Near-field probe optimization method suitable for terahertz nanometer spectrum technology

Abstract

The invention relates to a near-field probe optimization method suitable for a terahertz nano-spectrum technology, and the method comprises the steps: employing a customized AFM probe, carrying out the experiment, combining with the analysis of full-wave electromagnetic simulation, and systematically exploring the regulation and control rule of the curvature radius of the tip of the probe on the imaging and wide-spectrum signals of a scattering type scanning near-field optical microscope working at 0.5-1.5 THz. The relation between the curvature radius R of the tip end of the probe and the intensity of the near-field scattering signal is determined and used for optimizing the near-field probe, and the near-field scattering signal with the wide frequency spectrum of 0.5-1.5 THz can be remarkably improved by optimizing the size of the tail end of the tip end of the AFM probe.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of scattering-type scanning near-field optical microscopy, and particularly relates to a near-field probe optimization method suitable for terahertz nanospectroscopy. BACKGROUND

[0002] Scattering-type scanning near-field optical microscopy (s-SNOM) is an emerging terahertz nanoscale optical imaging and spectroscopy technique, which can realize nanoscale imaging and spectroscopy from visible light to terahertz frequency, and is applied to material science, semiconductor technology, photonics, biomedicine and other fields.

[0003] s-SNOM adopts a tapping mode, and the probe intermittently taps the sample surface at a certain frequency under the driving of a piezoelectric ceramic, and a displacement table moves the sample according to a set program. In s-SNOM, the incident laser is focused on the tip end of an atomic force needle through a parabolic mirror, and due to the probe antenna effect, lightning rod effect and the like, the incident electromagnetic field can be locally enhanced at the tip vertex region, forming a near-field spot with a characteristic scale of tens of nanometers (i.e. "nanofocusing" effect), so that a near-field interaction is generated between the near-field spot and the sample to be measured. Then, the scattering signal carrying the sample information is scattered through the needle tip and detected by a far-field optical detector.

[0004] However, the low signal-to-noise ratio of THz s-SNOM hinders its wide application to some extent. This problem is particularly serious in the low-frequency terahertz (<2 THz). Therefore, effectively improving the signal-to-noise ratio of THz s-SNOM has become a key problem to be solved in the current field. SUMMARY

[0005] In view of the low near-field scattering signal of s-SNOM working at 0.5-1.5 THz, a near-field probe optimization method suitable for terahertz nanospectroscopy is proposed. Through the combination of experimental research and theoretical simulation, the influence of the customized near-field probe with different geometric parameters on the near-field signal intensity is systematically explored, and the near-field scattering signal of THz s-SNOM is effectively improved.

[0006] The technical scheme of the present application is as follows: a near-field probe optimization method suitable for terahertz nanospectroscopy, specifically comprising the following steps:

[0007] 1) A six-axis micro-displacement table is used to coaxially calibrate the parabolic mirror, reference arm mirror and detector to ensure the best signal receiving conditions;

[0008] 2) Collect the 0th to 5th order near-field time-domain signals of the gold substrate sample under s-SNOM conditions with probes of different tip curvature radius sizes, including the amplitude and phase signals in the time domain;

[0009] 3) Convert the time-domain signals to frequency-domain signals using FFT, and then analyze the variation law of the signals with the tip curvature radius;

[0010] 4) In a commercial numerical simulation software, a model matching the experimental probe geometry in step 2) is constructed, and finite element simulation is performed to determine the scattering dipole moment p and the probe surface charge density σ s with frequency, and how they are affected by the tip curvature radius R;

[0011] 5) Through analysis of the experimental and simulation results, the relationship between the tip curvature radius R of the probe and the near-field scattering signal strength is established, which is used for near-field probe optimization.

[0012] Further, the specific implementation method of step 1) is as follows: After installing the probe, the parabolic mirror, reference arm mirror and detector in s-SNOM are coaxially calibrated by adjusting the six-axis micro-displacement table with an accuracy of +0.1 mm. During the coaxial calibration process, the key data of s-SNOM "signal-to-noise ratio" and "second-order amplitude" are observed in real time to ensure that the "signal-to-noise ratio" and "second-order amplitude" are close to the maximum values under the experimental conditions.

[0013] Further, the specific implementation method of step 3) is as follows:

[0014] A: Considering the influence of background signal and incident laser intensity, a group of second or third order time-domain signals in the near-field signal is selected;

[0015] B: Due to phase shift, the real and imaginary parts of the field are complex, and the original phase signal can be moved by adding a phase shift to the entire phase signal , which will ensure that the phase now oscillates between 0 and π;

[0016] C: Convert the time-domain signal to a frequency-domain signal by the following formula:

[0017]

[0018] where A corresponds to the time-domain amplitude in step 2), corresponds to the time-domain phase in step 2), the absolute value of the minimum negative value of the time-domain phase in step 2), and the real part A represents the near-field scattering signal strength.

