CN111521582A

CN111521582A - Near-infrared band double-D type photonic crystal fiber SPR sensor

Info

Publication number: CN111521582A
Application number: CN202010481562.8A
Authority: CN
Inventors: 杨宏艳; 黄善文; 刘梦银; 陈昱澎; 苑立波
Original assignee: Guilin University of Electronic Technology
Current assignee: Guilin University of Electronic Technology
Priority date: 2020-05-31
Filing date: 2020-05-31
Publication date: 2020-08-11

Abstract

A photonic crystal fiber sensor with a double D-shaped structure based on surface plasmon resonance comprises photonic crystal fibers, air holes and a nanogold film, wherein the air holes are distributed in the edge of the inner part of silicon dioxide, the two photonic crystal fibers are symmetrically distributed in parallel, the photonic crystal fibers are all laterally polished into a D shape, the nanogold film is coated on the laterally polished surface, the air holes in the middle of the laterally polished surface are semicircular, the air holes and the neighborhood of the air holes form a photonic crystal fiber core, and namely the area surrounded by the air holes forms the fiber core of the photonic crystal fiber. The sensor realizes coupling by using two parallel D-shaped optical fibers, can effectively improve the sensitivity of the sensor, and is a practical refractive index sensor.

Description

Near-infrared band double-D type photonic crystal fiber SPR sensor

(I) technical field

The invention relates to a near-infrared band double-D type photonic crystal fiber surface plasma resonance sensor

(II) background of the invention

In recent years, photonic crystal fiber surface plasmon resonance (PCF-SPR) sensors have been widely noticed by researchers because of their excellent sensing properties, which have great application potential in the fields of life sciences, drug screening, molecular recognition, immunoassay, and the like.

Photonic Crystal Fibers (PCFs), also called microstructured Fibers, were first proposed in 1992 by stj. rusell et al, the concept of regularly arranging air holes in a silica fiber and introducing a defect structure in the core of the fiber that destroys the periodicity of the cladding, which may be large air holes or solid silica. The first PCF sample was developed in 1996 by Russell et al at the university of Bath, uk.

Surface Plasmons (SPs) refer to an evanescent wave of electrons propagating along a metal Surface resulting from the interaction of freely vibrating electrons present at the metal Surface with photons. In 1902, Wood observed a metal diffraction grating, and found a new diffraction peak in a normal diffraction angle distribution spectrum, and the SPR phenomenon was first found. In 1909, a. sommerfeld introduced the concept of complex dielectric constant from Maxwell electromagnetic theory, and derived the expression of surface plasmon waves. In 1941, Fano et al explained the SPR phenomenon by generating electromagnetic waves at a metal-air interface. In 1957, ritchai bombarded metal flakes with higher energy electrons, and during this experiment it was found that there was a loss of energy at the plasma frequency, and he considered that the energy loss was related to the metal interface. Stren, in 1960, when studying the conditions under which the SPR phenomenon occurs, first proposed the concept of surface plasmons. In 1982, c.nylander et al first used SPR in the chemical field for gas detection. Since then, SPR sensors have become a hotspot for research. In order to be able to make measurements using SPR, special devices need to be used to achieve SPR.

The photonic crystal fiber sensor has the characteristics of remote real-time detection, small volume, easiness in integration, high sensitivity, flexibility in design and the like due to the unique advantages of the photonic crystal fiber sensor, and attracts excellent scientific researchers to invest much energy and resources to research the photonic crystal fiber sensor. The method combines the microstructure design of the photonic crystal fiber with the high-sensitivity technology of surface plasma resonance, and has the advantages of real-time monitoring, no need of marking and small interference. The photonic crystal fiber is used as a sensor instead of a common fiber, so that the phase matching problem can be solved.

The photonic crystal fiber sensor does not need an additional coupling device, can effectively solve the problem of phase matching through the arrangement design of air holes, realizes the mode coupling of a fiber core mode and a surface plasma mode, converts the change of the refractive index of a substance to be measured into the shift change of an absorption peak, and realizes sensing measurement.

