Measurement method and device, and imaging system

Abstract

An LD (101) emits linearly-polarized ultrashort pulse laser light (121). A branching unit (102) branches the laser light (121) into first light (122) and second light (123). A wavelength plate (103) turns the second light (123) obtained as a result of the branching by the branching unit (102) into circularly polarized light. A delay unit (104) makes the optical path length of the first light (122) longer than the optical path length of the second light (123). An irradiation site of a magnetic material (131) is irradiated with the first light (122) that has passed through the delay unit (104) and the second light (123) that has been turned into circularly polarized light.

Description

Measurement method and apparatus, and imaging system

[0001] The present invention relates to a measurement method and apparatus using the magneto-optical effect, and an imaging system.

[0002] Sensors and imaging devices have been developed that utilize the magneto-optical (MO) effect, in which the plane of polarization of light rotates when it propagates through or reflects off a magnetic material. This technology is used in a variety of fields, including magneto-optical disk readout, non-destructive testing, and semiconductor spintronics research. In particular, magnetic properties such as the magnetic domain structure at the interface between different materials in magnetic thin films are essential for the development of the spintronics field.

[0003] Although the use of an optical probe to measure the magnetic domain structure enables rapid testing, it adversely affects sensitivity due to the shot noise of the photodetector, etc. To improve this, highly sensitive imaging systems have been proposed, such as a scanning laser MO imaging system equipped with an RF lock-in amplifier, an acousto-optic modulator, and a photomultiplier tube (see Non-Patent Document 1).

[0004] Furthermore, instead of using the linear MO effect as in Non-Patent Document 1, a MO imaging method utilizing the nonlinear MO effect has also been proposed (see Non-Patent Documents 2 and 3). In this proposal, an MO imaging system is constructed using magnetization-induced second harmonic generation (MSHG) by irradiating an ultrashort pulse laser onto the interface of different materials, such as Pt / CoNi / Pt, where the inversion symmetry is broken. This nonlinear MO effect can rotate the polarization by two orders of magnitude more than the linear MO effect, depending on the angle of incidence and other conditions (Non-Patent Document 4), thereby achieving high sensitivity in MO imaging.

[0005] 6 shows the configuration of a conventional imaging system using the nonlinear magneto-optical Kerr effect (NOMOKE). This imaging system includes a light source 301, a polarizer 302, a filter 303, an analyzer 304, and a detector 305. The light source 301 irradiates a target magnetic material 331 with an ultrashort pulse laser 321 having a frequency ω. The polarizer 302 adjusts the polarization of the linearly polarized ultrashort pulse laser 321 irradiated from the light source 301 to a specific polarization direction. The ultrashort pulse laser 321 that has passed through the polarizer 302 passes through a first objective lens 306 and is irradiated onto a predetermined location on the magnetic material 331.

[0006] When an ultrashort pulse laser 321 is irradiated onto a magnetic material 331, reflected light 322 is reflected, and a harmonic 323 is generated by magnetization-induced second harmonic generation. The reflected light 322 and the harmonic 323 pass through a second objective lens 307, where the reflected light 322 is removed by a filter 303, and the harmonic 323 is transmitted. The polarization components of the harmonic 323 transmitted through the filter 303 are controlled by an analyzer 304. The intensity of the harmonic 323 transmitted through the analyzer 304 in this manner is measured by a detector 305. Figure 6 shows the wavefront of the ultrashort pulse laser 321 as viewed perpendicularly. A magnetic field is applied to the magnetic material 331 by a permanent magnet 332, and the magnetic material 331 is magnetized in the in-plane direction.

[0007] The nonlinear polarization expressed under the electric dipole approximation is described by the following equation, and the MSHG intensity depends on the nonlinear polarization P(ω) induced by the electric field E(ω) of the incident light, where E and P are vectors.

[0008]

[0009] In formula (1), χ (2) ijk is the second-order nonlinear susceptibility in the magnetization M, and is decomposed into even and odd terms for the magnetization M by parity. Note that M is a vector, and is shown in bold in the formula. Hereinafter, vectors are shown in bold in the formula.

[0010]

[0011] In equation (2), i, j, and k are components of a Cartesian coordinate system. For example, when the electric field of the ultrashort pulse laser 321 that is transmitted through the polarizer 302, passes through the first objective lens 306, and is irradiated onto a predetermined location on the magnetic material 331 is s-polarized, and the magnetization M of the irradiated location is parallel to that direction, the intensity I(2ω) of the p-polarized component of the MSHG can be expressed as in equation (3). Note that in Figure 6, the traveling direction of the ultrashort pulse laser 321 is the direction of the wave vector, but here, for simplicity, the s-polarized light is assumed to be parallel to the y-axis, and the direction of the wave vector is assumed to be the z-axis.

