Terahertz refractive index sensor based on Tamm plasmon-polaritons with graphene

Tamm and surface plasmon hybrid modes in anisotropic graphene-photonic-crystal structure for hemoglobin detection

We propose Tamm plasmon (TP) and surface plasmon (SP) hybrid modes for hemoglobin (Hb) detection in anisotropic graphene-photonic-crystal (GPC) structures. The proposed GPC sensor shows polarization-dependent responses due to the in-plane anisotropic property. The reflection profiles of the proposed sensor exhibit two reflectivity minima due to the simultaneous excitation of TP and SP modes. When used to detect Hb, the TP mode offers a greater figure-of-merit (FoM) than the SP mode. Using a Fourier mode spectral analysis, we observe energy coupling from the TP to the SP mode when the incident light's polarization changes, providing an option to enhance the sensor's sensitivity. We propose a double dips method (DDM) to detect Hb based on the simultaneous excitation of TP and SP modes. Using DDM, the proposed sensor offers a maximum sensitivity of 314.5 degrees/RIU and a FoM of 1746 RIU-1 when the Hb level is 189 g/L. The proposed anisotropic GPC sensor offers possible applications for highly sensitive bio-molecule detection with high FoM.

Near-field manipulation of Tamm plasmon polaritons

Tamm plasmon polaritons (TPPs) arise from electromagnetic resonant phenomena which appear at the interface between a metallic film and a distributed Bragg reflector. They differ from surface plasmon polaritons (SPPs), since TPPs possess both cavity mode properties and surface plasmon characteristics. In this paper, the propagation properties of TPPs are carefully investigated. With the aid of nanoantenna couplers, polarization-controlled TPP waves can propagate directionally. By combining nanoantenna couplers with Fresnel zone plates, asymmetric double focusing of TPP wave is observed. Moreover, radial unidirectional coupling of the TPP wave can be achieved when the nanoantenna couplers are arranged along a circular or a spiral shape, which shows superior focusing ability compared to a single circular or spiral groove since the electric field intensity at the focal point is 4 times larger. In comparison with SPPs, TPPs possess higher excitation efficiency and lower propagation loss. The numerical investigation shows that TPP waves have great potential in integrated photonics and on-chip devices.

Gyroidal graphene/porous silicon array for exciting optical Tamm state as optical sensor

In this study, the optical Tamm state is excited for the first time using gyroidal graphene/porous silicon one-dimensional photonic crystal terminated by a gyroidal graphene layer. The gyroidal graphene and porous silicon are used to enhance the figure of merit and sensitivity of the based Tamm resonance photonic crystal sensor. By tuning different parameters like the angle of incidence, the thickness of the sample layer, and the thickness of the gyroidal graphene layer, we have reached the optimized sensor. The observation of resonant dips in the reflectance spectra is strong evidence that Tamm plasmon-polaritons exist with higher sensitivity (188.8 THz/RIU) and figure of merit (355,384 RIU-1) than previously reported structures. The proposed sensor recorded sensitivity and FoM higher 38% and 747% respectively than a similar structure composed of graphene sheets and porous silicon.

Miniaturized spectroscopy with tunable and sensitive plasmonic structures

A broad linewidth and a lack of spectral analysis limit the applications of plasmonic sensors. In this Letter, a plasmonic sensor with a large sensitivity in the terahertz (THz) range is proposed based on high-quality factor (>1000) surface lattice resonance in subwavelength near-flat metallic gratings. Moreover, such a highly selective spectral manipulating scheme, plus the greatly localized plasmonic resonance, enables miniaturized spectroscopy based on a single detector by integrating an electro-optical material with the gratings. A spectral resolution of 0.1 GHz at a center frequency of 1.1 THz is predicted showing a four times improvement of measuring efficiency. This technique shows promising potential in on-site matter inspection and point-of-care testing.

Controlling Tamm phonons using hBN and a distributed Bragg reflector for narrowband refractive index sensing

Optical Tamm state with sharp reflection dip provides the sensing potential combined with high sensitivity. In this paper, we numerically demonstrate that narrowband refractive index sensing can be realized in a distributed Bragg reflector (DBR) structure with hexagonal boron nitride (hBN). Here, we show that the sensitivity and narrowband properties can not only be regularly governed by different analyte thickness but also exhibit dependence on the number of DBR pairs and the thickness of the hBN layer. With varying the analyte index and optimized analyte thickness, the deep reflectance dip can be sustained with the sensitivity (figure of merit, FOM) close to 3.02 µm/RIU (1093/RIU). In addition, the different analyte categories can be detected through adjusting the thickness of the analyte-filled cavity. High sensitivity, combined with ultra-high FOM originated from strong Tamm phonon mode, offers a promising platform to detect the smallest variation of the refractive index.

