S-polarized Modes Research Articles

Spectral responses of tilted fiber Bragg grating coated with a thin Ge2Sb2Se4Te1 layer

This article reports the spectral properties of a Ge2Sb2Se4Te1 (GSST) -coated tilted fiber Bragg grating to the changes of surrounding refractive index (SRI). The variations of the cladding mode resonance amplitudes and wavelengths under two orthogonal polarization modes are analyzed and validated with the simulation results. The introduction of the high index coating has induced non-degeneracy to the cladding modes, but the effect gradually diminishes with increasing SRI. The SRI sensitivities of the s-polarized modes have been substantially suppressed and became 20 times lower than that of p-polarized modes. The findings also reveal the association between the cladding resonance amplitudes, the imaginary part of the effective refractive indices and the evanescent field intensity distributions.

Leaky mode transition and enhanced resonance in tilted fiber Bragg grating

Leaky mode resonance in tilted fiber Bragg grating (TFBG) has been widely underestimated and ignored. Herein, we theoretically demonstrate that the leaky mode resonance can be greatly enhanced, and it presents a highly sensitive property by virtue of its mode transition. The leaky mode transition consists of five stages from the cladding guided mode to radiationlike and guidedlike leaky modes for both p- and s-polarized modes of bare TFBG. However, the graphene induces only one transition for the s-polarized guidedlike leaky mode, which, in turn, leads to an insensitive wavelength shift. The resonance amplitude depends on the imaginary part of the effective refractive index. Consequently, the enhanced leaky mode resonance (eLMR) in the s-polarized state is obtained and presents higher sensitivity than the widespread surface plasmon resonance (SPR). Furthermore, a general empirical rule associated with the refractive index of nanometer materials for exciting the eLMR in TFBG is obtained (the SPR is also discussed for comparison). The results obtained in this work have shed contemporary light on the undervalued leaky mode resonance, which is of great interest and importance to the development of novel multimodal and multifunctional eLMR-based TFBG devices, such as tunable lasers, optical modulators, and sensors.

Polarization-Dependent Tuning Property of Graphene Integrated Tilted Fiber Bragg Grating for Sensitivity Optimization: A Numerical Study

In this paper, the general characteristics of the polarization-dependent tuning property and sensitivity optimization of the graphene integrated tilted fiber Bragg grating (TFBG) are investigated theoretically. The tuning mechanism of the graphene on mode characteristics and transmission spectrum is thoroughly explored. The results reveal that the graphene induces a stronger influence on the s-polarized mode than its p-polarized counterpart, but the TFBGs with or without graphene in all the polarized states have nearly the same wavelength sensitivity ( $\sim \text{540}\; \text{nm/RIU}$ in average). In contrast, the resonance strength is highly dependent on the tunable state of the graphene, especially for the s-polarized case. In consequence, the normalized power response is increased in the low refractive index region. More importantly, the s-polarized leaky mode resonance that is ignored for regular TFBGs is significantly enhanced (by increasing field content in the fiber region and decreasing the confinement loss) whereas the guided counterpart is suppressed greatly by the graphene with lower chemical potential. The sensitivity of up to $ \text{6595.8 dB/RIU}$ (9.17 times higher than that of the regular counterpart) is achieved in response to an extremely small analyte perturbation ( $ \text{10}^{-\text{4}}$ close to 1.333) based on the graphene enhanced leaky mode resonance. This paper provides a novel technique for significantly optimizing the sensing performance of the TFBG, which has great potential in biological and biochemical sensing applications.

Theoretical analysis of tuning property of the graphene integrated excessively tilted fiber grating for sensitivity enhancement

The graphene integrated optical fiber devices show many excellent characteristics, especially their tunable optical properties. Here we investigate thoroughly the influence of the tuning property of the graphene on the resonance and sensing performance of the excessively tilted fiber grating (Ex-TFG) operating around the dispersion tuning point. An improved piecewise discretization method for the finite-difference mode solver combined with coupling mode theory is presented to explore the general variation rule of mode characteristics and polarization-dependent resonances corresponding to polarized TE/TM0,m and HE/EHv,m modes of the graphene integrated Ex-TFG. The results reveal that both p-polarized and s-polarized modes and their resonances are greatly influenced by the tunable graphene. On this basis, the sensitivity enhancement of the graphene coated Ex-TFG is explored in detail. It is shown that the sensing performance of both polarized cases can be greatly improved by tuning the chemical potential. In particular, the graphene induces a greater influence on both resonance wavelength and resonance strength of the s-polarized mode, but the p-polarized one has a higher sensitivity. We believe that these unique tuning properties make the graphene integrated Ex-TFG devices ideal for the telecommunication and sensing applications, such as tunable fiber modulators and biochemical sensors.

