Measuring system, and measuring method

Abstract

To enable measurement of optical excitation dynamics only by adding a light source unit to an existing atomic force microscope (AFM).SOLUTION: A measuring system comprises: a cantilever; an AFM controller 43 that outputs a change amount signal that is voltage or current according to a change in the resonance frequency or amplitude of the cantilever; and a light source unit. The light source unit includes: a pump light generation section 22 that outputs pump light in response to a reference pulse P1 that is input thereto; a probe light generation section 23 that outputs probe light in response to a delay pulse P2 that is input thereto; and a trigger generator and delay time controller that inputs the reference pulse P1 and the delay pulse P2 to the pump light generation section 22 and the probe light generation section 23 and repeatedly inputs signals while switching a delay time td in multiple ways, and outputs a reference signal to a lock-in amplifier 25 in switching the delay time td.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a measurement system and a measurement method. [Background technology]

[0002] Although STM (Scanning Tunneling Microscope) can obtain information such as conductivity, the measurement target is limited to conductive objects. AFM (Atomic Force Microscope) is generally considered easier to use than STM and is widely used as a means of observing the surface of a sample. Patent Document 1 discloses a method for determining the mechanical properties of a soft viscoelastic sample using an atomic force microscope (AFM)-based system. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-89945 Summary of the Invention [Problem to be solved by the invention]

[0004] The invention described in Patent Document 1 cannot measure photoexcitation dynamics. [Means for solving the problem]

[0005] a trigger generation and delay time controller that inputs the first and second signals to the first and second lasers, and repeatedly inputs the first and second signals by switching a variable delay value, which is the difference between the timing at which a signal is input to the first laser and the timing at which a signal is input to the second laser, among a plurality of values, and outputs a reference signal to a lock-in amplifier when switching the variable delay value; an auxiliary optical system that guides the pump light and the probe light to a sample; a cantilever having a probe tip arranged in close proximity to the sample; and a controller that applies a periodically varying voltage to the cantilever and outputs a change signal, which is a voltage or current corresponding to a change in the resonant frequency of the cantilever, and the lock-in amplifier measures the change signal based on the reference signal. A measurement method according to a second aspect of the present invention is a measurement method performed by a measurement system including a first laser that outputs pump light, which is a pulsed laser, in response to an input first signal; a second laser that outputs probe light, which is a pulsed laser, in response to an input second signal; auxiliary optical systems that guide the pump light and the probe light to a sample; a cantilever having a probe tip arranged in close proximity to the sample; and a lock-in amplifier, the measurement method including: inputting the first signal and the second signal to the first laser and the second laser; repeatedly inputting the first signal and the second signal by switching a variable delay value, which is the difference between the timing at which a signal is input to the first laser and the timing at which the first signal and the second signal are input to the second laser, in a plurality of ways; outputting a reference signal to the lock-in amplifier when switching the variable delay value; applying a periodically varying voltage to the cantilever and outputting a change signal, which is a voltage or current corresponding to a change in the resonant frequency of the cantilever; and measuring the change signal by phase-sensitive detection using the reference signal by the lock-in amplifier. [Effects of the Invention]

[0006] According to the present invention, photoexcitation dynamics can be measured simply by adding a light source unit to an existing microscope unit. [Brief explanation of the drawings]

[0007] [Figure 1] Overall configuration diagram of a measurement system according to a first embodiment [Figure 2] Diagram showing the processing of the delay time controller [Figure 3] Measurement example diagram [Figure 4] FIG. 10 is a diagram showing a light source unit according to a first modified example. [Figure 5] FIG. 10 is a diagram showing a light source unit according to a second modified example. [Figure 6] Overall configuration diagram of the measurement system in Modification 5 [Figure 7] Overall configuration diagram of a measurement system according to a second embodiment [Figure 8] Diagram showing force spectroscopy [Figure 9] Diagram showing the IV curve [Figure 10] Overall configuration diagram of a measurement system according to a third embodiment [Figure 11] A diagram explaining the output timing of the reference pulse [Figure 12] Diagram showing the problems that can occur if the frequency of the reference pulse is not adjusted [Figure 13] Diagram showing the problems that can occur if the phase of the reference pulse is not adjusted DETAILED DESCRIPTION OF THE INVENTION

[0008] -First embodiment- A first embodiment of the measurement system will be described below with reference to FIGS.

[0009] 1 is a diagram showing the overall configuration of a measurement system 1. The measurement system 1 includes a light source unit 2, a microscope unit 3, and an AFM unit 4. The AFM unit 4, which includes a sample stage (not shown), has a known configuration, and the measurement system 1 is characterized by combining the AFM unit 4 with the light source unit 2.

[0010] The light source unit 2 includes a trigger generator 21, a pump light generating unit 22, a probe light generating unit 23, a delay time controller 24, and a lock-in amplifier 25. The light source unit 2 is, for example, a delay time modulation excitation light source unit (UNISOKU Corporation, OPP-PS or OPP-NS). Hereinafter, the pump light generating unit 22 will also be referred to as the "first laser," and the probe light generating unit 23 will also be referred to as the "second laser."

[0011] The microscope unit 3 includes an LED light source 31, a CCD camera 32, an imaging lens 33, and an objective lens 34. The microscope unit 3 observes a sample 45 placed on a sample stage (not shown). The AFM unit 4 includes a tuning fork cantilever 41, a charge amplifier 42, an AFM controller 43, and a sample stage (not shown). The sample 45 is placed on the sample stage.

