Electric field measurement method and electric field sensor

Abstract

To provide an electric field measurement method and an electric field sensor with which it is possible to acquire the spatial distribution of a measurement electric field with measurement accuracy that is sufficient for practical uses.SOLUTION: Provided is an electric field measurement method for finding, utilizing a nonlinear optical effect, the electric field as a measurement object electric field, that is emitted by an object being measured. This measurement method includes adjusting one or more parameters selected from the group consisting of the beam diameter, pulse width, pulse energy and wavelength of an incident laser and a focal distance in light condensation on a condensing region, with a lower limit intensity value determined from the viewpoint that conversion efficiency as the efficiency of conversion of the incident laser with which the object being measured is irradiated into a nonlinear beam should be a given value or greater, with an upper limit intensity value determined from the viewpoint that plasma generation in a measurement space should be suppressed.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electric field measurement method and an electric field sensor. [Background technology]

[0002] Electric field measurement methods that utilize nonlinear optical effects to measure the electric field of a measurement target are known (Patent Documents 1 and 2). In particular, electric field measurement methods that utilize short-pulse lasers (wavelength: 800 nm or less), such as CARS (Coherent Anti-Stokes Raman Scattering) and E-FISH (Electric Field Induced Second Harmonic generation), have become popular in recent years (Non-Patent Documents 1 and 2). This type of electric field sensor uses a short-pulse laser to induce a nonlinear optical effect in the ambient gas, and detects the resulting response as a signal. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 6-88840 [Patent Document 2] Japanese Patent Application Publication No. 2020-118498 [Non-patent literature]

[0004] [Non-Patent Document 1] CARS (Coherent Anti-stokes Raman Scattering): T. Ito, K. Kobayashi, U. Czarnetzki and Satoshi Hamaguchi: “Rapid formation of electric field profiles in repetitively pulsed high-voltage high-pressure nanosecond discharges”, J. Phys. D: Appl. Phys. 43 (2010) 062001 [Non-patent document 2] E-FISH (Electric Field Induced Second Harmonic generation): A. Dogariu, BM Goldberg, S. O'Byrne and RB Miles: “Species-Independent Femtosecond Localized Electric Field Measurement”, Phys. Rev. Applied 7 (2017) 024024 Summary of the Invention [Problem to be solved by the invention]

[0005] In the above-described conventional electric field measurement method, in order to increase the detection efficiency of the target nonlinear light (to increase the probability of occurrence of the nonlinear optical effect), it is necessary to strongly focus the incident laser on the measurement target or shorten the pulse time width of the incident laser to increase the number density of incident photons per unit time and unit space. However, in this type of electric field measurement method, self-contraction or self-diffusion may occur during the propagation of the incident laser through space, making it difficult to achieve the desired focused state. In particular, when an incident laser with a high energy density in both time and space, such as a short-pulse laser, is used, the number density of incident photons per unit time increases, making self-contraction or self-diffusion more likely to occur. As a result, there is a problem in that the desired focused state cannot be achieved to ensure practically sufficient measurement accuracy.

[0006] In view of the above circumstances, an object of the present invention is to provide an electric field measurement method and an electric field sensor that can acquire the spatial distribution of the measured electric field with a measurement accuracy sufficient for practical use. [Means for solving the problem]

[0007] According to one aspect of the present invention, there is provided an electric field measurement method for determining an electric field emitted from a measurement target portion as a measurement target electric field by utilizing a nonlinear optical effect. In this electric field measurement method, an incident laser for irradiating the measurement target portion is focused on a focusing region where the measurement target portion is located and which extends over a predetermined length substantially perpendicular to the propagation direction of the incident laser, nonlinear light to be measured is extracted from transmitted light emitted from the measurement target portion, and the measurement target electric field is calculated based on the intensity of the extracted nonlinear light. Then, when the intensity of the incident laser is equal to or greater than a predetermined intensity lower limit value and equal to or less than an intensity upper limit value, The first intensity condition is satisfied, and the intensity of the incident laser satisfies the second intensity condition. To add, Measurement parameters Adjust. The second intensity condition is defined based on the light collection angle related to the light collection onto the light collection region and the intensity of the incident laser. In particular, the lower limit of the intensity is set to a certain value or more for the conversion efficiency, which is the efficiency with which the incident laser irradiated on the measurement part is converted into nonlinear light. The minimum strength is The upper limit of the intensity is set to suppress the generation of plasma in the measurement space. The maximum intensity is It is determined. The second intensity condition is defined as a relationship between the collection angle and the intensity of the incident laser that satisfies the first intensity condition and makes the Rayleigh length of the incident laser equal to or less than a certain value, and the power of the incident laser per unit area on a plane perpendicular to the direction of travel after passing through a focusing element used to focus the light onto the focusing region is used to determine whether the intensity of the incident laser satisfies the first intensity condition. Furthermore, the integrated value of the power on the plane is used as the intensity of the incident laser that defines the second intensity condition.

