Film, structure and method for enhancing mechanical light scattering spectral signal

Abstract

The invention discloses a film, a structure and a method for enhancing a mechanical light scattering spectrum signal, and belongs to the technical field of spectrum detection. The thin film can be used for covering the surface of a substrate coupled with plasmon metal. The thickness d of the thin film meets the condition: d = [(2m-1) * lambda] / (4n * cos theta), so that incident light for exciting the plasmon metal can interfere and grow in the thin film; wherein m is the interference series, lambda is the wavelength of the incident light, n is the refractive index of the thin film, and theta is the incident angle. According to the thin film, the structure and the method for enhancing the mechanical light scattering spectral signal provided by the invention, the detectability of a low-frequency mechanical Raman scattering signal can be effectively improved; the film thickness and material selection are optimized by utilizing the constructive interference effect, so that a mechanical Raman scattering signal is obviously enhanced; and due to the application of the interference film, the original undetectable signal becomes clear and distinguishable, and the application space of the mechanical Raman scattering technology in the aspect of deeper sample detection is widened.

Description

Technical Field

[0001] The present application belongs to the field of spectral detection technology, and specifically relates to a film, structure and method for enhancing mechanical light scattering spectral signals. Background Art

[0002] Mechanical Raman scattering spectroscopy is a spectroscopic technique based on the strong coupling of mechanical oscillators with localized surface plasmon cavities. By dynamically modulating the polarization tensor, mechanical Raman scattering spectroscopy can convert mechanical vibration signals into radiatively active optical signals, thereby achieving highly sensitive detection of mechanical vibration information. This technology makes up for the limitations of traditional spectroscopic methods in the detection of non-optically active vibrations and improves the ability to measure low-frequency vibration modes. Especially in low-dimensional materials, mechanical Raman scattering spectroscopy is widely used in the analysis of lattice vibration modes, the characterization of layer characteristics, and the detection of hidden defects in the lattice. At the same time, in larger-scale macroscopic mechanical vibration phenomena, such as the detection of shock waves and acoustic vibrations, this technology also demonstrates unique advantages, providing a new path for multi-scale vibration research.

[0003] Although mechanical Raman scattering spectroscopy has a wide range of applicability, it still faces several technical bottlenecks in practical applications. The existing mechanical Raman scattering technology relies on the strong coupling of mechanical oscillators with localized surface plasmons for signal acquisition sensitivity, while the locality of the plasmon light field limits the signal enhancement range and coverage depth. When the sample being measured is a thick layered crystal or has a complex interface structure, the intensity of the mechanical Raman signal decreases significantly and may even be completely submerged by the background noise. This signal attenuation problem is particularly evident in multilayer materials, rough surfaces or interfaces, resulting in a significant decrease in the detection accuracy of lattice vibration modes and hidden defects.

[0004] Therefore, it is necessary to provide a new solution to the above technical problems. Summary of the invention

[0005] The purpose of the present application is to provide a film, structure and method for enhancing mechanical light scattering spectral signals, which can effectively improve the intensity of mechanical light scattering signals and significantly expand the depth and sensitivity of signal detection.

[0006] To achieve the above objectives, the technical solutions provided by this application are as follows:

[0007] In the first aspect, the present application provides a film for enhancing mechanical light scattering spectral signals, wherein the film is made of an optically transparent material and is used to cover the surface of a substrate coupled with a plasmon metal; the thickness d of the film satisfies the condition: d = [(2m-1)·λ] / (4n·cosθ), so that the incident light used to excite the plasmon metal can interfere constructively in the film; wherein m is the interference order, λ is the wavelength of the incident light, n is the refractive index of the film, and θ is the angle of incidence.

[0008] In one or more embodiments, the film is made of PDMS, PMMA, PVA, PPC or PET.

[0009] In one or more embodiments, the light transmittance of the film is greater than 93%, and / or the refractive index of the film is 1.2-2.0.

[0010] In a second aspect, the present application provides a structure for measuring a mechanical light scattering spectral signal, comprising: a substrate and an optically transparent film; the substrate is coupled with a plasmon metal layer; the optically transparent film covers the surface of the plasmon metal layer; the thickness d of the film satisfies the condition: d = [(2m-1)·λ] / (4n·cosθ), so that the incident light used to excite the plasmon metal in the plasmon metal layer can interfere constructively in the film; wherein m is the interference order, λ is the wavelength of the incident light, n is the refractive index of the film, and θ is the angle of incidence.

[0011] In one or more embodiments, the material of the plasmon metal layer includes at least one of silver, gold, and copper.

[0012] In one or more embodiments, the incident light used to excite the plasmonic metal in the plasmonic metal layer, the forward incident light electric field vector amplitude and the backscattered light electric field vector amplitude enhanced by interference in the film are respectively expressed as:

[0013]

[0014] in, represents the electric vector amplitude of the forward excitation light, represents the sum of all forward excitation photoelectric vector amplitudes, represents the amplitude of the forward excitation photoelectric vector after two reflections each time, represents the electric vector amplitude of the backscattered light, represents the sum of all backscattered photoelectric vector amplitudes, represents the backscattered photoelectric vector amplitude after two reflections each time, t ij represents the ratio of the amplitude of the transmitted electric vector of light propagating from medium i to medium j at the medium interface, rij k represents the ratio of the reflected electric vector amplitude of light propagating from medium i to medium j at the medium interface, i represents the wave vector of the excitation light, k s represents the scattered light wave vector, z represents the z-direction coordinate, ω represents the frequency of light, t represents time, and d 薄膜 Indicates the film thickness, 0 represents air medium, 1 represents thin film medium, and 2 represents plasmon metal medium.

