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Abstract The ability to confine and guide wave makes topological physics a promising platform for large local field enhancement and strong scattering immunity, which enables efficient nonlinear processes. In this research, we employ a mirror-stacking approach to achieve resonance through two distinct frequency localized states (LSs) in one-dimensional topological circuits, introducing a novel method for validating topological states to facilitate harmonic enhancement. Experimental results reveal that the harmonic wave power increases significantly, by two orders of magnitude, when both the fundamental and harmonic waves are in LSs, in contrast to cases where only one wave is localized. The conversion efficiency is 15.7 times that when the fundamental wave is in a localized state and the harmonic is in a transmission mode. This method, leveraging double-resonance in topological LSs, not only advances harmonic generation in topolectrical circuits but also opens up possibilities for innovative applications in the broader field of photonic technology.

Abstract Manipulating wave propagation and energy collection plays a core role in modern physics, for which topological insulators hosting robust boundary states offer an ideal platform. However, there exist challenges in integrating multiple topological states like two-dimensional (2D) surface state, one-dimensional (1D) hinge state, and zero-dimensional (0D) corner state into a single three-dimensional (3D) architecture. Here we introduce a dimensional hierarchy acoustic structure with a piled 3D Kagome-chain crystal. By tuning the inter- and intra-layer hopping, we lift the 3D bulk states into 2D surface states. A further distortion on the in-plane unit cell makes the system support the 1D hinge and 0D corner states simultaneously. This hierarchy keeps the parent architecture unchanged. Analytically, we prove the robustness of our framework in different geometrical configurations. Our research offers insight for the practical use of the sonic or optical device with diversified topological modes like wave concentrations and transmissions.

Abstract Sensing devices are key nodes for information detection, processing, and conversion and are widely applied in different fields such as industrial production, environmental monitoring, and defense. However, increasing demand of these devices has complicated the application scenarios and diversified the detection targets thereby promoting the continuous development of sensing materials and detection methods. In recent years, Tin+1CnTx (n = 1, 2, 3) MXenes with outstanding optical, electrical, thermal, and mechanical properties have been developed as ideal candidates of sensing materials to apply in physical, chemical, and biological sensing fields. In this review, depending on optical and electrical sensing signals, we systematically summarize the application of Tin+1CnTx in nine categories of sensors such as strain, gas, and fluorescence sensors. The excellent sensing properties of Tin+1CnTx allow its further development in emerging intelligent and bionic devices, including smart flexible devices, bionic E-skin, neural network coding and learning, bionic soft robot, as well as intelligent artificial eardrum, which are all discussed briefly in this review. Finally, we present a positive outlook on the potential future challenges and perspectives of MXene-based sensors. MXenes have shown a vigorous development momentum in sensing applications and can drive the development of an increasing number of new technologies.

The 4 × 4 magneto-optical matrix is theoretically established to investigate the group delays of circular polarization, which are generated by the linearly polarized wave crossing through a sandwich composition that consists of a multi-Weyl semimetal (mWSM) layer and two identical Ag layers. Results denote that the cross-polarized transmission of right hand circularly polarized (RCP) and left hand circularly polarized (LCP) waves experience sharp increases from zero transmission to total transmission at their respective resonant tunneling wavelengths because of the nonzero off-diagonal components of mWSM. Based on the steep change of cross-polarized transmissions, the giant enlarged group delays of several picoseconds can be obtained. Furthermore, the resonant tunneling wavelength of RCP and LCP waves exhibit the dependence on tilt degree of Weyl cones, Fermi energy, Weyl nodes separation and topological charge, which provide feasible strategies to achieve the regulable enhanced group delays. Our findings reveal effective methods to acquire the tunable enlarged group delays of circular polarization with mWSM.

In this paper, we proposed a cavity containing an intracavity medium of five-level double-ladder-type atoms with electromagnetically induced transparency to enhance Goos-Hänchen shifts of reflected and transmitted light beams. The dependence of the Goos-Hänchen shifts has been analyzed. It is shown that Goos-Hänchen shifts can be controlled by modifying the intensity and detuning of the coherent control field without changing material and the structure of the dielectric interface. This work has considerable potential for applications such as optical devices in information processing, flexible optical-beam steering and alignment, optical sensors and optical switches.