[0008] According to another aspect of the present invention, there is provided an electric field sensor that utilizes a nonlinear optical effect to determine an electric field emitted by a measurement target portion as a measurement target electric field. This electric field sensor comprises a laser oscillator that emits an incident laser to be irradiated onto the measurement target portion, a focusing element that focuses the incident laser emitted from the laser oscillator onto a focusing region where the measurement target portion is located, an extractor that extracts the measurement target nonlinear light from the transmitted light emitted from the measurement target portion, and a calculator that calculates the measurement target electric field based on the intensity of the extracted nonlinear light. In this electric field sensor, when the intensity of the incident laser is equal to or greater than a predetermined intensity lower limit value and equal to or less than an intensity upper limit value, The first intensity condition is satisfied, and the intensity of the incident laser satisfies the second intensity condition. To add, Measurement parameters is adjusted. The second intensity condition is defined based on the light collection angle related to the light collection onto the light collection region and the intensity of the incident laser. In particular, the lower limit of the intensity is set to a certain value or more for the conversion efficiency, which is the efficiency with which the incident laser irradiated on the measurement part is converted into nonlinear light. The minimum strength is The upper limit of the intensity is set to suppress the generation of plasma in the measurement space. The maximum intensity is It is determined. The second intensity condition is defined as a relationship between the collection angle and the intensity of the incident laser that satisfies the first intensity condition and makes the Rayleigh length of the incident laser equal to or less than a certain value, and the power of the incident laser per unit area on a plane perpendicular to the direction of travel of the incident laser after passing through a focusing element used to focus the laser onto the focusing region is used to determine whether the intensity of the incident laser satisfies the first intensity condition. Furthermore, the integrated value of the power on the plane is used as the intensity of the incident laser that defines the second intensity condition. [Effects of the Invention]

[0009] According to the present invention, an electric field measurement method and an electric field sensor are realized that can acquire the spatial distribution of the measured electric field with a practically sufficient measurement accuracy. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram illustrating the overall configuration of an electric field sensor according to an embodiment of the present invention. [Figure 2] FIG. 2 is a block diagram illustrating the configuration of a measurement / calculation device. [Figure 3] FIG. 2 is a side view (yz plan view) of the measurement stage. [Figure 4] FIG. 1 is a plan view (zx plan view) of the measurement stage. [Figure 5] FIG. 2 is a perspective view of a main part of a measurement stage. [Figure 6] FIG. 1 is a diagram showing a high-resolution region. [Figure 7] FIG. 10 is a diagram showing the depth resolution of the electric field sensor of the embodiment. [Figure 8] FIG. 10 is a diagram showing the depth resolution of the electric field sensor of Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0012] 1 is a diagram illustrating the configuration of an electric field sensor 10 according to one embodiment of the present invention. The electric field sensor 10 of this embodiment is a sensor that focuses a pulsed laser (hereinafter also referred to as an "incident laser IL") generated from a predetermined laser generation source and irradiates the object O to be measured, detects the intensity of light (hereinafter also referred to as "nonlinear light EL") excited from the object O to be measured and contained in light (hereinafter also referred to as "transmitted light TL") that passes through the object O to be measured, and determines the electric field emitted by the object O to be measured based on the intensity.

[0013] The electric field sensor 10 of this embodiment described below is assumed to be an E-FISH type electric field sensor that irradiates the object to be measured O with an incident laser IL in a measurement space in the atmospheric environment and in an environment in which an electrostatic field is applied to the object to be measured O. However, the configuration of the electric field sensor 10 of this embodiment is not limited to the E-FISH type, and can be applied with appropriate modifications to other types of sensors that utilize nonlinear optical effects, such as a CARS type.

[0014] In addition, in this embodiment, the coordinate axis along the direction in which the incident laser IL or nonlinear light EL travels (optical axis direction) is defined as the z-axis, the coordinate axis along the focusing region F described below where the incident laser IL is focused is defined as the y-axis, and the coordinate axis along the direction perpendicular to the y-axis and z-axis is defined as the x-axis.

[0015] As shown in the figure, the electric field sensor 10 of this embodiment mainly comprises a laser oscillator 20, a lens set 22, a measurement stage 24, a filter set 26, and a measurement / calculation device .

[0016] The laser oscillation device 20 outputs a laser pulse of a predetermined wavelength as the incident laser IL. Specifically, the laser oscillation device 20 outputs a short pulse laser (particularly, an ultrashort pulse laser or an ultrashort pulse laser), such as a nanosecond pulse laser (ns pulse laser), a picosecond pulse laser (ps pulse laser), or a femtosecond pulse laser (fs pulse laser), more specifically, an fs pulse laser, as the incident laser IL. Furthermore, the laser oscillation device 20 outputs a laser pulse of a predetermined wavelength λ IL , pulse duration pd IL , and pulse energy Ep IL It is configured so that [J] can be adjusted arbitrarily.

[0017] The lens set 22 is composed of optical equipment for collimating the incident laser light IL oscillated from the laser oscillator 20 and for suitably focusing the light on a focusing region F within the measurement stage 24. More specifically, the lens set 22 of this embodiment is composed of a spherical concave lens 22a, a spherical convex lens 22b, and a cylindrical lens 22c, which are arranged in this order from the laser oscillator 20. In particular, the cylindrical lens 22c functions as a focusing element that focuses the incident laser light IL on a linear region (hereinafter also referred to as "focusing region F") extending over a predetermined length in the y-axis direction at a predetermined position z (beam waist position z0, described later) that corresponds to the focal length f.

[0018] The measurement stage 24 is a portion on which the object to be measured O is placed. The measurement stage 24 is also provided with electrodes 25 (see FIG. 3) for applying an electric field (hereinafter also referred to as a "measurement electric field") directed from the positive direction to the negative direction of the y-axis to the object to be measured O. That is, the object to be measured O is irradiated with the incident laser IL in a state in which the measurement electric field is applied.

[0019] The filter set 26 includes various filters for extracting nonlinear light EL having wavelength components to be measured from the transmitted light TL in response to irradiation with the incident laser IL. In particular, the filter set 26 of this embodiment is composed of a short-pass filter 26a that cuts long-wavelength components equal to or greater than a predetermined value, a long-pass filter 26b that cuts short-wavelength components equal to or less than a predetermined value after cutting the long-wavelength components, and a band-pass filter 26c that cuts components outside a predetermined frequency range (i.e., a predetermined wavelength range) after cutting the short-wavelength components.