[0019] Further, the specific implementation method of step 4) is as follows:

[0020] the scattering dipole moment p and the probe surface charge density σs The formula and the tip scattering cross section S were input into COMSOL Multiphysics software, and the frequency parameter was set in the parametric sweep status bar. The parametric sweep obtained the frequency-dipole moment spectrum in the frequency range of 0.5-1.5 THz;

[0021] The formula for the scattering dipole moment p is:

[0022] p=|∫∫ s σ s SdS|;

[0023] S probe surface charge density σ s The formula is:

[0024] σ s ∝E·n;

[0025] In the above formula, E is the incident electric field intensity, n is the external normal of the tip surface, and S is the tip scattering cross section, which can be calculated by photographing the curvature radius R of the probe tip with a scanning electron microscope.

[0026] Furthermore, in step 5), through analysis of experimental and simulation results, the near-field scattering signal is positively correlated with the tip curvature radius R of the probe in the wide frequency range of 0.5-1.5 THz, that is, the near-field scattering frequency domain signal is enhanced with the increase of the tip curvature radius R.

[0027] The present invention provides a near-field probe optimization method for terahertz nanospectroscopy. Using a customized AFM probe and combined with full-wave electromagnetic simulation, the method systematically explores how the probe tip radius of curvature influences the imaging and broad-spectrum signal of a scattering-type scanning near-field optical microscope operating in the 0.5-1.5 THz range. By optimizing the AFM probe tip size, the near-field scattering signal across the 0.5-1.5 THz wide spectrum can be significantly enhanced. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A schematic structural diagram of a probe according to the probe optimization method for terahertz scattering scanning near-field according to the present invention;

[0029] Figure 2 The frequency domain spectral signals of the second-order and third-order near-field scattering of the probe and the probe frequency domain signal fitting diagram obtained by the probe optimization method suitable for terahertz scattering scanning near field according to the present invention;

[0030] Figure 3 The dipole moment spectrum of the needle tip obtained by the probe optimization method of the present invention and the dependence diagram of the dipole moment on R change are shown. DETAILED DESCRIPTION

[0031] The application will be described in detail below with reference to the drawings and specific embodiments. The embodiments are implemented on the premise of the technical solutions of the application, and detailed implementation modes and specific operation processes are given, but the protection scope of the application is not limited to the following embodiments.

[0032] It is found through experiments that the THz s-SNOM signal intensity increases with the increase of the curvature radius of the needle tip, proving that the near-field signal intensity of THz s-SNOM is not determined by the electric field at the tip end alone, but by the scattering dipole moment at the tip end (the scattering dipole moment is directly proportional to the product of the end electric field and the needle scattering cross section). Through systematic simulation of the electric field and scattering dipole moment spectrum of different needle tip end curvature radii R, the following key mechanisms are revealed: (1) with the increase of the needle tip curvature radius R, the electric field intensity decreases with the increase of the radius, but the change trend is obviously slower than that in the visible-infrared-terahertz high frequency band. (2) Unlike the electric field, the scattering dipole moment at the tip end of the needle increases monotonously with the increase of the needle tip curvature radius R, which is highly consistent with the experimental results.

[0033] Reference Figures 1-3 A probe optimization method suitable for terahertz scattering type scanning near field, which adjusts the near-field scattering signal intensity by adjusting the curvature radius of the tip of the probe or the scattering dipole moment at the tip end of the needle.

[0034] The scattering dipole moment describes the dipole moment generated by the scatterer in the process of electromagnetic wave scattering. Dipole moment is a physical quantity describing the charge distribution, which is usually used to describe the charge distribution of molecules or atoms. In electromagnetic scattering theory, the scattering dipole moment refers to the electric or magnetic dipole moment generated by the scatterer under the action of electromagnetic waves. These dipoles can affect the characteristics of scattered waves, such as scattering intensity and scattering direction.

[0035] The scattering dipole moment at the tip end of the near-field probe is the key mechanism to determine the signal intensity of THz s-SNOM. Its calculation formula is:

[0036] E sca ∝p=|∫∫ s σ s SdS| (1)

[0037] Where E sca is the near-field scattering signal intensity received by the detector, p is the scattering dipole moment at the tip end of the probe, σ s is the surface charge density of the probe, and S is the scattering cross section of the needle tip.

