CN115657184B - Sub-wavelength asymmetric grating structure with infrared light modulation characteristic and manufacturing method - Google Patents
Sub-wavelength asymmetric grating structure with infrared light modulation characteristic and manufacturing method Download PDFInfo
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Abstract
The invention discloses a sub-wavelength asymmetric grating structure with infrared light modulation characteristics and a manufacturing method thereof, and relates to the technical field of advanced optical function metamaterial design and micro-nano processing, wherein the sub-wavelength asymmetric grating structure comprises a substrate, a metal film layer, a medium isolation layer, a metal adhesion layer and a metal grating, the metal grating is provided with an asymmetric dual-grating unit, the metal grating comprises a first grating unit with a first ridge and a second grating unit with a second ridge, the structural parameters of the metal grating comprise the widths of the first ridge and the second ridge, and a first gap and a second gap between the first ridge and the second ridges on two adjacent sides, the structure realizes the near-perfect absorption of a narrow band of infrared light within an incident light angle range of 0-50 degrees under the condition of TM polarization excitation by adjusting the structural parameters, and the problems of high process cost, complex preparation flow, high operation threshold, certain uncertainty of a processing result and single performance of the infrared light modulation surface structure are solved.
Description
Technical Field
The invention relates to the technical field of advanced optical function metamaterial design and micro-nano processing, in particular to a sub-wavelength asymmetric grating structure with infrared light modulation characteristics and a manufacturing method thereof.
Background
Surface Plasmons (SPs) formed by coupling of free electrons in metals with photons are one of the first polariton forms discovered and utilized. The metal Surface Plasmon can break through the optical diffraction limit, the energy of light waves is limited within the sub-wavelength scale, the Resonance excitation (SPR) can induce and generate obvious field enhancement effect (namely an electromagnetic field mode 'hot spot') in a metal-medium interface or a metal nano structure, and the transport properties of electromagnetic wave such as dispersion, wave speed and the like are regulated and controlled, so that the Surface Plasmon Resonance (SPR) nano structure is widely applied to the fields of novel light field regulation and control, ultrasensitive biosensing, surface interface enhanced spectroscopy, efficient nano energy device development and the like, and the SPR nano structure with high spatial activity is an important basis for promoting the sustainable development of the Surface Plasmon photonics in the fields.
In the aspect of structural design, by constructing an asymmetric configuration, the high-spatial-activity SPR super-surface utilizing a local/non-local mode coupling regulation and control mechanism can effectively realize high-Q-value light energy storage (such as perfect light trapping, bound states in a continuous domain, rainbow light trapping effect and the like) and nonlinear optical signal processing such as optical mixing, frequency up/down conversion and the like in specific or multiple frequency bands, so that the method has wide application prospects in the fields of nano light source design, quantum information processing, photonic integrated chip manufacturing and the like. The design and preparation of SPR super-surface structures with high spatial activity become the focus of recent scientific research personnel at home and abroad.
In terms of action wavelength, compared with a visible light band (380 nm to 780 nm) sensitive to human eyes, the infrared band comprises wide spectrum bands such as near infrared (a near infrared region I is 780 nm to 1.1 micron, a region II is 1.1 micron to 2.5 micron), middle infrared (2.5 micron to 25 micron), far infrared (terahertz is 30 micron to 3000 micron); in recent years, infrared optical absorbers based on SPR super-surface structures have received widespread attention for their important applications in medical diagnostics, biosensing and bolometers, etc. The current research result shows that various SPR super surface configurations such as sub-wavelength gratings, nano particles/nano particle aggregates and metal-medium-metal multilayer systems can be used for designing and developing specific or multiband infrared optical absorption devices.
The current main technology for manufacturing the infrared light modulation device includes processing technologies such as Electron Beam Lithography (EBL) and focused ion beam lithography (FIB). The processing technology has high cost, complex preparation flow, high operation threshold and certain uncertainty of a processing result, so that it is important to find a design scheme with high structural tolerance (i.e. the performance characteristics of a structural device can still be ensured within a larger processing error range) for the preparation of the structural device.
Disclosure of Invention
Aiming at the problems of high process cost, complex preparation flow, high operation threshold, certain uncertainty of a processing result, single performance and the like of the conventional infrared light modulation super-surface structure, the embodiment of the application provides a sub-wavelength asymmetric grating structure with an infrared light modulation characteristic and a manufacturing method thereof to solve the problems.
The first aspect of the invention provides a sub-wavelength asymmetric grating structure with infrared light modulation characteristics, which comprises a substrate, a metal film layer, a dielectric isolation layer, a metal adhesion layer and a metal grating which are sequentially stacked from bottom to top, wherein the metal grating is provided with an asymmetric dual-grating unit, the asymmetric dual-grating unit comprises a first grating unit with a first ridge and a second grating unit with a second ridge, the first ridge and the second ridge have different cycle widths and are alternately arranged at intervals, the metal adhesion layer is arranged below the first ridge and the second ridge and has the same width as the first ridge and the second ridge respectively, structural parameters of the metal grating comprise the width of the first ridge, the width of the second ridge, a first gap between the first ridge and the second ridge on one adjacent side and a second gap between the first ridge and the second ridge on the other adjacent side, and the tunable narrow-band infrared light absorption efficiency of 80-100% within the range of incident light angles of 0-50 DEG and frequency bands under the condition of TM polarization excitation.