Transmittance properties of tunable filter in a 1D photonic crystal doped by an anisotropic metamaterial

The influence of the variation in the optical axis and the incidence angle a the P and the S polarizations on the transmittance properties of the tunable filter in a 1D photonic crystal (1DPC) with an anisotropic metamaterial defect layer is theoretically investigated. By using the 4 × 4 transfer matrix method, the pronounced contrast in the behavior of the P-and the S-polarized modes were demonstrated. The results indicate that, the intensity and the peak wavelength of the defect mode under the P –polarized wave can be tuned by a variation of the optical axis of the anisotropic layer, whereas the intensity and the peak wavelength of the defect mode under the S polarized wave remains unchanged. In addition, we show that the peak wavelength and the intensity of the tunable filter can be tailored by varying the incidence angle of light. The results, therefore, illustrate that the peak wavelength of the defect mode shifts towards blue as the angle of incidence increases for both polarizations. These results lead to certain new findings concerning the design of new types of tunable narrowband filters.

Study of s-polarized photonic bandgap structure in three component optical fibonacci multilayers composed of nanoscale materials

In this communication dispersion relation, reflection and transmission coefficient for quasiperiodic optical multilayers arranged according to the three-component Fibonacci rule is derived using transfer matrix method. In this work Fibonacci multilayers using three different photonic band gap (PBG) materials is designed and its photonic band gap structure is presented. Also, a detailed calculation for allowed and forbidden frequency bands for s-polarized radiation of light incident on these structures at various angles is presented. It is demonstrated that in case of s-polarized mode of radiation, widening in the band gap and frequency shifts occur as one moves towards the higher frequency region, which shows that such a multilayer can be used as an efficient polarizer. The effect of an imaginary value for the propagation vector Q on the existence of forbidden photonic bands in these lattices is also studied.

Exciton Polaritons in One-Dimensional Metal-Semiconductor Photonic Crystals

We investigate theoretically the coupling of exciton with light in a one-dimensional photonic crystal. The unit cell of the crystal consists of two alternating layers, namely a metallic layer and a semiconductor one. The frequency-dependent dielectric function of the metal is described by the Drude model, whereas for the semiconductor we use a nonlocal excitonic dielectric function. The polariton dispersion for s-polarized modes in the metal-semiconductor photonic crystal is compared with that for a dielectric-semiconductor photonic crystal. Because of the metal layers, a low-frequency gap appears in the photonic band structure. The presence of the semiconductor gives rise to photonic bands associated with the coupling of light with size-quantized excitón states. At frequencies above the longitudinal exciton frequency, the photonic band structure exhibits anticrossing phenomena produced by the upper exciton-polariton mode and size-quantized excitons. It is found that the anticrossing phenomena in the metal-semiconductor photonic crystal occur at higher frequencies in comparison with the dielectric-semiconductor case.

Photonic Crystal Surface Waves for Optical Biosensors

We present a new optical biosensor technique based on registration of dual optical s-polarized modes on a photonic crystal surface. The simultaneous registration of two optical surface waves with different evanescent depths from the same surface spot permits the segregation of the volume and the surface contributions from an analyte, while the absence of metal damping permits an increase in the propagation length of the optical surface waves and the sensitivity of the biosensor. Our technique was tested with the binding of biotin molecules to a streptavidin monolayer that has been detected with signal/noise ratio of approximately 15 at 1-s signal accumulation time. The detection limit is approximately 20 fg of the analyte on the probed spot of the surface.

An Investigation of P- and S-polarized Long-range Optical Modes Supported by a Strongly Absorbing Normal Uniaxial Thin Organic Film

Abstract We have shown previously an analysis of the p-polarized long-range modes (LRMs) which may be supported by an isotropic thin organic film. Here we extend the analysis to a uniaxial system. In addition since the analysis of p-polarized LRMs revealed counter-intuitive results which could not be expected from conventional guided wave theory we also analysed the situation for s-polarized modes in the hope that these too would exhibit surprising behaviour. To this end we have discovered a second LRM which is s-polarized and can be supported by a strongly absorbing isotropic thin film. Experimental data for the optical excitation of both p- and s-polarized modes are presented over the wavelength range (400–720) nm.