[0012]

[0013] In this way, in order to increase the light intensity of the harmonic 323, it is necessary to increase the magnetization and control the direction. Note that general metal crystals have centrosymmetrical properties in the bulk, and the χ (2) Since the harmonic 323 due to MSHG is generated at the interface or material surface, high sensitivity is expected.

[0014] The advent of ultrashort pulse lasers has made it possible to easily generate pulses with a pulse width of approximately 100 fs. This has stimulated research in various optical fields, such as nonlinear optics and pump-probe techniques with high time resolution. One of these is the manipulation and measurement of magnetism using light. In the field of magneto-optics, AV Kimel et al. reported magnetic field pulses induced by the inverse Faraday effect, such as the excitation of magnetization oscillation modes by irradiation with circularly polarized femtosecond pulses (Non-Patent Document 5).

[0015] Generally, magnetization M in response to a spatiotemporally oscillating magnetic field H is defined by the following equation: In the following equation, χ is magnetic susceptibility, k is wave number, and magnetization and magnetic field are Fourier components.

[0016]

[0017] Furthermore, because the magnetization induced by irradiation with circularly polarized light is induced in the direction of the wave number (Non-Patent Document 5), time-varying magnetization is induced by the oscillation of the electromagnetic field of circularly polarized light and controlled by the direction of incidence of the circularly polarized light. Thus, magnetization control using circularly polarized pulses with angular momentum has attracted attention.

[0018] H. Murakami and M. Tonouchi, "High-sensitive scanning laser magneto-optical imaging system", Review of Scientific Instruments, vol. 81, no. 1, 013701, 2010.A. Kirilyuk et al., "Nonlinear magneto-optical imaging of interface magnetic structures", Journal of Magnetism and Magnetic Materials, vol. 198-199, pp. 620-623, 1999.Ru-Pin Pan et al., "Optical second-harmonic generation from magnetized surfaces", Physical Review B, vol. 39, no. 2, pp. 1229-1230, 1989.M. G. Koerkamp and T. Rasing, "Giant nonlinear magneto-optical Kerr effect from Fe interfaces ", Surface Scienc, vol. 352-354, pp. 933-936, 1996.A. V. Kimel et al., "Ultrafast non-thermal control of magnetization by instantaneous photomagnetic pulses", Nature, vol. 435, pp. 655-657, 2005.

[0019] However, magnetizing a magnetic material requires the application of an external magnetic field. In the conventional technology described above, magnetization is achieved using a permanent magnet, but this configuration does not result in uniform magnetization of the measurement area when imaging a magnetic material. This is due to misalignment or other factors, which can cause variations in the relative position between the magnet and the surface of the magnetic material, or differences in the magnetization time depending on the measurement area. Furthermore, the direction of magnetization depends on the magnet's placement. Therefore, controlling the magnetization direction can be limited by the device configuration, and it can be difficult to achieve uniform magnetization of the incident light focusing area due to factors such as accumulated magnetization.

[0020] Furthermore, conventional MO imaging systems such as those described in Non-Patent Document 1 use a linear MO effect, which has the problem of poor sensitivity due to a small rotation angle of the polarization plane of incident light. According to Non-Patent Document 4, the rotation angle of polarization due to linear MOKE on an iron surface is approximately 0.05°, which limits the types of materials that can be used for imaging to distinguish between different materials.

[0021] As described above, the prior art has had the problem that it is not easy to uniformly magnetize the target magnetic material, and the measurement sensitivity of the harmonics due to magnetization-induced second harmonic generation is low.

[0022] The present invention has been made to solve the above problems, and aims to more easily and uniformly magnetize the target magnetic material and to increase the measurement sensitivity of harmonics due to magnetization-induced second harmonic generation.

[0023] a fourth step of irradiating the irradiation location with the first light and the second light; a fifth step of transmitting measurement light having a frequency twice that of the laser light generated at the irradiation location irradiated with the first light and the second light; a sixth step of separating the transmitted light transmitted in the fifth step into first and second polarized light that are orthogonal to each other; a seventh step of measuring the light intensity of the first polarized light separated in the sixth step; an eighth step of measuring the light intensity of the second polarized light separated in the sixth step; and a ninth step of calculating an intensity ratio between the light intensity measured in the seventh step and the light intensity measured in the eighth step.

