• critical coupling
• graphene
• intersubband polariton
• microcavity
• plasmons
• quantum well
• THz photonics

Light and light-matter interaction not only lie at the heart of everyday life, but also are the foundations of many fields of contemporary science. Nowadays, photonic sciences play a central role in the development of new technologies and new systems for the research of exotic properties of nature. Most applications of modern photonics go in the direction of nanophotonics, which studies how to confine light in nanometric structures, down to atomic scales. Confining light on such small volumes yields a great enhancement of the field intensity, thus unlocking new regimes of light-matter interaction, such as the strong coupling regime.
Among the ways of confining light to the nanoscale, the use of surface plasmon modes (SPs) is one of the most exploited. Surface plasmons are collective oscillations of the surface charge density, resulting from the coupling of electromagnetic waves with electronic motion at the interface between a metal and a dielectric. An extremely appealing platform for plasmons at THz and Mid-IR frequencies is graphene, a one-atom-thick layer of carbon which exhibits broadband and tunable optical properties. Graphene sustains surface waves similar to surface plasmons, but with the advantage that its plasmonic properties can be electrically controlled with an external gate in a FET-like geometry. Another system widely used to confine light in THz photonics is represented by Metal Insulator Metal (MIM) microcavities. These structures support resonant modes where the electric field is strongly localized between the metal plates. MIM microcavities can be engineered to increase the efficiency with which light couples to the cavity modes, enabling a resonant transfer of energy inside the struc- ture. This condition is called critical coupling.
The goal of this thesis is to develop a system with tunable optical properties, by combining the concept of microcavities and graphene. We substitute one of the metallic faces of a MIM microcavity with CVD-grown graphene crystals featuring on top metallic antennas designed to launch graphene plasmons. Exploiting the tunability of graphene plasmons, we show that it is possible to electrically tune the efficiency with which light is trapped inside the structure, thus enabling for a dynamical control of the absorption at selected wavelengths. In my work, I present the design, fabrication and spectroscopic characterization of the proposed device. In particular, I perform numerical simulations with the Finite Element Method and the Rigorous Coupled Wave Analysis to design the system and optimize it for the fabrication. I compute the electric field distribution inside the structure, and I analyze the resonant behavior of graphene plasmons when their wavelength is comparable with the distance between the antennas, showing that they enter in a Fabry-Perot like resonant regime with a consequent discretization of their wavelengths. Furthermore, I compute the reflectivity of the proposed device when a doped quantum well is placed close to the surface. In this way I explore the possibility of coupling the intersubband excitations with the resonant mode of the structure for different doping levels and different distances from the surface of the device. I also present an overview of the fabrication process and I show the methods used to carry out the electron beam lithography of sub-micrometric patterns over millimetric areas. Then, I describe a setup to perform a Fourier Transform InfraRed (FTIR) spectroscopy in reflection and I analyze the experimental results for two different sets of samples. These measurements highlight the effect of the metallic pattern in trapping the light, but they do not yet show the enhancement effect of graphene. New measurements will be soon performed gating graphene in order to further determine its contribution.
In the end, I discuss how to improve the experimental setup for further measurements, and I show what are my prospects for the future of the project. This kind of research is of great interest both for practical applications and for fundamental physics research. For example, a control of the optical response of such structures could lead to the development of new microcavity-integrated graphene photodetectors with in situ spectral selectivity. Furthermore, this kind of microcavities could be integrated with quantum wells to exploit the increased energy density to enhance the coupling of the cavity mode with intersubband transitions. The electrical control of this coupling could be used to turn on and off the strong coupling regime, giving the possibility of studying new properties of intersubband polaritons, i.e. the quasiparticles emerging from the mixing of intersubband excitations and photonic modes.