Surface plasmon polarition (SPP), which travels along the metal-dielectric or metal-air interface and originates from the interaction between light and collective electron oscillation on metal surfaces, can limit or compress electromagnetic energy in a very small space, even smaller than the diffraction limit [1,2]. In addition, refractive sensors based on surface plasmon resonance (SPR) was widely used in the optical applications, such as label-free temperature detection, chemical process control [3,4]. Although the photonic crystal fibers (PCFs) have the excellent performance in refractive sensing [5–7], because of the extremely difficult fabrication process, the PCF sensors still have a long distance in achieving large scale commercialization; as a consequence, many various realizable and integrated resonance cavity sensors were proposed. For instance, Yan et al demonstrated a high sensitivity Knob-integrated grating sensor with 8.2 pm/°C [8], and Si₃N₄ phase grating with a sensitivity of 110 nmRIU⁻¹ was displayed by Joseph et al [9]. Newly, an optical fiber grating sensor with gaseous media was achieved a sensitivity of 78 nmRIU⁻¹ by Gonzalezvila et al [10], and the sensitivity of Fabry-Perot fiber with a sensitivity of 18.6 pm/°C was demonstrated by Shu et al [11].

In this letter, we demonstrate a cost-efficient integrated plasmonic period grating refractive sensor, which has outstanding performance in temperature sensing and operating wavelength tuning. Firstly, according to the quantitative analysis, we calculate the optimal structural parameters by the finite element method (FEM), including the period of grating, the surface area of graphene, the thickness of Ag grating and grating substrate. Secondly, we analyze the reflectance spectrum after filling the temperature sensing solution in the top of grating. Finally, compared with other studies, the sensitivity of the proposed sensor, which is further calculated by reflectance spectrum and validate by Matlab, obtained the greater improvements.

The Fig. 1 shows that the proposed sensor is composed of SiO₂ substrate, InGaAsP semiconductor material, Ag periodic grating and surface monolayer graphene from bottom to top and then the temperature sensing solution is filled above the Ag grating. In the proposed structure, the period grating structure can be used to excite SPP at the interface between Ag and graphene. Because of the outstanding optical characteristics and high carrier mobility, the graphene have the capability in enhancing the SPP, which is excited at the interface between Ag and graphene. Furthermore, the period graphene layer can decrease the propagation loss and supply the gain compensation, which is produce by the ohmic loss in the Ag.

 

Fig. 1 Schematic diagram of proposed temperature sensor.

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To begin with, the space is dug in the optical fiber by plasma etching technology. After that, InGaAsP semiconductor buffer layer, which can provide gain compensation and improve the propagation length of SPP, is grown above the optical fiber. Then, the Ag period grating and monolayer graphene are deposited on the InGaAsP buffer layer by chemical vapor deposition (CVD), respectively. Finally, the temperature sensing medium is filled above the Ag grating structure and encapsulated by glass. The total thickness of Ag grating is set to high, where the thickness of grating substrate is set as h_d. In addition, the width of graphene and the Ag grating are respectively set to d and width, where the width always fixes at 500 nm. The vertical incident light is used to excite SPP from the top of the structure.

As well known, the complex relative permittivity of Ag grating can be calculated by the Lorentz-Drude model in the theoretical analysis [12].

In additional, the monolayer graphene can be express by surface conductivity ${\text{σ}}_{\text{g}}$, which is related to the operation frequency $\omega$, chemical potential ${\mu }_c$, Fermi level ${\text{E}}_{\text{f}}$, the environmental temperature T, and relaxation time $\tau$. Based on the local random phase approximation (RPA), the conductivity of graphene can be calculated by Kubo formula [13]:

Finally, because the excellent thermo-optic coefficient (dn/dT) value plays the key role in temperature sensor, the solution of sunflower oil is selected in the proposed device. Furthermore, the relationship between the refractive index of the oil solution and the temperature can be expressed by:

Firstly, in the fundamental mode calculation, we fix the total thickness of Ag grating (high) at 200 nm and the thickness of grating substrate (h_d) at 80 nm, simultaneously. As shown in Fig. 2(a), the width of graphene, d, is adjusted from 15 nm to 40 nm and the incident wavelength is tuned from 1100 nm to 2100 nm by spacing 1 nm. Secondly, based on the optimum structural parameter of d, we further calculate the relationship between reflectance and incident wavelength for increasing thickness of Ag grating substrate-h_d from 40 nm to 150 nm, as shown in Fig. 2(b). Thirdly, the optimal structural parameters of the proposed sensor can be obtained by adjusting the thickness of Ag grating-high from 180 nm to 250 nm, as shown in Fig. 2(c). Lastly, based on the optimal structural parameters, we calculate the sensitivity of proposed sensor in the condition of d = 20 nm, h_d = 80 nm, high = 200 nm, and the temperature increase from −100 °C to 100 °C, spacing 0.5 nm, as shown in Figs. 2(d)