[0024] a wavelength plate that circularly polarizes the second light split by the splitting unit; a first optical system that irradiates the irradiation location with the first light; a second optical system that irradiates the irradiation location with the circularly polarized second light; a delay unit that makes the optical path length of the first light in the first optical system longer than the optical path length of the second light in the second optical system; a filter that transmits measurement light having twice the frequency of the laser light generated at the irradiation location irradiated with the first and second lights; a splitting unit that splits the transmitted light that has passed through the filter into first and second polarized lights that are orthogonal to each other; a first optical sensor that measures the light intensity of the first polarized light split by the splitting unit; a second optical sensor that measures the light intensity of the second polarized light split by the splitting unit; and a circuit that calculates the intensity ratio between the light intensity measured by the first optical sensor and the light intensity measured by the second optical sensor.

[0025] The imaging system according to the present invention uses the above-described measuring device to image the distribution of magnetization in a magnetic material.

[0026] As described above, according to the present invention, linearly polarized ultrashort pulse laser light is split into first and second light beams, the optical path length of the first light is made longer than the optical path length of the second light beam, and the first light beam and the circularly polarized second light beam are irradiated onto an irradiated portion of a magnetic material. This makes it possible to magnetize the target magnetic material more easily and uniformly, and to increase the measurement sensitivity of harmonics due to magnetization-induced second harmonic generation.

[0027] FIG. 1 is a diagram showing the configuration of a measurement device according to an embodiment of the present invention. FIG. 2 is a diagram showing the configuration of another measurement device according to an embodiment of the present invention. FIG. 3 is an explanatory diagram showing the branching section 102 and the demultiplexing section 106 as viewed from the negative direction of the x-axis. FIG. 4 is a characteristic diagram showing graphs of the derivative (a) with respect to the angle of equation (5) and the derivative (b) with respect to the angle of equation (6). FIG. 5 is a flowchart for explaining a measurement method according to an embodiment of the present invention. FIG. 6 is a diagram showing the configuration of a conventional measurement device.

[0028] A measurement device according to an embodiment of the present invention will be described below with reference to Fig. 1. This measurement device includes a light source (LD) 101, a branching unit 102, a wave plate 103, a first optical system 151, a second optical system 152, a delay unit 104, a filter 105, a branching unit 106, a first optical sensor (PD1) 107, a second optical sensor (PD2) 108, and a circuit 109.

[0029] The LD 101 emits linearly polarized ultrashort pulse laser light 121. The LD 101 may be a well-known femtosecond laser or picosecond laser. The LD 101 is, for example, a linearly polarized femtosecond laser that emits light with a wavelength of 835 nm, a pulse width of 100 fs, an average output of 300 mW, and a repetition rate of 82 MHz.

[0030] The splitter 102 is disposed between the LD 101 and a predetermined irradiation location of the target magnetic material 131, and splits the laser beam 121 into a first beam 122 and a second beam 123. The splitter 102 may be, for example, a well-known beam splitter. The magnetic material 131 may be, for example, Fe. The wave plate 103 converts the second beam 123 split by the splitter 102 into circularly polarized light. The wave plate 103 may be, for example, a λ / 4 plate. For example, in a coordinate system in which the surface of the magnetic material 131 is defined as the xy plane and the normal to the surface of the magnetic material 131 is defined as the z axis, the laser beam 121 is split into the first beam 122 and the second beam 123 so that the wavefront is parallel to the x axis.

[0031] The delay unit 104 makes the optical path length of the first light 122 in the first optical system 151 longer than the optical path length of the second light 123 in the second optical system 152. The delay unit 104 delays the time it takes for the first light 122 to reach the irradiated location relative to the time it takes for the second light 123 to reach the irradiated location. The delay unit 104 can be composed of, for example, a first mirror 141, a second mirror 142, a third mirror 143, and a fourth mirror 144.

[0032] The first light 122 incident on the delay unit 104 has its optical path changed by 90° by the first mirror 141 and is incident on the second mirror 142. The first light 122 incident on the second mirror 142 has its optical path changed by 90° by the second mirror 142 and is incident on the third mirror 143. The first light 122 incident on the third mirror 143 has its optical path changed by 90° by the third mirror 143 and is incident on the fourth mirror 144 and is further changed by 90°. The first light whose optical path has been changed by 90° by the fourth mirror 144 is incident on the multiplexing unit 155.

[0033] The optical path length control mechanism 110 may also be provided to control the optical path length of the delay unit 104. For example, the optical path length control mechanism 110 changes the optical path length in the delay unit 104 by changing the distance between the second mirror 142 and the third mirror 143 and the distance between the first mirror 141 and the fourth mirror 144. By changing the optical path length, it is possible to change the delay time between the arrival of the first light 122 at the irradiation location relative to the arrival of the second light 123 at the irradiation location.

