CN111722310A - Device and method for modulating fluorescence polarization of two-dimensional crystal forbidden band - Google Patents
Device and method for modulating fluorescence polarization of two-dimensional crystal forbidden band Download PDFInfo
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- CN111722310A CN111722310A CN201910216980.1A CN201910216980A CN111722310A CN 111722310 A CN111722310 A CN 111722310A CN 201910216980 A CN201910216980 A CN 201910216980A CN 111722310 A CN111722310 A CN 111722310A
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- G02F1/0136—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour for the control of polarisation, e.g. state of polarisation [SOP] control, polarisation scrambling, TE-TM mode conversion or separation
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Abstract
The invention discloses a device and a method for modulating fluorescence polarization of a two-dimensional crystal forbidden band, wherein the device comprises the following components: a substrate of highly conductive material; an insulating layer formed on the high conductive material substrate; a two-dimensional crystal monolayer formed on the insulating layer; a light-transmitting dielectric layer formed on the two-dimensional crystal single layer; and the metal nanometer metamaterial with the periodic unit structure is formed on the light-transmitting medium layer. The invention is based on the metal surface plasmon resonance principle, utilizes the metal nano metamaterial anisotropic resonance field to regulate and control the semiconductor fluorescence field, realizes polarized fluorescence and achieves the purpose of adjustable polarization. The device and the method for modulating the polarization of the two-dimensional crystal forbidden band fluorescence effectively widen the linear adjustable range and the temperature application range of the two-dimensional crystal forbidden band fluorescence.
Description
Technical Field
The invention belongs to the technical field of nano photoelectron, and particularly relates to a device and a method for modulating fluorescence polarization of a two-dimensional crystal forbidden band.
Background
The micro-nano light source with adjustable polarity has special advantages in the aspects of information safety, high-sensitivity sensing, structural analysis, pathogen diagnosis and the like. The novel two-dimensional crystal which is started in recent years has wide direct forbidden band selectivity, structural stability, photoelectric adjustability and substrate diversity and has excellent conditions of an on-chip micro-nano light source. However, the application range of the micro-nano light source is limited by the polarization randomness of fluorescence at room temperature, so that the light source on the chip needs to be subjected to polarization modulation.
The macroscopic light path modulates the polarization of light using a polarizer and a polarizing prism. The polaroid is mainly composed of a polyvinyl alcohol polarizing film and transparent cellulose triacetate protective films compounded on the upper and lower surfaces of the polyvinyl alcohol polarizing film. The main components of the polyvinyl alcohol polarizing film are polyvinyl alcohol and iodine, and the polyvinyl alcohol polarizing film is prepared by adopting a wet-process stretching process. The typical process is as follows: firstly, preparing a transparent polyvinyl alcohol thick film by a tape casting method or a melt extrusion method, immersing the transparent polyvinyl alcohol thick film into a compound solution containing iodine for reaction, and carrying out iodine dyeing on the thin film; and then, unidirectionally stretching the PVA film by 3-5 times in a certain direction between rollers rotating at different speeds to highly orient the molecular bonds of the polyvinyl alcohol, and simultaneously orienting the iodine molecules embedded therein to have dichroism, absorb light components with the polarization direction being the same as the stretching direction, and transmit the light components perpendicular to the stretching direction. The polyvinyl alcohol polarizing film is quickly deformed, shrunk, loosened, faded and the like in a warm environment, and has the advantages of low strength, brittle and breakable property and inconvenience in use and processing. Therefore, it is necessary to attach a triacetyl cellulose protective film having high light transmittance, good heat and temperature resistance, high strength, and optical isotropy to the upper and lower surfaces. This increases the thickness of the polarizer and prevents direct integration into the photonic device. In addition, the polarizer can be assembled from dichroic crystals, such as calcite, designed, cut, and polished according to specific aspect ratios and end face angle requirements. Although polarizers made with polarizing prisms have the advantages of high extinction ratio, high transmission, and high damage threshold. But the practicability and compatibility of the crystal in a micro-nano photonic system are very low due to the blocky crystal characteristics. Therefore, neither the material structure nor the fabrication process is suitable for the photonic chip.
The photonic device on chip is modulated with a specific theoretical basis and a working method thereof, which are different from a macroscopic optical element. In the aspect of two-dimensional crystal fluorescence polarization modulationThe former people use temperature and magnetic field as two modulation means for research. First, at low temperatures, linearly polarized fluorescence can be generated using a two-dimensional crystal valley polarization effect. The two-dimensional crystal valley polarization means that the single-layer crystal forms two valleys (K) in the K space due to the central symmetry defect+And K-). The two valleys can be detected by circularly polarized light, and can emit circularly polarized fluorescence with corresponding properties when being excited by external circularly polarized laser. Because the linearly polarized light is the coherent superposition of left and right optical rotation, when the linearly polarized light is used for exciting the two-dimensional crystal, the left and right components in the linearly polarized light respectively act with the left and right valleys, so that the left and right valley electrons jump to the conduction band from respective valence band simultaneously to emit left and right fluorescence, and the coherent superposition of the left and right valleys and the conduction band forms linear polarized fluorescence. This process occurs with the requirement that an important condition is met, namely that the de-coherence time of the left and right excitons is greater than the electron-hole recombination time. On the contrary, only randomly polarized fluorescence can be obtained when the left and right valley excitons are decohered before the electron hole recombination. The higher the temperature, the stronger the inter-valley scattering caused by phonon resonance, and the shorter the decoherence time, which is the main cause of polarization randomness at room temperature. The lower the temperature, the smaller the inter-valley scattering, the longer the decoherence time, and the better the fluorescence linear bias. Using this principle, at very low temperatures of 30K, the relevant work has measured a fluorescence linearity of 35%. Secondly, when no magnetic field is applied, the optical axis of the low-temperature linear polarization fluorescence is always parallel to the optical axis of the excitation laser; when a magnetic field perpendicular to the surface of the two-dimensional crystal is applied, the polarization direction of the linearly polarized fluorescence is deflected under the influence of the valley Zeeman splitting effect, and the polarization degree is reduced to 16%. It follows that temperature and magnetic field are viable means of modulating the fluorescence polarization of a two-dimensional crystal. However, both of these methods have their own drawbacks and limitations: firstly, the fluorescence polarization modulation intensity is small; and secondly, the wide application is difficult under extreme conditions.
