JP7134476B2 - Measuring device, near-field measuring method - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act APS March Meeting 2018, March 6, 2018 The Surface Science Society of Japan 3rd Kanto Branch Conference, April 7, 2018 2018 KPS Spring Meeting, April 26, 2018

The present invention relates to a measuring device and a near-field measuring method.

An Optical Pump-Probe Scanning Tunneling Microscopy (OPP-STM) has been proposed as a device for acquiring information on a sample by time-resolving it at the atomic level and the molecular level. In OPP-STM, the probe and the sample to be measured are first irradiated with the pump light, and the photoinduced dynamics caused by the pump light, such as the excitation and relaxation process of the sample, are measured between the pump light and the probe light. is observed by changing the delay time of and measuring the corresponding change in the tunneling current. By the way, it is known that the electric field waveform generated between the probe and the sample by the irradiation of the pump light does not match the original waveform of the pump light, and is affected by various factors such as the shape of the probe and the sample. , various hypotheses have been proposed. However, no method has been found to directly measure and evaluate the electric field waveform generated by the irradiation of the pump light on the spot under actual conditions. Regarding OPP-STM, for example, Patent Document 1 can be cited, but Patent Document 1 does not disclose a device or method for directly measuring a near-field generated by irradiation with pump light.

JP 2013-32993 A

Normally, a pulse contains an electric field of several cycles, and its phase (carrier envelope phase, CEP) is random between pulses and fluctuates with time. That was the reason why the time resolution was determined by the pulse width, ie not better. In this case, the electric field waveform does not matter and has not been paid attention so far. Recently, however, CEP has been locked (stable without fluctuations over time), and it has become possible to control it. , it has become very important to know the electric field waveform. In particular, terahertz-STM (THz-STM), which uses an electric field instead of an applied voltage, has been developed as pulsed light with one cycle of an electric field has become available. In order to develop such technology, it is indispensable to have a method to measure nanoscale metal structures and near-field waveforms right under the STM probe on the spot and evaluate them correctly. The conventional technique cannot directly measure the near-field generated by irradiation with the pump light.

A measuring apparatus according to a first aspect of the present invention comprises a first optical system that irradiates an object to be irradiated with first light whose carrier envelope phase is locked to form a near-field, and at least an irradiation range of the first light. A second optical system that irradiates the object to be irradiated with a second light having a pulse width shorter than that of the first light so as to partially overlap, and the second light is emitted when the object is irradiated. a measurement unit for measuring the photoelectrons or tunnel current generated by the first light, and the first light and the and a timing adjustment unit that adjusts irradiation timing of at least one of the second lights.
A near-field measuring method according to a second aspect of the present invention comprises irradiating an object to be irradiated with first light whose carrier envelope phase is locked to form a near-field; irradiating the object to be irradiated with a second light having a pulse width shorter than that of the first light so as to partially overlap; and photoelectrons emitted by irradiating the object to be irradiated with the second light. Alternatively, measuring a tunnel current, and the first light and the second light so that the object is irradiated with the second light while the near-field is present due to the first light. and adjusting the irradiation timing of at least one of

According to the present invention, it is possible to directly measure the near-field waveform generated by irradiation with CEP-controlled pump light.

Schematic configuration diagram of an electron microscope 1 according to a first embodiment Diagram for explaining delay time td FIG. 4 is a diagram showing a near-field measurement state in the first embodiment; Model diagram of the near-field measurement mechanism in the optoelectronic region A diagram showing the results of the first experiment A diagram showing the results of the second experiment Diagram showing the spatial distribution of measurements Diagram showing measurement results in the time domain Diagram showing the spatial distribution of measurements at different frequencies Diagram showing measurement results in the frequency domain Schematic configuration diagram of an electron microscope 1A according to a second embodiment The figure which shows the measurement state of the near-field in 2nd Embodiment. A model diagram of the near-field measurement mechanism in the tunneling current region and a diagram showing the measurement results in the tunneling current region

-First Embodiment-
A first embodiment of an electron microscope, which is a measuring apparatus according to the present invention, will be described below with reference to FIGS. 1 to 10. FIG.