[0012] The configuration of the light source unit 2 will now be described. The trigger generator 21 outputs a set voltage pulse at a predetermined cycle, for example, 1 MHz. The trigger generator 21 outputs the same voltage pulse at the same timing to the pump light generating unit 22 and the delay time controller 24. Hereinafter, this voltage pulse will be referred to as a reference pulse P1.

[0013] The pump light generating unit 22 outputs a laser pulse in response to an input voltage pulse, i.e., a reference pulse P1. The pump light generating unit 22 outputs one laser pulse when one voltage pulse is input. The time interval from when a voltage pulse is input to the pump light generating unit 22 to when the pump light generating unit 22 outputs a laser pulse is constant every time. In this embodiment, the laser pulse output by the pump light generating unit 22 is used as pump light.

[0014] The delay time controller 24 receives a reference pulse P1 from the trigger generator 21, outputs a delay pulse P2 to the probe light generator 23, and outputs a reference signal R to the lock-in amplifier 25. When the delay time controller 24 receives the reference pulse P1, it delays it by a delay time td and outputs a delay pulse P2. The delay time controller 24 periodically changes this delay time td and changes the reference signal R at the timing when the delay time td is changed. In other words, the delay time controller 24 achieves delay time modulation.

[0015] There are two types of delay time td: a measurement delay time and a reference delay time. The measurement delay time is shorter than the relaxation process of the excited state of the sample 45 due to light, while the reference delay time is longer than the relaxation time of the excited state of the sample 45 excited by light. The reference delay time can also be said to be the time at which the sample 45 excited by the pump light can be considered to have returned to a non-excited state. Note that the delay time td will also be referred to as the "variable delay value" below. The reference pulse P1 will also be referred to as the "first signal," and the delay pulse P2 will also be referred to as the "second signal."

[0016] FIG. 2 is a diagram illustrating the processing of the delay time controller 24, showing the relationship between the reference pulse P1, the delay pulse P2, and the reference signal R. The times t11 to t19 shown in FIG. 2 are shown for ease of explanation. The reference pulse P1 is output at a constant interval from time t11 to time t19. From time t11 to time t13, the delay time controller 24 sets the delay time td to a first time T1, which is a measurement delay time, and outputs a delay pulse P2 after the first time T1 has elapsed since the reception of the reference pulse P1. Hereinafter, the delay pulse P2 to which the measurement delay time has been applied will also be referred to as the first delay pulse P2-1.

[0017] The delay time controller 24 outputs the reference signal R at a high level, e.g., 5 V, from time t11 to time t13, and then changes it to a low level, e.g., 1 V, after time t13. The timing for switching the reference signal R is preset, for example, at predetermined intervals, each time the reference pulse P1 is received a predetermined number of times, or each time the delay pulse P2 is output a predetermined number of times. The delay time controller 24 switches the delay time between the measurement delay time and the reference delay time each time the reference signal R is switched. The delay time controller 24 outputs a delay pulse P2 after the reference delay time Tmax has elapsed in response to the reference pulse P1 received from time t14 to time t16. Hereinafter, the delay pulse P2 to which the reference delay time has been applied is also referred to as a second delay pulse P2-2.

[0018] After time t16, the delay time controller 24 switches the output of the reference signal R, changing it from Low to High. Thereafter, the delay time controller 24 outputs the first delay pulse P2-1 with the delay time td set to the measurement delay time T1, as in the case of times t11 to t13. Thereafter, the delay time controller 24 switches the output of the reference signal R, as in the case after time t13, and sets the delay time td to Tmax. Note that for ease of illustration, FIG. 2 shows the delay pulse P2 being output only three times without changing the delay time td, but in reality, the number of repetitions is very large. In this embodiment, measurements are performed by varying the short delay time td in various ways.

[0019] For example, in one measurement, the delay time td is set alternately to T1 and Tmax, in another measurement, the delay time td is set alternately to T2 and Tmax, and in yet another measurement, the delay time td is set alternately to T3 and Tmax. T1, T2, etc. may be input to the delay time controller 24 in advance, or may be set via communication from an external control device (not shown), such as a general-purpose computer. Tmax may be set to a predetermined value in advance, or may be set each time, like T1, etc.

[0020] In the example shown in Figure 2, the delay pulse P2 is delayed relative to the reference pulse P1, and this state is defined as a positive delay time. If the delay time is set to a negative value, the delay pulse P2 precedes the reference pulse P1. In other words, when the delay time is negative, the delay pulse P2 is irradiated onto the sample 45 first, followed by the reference pulse P1.

[0021] 1, the explanation will be continued. The probe light generation unit 23 outputs a laser pulse in response to a delay pulse P2 input from the delay time controller 24. The configuration of the probe light generation unit 23 is the same as that of the pump light generation unit 22, but the voltage pulse input thereto is different. In this embodiment, the laser pulse output by the probe light generation unit 23 is used as the probe light, and therefore the only difference is that the name is different from that of the pump light generation unit 22.

[0022] However, for the sake of convenience, the pump light generating unit 22 and the probe light generating unit 23 are simply assumed to be identical, and the hardware configurations of the two may be different. Furthermore, the wavelengths and intensities of the laser pulses output by the pump light generating unit 22 and the probe light generating unit 23 may or may not be the same. For example, the laser pulses output by the pump light generating unit 22 and the probe light generating unit 23 have a wavelength of 532 nm and a pulse width of 45 ps. The pump light output by the pump light generating unit 22 and the probe light output by the probe light generating unit 23 are guided to the same optical path by a mirror 81 and a half mirror 82.