Disclosure of Invention
Technical problem to be solved
In order to overcome the defects and shortcomings of the modulation method, the invention provides a device and a method for modulating the fluorescence polarization of a two-dimensional crystal forbidden band by adopting an anisotropic metamaterial, so as to solve the problems of small polarization modulation strength and temperature limitation.
(II) technical scheme
The invention provides a device for modulating fluorescence polarization of a two-dimensional crystal forbidden band, which comprises the following components:
a substrate of highly conductive material;
an insulating layer formed on the high-conductivity material substrate;
a one-dimensional crystal monolayer formed on the insulating layer;
a light-transmitting dielectric layer formed on the two-dimensional crystal single layer; and
and the metal nano metamaterial is formed on the light-transmitting medium layer and has a periodic unit structure.
Wherein the high conductive material substrate is a highly doped silicon or metal substrate with a thickness of 500 and 750 μm; the high-conductivity material plays a role in structural support and reflects the two-dimensional crystal single-layer fluorescence;
wherein the insulating layer is SiO2、Al2O3The thickness is 270-310 nm; the insulating layer is used as a growth substrate of the two-dimensional crystal monolayer;
wherein the two-dimensional crystal is a single-layer transition metal chalcogenide MX2Wherein M ═ Mo, W; x is S, Se, Te; the thickness of the two-dimensional crystal monolayer is less than 1 nm; the two-dimensional crystal monolayer emits fluorescence under the excitation of the pump light;
wherein the light-transmitting dielectric layer is Al2O3Or SiO2The thickness is 10-50 nm; the thickness of the light-transmitting medium layer influences the resonance transmission and absorption of the metal nano metamaterial at the excitation field, and further influences the fluorescence intensity; the light-transmitting medium layer protects the two-dimensional crystal material from water, oxygen or impurity ions;
wherein the metal nano metamaterial is a metal nano structure of gold, silver or aluminum and the like, and the thickness of the metal nano metamaterial is 30-100 nm;
the metal nano metamaterial with the periodic unit structure is provided with a linear, V-shaped or N-shaped air gap structure; and the included angle between the V-shaped or N-shaped air gap structure and the horizontal direction is an air gap inclination angle theta.
Based on the metamaterial polarization modulation device, the invention also provides a method for modulating the fluorescence polarization of the two-dimensional crystal forbidden band by using the device, and the method comprises the following steps:
determining the thickness of the light-transmitting medium layer according to the wavelength of the pumping light, namely the wavelength of the excitation field, so as to realize stronger resonance transmission and absorption of the metamaterial at the excitation field;
determining the fluorescence wavelength according to the forbidden bandwidth of the two-dimensional crystal monolayer, namely the monolayer of the transition metal chalcogenide;
determining the period range of the metal nano metamaterial unit structure according to the fluorescence wavelength;
and adjusting the inclination angle theta of the air gap in the metal nano metamaterial unit structure, and finely adjusting the position of the resonance mode to obtain the maximum fluorescence polarization degree.
In the step of determining the period range of the metal nano metamaterial according to the fluorescence wavelength, after the period is determined, the metamaterial surface plasmon resonance mode is ensured to be positioned near the fluorescence wavelength but not necessarily just positioned at the fluorescence wavelength; in the step of adjusting the inclination angle theta of the air gap in the metamaterial unit structure, the device can adjust the polarization degree of the fluorescence of the forbidden band of the two-dimensional crystal from high to low or from low to high by adjusting the inclination angle theta of the air gap.
(III) advantageous effects
According to the technical scheme, the device and the method for modulating the fluorescence polarization of the two-dimensional crystal forbidden band have the following beneficial effects:
(1) according to the device and the method for modulating the fluorescence polarization of the two-dimensional crystal forbidden band, the fluorescence polarization of the two-dimensional crystal forbidden band is modulated by the metamaterial, and the metamaterial surface plasmon resonance is a collective resonance behavior of light and metamaterial surface electrons and is not influenced by temperature, so that the polarization modulation method effectively widens the temperature application range.
(2) According to the device and the method for modulating the polarization of the two-dimensional crystal forbidden band fluorescence, the coupling degree between the metamaterial resonant mode and the two-dimensional crystal fluorescence is adjusted through optimizing the structural design, so that fluorescence signals with different linearity are generated, and the adjustable range of the linearity is widened.
(3) According to the device and the method for modulating the polarization of the fluorescence of the forbidden band of the two-dimensional crystal, the metamaterial is small in size, is not limited by the limit of optical diffraction and is suitable for on-chip fixed-point integration, so that the device is flexible and adjustable in size and is suitable for on-chip integration and micro-nano light source modulation. The adjustable minimum area is determined by the metamaterial structure unit, and the device foundation is provided for chip-level photonic devices, systems and function development in the hundred nanometer square magnitude.
