CN117074365A - Terahertz high-quality factor refractive index sensor based on metal super surface - Google Patents
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Abstract
The invention discloses a terahertz high-quality factor refractive index sensor based on a metal super-surface, which is of a super-surface structure and comprises a dielectric substrate layer at the bottom and a metal resonator layer arranged on the dielectric substrate layer; the metal resonator layer is composed of a periodic structure unit, the structure unit is a ring resonator with four openings, the four openings are distributed at the crossing of a cross line of a circle center and the ring, the top opening size is different from the other three openings, and the symmetry of the resonator is destroyed. The invention realizes high-sensitivity and high-merit sensing based on the high quality factor provided by the quasi-continuous domain bound state, and enables the resonance intensity to be larger and the refractive index change of the detected analyte to be better and accurately identified by the sensing system based on the strong local electromagnetic field property of the electric dipole mode.
Description
Technical Field
The invention relates to the field of micro-nano photonics, belongs to a terahertz super-surface refractive index sensor, and particularly relates to a terahertz high-quality factor refractive index sensor based on a metal super-surface.
Background
Terahertz supersurfaces with resonant high quality factors are one of the more important directions in recent years. The high-quality factor sensor has potential application in the fields of chemical identification, cancer detection, optical detection devices and the like. In terahertz super-surfaces, sharp resonances requiring high quality factors, such as refractive index sensors, terahertz wave control, and filtering, are increasingly applied. The high quality factor means that the loss rate of resonance is low, the interaction between light and substances is strong, and the sensitivity to environment is good, which is very important in manufacturing the high quality factor sensor.
Currently, the quality factor of currently reported terahertz high-quality factor refractive index sensors based on metallic supersurfaces is generally low due to radiative losses and non-radiative losses of the material itself (such as inherent thermal losses) (Applied Physics letters, volume 112, page 201111 (2018)). It is particularly important to find supersurfaces that further increase the quality factor, and many recent studies have focused on achieving higher quality factors for resonance. During this time, there have been many methods for achieving improvement of the quality factor, such as coupling of surface plasmon mode, surface lattice mode, etc. and other modes to further improve the quality factor (Nature photonics. Volume 11, page 543 (2017)). Generally speaking, the formation of a farot resonance with an asymmetric line shape due to interference between the bright and dark modes in the structure can provide an effective way for the system to achieve a high quality factor (Materials today, volume 32, page 108 (2020)). Many works have been reported based on the above method, but the quality factor is not yet very high. Recently, the continuous domain binding state, which ideally enables infinite quality factors, is a new approach, and recently, attention has been paid more and more (IEEE Photonics journal 15, volume 15, page 1 (2023)). Therefore, the introduction of the continuous domain bound state into the terahertz super-surface provides a new approach for realizing higher quality factor resonance. The sub-wavelength micro-nano scale longitudinal thickness of the terahertz high-quality factor sensor based on the metal super-surface is particularly beneficial to miniaturization, integration and low cost of the system. In addition, based on the ultrathin characteristic of the terahertz high-quality factor sensor on the metal super surface, the sensor can be processed and manufactured by using a plurality of methods such as an ion beam etching method, an electron beam etching method, a photoetching machine etching method, a metal liquid ink-jet printing technology and the like, and has the advantage of low-cost batch manufacturing. The excellent characteristic of the high-sensitivity high-quality sensing device on electromagnetic wave transmission is utilized, and the high-sensitivity high-quality sensing device is favorable for manufacturing the high-sensitivity high-quality sensing device.
Disclosure of Invention
The invention aims to provide a refractive index sensor based on a terahertz super surface, which can enter a quasi-continuous domain constraint state mode in a 0.1-1 terahertz wave band, and realize high-sensitivity and high-figure-of-merit sensing based on a high quality factor provided by the quasi-continuous domain constraint state.
In order to achieve the above purpose, the following technical scheme is adopted: a terahertz high-quality factor refractive index sensor based on a metal supersurface, the sensor being of a supersurface structure comprising a dielectric substrate layer at the bottom and a metal resonator layer disposed on the dielectric substrate layer; the metal resonator layer is composed of a periodic structure unit, the structure unit is a ring resonator with four openings, the four openings are distributed at the crossing of a cross line of a circle center and the ring, the top opening size is different from the other three openings, and the symmetry of the resonator is destroyed.