[0020] More specifically, for example, the short-pass filter 26a is an optical filter that reflects wavelength components of 700 nm to 900 nm and transmits wavelength components of 350 nm to 700 nm. The long-pass filter 26b is an ultraviolet cut filter that transmits only wavelength components of 400 nm or more. Furthermore, the band-pass filter 26c is a filter that filters out a frequency component that is twice the frequency of the incident laser IL (i.e., wavelength λ) contained in the transmitted light TL. ILTherefore, the filter set 26 mainly filters out the nonlinear light EL contained in the transmitted light TL, more specifically, the wavelength λ of the incident laser IL. IL 1 / 2 wavelength λ SHG Second Harmonic Generation (SHG) I SHG will be extracted.

[0021] In this embodiment, the second harmonic I SHG The nonlinear light EL is emitted from the incident laser IL onto the object O, and the third-order nonlinear optical effect (when the object O has a wavelength λ IL absorbs two photons of wavelength λ IL This component is produced by the induction of the photon energy effect (the effect of emitting one photon with twice the energy at 1 / 2).

[0022] The measurement / calculation device 28 is a device that mainly receives as input the signal of the transmitted light TL filtered by the filter set 26 and the signal of the incident laser IL to determine the electric field (hereinafter also referred to as the "electric field E to be measured") possessed by the object to be measured O, which is the object to be measured.

[0023] 2 is a block diagram illustrating the configuration of the measurement / calculation device 28. As shown in the figure, the measurement / calculation device 28 measures the intensity I of the incident laser IL. IL and a laser intensity acquirer 28a that acquires the intensity of SHG contained in the nonlinear light EL (hereinafter referred to as the SHG intensity I (2 ω ) ” (also referred to as “SHG intensity extractor 28b”), and the acquired intensity I IL and the extracted SHG intensity I (2 ω ) and an electric field calculator 28c that calculates the electric field E to be measured based on the above.

[0024] The laser intensity acquirer 28a acquires the intensity I of the incident laser IL. IL , in particular, the intensity I at the position where the incident laser IL is most focused (i.e., the position of the focused region F)IL (hereinafter referred to as "maximum incident laser intensity I ( ω ) probe ") or an intensity I close enough to be considered equivalent to it. IL The device is configured to detect or estimate

[0025] In particular, in this embodiment, the laser intensity acquirer 28a acquires the intensity I of the incident laser IL. IL (In particular, the maximum incident laser intensity I ( ω ) probe ) in the measurement space, the unit area [cm ] of the cross section of the plane (xy plane) perpendicular to the propagation direction (z-axis direction) of the incident laser IL. 2 ]. That is, the intensity I of the incident laser IL in this embodiment is calculated as IL is a physical quantity that can be equated with the magnitude of the optical electric field generated by the incident laser IL.

[0026] The SHG intensity extractor 28b includes a spectrometer that disperses the nonlinear light EL from the filter set 26, a CCD imaging sensor that is made up of a photomultiplier tube for obtaining an intensity spectrum of the dispersed nonlinear light EL, and an SHG intensity I from the obtained intensity spectrum. (2 ω ) It is composed of a calculator that calculates the

[0027] Then, the electric field calculator 28c calculates the maximum incident laser intensity I ( ω ) probe and SHG intensity I (2 ω ) Based on this, the electric field E to be measured is calculated using the following equation (1).

[0028]

number

[0029] In the formula, "A" is a calibration constant, "N" is the number density of molecules in the measurement space (especially the air atmosphere), "ω" is the angular frequency of the incident laser IL, and "χ (3)i,j,k,l (-2ω,0,ω,ω)" represents the third-order nonlinear susceptibility tensor. Note that the calibration constant A, the number density N, and the third-order nonlinear susceptibility tensor χ (3) i,j,k,l (-2ω, 0, ω, ω) may be determined in advance or may be determined appropriately depending on the measurement environment, etc.

[0030] Measurements performed by the electric field sensor 10 having the above-described configuration will now be described in further detail. The meanings of the parameters used in the following description are defined as follows. "Beam diameter D": The intensity I of the spatial distribution of the incident laser IL that actually affects the measurement results relative to the optical axis r IL The size of the range (especially the range on the xy plane) that takes on this value. "Spot size w(z)": The intensity I of the incident laser IL at the position z on the optical axis r IL a predetermined ratio (for example, 1 / e times or 1 / e 2 The size of the beam area (especially the length of the beam area in the x-axis direction) that is greater than or equal to 100 times the x-axis. "Beam waist position z0": Position z on the optical axis r where the incident laser IL is most focused (i.e., the position of the focused region F). · "Beam waist diameter w0": Spot size w(z0) at the beam waist position z0. "Rayleigh range z (Ra)": The range on the z-axis where the cross-sectional area of ​​the beam (incident laser IL or transmitted light TL) is less than twice the cross-sectional area at the beam waist position z0. Rayleigh length Z r ": The length from the beam waist position z0 on the z-axis to the end of the Rayleigh range z(Ra). "Convergence angle θ": The angle between the optical axis r and the outer edge of the beam area, starting from the beam waist position z0 (focusing area F). In other words, the convergence angle θ is a physical quantity that indicates the ease of focusing the incident laser IL in the focusing area F after passing through the cylindrical lens 22c.

[0031] 5, which will be described later, the distribution of the incident laser beam IL after passing through the cylindrical lens 22c, which is a focusing element, has a substantially uniform shape along the xz plane (more specifically, a triangle with the focusing region F as its apex). That is, in this embodiment, the magnitude of the angle of the apex of the triangle (i.e., a one-dimensional angle) is directly related to the ease of focusing the incident laser beam IL in the focusing region F, and therefore this is used as the focusing angle θ. On the other hand, depending on the shape of the distribution of the incident laser beam IL after focusing, if it is difficult to express the ease of focusing the incident laser beam IL in the focusing region F only by a one-dimensional angle, the focusing angle θ may be defined as an appropriate solid angle.

