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An innovative method has developed recently for biasing the varactors of a reconfigurable intelligent surface (RIS) by utilizing resonant standing waves on the “biasing transmission line (TL)” [E. Ayanoglu, F. Capolino, and A. L. Swindlehurst, “Wave-controlled metasurface-based reconfigurable intelligent surfaces,” IEEE Wireless Communications, vol. 29, no. 4, pp. 86-92,2022] located beneath the reflective surface. Using this approach, each RIS element does not require separate external biasing. For estimating the RIS reflection properties controlled by varactors, we analyze a planar array with phase gradient in one direction, of side length L, of reconfigurable elements. We employ the analytical model for predicting the reflection coefficients of the unit cells presented in [D. Hanna, M. Saavedra-Melo, F. Shan, and F. Capolino, “A versatile polynomial model for reflection by a reflective intelligent surface with varactors,” IEEE AP-S/URSI, 2022] and investigate how the standing wave biasing approach compares with the traditional way to generate field patterns of the reflected wave. 

A novel method for biasing the varactors of a reconfigurable intelligent surface (RIS) by using resonant standing waves on the biasing transmission line (TL) at a layer below the RF reflective surface to eliminate the need to bring external bias for each element of the RIS is described. We use an analytical model of the RIS to compare the field pattern of the reflected wave by (i) considering the ideal case, (ii) the case where reflection accounts for the varactor's model, and (iii) the case as in (ii) but where the biasing voltage distribution is constructed by using the wave control (i.e., standing waves). 

The physics of exceptional points leads to very high sensitivity because the perturbation of an exceptionally degenerate state is highly sensitive to a system’s perturbation. This property is indeed not shared with nondegenerate systems, and it relies in the fractional power expansion (Puiseux series) describing the perturbation of eigenvalues and eigenvectors. We discuss how this property is met in systems made of coupled resonators and with coupled modes in waveguides, whose eigenvalues are the resonant frequencies and the wavenumbers, respectively. We will also discuss the experimental implementation of this principle in unstable nonlinear systems to build extremely sensitive saturated oscillators. 

We design a three-way silicon optical waveguide with the Bloch dispersion relation supporting a stationary inflection point (SIP). The SIP is a third order exceptional point of degeneracy (EPD) where three Bloch modes coalesce forming the frozen mode with greatly enhanced amplitude. The proposed design consists of a coupled resonators optical waveguide (CROW) coupled to a parallel straight waveguide. At any given frequency, this structure supports three pairs of reciprocal Bloch eigenmodes, propagating and/or evanescent. In addition to full-wave simulations, we also employ a so-called “hybrid model” that uses transfer matrices obtained from full-wave simulations of sub-blocks of the unit cell. This allows us to account for radiation losses and enables a design procedure based on minimizing the eigenmodes’ coalescence parameter. The proposed finite-length CROW displays almost unitary transfer function at the SIP resonance, implying a nearly perfect conversion of the input light into the frozen mode. The group delay and the effective quality factor at the SIP resonance show an $N^3$ scaling, where N is the number of unit cells in the cavity. The frozen mode in the CROW can be utilized in various applications like sensors, lasers and optical delay lines. 

We will discuss two kinds of exceptional points of degeneracy in waveguides and their respective application in lasers. Such exceptional points occur in waveguides with balanced loss and gain (e.g., PT symmetry), and in waveguides without loss and gain (e.g., periodic Si waveguides). Waveguides with such exceptional points have a strong degeneracy of their wavenumbers and polarization states that enables specific wave physics, only found in these degenerate systems. We will discuss advantages and disadvantages of both concepts to conceive laser regimes, related to high power, high spectral purity, high efficiency, etc, and show some realistic designs involving Si ridge waveguides. 

We theoretically show that the optical chiral properties of tightly focused laser beams can be characterized by means of force detection. To measure the chiral properties of a beam of given handedness in the microscopic focal volume, we determine the photoinduced force exerted on a sharp tip, which is illuminated first by the beam of interest and second by an auxiliary beam of opposite handedness, in a sequential manner. We show that the difference between the force measurements is directly proportional to the chiral properties of the beam of interest. In particular, the gradient force difference Δ⟨Fgrad ,z⟩ is found to have exclusive correspondence to the time-averaged helicity density of the incident light, whereas the differential scattering force provides information about the spin angular momentum density of light. We further characterize and quantify the helicity-dependent Δ⟨Fgrad ,z⟩ using a Mie scattering formalism complemented with full wave simulations, underlining that the magnitude of the difference force is within an experimentally detectable range.