For a light source of 0.1-1THz, when the light source irradiates perpendicular to the resonator ring surface, the metal resonator layer is coupled with the electric field and the magnetic field of the incident electromagnetic wave in a resonance mode, an electric dipole mode is presented, and a quasi-continuous domain constraint state is generated. The resonance mode can be generally realized by adjusting the width of the annulus of the ring resonator and the size of the four openings.
The invention realizes high-sensitivity and high-merit sensing based on the high quality factor provided by the quasi-continuous domain bound state, and enables the resonance intensity to be larger and the refractive index change of the detected analyte to be better and accurately identified by the sensing system based on the strong local electromagnetic field property of the electric dipole mode.
The dielectric substrate layer is mainly used for supporting the metal resonator layer, and can be selected from a silicon dioxide substrate, a PEN (polyethylene naphthalate for short) plastic substrate, a polyimide substrate, a silicon substrate, a PET (polyethylene terephthalate for short) substrate and a sapphire substrate, and is preferably a polyimide substrate.
Preferably, the thickness of the dielectric substrate layer ranges from 20 μm to 70 μm.
Preferably, the thickness of the metal resonator layer ranges from 50nm to 200nm.
Further preferably, the thickness of the dielectric substrate layer is 50 μm and the thickness of the metal resonator layer is 100nm.
Preferably, the central angle of the top opening of the metal resonator layer structure unit is 30 degrees, the central angles of the other three openings are 40 degrees, the inner ring radius of the metal resonator layer is 85 μm to 105 μm, and the outer ring radius is 110 μm to 135 μm.
Preferably, the material of the metal resonator layer is one of gold, silver, copper, aluminum and titanium.
The sensor can be prepared by adopting a standard photoetching mask method, and comprises the following specific steps:
1) Pre-treating the dielectric substrate layer to clean surface pollutants;
2) Depositing a metal film layer on a substrate by using a magnetron sputtering technology;
3) Coating a positive photoresist layer on the metal film layer, and drying the photoresist to prevent the photoresist from polluting devices;
4) Performing exposure etching by using laser direct writing;
5) Dissolving the exposure area to develop the positive film structure, and removing the unexposed area to develop the negative film;
6) And obtaining the required terahertz super-surface refractive index sensor structure through pattern transfer and photoresist removal.
The beneficial effects are that:
the invention realizes high-sensitivity and high-merit sensing based on the high quality factor provided by the quasi-continuous domain bound state, and enables the resonance intensity to be larger and the change of the refractive index of incident light to be better accurately identified by a sensing system based on the strong local electromagnetic field property of the electric dipole mode. As for terahertz waves having a frequency in the range of 0.6THz to 0.8THz (see fig. 6), the detection refractive index is from 1.0 to 2.0, the quality factor of the refractive index sensor is between 1561 and 1940, the sensing sensitivity is 141GHz/RIU, and the sensing optimum is between 306 and 454, it can be seen that the sensor of the present invention has a high quality factor, high sensitivity, and high optimum. In addition, the invention adopts the metal super surface, can use a mature standard photoetching technology in technology, has simple structure, and has the characteristic of low manufacturing cost.
Drawings
FIG. 1 is a schematic view of a three-dimensional periodic structure of a terahertz high-quality factor refractive index sensor based on a metal subsurface prepared in example 1 according to the present invention;
FIG. 2 is a top view of a two-dimensional unit structure of a terahertz high-quality factor refractive index sensor based on a metal subsurface prepared in example 1 according to the present invention;
FIG. 3 is a transmission spectrum of the top opening of the terahertz high-quality factor refractive index sensor metal resonator based on the metal supersurface prepared in example 1 from 5 degrees to 40 degrees;
FIG. 4 is a surface vector electric field and quality factor fitting plot of a terahertz high-quality factor refractive index sensor based on a metal supersurface prepared in example 1;
FIG. 5 is a schematic diagram of the quality factor, resonance intensity, and the product of the quality factor and the resonance intensity when the asymmetry parameter of the terahertz high-quality factor refractive index sensor based on the metal subsurface prepared in example 1 is reduced from 100% to 0%;
FIG. 6 is a graph of the transmission shift of the sensed refractive index of the terahertz high-quality factor refractive index sensor based on a metal subsurface prepared in example 1 from 1.0 to 2.0;
FIG. 7 is a plot of a frequency shift fit of the sensed refractive index of the terahertz high-quality factor refractive index sensor based on a metal subsurface prepared in example 1 from 1.0 to 2.0;
FIG. 8 is a graph of the sensing quality factor and figure of merit of the terahertz high-quality factor refractive index sensor based on a metal subsurface prepared in example 1, increasing the sensing refractive index from 1.0 to 2.0;
