1,720,982 research outputs found
Nonlocal scattering matrix description of anisotropic polar heterostructures
No full textPolar dielectrics are a promising platform for mid-infrared nanophotonics, allowing for nanoscale electromagnetic energy confinement in oscillations of the crystal lattice. We recently demonstrated that in nanoscopic polar systems a local description of the optical response fails, leading to erroneous predictions of modal frequencies and electromagnetic field enhancements. In this Paper we extend our previous work providing a scattering matrix theory of the nonlocal optical response of planar, anisotropic, layered polar dielectric heterostructures. The formalism we employ allows for the calculation of both reflection and transmission coefficients, and of the guided mode spectrum. We apply our theory to complex AlN/GaN superlattices, demonstrating the strong nonlocal tuneability of the optical response arising from hybridisation between photon and phonon modes. The numerical code underlying these calculations is provided in an online repository to serve as a tool for the design of phonon-based mid-infrared optoelectronic devices
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
Quantum theory of longitudinal-transverse polaritons in nonlocal thin films
No full textWhen midinfrared light interacts with nanoscale polar dielectric structures, optical phonon propagation cannot be ignored, leading to a rich nonlocal phenomenology that we have only recently started to uncover. In properly crafted nanodevices this includes the creation of polaritonic excitations with hybrid longitudinal-transverse nature, which are predicted to allow energy funneling from longitudinal electrical currents to far-field transverse radiation. In this work we study the physics of these longitudinal-transverse polaritons in a dielectric nanolayer in which the nonlocality strongly couples epsilon-near-zero modes to longitudinal phonons. After having calculated the system’s spectrum solving Maxwell’s equations, we develop an analytical polaritonic theory able to transparently quantify the nonlocality-mediated coupling as a function of the system parameters. Such a theory provides a powerful tool for the study of longitudinal-transverse polariton interactions and we use it to determine the conditions required for the hybrid modes to appear
Polaritonic quantization in nonlocal polar materials
No full textIn the Reststrahlen region, between the transverse and longitudinal phonon frequencies, polar dielectric materials respond metallically to light, and the resulting strong light–matter interactions can lead to the formation of hybrid quasiparticles termed surface phonon polaritons. Recent works have demonstrated that when an optical system contains nanoscale polar elements, these excitations can acquire a longitudinal field component as a result of the material dispersion of the lattice, leading to the formation of secondary quasiparticles termed longitudinal-transverse polaritons. In this work, we build on previous macroscopic electromagnetic theories, developing a full second-quantized theory of longitudinal-transverse polaritons. Beginning from the Hamiltonian of the light–matter system, we treat distortion to the lattice, introducing an elastic free energy. We then diagonalize the Hamiltonian, demonstrating that the equations of motion for the polariton are equivalent to those of macroscopic electromagnetism and quantize the nonlocal operators. Finally, we demonstrate how to reconstruct the electromagnetic fields in terms of the polariton states and explore polariton induced enhancements of the Purcell factor. These results demonstrate how nonlocality can narrow, enhance, and spectrally tune near-field emission with applications in mid-infrared sensing
Real-space Hopfield diagonalization of inhomogeneous dispersive media
No full textWe introduce a real-space technique able to extend the standard Hopfield approach commonly used in quantum polaritonics to the case of inhomogeneous lossless materials interacting with the electromagnetic field. We derive the creation and annihilation polaritonic operators for the system normal modes as linear, space-dependent superpositions of the microscopic light and matter fields. We prove their completeness and invert the Hopfield transformation expressing the microscopic fields as functions of the polaritonic operators. As an example, we apply our approach to the case of a planar interface between vacuum and a polar dielectric, showing how we can consistently treat both propagative and surface modes, and express their nonlinear interactions, arising from phonon anharmonicity, as polaritonic scattering terms. We also show that our theory, including the proof of completeness, can be naturally extended to the case of dissipative materials
Theory of four-wave-mixing in phonon polaritons
No full textThird order anharmonic scattering in light–matter systems can drive a wide variety of practical and physically interesting processes from lasing to polariton condensation. Motivated by recent experimental results in the nonlinear optics of localized phonon polaritons, in this Letter we develop a quantum theory capable of describing four-wave mixing in arbitrarily inhomogeneous photonic environments. Using it, we investigate Kerr self-interaction and parametric scattering of surface and localized phonon polaritons, showing both processes to be within experimental reach
Surface phonon polaritons for infrared optoelectronics
No full textIn recent years, there has been significant fundamental research into surface phonon polaritons, owing to their ability to compress light to extremely small dimensions, low losses, and the ability to support anisotropic propagation. In this Perspective, after briefly reviewing the present state of mid-infrared optoelectronics, we will assess the potential of surface phonon polariton-based nanophotonics for infrared (3-100 μm) light sources, detectors, and modulators. These will operate in the Reststrahlen region where conventional semiconductor light sources become ineffective. Drawing on the results from the past few years, we will sketch some promising paths to create such devices and we will evaluate their practical advantages and disadvantages when compared to other approaches to infrared optoelectronics.</p
Non-equilibrium electrical generation of surface phonon polaritons
No full textNotwithstanding its relevance to many applications in sensing, security, and communications, electrical generation of narrow-band mid-infrared light remains highly challenging. Unlike in the ultraviolet or visible spectral regions {few materials possess direct electronic transitions}} in the mid-infrared and most that do are created through complex band-engineering schemes. An alternative mechanism, independent of dipole active material transitions, relies instead on energy lost to the polar lattice through the Coulomb interaction. Longitudinal phonons radiated in this way can be spectrally tuned through the engineering of polar nanostructures and coupled to localized optical modes in the material, allowing them to radiate mid-infrared photons into the far-field. A recent theoretical work explored this process providing for the first time an indication of its technological relevance when compared to standard thermal emitters. In order to do so it nevertheless used an equilibrium model of the electron gas, making this model difficult to inform the design of an optimal device to experimentally observe the effect. The present paper removes this limitation, describing the electron gas using a rigorous, self-consistent, non-equilibrium Green’s function model, accounting for variations in material properties across the device, and electron-electron interactions. Although the instability of the Schrodinger-Poisson iteration limits our studies to the low-bias regime, our results demonstrate emission rates comparable to that of room-temperature thermal emission despite such low biases. These results provide a pathway to design a confirmatory experiment of this new emission channel, that could power a new generation of mid-infrared optoelectronic devices
- …