[0032] Fig. 3 is a side view (yz plan view) of the measurement stage 24, Fig. 4 is a plan view (zx plan view) of the measurement stage 24, and Fig. 5 is a perspective view of the main parts of the measurement stage 24. Note that in Fig. 5, the object to be measured O and the electrodes 25 are omitted for simplicity of the drawing.

[0033] As shown in the figure, the incident laser IL is collected by the cylindrical lens 22c and irradiated onto the object to be measured O. Then, the transmitted light TL containing the nonlinear light EL emitted from the object to be measured O is emitted toward the filter set 26.

[0034] In the electric field measurement method using the electric field sensor 10 of this embodiment described above, the cylindrical lens 22c focuses the incident laser IL onto the light-focusing region F. Therefore, the electric field E to be measured of the object O to be measured can be obtained as a spatial distribution in the direction in which the light-focusing region F extends (y-axis direction) (hereinafter also simply referred to as "electric field spatial distribution").

[0035] In particular, in this embodiment, the intensity I of the incident laser IL IL By imposing the first condition (hereinafter also referred to as the "first intensity condition") on the intensity I of the incident laser IL, it is possible to achieve a measurement accuracy sufficient for practical use in measuring the above-mentioned electric field spatial distribution. ILBy imposing the second condition (hereinafter referred to as the "second intensity condition") on the premise that the first condition is satisfied, it is possible to achieve high depth resolution (the shortness of the area in the z-axis direction that can be distinguished during measurement) in measuring the electric field spatial distribution. The first intensity condition and the second intensity condition will be described in detail below.

[0036] [First strength condition] The inventors have found that the main reason why practically sufficient measurement accuracy cannot be obtained in measuring the spatial distribution of the electric field is the decrease in conversion efficiency due to the self-diffusion of the incident laser IL, or the generation of ionization (plasma) of atoms or molecules in the measurement space due to the self-contraction of the incident laser IL. As a result of extensive research, the inventors have found that the occurrence of self-diffusion or self-contraction, which are factors that reduce the measurement accuracy, is due to the intensity I IL We found that the distribution of

[0037] Therefore, the first strength condition in this embodiment is set as strength I IL However, the lower limit of the intensity I is determined from the viewpoint of maintaining the conversion efficiency. lo The upper limit of the intensity I is determined from the viewpoint of suppressing the generation of plasma. ul The condition is set as being within the following range: Then, the measurement parameters are adjusted so that this first intensity condition is satisfied.

[0038] In addition, the lower limit of strength I lo is the intensity I, which is the criterion for determining whether the conversion efficiency has reached a level that allows for practically acceptable measurement accuracy. IL Here, as can be understood by referring to the above formula (1), the SHG intensity I (2 ω ) is the intensity of the incident laser IL, I IL (In particular, the maximum incident laser intensity I ( ω ) probe), the electric field E to be measured, and the type and state of the atmospheric gas (pressure or temperature). Therefore, if the electric field E to be measured is set to a certain value and the type and state of the atmospheric gas are determined, the lower limit of the intensity I lo The intensity of the incident laser IL according to the minimum allowable detection sensitivity, I IL The lower limit of the strength I lo For example, 10 10 [J / (s·cm 2 )]~10 11 [J / (s·cm 2 ) range.

[0039] In addition, the upper limit of strength I ul is the intensity I that suppresses the generation of plasma caused by the spatial propagation of the incident laser IL to a range that maintains practical measurement accuracy. IL The upper limit of the strength I ul is the maximum incident laser intensity I that produces plasma emission ( ω ) probe The upper limit of the strength is defined as I ul The specific value of can be determined appropriately depending on necessary parameters such as the type and state of the atmospheric gas. ul For example, 10 13 [J / (s·cm 2 )]~10 15 [J / (s·cm 2 ) range.

[0040] In addition, the intensity I of the incident laser IL used to determine whether the first intensity condition is satisfied IL is the maximum incident laser intensity I ( ω ) probe It is preferable to use a value as close as possible to the intensity I at a position close to the beam waist position z0 (the position of the focused region F). IL It is preferable to use values ​​obtained by sensors (not shown) that are provided to estimate or detect the above.

[0041] The measurement parameters are the intensity of the incident laser IL, IL The electric field sensor 10 may include any one or more parameters that can affect the change of the electric field, and that can be adjusted by arranging each device included in the electric field sensor 10 for performing the electric field measurement method and / or manipulating the control parameters of each device (such as manipulating the output).

[0042] In particular, the measurement parameters in this embodiment are the beam diameter D and pulse width pd of the incident laser IL. IL , pulse energy Ep IL , and wavelength λ IL and a focal length f.

[0043] More specifically, in this embodiment, the intensity I of the incident laser IL IL , the upper limit of the intensity I ul Increase the beam diameter D within the range not exceeding the pulse width pd IL Reduce the pulse energy Ep IL Increasing the wavelength λ IL This can be specifically determined by shortening and / or reducing the focal length f.

[0044] On the other hand, the intensity of the incident laser IL is IL , the lower limit of the intensity I lo Reduce the beam diameter D within the range not to fall below the pulse width pd IL Increase the pulse energy Ep IL To reduce the wavelength λ IL and / or increase the focal length f.