description of the reference numerals
1. A dielectric substrate layer; 2. a metal resonator layer; 21. a ring resonator.
Detailed Description
The invention will be further elucidated with reference to the following figures. The purpose of parameter selection according to the present invention is merely to propose preferred examples, and the embodiments are only used for describing the technical solution of the present invention in detail, and are not to be taken as limiting basis of the present invention. Other modifications and applications can be made without departing from the principles and spirit of the invention.
Aiming at the problem that the resonance quality factor, sensitivity and sensing figure of merit of the current refractive index sensor are not high, the invention aims to provide the terahertz refractive index sensor based on the metal super surface, which not only has high quality factor and sensing figure of merit, but also has simple structure and is easy to process and manufacture.
Fig. 1 is a schematic view of a three-dimensional structure of a terahertz high-quality factor refractive index sensor based on a metal subsurface of the present invention, which is a subsurface structure including a dielectric substrate layer 1 at the bottom and a metal resonator layer 2 disposed on the dielectric substrate layer 1; the metal resonator layer 2 is composed of a periodic structure unit 21, as shown in fig. 2, the structure unit 21 is a ring resonator with four openings, the four openings are distributed at the intersection of the cross line of the circle center and the ring, and the top opening size is different from the other three openings, so as to break the symmetry of the resonator. When a light source of 0.1-1THz is adopted to irradiate perpendicularly to the resonator ring surface, the metal resonator layer 2 is coupled with the electric field and the magnetic field of the incident electromagnetic wave in a resonance mode, an electric dipole mode is presented, and a quasi-continuous domain constraint state is generated. The resonance mode can be generally realized by adjusting the width of the annulus of the ring resonator and the size of the four openings.
The invention realizes high-sensitivity and high-merit sensing based on the high quality factor provided by the quasi-continuous domain bound state, and enables the resonance intensity to be larger and the refractive index change of the detected analyte to be better and accurately identified by the sensing system based on the strong local electromagnetic field property of the electric dipole mode. In addition, the invention adopts the metal super surface, can use a mature standard photoetching technology in technology, has simple structure, and has the characteristic of low manufacturing cost.
Fig. 2 is a schematic two-dimensional structure of a structural unit 21 of the terahertz high-quality-factor refractive index sensor based on a metal super surface of the present invention. As a preferred embodiment, R is as shown in FIG. 2 1 Is the inner radius of the ring resonator, which preferably ranges from 80 μm to 105 μm; r is R 2 Is the outer radius of the ring resonator, which preferably ranges from 110 μm to 135 μm; delta 2 A center angle of the top opening of the ring resonator, preferably 30 degrees; delta 1 The central angle of the other three openings of the circular resonator is preferably 40 degrees; p is the period of the unit cell and the electric field is along the x-direction.
As a further preferred embodiment, the thickness of the dielectric layer substrate layer 1 ranges from 20 μm to 70 μm; the thickness of the metal resonator layer 2 ranges from 50nm to 200nm.
With the above dimensional changes, the subsurface structure resonance frequency will shift and the resonance bandwidth will also change. Beyond a certain size, the resonance bandwidth will be larger and the resonance quality factor will be reduced.
The material of the metallic resonator layer 2 suggests one of gold, silver, copper, aluminum, titanium. The dielectric substrate layer 1 supports the metal resonator layer 2, and is mainly used for localized terahertz waves, and can be selected from one of a silicon dioxide substrate, a PEN plastic substrate, a polyimide substrate, a silicon substrate, a PET substrate and a sapphire substrate, and is preferably a polyimide substrate.
The following examples are only for the detailed description of the technical scheme of the present invention and are not to be taken as limiting basis of the present invention.