To improve the sensing sensitivity, one strategy is to improve the coupling efficiency between the core guided mode and the plasmon mode by optimizing the structure. The coupling effect includes the interaction between adjacent multi-plasmon sensors, which has been applied to dual parallel optical fibers. However, there is little progress in the development of dual parallel D-shaped PCF-SPR sensors.

In the conventional research on PCF-SPR, the coupling between a fiber core guided mode and a surface plasmon polariton is mostly researched, and mode coupling other than the two modes is rarely involved, such as the coupling problem between a mode penetrating into a solution to be measured and the surface plasmon polariton, the problem of guided mode coupling (intermodal interference) between fiber cores in a multi-core optical fiber and the like. These problems have a more or less influence on the magnitude of the resonance intensity, the change in the resonance wavelength, the generation of spectral sub-peaks, and the like. These mode coupling problems are well studied and help to further optimize sensor performance.

Disclosure of the invention

In view of the above problems, an object of the present invention is to provide a surface plasmon resonance-based photonic crystal fiber sensor with a double-symmetric D-type structure, which utilizes the coupling between a fiber core guided mode and a surface plasmon mode and the guided mode coupling between two fiber cores, and combines the mode coupling of two modes to realize high-sensitivity sensing detection.

In order to achieve the purpose, the invention adopts the following technical scheme: a photonic crystal fiber sensor with a double-symmetrical D-shaped structure based on surface plasmon resonance comprises 6 photonic crystal fibers, 1 air holes, 2 nanogold films, wherein the air holes are distributed in the inner edge of each photonic crystal fiber, the two photonic crystal fibers are symmetrical about the geometric center of each photonic crystal fiber, the photonic crystal fibers are laterally polished into a D shape, the nanogold films are coated on the laterally polished surfaces, the air holes in the middle of the laterally polished surfaces are semi-circles, the air holes and the neighborhoods of the air holes form fiber cores of the photonic crystal fibers, and the areas surrounded by the air holes form the fiber cores of the photonic crystal fibers.

In the photonic crystal fiber sensor, the air holes are uniformly surrounded on the edge of the inner part of the substrate.

In the photonic crystal fiber sensor, the diameters of the air holes are 9um, the phases of the adjacent air holes are different by 30 degrees in sequence, and the air hole in the middle of the side throwing is a semicircle.

In the photonic crystal fiber sensor, the distance between two fibers is 900 nm.

Above-mentioned photonic crystal fiber sensor, go up D type optic fibre and D type optic fibre radius down and be 67.5um, the side is thrown the degree of depth and is 10 um.

In the photonic crystal fiber sensor, the thickness of the nano gold film is 40 nm.

Due to the adoption of the technical scheme, the invention has the following advantages:

1. the design of the air holes arranged in a surrounding manner limits light to be transmitted in the fiber core, so that the transmission loss of the light can be obviously reduced, and the sensitivity of the sensor is improved; the air hole has smaller size, can reduce the effective refractive index of the fiber core, and is convenient for the coupling of the surface plasma mode and the fiber core mode.

2. The gold-plated nano film is selected, so that the gold has stable chemical property and is not easy to oxidize.

3. The coupling between the fiber core guided mode and the surface plasma body mode and the guided mode coupling between the two fiber cores, and the superposition coupling of the two modes can realize the sensing detection with higher sensitivity and improved stability

(IV) description of the drawings

FIG. 1 is a three-dimensional block diagram of the present invention

FIG. 2 is a cross-sectional view of the structure of the present invention

FIG. 3 shows the limiting loss spectrum of the present invention in the refractive index range (n is 1.36-1.41)

FIG. 4 is a limiting loss spectrum for varying air hole diameter according to the present invention

FIG. 5 is a limiting loss spectrum of the present invention when the thickness of the nano-gold film is changed

FIG. 6 is a limiting loss spectrum of the present invention when changing the spacing between two optical fibers

FIG. 7 is a graph of refractive index amplitude sensitivity of the present invention

The reference numbers in the figures are: 1. an air hole; 2. a nano gold film; 3. a fiber core 1; 4. a fiber core 2; 5. an analyte-sensing region; 6. photonic crystal fiber

(V) detailed description of the preferred embodiments

In order to make the objects, technical solutions and advantages of the present invention more apparent, the following is further explained with reference to the accompanying drawings and examples.