Preferably, the infrared narrow-band absorption action band which is continuously angle-tunable and frequency-tunable comprises a near infrared I region and/or a near infrared II region.
Preferably, the ratio of the height of the first ridge portion to the width of the first ridge portion is 0.5 or more.
Preferably, the width of the first ridge is 70 to 90 nanometers, the width of the second ridge is 200 to 220 nanometers, the first gap is 190 to 210 nanometers, the second gap is greater than or equal to 100 nanometers, and the heights of the first ridge and the second ridge are greater than or equal to 40 nanometers.
Preferably, the near-perfect absorption of the infrared narrow band with the absorption efficiency of 80 to 100 percent is realized by adjusting the difference between the widths of the first ridge and the second ridge, and the action waveband for the near-perfect absorption of the infrared narrow band can be continuously adjusted by adjusting the second gap.
Preferably, the metal adhesion layer is made of chromium and has a thickness less than or equal to 1 nm.
Preferably, the refractive index of the medium isolation layer is 1.2 to 2.0, and the thickness regulation range is 180 to 400 nanometers.
Preferably, the dielectric isolation layer is made of silicon dioxide, the substrate is made of silicon, and the metal thin film layer and the metal grating are made of silver.
In a second aspect, the present application provides a method for manufacturing the sub-wavelength asymmetric grating structure with infrared light modulation characteristics, including the following steps:
s1, providing a substrate, and sequentially growing a metal thin film layer and a medium isolation layer on the substrate through an electron beam evaporation and plasma enhanced chemical vapor deposition process;
s2, spin-coating an electron beam photoresist on the medium isolation layer, and forming an asymmetric grating pattern after electron beam exposure and development;
and S3, growing an adhesion layer and a metal layer by adopting electron beam evaporation deposition, removing the residual photoresist and the metal adhesion layer and the metal layer above the photoresist, and transferring the asymmetric grating pattern to the adhesion layer and the metal layer to form the metal adhesion layer and the metal grating.
In a third aspect, the present application provides an application of the sub-wavelength asymmetric grating structure with infrared light modulation characteristics in a tunable infrared light absorption device and a nonlinear optical functional device.
Compared with the prior art, the invention has the following beneficial effects:
the invention provides a sub-wavelength asymmetric grating structure with infrared light modulation characteristics and a manufacturing method thereof, which can solve the problems of complex design scheme, high processing precision requirement and complex preparation process of the existing infrared light modulation device, thereby achieving the purpose of high-quality stable preparation, and the prepared product has adjustable multiple parameters, excellent large-angle polarization controllable near-perfect light absorption performance and flexible action wavelength and excitation angle expansibility in an infrared band, and meanwhile, the sub-wavelength asymmetric grating structure integrates the nonlinear optical signal processing function, and the excitation and emergent polarization of a conversion signal on the nonlinear frequency is controllable; in the process, the structural device prepared by the design scheme has high structural tolerance, can effectively simplify the process preparation flow, reduce the production cost and improve the production yield, and the silicon wafer is used as the sample substrate, so that the possibility is provided for the design and development of the industrialized and integrated multifunctional optical device in the microelectronic industry, such as a chip-level optical modulation or mixing device.
Drawings
The accompanying drawings are included to provide a further understanding of the embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and together with the description serve to explain the principles of the invention. Other embodiments and many of the intended advantages of embodiments will be readily appreciated as they become better understood by reference to the following detailed description.