Example 1
A terahertz high-quality factor refractive index sensor based on a metal super surface comprises a dielectric substrate layer 1 at the bottom and a metal resonator layer 2 arranged on the dielectric substrate layer 1; wherein, the material of the dielectric substrate layer 1 is polyimide material, and the thickness is 50 μm; the metal resonator layer 2 is silver and has a thickness of 100nm. The inner ring radius of the ring resonator 21 is 90 μm, the outer ring radius is 125 μm, the thickness is 100nm, and the period is 300 μm (i.e., P in FIG. 2 is 300 μm).
The structure may be prepared by a photolithographic process. To prepare the above sensor, first, a polyimide substrate layer 1 of 50 μm thickness is pretreated to effect cleaning of surface contaminants. A 100nm thick silver film was deposited on a substrate using magnetron sputtering techniques to form the metallic resonator layer 2. Then, a 2 μm thick positive photoresist layer was coated on the silver film, and the photoresist was dried to prevent the photoresist from contaminating the device. The next step is to use laser direct writing for the exposure etching process. Subsequently, the development of the positive film structure is performed by dissolving the exposed areas, while the negative film development is performed by removing the unexposed areas, usually by water rinsing. Finally, the desired terahertz super-surface metal resonator layer 2 structure is obtained by pattern transfer and photoresist removal, as shown in fig. 1.
Fig. 3 is a graph showing the transmission shift in the terahertz high-quality factor refractive index sensor based on the metal super-surface in the present embodiment, and as can be seen from fig. 3, the resonance bandwidth gradually becomes smaller as the top opening increases until the linewidth disappears when the top opening is equal to the other openings, and a continuous domain confinement state occurs, and the quality factor in the ideal state at this time tends to be infinite. When the top opening is 30 degrees, a sharp quasi-continuous domain constrained state resonance mode appears, and the quality factor is higher.
Fig. 4 (a) and (b) are fitted graphs of vector electric field distribution and quality factor with respect to asymmetry parameters in the terahertz high-quality factor refractive index sensor based on a metal super surface according to the present embodiment. We define here the asymmetry parameter as: alpha= (delta) 1 -δ 2 )/δ 1 X 100%. It can be seen that parallel and equidirectional current distribution occurs on the ring of the metal resonator at a resonant frequency of 0.72THz, presenting an electric dipole mode, providing a strong local electric field. At the same time, the quality factors under different asymmetric parameters are simulatedIn addition, the electric dipole mode was found to exhibit quasi-continuous domain confinement, providing high quality factor resonance.
Fig. 5 is a graph showing the joint relation between the quality factor and the resonance intensity in the terahertz high-quality-factor refractive index sensor based on the metal super-surface according to the present embodiment, and it can be found from the graph that as the asymmetry parameter decreases, the resonance approaches to the continuous-domain bound state, resulting in a gradual increase in the quality factor, and the resonance intensity gradually becomes weaker as the asymmetry parameter decreases. The overall performance parameter of resonance (product of quality factor and resonance intensity) increases gradually with decreasing asymmetry parameter, and then tends to decrease rapidly after being extremely large. At the asymmetry parameter α=25%, the best overall performance parameter is obtained. Therefore, this parameter is chosen as the optimal parameter for refractive index sensing according to the present invention.
Fig. 6 is a graph of transmission shift at different refractive indices in a terahertz high-quality factor refractive index sensor based on a metal supersurface according to this embodiment. It can be seen from the graph that as the sensed refractive index increases from 1.0 to 2.0, the resonance appears to be red shifted by 0.141THz. The structure is shown to have high sensitivity to refractive index.
Fig. 7 is a frequency shift diagram at different refractive indices in a terahertz high-quality factor refractive index sensor based on a metal subsurface in the present embodiment. f (f) n –f n0 Defined as the amount of frequency shift of resonance, where f n0 Is the resonance frequency (n) when air is assumed to be sensed 0 ＝1)，f n Is the resonant frequency when the sensed refractive index is n. It can be seen from the graph that the frequency shift amount is linear with refractive index. In general, for refractive index sensors, the sensing sensitivity is caused by the sensed refractive index change. For refractive index sensing, the sensing fit function is generally given by: y=kn+b, where y represents the amount of frequency shift caused by different refractive indices, n represents the refractive index of the sensor, the slope k is defined as the sensitivity of the sensor, and b is a fitting constant. The simulation data of quasi-continuous domain binding state resonance induced by different refractive indexes are fitted, the fitting function is y= -149.6+141n, and the sensing sensitivity can be obtained at 141GHz/RIU.