[0038] Where the surface charge density σ s σ s ∝E·n(2)

[0039] In the formula, E is the incident electric field intensity, n is the outer normal of the tip surface, S is the needle tip scattering cross section, which can be obtained by calculating after taking a probe tip radius R by scanning electron microscope (SEM).

[0040] Figure 1 Figure 1 is a schematic diagram of a gold substrate tested by a terahertz scattering scanning near-field optical microscope, L is the length of the probe, and the probe tip is equivalent to a small ball with a radius R.

[0041] A probe optimization method suitable for terahertz scattering scanning near-field is provided, and the specific steps are as follows:

[0042] Step 1: After installing the probe, the parabolic mirror, reference arm mirror and detector in s-SNOM are coaxially calibrated by six-axis micro-displacement table (adjustment accuracy +0.1mm). During the coaxial calibration process, the key data such as s-SNOM "signal-to-noise ratio" and "second-order amplitude" are observed in real time to ensure that the "signal-to-noise ratio" and "second-order amplitude" are close to the maximum value under the experimental conditions.

[0043] Step 2: For the s-SNOM calibrated and optimized in step 1, a standard sample-gold substrate is used as the sample to be tested, and probes with different tip curvature radius sizes are used to detect the sample-gold substrate to obtain terahertz near-field signals 0th to 5th order near-field time-domain signals of different tip curvature radius sizes, wherein the time-domain signals include amplitude signals and phase signals.

[0044] Step 3: The time-domain signals obtained in step 2 are converted to 0th to 5th order near-field frequency-domain signals by FFT.

[0045] The specific steps of the conversion are as follows:

[0046] 1) Considering the influence of background signal and incident laser intensity, a group of second or third order time-domain signals in step 2 is selected;

[0047] 2) Due to the phase shift, the real and imaginary parts of the field are complex, and the original phase signal can be moved by adding a phase shift to the entire phase signal , which will ensure that the phase now oscillates between 0 and π;

[0048] 3) The time-domain signal is converted to a frequency-domain signal by the following formula:

[0049]

[0050] where A corresponds to the time-domain amplitude in step 2, corresponds to the time-domain phase in step 2, the absolute value of the minimum negative value of the time-domain phase in step 2, and the real part A represents the near-field scattering signal intensity.

[0051] By steps 1, 2, 3, the frequency domain spectrum signals of the second-order and third-order near-field scattering can be obtained, such as Figure 2 On the left, the frequency domain spectrum signals of the second-order and third-order near-field scattering of the probes with the probe length of 150 μm and the tip curvature radius R = 20 nm, 40 nm and 500 nm are respectively shown. It can be clearly observed that the near-field scattering signals increase with the increase of the tip curvature radius in the 0.5-1.5 THz wide frequency range. Figure 2 The right side shows the curves of the second-order near-field scattering frequency domain signals with the tip curvature radius R at 0.52 THz. As shown in the left side of the figure, Figure 2 As shown in the right side, the near-field scattering signals are positively correlated with R, and the signal of the R = 500 nm probe is increased by nearly 3 times than that of the R = 20 nm probe.

[0052] Step 4, the finite element (FEM) method is used to reproduce the experimental results in the commercial numerical simulation software (COMSOL Multiphysics). In the simulation, the probe tip is modeled as a hemispherical tip end, and a cuboid is used to represent the probe cantilever. The numerical simulation probe geometry is very consistent with the probe geometry in the experiment. The probe length L is fixed at 150 μm, the probe tip curvature radius R is changed from 20 nm to 500 nm, and the sample to be tested is a gold substrate. The incident light field is set as the P-polarized light consistent with the experiment, and the incident wave vector is incident at 30° relative to the probe surface.

[0053] Step 5, the formula (4), (2), (5) is input into the COMSOL Multiphysics software, and the frequency parameter is parameterized in the parameterized scanning status bar. The frequency-dipole moment spectrum in the 0.5-1.5 THz frequency range is scanned, and the formula of the scattering dipole moment p is:

[0054] p = |∫∫ s σ s SdS| (4)

[0055] S is the tip scattering cross section. Since the probe tip is modeled as a hemispherical tip, the formula for calculating the tip scattering cross section is:

[0056] S = 4πR 2 (5)。

[0057] Figure 3 According to the dipole moment spectrum of the tip obtained by the probe optimization method of the application and the dependence of the dipole moment on R, (a) is the numerical calculation result of the dipole moment of the probe with the tip curvature radius R = 20 nm, 40 nm, 200 nm and 500 nm, and (b) is the dependence of the dipole moment on R.

