A dielectric-constant-near-zero-based d-type photonic crystal fiber refractive index sensor

Abstract

The application discloses a D-type photonic crystal fiber refractive index sensor based on dielectric constant near zero, which comprises a fiber core, the fiber core is a D-type photonic crystal fiber, the cross-sectional shape is composed of an arc and a chord, an open ring channel is arranged in the middle of the chord of the D-type photonic crystal fiber, and an ultrathin conductive oxide film is attached to the inner surface of the open ring channel; the ultrathin conductive oxide has a dielectric function near zero phenomenon in the communication C wave band under a set thickness. The ENZ mode of the ultrathin conductive oxide used in the application can replace the surface plasmon effect of gold in the communication C wave band, and has the advantages of high refractive index sensitivity and low cost.

Description

TECHNICAL FIELD

[0001] The application discloses a D-type photonic crystal fiber refractive index sensor based on epsilon near zero, and particularly relates to a metal oxide material with strong field confinement ability of ENZ mode, which can be applied to the technical fields of fiber sensing and optical communication. BACKGROUND

[0002] Epsilon Near Zero (ENZ) is the Chinese name of dielectric function near zero. When the partial metal oxide film thickness with the near-zero mode is in the deep subwavelength range, the dielectric function near zero (ENZ) mode can be observed. The thin film material can support the ENZ polariton mode with highly confined and enhanced electric field characteristics.

[0003] Surface Plasmon Resonance (SPR) is the Chinese name of surface plasmon resonance. When the frequency of electromagnetic waves matches the frequency of surface plasmons, the interface electron oscillation reaches the maximum value, and SPR is generated. Since the resonance frequency is largely dependent on the properties of the medium close to the material surface, the excitation of surface plasmons has become the most widely used sensing mechanism.

[0004] At present, the optical fiber SPR sensor widely used in the communication C band is a TiO2-Au SPR sensor prepared by attaching TiO2 with high dielectric constant real part to the surface of gold. The defects of this method are that the cost of Au is high, the process of attaching TiO2 is complex, and it is very inconvenient to measure the amount of biochemistry. Therefore, a new type of low-cost and high-performance refractive index ENZ fiber sensor is needed. SUMMARY

[0005] The technical problems to be solved by the application are that the cost of Au in the traditional noble metal SPR sensor is high, the process of attaching TiO2 is complex, and it is difficult to detect the amount of biochemistry.

[0006] To solve the above technical problems, the application provides a D-type photonic crystal fiber refractive index sensor based on epsilon near zero, which comprises a fiber core, the fiber core is a D-type photonic crystal fiber, the cross-sectional shape is composed of a circular arc and a chord, an open ring-shaped channel is arranged in the middle of the chord of the D-type photonic crystal fiber, and an ultrathin conductive oxide film is attached to the inner surface of the open ring-shaped channel, wherein the ultrathin conductive oxide film has the phenomenon of dielectric function near zero in the communication C band under the condition of a set thickness.

[0007] Six small circular air holes arranged at the vertices of a hexagon are arranged in the fiber core;

[0008] A small circular air hole is arranged on the outer periphery of the small circular air hole;

[0009] Two small-ellipse air holes arranged symmetrically left and right;

[0010] Two large-circle air holes arranged symmetrically left and right;

[0011] A large-ellipse air hole corresponding to the upper and lower positions of the open annular channel;

[0012] The outside of the fiber core is the substance to be measured.

[0013] The material of the ultra-thin conductive oxide film is fluorine-doped cadmium oxide, and the thickness is 30 nm (nanometers).

[0014] The material of the fiber core is quartz with a refractive index n = 1.45.

[0015] The small-ellipse air holes are arranged symmetrically on both sides of the fiber core, the angle between the long axis of the small-ellipse air hole and the horizontal axis of the fiber core is 45°, the long axis length a1 = 1.2 um (microns), and the short axis length b1 = 0.6 um.

[0016] The diameter d1 of the small-circle air hole arranged at the vertex of the hexagon is 0.8 um, and the distance Λ1 between the centers of the two small-circle air holes is 1.8 um.