[0034] The first optical system 151 irradiates the irradiation location with first light 122. The second optical system 152 irradiates the irradiation location with circularly polarized second light 123. In this example, the first optical system 151 includes a delay unit 104, a multiplexing unit 155, and a first objective lens 156. The second optical system 152 includes a fifth mirror 153, a sixth mirror 154, a wave plate 103, the multiplexing unit 155, and the first objective lens 156. In this example, the first light 122 that has passed through the delay unit 104 and the circularly polarized second light 123 are multiplexed (coaxially) by the multiplexing unit 155 to form multiplexed light 128, and the first optical system 151 and the second optical system 152 share the multiplexing unit 155 and the first objective lens 156.

[0035] The first light 122, which travels through the first optical system 151 as an optical path, and the second light 123, which travels through the second optical system 152 as an optical path, are irradiated onto a predetermined irradiation location on the magnetic material 131. In this example, the first light 122 and the second light 123 are coaxially coupled by the coupling unit 155 to form a coupled light 128, which is then irradiated onto the irradiation location. The first light 122 passing through the delay unit 104 is delayed relative to the circularly polarized second light 123 and then irradiated onto the irradiation location. Note that non-magnetic materials 132 are formed at multiple locations on the surface of the magnetic material 131. The non-magnetic materials 132 are portions where rust has occurred due to deterioration of the magnetic material 131 made of Fe. The irradiation location can be the interface between the magnetic material 131 and the non-magnetic material 132. For example, the first light 122 and the combined light 128 from the second optical system 152, which are made coaxial by the combining section 155, are focused by the first objective lens 156 onto the interface between the magnetic material 131 and the non-magnetic material 132 and irradiated thereon.

[0036] The irradiation spot irradiated with the circularly polarized second light 123 is locally magnetized. The irradiation spot within the range of the beam spot diameter of the second light 123 is magnetized. When the first light 122 is irradiated onto the magnetized irradiation spot, reflected light 124 of the first light 122 is reflected at the irradiation spot, and further, measurement light 125, which is a harmonic having a frequency twice that of the laser light 121, is generated by magnetization-induced second harmonic generation.

[0037] Here, since the magnetization induced by the circularly polarized second light 123 demagnetizes over time, the delay unit 104 is adjusted so that the delay time is approximately several ps or less. However, if the delay time is adjusted to 0 ps or less, a rapid increase in the rotation angle of the measuring light 125 due to the specular inverse Faraday effect (SIFE) or the specular inverse Kerr effect (SIKE) is expected (reference literature).

[0038] The filter 105 transmits measuring light 125 having a frequency double that of the laser light 121 generated at the irradiation point irradiated with the first light 122 and the second light 123 (combined light 128). The reflected light 124 reflected at the irradiation point and the generated measuring light 125 are incident on the filter 105 (diffused) through the second objective lens 157. The filter 105 can be a bandpass filter that attenuates the reflected light 124 and transmits the measuring light 125.

[0039] The demultiplexing unit 106 demultiplexes the transmitted light that has passed through the filter 105 into first polarized light 126 and second polarized light 127 that are orthogonal to each other. For example, the first polarized light 126 can be s-polarized light, and the second polarized light 127 can be p-polarized light. The demultiplexing unit 106 can be configured, for example, by a polarizing beam splitter. The PD 1107 measures the light intensity of the first polarized light 126 demultiplexed by the demultiplexing unit 106. The PD 2108 measures the light intensity of the second polarized light 127 demultiplexed by the demultiplexing unit 106. The PD 1107 and PD 2108 can be configured, for example, by photodiodes.

[0040] The circuit 109 calculates and outputs the intensity ratio between the light intensity measured by the PD 1107 and the light intensity measured by the PD 2108. The circuit 109 can be configured, for example, from an oscilloscope. The output of the PD 1107 and the output of the PD 2108 are connected to channels 1 and 2 of the oscilloscope, and the maximum value of the waveforms is taken as the light intensity. The intensity ratio can be calculated by calculating the ratio of these values.

[0041] Here, if the combining unit 155 is a polarizing beam splitter that transmits s-polarized light (first polarization) and reflects p-polarized light (second polarization) in the above-mentioned coordinate system, a portion of the circularly polarized second light 123 is irradiated onto the irradiation location. Furthermore, the s-polarized light of the first light 122 is transmitted and irradiated onto the irradiation location. According to Non-Patent Document 4, the rotation angle of the measuring light 125 generated by MSHG on the surface of the magnetic material 131 (Fe) is more than 10 times greater when the incident light is s-polarized than when it is p-polarized. Since the above-mentioned phenomenon varies depending on the type of magnetic material, the polarization of the first light 122 irradiated onto the irradiation location should be changed. In this example, the magnetic material 131 is assumed to be Fe (steel), and the combining unit 155 is a polarizing beam splitter so that the first light 122 irradiated onto the irradiation location is s-polarized.