[0023] The LED light source 31 outputs illumination light for imaging by the CCD camera 32. This illumination light is guided by a short-pass filter 83 to the same optical path as the pump light and probe light. The cutoff wavelength of the short-pass filter 83 is, for example, 550 nm. The CCD camera 32 images the sample 45 via an imaging lens 33 and an objective lens 34. However, the CCD camera 32 is provided for the convenience of checking the sample 45 and is not an essential component for the measurements described below. The short-pass filter 83 can be changed as appropriate depending on the wavelength of the light source 46 used and the purpose of observation; for example, a long-pass filter or a half mirror may also be used.

[0024] The pump light, probe light, and illumination light are irradiated onto the sample 45 via the half mirror 84 and the objective lens 34. As mentioned above, the CCD camera 32 is not an essential component for the measurements described below, and therefore the role of the microscope unit 3 in the measurements is to hold the sample 45 and guide the pump light and probe light to the sample 45. The CCD camera 32 can also be used when adjusting the positions of the probe tip 411 and the sample 45. Below, the mirror 81, half mirror 82, short-pass filter 83, and half mirror 84 are collectively referred to as the auxiliary optical system 80.

[0025] The tuning fork cantilever 41 is a quartz crystal resonator having a U-shape like a tuning fork, and is provided with a probe tip 411 at its tip. The probe tip 411 is placed close to a sample 45 and receives atomic force from the sample 45. The tuning fork cantilever 41 is connected to an AFM controller 43 by a first wire 451 and a second wire 452. A drive signal is input to the tuning fork cantilever 41 from the AFM controller 43 via the first wire 451. The drive signal is a voltage that periodically fluctuates in a sinusoidal waveform, and the frequency of this drive signal will be referred to below as the drive frequency fd.

[0026] When a voltage is applied to the tuning fork cantilever 41, distortion occurs, and the tuning fork cantilever 41 vibrates as the applied voltage fluctuates periodically. When the drive frequency fd matches the resonant frequency of the tuning fork cantilever 41, the vibration reaches a maximum. However, the resonant frequency of the tuning fork cantilever 41 is affected by an external force, i.e., the atomic force from the sample 45. When a voltage is applied to the tuning fork cantilever 41 and it distorts, it outputs a current, which is output to the AFM controller 43 via the second wire 452.

[0027] The charge amplifier 42 is connected to a second wire 452 that connects the tuning fork cantilever 41 and the AFM controller 43. The charge amplifier 42 amplifies the output of the tuning fork cantilever 41 and outputs the amplified signal to the AFM controller 43. The AFM controller 43 includes a PLL 431, an amplification controller 432, and a phase shifter 433. The PLL 431 is a phase locked loop that outputs a frequency to the amplification controller 432 so that the phase difference between the signal input from the tuning fork cantilever 41 via the second wire 452 and a reference signal is constant.

[0028] The PLL 431 also outputs a voltage or current corresponding to the difference between the current resonant frequency of the tuning fork cantilever 41, calculated from the output of the tuning fork cantilever 41, and the resonant frequency of a reference tuning fork cantilever 41, as a frequency deviation signal 71 to the lock-in amplifier 25. The amplification controller 432 amplifies the signal output by the PLL 431 and outputs it to the phase shifter 433. The phase shifter 433 outputs a voltage sine wave of a reference phase at a frequency specified by the PLL 431 to the tuning fork cantilever 41. The lock-in amplifier 25 performs lock-in detection of the frequency deviation signal 71 output by the AFM controller 43 using the reference signal R output by the delay time controller 24. The lock-in amplifier 25 outputs the measurement results to the storage device 9. Note that lock-in detection by the lock-in amplifier 25 is also called "phase-sensitive detection."

[0029] (Measurement procedure) First, the operator places the sample 45 and adjusts the positions of the sample 45 and tuning-fork cantilever 41 while watching the CCD camera 32. Next, the operator operates the AFM controller 43 and lock-in amplifier 25. Changing the relative position of the tuning-fork cantilever 41 and the sample 45 using an XYZ table (not shown) changes the atomic force depending on the distance between the probe tip 411 placed at the end of the tuning-fork cantilever 41 and the sample 45, thereby changing the signal output by the AFM controller 43. The operator determines the measurement point on the sample 45 to be measured for time-resolved measurements and fixes the relative position of the probe tip 411 and the measurement point. The operator then operates the trigger generator 21, pump light generator 22, probe light generator 23, and delay time controller 24. The pump light and probe light form a spot area with a diameter of several microns on the sample 45.

[0030] The operator sets a first delay time, for example, T1, in the delay time controller 24 and starts measurement. As explained with reference to Figure 2, the delay time controller 24 periodically switches the delay time between T1 and Tmax. The sample 45 is irradiated with the probe light after T1 has elapsed since the pump light was irradiated. When the pump light is irradiated, the sample 45 reaches a peak in its excited state after a very short time, then gradually relaxes and finally reaches a non-excited state. As will be described in detail later, the sample 45 exerts a force on the tuning-fork cantilever 41 depending on the excited state, changing the resonant frequency. The lock-in amplifier 25 measures and outputs this change in resonant frequency.

[0031] The operator can then measure the change in the resonant frequency with respect to the change in delay time by changing the delay time to T2, T3, etc. When the delay time is Tmax, the probe light is irradiated onto the sample 45 in a non-excited state, so the lock-in amplifier 25 evaluates the resonant frequency at each delay time, so to speak, with the delay time at Tmax as the reference.

