Plasmonic mode coupling and thin film sensing in metal-insulator-metal structures

SPP excitation and coupling mechanism based on micro/nano fibers

A hot trend in the development of optoelectronic devices is how to use the principle of surface plasmon resonance to enhance the performance of integrated photonics devices and achieve miniaturization. This paper proposes an accompanying waveguide coupling structure of micro/nano fibers, which consists of two parallel-placed micro/nano fibers (MNFs) coated with a silver film in the waist region and infused with a refractive index matching oil. In the overlapping region, there exists a segment of surface plasmon polaritons (SPPs) coupling area. The excitation and coupling characteristics of SPPs are studied through numerical simulation. Optimal coupling enhancement configuration is obtained by studying variables such as spacing distance, coupling length, and metal film thickness. A comparison is made with the SPP intensity of a single MNF, showing a 220% increase in electric field intensity, demonstrating its excellent coupling effect. By using this coupling structure, exploration of SPPs excitation and coupling mechanisms is enhanced, and structures resembling interferometric devices can be designed, providing new insights for high-performance miniaturized devices.

Deep Learning for Optical Sensor Applications: A Review

Over the past decade, deep learning (DL) has been applied in a large number of optical sensors applications. DL algorithms can improve the accuracy and reduce the noise level in optical sensors. Optical sensors are considered as a promising technology for modern intelligent sensing platforms. These sensors are widely used in process monitoring, quality prediction, pollution, defence, security, and many other applications. However, they suffer major challenges such as the large generated datasets and low processing speeds for these data, including the high cost of these sensors. These challenges can be mitigated by integrating DL systems with optical sensor technologies. This paper presents recent studies integrating DL algorithms with optical sensor applications. This paper also highlights several directions for DL algorithms that promise a considerable impact on use for optical sensor applications. Moreover, this study provides new directions for the future development of related research.

The Optimization of Metal Nitride Coupled Plasmon Waveguide Resonance Sensors Using a Genetic Algorithm for Sensing the Thickness and Refractive Index of Diamond-like Carbon Thin Films

We theoretically designed the Kretschmann configuration coupled plasmon-waveguide resonance (CPWR) sensors, composed of thin films of metal nitrides. The thicknesses of the layers of the CPWR sensors were optimized using a genetic algorithm. The optimized CPWR sensors were applied to simultaneously measure the thickness and refractive index (RI) of diamond-like carbon (DLC) films. The field profiles and the sensitivity of the CPWR sensors in response to thin DLC films were studied using the finite-different time-domain technique and the transfer matrix method. The genetic algorithm method predicted that the two-mode CPWR sensors could simultaneously analyze the thickness and RI of the DLC films as thin as 1.0 nm at a wavelength of 1550 nm. The simulations showed that the angular sensitivity toward the refractive index changes of the DLC films of the optimized CPWR sensors was comparable to that of traditional CPWR sensors.

Multiparameter Sensing Based on Tunable Fano Resonances in MIM Waveguide Structure with Square-Ring and Triangular Cavities

In this paper, a metal–insulator–metal (MIM) surface plasmon waveguide structure is proposed and numerically investigated. It is composed of a square-ring cavity with a silver baffle, an isosceles triangle cavity, and a bus waveguide with a silver baffle. The results show that the structure can produce triple Fano resonances that can be independently tuned by changing the structural parameters. The detection of refractive indexes at different positions in the structure was also accomplished, with a maximum sensitivity of 2259.56 nm/RIU. On the basis of this, the simultaneous measurement of multiple parameters (plasma concentration and glucose concentration) was performed. The numerical simulation results are beneficial to the applications of MIM waveguide structure in nanosensing and biosensing with time-sharing or simultaneous measurement of multiple parameters.

Design Strategy for Nanostructured Arrays of Metallodielectric Cuboids to Systematically Tune the Optical Response and Eliminate Spurious Bulk Effects in Plasmonic Biosensors

Plasmonic biosensors are a powerful tool for studying molecule adsorption label-free and with high sensitivity. Here, we present a systematic study on the optical properties of strictly regular nanostructures composed of metallodielectric cuboids with the aim to deliberately tune their optical response and improve their biosensing performance. In addition, the patterns were tested for their potential to eliminate spurious effects from sensor response, caused by refractive index changes in the bulk solution. Shifts in the plasmonic spectrum are exclusively caused by the adsorbing molecules. For this purpose, nanopatterns of interconnected and separated cubes with dimensions ranging from 150 to 600 nm have been fabricated from poly(methyl methacrylate) using electron-beam lithography followed by metallization with gold. It is shown that a small lateral pattern size, a high aspect ratio, and short connection lengths are favorable to generate extinction spectra with well-separated and pronounced peaks. Furthermore, for selected nanostructures, we have been able to identify reflection angles for which the influence of the bulk refractive index on the position of the plasmonic peaks is negligible. It is shown that sensor operation under these angles enables monitoring of in situ biomolecule adsorption with high sensitivity providing a promising tool for high-throughput applications.

Nanoantennas Inversely Designed to Couple Free Space and a Metal-Insulator-Metal Waveguide

The metal-insulator-metal (MIM) waveguide, which can directly couple free space photons, acts as an important interface between conventional optics and subwavelength photoelectrons. The reason for the difficulty of this optical coupling is the mismatch between the large wave vector of the MIM plasmon mode and photons. With the increase in the wave vector, there is an increase in the field and Ohmic losses of the metal layer, and the strength of the MIM mode decreases accordingly. To solve those problems, this paper reports on inversely designed nanoantennas that can couple the free space and MIM waveguide and efficiently excite the MIM plasmon modes at multiple wavelengths and under oblique angles. This was achieved by implementing an inverse design procedure using a topology optimization approach. Simulation analysis shows that the coupling efficiency is enhanced 9.47-fold by the nanoantenna at the incident wavelength of 1338 nm. The topology optimization problem of the nanoantennas was analyzed by using a continuous adjoint method. The nanoantennas can be inversely designed with decreased dependence on the wavelength and oblique angle of the incident waves. A nanostructured interface on the subwavelength scale can be configured in order to control the refraction of a photonic wave, where the periodic unit of the interface is composed of two inversely designed nanoantennas that are decoupled and connected by an MIM waveguide.

Ultrahigh Sensitivity of a Plasmonic Pressure Sensor with a Compact Size

This study proposes a compact plasmonic metal-insulator-metal pressure sensor comprising a bus waveguide and a resonator, including one horizontal slot and several stubs. We calculate the transmittance spectrum and the electromagnetic field distribution using the finite element method. When the resonator's top layer undergoes pressure, the resonance wavelength redshifts with increasing deformation, and their relation is nearly linear. The designed pressure sensor possesses the merits of ultrahigh sensitivity, multiple modes, and a simple structure. The maximum sensitivity and resonance wavelength shift can achieve 592.44 nm/MPa and 364 nm, respectively, which are the highest values to our knowledge. The obtained sensitivity shows 23.32 times compared to the highest one reported in the literature. The modeled design paves a promising path for applications in the nanophotonic field.