We theoretically set up a 4 × 4 magneto-optical matrix to investigate the transmittances induced by the forward and backward left hand circularly polarized (LCP) waves impinging the 4 μm thickness Weyl semimetal (WSM) material at normal incidence. Because of the different effective permittivity of the two LCP waves in WSM induced by the non-zero off-diagonal term, the two waves experience blocking transmission, bidirectional transmission and unidirectional transmission with the increase of the wavelength, respectively. Based on the wide wavelength of the unidirectional transmission, the broadband optical isolator can be realized. The threshold wavelength of the optical isolator can be regulated by changing the wavelength where the parallel component of the diagonal element is identical to the off-diagonal term. It is also found that the Fermi energy, tilt degree of Weyl cones and Weyl node separation reveal simple and available approaches to achieve the regulation. We believe that the adjustable broadband compact optical isolator can perform a significant role in fiber-optic communication system.

In this paper, we theoretically study the optical bistability (OB) of reflected beams in a composite multilayer structure with three-dimensional Dirac semimetals (3D DSMs) and one-dimensional photonic crystal (1D PC) on both sides of the spacer layer. The optical Tamm states (OTSs) excited by the 3D DSMs-1D PC multilayer structure significantly enhances the local electric field near the nonlinear 3D DSMs, which provides a positive condition for the realization of low threshold OB. Through parameter optimization, we obtain a threshold electric field with an incident electric field of 106 V/m levels. Furthermore, the influences of the Fermi energy of 3D DSMs, the angle of the incident light and dispersion characteristics of the spacer layer on the hysteretic behavior and the threshold of OB are clarified. We believe above results will provide possible options for nonlinear optical bistable devices.

The Kerr nonlinearity in two-dimensional (2D) nanomaterials is emerging as an appealing and intriguing research area due to their prominent light processing, modulation, and manipulation abilities. In this contribution, 2D black arsenic-phosphorus (B-AsP) nanosheets (NSs) were applied in nonlinear photonic devices based on spatial self-phase modulation (SSPM) method. By applying the Kerr nonlinearity in 2D B-AsP, an all-optical phase-modulated system is proposed to realize the functions of “on” and “off” in all-optical switching. By using the same all-optical phase-modulated system, another optical logic gate is proposed, and the logical “or” function is obtained based on the 2D B-AsP NSs dispersions. Moreover, by using the SSPM method, a 2D B-AsP/SnS2 hybrid structure is fabricated, and the result illustrates that the hybrid structure possesses the ability of the unidirectional nonlinear excitation, which helps in obtaining the function of spatial asymmetric light propagation. This function is considered an important prerequisite for the realization of diode functionalization, which is believed to be a factor in important basis for the design of isolators as well. The initial investigations indicate that 2D B-AsP is applicable for designing optical logical devices, which can be considered as an important development in all-optical information processing.

In this paper, we study the sensitivity-tunable terahertz (THz) liquid/gas biosensor in a coupling prism–three-dimensional Dirac semimetal (3D DSM) multilayer structure. The high sensitivity of the biosensor originates from the sharp reflected peak caused by surface plasmon resonance (SPR) mode. This structure achieves the tunability of sensitivity due to the fact that the reflectance could be modulated by the Fermi energy of 3D DSM. Besides, it is found that the sensitivity curve depends heavily on the structural parameters of 3D DSM. After parameter optimization, we obtained sensitivity over 100°/RIU for liquid biosensor. We believe this simple structure provides a reference idea for realizing high sensitivity and a tunable biosensor device.