(4) According to the device and the method for modulating the fluorescence polarization of the forbidden band of the two-dimensional crystal, provided by the invention, in the preparation process, the two-dimensional crystal is isolated from the outside through the light-transmitting medium layer prepared by the evaporation method, so that the influence of factors such as water, oxygen and the like on the two-dimensional crystal is reduced, and the service life of a device is prolonged.
(5) According to the device and the method for modulating the polarization of the forbidden band fluorescence of the two-dimensional crystal, the substrate material of the two-dimensional crystal has multiple selectivity and has good compatibility with a silicon-based photonic platform and a micro-nano preparation technology.
Drawings
Fig. 1 is a schematic structural diagram of an apparatus for modulating fluorescence polarization of a two-dimensional crystal forbidden band according to an embodiment of the invention.
Fig. 2 is a flowchart of the operation of an apparatus for modulating fluorescence polarization of a two-dimensional crystal forbidden band according to an embodiment of the invention.
Fig. 3 is a flow chart of a test of the apparatus for modulating fluorescence polarization of a two-dimensional crystal forbidden band shown in fig. 1.
FIG. 4A is a block diagram of WS according to an embodiment of the present invention2Single layer optical imaging;
FIG. 4B is a block diagram of WS according to an embodiment of the present invention2Fluorescence intensity maps of the monolayer in the TE and TM directions;
FIG. 4C is a WS according to an embodiment of the present invention2A graph of variation of single-layer fluorescence peak value with exit angle;
FIG. 4D is a block diagram of WS according to an embodiment of the present invention2Single layer fluorescenceA graph of linearity versus fluorescence wavelength;
FIG. 5A is a SEM image of a nanowire grid according to an embodiment of the invention;
FIG. 5B is a fluorescence spectrum of a device for modulating fluorescence polarization of a two-dimensional crystal forbidden band by a nanowire grid in TE and TM directions according to an embodiment of the invention;
FIG. 5C is a graph of the variation of the fluorescence peak with the exit angle of the device for modulating the fluorescence polarization of the two-dimensional crystal forbidden band by the nanogrid according to the embodiment of the invention;
FIG. 5D is a graph of the fluorescence linearity versus fluorescence wavelength for a device in which a nanowire grid modulates the fluorescence polarization of a two-dimensional crystal forbidden band in accordance with an embodiment of the present invention;
FIG. 6 is a schematic view of a V-shaped nano-gate surface structure according to an embodiment of the invention;
fig. 7A is an SEM image of a V-shaped metal nano-grid at 51 ° according to an embodiment of the present invention;
fig. 7B is a TE-directional white light reflection spectrum of a V-shaped nano-grid with θ being 51 ° according to an embodiment of the present invention;
fig. 7C is a fluorescence spectrum of a device for modulating fluorescence polarization of a two-dimensional crystal forbidden band by a V-shaped nano-grid at 51 ° according to an embodiment of the present invention in TE and TM directions;
fig. 7D is a graph of the variation of the fluorescence peak value of the device for modulating the fluorescence polarization of the two-dimensional crystal forbidden band with the V-shaped nano-grid at 51 ° according to the embodiment of the present invention as a function of the emission angle;
fig. 7E is a graph of the change of the fluorescence linearity of the device for modulating the fluorescence polarization of the two-dimensional crystal forbidden band by the V-shaped nano-grid with the fluorescence wavelength when θ is 51 °;
fig. 8A is a SEM image of a type V metal nano-grid at 63 ° θ in accordance with an embodiment of the present invention;
fig. 8B is a TE-directional white light reflection spectrum of the V-shaped nano-grid at 63 °;
fig. 8C is a fluorescence spectrum of the device for modulating fluorescence polarization of two-dimensional crystal forbidden band by using V-shaped nano-grid at 63 ° according to the embodiment of the present invention in TE and TM directions;
fig. 8D is a graph of the variation of the fluorescence peak value of the device for modulating the fluorescence polarization of the two-dimensional crystal forbidden band with the V-shaped nano-grid according to the embodiment of the invention when θ is 63 °;
fig. 8E is a graph of the change of the fluorescence linearity of the device for modulating the fluorescence polarization of the two-dimensional crystal forbidden band by the V-shaped nano-grid at 63 ° according to the embodiment of the present invention with the fluorescence wavelength;
FIG. 9 is a schematic view of an N-type nano-gate surface structure according to an embodiment of the invention;
fig. 10A is an SEM image of an N-type nano-gate at 30 ° θ in accordance with an embodiment of the present invention;
fig. 10B is a TE-directional white light reflection spectrum of an N-type nano-grid with θ being 30 ° according to an embodiment of the present invention;
fig. 10C is a fluorescence spectrum of a device in which the N-type nano-grid modulates the polarization of two-dimensional crystal forbidden band fluorescence in TE and TM directions when θ is 30 ° according to an embodiment of the present invention;
fig. 10D is a graph of the variation of the fluorescence peak value of the device for modulating the fluorescence polarization of the two-dimensional crystal forbidden band by the N-type nano-grid with the emission angle when θ is 30 ° according to the embodiment of the present invention;
fig. 10E is a graph of the change of the fluorescence linearity of the device for modulating the fluorescence polarization of the two-dimensional crystal forbidden band by the N-type nano-grid with the fluorescence wavelength when θ is 30 ° according to the embodiment of the present invention;
fig. 11A is an SEM image of an N-type nano-gate at 45 ° θ in accordance with an embodiment of the present invention;
fig. 11B is a TE-directional white light reflection spectrum of an N-type nano-grid when θ is 45 ° according to an embodiment of the present invention;
fig. 11C is a fluorescence spectrum of a device in which the N-type nano-grid modulates the polarization of two-dimensional crystal forbidden band fluorescence in TE and TM directions when θ is 45 ° according to an embodiment of the present invention;
fig. 11D is a graph of the variation of the fluorescence peak value of the device for modulating the fluorescence polarization of the two-dimensional crystal forbidden band by the N-type nano-grid with the emission angle when θ is 45 ° according to the embodiment of the present invention;
fig. 11E is a graph of the change of the fluorescence linearity of the device for modulating the fluorescence polarization of the two-dimensional crystal forbidden band by the N-type nano-grid with the fluorescence wavelength when θ is 45 °;
fig. 12A is an SEM image of an N-type nanogate at 55 °;
fig. 12B is a white light reflection spectrum of the N-type nano-grid TE direction at θ ═ 55 °;
fig. 12C is a fluorescence spectrum of the device for modulating the fluorescence polarization of the two-dimensional crystal forbidden band by the N-type nano-grid in TE and TM directions when θ is 55 °;
fig. 12D is a graph of the variation of the fluorescence peak with the emission angle of an apparatus for modulating the fluorescence polarization of the two-dimensional crystal forbidden band by an N-type nano-grid when θ is 55 ° according to an embodiment of the present invention;
fig. 12E is a graph of the fluorescence linearity of the device for modulating the fluorescence polarization of the two-dimensional crystal forbidden band by the N-type nano-grid at 55 ° according to the embodiment of the present invention, as a function of the fluorescence wavelength.