FIG. 1 is a schematic diagram of a sub-wavelength asymmetric grating structure with infrared light modulation properties according to an embodiment of the present application;
FIG. 2 is a Scanning Electron Microscope (SEM) top view of a sub-wavelength asymmetric grating structure with IR light modulation properties of an embodiment of the present application;
fig. 3 is an SEM side view of a sub-wavelength asymmetric grating structure with ir modulation properties of an embodiment of the present application, inset is an SEM top view tilted at 45 ° viewing angle (scale 200 nm);
fig. 4 and 5 are far-field angle-resolved reflectance spectra measured experimentally and simulated by computer simulation for given structural parameters (W1 =80 nm, W2=210 nm, g = s =200 nm, t =50 nm, D =180 nm, D =200 nm, n =1.467, h =0.5 nm), respectively, where the range of the incident angle of the light source is 0 to 50 °, the Polarization direction of the light source is perpendicular to the direction of the grating grooves, denoted as TM Polarization;
FIG. 6 is a far field reflectance spectrum from a computer simulation and experiments with given structural parameters, where the incident angle of the light source is 0, denoted Normal index, and the Polarization direction of the light source is parallel to the direction of the grating grooves, denoted TE Polarization;
fig. 7 is a reflection spectrogram of the sub-wavelength asymmetric grating structure simulated by computer simulation along with the height (t) of the first ridge and the second ridge of the metal grating under given structural parameters, wherein a black dotted line diagram is a far-field reflection spectrogram of the sub-wavelength asymmetric grating structure at t of 10 nm, 20 nm, 40 nm, 50 nm, 60 nm, 80 nm, 100 nm, 120 nm, and 140 nm, respectively;
FIG. 8 is a reflection spectrum of the sub-wavelength asymmetric grating structure as a function of the second gap(s) simulated by computer simulation for given structural parameters;
FIG. 9 is a reflection spectrum of the sub-wavelength asymmetric grating structure as a function of the refractive index (n) of the dielectric insulating layer simulated by computer simulation under given structural parameters;
fig. 10 is a reflection spectrogram of the sub-wavelength asymmetric grating structure simulated by computer simulation varying with the thickness (h) of the metal adhesion layer (in the discontinuous coverage state) under given structural parameters, wherein the black dotted line diagram is a far-field reflection spectrogram of the sub-wavelength asymmetric grating structure with h being 0.0 nm to 1.0 nm (at an interval of 0.1 nm), respectively, and the inset is a side view schematic diagram of the sub-wavelength asymmetric grating structure with the metal adhesion layer (in the discontinuous coverage state);
fig. 11 is a reflection spectrogram of the sub-wavelength asymmetric grating structure simulated by a computer according to the thickness (h) of the metal adhesion layer (in the continuous coverage state) under given structural parameters, wherein a black dotted line diagram is a far-field reflection spectrogram of the sub-wavelength asymmetric grating structure with h being 0.0 nm to 1.0 nm (interval of 0.1 nm), respectively, and an inset is a side view of the sub-wavelength asymmetric grating structure with the metal adhesion layer (in the continuous coverage state);
FIG. 12 is a far-field reflectance spectrum of the sub-wavelength asymmetric grating structure as a function of the dielectric insulating layer thickness (d is 180 nm to 400 nm) simulated by computer simulation given the structure parameters and incident Polarization conditions (Normal index, TM/TE Polarization);
fig. 13 is an electromagnetic field distribution diagram of the sub-wavelength asymmetric grating structure corresponding to the reflection absorption valley D1 under given incident Polarization conditions (Normal incorporation, TM Polarization) and different dielectric barrier layer thicknesses (D =180 nm, 280 nm, 310 nm, 320 nm, 360 nm), where: an electric field profile, a magnetic field z-component profile;
FIG. 14 is a graph of the power dependence of a Second Harmonic (SHG) signal measured in a nonlinear optical characterization experiment for a sub-wavelength asymmetric grating structure with IR light modulation properties according to an embodiment of the present application;
FIG. 15 is a schematic diagram illustrating the relationship between the incident/emergent polarization angle and the orientation of the sub-wavelength asymmetric grating structure in the polarization test according to the embodiment of the present application;
FIG. 16 is a spectrum diagram of an SHG signal corresponding to different incident/emergent polarization combinations of a sub-wavelength asymmetric grating structure with infrared light modulation characteristics under given structural parameters according to an embodiment of the present application;
fig. 17 is an intensity polarization distribution diagram of SHG signals corresponding to different incident/emergent polarization combinations with given structural parameters according to the sub-wavelength asymmetric grating structure with infrared light modulation characteristics of the embodiment of the present application, as a function of the thickness of the dielectric barrier layer (d =240 nm, 310 nm, 360 nm);
fig. 18 is a graph of signal spectra and intensity polarization distributions of an unmodified sub-wavelength asymmetric grating structure reference for a sub-wavelength asymmetric grating structure with ir modulation characteristics according to an embodiment of the present application under given structural parameters and different combinations of incident/exit polarizations.
Detailed Description
The present application will be described in further detail with reference to the drawings and examples. It is to be understood that the specific embodiments described herein are merely illustrative of the relevant invention and not restrictive of the invention. It should be noted that, for convenience of description, only the portions related to the related invention are shown in the drawings.
It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict. The present application will be described in detail below with reference to the embodiments with reference to the attached drawings.