In this article, we present a theoretical analysis of the optical bistability phenomenon of the transmitted light beam in a graphene-based one-dimensional photonic crystal heterostructure in terahertz frequency range. This low-threshold optical bistability originates from the enhancement of the local electric field owing to the excitation of topological edge state at the interface between the two proposed one-dimensional photonic crystals. The results of calculation and simulation show that the hysteretic behavior and the threshold of optical bistability can be adjusted continuously by changing the applied voltage of the graphene. Moreover, the optical bistability of this structure also can be modified by the angle of the incident light and the number of graphene layers. Our findings provide a new method for realizing low threshold and tunable optical bistability in terahertz range, indicating excellent application prospects in the field of all-optical switches and other optical bistable devices.

In order to enhance the photonic spin hall effect (PSHE) of the reflected light at terahertz frequencies, we propose a multilayer structure where the one-dimensional photonic crystal (1D PC) is coated with a piece of monolayer graphene separated by a spacer layer. Enhanced photonic spin hall effects are obtained by this structure based on the excitation of optical Tamm states (OTSs) on the interface between the monolayer graphene and 1D PC. According to the analytical results, the PSHE of the reflected light can be flexibly controlled by properly varying the Fermi energy and relaxation time of graphene. It is also proved that the spin behavior of the composite structure has a great sensitivity to the incidence angle and the dispersion characteristics of the spacer layer, thus making the proposed structure a competitive candidate for designs of new photonic devices on the basis of photonic SHE at terahertz band.

Label-free molecular-level quantification of MicroRNA (miRNA) remains challenging. Here, the authors develop a new surface plasmon resonance sensor based on two-dimensional nanomaterial of antimonene for the specific label-free detection of clinically relevant biomarkers such as miRNA-21 and miRNA-155.

A guided−wave long−range surface plasmon resonance (GW−LRSPR) sensor was proposed in this investigation. In the proposed sensor, high−refractive−index (RI) dielectric films (i.e., CH3NH3PbBr3 perovskite, silicon) served as the guided−wave (GW) layer, which was combined with the long−range surface plasmon resonance (LRSPR) structure to form the GW−LRSPR sensing structure. The theoretical results based on the transfer matrix method (TMM) demonstrated that the LRSPR signal was enhanced by the additional high#x2212;RI GW layer, which was called the GW−LRSPR signal. The achieved GW−LRSPR signal had a strong ability to perceive the analyte. By optimizing the low− and high−RI dielectrics in the GW−LRSPR sensing structure, we obtained the highest sensitivity (S) of 1340.4 RIU−1 based on a CH3NH3PbBr3 GW layer, and the corresponding figure of merit (FOM) was 8.16 × 104 RIU−1 deg−1. Compared with the conventional LRSPR sensor (S = 688.9 RIU−1), the sensitivity of this new type of sensor was improved by nearly 94%.

We demonstrate theoretically a wide-angle and tunable mid-infrared perfect absorber by coupling the graphene and black phosphorus plasmonics and utilizing the concept of critical coupling. With the black phosphorus nanoribbons alone, the localized surface plasmon resonances (LSPR) can be excited, while the absorption is small due to its single-layer thickness. A Fabry-Perot cavity is formed by adding an Au mirror as the substrate, which is separated from the black phosphorus by a dielectric spacer, hence the absorption is enhanced markedly due to critical coupling. In order to further enhance the absorption, a graphene grating is introduced on the top of black phosphorus and is separated by a dielectric slab. It is shown that the perfect absorption can be obtained due to the critical coupling of LSPR of black phosphorus and graphene. Based on these result, it is further demonstrated that the perfect absorption can work in large incident angle range, and the working wavelengths can be adjusted by the Fermi level of graphene. Our results provide a new avenue to achieve wide-angle and adjustable perfect absorption, and it may be beneficial for a variety of mid-infrared plasmonic applications.