[0042] As shown in FIG. 2, this measuring device can irradiate the irradiation location with first light 122 and second light 123 that has been made circularly polarized, separately.

[0043] In this example, the first optical system 151 includes a delay unit 104, a combining unit 155, and a first objective lens 156. The second optical system 152 includes a fifth mirror 153, a sixth mirror 154, a wave plate 103, and a third objective lens 158. The first light 122 that has passed through the delay unit 104 passes through the first objective lens 156 and is irradiated onto the irradiation location.

[0044] On the other hand, the second light 123 split by the splitter 102 is reflected by the fifth mirror 153 and the sixth mirror 154, passes through the wave plate 103, and passes through the third objective lens 158 to be irradiated onto the irradiation location. In this configuration, there is no need to use the multiplexer 155 of the measurement device described with reference to FIG.

[0045] From theoretical and experimental results, by making the incident angle of the circularly polarized second light 123 approach 0°, ijk It is expected that the contribution of the nonlinear magneto-optical Kerr effect (NOMOKE) will change and the rotation angle of the measuring beam 125 generated by the nonlinear magneto-optical Kerr effect (NOMOKE) will increase (reference literature).

[0046] The measurement device may also include a circularly polarized light incident angle control mechanism 111 that controls the angle of incidence of the circularly polarized second light 123 at the irradiation location. The circularly polarized light incident angle control mechanism 111 controls the angle of incidence of the second light 123 at the irradiation location by, for example, adjusting the optical path of the second optical system 152.

[0047] The measurement device may further include a linearly polarized light incident angle control mechanism 112 that controls the angle of incidence of the first light 122 at the irradiation location. The linearly polarized light incident angle control mechanism 112 adjusts the optical path of the first optical system 151 to control the angle of incidence of the first light 122 at the irradiation location.

[0048] In this way, when the beam lines of the first light 122 and the circularly polarized second light 123 are not coaxial, the incident angle of the second light 123 is adjusted so that the polarization rotation angle of the measurement light 125 is maximized. For example, although the x component may be taken into consideration even if the magnetization is not parallel to the x axis, a case where s-polarized light parallel to the y axis is irradiated onto a magnetic material 131 made of Fe magnetized in the x axis direction is considered here for simplicity. The rotation angle φ of the measurement light 125 is k (2) is expressed by the following formula:

[0049]

[0050] In equation (4), χ yyy (2)- and χ yyy (2)+ When the sensitivity is poor in a configuration in which the beam lines of the first light 122 and the circularly polarized second light 123 are coaxial, the sensitivity can be improved by adjusting the incident angle of the second light 123 using the circularly polarized light incident angle control mechanism 111.

[0051] Next, the reflected light 124 of frequency ω and the measuring light 125 of 2ω are expanded by the second objective lens 157, and the measuring light 125 is extracted by the filter 105. After that, the measuring light 125 is split into two by the splitting unit 106. In this splitting, the first polarized light 126, which has the same polarization direction as the first light 122, and the second polarized light 127, which is a polarized component perpendicular to the first polarized light 126, are split. After this splitting, the light intensity I of the first polarized light 126 is measured by the PD 1107, and the light intensity I of the second polarized light 127 is measured by the PD 2108.

[0052] 3 shows the branching unit 102 and the demultiplexing unit 106 as viewed from the negative direction of the x-axis. K (2) The light intensity entering the PD is proportional to the square of the electric field amplitude, so the following equation can be measured as an index of NOMOKE:

[0053]

[0054] Furthermore, when the demultiplexing unit 106 is an analyzer instead of a polarizing beam splitter, as in a crossed Nicol system, and the light intensity I1 of the first polarized light 126 is measured by the PD 1107, the following equation (6) is obtained.

[0055]

[0056] 4 shows graphs of the derivative (a) of equation (5) with respect to the angle and the derivative (b) of equation (6) with respect to the angle. As shown in FIG. 4, the derivative of equation (5) changes more sharply, and therefore NOMOKE can be detected with good sensitivity, although this depends on the extinction ratio of the polarizing beam splitter and analyzer that make up the demultiplexing unit 106.

[0057] By utilizing these mechanisms, when the first light 122 and the circularly polarized second light 123 are irradiated onto the surface of the magnetic material 131 where the non-magnetic material 132 does not exist, the χ (2) The behavior of MSHG is different from that when the second harmonic is focused on the interface between different materials, and this difference makes it possible to image the magnetic material portion and the non-magnetic material portion. According to the measuring device of the embodiment, the difference between the rotation angle of the polarization angle of the measuring light 125 of the second harmonic from the non-magnetic material and the rotation angle of the polarization angle of the measuring light 125 of the second harmonic from the magnetic material can be increased, thereby realizing high-sensitivity imaging.