[0045] Here, the beam diameter D, which is also used as a measurement parameter, is preferably a value detected or estimated at a position immediately before the incident laser beam on the cylindrical lens 22c, which is a focusing element (in this embodiment, a position between the spherical convex lens 22b and the cylindrical lens 22c). In particular, the beam diameter D can be any value correlated with the size of the cross-sectional shape of the incident laser beam IL (the extent of spread of the incident laser beam IL in the xy plane). For example, if the cross-sectional shape of the incident laser beam IL is a polygon such as a square, the length of one side, the length of a diagonal, and the area of ​​the incident laser beam IL can be used as the beam diameter D. Furthermore, if the cross-sectional shape of the incident laser beam IL is a circle, the diameter, perimeter, and the area of ​​the circle can also be used as the beam diameter D. In particular, from the viewpoint of realizing the first intensity condition, the beam diameter D is preferably selected from the range of 0.1 mm to 50 mm, for example.

[0046] In addition, the beam diameter D can also be adjusted by placing any optical device (optical device that replaces or is used in combination with the spherical concave lens 22a and spherical convex lens 22b of this embodiment) in front of the cylindrical lens 22c that ultimately focuses the incident laser IL, and / or by adjusting the area of ​​the exit surface of the incident laser IL by the laser oscillator device 20.

[0047] Furthermore, the pulse width pd IL , pulse energy Ep IL , and wavelength λ IL is adjusted to a desired value by selecting the specifications of the laser oscillator 20 and by operating the output setting of the laser oscillator 20. IL The adjustment of may be performed by providing a predetermined filter element. In particular, the pulse width pd IL is selected from the range of 10 [f·s] to 1 [n·s], for example. IL is selected from the range of, for example, 0.1 [mJ] to 1 [J]. Furthermore, the preferred wavelength λ IL is selected from the range of 0.8 [μm] to 5 [μm], for example.

[0048] Furthermore, the focal length f used as a measurement parameter is determined as the distance from the exit surface of the lens set 22 from which the incident light is incident to the imaging position (beam waist position z0), which is determined according to the structure (thickness, material, radius of curvature, etc.) of the lens set 22 (particularly, cylindrical lens 22c in this embodiment). In particular, from the viewpoint of realizing the first intensity condition, the preferred focal length f is selected from the range of 10 mm to 1000 mm, for example.

[0049] As mentioned above, the intensity of the incident laser IL is IL By adjusting the measurement parameters so that satisfies the first intensity condition, it is possible to achieve measurement accuracy sufficient for practical use in measuring the electric field spatial distribution, even when using a high-intensity incident laser IL such as a short-pulse laser (particularly, an ultrashort-pulse or ultrashort-pulse laser).

[0050] [Second strength condition] The inventors have realized a practically sufficient measurement accuracy in measuring the electric field spatial distribution by satisfying the first intensity condition, and furthermore, IL The idea is that by imposing a stricter second intensity condition on the depth of field, the depth resolution of the measurement can be further improved. The details of this are explained below.

[0051] First, assuming that the incident laser IL follows the characteristics of an ideal Gaussian beam (an electromagnetic wave whose intensity distribution around the optical axis r is approximately Gaussian), the Rayleigh length Z r is the beam waist diameter w0 or the focusing angle θ, and the wavelength λ of the incident laser IL IL Based on this, it is determined by the following formula (2).

[0052]

number

[0053] The beam waist diameter w0 is expressed by the following equation (3).

[0054]

number

[0055] Therefore, theoretically, the incident laser IL behaves in accordance with the characteristics expressed by the above formulas (2) and (3). That is, the above measurement parameters, the beam diameter D and wavelength λ included in formula (2) or formula (3), IL , or by adjusting the focal length f appropriately, the Rayleigh length Z r The shortness of the image (the height of the depth resolution) can be appropriately controlled.

[0056] On the other hand, as described above, when a nonlinear effect such as plasma generation occurs due to the use of a high-intensity incident laser IL, the focusing characteristics of the incident laser IL do not follow equations (2) and (3). Therefore, the Rayleigh length Z r As a result, it is difficult to improve the depth resolution.

[0057] In response to this, the present inventors have, as a result of intensive research, found that the intensity I IL If the second strength condition is satisfied, then the Rayleigh length Z r The inventors have come up with the idea that depth resolution can be improved by suppressing the occurrence of nonlinear effects that affect the control of the object.

[0058] Specifically, the second intensity condition of this embodiment is the convergence angle θ and the intensity peak P of the incident laser IL. L The intensity peak P in this embodiment is defined as a two-dimensional closed region (hereinafter also referred to as a "high-resolution region R") defined by the above two variables. L ([J / s]) is the intensity I IL [J / (s·cm 2 )] over a spread area in a plane perpendicular to the z-axis direction, is the maximum peak height (e.g., peak value).

[0059] 6 is a diagram showing a high-resolution region R. As shown in the figure, the high-resolution region R has an intensity I that is permissible from the viewpoint of improving the depth resolution when the collection angle θ varies within a range between its minimum and maximum values. IL The range of the strength I is determined from the viewpoint of satisfying the first strength condition. IL This will be explained in more detail below.

[0060] 5, when the incident laser beam IL is incident on the cylindrical lens 22c, if the collection angle θ is fixed, the area of ​​the beam spread region at the focal position (beam waist position z0) is H×w0. Therefore, by using the above formulas (2) and (3), the Rayleigh length Z r The collection angle θ and maximum incident laser intensity I ( ω ) probe The relationship can be defined as the following equation (4).