(Outline of the first embodiment)
In the first embodiment, the near field generated by irradiation with pump light is observed by measuring photoelectrons.

(Constitution)
FIG. 1 is a schematic configuration diagram of an electron microscope 1 according to the first embodiment. The electron microscope 1 includes a pump light output section 2 , an observation light output section 3 , a probe light output section 4 , a probe 5 , a measurement section 6 and an application section 7 . The electron microscope 1 observes the sample 900 by the pump-probe method. In this embodiment, the sample 900 is graphite (Highly oriented pyrolytic graphite: HOPG).

The outline of the electron microscope 1 is as follows. Pump light output unit 2 outputs pump light 21 . The observation light output unit 3 outputs observation light 31 . The probe light output unit 4 outputs probe light 41 . Pump light 21 and probe light 41 are pump light and probe light for observing sample 900 by a so-called pump-probe method. Observation light 31 is used for near-field observation. Next, each configuration will be described.

The pump light output unit 2 outputs CEP-controlled pump light 21 generated by an arbitrary method. Although the wavelength of the pump light 21 limits the wavelength of the observation light 31, the frequency of the pump light 21 is not particularly limited, although the details will be described later. The pump light output unit 2 outputs a THz pulse having a period of 1 picosecond and a width of about 1 picosecond, which is obtained by irradiating a LiNiO ₃ crystal with an IR pulse light having a wavelength of 1034 nm and a width of 300 femtoseconds. The spot diameter of the pump light 21 in this embodiment is about 1 millimeter. The pump light 21 has its CEP controlled and is output with a constant phase.

The observation light output unit 3 outputs the observation light 31 generated by an arbitrary method, the pulse width of which is sufficiently shorter than that of the pump light 21, and which is one cycle of pulsed light. The observation light output unit 3 converts, for example, IR light with a wavelength of 1035 nanometers and a width of 300 femtoseconds into a second harmonic with a wavelength of 517 nanometers using a BBO (β-Ba ₂ BO ₄ ) crystal, and generates pulsed light as observation light. output as 31. The spot diameter of the observation light 31 in this embodiment is approximately 10 micrometers. It is desirable that the CEP of the observation light 31 is also controlled, and the time resolution can be improved by controlling the CEP.

The observation light output section 3 includes a delay time adjustment section 3A and an irradiation position adjustment section 3B. The delay time adjuster 3A changes the timing at which the probe 5 is irradiated with the observation light 31 in a plurality of ways with reference to the timing at which the pump light 21 is irradiated. The timing adjusted by the delay time adjuster 3A is called a delay time td. For example, the delay time adjustment unit 3A adjusts the timing so that the probe is irradiated with the observation light 31 0.1 picoseconds later than the pump light 21 for the first time, and 0.2 picoseconds later than the pump light 21 for the second time. The timing is adjusted so that the probe is irradiated with the observation light 31 later by picoseconds, and the timing is shifted in the same way. The delay time adjusting section 3A is, for example, an optical path length adjusting device that changes the optical path length.

2A and 2B are diagrams for explaining the delay time td, where the upper graph shows the time change of the pump light 21 and the lower graph shows the time change of the observation light 31. FIG. In FIG. 2, time elapses from left to right. However, FIG. 2 does not show the order in which the delay times td are changed, and the delay times td may be changed in any order. The upper stage and the lower stage of FIG. 2 show the same time if the positions in the horizontal direction are the same. The definition of the delay time td in the present embodiment is the difference in the timing at which the intensity peak begins to be irradiated, and is negative when the observation light 31 reaches the probe 5 first.

In the state of td=-1 ps shown on the far left, the intensity peak of the observation light 31 is emitted earlier than the intensity peak of the pump light 21 by 1 picosecond. In the state of td=0 shown second from the left, the intensity peak of the observation light 31 and the intensity peak of the pump light 21 start at the same time. In the state of td=1 ps shown third from the left, the intensity peak of the observation light 31 is delayed from the intensity peak of the pump light 21 by 1 picosecond. In the state of td=2 ps shown on the far right, the intensity peak of the observation light 31 is delayed from the intensity peak of the pump light 21 by 2 picoseconds. Returning to FIG. 1, the description continues.