[0058] Step 6: The relationship between the tip radius R of the probe and the near-field scattering signal intensity is established by analyzing the experimental results of steps 1-3 and the simulation results of steps 4-5, which is used for the optimization of the near-field probe.

[0059] The steps for replacing the probe are as follows:

[0060] 1) Confirm that the controller power, laser switch, and software are all turned on;

[0061] 2) Open the top soundproof cover and gently pull out the scanning probe;

[0062] 3) Use a screwdriver to unscrew the probe holder and place it on the heating stage. Heat it to 100°C to dissolve the hot melt glue;

[0063] 4) Replace the probe with a different tip radius size. Place the probe holder on the sample stage and cool it for 2 minutes;

[0064] 5) Use tweezers to place the probe holder on the scanning probe and tighten the screws with a screwdriver to secure the probe holder;

[0065] 6) Install the scanning probe back to the initial position and close the soundproof cover.

[0066] The above-described embodiments only express the specific implementation of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be noted that for ordinary skilled in the art, without departing from the concept of the present application, a number of variations and improvements can be made, which are within the scope of the present application. Therefore, the scope of protection of the present application should be subject to the appended claims.

Claims

1. A near-field probe optimization method suitable for terahertz nanospectroscopy technology, characterized in that: The specific steps include: 1) Use a six-axis micro-displacement stage to perform coaxial calibration of the parabolic mirror, reference arm reflector, and detector to ensure optimal signal reception conditions; 2) Collect the 0th to 5th order near-field time domain signals of the probes with different tip curvature radii on the gold substrate sample under s-SNOM conditions. The signals include amplitude and phase signals in the time domain. 3) Use FFT to transform the time domain signal to obtain the frequency domain signal, and then analyze the change pattern of the signal with the tip curvature radius; 4) In commercial numerical simulation software, construct a model that matches the experimental probe geometry in step 2) and perform finite element simulation to determine the scattering dipole moment p and the probe surface charge density σ s how they vary with frequency, and how they are affected by the tip curvature radius R; 5) Through the analysis of experimental and simulation results, the relationship between the tip curvature radius R of the probe and the near-field scattering signal intensity is established for near-field probe optimization.

2. The near-field probe optimization method suitable for terahertz nanospectroscopy technology according to claim 1, characterized in that: Specific implementation method of step 1) is as follows: After installing the probe, the parabolic mirror, reference arm reflector, and detector in the s-SNOM are coaxially calibrated using a six-axis micro-displacement stage with an adjustment accuracy of +0.1mm. During the coaxial calibration process, the key data of the s-SNOM "signal-to-noise ratio" and "second-order amplitude" are observed in real time to ensure that the "signal-to-noise ratio" and "second-order amplitude" are close to the maximum value under the experimental conditions.

3. The near-field probe optimization method suitable for terahertz nanospectroscopy technology according to claim 1, characterized in that: The specific implementation method of step 3) is as follows: A: Considering the influence of background signal and incident laser intensity, a set of second-order or third-order time domain signals in the near-field signal is randomly selected; B: Due to the phase offset, the real and imaginary parts of the field are complex, which can be solved by adding a phase offset to the overall phase signal. to shift the original phase signal, which will ensure that the phase now oscillates between 0 and π; C: Convert the time domain signal into the frequency domain signal using the following formula: Where A corresponds to the time domain amplitude in step 2), Corresponding to the time domain phase in step 2), Take the absolute value of the minimum negative value of the time domain phase in step 2), where the real part A represents the near-field scattered signal intensity.

4. The near-field probe optimization method suitable for terahertz nanospectroscopy technology according to claim 1, characterized in that: The specific implementation method of step 4) is as follows: The scattering dipole moment p and the probe surface charge density σ s The formula and the tip scattering cross section S were input into COMSOL Multiphysics software, and the frequency parameter was set in the parametric sweep status bar. The parametric sweep obtained the frequency-dipole moment spectrum in the frequency range of 0.5-1.5 THz. The formula for the scattering dipole moment p is: p=|∫∫ s σ s SdS|; S probe surface charge density σ s The formula is: s s ∝E·n; In the above formula, E is the incident electric field intensity, n is the external normal of the tip surface, and S is the tip scattering cross section, which can be calculated by photographing the curvature radius R of the probe tip with a scanning electron microscope.

5. The near-field probe optimization method applicable to terahertz nanospectroscopy technology according to claim 1, characterized in that: In step 5), through analysis of experimental and simulation results, the near-field scattering signal is positively correlated with the tip curvature radius R of the probe in the wide frequency range of 0.5-1.5 THz, that is, the near-field scattering frequency domain signal is enhanced with the increase of the tip curvature radius R.

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