[0061]

number

[0062] Here, the maximum incident laser intensity I in Eq. (4) ( ω ) probe The lower limit value I of the strength related to the first strength condition lo By applying this, the intensity peak P according to the collection angle θ L On the other hand, the maximum incident laser intensity I ( ω ) probe The upper limit value I of the strength related to the first strength condition ul By applying this, the intensity peak P according to the collection angle θ L In other words, the high-resolution region R between the curves C1 and C2 has an upper limit of the intensity I that satisfies both the first intensity condition and the second intensity condition. IL (more precisely, the intensity peak P L Therefore, the intensity peak P LBy adjusting the measurement parameters so that is included in the high-resolution region R, it is possible to further improve the depth resolution while ensuring the required accuracy in the measurement.

[0063] Note that the specific means for adjusting the measurement parameters is not limited to the specific embodiments described with reference to Figures 1 and 2 above, and can be replaced by any means known in the art.

[0064] The effects of the electric field measuring method according to the present embodiment described above will now be described.

[0065] In this embodiment, an electric field measurement method is provided in which an electric field emitted from a measurement portion (measurement object O) is determined as a measurement target electric field E by utilizing a nonlinear optical effect. This electric field measurement method focuses an incident laser IL for irradiating the measurement target O on a light-focusing region F in which the measurement target O is placed and which extends over a predetermined length substantially perpendicular to the traveling direction of the incident laser IL (along the y-axis direction), extracts nonlinear light EL to be measured from transmitted light TL emitted from the measurement target O, and calculates the intensity (particularly, SHG intensity I (2 ω ) ) and calculate the electric field E to be measured.

[0066] In particular, in this electric field measurement method, the intensity I of the incident laser IL IL is the predetermined lower limit of strength I lo and the upper limit of the strength I ul The measurement parameters are adjusted to satisfy the first intensity condition: lo is a value determined from the viewpoint of ensuring that the conversion efficiency, which is the efficiency with which the incident laser IL irradiated onto the measurement object O is converted into nonlinear light EL, is a certain value or more. ul is a value determined from the viewpoint of suppressing the generation of plasma in the measurement space.

[0067] As a result, even if nonlinear optical phenomena such as self-contraction or self-diffusion that make it difficult to predict the focusing state of the incident laser IL occur during the propagation of the incident laser IL through the measurement space, the electric field E to be measured (particularly its spatial distribution) can be measured with high accuracy. In particular, even when a high-intensity incident laser IL is used, it is possible to ensure measurement accuracy sufficient for practical use.

[0068] Furthermore, in the electric field measurement method of this embodiment, the condensing angle θ of the condensing light on the condensing region F and the intensity I of the incident laser IL are IL Set a second intensity condition based on the intensity of the incident laser IL, IL The measurement parameters are adjusted so that the second intensity condition is satisfied. The second intensity condition is satisfied by adjusting the measurement parameters so that the first intensity condition is satisfied and the Rayleigh length Z of the incident laser IL is r The convergence angle θ and the intensity I of the incident laser IL are IL (Intensity peak P L ) (high-resolution region R).

[0069] This makes it possible to further improve the depth resolution in measuring the electric field E to be measured while ensuring a practically sufficient measurement accuracy.

[0070] Also, the intensity of the incident laser IL is I IL The power of the incident laser IL per unit area on a surface perpendicular to the traveling direction (z-axis direction) after passing through a focusing element (particularly, cylindrical lens 22c) used to focus the light onto the focusing region F (i.e., the maximum incident laser intensity I ( ω ) probe ) is used. In addition, the intensity I of the incident laser IL, which defines the second intensity condition, IL The integral value of the power of the incident laser IL on the above surface (intensity peak P L ) is used.

[0071] This allows the strength I to be used to determine whether the first strength condition and the second strength condition are satisfied. ILAs a parameter related to the above, a specific embodiment that is preferable from the viewpoint of further improving measurement accuracy will be realized.

[0072] In particular, the intensity of the incident laser IL, I IL The maximum incident laser intensity I ( ω ) probe By using a value as close as possible to the upper limit of the strength I ul This can further improve the accuracy of the comparison (the accuracy of determining whether or not plasma is generated in the actual measurement space). As a result, the measurement accuracy is further improved.

[0073] The measurement parameters are the beam diameter D and pulse width pd of the incident laser IL. IL , pulse energy Ep IL , and wavelength λ IL and a focal length f.

[0074] As a result, the intensity I of the incident laser IL that satisfies the first intensity condition and the second intensity condition is IL Specific measurement parameters that are preferable for adjusting the above will be provided.

[0075] Moreover, the beam diameter D of the incident laser IL is detected or estimated at a position immediately before it is incident on the light-collecting element (particularly the cylindrical lens 22c).

[0076] This allows the beam diameter D of the incident laser IL to be a more specific measurement parameter used in actual measurements based on the specific configuration of the measurement system.

[0077] Furthermore, in this embodiment, the pulse width pd IL , pulse energy Ep IL , and wavelength λ IL is adjusted by operating the laser oscillator 20 that oscillates the incident laser IL.

[0078] This allows the pulse width pd to satisfy the first intensity condition and / or the high-resolution region R. IL , pulse energy Ep IL , and wavelength λ IL A specific configuration for adjusting the

[0079] In particular, it is preferable to use an ultrashort pulse laser (or an extremely short pulse laser) as the incident laser IL, which makes it possible to more reliably achieve the desired conversion efficiency of the incident laser IL in measurement.

[0080] In the electric field measurement method of this embodiment, the second harmonic I SHG and extract the second harmonic I SHG Intensity (SHG intensity I (2 ω ) ) to calculate the electric field E to be measured.

[0081] This allows for a relatively high signal strength to be obtained from the second harmonic I SHG On the premise that the electric field E to be measured is measured by detecting the electric field E, it is possible to achieve a practically sufficient measurement accuracy and high depth resolution.