In this paper, we have shown that tunable nonlinear group delay of reflected light beam at terahertz frequencies can be achieved by a modified Otto configuration with the insertion of monolayer graphene and nonlinear substrate. This large nonlinear reflected group delay originates from the excitation of surface plasmon resonance at the interface of two dielectrics with graphene. Graphene sheet with unique optical properties can enhance the surface plasmon resonance and modulate bistable behavior of the reflected group delay by suitably adjusting the Fermi energy of the graphene and the relaxation time. Moreover, the large negative and positive of reflected group delay is sensitive to the thicknesses of the air layer. We believe these intriguing phenomena are promising for some potential applications in graphene-based optical delay devices. Keywords: Group delay, Graphene, Nonlinear surface plasmon, Otto configuration

One of the most important applications of THz frequencies is biomedical sensing. However, in a THz range, surface plasmon waves on flat metals are not confined and therefore cannot be used for subwavelength sensing. But, it has been shown that graphene can support surface waves at THz frequencies, which has similar properties as plasmonic waves in an optical range. In this paper, a highly sensitive gas sensor in the terahertz frequencies by exciting surface plasmon resonance (SPR) of graphene is proposed. The results show that the proposed SPR gas sensor has high stability and high sensitivity (S), and the highest Smax (~147°/RIU) has been obtained by optimizing the Fermi energy, the thickness of the dielectric layer, and the incident light frequency. Moreover, the S of the proposed THz sensor for different refractive index (RI) of gas sensing medium (n1) is also discussed.

Surface plasmon polaritons (SPPs) with noble metals lose the capacity to confine the optical field in the long wavelength region, while surface phonon polaritons (SPhPs) in polar dielectrics show greater ability to achieve high local field, sub-diffraction confinement, and low optical losses in the mid-IR to THz spectral ranges. We propose a hybrid structure based on SPhPs mode with CsI and SPPs mode with graphene in THz spectral ranges. The characteristics of the coupling of these two modes have been analyzed and strong coupling has been confirmed theoretically. The results are applied to an intensity-interrogated terahertz sensor; the highest imaging sensitivity as high as 954 RIU-1 and a large figure of merit of 74 740 RIU-1 have been demonstrated. The results could find potential applications in gas detection in the THz band.

A hybrid structure composed of periodic monolayer graphene nanoribbons and a dielectric multilayer structure was designed to generate a Fano resonance (FR). The strong interaction between the surface plasmon resonance of graphene and the dielectric waveguide mode results in the FR. The finite element method is utilized to investigate the behaviors of the FR, and it matches well with the theoretical calculations using rigorous coupled wave theory. The results demonstrate that the profile of the FR can be passively tuned by the period of the graphene nanoribbons and actively tuned by the Fermi level of the graphene. The decoupled nature of the FR gives it potential applications as a self-calibrated refractive index biosensor, and the sensitivity can reach as high as 4.615 μm/RIU. Thus, this work provides a new idea for an excellent self-referencing refractive index biosensor.

CH3NH3PbBr3 perovskite thin film is used as a guided-wave layer and coated on the surface of an Au film to form the Au-perovskite hybrid structure. Using the hybrid structure, a perovskite-based guided-wave surface plasmon resonance (GWSPR) biosensor is proposed with high angular sensitivity. First, it is found that the electric field at the sensing interface is improved by the CH3NH3PbBr3 perovskite thin film, thereby enhancing the sensitivity. The result demonstrates that the angular sensitivity of the Au-perovskite-based GWSPR biosensor is as high as 278.5°/RIU, which is 110.2% higher than that of a conventional Au-based surface plasmon resonance (SPR) biosensor. Second, the selection of the coupling prism in the configuration of the GWSPR biosensor is also analyzed, and it indicates that a low refractive index (RI) prism can generate greater sensitivity. Therefore, the low-RI BK7 prism is served as the coupling prism for the proposed GWSPR biosensor. Finally, the proposed GWSPR sensing structure can not only be used for liquid sensing, but also for gas sensing, and it has also been demonstrated that the GWSPR gas sensor is 2.8 times more sensitive than the Au-based SPR gas sensor.

Topological effects known from condensed matter physics have recently also been explored in photonic systems. Here, the authors directly observe topological surface-state arcs in momentum space by near-field scanning the surface of a chiral hyperbolic metamaterial.