As shown in fig. 1 and fig. 2, the device comprises a photonic crystal fiber, an air hole, a nano-gold film, a perfect matching layer and liquid to be measured. Silicon dioxide with stable performance is used as a substrate material, the refractive index is about 1.4525, air holes are distributed around the edge of the substrate and are arranged in a circular ring shape, the purpose is to reduce the average refractive index of the edge part of the silicon dioxide, the diameter is 9um, the phases of adjacent air holes are sequentially different by 30 degrees, and the air holes in the middle of side throwing are semicircular. The two D-type optical fibers have the radius R of 67.5um and the side-polishing depth of 9 um. The distance between the two fibers was 900 nm. And the plane sides of the upper D-shaped optical fiber and the lower D-shaped optical fiber are plated with 40nm nano gold films. A Perfect Matching Layer (PML) is added as a boundary condition to the outermost Layer of the simulation calculation region for absorbing radiation energy.

The formula for calculating the sensitivity is:

wherein, the delta lambda is the wavelength variation of the resonance absorption peak, and the delta n is the variation of the refractive index of the external medium to be measured.

Fig. 3 is a limiting loss spectrum of the present invention in a refractive index interval (n is 1.36-1.41), and when the refractive index is sequentially increased by 0.01 to 1.36, 1.37, 1.38, 1.39, 1.40, and 1.41, the resonance wavelength is gradually increased along with the increase of the refractive index, and the wavelength variation range is gradually increased. The loss peaks have peak offsets of 60nm, 20nm, 80nm, 140nm and 460nm, sensitivities of 6000RIU-1, 2000RIU-1, 8000RIU-1, 14000RIU-1 and 46000RIU-1, respectively, and an average sensitivity of 15200RIU-1 in a refractive index interval (n is 1.36 to 1.41).

FIG. 4 is a limiting loss spectrum for different air hole diameters. The introduction of air holes affects the average refractive index of the photonic crystal fiber, so the size of the air holes inevitably affects the phase matching between the core guided mode and the plasmon mode. The resonance wavelength is slightly red-shifted when the air hole diameter is increased from 7um to 12 um. The resonance intensity is gradually reduced along with the increase of the diameter of the air hole, because the larger the air hole is, the smaller the average refractive index of the equivalent cladding is, the larger the difference value of the refractive indexes of the equivalent cladding and the equivalent core is, the stronger the light constraint is, the less light can reach the nano gold film, and the weaker the resonance is.

FIG. 5 is a limiting loss spectrum of the thickness of the nanogold film, assuming that the refractive index of the liquid is 1.40, other structural parameters of the sensor are kept unchanged, only the thickness of the nanogold film is changed, so that the thicknesses of the nanogold film are respectively 35nm, 40nm, 45nm and 50nm, the values of the loss of the fiber core guided modes corresponding to the nanogold films with different thicknesses, which change along with the wavelength, are obtained through simulation analysis, and in order to explain the relationship between the resonance wavelength and the thickness of the nanogold film, the penetration depth (d) of evanescent waves needs to be introduced (d is used for explaining the relationship between_p) The concept of (a) is that,

the calculation formula is as follows:

wherein k is the wave number;

beta is a decay constant;

λ is the wavelength of the incident light.

The formula shows that the penetration depth of the evanescent wave is in direct proportion to the wavelength of the incident light. The surface plasma resonance phenomenon appears in the nano-gold film and the solution to be measured, when the thickness of the nano-gold film is increased from 35nm to 50nm, the resonance intensity is gradually reduced, which shows that the evanescent wave penetrating through the nano-gold film is reduced. In addition, the resonance wavelength is red-shifted with the increase of the thickness of the nanogold film. The surface plasma resonance phenomenon appears in the nano gold film and the solution to be measured, when the thickness of the nano gold film is increased, the penetration depth of evanescent waves is increased, the required incident light wavelength is increased, and therefore the resonance wavelength is red-shifted.