The irradiation position adjuster 3B adjusts the position at which the probe 5 is irradiated with the observation light 31 . Since the spot diameter of the observation light 31 is sufficiently smaller than that of the probe 5, various positions on the probe 5 can be selectively irradiated. The irradiation position adjusting section 3B is composed of, for example, a mirror, a piezo element for tilting the mirror, and a controller for controlling the piezo element.

The probe light output unit 4 outputs probe light 41 generated by an arbitrary method. The probe light 41 may be suitable for observing the sample 900 together with the pump light 21 . However, the details of the measurement using the pump light 21 and the probe light 41 are omitted in the description of this embodiment. The probe light output unit 4 may be provided with a mechanism for adjusting the irradiation timing of the probe light 41 based on the irradiation timing of the pump light 21 .

The probe 5 is tapered and applied with an arbitrary voltage by the applying section 7 . The material of the probe 5 in this embodiment is tungsten, which is coated with Pt/Ir. The probe 5 is made by electropolishing, for example. The shape of the probe 5 differs from individual to individual, and it is not easy to produce exactly the same probe 5 . In this embodiment, a first probe 5a and a second probe 5b having slightly different shapes are used. However, when there is no particular distinction between the two, they are referred to as "probe 5".

The measurement unit 6 uses a current preamplifier to detect electrons flowing into the sample 900, which serves as a counter electrode for the probe 5 in this embodiment, as a total current I _ALL . However, since the signal is weak, the pump light 21, that is, the THz pulse is chopped at a predetermined frequency, for example, 430 Hz, and the change in current with respect to the delay time td is lock-in detected as the differential current ΔI.

The application unit 7 applies a bias voltage Vdc to the probe 5 .

(Measurement state)
FIG. 3 is a diagram showing a near-field measurement state in the first embodiment. However, FIG. 3 shows an outline of the positional relationship, etc., and the dimensions and distances are exaggerated. A bias voltage Vdc is applied to the probe 5 by the application unit 7 . The probe 5 is placed at least 100 nm away from the sample 900 so that no tunneling current flows between the tip of the probe 5 and the sample 900 . The environment shown in FIG. 3 is room temperature in a vacuum.

The probe 5 and the sample 900 are irradiated with the pump light 21 emitted by the pump light output unit 2 . In FIG. 3, the irradiation range of the pump light 21 is indicated by a large dashed circle. As described above, the spot diameter of the pump light 21 is relatively large, and a wide range including the tip of the probe 5 is irradiated with the pump light 21 . This spot diameter is about millimeters for THz pulses, for example. In this embodiment, the irradiation position of the pump light 21 is fixed.

The observation light 31 emitted by the observation light output unit 3 is applied to a partial area of the probe 5 . In FIG. 3, the irradiation range of the observation light 31 is indicated by a small solid circle. As described above, the spot diameter of the observation light 31 is relatively small, for example, about 10 micrometers, and only a very small portion of the probe 5 is irradiated with it at one time. In FIG. 3, the observation light 31 is applied to the tip and middle of the probe 5, but is applied to various positions of the probe 5 by the irradiation position adjusting section 3B.

(measurement principle)
FIG. 4 is a model diagram of the near-field measurement mechanism in the optoelectronic region. As shown in FIG. 4, first, a near-field is formed by the pump light 21, and the barrier between the surface of the probe 5 and the vacuum is modulated by the magnitude of the electric field combined with the bias voltage Vdc of the applying section 7. As shown in FIG. When the probe 5 is irradiated with the observation light 31 , photoelectrons are generated by the multiphoton absorption process according to the height of the barrier and the energy of the observation light.

In addition, the amount of emitted photoelectrons represents the height and width of the barrier at the irradiated position at the moment when the observation light 31 is irradiated, that is, the strength of the near-field that modulates the barrier. By changing the irradiation position of the observation light 31, it is possible to measure the change in near-field strength over time and the difference in the near-field depending on the position of the probe 5. FIG. By adjusting Vdc so that the photoelectron current flows even in the absence of the pump light 21, it is possible to measure across the positive and negative regions of the electric field of the pump light 21, as will be described later.