[ notation ] to show
1: substrate
2: insulating layer
3: two-dimensional crystal monolayer
4: light-transmitting medium layer
5: metal nano metamaterial
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to specific embodiments and the accompanying drawings.
The invention provides a device for modulating fluorescence polarization of a two-dimensional crystal forbidden band, which has a structure schematic diagram shown in figure 1 and comprises: a substrate of highly conductive material; an insulating layer formed on the high-conductivity material substrate; a one-dimensional crystal monolayer formed on the insulating layer; a light-transmitting dielectric layer formed on the two-dimensional crystal single layer; and the metal nanometer metamaterial with a periodic unit structure is formed on the light-transmitting medium layer.
In the present invention, the substrate of high conductive material is a highly doped silicon or metal substrate with a thickness of 500-750 μm. The substrate material is flexible and various, has multiple selectivity, and has good compatibility with a silicon-based photon platform and a micro-nano preparation technology; insulating layerSiO can be selected2、Al2O3Oxide with the thickness of 270-310 nm; the two-dimensional crystal monolayer may be transition metal chalcogenide MX2The compound is a direct forbidden band semiconductor which emits light and has stronger fluorescence. Wherein M is Mo, W; x is S, Se, Te; the thickness is less than 1 nm; the transparent dielectric layer can be Al2O3Or SiO2. The light-transmitting medium layer isolates the two-dimensional crystal from the outside, reduces the influence of factors such as water, oxygen and the like on the two-dimensional crystal, and effectively prolongs the service life of a device. The thickness of the light-transmitting dielectric layer is selected in relation to the dielectric constant of the medium. The larger the dielectric constant, the smaller the dielectric layer thickness. Because the thickness of the dielectric layer influences the resonant transmission and absorption of the metamaterial on the excitation field, the optimal thickness is the corresponding thickness when the resonant transmission peak of the metamaterial is matched with the wavelength of the pump laser. In the invention, the thickness of the light-transmitting medium layer is 10-50 nm; the metal nano-structure metamaterial can be made of metal materials such as gold, silver, aluminum and the like, and the thickness of the metal nano-structure metamaterial is 30-100 nm.
As the metamaterial adopted by the device is small in size, is not limited by the limit of optical diffraction, and is suitable for on-chip fixed-point integration, the size of the device is flexible and adjustable, and the device is suitable for modulation of micro-nano light sources. The adjustable minimum area is determined by the metamaterial structure unit, and the device foundation is provided for chip-level photonic devices, systems and function development in the hundred nanometer square magnitude.
Meanwhile, the invention also provides a method for modulating the polarization of the fluorescence of the forbidden band of the two-dimensional crystal. The method is based on the metal surface plasmon resonance principle, utilizes the surface plasmon anisotropic resonance mode generated by the metamaterial at the fluorescence wavelength to enhance the resonance electric field in a specific direction, influences the electric field intensity of the emergent fluorescence field in the direction, generates polarized fluorescence and achieves the purpose of adjustable polarization. The metamaterial surface plasmon resonance is a collective resonance behavior of light and metamaterial surface electrons and is not influenced by temperature, so that the polarization modulation method provided by the invention effectively widens the temperature application range; meanwhile, the modulation method can widen the adjustable range of linearity. By optimizing the structural design, the coupling degree between the metamaterial resonant mode and the two-dimensional crystal fluorescence is adjusted, and fluorescence signals with different linearity degrees are generated. The method has a flow chart as shown in fig. 2, and specifically includes:
step S201: firstly, the thickness of the light-transmitting medium layer is determined according to the wavelength of the pumping light (namely the wavelength of the excitation field), so that the metamaterial can realize stronger resonance transmission and absorption at the excitation field. (note: determining dielectric layer thickness can be performed by numerical simulation).
Step S202: the fluorescence wavelength is determined from the forbidden bandwidth of the transition metal chalcogenide as a two-dimensional crystal monolayer.
Step S203: and determining the period range of the metamaterial nanostructure according to the fluorescence wavelength. The nanostructure resonant mode in this period overlaps with the fluorescence wavelength, but does not necessarily coincide exactly.