Fig. 8 is a graph of quality factors and figures of merit at different refractive indices in a terahertz high-quality factor refractive index sensor based on a metal subsurface according to this embodiment. The calculation of the quality factor is given by the following fitting equation:quality factor = ω 0 2 gamma, where T is the transmittance of the transmission spectrum, omega 0 Is the resonance frequency, gamma is the decay rate of the resonance, a 1 ,a 2 And b are fitting constants. It can be seen from the graph that the refractive index changes from 1.0 to 2.0, and the quality factor increases from 1561 to a maximum 1940, and then gradually decreases, which are very high resonance quality factors. Generally, the sensor uses a merit value to evaluate the actual sensing performance of the system. There are generally the following definitions: figure of merit = sensitivity x quality factor/resonant frequency. As with the quality factor, the figure of merit increases and decreases from 306 to a maximum 454. Based on the method, the terahertz high-quality factor refractive index sensor based on the metal super surface has good sensing performance (high quality factor and high figure of merit) on the micron film, and has potential application in biological cell detection, sensing and identification.
The above examples of the present invention are not intended to limit the scope of the present invention, and the specific embodiments of the present invention are not limited to the specific scope. In addition, according to the above basic content of the present invention, other forms of changes, such as modification or substitution, of the above structure of the present invention are all within the scope of the present invention by the ordinary knowledge and technical means in the art without departing from the actual basic knowledge of the present invention.
Claims (7)
1. A terahertz high-quality factor refractive index sensor based on a metal super surface, characterized in that the sensor is a super surface structure, and comprises a dielectric substrate layer at the bottom and a metal resonator layer arranged on the dielectric substrate layer; the metal resonator layer is composed of a periodic structure unit, the structure unit is a ring resonator with four openings, the four openings are distributed at the crossing of a cross line of a circle center and the ring, the top opening size is different from the other three openings, and the symmetry of the resonator is destroyed.
2. The terahertz high-quality factor refractive index sensor based on metal super surface according to claim 1, wherein when the light source irradiates perpendicularly to the resonator ring surface, the metal resonator layer resonantly couples with the electric field and the magnetic field of the incident electromagnetic wave, and presents an electric dipole mode, generating a quasi-continuous domain confinement state.
3. The terahertz high-quality factor refractive index sensor based on a metal super surface according to claim 1 or 2, characterized in that the central angle of the top opening of the metal resonator layer structure unit is 30 degrees, the central angles of the other three openings are 40 degrees, the inner ring radius of the metal resonator layer ranges from 85 μm to 105 μm, and the outer ring radius ranges from 110 μm to 135 μm.
4. The metal-subsurface-based terahertz high-quality factor refractive index sensor according to claim 3, wherein the thickness of the dielectric substrate layer ranges from 20 μm to 70 μm; the thickness of the metal resonator layer ranges from 50nm to 200nm.
5. The terahertz high-quality-factor refractive index sensor based on a metal subsurface according to claim 4, wherein the thickness of the dielectric substrate layer is 50 μm and the thickness of the metal resonator layer is 100nm.
6. The terahertz high-quality factor refractive index sensor based on a metal super surface according to claim 5, wherein the dielectric substrate layer is used for supporting the metal resonator layer, and is selected from one of a silicon dioxide substrate, a PEN plastic substrate, a polyimide substrate, a silicon substrate, a PET substrate, and a sapphire substrate, preferably a polyimide substrate.
7. The terahertz high quality factor refractive index sensor based on a metal super surface of claim 6, wherein the material of the metal resonator layer is one of gold, silver, copper, aluminum, and titanium.
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| CN117907280A (en) * | 2024-01-26 | 2024-04-19 | 中国矿业大学 | Super-surface near-infrared refractive index sensor based on electromagnetic multipole resonance |
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| CN117907280A (en) * | 2024-01-26 | 2024-04-19 | 中国矿业大学 | Super-surface near-infrared refractive index sensor based on electromagnetic multipole resonance |
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