Federico Valmorra

Low-bias active control of terahertz waves by coupling large-area CVD graphene to a terahertz metamaterial

We propose an hybrid graphene/metamaterial device based on terahertz electronic split-ring resonators directly evaporated on top of a large-area single-layer CVD graphene. Room temperature time-domain spectroscopy measurements in the frequency range from 250 GHz to 2.75 THz show that the presence of the graphene strongly changes the THz metamaterial transmittance on the whole frequency range. The graphene gating allows active control of such interaction, showing a modulation depth of 11.5% with an applied bias of 10.6 V. Analytical modeling of the device provides a very good qualitative and quantitative agreement with the measured device behavior. The presented system shows potential as a THz modulator and can be relevant for strong light-matter coupling experiments.

Highly tunable hybrid metamaterials employing split-ring resonators strongly coupled to graphene surface plasmons

Metamaterials and plasmonics are powerful tools for unconventional manipulation and harnessing of light. Metamaterials can be engineered to possess intriguing properties lacking in natural materials, such as negative refractive index. Plasmonics offers capabilities of confining light in subwavelength dimensions and enhancing light–matter interactions. Recently, the technological potential of graphene-based plasmonics has been recognized as the latter features large tunability, higher field-confinement and lower loss compared with metal-based plasmonics. Here, we introduce hybrid structures comprising graphene plasmonic resonators coupled to conventional split-ring resonators, thus demonstrating a type of highly tunable metamaterial, where the interaction between the two resonances reaches the strong-coupling regime. Such hybrid metamaterials are employed as high-speed THz modulators, exhibiting ∼60% transmission modulation and operating speed in excess of 40 MHz. This device concept also provides a platform for exploring cavity-enhanced light–matter interactions and optical processes in graphene plasmonic structures for applications including sensing, photo-detection and nonlinear frequency generation.

Ultrastrong coupling in the near field of complementary split-ring resonators

Ultrastrong coupling of split ring resonators to the cyclotron transition in two-dimensional electron gases is studied in the terahertz regime, clarifying the importance of the resonator geometry. The use of the complementary type of resonator allows removal of the signal from the uncoupled areas. The experimental results are of spectacular quality and quantity. A record high light-matter coupling ratio (normalized vacuum Rabi frequency) of 0.87 is achieved.

Strong Coupling in the Far-Infrared between Graphene Plasmons and the Surface Optical Phonons of Silicon Dioxide

We study plasmonic resonances in electrostatically gated graphene nanoribbons on silicon dioxide substrates. Absorption spectra are measured in the mid-far-infrared and reveal multiple peaks, with width-dependent resonant frequencies. We calculate the dielectric function within the random phase approximation and show that the observed spectra can be explained by surface-plasmon–phonon–polariton modes, which arise from coupling of the graphene plasmon to three surface optical phonon modes in silicon dioxide.

Electrically tunable graphene anti-dot array terahertz plasmonic crystals exhibiting multi-band resonances

Graphene-based plasmonic structures feature large tunability, high spatial confinement, and potentially low loss, and are therefore an emerging technology for unconventional manipulation of light. In this paper, we demonstrate electrically tunable terahertz plasmonic crystals consisting of square-lattice graphene periodic anti-dot arrays on a SiO2/Si substrate. Transmission spectroscopy reveals multiple distinct resonances arising from excitations of graphene surface-plasmon–polariton (SPP) modes on different branches of the SPP dispersion curves inherent to the periodic structures. The resonance frequencies are readily tuned electrostatically with the Si back-gate and exhibit the dependency on the carrier density unique to SPP in graphene. Simulations show excellent agreement with the experiments and further illustrate the symmetry-based selection rule for the excited graphene SPP modes. Such graphene plasmonic crystals may lead to a broad range of applications including plasmonic waveguide and transformation optics. Exploiting higher-order graphene SPP modes is an effective way to further facilitate field localization and enhancement.

Superconducting complementary metasurfaces for THz ultrastrong light-matter coupling

A superconducting metasurface operating in the THz range and based on the complementary metamaterial approach is discussed. Experimental measurements as a function of temperature and magnetic field display a modulation of the metasurface with a change in transmission amplitude and frequency of the resonant features. Such a metasurface is successively used in a cavity quantum electrodynamic experiment displaying ultrastrong coupling to the cyclotron transition of two-dimensional electron gas. A finite element modeling is developed and its results are in good agreement with the experimental data. In this system a normalized coupling ratio of is measured and a clear modulation of the polaritonic states as a function of the temperature is observed.

InGaAs/AlInGaAs THz quantum cascade lasers operating up to 195 K in strong magnetic field

Terahertz quantum cascade lasers based on InGaAs wells and quaternary AlInGaAs barriers were measured in magnetic field. This study was carried out on a four-quantum-well active-region design with photon energy of 14.3 meV processed with both Au and Cu waveguides. The heterostructure operates up to 148 K at B = 0 T in a Cu waveguide. The complete magneto-spectroscopic study allowed the comparison of emission and transport data. Increasing the magnetic field, the low effective mass of the InGaAs wells allowed us to reach the very strong confinement regime. At B = 12 T, where the cyclotron transition is almost resonant with the LO-phonon, we recorded a maximum operating temperature of 195 K for the devices with Cu waveguide. Additional lasing at 5.9 meV was detected for magnetic fields between 7.3 and 7.7 T.

Enhanced current injection from a quantum well to a quantum dash in magnetic field

Resonant tunneling injection is a key ingredient in achieving population inversion in a putative quantum dot cascade laser. In a quantum dot based structure, such resonant current requires a matching of the wavefunction shape in k-space between the injector and the quantum dot. We show experimentally that the injection into an excited state of a dash structure can be enhanced tenfold by an in-plane magnetic field that shifts the injector distribution in k-space. These experiments, performed on resonant tunneling diode structures, show unambiguously resonant tunneling into an ensemble of InAs dashes grown between two AlInAs barrier layers. They also show that interface roughness scattering can enhance the tunneling current.

Interaction of single-layer CVD graphene with a metasurface of terahertz split-ring resonators

The interaction of large-area single-layer CVD-graphene with a metasurface constituted by THz split-ring resonators was studied via THz Time-Domain Spectroscopy in the frequency range 250 GHz÷2.75 THz. Transmission measurements showed that the presence of the graphene shifts the resonances of the THz-metasurface towards lower energies and increases the transmittance, mainly at resonance. A comparison between two possible configuration is here presented revealing a much stronger interaction for the case of split-ring resonators evaporated directly onto the CVD-graphene layer with respect to the opposite configuration. From the recent literature the presented system is a good candidate for THz modulators with possible use also in cavity-QED experiments.

THz surface plasmon polariton modes coupled to complementary metasurfaces tuned by inter meta-atom distance

Tailoring the electro-magnetic response of materials beyond naturally occurring properties is possible with the concept of meta-materials [1]. Subwavelength elements which are usually closely spaced can influence the electro-magnetic response and form a fundamental building block of modern optics. The influence of the spacing of the meta-atoms has been investigated for direct meta-materials [2] but only little for complementary metamaterials [3], which are of interest e.g. in ultra-strong coupling experiments at THz frequencies [4]. The effective medium condition is changing due to the presence of a metal sheet in between the meta-atoms which has a very high refractive index in the THz.