[0058] As described above, by using the measuring device according to the embodiment, it is possible to construct an imaging system that images the distribution of magnetization in the magnetic material 131. For example, by changing the relative positional relationship between the magnetic material 131 and the measuring device and scanning the irradiation location, it is possible to two-dimensionally image the distribution of material differences (magnetization) on the surface of the magnetic material 131.

[0059] Next, a measurement method according to the embodiment will be described with reference to Fig. 5. First, in a first step S101, linearly polarized ultrashort pulse laser light 121 emitted from light source 101 is split into first light 122 and second light 123 by splitter 102.

[0060] Next, in a second step S102, the split second light 123 is made circularly polarized using a wave plate 103. In a third step S103, the delay unit 104 makes the optical path length of the first light 122 to the set irradiation location of the magnetic material 131 longer than the optical path length of the second light 123 to the irradiation location.

[0061] Next, in a fourth step S104, the irradiation location is irradiated with the first light 122 and the circularly polarized second light 123. Next, in a fifth step S105, the filter 105 transmits measurement light 125 having a frequency double that of the laser light 121 generated at the irradiation location irradiated with the first light 122 and the second light 123.

[0062] Next, in a sixth step S106, the demultiplexing unit 106 demultiplexes the transmitted light transmitted in the fifth step S105 into a first polarized light 126 and a second polarized light 127. Next, in a seventh step S107, the light intensity of the first polarized light 126 demultiplexed in the sixth step S106 is measured. Also, in an eighth step S108, the light intensity of the second polarized light 127 demultiplexed in the sixth step S106 is measured.

[0063] Next, in a ninth step S109, the circuit 109 calculates the intensity ratio between the light intensity measured in the seventh step S107 and the light intensity measured in the eighth step S108. For example, the circuit 109 scans the irradiated area to obtain the above-mentioned intensity ratios at a plurality of areas, and generates a two-dimensional image of the distribution of material differences (magnetization) on the surface of the magnetic material 131 from the distribution of the obtained plurality of intensity ratios.

[0064] As described above, in this embodiment, a sample containing a non-magnetic material is irradiated with an ultrashort pulse laser beam 121, which is split into a circularly polarized beam (second light 123) for magnetizing the object of observation and a linearly polarized beam (first light 122) for generating harmonics by magnetization-induced second harmonic generation at the surface or interface of the object of observation. The linearly polarized beam is then delayed and irradiated. In this manner, the linearly polarized beam is irradiated with a delayed beam at an irradiated location magnetized by the circularly polarized beam, and the intensity ratio between orthogonal first and second polarized light of harmonics (measurement light 125) generated depending on the nonlinear susceptibility of the irradiated location is measured. The difference in the intensity ratio between the first and second polarized light of the harmonics generated at multiple locations is imaged by scanning the irradiated location.

[0065] In the embodiment, in order to uniformly magnetize multiple irradiation locations, a circularly polarized pulse is used as pump light for magnetizing the magnetic material to be observed, and local magnetization can be achieved within the beam spot diameter of the irradiation location, which is the location to be measured (observed). Furthermore, by maintaining the power of the circularly polarized pulse constant, the strength of magnetization can be made constant regardless of the measurement location. In addition, the magnetization direction can be controlled by changing the incident angle of the pump light, allowing for flexibility in magnetization methods suited to the material.

[0066] Furthermore, according to the embodiment, the linearly polarized beam serving as the so-called probe light is an ultrashort pulse laser light, which makes it possible to generate the nonlinear magneto-optical Kerr effect (NOMOKE), thereby increasing the rotation angle of the polarization of the measurement light and achieving even higher sensitivity.

[0067] Furthermore, by controlling the delay time between the pump light and the probe light, it is possible to irradiate the irradiation location with the probe light, taking into consideration the time change in magnetization caused by the pump light.

[0068] By combining the above configurations, it is possible to locally magnetize the region to be observed without using an external magnetic field application device, and to observe a larger rotation angle of polarization compared to linear MOKE using NOMOKE.

[0069] Furthermore, as described with reference to FIG. 2 , further optimization can be achieved by irradiating the first light 122 and the circularly polarized second light 123 at different incident angles. First, the circularly polarized incident angle control mechanism 111 adjusts the incident angle of the second light 123 so that the rotation of the polarization angle of the measurement light 125 generated by magnetization-induced second harmonic generation is maximized. Furthermore, since the greater the power of the second light 123, the greater the magnetization, so the power of the second light 123 (LD 101) is adjusted within a range that does not damage the target magnetic material 131. Next, the linearly polarized incident angle control mechanism 112 controls the incident angle of the first light 122 at the irradiation location so that the rotation of the polarization angle of the measurement light 125 is maximized. These controls enable measurements to be performed with the polarization angle of the measurement light 125 always rotated at its maximum, leading to higher sensitivity.