[0082] Furthermore, in this embodiment, an electric field sensor 10 is provided that determines, as the electric field E to be measured, an electric field emitted from a measurement target portion (measurement target O) by utilizing a nonlinear optical effect.

[0083] In particular, the electric field sensor 10 includes a laser oscillator 20 that emits an incident laser beam IL to be irradiated onto the object to be measured O, a focusing element (particularly a cylindrical lens 22c) that focuses the incident laser beam IL emitted from the laser oscillator 20 onto a focusing region F where the object to be measured O is located, an extractor (SHG intensity extractor 28b) that extracts the nonlinear light EL to be measured from the transmitted light TL emitted from the object to be measured O, and an extractor (SHG intensity extractor 28b) that extracts the intensity of the extracted nonlinear light EL (particularly the SHG intensity I (2 ω ) and a calculation device (electric field calculator 28c) that calculates the electric field E to be measured based on the electric field E.

[0084] In this electric field sensor 10, the intensity I of the incident laser IL IL is the predetermined lower limit of strength I lo and the upper limit of the strength I ul The measurement parameters are adjusted to satisfy the first intensity condition: lo is a value determined from the viewpoint of ensuring that the conversion efficiency, which is the efficiency with which the incident laser IL irradiated onto the measurement object O is converted into nonlinear light EL, is a certain value or more. ul is a value determined from the viewpoint of suppressing the generation of plasma in the measurement space.

[0085] This provides an electric field sensor 10 having a configuration suitable for carrying out an electric field measurement method that achieves a measurement accuracy of the electric field E to be measured that is sufficient for practical use.

[0086] The electric field measurement method and electric field sensor 10 of the present embodiment described above can be suitably applied to various fields, such as cell analysis in medicine, analysis of natural phenomena such as lightning strikes, and monitoring of processing accuracy in precision processing such as semiconductor etching, even in cases where measurement using existing sensors (especially sensors that use a laser source with a high photon number density) is difficult. [Example]

[0087] The present invention will be explained in more detail below with reference to Examples 1 and 2, but the present invention is not limited to these Examples.

[0088] Example 1 The measurement accuracy was evaluated for the electric field measurement method based on the following equipment and measurement conditions.

[0089] [Device configuration] Laser oscillator 20: PHAROS PH2-SP-1mJ (Light Conversion) Lens set 22 (22a: concave cylindrical lens (f=-20mm), 22b and 22c: convex spherical lens (f=80mm)) Filter Set 26 (26a: #84-714 (Edmund), 26b: #65-132 (Edmund), 26c: #39-349 (Edmund))

[0090] Measurement / calculation unit 28: istar DH320T-18F-03-LP (Andor)

[0091] [Measurement parameter settings] Intensity I of the incident laser IL near the focal region F IL The estimated value of (≒ maximum incident laser intensity I ( ω ) probe The measurement parameters were adjusted so that the first intensity condition was satisfied. The specific measurement parameters that were adjusted are as follows:

[0092] Pulse width: 190fs Pulse energy 0.5mJ Incident laser wavelength λ IL :1030nm Beam diameter D: 5mm ·Focal length f:80mm

[0093] [Evaluation of measurement accuracy] Under the above conditions, the transmittance T(z) of the incident laser IL was measured for each position z on the object O. Meanwhile, the ideal value of the transmittance T(z) in the electric field measurement was determined based on the following equations (5) and (6).

[0094]

number

[0095]

number

[0096] In the formula, "β" is the two-photon absorption coefficient, and "I0(t)" is the maximum incident laser intensity I ( ω ) probe, the time profile of the object O, "L" represents the thickness (length along the z-axis) of the object O, and "z" represents the position of the object O on the z-axis.

[0097] The plot of the graph in Fig. 7 shows the measured values ​​of the transmittance T(z) according to Example 1. The solid line in Fig. 7 is an approximation curve of the transmittance T(z) under the measurement parameters of Example 1 generated from Equations (5) and (6).

[0098] Furthermore, the Rayleigh length z based on the approximation curve r Calculate and double it (2z r ) was used as an index to indicate the depth resolution. r was 400 μm.

[0099] Example 2 The measurement accuracy and depth resolution were evaluated for the electric field measurement method based on the following equipment and measurement conditions.

[0100] [Device configuration] The same as in Example 1 was used except that lens set 22 (22a: convex spherical lens f=-40 mm, 22b: convex spherical lens f=150 mm, 22c: convex cylindrical lens f=80 mm) was used.

[0101] [Measurement parameter settings] (1) An appropriate collection angle θ was set in advance through experiments etc., assuming the same measurement parameters as in Example 1. Specifically, the collection angle θ was set to 0.25 [rad].

[0102] (2) Convergence angle θ vs. intensity I shown in Figure 6 IL By referring to the table (the table that defines the second measurement conditions), the intensity peak P is calculated from the set collection angle θ. L The measurement parameters were adjusted so that the measured value was included in the high-resolution region R. The specific measurement parameters adjusted are as follows:

[0103] Pulse width: 190fs Pulse energy 0.1mJ Incident laser wavelength λ IL :1030nm Beam diameter D: 20mm ·Focal length f:80mm

[0104] [Evaluation of measurement accuracy] The measurement accuracy was evaluated in the same manner as in Example 1.

[0105] The plot of the graph in Fig. 8 represents the measured values ​​of the transmittance T(z) in Example 2. The solid line in Fig. 8 is an approximation curve generated from Equation (5) and Equation (6) for the transmittance T(z) under the measurement parameters in Example 2. As shown in Fig. 8, the measured values ​​in Example 2 generally follow the approximation curve of the transmittance T(z). The depth resolution 2z r The depth resolution 2z r was 46 μm.

