full textPolar dielectric nanoresonators can support hybrid photon-phonon modes termed surface phonon polaritons with length scales below the diffraction limit. In the deep subwavelength regime the optical response of these systems was shown to diverge from that predicted through a standard dielectric description. Recently, we developed an analytical, dielectric approach and applied it to spheres and planar heterostructures, reproducing anomalous features observed in experiment and microscopic calculations. In this Rapid Communication we develop tools to describe the nonlocal response of polar nanoresonators of arbitrary symmetry, and use them to investigate systems with nanogaps and nanolayers of practical technological relevance. We demonstrate that the available field enhancement is strongly reduced, as the electromagnetic energy leaks away from the hot spots, while phononic resonances are shifted by resonator effects
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