[0032] (Measurement example) An example of measurement will be described below, in which bulk tungsten selenide WSe2 was used as the sample 45. The LED light source 31 was 28 μW, had a spot diameter of 200 μm, a wavelength of 550 to 750 nm, and an excitation density of approximately 90 pW / μm. 2 The pump and probe beams were 1 mW, with a spot diameter of approximately 5 μm, a central wavelength of 532 nm, and an excitation density of approximately 50 μW / μm. 2 The measurement was performed by feedback control of the AFM controller 43 so that the frequency was 1 Hz higher than the resonance frequency in the repulsion region. In this setup, the force acting between the probe tip 411 and the sample 45 corresponds to 10 nN or less. The amplitude of the tuning fork cantilever 41 is 4 nm.

[0033] Figure 3 shows an example of a measurement. In this measurement, the delay time td was varied by approximately ±1 microsecond, specifically between -800 nanoseconds and +800 nanoseconds. As shown in Figure 3, we succeeded in obtaining a time-resolved signal with a very high S / N ratio. The data was then analyzed using a two-component exponential function (A fast exp (-t / τ fast )+ A slow exp (-t / τ slow )) and fitting, τ fast is about 30 ns, τ slow was approximately 150 ns.

[0034] According to the first embodiment described above, the following advantageous effects can be obtained. (1) The measurement system 1 includes a pump light generator 22 that outputs pump light, which is a pulse laser, in response to an input reference pulse P1, a probe light generator 23 that outputs probe light, which is a pulse laser, in response to an input delayed pulse P2, and a pump light generator 22 that inputs the reference pulse P1 and the delayed pulse P2 to the pump light generator 22 and the probe light generator 23, and repeatedly inputs a signal by switching a delay time td, which is the difference between the timing at which the reference pulse P1 is input to the pump light generator 22 and the timing at which the delayed pulse P2 is input to the probe light generator 23, in a plurality of ways, and The system includes a trigger generator 21 and a delay time controller 24 that output a reference signal R to a lock-in amplifier 25 when switching between the two beams; an auxiliary optical system 80 that guides the pump and probe beams to a sample 45; a tuning-fork cantilever 41, which is a quartz oscillator and has a probe tip 411 positioned close to the sample 45; an AFM controller 43 that applies a periodically varying voltage to the tuning-fork cantilever 41 and outputs a change signal, which is a voltage or current corresponding to the change in the resonant frequency of the tuning-fork cantilever 41; and a lock-in amplifier 25 that measures the change signal based on the reference signal R. Therefore, photoexcitation dynamics can be measured simply by adding a light source unit 2 to an existing microscope unit 3 equipped with a tuning-fork cantilever 41. Furthermore, the use of the tuning-fork cantilever 41 eliminates the need for an optical system for detecting probe vibration, allowing the measurement system 1 to be used in complex equipment such as an ultra-high vacuum chamber. Furthermore, the spring constant of the tuning-fork cantilever 41 can be increased, thereby reducing the vibration amplitude.

[0035] (2) The delay time controller 24 switches the delay time td between a measurement delay time such as the first time T1 and a reference delay time Tmax, and the reference delay time Tmax is a time longer than the time it takes for the excited state of the sample 45 excited by the pump light to relax.

[0036] (3) The lock-in amplifier 25 measures the first change amount signal in a state where the delay time controller 24 sets the measurement delay time to a first time T1 and the reference delay time to Tmax, and measures the second change amount signal in a state where the delay time controller 24 sets the measurement delay time to a second time T2 and the reference delay time to Tmax.

[0037] (Variation 1) FIG. 4 is a diagram showing a light source unit 2A in Modification 1. The configuration other than the light source unit 2A is the same as that of the first embodiment, so description and explanation will be omitted. In this modification, a trigger generation and delay time controller 21A is provided instead of the trigger generator 21 and the delay time controller 24. The trigger generation and delay time controller 21A has the functions of the trigger generator 21 and the delay time controller 24. In other words, the trigger generation and delay time controller 21A is an integrated configuration of the trigger generator 21 and the delay time controller 24. The trigger generation and delay time controller 21A outputs a reference pulse P1 to the pump light generation unit 22, outputs a delay pulse P2 to the probe light generation unit 23, and outputs a reference signal R to the lock-in amplifier 44. The operation of the trigger generation and delay time controller 21A is the same as the operation of the trigger generator 21 and the delay time controller 24 in the first embodiment described above, so description will be omitted.

[0038] (Variation 2) FIG. 5 is a diagram showing a light source unit 2B in Modification 2. The configuration other than that of the light source unit 2A is the same as that of the first embodiment, so description and explanation will be omitted. The light source unit 2B includes a multiple-pulse generator 21B, a switch 26, and a flip-flop 27 instead of the trigger generator 21 and the delay time controller 24. The multiple-pulse generator 21B combines some of the functions of the trigger generator 21 and the delay time controller 24. That is, the multiple-pulse generator 21B outputs a reference pulse P1, a first delay pulse P2-1, a second delay pulse P2-2, and a reference signal pulse Rp. The reference signal pulse Rp is a pulse signal that indicates the timing at which the polarity of the reference signal R switches. In the first embodiment described above, the delay time controller 24 switches between outputting the first delay pulse P2-1 and the second delay pulse P2-2, but in this modification, the multiple-pulse generator 21B outputs the first delay pulse P2-1 and the second delay pulse P2-2.