[0015] In one or more embodiments, the total interference enhancement factor Factor of the film is expressed as:

[0016]

[0017] In a third aspect, the present application provides a method for measuring a mechanical light scattering spectral signal, comprising:

[0018] An optically transparent film is covered on the surface of a sample coupled with a plasmon metal; the film is cured to make it adhere to the surface of the sample; a laser with a plasmon resonance wavelength is used to irradiate the sample through the film to excite the plasmon metal in the sample, and a mechanical light scattering spectrum signal of the sample is collected; wherein the thickness d of the film satisfies the condition: d=[(2m-1)·λ] / (4n·cosθ), so that the incident light used to excite the plasmon metal can interfere constructively in the film, m is the interference order, λ is the wavelength of the incident light, n is the refractive index of the film, and θ is the angle of incidence.

[0019] In one or more embodiments, the plasmon metal is deposited on the sample surface by evaporation or sputtering; and / or the thin film is coated on the sample surface by spin coating or the cured thin film is directly adhered to the sample surface.

[0020] In one or more embodiments, the curing temperature of the film is 60 to 200° C., and the curing time is 1 to 150 min.

[0021] Compared with the prior art, the film, structure and method for enhancing the mechanical light scattering spectral signal provided in the present application can effectively improve the detectability of low-frequency mechanical Raman scattering signals; the film thickness and material selection are optimized by utilizing the constructive interference effect, thereby achieving a significant enhancement of the mechanical Raman scattering signal; the application of the interference film makes the originally undetectable signal clearly discernible, broadening the application space of mechanical Raman scattering technology in deeper sample detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings required for use in the embodiments or the prior art description are briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present application, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0023] Figure 1 This is a schematic diagram of the direction of the electric field of the incident light and the scattered light of mechanical Raman scattering in one embodiment of the present application;

[0024] Figure 2 This is a schematic diagram of the direction of the optical field of mechanical Raman scattering enhanced by thin film interference in one embodiment of the present application;

[0025] Figure 3 This is a flow chart of a method for measuring a mechanical light scattering spectrum signal in one embodiment of the present application;

[0026] Figure 4 This is a mechanical Raman scattering spectrum diagram of the sample based on interference enhancement in one embodiment of the present application;

[0027] Figure 5 This is an atomic force microscope test image of an interference enhancement film sample in one embodiment of the present application;

[0028] Figure 6 This is a mechanical Raman scattering spectrum of a thin film protection sample in one embodiment of the present application. DETAILED DESCRIPTION

[0029] In order to enable those skilled in the art to better understand the technical solutions in the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work should fall within the scope of protection of the present application.

[0030] It should be noted that, unless otherwise indicated, all numbers used in the specification and claims to represent feature sizes, quantities and physical properties should be understood to be modified by the term "about" in all cases. Therefore, unless otherwise indicated, the numerical parameters listed in the above specification and the attached claims are approximate values, and those skilled in the art can use the teachings disclosed herein to seek to obtain the desired properties and appropriately change these approximate values. The use of numerical ranges expressed as endpoints includes all numbers within the range and any range within the range, for example, 1 to 5 includes 1, 1.2, 1.4, 1.55, 2, 2.75, 3, 3.80, 4 and 5, etc.

[0031] For ease of understanding, the nouns involved in the embodiments of the present application are first explained below.

[0032] Mechano-Raman Spectroscopy (MRS): Mechano-Raman Spectroscopy is a spectroscopic method that uses the mechanical Raman scattering effect to detect mechanical vibration information. The mechanical Raman scattering effect refers to the periodic vibration of a mechanical oscillator (frequency ω mech ) driven by the plasmon cavity to vibrate synchronously. Under the resonant excitation condition, the photons in the plasmon cavity will mechanically couple with the mechanical oscillator and produce inelastic scattering, ultimately generating a frequency of ν0±ω mech For a series of low-frequency mechanical oscillators, such as lattice shear phonons, there is a lack of sufficient electric dipole to produce Raman scattering with the incident light. By coupling the interaction between plasmon metals and mechanical oscillators, the scattering efficiency can be effectively enhanced, so that such non-electric dipole active vibrations can also be measured. This technology can provide a new means for characterizing the microstructure, interface characteristics and defects of materials.

[0033] Ultra-Low Frequency Raman Spectroscopy: Ultra-low frequency Raman spectroscopy is the ability to detect and analyze Raman scattering signals with wavenumbers much lower than those of conventional Raman spectroscopy analysis systems, usually in the range of 5 cm-1 or less. These low-frequency signals usually involve low-frequency vibration modes of materials, such as acoustic phonons, low-frequency lattice vibrations, or mechanical vibrations (such as shear vibrations or bending modes).

[0034] Localized Surface Plasmon Resonance (LSPR): LSPR refers to the collective oscillation phenomenon generated when free electrons on the surface of metal nanoparticles or metal nanostructures interact with incident light at a specific frequency. The plasmon cavity formed by this collective oscillation can significantly enhance the local light field, resulting in a significant enhancement of the interaction between light and matter. By precisely designing the shape, size, and arrangement of metal nanostructures, the resonant frequency of the plasmon cavity can be controlled to optimize its performance in specific applications. This effect is of great significance for enhancing the intensity and accuracy of mechanical Raman scattering signals.

[0035] Interference effect: Interference effect is an important phenomenon in wave phenomena, which refers to the constructive or destructive effect caused by different phase differences when waves emitted by different wave sources are superimposed in space. When two waves meet, if their phase difference is an integer multiple of the wavelength (Δφ=mλ), constructive interference will occur and the amplitude of the wave will be enhanced; if the phase difference is a half-integer multiple of the wavelength (Δφ=(m+1 / 2)λ), destructive interference will occur and the amplitude of the wave will be reduced, where Δφ is the optical path difference, λ is the wavelength, and m is the interference order. In this application, the optical interference effect of the film is used to adjust the intensity and phase of light. By selecting appropriate film materials and thicknesses, the reflection and transmission of light waves interfere under specific conditions, which can enhance the scattering signal within a specific wavelength range, thereby achieving the effect of enhancing the mechanical Raman scattering signal.

