Dipartimento di Fisica - Tesi di Dottorato

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Photon dynamics in metal insulator anisotropic active and passive systems
(Università della Calabria, 2023-06-12) Aniket, Patra; Cipparrone, Gabriella; Caligiuri, Vincenzo
In this thesis, we analyzed the photon dynamics in different metal-dielectric anisotropic structures theoretically as well as experimentally, with and without embedded gain materials. The simplest system is a metal-dielectric bilayer that acts as an open cavity and can support some optical modes, which we call pseudo-cavity modes. This system can be used to study strong light matter interaction, which is demonstrated by placing an optical active material or gain medium on top of the open cavity. Since in such an open cavity structure the gain material is directly accessible to the optical stimulation (for example a pump laser), the photoluminescence from the active material can be straightforwardly enhanced. This open cavity also preserves the polarization state, which enables such structures as polarization dependent optical sources. A Metal-Dielectric-Metal (MDM) structure forms a closed cavity with two reflective metal layers. In recent studies, it has been shown that such structures support epsilon near zero (ENZ) modes, and such vanishing dielectric permittivity triggers numerous intriguing new properties such as: cloaking with negative refractive index, Purcell factor enhancement, energy squeezing and electric levitation, etc. In our work, we fabricated MDM cavities and demonstrated the polariton generation inserting a light emitting gain material inside the dielectric layer. By matching the ENZ cavity resonance with gain medium we obtained strong coupling, and found that the polaritons in this system maintained the ENZ properties of the MDM cavity. In the second part of the work, the photon dynamics in the extreme anisotropic medium are analyzed without an embedded gain medium. With the help of a combined system composed of open and closed cavities, a strong plasmonic PUF (Physical Unclonable Function) has been demonstrated for practical application. With the help of a morphology study of the plasmonic materials and image processing, high-level anticounterfeiting functionality of silver nano island on top of MDM cavities is demonstrated. The photon dynamics in another highly anisotropic system are investigated by combining two different optical resonators, a MDM cavity and a Guided mode resonance (GMR) cavity. The origin of the strong coupling regime occurring in a GMR-MDM architecture has been investigated numerically and experimentally. The role of the polarization of the incident light on the strong coupling between the modes is investigated and plausible applications such as polarization-sensitive switching are discussed.
Dielectric and resonant Gain singularities in multilayered nanostructures
(2017-05-03) Caligiuri, Vincenzo; Carbone, Vincenzo; De Luca, Antonio
In this thesis work, the dielectric and gain singularity regimes in Hyperbolic Metamaterials (HMM) have been conceived and both theoretically and experimentally studied. For the first one it has been demonstrated how, in order to induce a dielectric singularity in the dielectric permittivities of an HMM specific conditions on both the geometry and optical properties of the fundamental metal/dielectric components have to be fulfilled. An HMM respecting these constrains is named Epsilon- Near-Zero-and-Pole (eNZP). Such a system manifests both the so-called Type I and Type II within the visible range and, noticeably, allows to cancel the usually found effective dielectric (or metallic) frequency gap betwen them, showing a inversion point of these two coexisting anisotropies, called Canalization Wavelength or Transition Wavelength. It has been demonstrated how a light wave propagating inside the eNZP HMM, remains perfectly subwavelength collimated, proceeding as a straight soliton for more than 100 Rayleigh lengths. Many fascinating new properties are unlocked in such regime, among which the supercollimation and the perfect lensing have been theoretically studied as well as experimentally demonstrated. Due to the specific stringent conditions to be respected, it has been demonstrated that with a classic two-component HMM it is not possible to tune the eNZP wavelength and a new configuration has to be adopted based on three components: a high index dielectric, a low index dielectric and a metal. By means of this new configuration, a full visible range design range of the eNZP wavelength has been demonstrated, keeping the same three fundamental materials and only acting on their thickness. The possibility of introducing thermal tunability of the optical features of a classic HMM has been demonstrated, thus overcoming the well known lack of tunability such structures usually are affected by. Basing on a sol-gel TiO2 matrix, a new material has been conceived, embedding a low index dielectric (Polyvinylpyrrolidone, PVP) and an organic fluorescent medium (Coumarin C500). It has been found that the unsintered sol-gel TiO2 remains extremely sensitive to any temperature change, endowing the HMM embedding such new mixture with thermally tunable features. The possibility to thermally reversibly reconfigure the most significant properties of an HMM embedding such a new dielectric has been both theoretically and experimentally demonstrated as well those one of a complete reconfiguration of the system, irreversibly switching from an effective metal to an effective dielectric, when exposed to high temperatures. In the end, it has been possible to theorize and study a new propagation regime called Resonant Gain, occurring in specifically modified eNZP HMMs. In order the resonant gain singularity to occur in the perpendicular dielectric permittivity of the eNZP HMM, a fluorescent medium has to embedded in the dielectric layers. Conditions to fulfil are very stringent but, once reached, it has been demonstrated that light propagating in such a regime is extremely subwavelength confined and amplificated inside the HMM, giving rise to a self-amplifying perfect lens and leading this system to configure as a promising candidate for LASER effect at the nanoscale. The same phenomenon has been verified in the framework of newly conceived system consisting in dye embedding metal/dielectric multishell nanoparticle, configuring as promising candidates for SPASER effect.

Dipartimento di Fisica - Tesi di Dottorato

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