(Total current I _ALL and differential current ΔI)
The pump light 21 irradiated to the probe 5 forms a near-field and changes the barrier between the probe surface and the vacuum. The current I measured by the measurement unit 6 is affected by the pump light 21 , the observation light 31 , and the bias voltage Vdc from the application unit 7 . Here, the voltage generated by the electric field of the observation light 31 is represented by _Vov , and the ease of passage of photoelectrons in the barrier between the probe surface and the vacuum is represented by the function Ba. 1 holds.
I _ALL =Ba(Vdc+Vov) (Formula 1)

However, Vov in Equation 1 is for convenience. When the probe 5 is irradiated with the observation light 31, photoelectrons are emitted through a multiphoton absorption process, and the number of emitted photoelectrons depends on the height of the barrier between the probe surface and the vacuum. Therefore, Vov is set for convenience in order to clearly show that it is affected by the probe surface-vacuum barrier in the same way as the bias voltage Vdc. Further, hereinafter, the current obtained by excluding the influence of the bias voltage Vdc by lock-in detection and evaluating only the influence of the observation light 31 is referred to as "difference current ΔI".

Two experimental results for this Equation 1 are shown. In the first experiment, the bias voltage Vdc is set to +10 V, the intensity of the pump light 21 is changed in six ways, and the total current I _ALL is measured by the measurement unit 6 . In the second experiment, the pump light 21 having a constant intensity, that is, the maximum amplitude, is irradiated, the bias voltage Vdc is varied, and the total current I _ALL is measured by the measurement unit 6 . In the first experiment, the electric field waveform is known by measuring the delay time dependence of the total current I _ALL . In the second experiment, Vov is zero and the observation light is on. In the experiment, a monocycle THz electric field with a pulse width of 1 picosecond is irradiated as pump light. Since the spot diameter is about millimeters, the entire probe is uniformly illuminated.

FIG. 5 shows the results of the first experiment. FIG. 5(a) is a diagram showing a change in the total current I _ALL over time, and FIG. 5(b) is a summary of the peak values extracted from FIG. 5(a). FIG. 5A summarizes a plurality of measurement results in one figure, with the time at which the maximum intensity of the pump light 21 is applied as zero. From FIG. 5(a), it can be seen that the total current I _ALL changes in time series, and the changes are similar. FIG. 5B is a diagram showing the correlation between the value of the total current I _ALL at time zero in FIG.

FIG. 6 is a diagram showing the results of the second experiment, showing changes in current with respect to changes in voltage when only the probe light is irradiated. The horizontal axis of FIG. 6 is the bias voltage Vdc, and the vertical axis is the total current I _ALL . In FIG. 6, the total current I _ALL increases as the bias voltage Vdc increases, and especially near the bias voltage Vdc=+10V, the total current I _ALL changes linearly with respect to the bias voltage Vdc.

From the above two experiments, if the bias voltage is set to Vdc=+10 V, there is a linear relationship between the change in the detected current I and the intensity of the near-field, that is, the delay time dependence of the current intensity can be measured. It can be seen that the electric field waveform can be obtained with

(measurement results in the time domain)
FIGS. 7(a) and 7(d) are optical microscope photographs of the first probe 5a and the second probe 5b. The first probe 5a has a sharper shape than the second probe 5b. 7(b) and (c), which will be described later, are spatially aligned with FIG. 7(a), and FIGS. 7(e) and (f) are spatially aligned with FIG. 7(d). I am letting

Measurement was performed by moving the irradiation position of the observation light 31 by 4.5 micrometers in the vertical direction and the horizontal direction using the irradiation position adjusting unit 3B. Measurements were taken at 31 points in the horizontal direction and at 7 points in the vertical direction. Each measurement point corresponds to a square area in the figure. FIGS. 7(b) and 7(e) are diagrams showing the magnitude of the differential current ΔI measured with a delay time of zero seconds using the first probe 5a and the second probe 5b. In FIGS. 7(b) and 7(e), the magnitude of the photoelectron current obtained at each irradiation position of the observation light 31 is represented by color densities. Moreover, each of FIGS. 7(b) and 7(e) matches the spatial positions of FIGS. 7(a) and 7(d). 7(b) and 7(e) almost reflect the shape of the probe, but since the diameter of the probe is smaller than the spot size of the observation light 31 at the tip portion, the intensity is reduced.