[0036] Accurately measuring vibrations plays a key role in many fields, including basic science, materials research, geological exploration, engineering monitoring, and military applications. Existing spectral analysis methods, such as infrared absorption spectroscopy and Raman scattering spectroscopy, are the main tools for studying molecular vibrations and lattice atomic motions, and are known for their picometer-level displacement resolution. However, these traditional spectroscopy methods have technical bottlenecks when detecting specific vibration modes, especially non-optically active vibrations, and it is difficult to effectively obtain relevant information.

[0037] Taking the vibration of mechanical oscillators in a vibrating system as an example, it plays an important role in multi-scale systems from macroscopic to microscopic. However, since mechanical oscillators usually do not have a non-zero electric dipole polarization tensor, their vibration modes are difficult to directly characterize in classical spectroscopy. For example, in layered crystals, ultra-low frequency shear phonons are typical mechanical oscillator vibration modes. Although the interlayer atomic motion has clear mechanical characteristics, it cannot form radiation-active electric dipoles due to the mutual cancellation of interlayer polarizability changes. Therefore, traditional infrared absorption or Raman scattering spectroscopy techniques are difficult to detect this type of vibration signal. This challenge limits the acquisition of mechanical vibration information and also hinders the in-depth application of related technologies in multiple scientific and engineering fields.

[0038] This dilemma is not limited to the microscopic level. Many important macroscopic vibration phenomena (such as shock waves, geological waves, gravitational waves, etc.) carry a large amount of physical information, and it is of great significance to detect them with high sensitivity. However, these vibration signals lack effective coupling methods with photons in existing optical technologies, resulting in limited detection capabilities. How to effectively obtain the vibration information of mechanical oscillators is not only a technical problem in the field of spectroscopy, but also an important bottleneck restricting the research and application of mechanical system vibrations.

[0039] In order to overcome this technical bottleneck, mechanical Raman scattering spectroscopy has become one of the breakthrough methods. It is based on the strong coupling of mechanical oscillators and localized surface plasmon cavities, and converts mechanical vibrations into radiatively active optical signals by dynamically modulating the polarization tensor. The emergence of this method has greatly improved the ability to measure non-optically active vibrations, and has shown wide applicability in the structural characterization of low-dimensional materials, mode analysis of lattice vibrations, detection of hidden lattice defects, and detection of macroscopic mechanical vibrations.

[0040] However, mechanical Raman scattering spectroscopy technology still has problems such as limited signal intensity and limited detection depth in practical applications. Especially when characterizing thicker layered crystals or complex interface structures, the signal is easily submerged in the background noise, resulting in a decrease in measurement sensitivity.

[0041] After analysis, it was found that the lack of signal strength in the existing technology is due to the low efficiency of coupling between the mechanical oscillator and the plasmon, while the limitation of the detection depth is directly related to the penetration ability of the excitation light and the effective collection range of the scattered signal. In addition, the plasmon metal is susceptible to oxidation and corrosion when exposed to air, resulting in the gradual attenuation of the signal and difficulty in maintaining long-term stability.

[0042] The intensity and signal-to-noise ratio of the mechanical Raman scattering signal are affected by many factors in the detection environment, including the matching degree between the laser wavelength and the plasmonic metal layer, the surface quality of the sample, and the stability of the test environment. In particular, when the plasmonic metal surface is exposed to air, oxidation and contamination will weaken the resonance effect of the localized surface plasmons, thereby further reducing the intensity of the scattered signal. In addition, the traditional mechanical Raman scattering technology is limited by the optical penetration depth when collecting signals in higher depth layers, which makes the characterization of thick layer materials and interface structures more difficult.

[0043] In response to these problems, this application proposes an enhancement method based on the constructive effect of optical interference. By covering the surface of the plasmon metal with a thin film with specific optical properties, the signal intensity and detection depth are simultaneously improved, while the environmental stability of the signal is significantly improved. The interference effect is an optical phenomenon that refers to the effect of enhancing or weakening the light intensity due to the interaction of phases when multiple beams of light overlap in space. By designing the propagation path and phase relationship of light, light in a specific direction can be made to interfere constructively, thereby greatly improving the intensity of the optical signal.

[0044] The implementation ideas of this application can be summarized as follows: by analyzing the interaction law between the mechanical scattered light signal and the excitation light field, it is found that the optical interference effect can be used to optimize the enhancement mechanism of the scattered signal; by covering the surface of the plasmon metal with a transparent film, the incident light forms constructive interference after multiple reflections in the film, thereby significantly improving the intensity of the forward incident light field and the backscattered light field; selecting a thin film material with optical transparency and environmental stability, and utilizing its protective effect on the plasmon metal to delay the oxidation degradation process of the metal surface and ensure the long-term stability of the signal.

[0045] This application combines the optical interference effect and material protection characteristics, using a thin film as a medium, not only improving the detection capability of mechanical scattered light signals, but also fundamentally improving signal attenuation and environmental stability issues, providing an innovative technical path for the characterization of complex interfaces and thick materials. This application can expand the applicability of mechanical scattering spectroscopy technology to deeper sample detection and longer stable testing.

[0046] In one embodiment of the present application, a film for enhancing a mechanical light scattering spectral signal is provided. The film is made of an optically transparent material and can be used to cover the surface of a substrate coupled with a plasmon metal.

[0047] The thickness d of the film satisfies the condition: d = [(2m-1)·λ] / (4n·cosθ), so that the incident light used to excite the plasmon metal can interfere constructively in the film; where m is the interference order, λ is the wavelength of the incident light, n is the refractive index of the film, and θ is the incident angle.

[0048] The film that enhances the mechanical light scattering spectrum signal is a structure designed by optimizing optical properties. Its core function is to achieve constructive interference of incident light after multiple reflection and transmission paths in the film through the optical interference effect, so as to enhance the intensity of the mechanical light scattering spectrum signal and improve the environmental stability of the signal. The film is made of optically transparent material and is specifically used to cover the surface of the substrate coupled with plasmon metal to play a role in signal enhancement and protection.