Cylindrical vector vortex (CVV) beam, which possesses both helical phase front and spatially inhomogeneous polarization, is a promising structured light for its various applications ranging from optical communication to optical field manipulation and optical microscopy. However, approaches to generate CVV beams with switchable and tunable polarization order and topological charge are still immature, which hinders the wide application of CVV beams. In this paper, we have experimentally demonstrated that order-controllable CVV beams can be produced by using spatial light modulator (SLM) and equivalent q-plate system at wavelength of 1550.8 nm. It is shown that the topological charge of the CVV beam can be switched by directly programming the SLM. We have also demonstrated that the polarization order of the CVV beam can be tuned to as high as eight by employing an equivalent q -plate system, which consists of two cascaded metasurfaces and a half-wave plate. To further verify the helical phase of the CVV beam, we have proposed a novel measurement method based on first removing the vector property and then interfering the remaining helical phase with plane wave or spherical wave.

Due to the weak spin-orbit interaction, the photonic spin Hall effect (SHE) is generally very weak, which is disadvantageous for potential applications of nanophotonic devices. Surface plasmon resonance (SPR) has been proposed to enhance the photonic SHE. In this paper, by covering the heterostructure material composed of graphene and molybdenum sulfide (MoS2) on the SPR structure, it is demonstrated that the two-dimensional (2-D) material heterostructure can effectively enhance the photonic SHE. The transverse shift of light beam in the heterostructure is larger than those in SPR structure due to the larger refractive index gradient in the heterostructure. The maximum transverse shift can be up to 27.5 μm under the optimized parameters, which is much larger than the previously reported values. This interesting phenomenon is attributed to the larger light absorption and hence the refractive index variation gradient of graphene/MoS2 heterostructure. These findings provide us with a new way to modulate the photonic SHE, and also establish foundation for developing nanophotonic devices based on 2-D nanomaterials heterostructure.

Beams carrying orbital angular momentum (OAM) are very helpful in enhancing the information carrying capacity in free-space optical communications. However, atmospheric turbulence and energy attenuation will seriously affect communication quality and signal transmissions. Here, a novel coherent separation detection technology for OAM mode division multiplexing is proposed, and numerical simulation work is conducted. With the proposed structure, two light beams with different OAM states, each encoded with 16-quadrature-amplitude modulation orthogonal frequency-modulated (QAM-OFDM) signals, can be demultiplexed and allow two orders of magnitude bit error rate (BER) lower than direct separation detection. Moreover, we show the scalability of multiplexing four OAM beams, achieving a 20 m free-space transmission with BER below 3.8 × 10–3 for all channels at signal-noise ratio (SNR) 18 dB. Our results show that coherent separation detection has excellent antinoise performance and can effectively extend the communication distance. It also paves the way for entirely new coherent optical OAM communications.

As a latest novel two-dimensional (2D) material with naturally occurring van der Waals heterostructure, franckeite has the unique advantages compared with the artificial stacking heterostructures. Herein, we propose a surface plasmon resonance (SPR) sensor with 2D franckeite nanosheets to improve the sensitivity. In this structure, franckeite is covered on the surface of 50 nm silver film, so that the SPR sensitivity is related to the thickness of the franckeite overlayer. The sensitivity as high as 188 °/RIU in theory has been obtained, which is a 62% increasing compared with the conventional SPR biosensor. Moreover, an additional film of graphene is considered for the virtue of bio-compatible, and the sensitivity of the franckeite-graphene heterostructures is furthermore improved. As an air-stable 2D material, we believe that 2D franckeite nanosheets could have potential applications in the field of chemical and biological sensors in the future. Keywords: Franckeite, van der Waals heterostructure, Sensor, Surface plasmon resonance