[0017] The large-circle air holes are arranged symmetrically on both sides of the fiber core, and the diameter d2 of the large-circle air hole is 2 um.

[0018] The distance from the center of the large-ellipse air hole to the center of the fiber core is 3.464 um, the long axis of the large-ellipse air hole is parallel to the horizontal axis of the fiber core, the long axis length a2 of the large-ellipse air hole is 2 um, and the short axis length b2 of the large-ellipse air hole is 1 um.

[0019] For the cross section of the fiber core, the vertical axis of the open annular channel coincides with the vertical axis of the fiber core.

[0020] The vertical distance d3 from the center of the fiber core to the D-shaped string is 4 um.

[0021] The ultra-thin conductive oxide has the characteristic of near-zero dielectric function at a specific wavelength and thickness, and when using light of a specific wavelength for transmission in the communication C band, the light is confined in the metal oxide thin film.

[0022] The application has the beneficial effects that: the sensor of the application introduces two sizes of elliptical air holes, effectively destroys the symmetry of the overall fiber structure, makes the y direction polarization of the fundamental mode significantly stronger than the x direction polarization, enhances the birefringence effect, and due to the high loss characteristics of the ENZ material, the loss in the y direction is further increased, and the performance of the sensor is further enhanced. The D-shaped photonic crystal fiber refractive index sensor has high wavelength sensitivity and high amplitude sensitivity, and also has a narrow full width at half maximum, further improving the resolution of the sensor; compared with the traditional gold-based SPR fiber sensor, the D-shaped photonic crystal fiber refractive index sensor can realize sensing of the communication C band, avoids high cost and complex secondary coating process, and can directly contact the measured substance, greatly increasing the use scene. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 It is a model structure diagram of a D-shaped photonic crystal fiber refractive index sensor based on a dielectric constant near zero in an embodiment of the application.

[0024] Figure 2 It is a dielectric function real part diagram of a super-thin conductive oxide film used in the embodiment of the application.

[0025] Figure 3 It is a dispersion curve diagram of a super-thin conductive oxide film used in the embodiment of the application in the ENZ mode.

[0026] Figure 4 It is a mode field diagram in the ENZ mode when the D-shaped photonic crystal fiber refractive index sensor based on the dielectric constant near zero in embodiment 1 of the application detects a medium with a refractive index of 1.34.

[0027] Figure 5 It is a detailed distribution diagram of the internal electric field of the film at the loss spectrum peak value when the D-shaped photonic crystal fiber refractive index sensor based on the dielectric constant near zero in embodiment 1 of the application detects a medium with a refractive index of 1.34.

[0028] Figure 6 It is a mode field diagram in the ENZ mode when the D-shaped photonic crystal fiber refractive index sensor based on the dielectric constant near zero in embodiment 2 of the application detects a medium with a refractive index of 1.34.

[0029] Figure 7 It is a detailed distribution diagram of the internal electric field of the film at the loss spectrum peak value when the D-shaped photonic crystal fiber refractive index sensor based on the dielectric constant near zero in embodiment 2 of the application detects a medium with a refractive index of 1.34.

[0030] Figure 8This is the loss spectrum under y-polarization of a D-type photonic crystal fiber refractive index sensor based on a near-zero dielectric constant, as described in Embodiment 1 of the present invention, when detecting a medium with a refractive index of 1.33-1.37.

[0031] Figure 9 This is the loss spectrum under y-polarization of a D-type photonic crystal fiber refractive index sensor based on a near-zero dielectric constant, as described in Embodiment 2 of the present invention, when detecting a medium with a refractive index of 1.33-1.37. Detailed Implementation

[0032] The present invention will now be described in detail with reference to specific embodiments and accompanying drawings. These embodiments are implemented based on the technical solutions of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0033] Example 1

[0034] like Figure 1 As shown, a refractive index sensor based on a near-zero dielectric constant D-type photonic crystal fiber includes a fiber core, which is a D-type photonic crystal fiber with a cross-sectional shape composed of circular arcs and chords. An open annular channel is provided in the middle of the chord of the D-type photonic crystal fiber. An ultrathin conductive oxide film is attached to the inner surface of the open annular channel. The ultrathin conductive oxide film has a near-zero dielectric function in the C-band of communication when it has a thickness of 5-80 nm.