Next, the results of measurement with the bias voltage Vdc=+10V are shown in FIGS. FIG. 8(a) is a diagram showing the temporal change of the pump light 21, and the time series of FIGS. 8(a) to 8(e) all match. However, in FIGS. 8A to 8E, the timing at which the intensity of the pump light 21 is the strongest is the delay time of zero. These values are mapped in FIGS. 7(c) and 7(f). The waveform of the pump light 21 shown in FIG. 8(a) is obtained by EO sampling.

7(c) and (f) are diagrams showing the differential current ΔI at zero delay time for each position irradiated with the observation light 31 when the first probe 5a and the second probe 5b are used. In FIGS. 7C and 7F as well, the magnitude of the difference current ΔI is represented by the shade of color. It can be seen from FIGS. 7(c) and (f) that the intensity of the near-field is strong in the narrow region at the tip of the probe 5. FIG. Positions A, B, and C shown in FIGS. 7(c) and (f) are positions corresponding to FIGS. 8(b) to (d), respectively. In particular, position C shown in FIGS. 7(c) and (f) indicates the tips of the first probe 5a and the second probe 5b.

As shown in FIG. 6, at the bias voltage Vdc=+10 V, the total current I _ALL is almost linear with respect to the voltage change, so the differential current ΔI is proportional to the magnitude of the near-field at the moment when the photoemission occurs, and the delay The time-dependent differential current ΔI is a direct reflection of the near-field waveform. The differential current ΔI at positions A, B, and C shown in FIGS. 7(c) and 7(f) was measured by changing the delay time, and the results converted into voltage by a method described later are shown in FIGS. are shown respectively. For conversion to voltage, the slope at V=+10 V shown in FIG. 6, ie, the value of dI/dV, was used to convert the current values into effective voltages for each waveform. In FIGS. 8B to 8D, the results using the first probe 5a and the results using the second probe 5b are shown together. The voltage V _THz at the position C of the first probe 5a is 0.58 V, which is about 6 times larger than that at the position A due to the enhancement effect of the tip.

In FIGS. 8B to 8D, the negative values can be measured because the bias voltage Vdc is applied so that a constant photoelectron current flows even if the near field is not formed by the pump light 21. because there is If the bias voltage Vdc were not applied, the negative values in FIGS. 8(b)-(d) would simply be measured as zero. However, in this embodiment, since a constant photoelectron current is allowed to flow, it can be evaluated as a negative value when the photoelectron current decreases.

FIG. 8(e) is a diagram obtained by integrating FIG. 8(a). In a simple model of antenna theory, the electric field due to the enhancement of the probe is assumed to be the integral of the original electric field, but the shape of the waveform shown in FIG. It does not match. Therefore, it is important to directly measure the near field as described in this embodiment.

(measurement results in the frequency domain)
9 and 10 are diagrams showing the frequency dependence of the near field formed by the pump light 21. FIG. 9(a) to (c) and FIG. 10(a) are descriptions of the first probe 5a, and FIGS. 9(d) to (f) and FIG. 10(b) are descriptions of the second probe 5b. be. FIGS. 9(a) and (d) are the same as FIGS. 7(a) and (d) shown above. FIGS. 10(a) and 10(b) show the electric field intensity spectra of the first probe 5a and the second probe 5b, respectively, and the positions of A, B, and C are the same as in FIG. Solid lines in FIGS. 10(a) and 10(d) indicate the frequency dependence of the pump light 21, which has a central component at 1 THz. On the other hand, it can be seen that the near field shifts to the low frequency side around 0.5 THz. This reduction in bandwidth can be explained to some extent by the dipole antenna model that the integral value corresponds to, but a more detailed study is actually required.

FIG. 9(b) is a diagram spatially mapping the 0.3 THz frequency component of the near-field formed by the pump light 21 when the first probe 5a is used, and FIG. It is the figure which similarly mapped the frequency component of 0.7 THz spatially. FIGS. 9(e) and 9(f) are diagrams in which frequency components of 0.3 THz and 0.7 THz are similarly spatially mapped when the second probe 5b is used. These figures reveal that the band reduction effect is more pronounced at the root than at the tip of the probe.