Compared with homogeneous two-dimensional materials, two-dimensional nanomaterial heterostructures which consist of different kinds of two-dimensional materials exhibit stronger light-matter interaction. The topological insulator of Bi2Se3-Bi2Te3 heterostructures were synthesised by two-step reaction proceeding. The reaction arised in two steps: at first, Bi2Se3 nanosheets were prepared in the solvent of ethylene glycol (EG); secondly, Bi2Te3 was epitaxial grown on the Bi2Se3 nanosheets to form heterostructure materials. The spatial self-phase modulation (SSPM) effect of 457nm, 532nm and 671nm was achieved by dispersing the as-prepared Bi2Se3-Bi2Te3 heterostructure materials into ethanol as an optical medium. Furthermore, based on the effect of SSPM, a device called all-optical switching was also realized. As an interesting nonlinear optical materials, topological insulator Bi2Se3-Bi2Te3 heterostructures might be an effective proposal for photonics devices such as optical switchings, optical modulators, photodetectors, etc.

A novel long-range surface plasmon with graphene is proposed to enhance the sensitivity and detection accuracy (DA) of the biosensor. Compared with the conventional long-range surface plasmon resonance (LRSPR) structure with Au, the coating of the metal surface with graphene is employed to increase the biomolecules adsorption, prevent oxidation, and enhance the sensitivity and DA. Furthermore, we demonstrated that the sensitivity has a nearly tenfold improvement with a huge increasing DA in the proposed LRSPR biosensor compared with the SPR biosensor. Finally, we discuss the influence of the refractive index of sensing medium on the LRSPR biosensor and find that the sensitivity is changing with the refractive index of the sensing medium and that an optimal sensitivity can be obtained at a suitable refractive index. We believe that this scheme could find potential applications in chemical examination, medical diagnosis, and biological detection.

In the conventional surface plasmon resonance biosensor, the sensitivity using angular interrogation is low. Graphene/MoS2 has been attached on the metal surface to improve the sensitivity; however, the ability to improve sensitivity is limited. Here, by sandwiching the MoS2 sheets between two gold films in the Kretschmann configuration, we demonstrate an ultrasensitive surface plasmon resonance sensor. By optimizing the structure of sensor, we find that the sensitivity as high as 182°/RIU can be realized with 4-layers MoS2 and a monolayer graphene. Moreover, it is indicated that the sensitivity can be tuned and controlled by changing the layer numbers of graphene and MoS2.

We have investigated the nonlinear and ultrafast photonics of chemically processed black phosphorus quantum dots (BPQDs) synthesized by the solvothermal treatment approach, with an average lateral size of about 2.48 ± 0.4 nm. BPQDs exhibit optical saturable absorption measured by the balanced twin-detector measurement system at the telecommunication band and have been demonstrated for the generation of passively mode-locking operation in an erbium-doped fiber laser. Either two-pulse or three-pulse bound state of soliton pulse has been obtained, making the best use of the BPQDs-based saturable absorber. Our work suggests that BPQDs might be developed as an efficient optical saturable absorber for ultrafast photonics applications.

In this paper, we propose a high-sensitivity optical sensor at terahertz frequencies based on a composite structure containing a one-dimensional photonic crystal (1D PC) coated with a layer of monolayer graphene. Between the 1D PC and the graphene there is a sensing medium. This high-sensitivity phenomenon originates from the excitation of optical resonance between the graphene and the 1D PC. The proposed sensor is highly sensitive to the Fermi energy of graphene, the thickness and refractive index of the sensing medium, and the number of graphene layers. By selecting appropriate parameters, the maximum sensitivity ( 407.36 ∘ / RIU ) is obtained. We believe the proposed configuration is promising for fabricating graphene-based biosensor- or gas-sensor devices and other related applications in the terahertz band.

The mutual energy-exchange interactions in two orthogonally polarized components of vector dark soliton, which is automatically formed in a normal-dispersion fiber laser cavity in the presence of reverse saturable absorption, and dissipative cavity effects, had been theoretically and numerically investigated. Under specific cavity parameters, a vector dark soliton with periodic coherent energy exchange could be formed and well described by a semianalytic method. The multiple vector dark solitons with energy exchange were also demonstrated by combining gain bandwidth and a reverse saturable absorption effect.