[0035] Six small circular air holes arranged in a hexagonal shape are provided inside the fiber core (2);

[0036] Around the small round vent are:

[0037] Two small oval pores arranged symmetrically from left to right (1);

[0038] Two large circular pores arranged symmetrically on the left and right (3);

[0039] Large elliptical pores (4) corresponding to the upper and lower positions of the open annular channel;

[0040] The material to be tested is located outside the fiber core (7).

[0041] The ultrathin conductive oxide film (6) is made of fluorine-doped cadmium oxide and has a thickness of 30 nm.

[0042] The core material is quartz with a refractive index n = 1.45.

[0043] The small elliptical pores are symmetrically arranged on both sides of the fiber core. The major axis of the small ellipse makes an angle of 45° with the horizontal axis of the fiber core. The length of the major axis is a1 = 1.2 μm and the length of the minor axis is b1 = 0.6 μm.

[0044] The diameter d1 of the small circular pores arranged at the vertices of the hexagon is 0.8 μm, and the distance Λ1 between the centers of two circular pores is 1.8 μm.

[0045] The large circular pores are symmetrically arranged on both sides inside the fiber core, and the diameter of the large circular pores is d2 = 2 μm.

[0046] The distance between the large elliptical pore and the center of the fiber core is 3.464 μm. The major axis of the large elliptical pore is parallel to the horizontal axis of the fiber core. The length of the major axis of the large elliptical pore is a2 = 2 μm, and the length of the minor axis of the large elliptical pore is b2 = 1 μm.

[0047] The vertical distance d3 from the center of the fiber core to the D-shaped chord is 4 μm.

[0048] The ultrathin conductive oxide has a near-zero dielectric function at specific wavelengths and thicknesses. In the C-band of communication, when light of a specific wavelength is used for transmission, the light is confined within the metal oxide film.

[0049] The fabrication method of the D-type photonic crystal fiber refractive index sensor based on near-zero dielectric constant of the present invention is as follows:

[0050] 1) A flat surface is ground out in the cladding of micro / nano optical fibers using a grinding method to serve as a sensing layer;

[0051] 2) Then, using a femtosecond laser, remove the corresponding parts of the following: two horizontally symmetrical small elliptical air holes (1) with a major axis a1 = 1.2 μm and a minor axis b1 = 0.6 μm at a distance of 3.464 μm from the fiber core; six small circular air holes (2) with a radius of 0.4 μm and symmetrically arranged at the vertices of a regular hexagon at a distance of 1.8 μm from the fiber core; three large circular air holes (3) with a radius of 1 μm and symmetrically arranged at the vertices of an equilateral triangle at a distance of 3.464 μm from the fiber core; and a horizontally large elliptical air hole (4) with a major axis a2 = 2 μm and a minor axis b2 = 1 μm at a distance of 6.928 μm from the fiber core, to obtain a photonic crystal fiber;

[0052] 3) Then, ultrathin conductive oxide is sputtered into an open annular channel by dual-gun radio frequency magnetron sputtering. The thickness of the sputtered ultrathin conductive oxide film is controlled by changing the working time, working temperature and sputtering power temperature.

[0053] like Figure 2 As shown, the dielectric function of ultrathin conductive oxides varies with the wavelength of electromagnetic waves. In this embodiment, a Drude-Lorentz model describing the dispersion relation of metals is selected to determine the dielectric constant at a certain band or wavelength. For the near-infrared wavelength of 1508nm, the real part of the dielectric constant of the corresponding metal oxide is -0.362.

[0054] Figure 3is the dispersion curve of the ultra-thin conductive oxide, the selected film thickness d is 50nm, the model of the IMI structure of medium-metal-medium is calculated by using COMSOL Multiphysics, and the dispersion curve is similar to the dispersion curve of the three-layer coupled mode LRSPP of medium-metal-medium. wherein ω p is the plasma frequency of the metal oxide, ε ∞ is the infinite dielectric of the ultra-thin conductive metal oxide, and Γ is the collision frequency of the ultra-thin conductive metal oxide.