Referring to fig. 1, in an embodiment of the present application, a sub-wavelength asymmetric grating structure with an infrared light modulation characteristic is provided, which includes a substrate, a metal thin film layer, a dielectric isolation layer, a metal adhesion layer, and a metal grating, which are sequentially stacked from bottom to top, where the metal grating has an asymmetric dual-grating unit, the asymmetric dual-grating unit includes a first grating unit having a first ridge and a second grating unit having a second ridge, the first ridge and the second ridge have different period widths and are alternately arranged at intervals, the metal adhesion layer is disposed below the first ridge and the second ridge and has the same width as the first ridge and the second ridge, respectively, structural parameters of the metal grating include a width (W1) of the first ridge, a width (W2) of the second ridge, a first gap (g) between the first ridge and the second ridge on an adjacent side, and a second gap(s) between the first ridge and the second ridge on an adjacent other side, and a ratio of the height of the first ridge to the width of the first ridge is greater than or equal to 0.5. Specifically, the width of the first ridge is 70 to 90 nanometers, the width of the second ridge is 200 to 220 nanometers, the first gap is 190 to 210 nanometers, the second gap is greater than or equal to 100 nanometers, and the heights of the first ridge and the second ridge are greater than or equal to 40 nanometers. The metal adhesion layer is made of chromium and has a thickness less than or equal to 1 nanometer. The refractive index of the medium isolation layer is 1.2 to 2.0, and the thickness regulation range is 180 to 400 nanometers. The dielectric isolation layer is made of silicon dioxide, the substrate is made of silicon, and the metal film layer and the metal grating are made of silver.
In a specific embodiment, the first gap (g) of the metal grating is 200 nm, the widths (W1 and W2) of the first ridge and the second ridge are 80 nm and 210 nm, respectively, the second gap(s) is 200 nm, and the period (P) of the metal grating is 690 nm; the height (t) of the first ridge part and the second ridge part of the metal grating is 50 nanometers, the thickness (h) of the metal adhesion layer is 0.5 nanometers, the thickness (D) of the medium isolation layer has a specific regulation window value, namely the corresponding regulation range is 180 to 400 nanometers, and the thickness (D) of the metal film layer is 200 nanometers. As shown in fig. 2, the resulting sub-wavelength asymmetric grating structure lies in the x-y plane and is uniformly reproducible over a large area with a period (P) of 690 nm. As shown in fig. 3, a plurality of structural parameters of the manufactured sub-wavelength asymmetric grating structure, such as the widths (W1 and W2) of the first and second ridges, the first gap (g), the second gap(s), and the heights (t) of the first and second ridges, are clearly observable, which indicates that the manufactured structural device conforms to the design scheme shown in fig. 1.
As shown in fig. 4 and 5, the experimental results are substantially consistent with the far-field angle-resolved reflectance spectrum results of computer simulation under the given structural parameters and incident Polarization conditions (the incident angle range of the light source is 0 to 50 °, TM Polarization). The above results show that the sub-wavelength asymmetric grating structure of the embodiment of the present application has a distinct reflection absorption valley D1 in the near infrared band (the region shown by the black dashed line), and the spectral position of the reflection absorption valley is cleaved with the increase of the incident light angle (from 0 ° to 50 °) to form two branches (as shown by the white dashed line arrows), wherein the spectral position of one branch is red-shifted with the increase of the incident light angle, and the spectral position of the other branch is blue-shifted. The above-mentioned Folded Dispersion (Folded Dispersion) phenomenon reveals that the resonant mode excited by the sub-wavelength asymmetric grating structure has the propagation property of photon-like mode. Meanwhile, based on the existence of the metal film layer with the thickness of 200 nanometers, the sub-wavelength asymmetric grating structure has no optical transmission, so that according to the formula A =1-R (wherein A is absorption efficiency, and R is reflection efficiency), the light reflection and absorption efficiencies of the sub-wavelength asymmetric grating structure under each corresponding action wavelength are all over 80%, namely, the near-perfect light trapping (light absorption) effect of an infrared region occurs. The test results prove that the sub-wavelength asymmetric grating structure can realize the near-perfect absorption of the continuous incident light angle and the infrared narrow band with tunable multi-frequency bands (near-infrared I/II regions) in a larger incident light angle range (0 to 50 degrees) under the condition of TM polarization excitation, namely the adjustment step of the incident light angle can be as fine as 1 degree or increased to the adjustment ranges of parameters such as 5 degrees, 10 degrees, 15 degrees and the like, the near-perfect absorption of light (corresponding to the light absorption efficiency of 80 to 100 percent) can be realized, the wavelength modulation range of the near-perfect absorption of light is in the near-infrared I/II region, and the corresponding specific acting wavelength can also be one-to-one finely adjusted corresponding to the incident light angle. In addition, as shown in FIG. 6, a distinct reflection absorption valley D2 is observed at 770 nm for the sub-wavelength asymmetric grating structure under a given incident Polarization condition (Normal index, TE Polarization). The results can be simultaneously verified by computer simulation and experimental characterization, and further show that the structural model established by simulation calculation conforms to the configuration of the manufactured structural device.