In this paper, we put forward a novel method for the management of optical collapse through controlling anomalous nonlinear diffraction (NLD) originating from the vectoral effect in metamaterials with simultaneous cubic electric and magnetic nonlinearity, particularly focusing on the unusual behaviors different from those in ordinary positive-index materials. First, NLD in metamaterials can be positive or negative, which makes it possible for us to offer greatly enhanced design freedom to manipulate the behavior of optical collapse at will. Then, it is further demonstrated that the negative NLD can arrest the catastrophic self-focusing of optical wave; however, the positive NLD will lead to the acceleration of its occurrence, as opposed to the case in ordinary positive-index materials. Our analysis is performed by directly numerically solving the nonlinear Schrödinger equation, as well as by using the modulation theory, both showing consistent results. These findings show that NLD in metamaterials can provide a more powerful tool in controlling the collapse of optical wave when compared with the corresponding case in ordinary positive-index materials.

Tunable terahertz (THz) angular/frequency filters in the modified Kretschmann-Raether configuration with the insertion of single-layer graphene have been numerically demonstrated. Due to the excitation of the transverse magnetic (TM) polarized surface plasmons at the interface of two dielectrics with the insertion of the graphene sheet, the transmission resonance occurs at some range incident angles and frequencies in the THz frequency range, which can be adopted for designing the angular/frequency filters. It is shown that the resonant angle and the resonant wavelength can be tuned by varying the Fermi energy of the graphene sheets via electrostatic biasing. Moreover, we show that the resonant behaviors can be engineered by changing the gap thickness or the incident angle.

In this paper, we proposed a cavity containing colloidal ferrofluids to enhance the Goos-Hänchen (GH) shifts of reflected and transmitted light beams. By optimizing the parameters of the cavity, the resonance inside the cavity was strengthened, and we obtained huge GH shifts. Magnetocontrolled negative GH shifts have been also analyzed. We found that a suitable cavity with optimized structural parameters can enhance the effect of the GH shifts on ferrofluids, which have an extremely small volume parameter and a small shell thickness parameter. Numerical simulations confirm the theoretical analysis. This paper has considerable potential for applications such as magnetic field sensors and optical switches.

We propose a graphene/planar waveguide hybrid structure, and demonstrate total absorption in the visible wavelength range by means of attenuated total reflectance. The excitation of planar waveguide mode, which has strong near field enhancement and increased light interaction length with graphene, plays a vital role in total absorption. We analyze the origin and physical insight of total absorption theoretically by using an approximated reflectance, and show how to design such hybrid structure numerically. Utilizing the tunability of doped graphene, we discuss the possible application in optical modulators. We also achieve broadband absorption enhancement in near-IR range by cascading multiple graphene-waveguide hybrid structures. We believe our results will be useful not only for potential applications in optical devices, but also for studying other two-dimension materials.

The optical bistability in one-dimensional photonic crystal (1DPC) with a nonlinear defect is investigated. It is demonstrated that, by introducing graphene layers into the nonlinear defect, the optical bistability in 1DPC can be changed significantly. The hysteresis threshold increases with the number of graphene monolayers and can be lowered or enhanced by tuning the Fermi energy of graphene. On the other hand, the hysteresis width and the nonlinear lateral shift can also be controlled by varying the Femi energy and the number of graphene monolayers. These results may be useful for controlling the optical bistability and nonlinear lateral shift in 1DPCs.

We have theoretically investigated the group delay of the TE-polarized beam reflected from a Fabry-Perot cavity with the insertion of the graphene sheets in the near-infrared band. It is shown that even a single-layer graphene allows for notable variation of group delay. Group delay can be enlarged negatively and can be switched from positive to negative, or vice versa. Importantly, the group delay depends on the Fermi energy of the graphene sheets, and thus, it can be actively controlled through electrical or chemical modification of the charge carrier density of the graphene. Furthermore, the influences of the position of graphene in the Fabry-Perot cavity, the mirror transmittance, and the number of graphene layers on group delay are clarified.