[0049] The material of the film is limited to optically transparent materials. The design of this feature is mainly based on the need for signal enhancement. The enhancement process of mechanical Raman scattering spectrum depends on the interference effect formed by multiple reflections and transmissions of incident light and scattered light in the film. Optically transparent materials can effectively transmit incident laser and scattered light signals, avoiding energy loss caused by material absorption or scattering, thereby retaining signal intensity to the greatest extent. The selection of transparent materials also needs to consider the matching of refractive index in order to optimize the interference constructive conditions.

[0050] The core of the film design is that its thickness d satisfies a specific interference condition, namely

[0051] d=[(2m-1)·λ] / (4n·cosθ).

[0052] The thickness design of the film is derived from the optical interference theory, which describes the propagation path and phase relationship of light in the film. When the film thickness meets this condition, the phase difference of the incident light after multiple reflections is just a multiple of the entire wavelength, thus forming a constructive interference effect in the propagation direction. Constructive interference significantly enhances the amplitude of light, thereby greatly improving the output intensity of the mechanical Raman scattering signal. Especially for weak signal detection scenarios, such as the detection of low-frequency mechanical vibrations, this interference effect can significantly improve the signal's measurability and signal-to-noise ratio.

[0053] The principle of optical interference ensures that under specific wavelengths and incident conditions, the electric field vector can achieve constructive interference after multiple superpositions of the reflection and transmission paths of light in the film. Constructive interference means that two phase-matched light waves are superimposed, and the electric field amplitude increases, thereby generating a higher light field intensity. If this interference condition is not met, the incident light may exhibit destructive interference after reflection and transmission in the film, resulting in a decrease in the electric field amplitude, and ultimately reducing the enhancement amplitude of the mechanical light scattering signal.

[0054] When the film is covered on the surface of the substrate coupled with plasmon metal, it can play a dual role of enhancing the signal and protecting the metal. Plasmon metals such as silver and gold can produce localized surface plasmon resonance (LSPR) with incident light, thereby exciting a strong localized light field, but the metal surface is easily corroded by oxygen and moisture when exposed to air, resulting in oxidation degradation and significantly reducing the optical performance.

[0055] After covering with an optically transparent film, the metal surface is effectively protected from oxygen and moisture in the environment, while the chemical stability of the film ensures its integrity and consistency in long-term testing. This protection is particularly suitable for samples that need to be stored for a long time or repeatedly tested, and can extend the service life of the plasmonic metal while maintaining the stability of the scattering signal.

[0056] In an exemplary embodiment, the material of the film is preferably a material with good light transmittance and suitable refractive index such as polydimethylsiloxane (PDMS), polymethyl methacrylate (PMMA), PVA (polyvinyl alcohol), PPC (polymethyl ethylene carbonate) or polyethylene terephthalate (PET). These materials are suitable as a medium for achieving signal enhancement due to their excellent optical properties, chemical stability and physical properties.

[0057] PDMS is a flexible silicone material with high optical transparency. Its transmittance in the visible light range of 400nm to 700nm is usually over 95%, and its refractive index is about 1.40. This high transparency ensures that the film does not introduce significant light loss during the enhancement of the mechanical light scattering spectral signal. In addition, PDMS has good flexibility and mechanical stability, and can be evenly covered on substrates of different shapes or with high surface roughness. Its easy processing characteristics are also one of its important advantages. The preparation of thin films with uniform thickness can be easily achieved through spin coating.

[0058] PMMA is a common organic glass material with a refractive index of about 1.49 and a light transmittance of over 93%. Compared with PDMS, PMMA has higher hardness and stronger solvent resistance, so it is particularly suitable for protecting samples that require long-term testing or frequent handling.

[0059] PET is a polymer material with excellent chemical stability and environmental tolerance, and is widely used in industry and experimental research. The refractive index of PET film is around 1.50, with good optical transparency and mechanical strength. Its excellent moisture resistance and oxidation resistance make it particularly suitable for protecting plasmonic metal surfaces.

[0060] Specifically, the light transmittance of the film is preferably greater than 93%, and / or the refractive index of the film is preferably 1.2-2.0.

[0061] Transmittance refers to the proportion of light that a film material allows to pass through. A design with a transmittance greater than 93% ensures efficient transmission of incident light and scattered light signals in the film, reducing energy loss caused by the film's own absorption or scattering. Mechanically scattered light signals are essentially weak signals with weak energy and are easily interfered by background noise. If the film's transmittance is insufficient, the transmission efficiency of the light signal will be significantly weakened, resulting in a weak interference enhancement effect. Therefore, choosing materials with higher transmittance (such as PDMS, PMMA, or PET) can help achieve signal enhancement.

[0062] The refractive index is a key parameter that affects the propagation path and phase of light in the film. Its range is designed to be 1.2 to 2.0, mainly to optimize the constructive interference conditions. The interference enhancement effect requires that the propagation path and phase difference of the incident light in the film meet specific conditions. The refractive index determines the propagation speed and optical path difference of light, which in turn affects the design of the film thickness. For example, for a laser with a wavelength of 532nm, when a PDMS film with a refractive index of 1.40 is selected, constructive interference of the incident light can be achieved by adjusting the thickness to about 95nm, thereby significantly amplifying the scattered signal. In addition, the design with a refractive index range of 1.2 to 2.0 can cover a variety of common transparent materials, ensuring the flexibility and wide applicability of film design.

[0063] In one embodiment of the present application, a structure for measuring a mechanical light scattering spectrum signal is provided, which includes: a substrate and an optically transparent film.