As shown in fig. 7 to 10, the wavelength of the sub-wavelength asymmetric grating structure can be controllably modulated according to the structure parameters under the given incident Polarization condition (Normal incorporation, TM Polarization). Under given structural parameters, as shown in fig. 7, the absorption wavelength and the reflection absorption valley half-peak width of the sub-wavelength asymmetric grating structure to infrared light remain unchanged as the heights (t) of the first ridge and the second ridge of the grating increase to 40 nm or more, which indicates that the sub-wavelength asymmetric grating structure has high structural tolerance under a given structural design scheme, i.e., the performance characteristics of a structural device can still be ensured within a larger processing error range, and the ratio of the height of the first ridge to the width of the first ridge of the metal grating is greater than or equal to 0.5, and the value of the small aspect ratio (as small as 0.5) of the sub-wavelength asymmetric grating structure meets the large-scale manufacturing requirement of the existing EBL process, and has low process requirement and is easy to implement. As shown in fig. 8 and 9, as the second gap(s) of the sub-wavelength asymmetric grating structure and the refractive index (n) of the dielectric insulating layer increase, the absorption wavelength of the infrared light by the sub-wavelength asymmetric grating structure continuously generates a red shift but the reflection absorption efficiency of the infrared light keeps unchanged, which indicates that the sub-wavelength asymmetric grating structure can adjust the structural parameters according to actual requirements to satisfy the near-perfect absorption of the narrow band of the infrared light in different frequency bands within a larger spectral modulation range (near-infrared I/II region), and the sub-wavelength asymmetric grating structure can realize the infrared light narrow-band absorption efficiency of 80 to 100% within a continuous angle and tunable frequency band within an incident light angle range of 0 to 50 ° under the TM polarization excitation condition by adjusting the structural parameters.
As shown in fig. 10, the metal adhesion layer (e.g. cr) added in the discontinuous coverage state is used to improve the adhesion of the metal grating on the dielectric isolation layer, and the thickness (h) of the metal adhesion layer has little influence on the wavelength and half-peak width of the near-perfect trapping effect of the infrared narrow band of the sub-wavelength asymmetric grating structure in the variation range of 0.0 to 1.0 nm, which indicates that the loss caused by the design of the metal adhesion layer to the infrared resonance absorption is negligible, i.e. the metal adhesion layer has high structure tolerance in the actual manufacturing process. In contrast, as shown in fig. 11, the structural design is modified to make the metal adhesion layer (e.g. chromium) in a continuous coverage state, and then as the thickness (h) increases, the wavelength of the near-perfect light trapping effect of the infrared narrow band of the sub-wavelength asymmetric grating structure remains unchanged, but the half-peak width of the reflection absorption valley of the sub-wavelength asymmetric grating structure is obviously widened, which indicates that the metal adhesion layer added with continuous coverage causes loss to the infrared resonance absorption. In conclusion, an optimized design idea is provided for realizing high-performance infrared light energy absorption under a given action wavelength based on the sub-wavelength asymmetric grating structure. Meanwhile, in this embodiment, the thickness (h) of the metal adhesion layer used in the simulation calculation is set to be 0.5 nm in consideration of the actual manufacturing process requirements.
As shown in fig. 12 and fig. 13, the reflection and absorption mechanism of infrared light by the sub-wavelength asymmetric grating structure can be further revealed. On the one hand, as shown in the left diagram in fig. 12, under given structural parameters and incident Polarization conditions (Normal incorporation, TM Polarization), the far-field reflection spectrum of the sub-wavelength asymmetric grating structure mainly exhibits reflection absorption valleys D1 in the infrared region, and the reflection absorption efficiency is 80 to 100% in the thickness variation range (180 nm to 400 nm) of the dielectric insulating layer, i.e., near-perfect light trapping in the infrared region occurs, indicating that the sub-wavelength asymmetric grating structure has high structural tolerance. Meanwhile, considering the requirement of an actual micro-nano processing technology (such as EBL), the thickness variation range of the dielectric isolation layer in the given structure design scheme has a specific regulation window value (namely 180 nanometers to 400 nanometers), namely, the non-conductivity of the dielectric isolation layer needs to be considered when the sub-wavelength asymmetric grating structure is prepared outside the regulation window value (for example, more than 400 nanometers), and the non-conductivity causes the accumulation of charges on the surface to further cause the distortion of a graph during processing. In order to solve the above problems, it is necessary to consider introducing a conductive layer above a dielectric isolation layer to assist processing, however, the introduction of the conductive layer increases process complexity and uncertainty of processing results, such as introduction of a conductive polymer, which is costly and short in storage time, and the introduction of a metal layer has problems of acid-base and dissolution and carbonization of a photoresist when the metal layer is removed at high temperature. In summary, a given dielectric barrier tuning window value is based on the optimal value in existing structural designs, which takes into account both structural tolerance and processing feasibility.
As shown in fig. 13, the electric field modulus spatial distribution results (wherein,Eis an electric field, <EI is electric field modulus, lg is not countingEI is a logarithmic scale on the electric field modulus) can be used to determine the electromagnetic field mode coupling mechanism of the reflective absorption valley D1:
1) Hybridization (marked as outer-surface modes) between the propagating surface polaritons and local surface plasmon resonances at the interface on the metal grating-medium isolation layer;
2) Propagating surface polaritons (denoted as inner-surface modes) at the interface below the dielectric isolation layer and the metal thin film layer;
3) The coupling hybridization between the electromagnetic field modes of the upper and lower interfaces (denoted as super-modes).