[0070] According to the above-described embodiment, imaging with a contrast ratio of, for example, 2.1 times is possible.

[0071] For example, let us consider a case where the magnetic material 131 is an iron plate and the non-magnetic material 132, which is a rusted portion on the surface, is present. The LD 101 emits linearly polarized laser light 121 with a wavelength of 835 nm, a pulse width of 100 fs, an average output of 300 mW, and a repetition rate of 82 MHz. The splitter 102 is a polarizing beam splitter that transmits s-polarized light (first polarized light) and reflects p-polarized light (second polarized light).

[0072] First, the incident angle of the circularly polarized second light 123, the power of the second light 123, and the delay unit 104 are controlled to adjust the rotation of the polarization angle of the measurement light 125 to a maximum.

[0073] In addition, the incident angle of the circularly polarized second light 123 is adjusted so that the magnetization becomes as parallel as possible to the wavefront and the surface of the magnetic material 131 (to the x-axis), and the s-polarized first light 122 is irradiated onto the magnetic material 131 at an incident angle of 15° so that the delay time by the delay unit 104 becomes 0 ps.

[0074] For these reasons, the rotation angle of the measuring beam 125 is φ K (2)At the interface between the magnetic material 131 and the non-magnetic material 132, the rotation angle of the measuring light 125 is about 75% φ K (2) When the angle is set to 30°, I2 / I1 becomes 0.33, and the magnetic material 131 and the non-magnetic material 132 can be imaged with a contrast ratio of 2.1.

[0075] Furthermore, when the conventional linear magneto-optical Kerr effect is used in the above-mentioned device, the rotation angle of the measuring light 125 is φ K (1) 0.004°, and from equation (5), I2 / I1 is 4.9 × 10 -7 At the interface between the magnetic material 131 and the non-magnetic material 132, the rotation angle of the measurement light is about 75% φ K (1) If the contrast ratio is 1.8 times, imaging can be performed with a contrast ratio of 1.8 times. -7 In the case of a measurement ratio of the order of φ K (1) Since it is unlikely that the difference in intensity between the angle of 0.04° and the angle of 0.03° can be measured, this method is not considered suitable for imaging differences in materials.

[0076] As a method of utilizing an imaging system using a measuring device according to an embodiment, for example, when steel material has deteriorated and rust has appeared on the surface of the steel material, by using the imaging system according to an embodiment, 2D mapping (two-dimensional image) that distinguishes between the steel material (iron) and the rust (iron oxide) can be obtained.

[0077] The circuit 109 may be a computer device including a CPU (Central Processing Unit), a main memory device, an external memory device, a network connection device, etc., and the above-described function (the method of the ninth step) may be realized by the CPU operating (executing) a program loaded in the main memory device. The program is a program for causing a computer to execute the method of the ninth step shown in the above-described embodiment. The network connection device is connected to a network.

[0078] As described above, according to the present invention, linearly polarized ultrashort pulse laser light is split into first and second light beams, the optical path length of the first light is made longer than the optical path length of the second light, and the first light and the circularly polarized second light are irradiated onto an irradiated portion of a magnetic material, thereby making it easier to uniformly magnetize the target magnetic material and increasing the measurement sensitivity of harmonics due to magnetization-induced second harmonic generation. According to the present invention, a highly sensitive nonlinear magneto-optical imaging technique can be realized by controlling the magnetization direction and causing NOMOKE at an irradiated portion of a magnetized magnetic material without using devices such as permanent magnets or coils.

[0079] Some or all of the above-described embodiments may also be described as, but are not limited to, the following supplementary notes.

[0080] a fifth step of transmitting measurement light having a frequency twice that of the laser light generated at the irradiation location irradiated with the first and second lights; a sixth step of separating the transmitted light transmitted in the fifth step into first and second polarized lights that are orthogonal to each other; a seventh step of measuring the light intensity of the first polarized light separated in the sixth step; an eighth step of measuring the light intensity of the second polarized light separated in the sixth step; and a ninth step of calculating an intensity ratio between the light intensity measured in the seventh step and the light intensity measured in the eighth step.