[0039] The flip-flop 27 receives a reference signal pulse Rp from the trigger generator 21 and outputs a reference signal R to the switch 26 and the lock-in amplifier 25. The flip-flop 27 is capable of outputting High and Low, and outputs a different signal to the switch 26 and the lock-in amplifier 25 each time a reference signal pulse Rp is input from the trigger generator 21. The reference signal R input to the lock-in amplifier 25 is the same in this modification as in the first embodiment.

[0040] The switch 26 receives the first delay pulse P2-1 and the second delay pulse P2-2 from the trigger generator 21 and outputs either of them to the probe light generation unit 23. Which signal to output to the probe light generation unit 23 is determined by the reference signal R input from the flip-flop 27. For example, when the reference signal R is High, the switch 26 outputs the first delay pulse P2-1, and when the reference signal R is Low, the switch 26 outputs the second delay pulse P2-2. As described above, the configuration of the light source unit 2 is not limited to that of the first embodiment, and may be the configuration shown in FIG. 4.

[0041] (Variation 3) In the first embodiment described above, the reference pulse P1 is input to the pump light generating unit 22, and the delay pulse P2 is input to the probe light generating unit 23. However, the delay pulse P2 may be input to the pump light generating unit 22, and the reference pulse P1 may be input to the probe light generating unit 23.

[0042] (Variation 4) The AFM controller 43 in the first embodiment described above employs a so-called FM-AFM, which detects the amount of frequency shift in the resonance frequency of the tuning-fork cantilever 41. However, the AFM controller 43 may employ a so-called AM-AFM, which detects changes in the vibration amplitude of the tuning-fork cantilever 41 and controls the distance between the tuning-fork cantilever 41 and the sample 45 using a feedback circuit so that the vibration amplitude remains constant.

[0043] (Variation 5) In the first embodiment described above, tuning fork cantilever 41, which is a tuning fork cantilever made of a quartz crystal oscillator, is used. However, a cantilever using a more general leaf spring may also be used.

[0044] 6 is a diagram showing the overall configuration of a measurement system 1 in Modification 5. The AFM unit 4 in this modification includes a leaf spring cantilever 41A, a light source 46, a photodetector 47, an amplifier 42A, a piezoelectric diaphragm 48, an AFM controller 43A, and a sample stage (not shown).

[0045] A light source 46 irradiates the back surface of the leaf spring cantilever 41A with laser light, and the reflected light is detected by a photodetector 47. The light source 46 and the photodetector 47 use a so-called optical lever system. The leaf spring cantilever 41A is vibrated at a predetermined frequency by a piezoelectric diaphragm 48. The resonant frequency and amplitude of the leaf spring cantilever 41A change depending on the force it receives from the sample 45.

[0046] Changes in the frequency and amplitude of the vibration of the leaf spring cantilever 41A are detected by the photodetector 47 as changes in the behavior of the reflected light. The output signal of the photodetector 47 is amplified by an amplifier 42A and input to an AFM controller 43A. The configuration of the AFM controller 43A is the same as that of the first embodiment described above, except that the output source of the input signal has been changed from the tuning fork cantilever 41 to the photodetector 47. The operation of the AFM controller 43A is the same as that of the first embodiment, so a description thereof will be omitted. The AFM controller 43A performs feedback control using the output of the photodetector 47, and vibrates the leaf spring cantilever 41A using a piezoelectric diaphragm 48.

[0047] In this modification, the vibration of the leaf spring cantilever 41A is detected by a so-called optical lever method, but a self-detecting cantilever may also be used. In this case, a piezoresistor, for example, is built into the cantilever, and the light source 46 and photodetector 47 are not required, and no space is required for arranging measurement equipment near the sample 45.

[0048] --Second embodiment-- A second embodiment of the measurement system will be described with reference to Figures 7 to 9. In the following description, the same components as in the first embodiment are denoted by the same reference numerals, and differences will be mainly described. Points that are not particularly described are the same as in the first embodiment. This embodiment differs from the first embodiment mainly in that tunnel current is measured.

[0049] 7 is an overall configuration diagram of measurement system 1A. In addition to the configuration of the first embodiment, measurement system 1A includes a current measurement unit 51 and a voltage source 52. Current measurement unit 51 measures the tunneling current flowing through probe tip 411 and inputs it to lock-in amplifier 25 as tunneling current signal 72. Tunneling current signal 72 is subjected to lock-in detection in the same manner as frequency deviation signal 71 output by AFM controller 43 and output to storage device 9. Because current measurement unit 51 measures tunneling current, it can also be called a "tunneling current measurement unit."

[0050] The voltage source 52 applies a bias voltage Vs, which is an arbitrary DC voltage, to the sample 45. By applying the bias voltage Vs, the state of the sample 45 is statically changed, and the sample 45 can be measured at different potentials.

[0051] Figure 8 shows force spectroscopy. Specifically, the change in resonant frequency was measured by varying the illumination and bias voltage Vs. Of the three graphs shown in Figure 8, the top graph shows the case without illumination (hereinafter also referred to as the "dark state"). The middle graph shows the case with irradiation of light generated by the LED light source 31 (hereinafter also referred to as the "weak excitation state"). The bottom graph shows the case with irradiation of light generated by the LED light source 31 and the pump light generator 22 (hereinafter also referred to as the "strong excitation state"). The bias voltage Vs was varied from -3 V to +3 V. In the middle and bottom graphs of Figure 8, the LED light source 31 was always on, and the pump light generator 22 output a signal at 500 kHz. The LED light source 31 and the pump light intensity were the same as those in the first embodiment. However, in this measurement, delay time modulation was not performed, and the frequency deviation signal 71 and the tunnel current signal 72 were measured directly without lock-in detection.