Near-field waveform information is important information that is indispensable for obtaining accurate and reliable results in THz-SNOM, THz near-field of nanogap structures, and THz-STM measurements. It is known that the near-field waveform depends on the geometry of the antenna, but from the measurement results using the first probe 5a and the second probe 5b, which have slightly different shapes, when using the same pump light 21 However, it was shown that the near-field waveforms generated at the tip of the probe differ greatly. Since it is very difficult to predict the near-field waveform for each experimental condition, it is important to directly measure the near-field as described in this embodiment.

According to the first embodiment described above, the following effects are obtained.
(1) The electron microscope 1 includes a pump light output unit 2 that irradiates the probe 5 with the pump light 21 to form a near-field, and a pump light 21 that is also pulsed so as to overlap at least a part of the irradiation range of the pump light 21. An observation light output unit 3 that irradiates the probe 5 with the observation light 31 having a short width, a measurement unit 6 that measures photoelectrons or tunnel current emitted by the irradiation of the probe 5 with the observation light 31, and a pump light. 21, in other words, while the sample 900 is excited by the pump light 21, the probe 5 is irradiated with the observation light 31. A delay time adjustment unit 3A for adjusting the irradiation timing of at least one of the observation lights 31 is provided. Therefore, the near field can be measured directly.

(2) The electron microscope 1 includes a delay time adjustment unit that changes the delay time, which is the time from the start of irradiation of the pump light 21 to the start of irradiation of the observation light 31, in a plurality of ways. Therefore, the time change of the near field, that is, the waveform of the near field can be measured.

(3) Pump light 21 is pump light in the pump probe method. The object to be irradiated is the probe in the pump-probe method. The electron microscope 1 includes a probe light output unit 4 that irradiates a sample 900 with a probe light 41 that is probe light in the pump probe method. Therefore, the near-field measurement and the analysis of the sample 900 by the pump-probe method can be realized by an integrated device.

(4) Make the sample 900 function as a counter electrode that receives photoelectrons from the probe 5 . The measurement unit 6 measures photoelectrons through the sample 900 . When the probe 5 is irradiated with the observation light 31, photoelectrons are emitted through a multiphoton absorption process. Therefore, the near-field can be measured in the experimental environment using the sample 900 that is the object of measurement.

(5) The electron microscope 1 includes an irradiation position adjusting section 3B that moves the irradiation area of the observation light 31 . Therefore, as shown in FIGS. 7 and 8, the characteristics of the probe 5 can be measured at various positions.

(6) The electron microscope 1 includes an application section 7 that applies a bias voltage Vdc to the probe 5 . A bias voltage Vdc is applied by the application unit 7 to allow a constant photoelectron current to flow even if the near field is not formed by the pump light 21, so that a negative voltage is obtained as shown in FIGS. can be measured.

-Second Embodiment-
A second embodiment of the measuring apparatus according to the present invention will be described with reference to FIGS. 11 to 13. FIG. In the following description, the same components as those in the first embodiment are assigned the same reference numerals, and differences are mainly described. Points that are not particularly described are the same as those in the first embodiment.

(Overview of Second Embodiment)
In the second embodiment, the near field generated by irradiation with pump light is observed by measuring the tunnel current.

(Constitution)
FIG. 11 is a schematic configuration diagram of an electron microscope 1A according to the second embodiment. The configuration of the electron microscope 1A is the same as that of the electron microscope 1 in the first embodiment except for the following points. The electron microscope 1A includes an observation light output section 3K instead of the observation light output section 3. FIG. The observation light 31A output by the observation light output unit 3K has a wavelength different from that of the observation light 31 in the first embodiment. The observation light output section 3K may include the delay time adjustment section 3A but may not include the irradiation position adjustment section 3B. In the second embodiment, the distance between the tip 51a of the probe 5 and the sample 900 is set to the distance through which the tunnel current flows, that is, 1 nanometer or less.