[0081] [Supplementary Note 2] A light source that emits linearly polarized ultrashort pulse laser light, a branching unit that is disposed between the light source and a set irradiation location of a target magnetic material and that branches the laser light into first light and second light, a wave plate that circularly polarizes the second light branched by the branching unit, a first optical system that irradiates the irradiation location with the first light, a second optical system that irradiates the irradiation location with the circularly polarized second light, a delay unit that makes the optical path length of the first light in the first optical system longer than the optical path length of the second light in the second optical system, and a delay unit that makes the optical path length of the first light in the first optical system longer than the optical path length of the second light in the second optical system. a first optical sensor that measures the intensity of the first polarized light separated by the splitting unit; a second optical sensor that measures the intensity of the second polarized light separated by the splitting unit; and a circuit that calculates the intensity ratio of the light intensity measured by the first optical sensor to the light intensity measured by the second optical sensor.

[0082] [Supplementary Note 3] The measurement device according to Supplementary Note 2, further comprising a circularly polarized light incident angle control mechanism that controls the incident angle of the circularly polarized second light onto the irradiation location.

[0083] [Supplementary Note 4] The measurement device according to Supplementary Note 2 or 3, further comprising a linearly polarized light incident angle control mechanism that controls the incident angle of the first light onto the irradiation location.

[0084] [Supplementary Note 5] The measurement device according to any one of Supplementary Notes 2 to 4, further comprising an optical path length control mechanism that controls the optical path length of the delay section.

[0085] [Supplementary Note 6] The measurement device according to any one of Supplementary Notes 2 to 5, further comprising a multiplexing unit that multiplexes the first light that has passed through the delay unit and the second light that has been made circularly polarized.

[0086] [Supplementary Note 7] An imaging system that uses the measuring device according to any one of Supplementary Notes 2 to 6 to image the distribution of magnetization in the magnetic material.

[0087] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0088] [Reference] S. Iihama et al., "Interface-induced field-like optical spin torque in a ferromagnet / heavy metal heterostructure", Nanophotonics, vol. 10, Issue 3, pp. 1169-1176, 2021.

[0089] 101...light source (LD), 102...branching section, 103...wave plate, 104...delay section, 105...filter, 106...wavelength branching section, 107...first optical sensor (PD1), 108...second optical sensor (PD2), 109...circuit, 110...optical path length control mechanism, 111...circularly polarized light incident angle control mechanism, 112...linearly polarized light incident angle control mechanism, 121...laser light, 122...first light, 123...second light, 124...reflected light, 125... Measurement light, 126...first polarized light, 127...second polarized light, 128...combined light, 131...magnetic material, 132...non-magnetic material, 141...first mirror, 142...second mirror, 143...third mirror, 144...fourth mirror, 151...first optical system, 152...second optical system, 153...fifth mirror, 154...sixth mirror, 155...combining section, 156...first objective lens, 157...second objective lens, 158...third objective lens.

Claims

a fifth step of transmitting measurement light having twice the frequency of the laser light generated at the irradiation point irradiated with the first and second lights; a sixth step of separating the transmitted light transmitted in the fifth step into first and second polarized lights that are orthogonal to each other; a seventh step of measuring the intensity of the first polarized light separated in the sixth step; an eighth step of measuring the intensity of the second polarized light separated in the sixth step; and a ninth step of calculating the intensity ratio between the light intensity measured in the seventh step and the light intensity measured in the eighth step.

2. A light source that emits linearly polarized ultrashort pulse laser light; a branching unit that is arranged between the light source and a set irradiation location on the target magnetic material and that branches the laser light into first light and second light; a wave plate that circularly polarizes the second light branched by the branching unit; a first optical system that irradiates the irradiation location with the first light; a second optical system that irradiates the irradiation location with the circularly polarized second light; a delay unit that makes the optical path length of the first light in the first optical system longer than the optical path length of the second light in the second optical system; a filter that transmits measurement light having twice the frequency of the laser light generated at the irradiation location irradiated with the first light and the second light; a branching unit that separates the transmitted light that has passed through the filter into first and second polarized light that are orthogonal to each other; a first optical sensor that measures the light intensity of the first polarized light separated by the branching unit; and a second optical sensor that measures the light intensity of the second polarized light separated by the branching unit. a circuit for determining an intensity ratio between the light intensity measured by the first optical sensor and the light intensity measured by the second optical sensor.

3. A measuring device according to claim 2, further comprising a circularly polarized light incident angle control mechanism for controlling the incident angle of the second light, which is circularly polarized light, on the irradiation location.

4. A measuring device according to claim 2, further comprising a linearly polarized light incident angle control mechanism for controlling the incident angle of said first light onto said irradiation location.

5. The measuring device according to claim 2, further comprising an optical path length control mechanism for controlling the optical path length of said delay section.

6. A measuring device according to claim 2, further comprising a multiplexing section for multiplexing the first light that has passed through the delay section and the second light that has been made circularly polarized.

7. An imaging system for imaging the distribution of magnetization in a magnetic material using the measuring device according to any one of claims 2 to 6.

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