• QCLs
• graphene
• THz

In recent decades, Quantum Cascade Lasers (QCLs) have emerged as one of the most promising radiation sources at Terahertz (THz) frequencies. Basically, QCLs are unipolar devices exploiting optical transitions between electronic conduction subbands created by spatial confinement in semiconductor multi-quantum-wells.
THz QCLs exhibit excellent coherence properties and good optical powers in the frequency range between 1.2 and 5.4 THz, making them suitable for spectroscopic applications including gas sensing, medical diagnostics, environmental monitoring, security and observational astrophysics.
Despite their remarkable properties, some challenges still remain open for technological applications. One of the main goals is the achievement of a high confinement low-loss waveguide for such long wavelengths, allowing the realization of low threshold lasers and their consequent miniaturization.
The waveguides commonly used for THz QCLs are double metal and semi-insulating surface-plasmon waveguides, which exploit metal Surface Plasmon Polaritons (SPPs).
Between them, double metal ones are the most suitable since they guarantee near unity confinement factor (i.e. the overlap of the guided mode with the active region) and low lasing threshold current. Moreover, thanks to the lack of a cut-off frequency, they enable to confine a transverse magnetic mode (that couples to the intersubband electronic transition of the active region) inside the cavity independently from its transverse dimension. This property has been useful for device miniaturization since it allows to focus on the reduction of planar size. Several subwavelength microcavities have been realized with double metal waveguides; in particular, microdisk resonators, operating on whispering-gallery modes (WGMs), are known to yield lasers with very low threshold currents thanks to the combination of a small-mode size and high mode quality factors. However, being the used metals not perfect conductors, there is a relevant light absorption that increases as the wavelength exceeds the waveguide thickness.
Thus, the ohmic losses in the metal are the main factor that limits the lowest possible achievable current threshold.
Hence, the idea of this thesis is to exploit an innovative low-loss waveguide based on graphene SPPs. Owing to the two-dimensional collective nature of these excitations, graphene SSPs are confined much more strongly than in conventional metals, meaning that they would allow a further miniaturization and reduction of the injection power.
This novel structure consists in a WGM resonator exploiting a waveguide similar to the double metal one, which instead of having gold as top surface, adopts graphene embedded in two layers of hexagonal-Boron Nitride (h-BN). In this configuration, the already peculiar graphene electronic properties are further increased. Indeed, it has been proved that encapsulated graphene leads to electronic mobilities comparable with the ones reached in free-standing graphene (200000 cm^2/Vs in suspended graphene and 140000 cm^2/Vs in h-BN/graphene/h-BN heterostructures at room temperature).
Here, a microdisk resonator based on h-BN/graphene/h-BN heterostructure was studied via Finite Element Method (FEM) simulations and compared with a microdisk exploiting double metal waveguides. The system was investigated at first by varying its geometry and, then, by varying the graphene electronic properties, like its Fermi energy and the electronic scattering time. Simulations revealed that graphene SPPs confine the electromagnetic energy inside the active region in a deeply sub-wavelength effective mode volume. The reduction of the mode volume leads to a strong enhancement of the Purcell factor. The higher Purcell factor together with the low ohmic losses could result in ultra-small lasing threshold. Moreover, the possibility to tune graphene conductivity by changing the carrier density via chemical doping or electrostatic gating, leads to tunable SPPs resulting in controlled laser emission properties (e.g the laser frequency and far-field profile).
In addition, simulations showed that placing two identical subwavelength microdisk resonators in close proximity leads to a coupling between separate optical modes that results in a confined and vertical laser emission. Vertical emission in addition with the extremely sub-wavelength dimensions of the proposed device can allow massive parallelization of THz QCLs with high brightness and very low power consumption.
Besides the simulation study, all the steps for the final fabrication of the microdisk resonator were implemented using different nano-fabrication techniques, such as Electron Beam Lithography (EBL), Reactive Ion Etching (RIE), thermal metal deposition and Inductively Coupled Plasma-Reactive Ion Etching (ICP-RIE). Both graphene and h-BN were mechanical exfoliated and assembled in h BN/graphene/h-BN heterostructures using the pick up technique. The characterization of the h-BN/graphene/h-BN stacks were performed, at first, by Atomic Force Microscopy (AFM) and Raman spectroscopy, that has made possible to estimate the carrier density of graphene encapsulated in the two h-BN layers. Moreover, by electrical measurements, some information were obtained through a Field Effect Transistor (FET) based on h-BN/graphene/h-BN hetrostructure.
In summary, this thesis revealed an innovative waveguide for THz QCLs that exploits SSPs of graphene encapsulated in two h-BN layers. The extremely high Purcell factor combined with the low ohmic losses achieved in this waveguide suggests the presented device as a candidate for the realization of unprecedented sub-wavelength low threshold THz QCLs.