Step S204: adjusting the inclination angle theta of an air gap in the metal nano metamaterial unit structure, finely adjusting the position of a resonance mode to obtain the maximum fluorescence polarization degree, and adjusting the fluorescence polarization degree of the forbidden band of the two-dimensional crystal from high to low or from low to high by changing the inclination angle theta of the air gap.
As the surface plasmon resonance characteristics of the metamaterial are mainly influenced by the shape of the metamaterial, the metamaterial with three shapes is designed in the embodiment of the invention to modulate the fluorescence linearity, namely the nano wire grid, the V-shaped nano grid and the N-shaped nano grid.
Specifically, selecting WS2As a two-dimensional crystal single layer in the device, an N-type silver nano-grid is selected as a metal nano-structure metamaterial.
Through the step S201, the thickness of the light-transmitting medium layer is determined to be 30nm according to the wavelength of the pump light of 532 nm;
through step S202, according to WS2The fluorescent wavelength is determined to be 630nm by the forbidden band width;
according to WS, through step S2032The period of the N-type silver nano structure in the TE and TM polarization directions is determined to be 200nm and 400nm respectively by the fluorescence wavelength of the N-type silver nano structure;
by step S204, different air gap inclinations θ (θ ═ 30 °, 45 °, 55 °) are set, and the corresponding fluorescence linearity is calculated:
(1) when θ is 30 °, the linearity of the device in the vicinity of the fluorescence peak 630nm is less than 40%, because when θ is 30 °, the N-type grating surface plasmon resonance position is at 570nm, which is largely deviated from the fluorescence peak position, and thus the influence of the resonance field on the polarization of the fluorescence field is weak. High background fluorescence occurs because of weak signal and low signal-to-noise ratio.
(2) When θ is 45 °, the linearity of the device around the fluorescence peak of 630nm is 80%. The reason why the structure has super-strong linearity is that when theta is 45 degrees, the surface plasmon resonance of the N-type nano grid in the TE direction is just at the fluorescence wavelength of 630nm, and the coupling with the fluorescence field is best. In this mode driving, the fluorescence in the TE direction smoothly passes through the nanovoids, while the fluorescence in the TM direction is suppressed, so that high fluorescence linearity occurs. The fluorescence signal is strong, and the signal-to-noise ratio is high.
(3) When θ is 55 °, the linearity of the device around the fluorescence peak of 630nm is 60%, and the linearity outside the fluorescence peak decreases rapidly. The surface plasmon resonance position of the structure is 660nm and WS2The linearity of fluorescence generated after integration is between N-type grids when theta is 30 DEG and theta is 45 deg. The fluorescence signal is strong, and the signal-to-noise ratio is high.
The above results indicate that changing the angle can change the surface plasmon resonance position while adjusting the fluorescence linearity. The highest fluorescence linearity corresponds to the optimal coupling of the fluorescence wavelength with the metamaterial resonant mode.
In order to verify the modulation effect of the device for modulating the fluorescence polarization of the two-dimensional crystal forbidden band on the fluorescence polarization of the two-dimensional crystal forbidden band, the test and calculation process shown in fig. 3 is carried out on the device provided by the invention, and the test and calculation process specifically comprises the following steps:
step S301: and converging the pump light on the sample by using a 50X objective lens, exciting the two-dimensional crystal monolayer to emit fluorescence, and collecting a fluorescence signal. Firstly, fluorescent image acquisition is carried out, and an excitation light path, a side-band light filter and a high-sensitivity imaging CCD (charge coupled device) are carried on a common optical microscope to form an imaging system for acquiring the fluorescent image.
Step S302: and collecting the emergent fluorescence spectrum of the sample in the TE/TM direction. Wherein the TE/TM direction refers to the direction parallel/perpendicular to the pump laser polarization. In the step, a fluorescence spectrometer (Andor SR-500) is adopted to collect fluorescence signals. During collection, an analyzer needs to be placed in front of the band-pass filter, and fluorescence collection in different polarization directions is completed by rotating the analyzer.
Step S303: measuring the spectrum of the fluorescence of the sample in the range of 0-360 degrees of exit angle, and drawing a graph of the change of the fluorescence peak value with the exit angle.
Step S304: by the formula L ═ ITE-ITM)/(ITE+ITM) And calculating the fluorescence linearity, and drawing a curve graph of the fluorescence linearity with the fluorescence wavelength.
Example 1: WS2Single layer fluorescence polarization assay
First, in SiO2On a/Si substrate, by chemical vapor deposition to produce WS2Single layer: respectively heating sulfur and tungsten powder to generate vapor, and carrying the vapor to a clean substrate by using argon as carrier gas to deposit and grow to obtain WS2A single layer.
FIG. 4A is the WS2Optical microscopy of monolayers. Fig. 4B is the emission fluorescence spectrum in the TE, TM directions obtained by the test of step S302. As can be seen from FIG. 4B, the fluorescence intensity of the sample in TE and TM directions is equal, WS2The single layer has a fluorescence peak of 630nm, a full width at half maximum of 30nm and a forbidden band width of 1.9-2.0 eV. Fig. 4C is a graph of the fluorescence spectrum obtained by the test of step S303 as a function of the exit angle. As can be seen from fig. 4C, the intensity of the sample at each exit angle is equal. Fig. 4D is a curve of the fluorescence linearity with the fluorescence wavelength obtained by the test in step S304, and it can be seen from fig. 4D that the fluorescence linearity of the sample in the forbidden band range is-0.
Example 2: ag nano wire grating pair WS2Device for modulating polarization of forbidden band fluorescence
The modulation method provided by the invention is based on the metamaterial surface plasmon resonance principle, which is influenced by the shape of the metamaterial, so that three shapes of metal nanometer metamaterials are designed in the example to modulate the fluorescence linearity, namely a nanometer wire grid, a V-shaped nanometer grid and an N-shaped nanometer grid.