In the second embodiment, the wavelength of the observation light 31A is determined so that even if the probe 5 is irradiated with the observation light 31A, no photoelectrons are emitted due to the multiphoton absorption process. For example, if the probe 5 is tungsten coated with Pt/Ir as in the first embodiment, the observation light 31 is IR light of 1030 nm. Further, in this embodiment, bismuth selenide (Bi ₂ Se ₃ ) is used as the sample 900 in addition to graphite (HOPG) used in the first embodiment. The surface of bismuth selenide was prepared by vacuum cleavage.

(Measurement state)
FIG. 12 is a diagram showing a near-field measurement state in the second embodiment. However, FIG. 12 shows an outline of the positional relationship, etc., and the dimensions and distances are exaggerated. Differences from the first embodiment shown in FIG. 2 are that the distance between the probe 5 and the sample 900 is short, and that the observation light 31A is irradiated only at one point on the probe 5. . In the present embodiment, the distance between the probe 5 and the sample 900 must be shortened, so the voltage applied by the applying section 7 is set low in order to prevent the sample from being destroyed when the current increases.

(measurement principle)
FIG. 13(a) is a model diagram of the near-field measurement mechanism in the tunneling current region. As shown in FIG. 13(a), in the tunnel junction, the tunnel barrier is substantially lowered due to the effect of the mirror potential in addition to the electric field of the pump probe light, making it easier for hot electrons to tunnel. When the probe 5 is irradiated with the pump light 21 and the near field is generated, the observation light 31A is applied to a portion where the barrier is lowered, whereby the hot electrons tunnel efficiently. The tunneling current It depends on the barrier that changes when the probe 5 is irradiated with the observation light 31A, and hot electron relaxation is fast, on the order of femtoseconds, as in the case of photoelectron emission. By being able to measure, a spectrum close to the near-field waveform can be obtained with high resolution.

(Measurement result)
13B and 13C are diagrams showing time-series changes in the tunneling current It when graphite and bismuth selenide are used for the sample 900, respectively. The time-series change of the pump light 21 in this case is as shown in FIG. 8(a). In the measurement results of graphite shown in FIG. 13(b), the signal is small and the noise level is observed except for the peak at which the delay time is zero. On the other hand, in the measurement result of bismuth selenide shown in FIG. 13(c), a signal reflecting the pump light 21 more clearly is obtained. This makes it possible to reduce the distance between the probe 5 and the sample 900 by lowering the voltage applied by the applying unit 7, but bismuth selenide, which has a higher sample resistance than graphite, is more sensitive to the probe than graphite. It is considered that the strong signal was observed because the distance to 5 was close.

By the way, the waveform of the tunneling current shown in FIG. 13(c) is positively and negatively asymmetric compared to the waveform of the pump light 21 shown in FIG. 8(a) and the voltage change obtained by photoelectron emission in the first embodiment. characteristics, in other words, the ratio of the first peak to the second peak is large. The reason for this is explained.

FIG. 13(d) is a diagram showing an IV curve of tunnel current. The lower part of FIG. 13(d) shows the waveform of the pump light 21 shown in FIG. 8(a), and the right side of FIG. 13(d) shows the waveform of FIG. 13(c). Actually, the electric field in the lower part of FIG. 13(d) is the electric field induced by the pump light 21. The IV curve shown in FIG. 13(d) has nonlinearity like a higher-order function, in which I sharply changes when the absolute value of V exceeds a certain threshold. The waveform of the tunneling current shown in FIG. 13(c) is obtained by rectifying the pump light 21 by the non-linearity of the IV curve shown in FIG. 13(d), so the positive and negative asymmetry is large.

According to the second embodiment described above, the following effects are obtained.
(7) The measurement unit 6 measures the tunnel current. Therefore, the near field that is enhanced by the probe 5 and that is affected by the sample 900 can be directly measured. An accurate electric field waveform can be obtained by correcting the asymmetry using the IV curve.

(various modifications)
Each of the embodiments described above may be modified as in A to E below.
(A) The delay time adjustment section 3A may be provided in the pump light output section 2 instead of the observation light output section 3 . (B) The electron microscope 1 does not have to include the probe light output section 4 . (C) The electron microscope 1 does not have to include the application section 7 . (D) The electron microscope 1 does not have to include the delay time adjusting section 3A and the irradiation position adjusting section 3B. (E) When the signal from the observation light 31 is weak, the phases of the pump light 21 and the observation light 31 may be modulated for lock-in detection.

