Georgios Sinatkas

We report on the extraction of silver losses in the range 10 K-180 K by performing temperature-dependent micro-photoluminescence measurements in conjunction with numerical simulations on silver-coated nanolasers around near-infrared telecommunication wavelengths. By mapping changes in the quality factor of nanolasers into silver-loss variations, the imaginary part of silver permittivity is extracted at cryogenic temperatures. The latter is estimated to reach values an order of magnitude lower than room-temperature values. Temperature-dependent values for the thermo-optic coefficient of III-V semiconductors occupying the cavity are estimated as well. This data is missing from the literature and is crucial for precise device modeling. Our results can be useful for device designing, the theoretical validation of experimental observations as well as the evaluation of thermal effects in silver-coated nanophotonic structures.

Electro-optic modulators are an indispensable part of photonic communication systems, largely dictating the achievable transmission rate. Recent advances in materials and fabrication/processing techniques have brought new elements and a renewed dynamic to research on optical modulation. Motivated by the new opportunities, this Perspective reviews the state of the art in integrated electro-optic modulators, covering a broad range of contemporary materials and integrated platforms. To provide a better overview of the status of current modulators, an assessment of the different material platforms is conducted on the basis of common performance metrics: extinction ratio, insertion loss, electro-optic bandwidth, driving voltage, and footprint. The main physical phenomena exploited for electro-optic modulation are first introduced, aiming to provide a self-contained reference to researchers in physics and engineering. Additionally, we take care to highlight topics that can be overlooked and require attention, such as the accurate calculation of carrier density distribution and energy consumption, the correct modeling of thin and two-dimensional materials, and the nature of contact electrodes. Finally, a future outlook for the different electro-optic materials is provided, anticipating the research and performance trends in the years to come.

Graphene exhibits high third-order nonlinearity in both THz and NIR regimes. Here, we consider single- and multi-channel nonlinear phenomena in resonators comprising graphene to exploit its high nonlinearity. Specifically, we study optical bistability in the NIR with a graphene-covered silicon slot microring resonator and degenerate four-wave mixing in the THz regime with a graphene ribbon resonator. For calculating the nonlinear response, we employ a recently developed perturbation theory/coupled mode theory framework which is both efficient and accurate. We find low power requirements and high conversion efficiencies, highlighting graphene's potential for nonlinear applications.

We discuss arrays of metal-semiconductor-metal cavities as electrically tunable terahertz metasurfaces. The operation of the considered device is based on reverse biasing the Schottky junction formed between top metal strips and the n-type semiconductor buried beneath. The effective Drude permittivity of the cavity array is tuned by changing the depletion layer thickness via a gate bias applied between the strips and a back metal reflector. Combining Maxwell equations for terahertz waves with a drift-diffusion model for the semiconductor carriers into a multiphysics framework, we show that the proposed modulation concept is promising for a large part of the terahertz spectrum.

A rigorous mathematical framework combining temporal coupled-mode theory (CMT) and perturbation theory for modeling wave mixing in resonant systems involving 2D (sheet) materials is developed, expanding the existing literature that focuses on wave mixing in bulk systems [1]. Throughout this work, conductive 2D materials are naturally modeled with a nonlinear surface current term, rigorously implemented in the nonlinear CMT formalism to accurately describe the system response using coefficients calculated after linear finite-element (FEM) simulations. The developed framework is flexible, allowing the systematic incorporation of other nonlinear effects such as the Kerr effect, two photon absorption, and saturable absorption [2], unfolding the complex dynamic behavior of the system, that can also demonstrate optical bistability and limit cycles [1,3].

The correct numerical calculation of the resonance characteristics and, principally, the quality factor Q of contemporary photonic and plasmonic resonant systems is of utmost importance, since Q defines the bandwidth and affects nonlinear and spontaneous emission processes. Here, we comparatively assess the commonly used methods for calculating Q using spectral simulations with commercially available, general-purpose software. We study the applicability range of these methods through judiciously selected examples covering different material systems and frequency regimes from the far-infrared to the visible. We take care in highlighting the underlying physical and numerical reasons limiting the applicability of each one. Our findings demonstrate that in contemporary systems (plasmonics, 2D materials) Q calculation is not trivial, mainly due to the physical complication of strong material dispersion and light leakage. Our work can act as a reference for the mindful and accurate calculation of the quality factor and can serve as a handbook for its evaluation in guided-wave and free-space photonic and plasmonic resonant systems.

We propose metal-semiconductor-metal cavity arrays as the active element of electrically tunable metasurfaces operating in the terahertz spectrum. Their function is based on reverse biasing the Schottky junction formed between top metal strips and the n-type semiconductor buried beneath. A gate bias between the strips and a back metal reflector controls the electron depletion layer thickness thus tuning the Drude permittivity of the cavity array. Using a rigorous multiphysics framework which combines Maxwell equations for terahertz waves and the drift-diffusion model for describing the carrier behavior in the semiconductor, we find a theoretically infinite extinction ratio, insertion loss of around 10% and picosecond intrinsic switching times at 1 THz, for a structure designed to enter the critical coupling regime once the depletion layer reaches the bottom metal contact. We also show that the proposed modulation concept can be used for devices operating at the higher end of the terahertz spectrum, discussing the limitations on their performance.