The unit structure period is selected according to two-dimensional crystalThe fluorescence wavelength. For example, the periods of the N, V-type nano-structure in the TE and TM polarization directions can be respectively set to 200nm and 400nm, so that the surface plasmon resonance of the metamaterial can be realized in the WS single layer2Fluorescence occurs at a wavelength around 630 nm. The inclination angle of the air gap in the structural unit is further adjusted, and resonance response can be adjusted to the fluorescent wavelength, so that the metamaterial and the two-dimensional crystal are coupled to a higher degree.
The three metal nanometer metamaterials have different periods in the TM direction and the TE direction. Wherein the TM direction period of the wire grid structure is 600-800 nm; the period of the TE direction of the N, V type nano gate is 150-300nm, and the period of the TM direction is 300-600 nm. The N, V-shaped nanostructure has more adjustable structural dimensions than the wire grid structure, especially the air gap tilt angle theta in the structural unit. The invention can accurately adjust the resonance position by adjusting theta.
The process of modulating the fluorescence polarization of the two-dimensional crystal forbidden band by the three structures of the nano-wire grid, the V-shaped nano-grid and the N-shaped nano-grid is described in detail below.
Ag nanowire grid pair WS2Device for modulating polarization of forbidden band fluorescence
Firstly, a layer of SiO is obtained on a Si substrate by wet oxidation2An insulating layer; then, a layer of WS is grown on the insulating layer by chemical vapor deposition2A single layer; then evaporating and coating by an electron beam coating instrument on the single layer to obtain a layer of Al2O3A light-transmitting medium layer; spin-coating electron beam negative resist MA2403 on the light-transmitting dielectric layer; exposing the negative photoresist MA2403 by adopting an electron beam direct writing technology, and writing a structure on the negative photoresist by using a focused electron beam; exposed negative resist MA2403 is remained after development; finally, forming Ag nano metamaterial through metal evaporation and degumming processes to prepare the nano wire grid-WS2A single-layer fluorescence polarization modulation device, wherein fig. 5A is an SEM image of the Ag nanowire grid in the structure. The period and the line width b of the silver nanowire grid are 600nm, and w is 480 nm.
FIG. 5B is a nanowire grid-WS obtained by the test of step S3022The single-layer forbidden band fluorescence polarization modulation device emits fluorescence spectra in the TE and TM directions. As shown in FIG. 5B, the nanowire grid-WS2The fluorescence intensity of the single-layer forbidden band fluorescence polarization modulation device in the TM direction is 40% of that in the TE direction. Fig. 5C is a change law of the fluorescence peak value with the emission angle obtained by the test of step S303. As can be seen from FIG. 5C, the fluorescence exhibits linear polarization, with the polarization direction parallel to the nanowire grid. Fig. 5D is a graph showing the linearity of fluorescence according to the wavelength of fluorescence obtained by the test of step S304. As can be seen from fig. 5D, although the fluorescence peak is at 630nm, the fluorescence linearity is not enhanced at this position, but shows a broad spectrum response. This is caused by the wide spectrum resonance of the wire grid.
(II) V type Ag nano grid pair WS2Device for modulating polarization of forbidden band fluorescence
Firstly, a layer of SiO is obtained on a Si substrate by wet oxidation2As an insulating layer; then, a layer of WS is grown on the insulating layer by chemical vapor deposition2A single layer; then evaporating a layer of Al on the single layer by using an electron beam coating instrument2O3A light-transmitting medium layer; spin-coating electron beam negative resist MA2403 on the light-transmitting dielectric layer; exposing the negative photoresist MA2403 by adopting an electron beam direct writing technology, and writing a structure on the negative photoresist by using a focused electron beam; exposed negative resist MA2403 is remained after development; and finally, forming an Ag nano structure through metal evaporation and glue removal processes to obtain the device, wherein fig. 6 is a structural diagram of a V-shaped Ag nano wire grid in the structure. Wherein, the period and line width a of the nano grid is 200nm, and b is 400 nm; s is 50nm and the air gap angle theta is a variable.
In the following, V-shaped nanograting-WS was performed for θ 51 ° and θ 63 °2Single layer coupling device fluorescence polarization measurements.
(1)θ＝51°
Fig. 7A is an SEM image of a V-shaped nanogate when θ is 51 °. FIG. 7B is the white light reflectance spectrum of the device in the TE direction, with the reflection valley of 600-650nm representing the surface plasmon resonance position of the device. Fig. 7C is the fluorescence spectrum of the device in the TE, TM directions obtained by the test of step S302. As can be seen from FIG. 7C, the fluorescence intensity in the TM direction is only 25% of that in the TE direction, compared to the nanowire grid-WS2The difference in TE and TM intensities of the coupling device is increased. FIG. 7D is the variation of the fluorescence peak value with the emission angle obtained by the test of step S303And (5) regularity. As can be seen from FIG. 7D, the fluorescence exhibits strong linear polarization, with the polarization direction parallel to the long axis of the V-shaped grating. FIG. 7E is a plot of fluorescence linearity as a function of fluorescence wavelength, with a peak at 625nm, aligned with the surface plasmon resonance position, up to 60%, as measured in step S304. The linearity of the fluorescence remains high after 650nm wavelength, due to WS2Impurity fluorescence in this range is due to metamaterial modulation.