Single layers of tin selenide (SnSe), which have a similar structure as graphene and phosphorene, also show excellent optoelectronic properties, and have received much attention as a two-dimensional (2D) material beyond other 2D material family members. Surface plasmon resonance (SPR) sensors based on three monolayer SnSe allotropes are investigated with the transfer matrix method. The simulated results have indicated that the proposed SnSe-containing biochemical sensors are suitable to detect different types of analytes. Compared with the conventional Ag-only film biochemical sensor whose sensitivity is 116°/RIU, the sensitivities of these SnSe-based biochemical sensors containing α-SnSe, δ-SnSe, ε-SnSe, were obviously increased to 178°/RIU, 156°/RIU and 154°/RIU, respectively. The diverse biosensor sensitivities achieved with these three SnSe allotropes suggest that these 2D materials can adjust SPR sensor properties.

Surface exciton polaritons (SEPs) are one of the three major elementary excitations: Phonons, plasmons and excitons. They propagate along the interface of the crystal and dielectric medium. Surface exciton polaritons hold a significant position in the aspect of novel sensor and optical devices. In this article, we have realized a sharp Fano resonance (FR) by coupling the planar waveguide mode (WGM) and SEP mode with Cytop (perfluoro (1-butenyl vinyl ether)) and J-aggregate cyanine dye. After analyzing the coupling mechanism and the localized field enhancement, we then applied our structure to the imaging biosensor. It was shown that the maximum imaging sensitivity of this sensor could be as high as 5858 RIU−1, which is more than three times as much as classical FR based on metal. A biosensor with ultra-high sensitivity, simple manufacturing technique and lower cost with J-aggregate cyanine dye provides us with the most appropriate substitute for the surface plasmon resonance sensors with the noble metals and paves the way for applications in new sensing technology and biological studies.

Transparent conducting oxides (TCOs) have appeared in the past few years as potential plasmonic materials for the development of optical devices in the near infrared regime (NIR). However, the performance of biosensors with TCOs has been limited in sensitivity and figure of merit (FOM). To improve the performance of the biosensors with TCOs, a biosensor based on long-range surface plasmon with Ga-doped zinc oxide (GZO) is proposed. It is shown that a larger FOM with a 2~7 times enhancement compared to the traditional surface plasmon polaritons (SPPs) sensor and higher detection accuracy (DA) can be realized in our proposed sensor compared with the surface plasmon resonance (SPR) sensor with GZO. Therefore, this sensor can be used to detect biological activity or chemical reactions in the near infrared region.

Graphene terahertz (THz) surface plasmons provide hope for developing functional devices in the THz frequency. By coupling graphene surface plasmon polaritons (SPPs) and a planar waveguide (PWG) mode, Fano resonances are demonstrated to realize an ultrasensitive terahertz biosensor. By analyzing the dispersion relation of graphene SPPs and PWG, the tunable Fano resonances in the terahertz frequency are discussed. It is found that the asymmetric lineshape of Fano resonances can be manipulated by changing the Fermi level of graphene, and the influence of the thickness of coupling layer and air layer in sandwich structure on the Fano resonances is also discussed in detail. We then apply the proposed Fano resonance to realize the ultrasensitive terahertz biosensors, it is shown that the highest sensitivities of 3260 RIU−1 are realized. Our result is two orders of a conventional surface plasmon resonance sensor. Furthermore, we find that when sensing medium is in the vicinity of water in THz, the sensitivity increases with increasing refractive index of the sensing medium.

We have theoretically investigated the Goos-Hänchen (GH) shift of the TM-polarized beam reflected from a graphene-on-dielectric surface near the Brewster angle. It is shown that even a single-layer graphene allows for notable variation of the GH shift. The GH shift can be enlarged and switched from positive to negative or vice versa. Importantly, the GH shift depends on the Fermi energy, and thus, it can be electrically controlled through electrical or chemical modification of the charge carrier density of the graphene. Furthermore, the relationship between the GH shift and the electron-phonon relaxation time and the number of graphene layers is clarified.