[0106] [Results and Discussion] 7 and 8, in Examples 1 and 2 that satisfy the first intensity condition, the transmittance T(z) generally follows the curve of the ideal value. That is, it is considered that the measurement method of Examples 1 or 2 ensures at least practically sufficient measurement accuracy in measuring the electric field spatial distribution.

[0107] Furthermore, in Example 2, which satisfies the second intensity condition, the depth resolution is significantly improved compared to Example 1. This is thought to be due to Example 2 satisfying the second intensity condition.

[0108] Although the embodiments of the present invention have been described above, the above embodiments merely illustrate some of the application examples of the present invention, and it is not intended that the technical scope of the present invention be limited to the specific configurations of the above embodiments.

[0109] For example, in the electric field sensor 10, a polarization splitter or the like may be disposed to separate the nonlinear light EL to be measured into a plurality of polarization directions (e.g., the y-axis direction and the z-axis direction), and the measurement / calculation device 28 may determine the electric field E to be measured corresponding to the plurality of polarization directions based on the nonlinear light EL separated into the plurality of directions by the polarization splitter. [Explanation of symbols]

[0110] 10 Electric field sensor 20 Laser Oscillator 22c cylindrical lens 28 Measurement / Calculation Device 28a Laser intensity acquisition device 28b SHG intensity extractor 28c Electric field calculator

Claims

1. An electric field measurement method for determining an electric field emitted from a measurement target portion as a measurement target electric field by utilizing a nonlinear optical effect, an incident laser beam for irradiating the measurement portion is focused on a focusing region in which the measurement portion is disposed and which extends over a predetermined length substantially perpendicular to the traveling direction of the incident laser beam; extracting nonlinear light to be measured from transmitted light emitted from the measurement target portion; calculating the electric field to be measured based on the extracted intensity of the nonlinear light; adjusting measurement parameters so that the intensity of the incident laser satisfies a first intensity condition that is equal to or greater than a predetermined intensity lower limit value and equal to or less than a predetermined intensity upper limit value, and so that the intensity of the incident laser satisfies a second intensity condition; the second intensity condition is defined based on a collection angle related to collection of light onto the light collection region and an intensity of the incident laser, the intensity lower limit value is predetermined as a minimum value of intensity that makes a conversion efficiency, which is an efficiency at which the incident laser irradiated on the measurement portion is converted into the nonlinear light, equal to or greater than a certain value, the upper intensity limit value is predetermined as a maximum value of the intensity that suppresses the generation of plasma in the measurement space, The second strength condition is a relationship between the collection angle and the intensity of the incident laser that satisfies the first intensity condition and makes the Rayleigh length of the incident laser equal to or less than a certain value, determining whether the intensity of the incident laser satisfies the first intensity condition using the power of the incident laser per unit area on a plane perpendicular to the traveling direction after passing through a focusing element used to focus the light onto the focusing region; an integrated value of the power in the plane is used as the intensity of the incident laser that defines the second intensity condition; Electric field measurement method.

2. 2. The electric field measurement method according to claim 1, The measurement parameters are: The incident laser includes one or more parameters selected from the group consisting of a beam diameter, a pulse width, a pulse energy, and a wavelength of the incident laser, and a focal length at which light is focused on the focused region. Electric field measurement method.

3. 3. The electric field measurement method according to claim 2, the measurement parameters include the beam diameter; The beam diameter is detected or estimated at a position immediately before the beam is incident on a focusing element used to focus the beam onto the focusing region. Electric field measurement method.

4. The electric field measurement method according to any one of claims 1 to 3, Furthermore, the nonlinear light is split into a plurality of polarization directions, determining the electric field to be measured corresponding to the plurality of polarization directions; Electric field measurement method.

5. The electric field measurement method according to any one of claims 1 to 4, The incident laser is an ultrashort pulse laser. Electric field measurement method.

6. The electric field measurement method according to any one of claims 1 to 5, extracting a second harmonic as the nonlinear light; calculating the electric field to be measured from the intensity of the incident laser and the intensity of the second harmonic; Electric field measurement method.

7. An electric field sensor that utilizes a nonlinear optical effect to determine an electric field emitted from a measurement target as a measurement target electric field, a laser oscillator that emits an incident laser to be irradiated onto the measurement target portion; a focusing element that focuses the incident laser emitted from the laser oscillator on a focusing region where the measurement target is located; an extracting device that extracts nonlinear light to be measured from transmitted light emitted from the measurement target portion; a calculation device that calculates the electric field to be measured based on the extracted intensity of the nonlinear light, measurement parameters are adjusted so that the intensity of the incident laser satisfies a first intensity condition that is equal to or greater than a predetermined intensity lower limit value and equal to or less than a predetermined intensity upper limit value, and so that the intensity of the incident laser satisfies a second intensity condition; the second intensity condition is defined based on a collection angle related to collection of light onto the light collection region and an intensity of the incident laser, the intensity lower limit value is predetermined as a minimum value of intensity that makes a conversion efficiency, which is an efficiency at which the incident laser irradiated on the measurement portion is converted into the nonlinear light, equal to or greater than a certain value, the upper intensity limit value is predetermined as a maximum value of the intensity that suppresses the generation of plasma in the measurement space, The second strength condition is a relationship between the collection angle and the intensity of the incident laser that satisfies the first intensity condition and makes the Rayleigh length of the incident laser equal to or less than a certain value, determining whether the intensity of the incident laser satisfies the first intensity condition using the power of the incident laser per unit area on a plane perpendicular to the direction of travel of the incident laser after passing through a focusing element used to focus the laser on the focusing region; an integrated value of the power in the plane is used as the intensity of the incident laser that defines the second intensity condition; Electric field sensor.

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