[0052] The contact potential difference (Vcpd) under three illumination conditions was calculated by fitting each of the Δf-Vs curves with a quadratic function. It was -0.13 ± 0.02 V in the dark state, 0.37 ± 0.01 V in the weak excitation state, and 0.92 ± 0.02 V in the strong excitation state. This indicates the presence of a surface photovoltage (SPV). SPV is the difference in Vcpd between the presence and absence of illumination, which generates a force between the probe tip 411 and the sample 45.

[0053] FIG. 9 shows IV curves. Similar to FIG. 8, this IV curve was obtained by measuring the current flowing between the probe tip 411 and the sample 45 using the current measurement unit 51 while measuring the Δf-Vs curves under three illumination conditions. FIG. 9 shows a first curve 811, which is an IV curve in a dark state, a second curve 812, which is an IV curve in a weakly excited state, and a third curve 813, which is an IV curve in a strongly excited state. The first curve 811 and the second curve 812 almost overlap over the entire range from −3 V to +3 V, resulting in almost no current flow at 0 A. On the other hand, the third curve 813, which is in a strongly excited state, shows a negative current flow when the bias voltage Vs is negative and a positive current flow when the bias voltage Vs is positive. This result is consistent with the measurement results shown in FIG. 8.

[0054] According to the second embodiment described above, the following advantageous effects can be obtained. (4) The measurement system 1A includes a current measurement unit 51 that measures the tunneling current flowing through the probe tip 411. Therefore, the tunneling current can be measured in parallel with the measurement of the atomic force by the AFM.

[0055] -Third embodiment- A third embodiment of the measurement system will be described with reference to Figures 10 to 13. In the following description, the same components as those in the first embodiment are denoted by the same reference numerals, and differences will be mainly described. Points that are not particularly described are the same as those in the first embodiment. This embodiment differs from the first embodiment mainly in that it further includes an indicator device.

[0056] FIG. 10 is a diagram showing the overall configuration of measurement system 1B. Measurement system 1B further includes an indicator 28 in addition to the configuration of the first embodiment. In this embodiment, trigger generator 21C outputs reference pulse P1 at a frequency specified by indicator 28, rather than at a predetermined frequency. The phase, i.e., the timing, at which reference pulse P1 is output is also specified by indicator 28. Indicator 28 is a computer with a computing function, and can be realized by combining a general-purpose personal computer, a microcomputer, a logic circuit, or the like with an interface circuit. For example, indicator 28 includes a central processing unit (CPU), a ROM for storing programs, and a RAM where the programs stored in the ROM are expanded for execution by the central processing unit.

[0057] The indicator 28 receives the outputs of the PLL 431, the lock-in amplifier 25, and the voltmeter 53. The voltmeter 53 measures the voltage output by the charge amplifier 42. However, the voltmeter 53 may be built into the indicator 28. The indicator 28 determines the frequency and phase at which the trigger generator 21C outputs the reference pulse P1, and instructs the trigger generator 21C. The trigger generator 21C changes the frequency and phase at which the reference pulse P1 is output based on the output of the indicator 28.

[0058] In this embodiment, the frequency of the reference pulse P1 is an integer multiple or an integer fraction of the vibration frequency of the tuning fork cantilever 41. Hereinafter, the value obtained by dividing the frequency of the reference pulse P1 by the vibration frequency of the tuning fork cantilever 41 will be referred to as the specified magnification W. When the specified magnification W is greater than 1, the frequency of the reference pulse P1 is greater than the vibration frequency of the tuning fork cantilever 41, and when the specified magnification W is less than 1, the frequency of the reference pulse P1 is less than the vibration frequency of the tuning fork cantilever 41. The specified magnification W is an integer such as "1," "2," or "3," or a decimal such as "1 / 2," "1 / 3," "1 / 4," etc. The specified magnification W is set in advance by the user.

[0059] The indicator 28 obtains the cantilever frequency f and the cantilever phase p from the output of the voltmeter 53 or the output of the PLL 431. For example, the indicator 28 can obtain the cantilever frequency f and the cantilever phase p by detecting, as a trigger, that the sinusoidally varying outputs of the charge amplifier 42 or the PLL 431 exceed a predetermined threshold. The indicator 28 only needs to receive at least one of the output of the voltmeter 53 and the output of the PLL 431; it does not necessarily need to receive the output of the lock-in amplifier 25. That is, the indicator 28 performs one of the following operations: a first operation using the output of the voltmeter 53, a second operation using the output of the PLL 431, a third operation using the outputs of the voltmeter 53 and the lock-in amplifier 25, or a fourth operation using the outputs of the PLL 431 and the lock-in amplifier 25.

[0060] In the first and second operations, the indicator 28 obtains the cantilever frequency f and the cantilever phase p using the output of the voltmeter 53 or the output of the PLL 431 as described above. In the third and fourth operations, the final cantilever frequency f is calculated by adding the frequency deviation corresponding to the output value of the lock-in amplifier 25 to the cantilever frequency f obtained using the output of the voltmeter 53 or the output of the PLL 431. The third and fourth operations are useful when there is a lot of noise and it is difficult to calculate an accurate frequency from the output of the voltmeter 53 or the PLL 431.