[0064] A substrate is coupled with a plasmon metal layer; an optically transparent film covers the surface of the plasmon metal layer; the thickness d of the film satisfies the condition: d=[(2m-1)·λ] / (4n·cosθ), so that incident light used to excite the plasmon metal in the plasmon metal layer can interfere constructively in the film; wherein m is the interference order, λ is the wavelength of the incident light, n is the refractive index of the film, and θ is the incident angle.

[0065] The substrate is the load-bearing part of the entire structure, providing physical support for the plasmonic metal layer and transparent film. The selection of the substrate needs to consider its compatibility with the plasmonic metal layer and its ability to maintain stability during the test. Commonly used substrate materials include glass, silicon wafers and aluminum oxide films, which have good surface flatness and mechanical stability, and can ensure the uniformity of the adhesion of the metal layer and the film.

[0066] The plasmon metal layer is the core component of signal enhancement. It concentrates the energy of the incident light into a very small area on the metal surface through the localized surface plasmon resonance (LSPR) mechanism, thereby significantly enhancing the intensity of the light field. The plasmon metal layer is preferably made of silver, copper or gold, which have excellent plasmon resonance properties in the visible and near-infrared bands. Silver has a higher resonance intensity, but its surface is susceptible to oxidation and corrosion; gold performs better in terms of chemical stability. By selecting a suitable plasmon metal layer and utilizing its light field enhancement properties, the intensity of the mechanical light scattering signal can be effectively improved.

[0067] The optically transparent film covering the surface of the plasmon metal layer is a key part to achieve the constructive interference effect. The design requirements of the film have been described in detail in the above embodiments and will not be repeated here.

[0068] Please refer to Figure 1 and Figure 2 , which is a schematic diagram of the principle of the interference effect enhancement mechanism in one embodiment of the present application. Figure 1 and Figure 2 The optical interference effect of thin films is demonstrated in detail, especially the process of enhancing the mechanical Raman scattering signal through constructive interference. Figure 1 Schematic diagram of the direction of the electric field of the incident light and scattered light in mechanical Raman scattering. Figure 2 Schematic diagram of the optical field direction of mechanical Raman scattering enhanced by thin film interference.

[0069] Ginseng Figure 1 and Figure 2As shown, the incident light used to excite the plasmon metal in the plasmon metal layer, the forward incident light electric field vector amplitude and the backscattered light electric field vector amplitude enhanced by interference in the film can be expressed as:

[0070]

[0071] in, represents the electric vector amplitude of the forward excitation light, represents the sum of all forward excitation photoelectric vector amplitudes, represents the amplitude of the forward excitation photoelectric vector after two reflections each time, represents the electric vector amplitude of the backscattered light, represents the sum of all backscattered photoelectric vector amplitudes, represents the backscattered photoelectric vector amplitude after two reflections each time, t ij represents the ratio of the amplitude of the transmitted electric vector of light propagating from medium i to medium j at the medium interface, r ij k represents the ratio of the reflected electric vector amplitude of light propagating from medium i to medium j at the medium interface, i represents the wave vector of the excitation light, k s represents the scattered light wave vector, z represents the z-direction coordinate, ω represents the frequency of light, t represents time, and d 薄膜 Indicates the film thickness, 0 represents air medium, 1 represents thin film medium, and 2 represents plasmon metal medium.

[0072] For forward incident light, the total electric field is the initial incident light electric field The result of the superposition of transmission and reflection when passing through the interface of air (medium 0), thin film (medium 1) and plasmon metal layer (medium 2). Similarly, the total electric field of the backscattered light is the scattered light After multiple reflections and transmissions, the denominators of the two formulas reflect the core of the interference effect. When the thickness d of the film meets the constructive interference condition, the denominator approaches the minimum value and the electric field intensity is significantly amplified. In actual implementation, the above formulas can be used to guide the design and parameter selection of the film.

[0073] From a physical mechanism perspective, these two formulas illustrate the dual effects of light field enhancement. First, the enhancement of the forward incident light increases the local light field intensity of the plasmon metal layer, making the coupling between the metal layer and the mechanical oscillator more significant, thereby amplifying the output of the mechanical Raman scattering signal. Second, the enhancement of the backscattered light field intensity ensures that the scattered signal can be effectively transmitted back to the detector, reducing signal attenuation and improving the sensitivity of signal acquisition; this dual enhancement effect is particularly important for weak signal detection (such as low-frequency mechanical vibration modes).

[0074] Furthermore, the total interference enhancement factor Factor of the film can be expressed as:

[0075]

[0076] The formula of the film interference enhancement factor (Factor) describes the total field intensity enhancement effect of the incident light and scattered light in the multi-layer reflection and transmission process of the film. This formula is based on the theory of optical interference and combines the transmission and reflection characteristics of the film to quantitatively express the amplification effect of the film structure on the mechanical light scattering spectrum signal.

[0077] The enhancement factor in the formula consists of two main components: the total field intensity of the forward incident light and the total field intensity of the backscattered light These two components describe the change in field intensity caused by multiple reflections and transmissions during the forward and backward propagation of light in the thin film structure. Specifically, the reflectivity and transmittance of the upper and lower interfaces of the film and the phase change of the light determine the interference effect. The denominator of the formula is the constructive or destructive condition caused by the superposition of multiple reflections of light. When the film thickness d meets the constructive interference condition, the denominator value is the smallest and the total field intensity is significantly amplified.

[0078] In addition to mechanical Raman scattering, other spectral techniques based on monochromatic light excitation, such as Brillouin scattering, also involve multiple reflections and interference processes between light and thin film media during signal detection. Their physical mechanisms are highly similar to the interference effects of the aforementioned mechanical Raman scattering in thin films. Therefore, the design concept of this application is not only suitable for the enhancement of mechanical Raman scattering spectral signals, but can also be widely used in other types of mechanical light scattering spectral techniques.