In conjunction with the magnetic field pattern analysis results in figure 13 (where,His a magnetic fieldHL is the modulus of the magnetic field,H z a z-component of a magnetic field), the excitation of the super-modes can combine the symmetric/antisymmetric PSPP mode characteristics formed by the symmetric and antisymmetric coupling between the upper and lower interfaces PSPP and the dipole resonance mode characteristics formed by the adjacent grating ridges in the medium region in a single asymmetric grating unit. The optical resonance mode characteristics can realize the coupling-concentration of incident light field energy in a dielectric isolation layer region, further realize the energy transmission between metal-dielectric material components, and effectively reduce the absorption (ohm) loss in the metal plasmon material and the radiation loss from mode resonance. Thus, excitation of super-modes has high-quality resonance mode characteristics, enabling near-perfect absorption of infrared light. In particular, when the thickness d of the dielectric isolation layer is increased to 310 nm, the incident light field energy cannot be coupled into the structural device through the grating, i.e., most of the incident light field energy is reflected to the free space to form a standing wave distribution. The change of the spatial distribution of the coupling mode of the above electromagnetic field causes the corresponding operation of the sub-wavelength asymmetric grating structure at d =310 nmThe reflectivity was mutated to 1.0 with wavelength (about 1018 nm), i.e. without any infrared light absorption properties. Singular points on the optical resonance mode can be used for disclosing the conversion process of the symmetrical and anti-symmetrical PSPP modes in the sub-wavelength asymmetrical grating structure and the corresponding mode resonance characteristics. Following the above mode analysis concept, the reflection absorption valleys D3 and D4 exhibited by the far-field reflection spectrum of the sub-wavelength asymmetric grating structure in the visible light region (high frequency band) can also be attributed to the anti-symmetric coupling between the upper and lower interface electromagnetic field modes; meanwhile, because the two reflection absorption valleys are in the visible light region, the discussion of the electromagnetic field mode coupling mechanism is omitted. On the other hand, as shown in the right diagram in fig. 12, the far-field reflection spectrum of the sub-wavelength asymmetric grating structure exhibits mainly reflection absorption valleys D2 in the infrared region, the spectral position of which is continuously red-shifted with increasing D and the variation range of which covers the near-infrared I/II region, given the structural parameters and incident Polarization conditions (Normal incorporation, TE Polarization).
In the embodiment of the application, a confocal micro-spectrum acquisition system (provided with a femtosecond laser: origami-10 XP \400fs \1028nm, an EMCCD camera: iXon Ultra 888 and a monochromatic spectrometer: andor SR 500) can be adopted to detect the Second Harmonic (SHG) signals of the prepared sub-wavelength asymmetric grating structure at room temperature, and the method comprises the following steps: measuring the power dependence relationship, the polarization dependence relationship and the absolute intensity contrast relationship of the signals; and characterizing the morphology information of the structural device by adopting a scanning electron microscope imaging technology.
As shown in FIG. 14, the square dependence of the measured second harmonic signal intensity (@ 514 nm) and the average power of the incident light (@ 1028 nm) can be used to detect the authenticity of the test signal and the second-order nonlinear optical characteristics thereof, thereby verifying that the manufactured structure device can effectively induce and generate the second harmonic effect. As shown in fig. 15 to 18, the sub-wavelength asymmetric grating structure integrates a nonlinear optical signal processing function. As shown in fig. 15 and 16, the sub-wavelength asymmetric grating structure rotates the excitation sample with the incident polarization direction of 90 ° and 0 ° respectively under the given structural parameters and the emergent polarization orientation (denoted as EP90, i.e. the polarization direction is perpendicular to the grating groove direction), and collects signals (denoted as IP90-EP90/IP0-EP90, as shown in labels 1 and 3), and the measured second harmonic signal intensity strongest point appears under the 90 ° incident polarization condition (denoted as IP90-EP90, as shown in label 1). For comparison, an excitation sample with an exit polarization orientation of 0 ° (denoted as EP0, i.e. a polarization direction along the grating groove direction) is set, while the incident polarization directions are rotated by 90 ° and 0 °, respectively, and signals (denoted as IP90-EP0/IP0-EP0, as shown in labels 2 and 4) are collected, which measure very weak second harmonic signal intensities, but relatively strong signals still appear under the 90 ° incident polarization condition (as shown in label 2). For labeling convenience, the signal intensity polarization distributions shown in labels 1 and 2 in the above description can be denoted as TM Resonant State On, and the polarization distributions shown in labels 3 and 4 can be denoted as TE Resonant State Off. It can be known that the excitation and emission polarization of the nonlinear frequency up-conversion signal of the sub-wavelength asymmetric grating structure is controllable.