[0055] Figure 4 is the electric field distribution diagram at the peak of the loss spectrum when the refractive index of the measured object is 1.34, and it can be seen that the enz mode is excited by the fiber under the incident light wavelength of 1600nm.

[0056] Figure 5 is the detailed internal electric field distribution diagram of the film at the peak of the loss spectrum when the refractive index of the measured object is 1.34, and under the resonant wavelength, the electric field energy leaked out of the core appears the standard ENZ mode characteristics, which is completely limited in the ultra-thin conductive metal oxide film.

[0057] Figure 8 is the loss spectrum diagram in the wavelength range of 1250nm to 1950nm when the refractive index of the measured object is 1.33-1.37, and the loss is defined as wherein Im(n eff ) is the effective refractive index imaginary part, and λ0 is the incident light wavelength.

[0058] Embodiment 2

[0059] As Figure 1As shown, a D-shaped photonic crystal fiber refractive index sensor based on dielectric constant near zero includes a core, the core is a D-shaped photonic crystal fiber, an open annular channel is arranged on the upper plane of the D-shaped photonic crystal fiber, and an ultrathin conductive oxide film is attached to the inner surface of the open annular channel, wherein the ultrathin conductive oxide has a dielectric function near zero phenomenon in the communication C band under the condition of 5-80nm thickness.

[0060] Six small circular air holes (2) arranged in the form of hexagonal vertices are arranged inside the core;

[0061] An outer periphery of the small circular air hole is provided with:

[0062] Two small elliptical air holes (1) arranged symmetrically on the left and right;

[0063] Two large circular air holes (3) arranged symmetrically on the left and right;

[0064] A large elliptical air hole (4) corresponding to the upper and lower positions of the open annular channel;

[0065] The outside of the core is a to-be-measured substance (7).

[0066] The material of the ultrathin conductive oxide film (6) is fluorine-doped cadmium oxide, and the thickness is 30nm (nanometer).

[0067] The material of the core is quartz with a refractive index n=1.45.

[0068] The small elliptical air holes are arranged symmetrically on both sides of the core, the long axis of the small elliptical air hole makes an angle of 45° with the horizontal axis of the core, the long axis length a1=1.2um, and the short axis length b1=0.6um.

[0069] The diameter of the small circular air hole arranged in the form of hexagonal vertices is d1=0.8um, and the distance between the centers of the two circular air holes is Λ1=1.8um.

[0070] The large circular air holes are arranged symmetrically on both sides of the core, and the diameter of the large circular air hole is d2=2.4um.

[0071] The distance between the center of the core and the D-shaped chord is d3=4um.

[0072] The large elliptical air hole has a long axis parallel to the horizontal axis of the core, the long axis length a2=2um, and the short axis length b2=1um.

[0073] The ultrathin conductive oxide has the characteristic of dielectric function near zero under the condition of specific wavelength and thickness, and when specific wavelength light is used for transmission in the communication C band, the light is confined in the metal oxide film.

[0074] As Figure 2 shown, the dielectric function of the ultra-thin conductive oxide changes with the change of the wavelength of the electromagnetic wave, and a Drude-Lorentz model describing the metal dispersion relationship is selected to determine the dielectric constant at a certain wavelength or a certain wavelength band, and for the near-infrared wavelength 1508 nm, the real part of the dielectric constant corresponding to the metal oxide is -0.362.

[0075] Figure 3 is the dispersion curve of the ultra-thin conductive oxide, the selected film thickness d is 50 nm, and COMSOL Multiphysics is used to calculate the model of the IMI structure of medium-metal-medium, and the dispersion curve is similar to the dispersion curve of the three-layer coupled mode LRSPP of medium-metal-medium. With the increase of frequency, the real part of the dielectric constant of the metal oxide increases, and when the real part of the dielectric constant is 0, the frequency where ω p is the plasma frequency of the metal oxide, ε ∞ is the infinite dielectric of the ultra-thin conductive metal oxide, and Γ is the collision frequency of the ultra-thin conductive metal oxide. Near the ENZ wavelength, the electric field is localized inside the ENZ film material, showing a strong field enhancement effect, and the optical fiber loss increases, and when the refractive index of the external medium layer changes, the ENZ wavelength shifts, and the sensor of the present application is based on this principle to sense the refractive index.