(Modification 1)
Each embodiment described above may be further modified as follows. That is, the light whose waveform is to be measured is not limited to the pump light in the pump-probe method, and may be light used in any experiment, or light used in a non-experiment. . For example, by using IR light as pump light to excite a sample and THz light as probe light, a phase transition or the like can be measured. The waveform of the THz light at this time may be measured. Subcycle spectroscopy becomes possible by using 1 picosecond THz light as pump light and 30 femtosecond THz light as probe light. In this case, any THz light may be measured.

(Modification 2)
An example in which the present invention is applied to the photoelectron region in the first embodiment and to the tunnel region in the second embodiment has been described. However, the invention may also be applied to field emission regions. That is, the present invention indirectly measures the change in the observation target light by measuring the amount of electrons corresponding to the change in the barrier because the observation target light changes the barrier. It is possible.

Each of the embodiments and modifications described above may be combined. Although various embodiments and modifications have been described above, the present invention is not limited to these contents. Other aspects conceivable within the scope of the technical idea of the present invention are also included in the scope of the present invention.

DESCRIPTION OF SYMBOLS 1... Electron microscope 2... Pump light output part 3... Observation light output part 3A... Delay time adjustment part 3B... Irradiation position adjustment part 4... Probe light output part 5... Probe 6... Measurement part 7... Application part 21... Pump light 31... Observation light 41... Probe light 900... Sample

Claims (11)

a first optical system that irradiates an object to be irradiated with first light whose carrier envelope phase is locked to form a near-field;
a second optical system that irradiates the object with a second light having a pulse width shorter than that of the first light so as to overlap at least a part of an irradiation range of the first light;
a measurement unit that measures photoelectrons or tunnel current emitted when the object is irradiated with the second light;
Adjusting the irradiation timing of at least one of the first light and the second light so that the second light is applied to the object while the object is being excited by the first light. A measuring device comprising a timing adjustment unit that In the measuring device according to claim 1,
The measurement apparatus further includes a delay time adjustment unit that changes a delay time, which is the time from the start of irradiation of the first light to the start of irradiation of the second light, in a plurality of ways. In the measuring device according to claim 1,
The first light is pump light in a pump probe method,
The object to be irradiated is a probe in a pump-probe method,
A measuring apparatus further comprising a third optical system for irradiating a sample with probe light in a pump-probe method. In the measuring device according to claim 3,
causing the sample to function as a counter electrode that receives the photoelectrons from the probe;
The measurement unit measures the photoelectrons through the sample,
A measurement apparatus for emitting photoelectrons through a multiphoton absorption process when the probe is irradiated with the second light. In the measuring device according to claim 4,
The measuring device further comprising an irradiation position adjusting unit that moves the irradiation area of the second light. In the measuring device according to claim 3,
A measuring device further comprising an applying unit that applies a bias voltage to the probe. In the measuring device according to claim 3,
The measuring unit is a measuring device that measures a tunnel current. irradiating an object to be irradiated with the first light whose carrier envelope phase is locked to form a near-field;
irradiating the object with a second light having a pulse width shorter than that of the first light so as to overlap at least a part of the irradiation range of the first light;
measuring photoelectrons or tunnel current emitted by irradiating the object with the second light;
Adjusting the irradiation timing of at least one of the first light and the second light so that the second light is applied to the object while the object is being excited by the first light. A near-field measurement method comprising: In the near-field measuring method according to claim 8,
A near-field measuring method further comprising changing a delay time, which is a time from the start of irradiation of the first light to the start of irradiation of the second light, in a plurality of ways. In the near-field measuring method according to claim 8,
The first light is pump light in a pump probe method,
The object to be irradiated is a probe in a pump-probe method,
A near-field measuring method further comprising irradiating a sample with probe light in a pump-probe method. In the near-field measuring method according to claim 10,
causing the sample to function as a counter electrode that receives the photoelectrons from the probe;
measuring the photoelectrons through the sample;
A near-field measuring method in which the distance between the sample and the probe is set to a distance at which tunnel current does not flow.

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