Thomas Christopoulos

A light-emitting metasurface for highly efficient lasing in the visible is proposed and thoroughly theoretically studied. The metasurface comprises split-square slots etched in a thin TiO ₂ layer and periodically arrayed in a subdiffractive lattice. The slots are filled with highly fluorescent rhodamine 6G (R6G) molecules in a methanol matrix, providing optically pumped gain under pulsed excitation. Lasing is enabled by a slot-type quasi-bound state in the continuum (qBIC) offering enhanced light-matter interaction. Through a meticulous design process, the lasing threshold is minimized by mitigating reabsorption losses and effectively coupling the pump light to a bright mode exhibiting significant angular dispersion. This enables relaxed resonant pumping conditions at the desired wavelength by appropriately choosing the incidence angle. To assess the lasing response, a rigorous temporal coupled-mode theory (CMT) framework is employed, which is adapted to organic dye gain media accounting for the critical reabsorption effect. The proposed metasurface laser has a very low lasing threshold of 1 kW/cm ² and remarkably high efficiency of 15% at 570 nm.

A careful design approach is presented for an integrated nanophotonic laser cavity, consisting of a silicon-rich silicon nitride ring resonator and a transition metal dichalcogenide (TMD) monolayer (WSe ₂ ) on-top as the optically pumped gain medium. The structure is analyzed and designed utilizing full-wave finite-element simulations and a comprehensive theoretical framework based on perturbation theory, temporal coupled-mode theory (CMT), and Maxwell–Bloch equations that rigorously capture both the temporal cavity (photonic) response and the carrier dynamics. The cavity is carefully designed following directives dictated by the utilized computational framework, ensuring a satisfying compromise between efficient pump absorption, low lasing threshold, and extended bandwidth up to 65 GHz. The dynamic response of the laser is in-depth examined by the CMT to estimate its direct modulation bandwidth in terms of system design parameters (cavity lifetime) and gain material properties (metastable carrier relaxation lifetime).

Temporal coupled-mode theory (CMT) is an acclaimed and widely used theoretical framework for modeling the continuous-wave response and temporal dynamics of any integrated or free-space photonic resonant structure. It was initially employed to understand how energy is coupled into and out of a cavity and how it is exchanged between different resonant modes. In the 30 years that followed its establishment, CMT has been expanded to describe a broad range of nonlinear interactions as well (self- and cross-phase modulation, saturable absorption, frequency generation, gain, etc.). In this Tutorial, we thoroughly present the basic principles and the evolution of CMT throughout the years, showcasing its immense capabilities for the analysis and design of linear and nonlinear resonant photonic systems. Importantly, we focus on the examples of modern, open nanophotonic resonators incorporating contemporary bulk or sheet (2D) materials that may be lossy and dispersive. For each linear/nonlinear effect under study, we follow a meticulous, step-by-step approach, starting from an accurate model of the physical phenomenon and proceeding to its introduction in the CMT framework all the way to the efficient solution of the resulting system of equations. Our work highlights the merits of CMT as an efficient, accurate, and versatile theoretical tool. We envision that it can serve both as an introductory reference for any reader and as a comprehensive handbook on how to incorporate a broad range of linear and nonlinear effects in the CMT framework.

A multimode framework that can be utilized in the analysis and design of general, non-Hermitian periodic systems that comprise contemporary 2D materials is developed. The theoretical framework is based on the concept of Quasinormal Modes (QNMs) that can be used to efficiently retrieve the spectral response of periodic systems. Both sub- and super-wavelength lattice constants are examined with extra care to include propagating higher diffraction orders. The framework is employed in two indicative configurations, a graphene-based periodic metasurface that supports tightly confined graphene surface plasmons, and a periodic dielectric metasurface enhanced with a transition metal dichalcogenide layer that supports quasi bound states in the continuum. Both configurations are of fundamental and practical interest since they exhibit controllable resonant response and, in addition, the electromagnetic properties of the involved 2D materials can be electrically tuned. The Finite Element Method (FEM) is used to retrieve the QNMs, which are then fed to the framework to specify the spectral response for the zeroth and higher diffraction orders. Full-wave FEM simulations are used to verify the validity of obtained results.

A nanophotonic passively Q‐switched lasing element in the near infrared is proposed and theoretically investigated. It consists of a silicon‐rich nitride disk resonator enhanced with the contemporary hetero‐bilayer and a graphene monolayer to provide gain and saturable absorption, respectively. The two‐dimensional materials are placed on top of the disk resonator and are separated by a spacer of hexagonal boron nitride. emits at 1128 nm due to the radiative recombination of interlayer excitons after being optically pumped at 740 nm. Optical pumping is conducted in a guided‐wave manner aiming at achieving a high overall efficiency by critically coupling to a cavity mode near the pump transition. The response of the proposed pulsed laser is assessed by utilizing a coupled‐mode theory framework fed with linear finite‐element method simulations, rigorously derived from the Maxwell–Bloch equations. Following a meticulous design process and exploiting the guided pumping scheme, an ultralow lasing threshold of just μW is obtained. Overall, the Q‐switched laser delivers pulsed light inside an integrated bus waveguide with mW peak power, ps duration, and GHz repetition rates requiring sub‐mW continuous wave pumping. These properties are highly promising for communication applications and highlight the potential of two‐dimensional materials for nanophotonic light sources.