[0076] Figure 6 is the electric field distribution diagram in the near-infrared wavelength band when the refractive index of the measured object is 1.34. It can be seen that at the resonance wavelength, the electric field energy leaked out of the core appears the standard ENZ mode characteristics, which is completely limited in the ultra-thin conductive metal oxide film.

[0077] Figure 7 is the detailed distribution diagram of the internal electric field of the film at the peak of the loss spectrum when the refractive index of the measured object is 1.34. At the resonance wavelength, the electric field energy leaked out of the core appears the standard ENZ mode characteristics, which is completely limited in the ultra-thin conductive metal oxide film.

[0078] Figure 9 is the loss spectrum diagram in the wavelength band of 1250 nm to 1950 nm when the refractive index of the measured object is 1.33-1.37, and the loss is defined as where Im(n eff) is the effective refractive index imaginary part, λ0 is the incident light wavelength, unlike the traditional fiber SPR sensor, only one kind of low-cost ultra-thin conductive metal oxide with ENZ effect can be used to realize high-sensitivity sensing in the communication C window, the maximum wavelength sensitivity is 2000nm / RIU, the minimum full width at half maximum is about 100nm, and the overall sensing resolution FOM reaches 20.

[0079] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit it, although the present application has been described in detail with reference to the above examples, those skilled in the art should understand that: the specific embodiments of the present application can still be modified or replaced by the equivalent, without departing from the spirit and scope of the present application, any modification or equivalent replacement, which should be covered within the protection scope of the claims of the present application.

Claims

1. A dielectric-constant-near-zero-based D-type photonic crystal fiber refractive index sensor comprising a core, characterized in that: The fiber core is a D-shaped photonic crystal fiber, the cross-sectional shape is composed of a circular arc and a chord, an open annular channel is arranged in the middle of the chord of the D-shaped photonic crystal fiber, and an ultrathin conductive oxide film is attached to the inner surface of the open annular channel, wherein the ultrathin conductive oxide has a dielectric function close to zero in the communication C band under a set thickness, and the thickness of the ultrathin conductive oxide is 5-80 nm. Six small circular air holes (2) arranged at the vertices of a hexagon are arranged in the fiber core; An outer periphery of the small circular air hole is provided with: Two small elliptical air holes (1) arranged symmetrically left and right; Two large circular air holes (3) arranged symmetrically left and right; A large elliptical air hole (4) corresponding to the upper and lower positions of the open annular channel; The small elliptical air hole has an angle of 45° between the major axis and the horizontal axis of the fiber core, the major axis length a1 is 1.2 um, and the minor axis length b1 is 0.6 um; The diameter d1 of the hexagonal vertex arranged small circular air holes is 0.8 um, and the center distance of two circular air holes is 1 is 1.8 um; The large circular air holes are symmetrically arranged on both sides of the fiber core, and the diameter d2 of the large circular air hole is 2 um; The large elliptical air hole has a distance of 3.464 um from the center of the fiber core, the major axis of the large elliptical air hole is parallel to the horizontal axis of the fiber core, the major axis length a2 of the large elliptical air hole is 2 um, and the minor axis length b2 of the large elliptical air hole is 1 um.

2. The dielectric constant near zero based D-type photonic crystal fiber refractive index sensor according to claim 1, characterized in that: The material of the ultrathin conductive oxide film (6) is fluorine-doped cadmium oxide, and the thickness is 30 nm.

3. The dielectric constant near zero based D-type photonic crystal fiber refractive index sensor according to claim 1, wherein: The material of the fiber core is quartz with a refractive index n of 1.

45.

4. The dielectric constant near zero based D-type photonic crystal fiber refractive index sensor according to claim 1, wherein: For the cross section of the fiber core, the vertical axis of the open annular channel coincides with the vertical axis of the fiber core.

5. The dielectric constant near zero based D-type photonic crystal fiber refractive index sensor according to claim 1, wherein: The vertical distance d3 from the center of the fiber core to the D-shaped chord is 4 um.

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