Further, as shown in fig. 17, structural devices with different dielectric barrier layer thicknesses d (e.g., d =240 nm, 310 nm, 360 nm) under given structural parameters are introduced and characterized with their corresponding nonlinear optical signal intensity polarization distributions under a given excitation average power (0.5 mw). When d =240 nm, the corresponding signal strength of IP0-EP90/IP0-EP0 is greater than that of IP90-EP90/IP90-EP0, and then TE Resonant State On and TM Resonant State On are known. In contrast, as d is increased to 310 nanometers, TE Resonant State On and TM Resonant State On corresponding signal strength continues to be suppressed; when d is increased to 360 nm, as shown by the black dashed circle in fig. 12, the reflection and absorption valley position of the sub-wavelength asymmetric grating structure under the TM polarization excitation condition is just near the wavelength position (1028 nm) of the excitation light source, and the reflection and absorption efficiency is close to 100%, that is, the narrow-band perfect absorption of infrared light occurs, so that the TM Resonant State On corresponding signal intensity is effectively amplified, and the TE Resonant State On corresponding signal intensity is effectively suppressed (TE Resonant State Off). It is noted that with increasing average power of excitation light (e.g., greater than 0.5 mw), the nonlinear optical signals of the samples with d =240 nm and 310 nm under TE polarization excitation will experience a sudden increase. At the moment, the optical imaging module of the system can observe that the sample is subjected to a photo-induced damage phenomenon, namely the damage threshold of the sample is lower under the TE polarization excitation condition. As shown in fig. 18, the sub-wavelength asymmetric grating structure has nearly one order of magnitude enhancement compared to the dielectric isolation layer-metal thin film layer-silicon substrate reference signal of the unmodified sub-wavelength asymmetric grating structure under the condition of modulating the structural parameters (d =360 nm) and given polarization combination (IP 90-EP 90) and excitation average power (2.0 mw); meanwhile, compared with the signal intensity polarization distribution of the non-modified structural reference sample, the polarization selection enhancement characteristic of the structural device is remarkable (as shown in an inset of fig. 18). In summary, the TM Resonant State On corresponding signal amplification mechanism in the sub-wavelength asymmetric grating structure follows the super-modes excitation mechanism as shown in fig. 12 and fig. 13. Meanwhile, the sub-wavelength asymmetric grating structure induces and amplifies a nonlinear optical signal under the excitation condition of TE polarization, and the nonlinear optical signal is mainly modulated by a TE resonance field enhancement effect formed by a fundamental frequency region (shown as a white dotted line in FIG. 12). The reflection absorption valley under the condition of TE polarization excitation is widened greatly, the Q value of the resonance mode is low (namely, the corresponding non-radiative loss is high), so that when the TE resonance field is enhanced and matched with a fundamental frequency region, the photoinduced structure damage threshold of a sample is low, and further the improvement of the nonlinear optical amplification conversion efficiency is limited.
Correspondingly, an embodiment of the present application further provides a method for manufacturing the sub-wavelength asymmetric grating structure with the infrared light modulation characteristic, including the following steps:
s1, providing a substrate, and sequentially growing a metal thin film layer and a dielectric isolation layer on the substrate through an electron beam evaporation and plasma enhanced chemical vapor deposition process, wherein the substrate is silicon, the metal thin film layer is silver, and the dielectric isolation layer is silicon dioxide.
And S2, spin-coating electron beam photoresist on the dielectric isolation layer, forming an asymmetric grating pattern after electron beam exposure and development, specifically, spin-coating polymethyl methacrylate (PMMA) on the dielectric isolation layer, and forming the asymmetric grating pattern on the PMMA after electron beam exposure and development.
And S3, growing an adhesion layer and a metal layer by adopting electron beam evaporation deposition, removing the residual photoresist and the metal adhesion layer and the metal layer above the photoresist, and transferring the asymmetric grating pattern to the adhesion layer and the metal layer to form the metal adhesion layer and the metal grating.
In the process, the sub-wavelength asymmetric grating structure prepared by adopting the design scheme has high structure tolerance, the process preparation flow can be effectively simplified, the production cost is reduced, and the production yield is improved.
In order to facilitate understanding of the above-described technical aspects of the present invention, the above-described technical aspects of the present invention will be described in detail below in terms of specific usage.
When the tunable infrared optical grating structure is used specifically, the sub-wavelength asymmetric grating structure with the infrared light modulation characteristic provided by the embodiment of the application has a great application prospect in the direction of an adjustable infrared light absorption device and a nonlinear optical functional device, particularly in the aspects of near-perfect absorption of multiband infrared light energy, effective induction generation and flexible regulation of nonlinear optical second harmonic signals. The prepared sub-wavelength multilayer structure can realize high-efficiency infrared light energy absorption by utilizing the symmetrical/antisymmetric coupling between the upper and lower interface electromagnetic field modes of the excitation frequency band and combining an asymmetric optical field perturbation regulation and control mechanism, and the measured SHG signal has remarkable incident polarization excitation and radiation polarization selection characteristics. The sub-wavelength asymmetric grating structure has wide application prospect in the field of surface plasmon application, and is expected to become a nano structure with wide spectrum absorption and perfect/rainbow light trapping functions. The shape parameters of the sub-wavelength asymmetric grating structure comprise a nano grating period, the width of a grating ridge, the thickness of a medium isolation layer, the type of a vapor plating metal material and the like, are flexible and adjustable, and can be prepared in a large area. If the electron beam evaporation metal is aluminum and other metals with abundant reserves and low price, the cost can be further reduced; if the dielectric isolation layer is made of semiconductor materials such as zinc oxide, the nonlinear frequency up-conversion efficiency of the structural device can be further improved by utilizing the larger intrinsic nonlinear polarizability coefficient of the materials, and the development and application of the structural device in the fields of nonlinear optical functional devices and nano integrated optoelectronics are expanded.
