arXiv subjects

Gian Paolo Papari

Publications and source records attributed to Gian Paolo Papari.

5 recordsLinked to original sources

arXiv2026

A Reflective Metasurface for High-Efficiency Terahertz Cross-Polarization Conversion

We demonstrate a reflective terahertz (THz) polarization conversion metasurface based on a double-ring aluminum resonator on a quartz substrate. Full-wave simulations are used to optimize the unit-cell geometry and investigate the polarization conversion mechanism through phase analysis, polarization ellipses, and surface-current distributions. The proposed metasurface exhibits a near-unity polarization conversion ratio (PCR) at resonance frequencies of 0.333 THz and 0.361 THz. Reflection-mode terahertz time-domain spectroscopy (THz-TDS) measurements show good agreement with the simulated results. The measured PCR exceeds 87% at both resonance frequencies, demonstrating efficient reflection-mode polarization conversion.

physics.optics↗

arXiv2026

Giant Effective Permeability in Drude Thin Films Probed by THz Time-Domain Spectroscopy

The electromagnetic response of metallic films is commonly analyzed in terahertz spectroscopy by assuming unit relative magnetic permeability. In this work we show that this assumption introduces significant distortions in the electrodynamic retrieval of highly conductive films. Aluminum and copper films, 10 nm thick, were investigated by terahertz time-domain spectroscopy in both transmission and reflection configurations. By applying a self consistent retrieval method that independently determines the complex permittivity and permeability, we show that the Drude-type dielectric response is systematically accompanied by a permeability that strongly departs from unity. This deviation is intrinsically linked to the reactive impedance of the films, which clarifies the light induced onset of large screening currents within a transversally confined geometry. A phenomenological interpretation based on the Faraday Neumann Lenz mechanism and a lumped-element model of the film impedance accounts for the observed trends. These results indicate that the common assumption $\tildeμ=1$ in non-magnetic Drude films can lead to an incomplete or biased electrodynamic characterization in the terahertz regime.

cond-mat.other↗

arXiv2025

Accurate THz Measurements of Permittivity and Permeability of BiFeO$_3$ Thin Films

The electrodynamic properties of BiFeO$_3$ films in the THz region are investigated via time domain spectroscopy. Combining the use of transmission ($\tilde{T}$) and reflection ($\tilde{R}$) measurements under normal incidence, the refractive index and impedance of the samples under test are evaluated using a retrieval method. From $\tilde{T}$(($\tilde{R}$)) data two complex functions describing the refractive index $\tilde{n}_T$ ($\tilde{n}_R$) and impedance $\tilde{z}_T$ ($\tilde{z}_R$) are extracted from the independent minimization of the error functions given by the difference between the theoretical model and measurements. Knowledge of the pairs ($\tilde{n}_T, \tilde{n}_R$) and ($\tilde{z}_T, \tilde{z}_R$) enables to calculate with a high accuracy both complex permittivity $\tilde{\varepsilon}$ and permeability $\tildeμ$ of the sample. Signatures of magnetoelectric effects and phononic resonances are observed in the permittivity and permeability functions and discussed in detail.

cond-mat.mtrl-sci↗

arXiv2022

Sensing enhancement of a Fabry-Perot THz cavity using switchable VO2 mirrors

We experimentally investigate the sensing properties of an open cavity operating in the THz regime and realized by employing as mirrors two thin vanadium dioxide (VO2) films grown on silicon parallel plates and separated by a variable length. The phase transition of VO2 is used to control the behavior of the system between two different responses: a high transmission mode to the incident radiation (VO2 in the insulating state) and a high sensitivity to tiny changes in the cavity refractive index (VO2 in the conducting state). In the first state, the low loss regime enables to adjust the cavity length and easily optimize the resonances due to the Fabry-Perot (FP) effect in the Si plates and in the cavity volume. The activation of the metallic-like state instead, by damping the FP oscillations in the plates, promotes the onset of a comb-like spectrum that can be exploited as a versatile tool for accurate sensing applications. Using both an analytical model and full-wave simulations, we estimate the device response to variation in the refractive index of the cavity volume, showing that the proposed structure can achieve sensitivity values among the highest reported for THz sensors.

physics.optics↗

arXiv2021

Thermoplastic polyurethane-graphene nanoplatelets microcellular foams for electromagnetic interference shielding

The incorporation of graphene-related materials as nanofiller can produce multifunctional foams with enhanced specific properties and density reduction. Herein we report on the preparation of microcellular thermoplastic polyurethane/graphene foams by batch foaming. Solution blending was first adopted to disperse graphene nanoplatelets (GNP) in the elastomeric matrix. Then, a foaming process based on the use of supercritical CO2 was adopted to produce the microcellular TPU/GNP composite foams with graphene content up to 1 wt%. The EMI shielding behaviour of the TPU/GNP foams has been assessed in the THz range, and has revealed their potential in comparison with other graphene-filled foams presented in the literature, that exhibit similar specific shielding effectiveness but at much higher content of graphene-related materials (10-30 wt%).

physics.app-ph↗