[0079] Take Brillouin scattering as an example. This is a spectral technique that detects the acoustic properties and elastic modulus of materials through the interaction of sound and light. The signal is usually weak and easily interfered by background noise. By introducing the optically transparent film designed in this application on the surface of the sample and utilizing the constructive interference effect to enhance the forward field intensity of the incident light and the back output of the scattered signal, the intensity and signal-to-noise ratio of the Brillouin scattering signal can be significantly improved, thus expanding its detection capability.

[0080] Similarly, for other monochromatic light-excited spectroscopy techniques, such as ultra-low frequency Raman spectroscopy and photoacoustic spectroscopy, the thin film interference enhancement method proposed in this application can also provide effective support. By optimizing the thin film material, thickness and refractive index parameters, it can flexibly adapt to the needs of different laser wavelengths and spectral techniques, and has strong applicability and versatility. Therefore, this application not only provides an innovative solution for the enhancement of mechanical light scattering spectroscopy, but also opens up a new path for signal optimization of other spectral techniques.

[0081] Based on the same inventive concept as the aforementioned structure for measuring mechanical light scattering spectral signals, please refer to Figure 3 As shown, the present application also provides a method for measuring a mechanical light scattering spectral signal, which comprises the following steps:

[0082] S301: Covering an optically transparent film on the surface of the sample coupled with the plasmon metal.

[0083] Before the thin film is spin-coated, a sample coupled with a plasmon metal is prepared. Plasmon metals usually refer to metal materials, such as silver and gold, which have the property of exciting localized surface plasmon resonance. In this application, the plasmon metal layer is usually a thin layer, which is evenly covered on the surface of the sample by evaporation, sputtering, etc., and can be coupled with a mechanical oscillator, so that it can be excited by light to generate a mechanical Raman scattering signal.

[0084] The deposition of plasmonic metal layers uses evaporation or sputtering processes, both of which are typical techniques of physical vapor deposition (PVD) and are suitable for preparing uniform and dense metal thin layers. In the electron beam evaporation process, metal materials (such as silver or gold) are placed in an evaporation boat under a high vacuum environment, heated by an electron beam to evaporate them into metal atoms, and deposited on the surface of the sample to form a thin film. This method can control the thickness of the metal film, usually with an accuracy of nanometers, and is very suitable for preparing plasmonic metal layers to achieve efficient excitation of localized surface plasmon resonance. The sputtering process uses high-energy ions to bombard metal targets, causing metal atoms to pop out of the target surface and deposit on the sample surface. The advantage of sputtering is that the film has a strong uniformity and adhesion, and is particularly suitable for metal deposition on rough or non-flat samples.

[0085] Next, select a suitable film material for covering. For example, PDMS or PMMA can be selected as the material of the interference film. These materials have appropriate refractive index, transparency and chemical stability, and can form a significant interference effect under incident light. PDMS and PMMA, as film materials, have different physical properties, but both can meet the requirements of this application for film thickness and stability. PDMS materials are usually supplied in liquid form and need to be mixed with a cross-linking agent and degassed to ensure the uniformity and quality of the film during spin coating; PMMA is prepared by a solution method, dissolving methyl methacrylate (MMA) in a solvent to form a coating liquid for easy spin coating.

[0086] The coverage of the thin film on the sample surface can be achieved by a spin coating process. In one embodiment of the present application, an appropriate amount of PDMS or PMMA solution is added dropwise to the prepared sample surface, and the sample is fixed on the rotating table of the spin coater. By driving the sample to rotate, the coating is evenly spread on the sample surface to form a thin film. During the spin coating process, the thickness of the film is closely related to the rotation speed and time. Generally speaking, the faster the spin coating speed, the longer the time, the thinner the film. In order to ensure that the constructive conditions of the interference effect are met, it is necessary to accurately control the thickness d of the film, and adjust the spin coating process according to the thin film interference constructive formula: d = [(2m-1)·λ] / (4n·cosθ) to achieve the optimal film thickness and ensure that the interference effect significantly enhances the mechanical scattered light signal. Wherein, m is the interference order, λ is the wavelength of the incident light, n is the refractive index of the film, and θ is the angle of incidence.

[0087] S302: performing a curing treatment on the film to make the film adhere to the surface of the sample.

[0088] After the film is coated, it needs to be cured so that the film forms a stable and strong bond with the sample surface to ensure the stability of the film. The meaning of curing is to convert the liquid film material coated on the sample surface into a stable solid film by chemical crosslinking or physical drying. This conversion process depends on the characteristics of the film material. For example, PDMS film is crosslinked and cured under heating conditions through the chemical action of the crosslinking agent; PMMA film forms a uniform solid film through the volatilization of the solvent and heat treatment. The cured film not only has good optical transparency, but also can form a tight adhesion on the sample surface to prevent the film from falling off due to external force or environmental factors.

[0089] In terms of specific implementation, the curing conditions (such as temperature and time) can be optimized according to the characteristics of the film material. The curing temperature is preferably 60 to 200°C, and the curing time is preferably 1 to 150 minutes. For example, for PDMS film, after coating, the sample can be placed in a constant temperature oven at 80°C for 2 hours to make the film completely cross-linked and firmly bonded to the sample surface. This heating treatment can significantly improve the mechanical properties and surface uniformity of the film, ensuring its reliability in subsequent tests. For PMMA film, by quickly heating to 120°C and maintaining it for 15 minutes, the solvent can be completely evaporated to form a uniform and transparent film structure.

[0090] In other embodiments, the cured film may be directly attached to the sample surface to achieve coverage of the sample surface by the film.

[0091] S303: Using a laser with a plasmon resonance wavelength to irradiate the sample through the film to excite the plasmon metal in the sample, and collecting a mechanical light scattering spectrum signal of the sample.

[0092] The plasmon metal layer in the sample is excited by laser of a specific wavelength, and the interference enhancement effect of the optically transparent film is used to amplify the mechanical light scattering signal and collect the signal. Plasmon resonance is a resonance phenomenon caused by the interaction between free electrons in metal nanostructures or films and incident light. By selecting a laser wavelength that matches the characteristics of plasmon metals, the local light field can be excited to the greatest extent, thereby significantly enhancing the coupling between light and mechanical vibrations.