[0061] The instruction device 28 causes the trigger generator 21C to output a reference pulse P1 at a frequency obtained by multiplying the cantilever frequency f obtained in any one of the first to fourth operations by a specified magnification W. For example, if the cantilever frequency f is "1234 Hz" and the specified magnification W is "10", the output frequency of the reference pulse P1 is "12340 Hz". Alternatively, if the cantilever frequency f is "12 kHz" and the specified magnification W is "0.5", the output frequency of the reference pulse P1 is "6 kHz".

[0062] The indicator 28 obtains the cantilever phase p, for example, as follows. Here, a case where the output of the voltmeter 53 is used will be described, but the procedure is similar when the output of the PLL 431 is used. The indicator 28 measures the minimum and maximum values ​​of the output of the voltmeter 53, which changes in a sinusoidal manner. Then, the indicator 28 calculates the cantilever phase p using the timing at which the output of the voltmeter 53 exceeds a predetermined value between the minimum and maximum values ​​and the reciprocal of the cantilever frequency f. However, the cantilever phase p may also be calculated as the timing at which the next reference pulse P1 should be output. The indicator 28 determines the timing at which the next reference pulse P1 should be output, for example, as follows.

[0063] FIG. 11 is a schematic diagram illustrating the output timing of the reference pulse P1. The upper part of FIG. 11 shows the tip position of the tuning-fork cantilever 41, and the middle part shows the measurement value of the voltmeter 53. The lower part of FIG. 11 will be described later. Note that in FIG. 11, for convenience of illustration, the tuning-fork cantilever 41 is shown as a leaf spring. As shown in FIG. 11, the tip position of the tuning-fork cantilever 41 and the measurement value of the voltmeter 53 are synchronized. Here, the average value of the minimum and maximum values ​​of the output of the voltmeter 53 is set as the threshold. The timing when the tip of the tuning-fork cantilever 41 is at its lowest position is defined as the phase "0." In this case, the timing when the measurement value exceeds the threshold is when the phase is "0.5π," and the timing when the phase next becomes "2π" or "0" is 3 / 4 of a period later. Since the period is obtained as the reciprocal of the calculated cantilever frequency f, the indicator 28 can calculate the timing when the next reference pulse P1 should be output.

[0064] The voltage measurement value may also be differentiated and used to determine the output timing of the reference pulse P1. The bottom part of FIG. 11 shows a differential value obtained by differentiating the measurement value shown in the middle part of FIG. 11. This differential value may be calculated by the indicator 28 differentiating the voltage measurement value with respect to time, or may be obtained by measuring the output of a differentiation circuit. The differential value is 90 degrees out of phase with the measurement value. Therefore, the indicator 28 detects when the voltage exceeds a threshold value set near 0 V from the negative side to the positive side, and immediately outputs the reference pulse P1 to the trigger generator 21C upon detection. Two methods for calculating the timing at which the reference pulse P1 should be output have been described here, but other methods may also be used.

[0065] The necessity for adjusting the frequency and phase of the reference pulse P1 will be explained with reference to FIGS. 12 and 13. In any of the embodiments and modifications described herein, atomic force, SPV (Surface Photo-Voltage)-induced force, and image dipole force act on the tuning-fork cantilever 41 or the leaf spring cantilever 41A. Of these, atomic force is not affected by the excited state of the sample 45 due to the pump light or the probe light, but the SPV-induced force and the image dipole force are closely correlated with the excited state of the sample 45. Therefore, to measure the SPV-induced force and the image dipole force with high sensitivity, it is desirable that the probe tip 411 of the tuning-fork cantilever 41 or the leaf spring cantilever 41A be in close proximity to the sample 45 when the sample 45 is excited. The tuning-fork cantilever 41 or the leaf spring cantilever 41A vibrates, and the distance between the probe tip 411 and the sample 45 changes at an approximately constant period.

[0066] FIG. 12 illustrates a problem that occurs when the frequency of the reference pulse P1 is not adjusted. In FIG. 12, time passes from left to right. Times t1, t2, t3, t4, and t5 shown at the bottom of FIG. 12 are the timings at which the reference pulse P1 is output. However, on the time axis of this figure, the delay time td is sufficiently short, and the pump light and probe light are irradiated onto the sample 45 at approximately the same timing as the reference pulse P1. The output period of the reference pulse P1 in this figure is constant, and the time interval between times t1 and t2, the time interval between times t2 and t3, the time interval between times t3 and t4, and the time interval between times t4 and t5 are all equal.

[0067] The middle part of Figure 12 shows the position of the cantilever tip before the frequency change. In other words, the middle part of Figure 12 shows the change in the distance between the probe tip 411 and the sample 45 over time. The oscillation frequency of the cantilever before the frequency change is, for example, 300 kHz, which is an integer multiple of 100 kHz, the frequency at which the reference pulse P1 is output. In this example, the specified magnification W is "1 / 3." In this case, if the cantilever tip is closest to the sample 45 at time t1, the cantilever tip is also closest to the sample 45 at all of times t2, t3, t4, and t5, allowing the SPV-induced force and image dipole force to be measured with high sensitivity at all times.

[0068] The upper part of Figure 12 shows the position of the cantilever tip after a frequency change. "After a frequency change" here refers to the time after the cantilever oscillation frequency has changed due to the effects of atomic forces, SPV-induced forces, and image dipole forces. If the output timing of the reference pulse P1 remains unchanged, even if the cantilever tip is closest to the sample 45 at time t1, the phase shift due to the mismatch in the periodicity becomes significant over time. In the example shown in Figure 12, the reference pulse P1 is output at time t5 when the cantilever tip is farthest from the sample 45. Therefore, if the frequency of the reference pulse P1 is not adjusted, it may be difficult to detect the SPV-induced force and image dipole force even if the sample 45 is excited, due to the large distance, resulting in reduced measurement sensitivity.