Refractive index sensor based on a Tamm Fabry-Perot hybrid resonance

A highly sensitive refractive index sensor is proposed by using the resonant coupling of a Tamm state to a Fabry-Perot resonance in slits periodically pierced in a metal film. This new hybrid resonance exists at the interface between a dielectric Bragg mirror and a subwavelength metal grating. Contrary to a classic Tamm plasmon, it is sensitive to the ambient media refractive index due to its confinement inside the grating slits. The proposed sensor output can be either the reflected intensity or preferably the wavelength, and its sensitivity and figure of merit are numerically investigated with the help of rigorous coupled wave analysis. The sensitivity of the studied sensor is 87 nm/RIU for a refractive index range from 1.25 to 1.38, and, at the expense of the resolution, it can reach up to 160 nm/RIU for a grating duty cycle of 50%. The figure of merit is around ${{7.5}}\;{{\rm{RIU}}^{- 1}}$ with a large measuring range. The index sensor performances and operating resonance wavelength can be modified by adjusting the grating geometric parameters (height and duty cycle). The possibility to achieve wavelength modulation in any spectral range makes the proposed grating Tamm structure an attractive candidate to design refractive index sensors and photonic components in the infrared and terahertz domains.

Modeling of a biosensor using Tamm resonance excited by graphene

In this paper, nanoscale pores in silicon layers are exploited to model and optimize a one-dimensional hybrid graphene-porous silicon photonic crystal biosensor. The physical nature of the proposed sensor is based on Tamm resonance. The transfer matrix method is applied to detect the change of the index of refraction in an aqueous solution. The proposed model is (PSi₁/PSi₂)^N/G/Substrate, in which PSi₁ and PSi₂ are porous silicon layers with different porosities, N is the number of periods, and G is the number of graphene layers. The numerical simulations show that the proposed sensor has good performance. The variation of the number of periods, number of graphene layers, porosities, thicknesses of silicon layers, incident angles, and the sample layer thickness affect the performance of the sensor. By varying these parameters, the sensitivity and figure of merit of the sensor can be controlled. The study shows that the sensitivity and figure of merit of the proposed sensor reach 4.75 THz/RIU and 475RIU^-1, respectively. The proposed sensor has a good capability in biological detection within terahertz. It is the first time, to our knowledge, that graphene has been used to excite the Tamm resonance using the photonic crystal of porous silicon and using it in biosensing applications.

Model of a tunable hybrid Tamm mode-liquid crystal device

A concept of an easily tunable device based on hybrid Tamm modes is proposed. The device can be controlled using a high-sensitivity chiral liquid crystal serving as a mirror. The coupling of the chiral optical Tamm state with the Tamm plasmons is predicted. The Tamm plasmons are excited at different frequencies for the orthogonal linear polarizations, while the chiral Tamm state is excited at only one frequency. The properties of the proposed model are analytically and numerically calculated. The possibility of creating a two- and three-mode laser with tunable characteristics on the basis of the proposed model is discussed.

Self-referenced terahertz refractive index sensor based on a cavity resonance and Tamm plasmonic modes

A self-referenced terahertz (THz) refractive index sensor is proposed. The structure consists of two opposite-facing, graphene-covered distributed Bragg reflectors (DBRs), with a cavity formed in between. The cavity is filled with the ambient medium, and its resonance frequency is sensitive to the changes of the ambient refractive index. On the other hand, Tamm-plasmonic modes, which are excited at the DBR-graphene boundaries, are insensitive to the ambient refractive index and thus provide a frequency reference. The proposed structure is studied using a semi-analytical transfer matrix method (TMM). The sensor, studied for gas sensing, achieves a sensitivity of 0.982 THz per refractive index unit (THz/RIU), and a figure of merit (FoM) of 142RIU^-1 at the cavity resonance frequency of 1.1 THz. The effects of different parameters on the sensor's performance are also investigated. Compared to the previous high-performance THz refractive index sensing approaches, the proposed structure is simpler because it requires no phase- or polarization-matching devices, such as polarizers, prisms, and gratings. Moreover, it provides a self-referenced operation, which was rarely achievable using previous methods.