[0093] The laser irradiates the sample through the film, and the optical interference effect of the film is combined to further amplify the intensity of the local light field. The thickness d of the film satisfies the constructive interference condition: d = [(2m-1)·λ] / (4n·cosθ), so that the amplitude of the light field formed after multiple reflections of the incident light is superimposed to achieve the maximum enhancement effect. At this time, the interaction efficiency between the local light field of the plasmon metal layer and the vibration of the mechanical oscillator is greatly improved, and the output intensity of the mechanical light scattering signal is also significantly improved.

[0094] The signal is collected by a matching spectral detection system, which usually includes a highly sensitive photomultiplier tube (PMT) or a spectral camera. For the scattered signals of low-frequency mechanical vibrations (such as shear phonon modes), an ultra-low frequency Raman spectrometer is a suitable choice. Based on the collected spectral signals, the enhanced signal intensity can be further analyzed and evaluated to verify the enhancement effect brought by the interference effect.

[0095] Figure 4 Shown is a mechanical Raman scattering spectrum diagram of a sample based on interference enhancement in one embodiment of the present application. Figure 4 It was demonstrated that under the interference enhancement of PDMS film of appropriate thickness, the mechanical Raman scattering signal excited by 532nm incident light was significantly enhanced.

[0096] Figure 4 The blue solid line in the figure shows the mechanical Raman scattering spectrum of graphite lattice oscillators without film interference enhancement. The inset is the optical microscopic imaging of the test sample, and the blue dots are the sample excitation area. Without interference enhancement, for graphite lattices with optical contrast less than 40 layers (thickness < 13.4nm), the mechanical Raman scattering signal is almost indistinguishable, indicating that the mechanical Raman scattering method that relies solely on plasmon-mediated coupled mechanical oscillators can no longer effectively detect layered materials of this thickness.

[0097] However, after thin film interference enhancement, e.g. Figure 4The mechanical Raman scattering spectra of 66 layers (thickness about 22.11nm), 51 layers of graphite lattice (thickness about 17.805nm) and 41 layers of graphite (thickness about 13.735nm) are shown. The inset is the optical microscopic imaging of the tested sample, and the dots are the sample excitation area. The mechanical Raman scattering signal shows an obvious enhancement effect, and the low-frequency shear phonon mode becomes fully distinguishable, such as the individual shear modes S shown by the gray arrows. 66,57 ,S 51,46 ,S 41,37 , etc., can directly correspond the spectral peak frequency of the mechanical Raman signal with the shear phonon frequency of the graphite lattice calculated by the linear chain model, and accurately identify the samples as 66-layer, 51-layer and 41-layer graphite lattices. Through the interference effect, the film enhances the mechanical Raman signal while greatly improving the identifiability of the low-frequency signal, thus verifying the effectiveness of the interference enhancement method proposed in this application in enhancing the Raman scattering signal in the ultra-low wavenumber range.

[0098] Figure 5 This is an atomic force microscope (AFM) test image of an interference enhancement film sample in an embodiment of the present application. Figure 5 The surface morphology data of the PDMS film on the sample surface obtained by the AFM probe is shown. In the AFM scanned image, the thickness of the film can be calculated by analyzing the height difference between the film and the substrate. The high resolution of AFM can accurately capture the tiny height changes of the film and provide nanometer-level thickness accuracy.

[0099] According to the constructive interference formula, the film thickness d = [(2m-1)·λ] / (4n·cosθ), where the incident light wavelength λ = 532nm, the PDMS film refractive index n = 1.4, the vertical incident angle θ = 0, and the interference order m = 1, the calculated thickness of the constructive interference film d = 95.14nm. In the embodiment of the present application, the thickness of the PDMS film measured by AFM is about 90nm, which is very close to the calculated constructive interference condition, that is, it meets the thickness requirement in the formula. It shows that the film thickness prepared under this condition can produce the best constructive interference effect with the incident light wavelength, thereby significantly enhancing the mechanical Raman scattering signal.

[0100] In application scenarios where plasmonic metals are prone to oxidation, the film not only plays a role in interference enhancement, but also has the function of protecting the metal surface. In particular, PDMS and PMMA materials have good chemical stability and can effectively prevent the metal surface from contacting with moisture or oxygen in the air, thereby avoiding metal oxidation or corrosion. Due to its excellent hydrophobicity and chemical stability, PDMS can effectively isolate moisture and oxygen in the air and protect the plasmonic metal surface from environmental factors such as oxidation. PMMA has high chemical stability and can provide an effective protective film for the plasmonic metal surface, reducing potential risks such as oxidation and corrosion, and extending the service life of the sample.

[0101] Figure 6 This is a mechanical Raman scattering spectrum of a thin film protected sample in an embodiment of the present application. In this embodiment, the effects of sample storage conditions under different protection measures on the mechanical Raman scattering signal intensity are further compared. The mechanical Raman scattering spectrum signals of three groups of samples that were always stored in a vacuum environment of -0.1MPa under different conditions: the first group is a graphite lattice sample without film protection and stored for 3 days; the second group is a graphite lattice sample protected by PDMS film and stored for 40 days; the third group is a graphite lattice sample without protection and stored for 15 days. By analyzing these spectra, it can be clearly seen that the film protection has a significant impact on the signal intensity and stability.

[0102] In the unprotected samples stored for 3 days, resolvable mechanical Raman scattering spectral signals can still be observed, indicating that in a short period of time, the unprotected sample surface has not undergone obvious oxidation or other destructive changes, and a certain signal intensity can be maintained. However, in the unprotected samples stored for 15 days, the mechanical Raman scattering signal is significantly weakened or even completely disappears. This phenomenon can be attributed to the oxidation reaction of the metal surface after contact with oxygen and moisture in the air, resulting in the formation of a non-conductive oxide layer on the metal surface, which inhibits the excitation of surface plasmons and the LSPR effect, thereby significantly reducing the intensity of the mechanical Raman scattering signal. Even in a vacuum environment, although there is no formation of an oxide layer, the long-term contact of the metal surface with moisture and pollutants in the air may still have an impact on the signal.