FIG. 6 is a plot of the limiting loss spectrum for the two fiber separation distance D. As shown, as the fiber separation increases from 860nm to 900nm, the resonant intensity gradually decreases, since the increase in separation prevents the mutual coupling between the two fibers, and the sensor tends to behave as a single fiber sensor. In addition, the resonance peak gradually shifts in the short wavelength direction as the pitch increases, because photons require more energy to pass through the pitch as the pitch increases, and resonance occurs more easily at short wavelengths.

FIG. 7 shows the variation of amplitude sensitivity with wavelength when the refractive index of the solution to be measured changes by 0.01, and another important parameter of the PCF-SPR sensor is amplitude sensitivity SA (λ). The amplitude sensitivity can be defined as:

wherein α (λ, n)_ana) Limiting loss when the refractive index of the solution to be measured is nana;

the variable quantity of the refractive index of the solution to be detected;

the difference in resonance intensity between adjacent loss spectra due to the change in refractive index of the solution to be measured.

The refractive index intervals of the solution to be detected corresponding to the invention are 1.36-1.37, 1.37-1.38, 1.38-1.39, 1.39-1.40 and 1.40-1.41 respectively. The maximum amplitude sensitivities at positions in the refractive index interval of 1.40 to 1.41 were 646RIU-1, 819RIU-1, 1082RIU-1, 1856RIU-1, and 1553RIU-1, respectively, and in addition, 1856RIU-1, which is the maximum amplitude sensitivity of the present invention, was obtained.

It should be noted that, although the above-mentioned embodiments of the present invention are illustrative, the present invention is not limited thereto, and therefore, the present invention is not limited to the above-mentioned embodiments. Other embodiments, which can be made by those skilled in the art in light of the teachings of the present invention, are considered to be within the scope of the present invention without departing from the inventive concept.

Claims

1. A double D type photonic crystal fiber sensor based on surface plasmon resonance comprises a photonic crystal fiber cladding and a fiber core, and is characterized in that: air holes are distributed on the inner edge of the silicon dioxide, the two photonic crystal fibers are symmetrical about the geometric center of the photonic crystal fibers, the photonic crystal fibers are all in a side-polished D shape, the surface of the side-polished photonic crystal fibers is coated with a gold film, the air holes in the middle of the side-polished surface are semi-circles, the air holes and the neighborhood of the air holes form a photonic crystal fiber core, and namely the area surrounded by the air holes forms the photonic crystal fiber core.

2. The dual-D photonic crystal fiber sensor of claim 1, wherein: the two fibers are symmetrical about the geometric center of the photonic crystal fiber.

3. The dual-D photonic crystal fiber sensor of claim 1, wherein:

the thickness of the nano gold film is 40 nm.

4. The dual-D photonic crystal fiber sensor of claim 1, wherein:

the diameter of the air hole is 9um, the phase difference of adjacent air holes is 30 degrees in sequence, wherein the air hole in the middle of the side throw is a semicircle.

5. The dual-D photonic crystal fiber sensor of claim 1, wherein: the distance between the two PCF fibers is 900 nm.

6. The dual-D photonic crystal fiber sensor of claim 1, wherein: the radiuses of the upper D-shaped optical fiber and the lower D-shaped optical fiber are both 67.5um, and the lateral polishing depth is both 10 um.

7. The dual-D photonic crystal fiber sensor of claim 1, wherein: the detection range of the refractive index of the external medium to be detected is 1.36-1.41.

8. The dual-D photonic crystal fiber sensor of claim 1, wherein: the wavelength is in the near infrared region, when the refractive index of the medium to be measured changes, the position change of the resonance absorption peak can be influenced, and the change of the refractive index of the medium to be measured can be demodulated by measuring the position change of the resonance absorption peak.