(2)θ＝63°
Fig. 8A is an SEM image of the V-shaped nanogate when θ is 63 °. Fig. 8B is a white light reflection spectrum of the device in the TE direction, and reflection valleys around 650nm represent surface plasmon resonance positions of the V-shaped nanogate structure. Fig. 8C shows the fluorescence spectrum of the device in TE and TM directions obtained by the test in step S302, and the spectrum result shows that the fluorescence intensity in two perpendicular directions has a large difference. Fig. 8D shows the change rule of the fluorescence peak value with the emission angle obtained by the test in step S303, and it can be seen from fig. 8D that the fluorescence has strong linear polarization, and the polarization direction is parallel to the long axis of the V-shaped grating. FIG. 8E is a curve showing the change of the linearity of fluorescence with the fluorescence wavelength obtained by the test in step S304, and it can be seen from FIG. 8E that the linearity has a distinct peak at about 640nm, the peak value is 60%, and the external linearity of the peak is rapidly reduced. This is because when θ is 63 °, surface plasmon resonance of the V-shaped nanogrid in the TE direction occurs in the vicinity of 650nm, and only strong modulation is exerted on the fluorescence signal there. The reason for the high background fluorescence may be caused by poor glue removal process and residual glue luminescence.
(III) N type Ag nano grid pair WS2Device for modulating polarization of forbidden band fluorescence
Firstly, a layer of SiO is obtained on a Si substrate by wet oxidation2An insulating layer; then, a layer of WS is grown on the insulating layer by chemical vapor deposition2A single layer; then evaporating a layer of Al on the single layer by using an electron beam coating instrument2O3A light-transmitting medium layer; spin-coating electron beam negative photoresist MA2403 on the transparent medium layer, and writing the structure on the negative photoresist by using the focused electron beam; exposing the negative photoresist MA2403 by adopting an electron beam direct writing technology; exposed negative resist MA2403 is remained after development; finally, theThe device is manufactured by forming an Ag nano structure through metal evaporation and degummed processes, wherein fig. 9 is a structural diagram of an N-type Ag nano wire grid in the structure. The period and line width a of the nano grid in the structure are 200nm, and b is 400 nm; s is 50nm and the air gap angle theta is a variable.
In the following, for the case where θ is 30 °, θ is 45 °, and θ is 55 °, N-type nanogrid-WS2The single layer coupling device was tested for fluorescence polarization.
(1)θ＝30°
Fig. 10A is an SEM image of an N-type nanogate at θ ═ 30 °. Fig. 11B shows a white light reflection spectrum of the device in the TE direction, from which it is understood that the surface plasmon resonance position is at 570 nm. Fig. 10C is the fluorescence spectrum of the device in the TE, TM directions obtained by the test of step S302. As can be seen from FIG. 10C, the fluorescence intensity in the two perpendicular directions is greatly different, and the background fluorescence is very high. Fig. 10D is a graph showing the change of the fluorescence peak value with the emission angle obtained by the test in step S303, and it can be seen that the fluorescence exhibits linear polarization, but is not strong, and the polarization direction is parallel to the long axis of the N-type grating. Fig. 10E is a graph of the change in the fluorescence linearity with the fluorescence wavelength obtained by the test in step S304, and the linearity in the vicinity of the fluorescence peak 630nm is less than 40%, because when θ is 30 °, the surface plasmon resonance position of the N-type grating is at 570nm, and the deviation from the fluorescence peak position is large, and therefore the polarization effect of the resonance field on the fluorescence field is weak. The reasons for the high background fluorescence are that θ is small, the emitted fluorescence is weak, and the signal-to-noise ratio is low.
(2)θ＝45°
Fig. 11A is an SEM image of an N-type nanogate at θ ═ 45 °. Fig. 11B shows a white light reflection spectrum of the device in the TE direction, from which it is understood that the surface plasmon resonance position is 630 nm. Fig. 11C shows the fluorescence spectra of the device in TE and TM directions obtained by the test in step S302. As can be seen from FIG. 11C, the maximum difference in fluorescence intensity occurred in the two perpendicular directions. Meanwhile, the emergent fluorescence signal is strong, the background is weak, and the signal-to-noise ratio is high. Fig. 11D is a graph of the variation of the fluorescence peak value with the emission angle obtained by the test in step S303, and it can be seen that the fluorescence has super-strong linear polarization, and the polarization direction is parallel to the long axis of the N-type grating. FIG. 11E is a graph showing the linearity of fluorescence with the wavelength of fluorescence measured in step S304. From this change curve, the linearity at around 630nm of the fluorescence peak was found to be 80%. The reason why the structure has super-strong linearity is that when theta is 45 degrees, the surface plasmon resonance of the N-type nano grid in the TE direction is just positioned at the fluorescent wavelength, and the modulation capability to the fluorescent field is strongest. In this mode driving, the fluorescence in the TE direction smoothly passes through the nanovoids, while the fluorescence in the TM direction is suppressed, so that high fluorescence linearity occurs.
(3)θ＝55°
Fig. 12A is an SEM image of a 55 ° N-type nanogate. Fig. 12B shows a white light reflection spectrum of the device in the TE direction, from which it is understood that the surface plasmon resonance position is 660 nm. Fig. 12C shows the emission fluorescence spectrum of the device in the TE and TM directions obtained by the test in step S302. As can be seen from FIG. 12C, the intensity of the fluorescence light in two perpendicular directions is greatly different, and the fluorescence signal emitted from the fluorescence light is strong, the background is weak, and the signal-to-noise ratio is high. Since the TM-direction fluorescence suppression ability is reduced, the difference in the TE/TM direction of the fluorescence intensity is reduced compared to an N-type grid having θ equal to 45 °. Fig. 12D is a graph of the variation of the fluorescence peak value with the emission angle obtained by the test in step S303, and it can be seen that the fluorescence exhibits strong linear polarization, and the polarization direction is parallel to the long axis of the N-type grating. FIG. 12E is a graph showing the linearity of fluorescence with the wavelength of fluorescence measured in step S304. As is clear from the graph, the linearity in the vicinity of the fluorescence peak at 630nm is 60%, and the linearity outside the fluorescence peak decreases rapidly. The surface plasmon resonance position of the structure is 660nm and WS2The linearity of fluorescence generated after integration is between N-type grids when theta is 30 degrees and theta is 45 degrees, which shows that changing the angle can change the surface plasmon resonance position and adjust the linearity of the fluorescence.