[0103] In the PDMS film-protected sample stored for 40 days, we can see a significantly enhanced and stable mechanical Raman scattering signal, which shows that the PDMS film protects the sample and effectively maintains the integrity of the Raman scattering signal after long-term storage. This shows that the PDMS film can effectively isolate the influence of environmental factors such as oxygen and moisture on the metal surface, thereby maintaining the plasmon effect and scattering enhancement effect, extending the service life of the sample, while ensuring the stability and repeatability of the scattering signal.

[0104] In summary, the film, structure and method for enhancing the mechanical light scattering spectral signal provided in the present application can effectively improve the detectability of low-frequency mechanical Raman scattering signals; the film thickness and material selection are optimized by utilizing the constructive interference effect, thereby achieving a significant enhancement of the mechanical Raman scattering signal; the application of the interference film makes the originally undetectable signal clearly discernible, broadening the application space of mechanical Raman scattering technology in deeper sample detection.

[0105] In addition, by using thin film materials such as PDMS, this application not only enhances the scattering signal, but also provides long-term protection for the sample, prevents the influence of environmental factors such as surface oxidation, ensures the signal stability under different storage conditions, maintains the reliability and repeatability of the experimental results, and provides a guarantee for the long-term application of mechanical Raman scattering technology.

[0106] It is obvious to those skilled in the art that the present application is not limited to the details of the exemplary embodiments described above, and that the present application can be implemented in other specific forms without departing from the spirit or essential features of the present application. Therefore, the embodiments should be regarded as exemplary and non-limiting in all respects, and the scope of the present application is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims be included in the present application. Any reference numeral in a claim should not be regarded as limiting the claim to which it relates.

[0107] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A film for enhancing mechanical light scattering spectral signals, characterized in that: The material of the film is an optically transparent material, and the film is used to cover the surface of the substrate coupled with the plasmon metal; The thickness d of the film satisfies the condition: d = [(2m-1)·λ] / (4n·cosθ), so that the incident light used to excite the plasmon metal can interfere constructively in the film; wherein m is the interference order, λ is the wavelength of the incident light, n is the refractive index of the film, and θ is the incident angle.

2. The film for enhancing mechanical light scattering spectral signal according to claim 1, characterized in that: The material of the film is PDMS, PMMA, PVA, PPC or PET.

3. The film for enhancing mechanical light scattering spectral signal according to claim 1, characterized in that: The light transmittance of the film is greater than 93%, and / or the refractive index of the film is 1.2-2.

0.

4. A structure for measuring mechanical light scattering spectral signals, characterized in that: include: A substrate coupled with a plasmonic metal layer; An optically transparent film covering the surface of the plasmon metal layer; The thickness d of the film satisfies the condition: d = [(2m-1)·λ] / (4n·cosθ), so that the incident light used to excite the plasmon metal in the plasmon metal layer can interfere constructively in the film; wherein m is the interference order, λ is the wavelength of the incident light, n is the refractive index of the film, and θ is the incident angle.

5. The structure for measuring mechanical light scattering spectrum signal according to claim 4, characterized in that: The material of the plasmon metal layer includes at least one of silver, gold and copper.

6. The structure for measuring mechanical light scattering spectrum signals according to claim 4, characterized in that: The incident light used to excite the plasmon metal in the plasmon metal layer, the forward incident light electric field vector amplitude and the backscattered light electric field vector amplitude enhanced by interference in the film are respectively expressed as: in, represents the electric vector amplitude of the forward excitation light, represents the sum of all forward excitation photoelectric vector amplitudes, represents the amplitude of the forward excitation photoelectric vector after two reflections each time, represents the electric vector amplitude of the backscattered light, represents the sum of all backscattered photoelectric vector amplitudes, represents the backscattered photoelectric vector amplitude after two reflections each time, t ij represents the ratio of the amplitude of the transmitted electric vector of light propagating from medium i to medium j at the medium interface, r ij k represents the ratio of the reflected electric vector amplitude of light propagating from medium i to medium j at the medium interface, i represents the wave vector of the excitation light, k s represents the scattered light wave vector, z represents the z-direction coordinate, ω represents the frequency of light, t represents time, and d 薄膜 Indicates the film thickness, 0 represents air medium, 1 represents thin film medium, and 2 represents plasmon metal medium.

7. The structure for measuring mechanical light scattering spectrum signals according to claim 6, characterized in that: The total interference enhancement factor of the film is expressed as:

8. A method for measuring mechanical light scattering spectral signals, characterized in that: include: An optically transparent film is covered on the surface of the sample coupled with the plasmonic metal; Performing a curing treatment on the film so that the film adheres to the surface of the sample; Using a laser with a plasmon resonance wavelength to irradiate the sample through the film to excite the plasmon metal in the sample, and collecting a mechanical light scattering spectrum signal of the sample; Among them, the thickness d of the film satisfies the condition: d=[(2m-1)·λ] / (4n·cosθ), so that the incident light used to excite the plasmon metal can interfere constructively in the film, m is the interference order, λ is the wavelength of the incident light, n is the refractive index of the film, and θ is the incident angle.

9. The method for measuring mechanical light scattering spectrum signal according to claim 8, characterized in that: The plasmon metal is deposited on the sample surface by evaporation or sputtering process; and / or the film is coated on the sample surface by spin coating process or the cured film is directly attached to the sample surface.

10. The method for measuring mechanical light scattering spectrum signal according to claim 8, characterized in that: The curing temperature of the film is 60-200° C., and the curing time is 1-150 minutes.

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