Recently, non-Hermitian Hamiltonians have gained a lot of interest, especially in optics and electronics. In particular, the existence of real eigenvalues of non-Hermitian systems has opened a wide set of possibilities, especially, but not only, for sensing applications, exploiting the physics of exceptional points. In particular, the square root dependence of the eigenvalue splitting on different design parameters, exhibited by 2 × 2 non-Hermitian Hamiltonian matrices at the exceptional point, paved the way to the integration of high-performance sensors. The square root dependence of the eigenfrequencies on the design parameters is the reason for a theoretically infinite sensitivity in the proximity of the exceptional point. Recently, higher-order exceptional points have demonstrated the possibility of achieving the nth root dependence of the eigenfrequency splitting on perturbations. However, the exceptional sensitivity to external parameters is, at the same time, the major drawback of non-Hermitian configurations, leading to the high influence of noise. In this review, the basic principles of PT-symmetric and anti-PT-symmetric Hamiltonians will be shown, both in photonics and in electronics. The influence of noise on non-Hermitian configurations will be investigated and the newest solutions to overcome these problems will be illustrated. Finally, an overview of the newest outstanding results in sensing applications of non-Hermitian photonics and electronics will be provided.

This theoretical modeling and simulation paper presents designs and projected performance of non-volatile broadband on-chip 1×2 and 2×2 electro-optical switches operating in the telecommunication C-band and based on the silicon-on-insulator technological platform. These optical switches consist of an asymmetric two-waveguide directional coupler and a symmetric three-waveguide directional coupler in which the optical phase change material (O-PCM) Ge2Sb2Se4Te1 is the top cladding layer for one of the silicon strip waveguides. Reversible cross-bar switching is attained by the amorphous (Am) to crystalline (Cr) and Cr-to-Am phase transitions in the GSST induced by heating the GSST in contact with an ITO microstrip through Joule heating. We examined device performance in terms of mid-band insertion loss (IL), crosstalk (CT) and 0.3-dB-IL bandwidth (BW). The 2×2 results for the coupling length Lc of 15.4 μm were IL = -0.018 dB, CT < 31.3 dB and BW = 58 nm for $\rm 17.4 \mu m L_c$ , IL = 0.046 dB, CT < 38.1 dB and BW = 70 nm. Simulations of the 1×2 devices at 16.7 μm Lc revealed that IL = 0.083 dB, CT < 12.8 dB along with an expanded BW of 95 nm. Thermal simulations showed that a 5 V pulse train applied to $\rm 10^{19} cm^{-3}$ -doped ITO would produce crystallization; however the process of amorphization required a 24 V pulse of 2.9 μs duration to raise the GSST temperature above the 900 K melting temperature.

Recently, the Vernier effect has been proved to be very efficient for significantly improving the sensitivity and the limit of detection (LOD) of chemical, biochemical and gas photonic sensors. In this paper a review of compact and efficient photonic sensors based on the Vernier effect is presented. The most relevant results of several theoretical and experimental works are reported, and the theoretical model of the typical Vernier effect-based sensor is discussed as well. In particular, sensitivity up to 460 μm/RIU has been experimentally reported, while ultra-high sensitivity of 2,500 μm/RIU and ultra-low LOD of 8.79 × 10⁻⁸ RIU have been theoretically demonstrated, employing a Mach-Zehnder Interferometer (MZI) as sensing device instead of an add drop ring resonator.

We propose an alternative readout scheme for the ultrasensitive gyroscope shown in [1], which uses the physics of the exceptional points of a PT-symmetric system to enhance the device sensitivity. In particular, we demonstrate that the full width at half maximum linewidth (ΔωFWHM) of the light exiting the ring resonator system is proportional to the square root of the rotation rate (Ω). Moreover, we design a PT-symmetric gyroscope, kept at the exceptional point (at rest) with two 100 μm-radius rings, obtaining a theoretical ΔωFWHM of 3.37ċ104 rad/s for an angular velocity of 100 °/h. One algorithm capable to keep the system at the exceptional point and another one to identify the direction of rotation are also proposed.

In this paper, a spectroscopic sensor formed by a silicon-on-insulator waveguiding Bragg grating ring resonator working in linear and non-linear regime is proposed. In linear regime, the device shows a spectral response characterized by a photonic band gap (PBG). Very close to the band gap edges, the resonant structure exhibits split modes having a splitting magnitude equal to the PBG spectral extension, whose characteristics can be exploited to obtain a RI optical sensor almost insensitive to the fabrication tolerances and environmental perturbations. When the device operates in nonlinear regime, exactly in the spectral region showing the split resonant modes, the RI sensing performance is strongly improved with respect to the linear regime. This improvement, demonstrated by taking into account all the non-linear effects excited in the integrated silicon structure (i.e., Two Photon Absorption (TPA), TPA-induced Free Carrier Absorption, plasma dispersion, Self-Phase-Modulation and Cross-Phase-Modulation effects as induced by Kerr nonlinearity) as well as the deleterious thermal and stress effects, allows enhancing the performance of the RI split mode resonant sensors, while achieving good immunity to the fabrication tolerances and environmental perturbations. The improvement in terms of sensor resolution can be at least one order of magnitude, still without using optimal parameters.

An accurate design of an innovative fiber optic temperature sensor is developed. The sensor is based on a cascade of three microstructured optical fibers (MOFs). In the first one a suitable cascade of long period gratings is designed into the core. A single mode intermediate and a rare-earth activated Fabry-Perot optical cavity are the other two sensor MOF sections. An exhaustive theoretic feasibility investigation is performed employing computer code. The complete set-up for temperature monitoring can be obtained by utilizing only a low cost pump diode laser at 980 nm wavelength and a commercial optical power detector. The simulated sensitivity S = 315.1 μW/°C and the operation range ΔT = 100 °C is good enough for actual applications.