Example 3:
in addition to the above systems, two-dimensional crystals which may be preferred have a single layer of WSe2。WSe2The monolayer has higher quantum efficiency and longer fluorescence wavelength of 750 nm. From the WSe2The structure and process of preparing the metamaterial-two-dimensional crystal forbidden band fluorescence polarization modulation device by a single layer are the same as those of the embodiment 2, the period of the selected nanostructure is determined according to the fluorescence wavelength, the thickness of the selected dielectric layer is determined by the wavelength of the excitation light, and the establishment principle is explained in the technical scheme introduction.Generally, the wavelength is increased, the metal dissipation is reduced, the surface plasmon resonance response is enhanced, and the modulation effect on the fluorescence linearity is better.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention, and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. An apparatus for modulating fluorescence polarization of a two-dimensional crystal forbidden band, comprising:
a substrate of highly conductive material;
an insulating layer formed on the high-conductivity material substrate;
a one-dimensional crystal monolayer formed on the insulating layer;
a light-transmitting dielectric layer formed on the two-dimensional crystal single layer; and
and the metal nano metamaterial is formed on the light-transmitting medium layer and has a periodic unit structure.
2. The device for modulating the polarization of fluorescence of a two-dimensional crystal forbidden band according to claim 1, wherein the substrate made of a high conductive material is a highly doped silicon or metal substrate with a thickness of 500-750 μm; the highly conductive material plays a role of structural support and reflects fluorescence emitted by the two-dimensional crystal monolayer.
3. The apparatus for modulating fluorescence polarization of two-dimensional crystal forbidden bands according to claim 1, wherein the insulating layer is SiO2、Al2O3The thickness is 270-310 nm; the insulating layer serves as a growth substrate for the two-dimensional crystal monolayer.
4. The device for modulating fluorescence polarization of forbidden band of two-dimensional crystal according to claim 1, wherein the monolayer of two-dimensional crystal is transition metal sulfurGroup compound MX2Wherein M ═ Mo, W; x is S, Se, Te; the thickness of the two-dimensional crystal monolayer is less than 1 nm; the two-dimensional crystal monolayer emits fluorescence under the excitation of the pump light.
5. The device for modulating fluorescence polarization of two-dimensional crystal forbidden band according to claim 1, wherein the light-transmitting medium layer is Al2O3Or SiO2The thickness is 10-50 nm; the thickness of the light-transmitting medium layer influences the resonance transmission and absorption of the metal nano metamaterial at the excitation field, and further influences the fluorescence intensity; the light-transmitting dielectric layer protects the two-dimensional crystal material from water, oxygen or impurity ions.
6. The device for modulating the polarization of fluorescence of a two-dimensional crystal forbidden band according to claim 1, wherein the metal nano metamaterial is a nano structure of gold, silver or aluminum, and the thickness of the nano metamaterial is 30-100 nm.
7. The apparatus for modulating fluorescence polarization of two-dimensional crystal forbidden bands according to claim 1, wherein:
the metal nano metamaterial with the periodic unit structure is provided with a linear, V-shaped or N-shaped air gap structure;
and the included angle between the V-shaped or N-shaped air gap structure and the horizontal direction is an air gap inclination angle theta.
8. A method for modulating fluorescence polarization of a two-dimensional crystal forbidden band, which is based on the device for modulating fluorescence polarization of a two-dimensional crystal forbidden band of any one of claims 1 to 7, and comprises the following steps:
determining the thickness of the light-transmitting medium layer according to the wavelength of the pumping light, namely the wavelength of the excitation field, so as to realize stronger resonance transmission and absorption of the metamaterial at the excitation field;
determining the fluorescence wavelength according to the forbidden bandwidth of the two-dimensional crystal monolayer, namely the monolayer of the transition metal chalcogenide;
determining the period range of the metal nano metamaterial unit structure according to the fluorescence wavelength;
and adjusting the inclination angle theta of the air gap in the metal nano metamaterial unit structure, and finely adjusting the position of the resonance mode to obtain the maximum fluorescence polarization degree.
9. The method for modulating the polarization of forbidden fluorescence of two-dimensional crystal according to claim 8, wherein in the step of determining the periodic range of the metal nano metamaterial according to the fluorescence wavelength, the periodic range is determined to ensure that the metamaterial surface plasmon resonance mode is located near the fluorescence wavelength but not necessarily just at the fluorescence wavelength.
10. The method for modulating fluorescence polarization of a two-dimensional crystal forbidden band according to claim 8, wherein in the step of adjusting the inclination angle θ of the air gap in the metamaterial unit structure, the device adjusts the fluorescence polarization degree of the two-dimensional crystal forbidden band from high to low or from low to high by adjusting the inclination angle θ of the air gap.
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| CN113528120B (en) * | 2021-07-19 | 2022-11-15 | 东南大学 | Method for realizing exciton valley polarization at non-resonance wavelength based on double resonance effect |
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