 

Fig. 2 (a)Reflectance spectrum of the proposed sensor versus incident wavelength for different width of graphene-d. The black asterisk, blue circle, magenta diamond, and red plus sign correspond to d of 15 nm, 20nm, 30nm, and 40nm, respectively. (b) Reflectance spectrum for different thickness of grating substrate-h_d correspond to 40 nm, 80nm, 100nm, 150nm, respectively. (c) Reflectance spectrum for different thickness of Ag grating-high. (d) Reflectance spectrum of proposed plasmonic grating sensor versus incident wavelength for different temperature, where black asterisk, blue circle, and magenta diamond correspond to −100, 0, 100 degree Celsius, respectively.

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Figure 2(a) shows the reflectance of the proposed plasmonic grating versus incident frequency for different width of graphene-d and all lines display the falling-rising trend. In addition, when the width of graphene is increased from 15 nm to 40 nm, the full width at half maximum (FWHM) gradually increases from 268 nm to 408 nm, and the resonance peak blue shifts 276 nm. The reason is that due to the extremely excellent propagation performance in graphene, the range of resonance wavelength will increase with the rising surface area of graphene. At that time, the minimum values of the reflectance spectrum for different d is 0.0064, 0.0008, 0.0038, and 0.0143, respectively. Because the smaller values of FWHM and reflectance spectrum can make the detection easier in the practical sensor application, the optimal structural parameter of width of grapheme, d = 20nm, can be obtained at the coupled wavelength, 1570nm. Then, based on the optimal structural parameter of d, we analyze reflectance spectrum for different thickness of Ag grating substrate-h_d, as shown in Fig. 2 (b). The resonance peaks blue shift 846 nm in changing the h_d from 40 nm to 150 nm, which shows the outstanding performance of working wavelength tunable. Since the ohmic loss increases as the thickness of the silver substrate increases, the resonance interval decreases, that is, the FWHM decreases from 374 nm to 74 nm. Finally, we further calculate the reflectance spectrum by changing high from 180 nm to 250 nm based on the above optimal structural parameters, d = 20 nm, h_d = 80nm, as shown in Fig. 2 (c) and obtain the optimal high = 200 nm. Furthermore, the resonance wavelength red shifts from 1380 nm to 2030 nm. In order to quantitative analyze the sensitivity of proposed device, we change the temperature from −100 degree Celsius to 100 degree Celsius, spacing 100 degree Celsius, as shown in Fig. 2 (d).

It shows that the resonance peaks blue shift 90 nm with increasing temperature. Then, for further calculate the sensitivity performance in the proposed device, we adjust the temperature from −100 degree Celsius to 100 degree Celsius, spacing 50 degree Celsius, as shown in Fig. 3 and the sensitivity can reach 0.455 nm/°C or 1625 nmRIU⁻¹.

 

Fig. 3 Resonance peaks versus different temperature, where resonance wavelengths correspond to 1615 nm, 1592 nm, 1569.5 nm, 1546.5 nm, and 1524 nm, respectively.

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Compared with other studies, the sensitivity has excellent performance in refractive index and temperature grating sensor, as shown in Table 1.

Table 1. The sensitivity reported in other article

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As shown in Table. 1, the sensitivity of proposed Ag/graphene plasmonic grating sensor has outstanding performance. Compared with the Refs [8,11,18], the temperature sensitivity of proposed device improve more than 1 to 2 orders and in addition, compared with the Refs [9,10,15,16], the refractive sensitivity of proposed sensor improve at least 1 order.

In summary, we propose a high sensitivity temperature sensor based on plasmonic Ag/graphene grating structure and sunflower oil is filled to the top of this structure as temperature sensitive medium. The sensitivity of proposed device can simultaneously reach 0.455 nm/°C or 1625 nmRIU⁻¹ and compared with other studies, it improve more than 1 or 2 orders. Furthermore, the operating wavelength can be tuned to the range of 1μm to 2μm by changing the structural parameters, including the thickness of Ag grating substrate, Ag grating, and the surface are of graphene. The proposed plasmonic Ag grating sensor can used in the field of real-time detection, integrated optical fiber sensing and biological/chemical measurements