In conclusion, by means of the technical scheme, the problems that a micro-nano structure device with an advanced optical function in the existing nano structure processing field is complex in design scheme, high in processing precision requirement and complex in preparation process can be solved, so that high-quality long-range ordered preparation is facilitated, the uniformity of a prepared product is good, the structure tolerance is high, multiple parameters are adjustable, and the prepared structure has remarkable infrared light absorption and nonlinear harmonic polarization enhancement characteristics. In the process, a silicon wafer is used as a sample substrate, so that the possibility is provided for the design and development of industrialization and integrated multifunctional optical devices such as chip-level light modulation or frequency mixing devices in the microelectronic industry in the future.
While the present invention has been described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.
Claims (9)
1. A sub-wavelength asymmetric grating structure with infrared light modulation characteristics is characterized by comprising a substrate, a metal thin film layer, a dielectric isolation layer, a metal adhesion layer and a metal grating which are sequentially stacked from bottom to top, wherein the metal grating is provided with an asymmetric double-grating unit, the asymmetric double-grating unit comprises a first grating unit with a first ridge and a second grating unit with a second ridge, the first ridge and the second ridge have different cycle widths and are alternately arranged at intervals, the metal adhesion layer is arranged below the first ridge and the second ridge and has the same width as the first ridge and the second ridge respectively, the structural parameters of the metal grating comprise the width of the first ridge, the width of the second ridge, a first gap between the first ridge and the second ridge on the adjacent side and a second gap between the first ridge and the second ridge on the adjacent other side, the infrared narrow-band absorption efficiency of the sub-wavelength asymmetric grating structure, which is continuously angle-tunable and frequency-tunable within an incident light angle range of 0-50 degrees, is 80-100% under the condition of TM polarization excitation by adjusting the structural parameters, specifically, the infrared narrow-band absorption efficiency is 80-100% by adjusting the difference of the widths of the first ridge and the second ridge, and the action waveband of infrared narrow-band absorption can be continuously adjusted by adjusting the second gap.
2. The sub-wavelength asymmetric grating structure with infrared light modulation characteristics of claim 1, wherein the continuous angle and frequency band tunable infrared light narrow band absorption band comprises a near infrared I region and/or a near infrared II region.
3. The sub-wavelength asymmetric grating structure with infrared light modulation characteristics according to claim 1, wherein a ratio of a height of the first ridge portion to a width of the first ridge portion is 0.5 or more.
4. The sub-wavelength asymmetric grating structure with infrared light modulation characteristics of claim 1, wherein the width of the first ridge is 70-90 nm, the width of the second ridge is 200-220 nm, the first gap is 190-210 nm, the second gap is greater than or equal to 100 nm, and the height of the first ridge and the second ridge is greater than or equal to 40 nm.
5. The asymmetric grating structure of claim 1, wherein the metal adhesion layer is made of chromium and has a thickness of 1 nm or less.
6. The sub-wavelength asymmetric grating structure with infrared light modulation characteristics of claim 1, wherein the refractive index of the dielectric isolation layer is 1.2-2.0, and the thickness regulation range is 180-400 nm.
7. The sub-wavelength asymmetric grating structure with infrared light modulation characteristics of claim 1, wherein the material of the dielectric isolation layer is silicon dioxide, the material of the substrate is silicon, and the materials of the metal thin film layer and the metal grating are silver.
8. A method for fabricating the sub-wavelength asymmetric grating structure with infrared light modulation characteristics according to any one of claims 1 to 7, comprising the steps of:
s1, providing a substrate, and sequentially growing a metal thin film layer and a medium isolation layer on the substrate through an electron beam evaporation and plasma enhanced chemical vapor deposition process;
s2, spin-coating electron beam photoresist on the medium isolation layer, and forming an asymmetric grating pattern after electron beam exposure and development;
and S3, growing an adhesion layer and a metal layer by adopting electron beam evaporation deposition, removing the residual photoresist and the metal adhesion layer and the metal layer above the photoresist, and transferring the asymmetric grating pattern to the adhesion layer and the metal layer to form the metal adhesion layer and the metal grating.
9. Use of a sub-wavelength asymmetric grating structure with infrared light modulation properties according to any of claims 1-7 in a tunable infrared light absorbing device and a nonlinear optical functional device.
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