Simone Zanotto

Magneto-optic transmittance modulation observed in a hybrid graphene–split ring resonator terahertz metasurface

By placing a material in close vicinity of a resonant optical element, its intrinsic optical response can be tuned, possibly to a wide extent. Here, we show that a graphene monolayer, spaced a few tenths of nanometers from a split ring resonator metasurface, exhibits a magneto-optical response which is strongly influenced by the presence of the metasurface itself. This hybrid system holds promises in view of thin optical modulators, polarization rotators, and nonreciprocal devices, in the technologically relevant terahertz spectral range. Moreover, it could be chosen as the playground for investigating the cavity electrodynamics of Dirac fermions in the quantum regime.

Optical critical coupling into highly confining metal-insulator-metal resonators

We demonstrate controlled optical critical coupling into highly confining metal-insulator-metal grating-based resonators. We achieve the coupling—and hence the absorption—of more than 95% of the incoming photons in a gallium arsenide based system confined between a metallic ground plane and a metallic grating. The demonstration is given in the terahertz range of the electromagnetic spectrum, at 75 μm ≤ λ ≤ 120 μm, for a semiconductor core thickness of only 10 μm. It is valid, however, at any wavelength, upon linear scaling. The critical coupling regime is judiciously tuned by precise etching of the semiconductor material in between the metallic fingers. The experimental results are in accordance with the universal behaviour predicted by temporal coupled mode theory.

Intersubband polaritons in a one-dimensional surface plasmon photonic crystal

In this work we demonstrate the achievement of light-matter strong coupling regime between an intersubband transition and the photonic bandlike mode supported by a metallic grating. Polaritonic resonances have been identified in the reflectivity spectra at 10° incidence, with a clear anticrossing appearing when the photonic mode is tuned across the transition line. Experimental results are in good agreement with the simulations performed with a scattering-matrix approach. This cavity design can be further optimized and is likely to open the way to a new class of time-resolved measurements, as it allows pump-and-probe measurements to be performed in collinear geometries.

Interferometric control of absorption in thin plasmonic metamaterials: general two port theory and broadband operation

In order to extend the Coherent Perfect Absorption (CPA) phenomenology to broadband operation, the interferometric control of absorption is investigated in two-port systems without port permutation symmetry. Starting from the two-port theory of CPA treated within the Scattering Matrix formalism, we demonstrate that for all linear two-port systems with reciprocity the absorption is represented by an ellipse as function of the relative phase and intensity of the two input beams, and it is uniquely determined by the device single-beam reflectance and transmittance, and by the dephasing of the output beams. The basic properties of the phenomenon in systems without port permutation symmetry show that CPA conditions can still be found in such asymmetric devices, while the asymmetry can be beneficial for broadband operation. As experimental proof, we performed transmission measurements on a metal-semiconductor metamaterial, employing a Mach-Zehnder interferometer. The experimental results clearly evidence the elliptical feature of absorption and trace a route towards broadband operation.

Mid-infrared intersubband polaritons in dispersive metal-insulator-metal resonators

We demonstrate room-temperature strong coupling between a mid-infrared (λ = 9.9 μm) intersubband transition and the fundamental cavity mode of a metal-insulator-metal resonator. Patterning of the resonator surface enables surface-coupling of the radiation and introduces an energy dispersion which can be probed with angle-resolved reflectivity. In particular, the polaritonic dispersion presents an accessible energy minimum at k = 0 where—potentially—polaritons can accumulate. We also show that it is possible to maximize the coupling of photons into the polaritonic states and—simultaneously—to engineer the position of the minimum Rabi splitting at a desired value of the in-plane wavevector. This can be precisely accomplished via a simple post-processing technique. The results are confirmed using the temporal coupled mode theory formalism and their significance in the context of the strong critical coupling concept is highlighted.

Distributed feedback terahertz frequency quantum cascade lasers with dual periodicity gratings

We have developed terahertz frequency quantum cascade lasers that exploit a double-periodicity distributed feedback grating to control the emission frequency and the output beam direction independently. The spatial refractive index modulation of the gratings necessary to provide optical feedback at a fixed frequency, and simultaneously, a far-field emission pattern centered at controlled angles, was designed through use of an appropriate wavevector scattering model. Single mode terahertz (THz) emission at angles tuned by design between 0° and 50° was realized, leading to an original phase-matching approach for highly collimated THz quantum cascade lasers.

Saturation and bistability of defect-mode intersubband polaritons

In this paper we report about linear and nonlinear optical properties of intersubband cavity polariton samples, where the resonant photonic mode is a defect state in a metallodielectric photonic crystal slab. By tuning a single geometric parameter of the resonator, the cavity

Photonic bands and defect modes in metallo-dielectric photonic crystal slabs

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Universal lineshapes at the crossover between weak and strong critical coupling in Fano-resonant coupled oscillators

factor can reach values as large as 85, with a consequent large cooperativity for the light-matter interaction. We show that a device featuring large cooperativity leads to sharp saturation, or even bistability, of the polariton states. This nonlinear dynamics occurs at the crossover between the weak- and the strong-coupling regimes, where the weak critical coupling concept plays a fundamental role.

Symmetry enhanced non-reciprocal polarization rotation in a terahertz metal-graphene metasurface

Photonic components based on structured metallic elements show great potential for device applications where field enhancement and confinement of the radiation on a subwavelength scale is required. In this paper, we report on a detailed study of a prototypical metallo-dielectric photonic structure, where features well known in the world of dielectric photonic crystals such as bandgaps and defect modes are exported to the metallic counterpart. Such a structure may have interesting applications in infrared science and technology, for instance, in quantum well infrared photodetectors, narrowband spectral filters, and tailorable thermal emitters.