We theoretically investigate the enhancement of optical absorption in thin-film organic solar cells in which the top transparent electrode is partially substituted by a periodic metallic grating. We show that the grating can result in broadband optical absorption enhancement for TM-polarized light, due to the large field enhancement in the vicinity of the strips of the grating, associated with the excitation of plasmonic modes. The overall optical absorption in the organic layers can be greatly enhanced up to $\sim 50\mathrm{\%}$ for such solar cell structures.

We theoretically investigate the properties of absorption switches for metal-dielectric-metal (MDM) plasmonic waveguides. We show that a MDM waveguide directly coupled to a cavity filled with an active material with tunable absorption coefficient can act as an absorption switch, in which the on/off states correspond to the absence/presence of optical pumping. We also show that a MDM plasmonic waveguide side-coupled to a cavity filled with an active material can operate as an absorption switch, in which the on/off states correspond to the presence/absence of pumping. For a specific modulation depth, the side-coupled-cavity switch results in more compact designs compared to the direct-coupled-cavity switch. Variations in the imaginary part of the refractive index of the material filling the cavity of Δκ = 0.01 (Δκ = 0.15) result in ~60% (~99%) modulation depth. The properties of both switches can be accurately described using transmission line theory.

In this paper, we provide a comprehensive review of unidirectional reflectionless light propagation in photonic devices at exceptional points (EPs). EPs, which are branch point singularities of the spectrum, associated with the coalescence of both eigenvalues and corresponding eigenstates, lead to interesting phenomena, such as level repulsion and crossing, bifurcation, chaos, and phase transitions in open quantum systems described by non‐Hermitian Hamiltonians. Recently, it was shown that judiciously designed photonic synthetic matters could mimic the complex non‐Hermitian Hamiltonians in quantum mechanics and realize unidirectional reflection at optical EPs. Unidirectional reflectionlessness is of great interest for optical invisibility. Achieving unidirectional reflectionless light propagation could also be potentially important for developing optical devices, such as optical network analyzers. Here, we discuss unidirectional reflectionlessness at EPs in both parity‐time ( PT )‐symmetric and non‐ PT ‐symmetric optical systems. We also provide an outlook on possible future directions in this field.

This paper shows how to analyze plasmonic metal-insulator-metal waveguides using the full modal structure of these guides. The analysis applies to all frequencies, particularly including the near infrared and visible spectrum, and to a wide range of sizes, including nanometallic structures. We use the approach here specifically to analyze waveguide junctions. We show that the full modal structure of the metal-insulator-metal (MIM) waveguides—which consists of real and complex discrete eigenvalue spectra, as well as the continuous spectrum—forms a complete basis set. We provide the derivation of these modes using the techniques developed for Sturm-Liouville and generalized eigenvalue equations. We demonstrate the need to include all parts of the spectrum to have a complete set of basis vectors to describe scattering within MIM waveguides with the mode-matching technique. We numerically compare the mode-matching formulation with finite-difference frequency-domain analysis and find very good agreement between the two for modal scattering at symmetric MIM waveguide junctions. We touch upon the similarities between the underlying mathematical structure of the MIM waveguide and the $\mathcal{P}\mathcal{T}$ symmetric quantum-mechanical pseudo-Hermitian Hamiltonians. The rich set of modes that the MIM waveguide supports forms a canonical example against which other more complicated geometries can be compared. Our work here encompasses the microwave results but extends also to waveguides with real metals even at infrared and optical frequencies.

We introduce a plasmonic waveguide system, which supports a subwavelength broadband slow-light guided mode with a tunable slowdown factor at a given wavelength. The system consists of a metal–dielectric–metal (MDM) waveguide side-coupled to a periodic array of MDM stub resonators. The slowdown factor at a given wavelength can be tuned by adjusting the geometrical parameters of the system. In addition, there is a trade-off between the slowdown factor and the propagation length of the supported optical mode. Finally, we show that light can be coupled efficiently from a conventional MDM waveguide to the plasmonic waveguide system.

We numerically design and experimentally test a SERS-active substrate for enhancing the SERS signal of a single layer of graphene (SLG) in water. The SLG is placed on top of an array of silver-covered nanoholes in a polymer and is covered with water. Here we report a large enhancement of up to 2 × 10⁵ in the SERS signal of the SLG on the patterned plasmonic nanostructure for a 532 nm excitation laser wavelength. We provide a detailed study of the light-graphene interactions by investigating the optical absorption in the SLG, the density of optical states at the location of the SLG, and the extraction efficiency of the SERS signal of the SLG. Our numerical calculations of both the excitation field and the emission rate enhancements support the experimental results. We find that the enhancement is due to the increase in the confinement of electromagnetic fields on the location of the SLG that results in enhanced light absorption in the graphene at the excitation wavelength. We also find that water droplets increase the density of optical radiative states at the location of the SLG, leading to enhanced spontaneous emission rate of graphene at its Raman emission wavelengths.