[0069] FIG. 13 illustrates the problems that arise when the phase of the reference pulse P1 is not adjusted. FIG. 13 explains the differences from FIG. 12. The times t1 to t5 shown at the bottom of FIG. 13 are the same as those in FIG. 12, so their explanation is omitted. The middle and top sections of FIG. 13 show the time series changes in the cantilever tip position in Examples 1 and 2. The frequencies in Examples 1 and 2 are the same as those before the frequency change in FIG. 12, and the specified magnification W is 1 / 3. Therefore, the output timing of the reference pulse P1 has the same phase in each example. If the phase is not explicitly adjusted, even if the frequency of the reference pulse P1 is an integer multiple or an integer fraction of the vibration frequency of the tuning fork cantilever 41, the only guarantee is that the phase of the cantilever tip position at the output timing of the reference pulse P1 is constant.

[0070] As a result, there are cases where the reference pulse P1 is output each time when the cantilever tip is in an approximately central position, as in Example 1, and cases where the reference pulse P1 is output when the cantilever tip is in a position farthest from the sample 45, as in Example 2. In Example 1, the sensitivity of measuring the SPV induced force and image dipole force is low, and in Example 2, the SPV induced force and image dipole force are barely measurable. Therefore, it can be seen that not adjusting the phase of the reference pulse P1 can make it difficult to detect the SPV induced force and image dipole force even if the sample 45 is excited because the distance is too far, resulting in a problem of low measurement sensitivity.

[0071] 12 and 13, if the frequency or phase of the reference pulse P1 is not adjusted, the problem of low measurement sensitivity for detecting the SPV-induced force and the image dipole force occurs. However, the trigger generator 21C in this embodiment increases or decreases the frequency at which the reference pulse P1 is output based on the output of the lock-in amplifier 25, and changes the phase of the reference pulse P1 based on the output of the voltmeter 53, so that the above problem does not occur and the SPV-induced force and the image dipole force can be measured with high sensitivity.

[0072] According to the above-described third embodiment, the following advantageous effects can be obtained. (5) The measurement system 1B includes an indicator 28 that causes the trigger generator 21C and the delay time controller 24 to output a reference pulse P1 and a delay pulse P2 based on the movement of the tuning fork cantilever 41. This allows for highly sensitive detection of changes in the vibration frequency of the tuning fork cantilever 41.

[0073] (6) The indicator 28 outputs a reference pulse P1 and a delay pulse P2 at a frequency that is an integer multiple of the vibration frequency of the tuning fork cantilever 41 or a frequency that is an integer fraction of the vibration frequency of the tuning fork cantilever 41.

[0074] (7) The indicator 28 adjusts the phases of the reference pulse P1 and the delayed pulse P2 so that the pump light and the probe light are irradiated onto the sample 45 when the tip of the tuning fork cantilever 41 is closest to the sample 45.

[0075] (Modification 1 of the third embodiment) In the third embodiment, the trigger generator 21C and the indicator 28 are described as separate hardware devices. However, the trigger generator 21C and the indicator 28 may be realized by the same hardware device. Also, the trigger generator 21C, the indicator 28, and the delay time controller 24 may be realized by the same hardware device. The indicator 28 may specify only the frequency at which the reference pulse P1 is output to the trigger generator 21C.

[0076] (Modification 2 of the third embodiment) The measurement system 1B may include a current amplifier instead of the charge amplifier 42. Even when the charge amplifier 42 is replaced with a current amplifier, the same effects can be obtained with the same configuration.

[0077] In each of the above-described embodiments and modifications, the functional block configurations are merely examples. Some functional configurations shown as separate functional blocks may be configured as an integrated unit, or a configuration shown in a single functional block diagram may be divided into two or more functions. Furthermore, some of the functions of each functional block may be provided by other functional blocks.

[0078] The above-described embodiments and modifications may be combined with each other. In particular, the fifth modification of the first embodiment may be combined with the third embodiment. Although various embodiments and modifications have been described above, the present invention is not limited to these. Other aspects conceivable within the scope of the technical concept of the present invention are also included within the scope of the present invention. [Explanation of symbols]

[0079] 1, 1A: Measurement system 2, 2A, 2B: Light source unit 3: Microscope unit 4: AFM unit 21, 21C: Trigger generator 21A: Trigger generation and delay time controller 21B: Multiple pulse generator 22: Pump light generation unit 23: Probe light generation unit 24: Delay time controller 25: Lock-in amplifier 28: Indicating device 35: Sample 41: Tuning fork cantilever 43:FMAM controller 44: Lock-in amplifier 51: Current measurement section 411: Probe tip

Claims

1. a first laser that outputs pump light, which is a pulse laser, in response to an input first signal; a second laser that outputs a probe light, which is a pulse laser, in response to an input second signal; a trigger generation and delay time controller that inputs the first signal and the second signal to the first laser and the second laser, switches a variable delay value, which is the difference between the timing of inputting a signal to the first laser and the timing of inputting a signal to the second laser, between a plurality of different values ​​to repeatedly input the first signal and the second signal, and outputs a reference signal to a lock-in amplifier when switching the variable delay value; an auxiliary optical system for directing the pump light and the probe light to a sample; a cantilever having a probe tip positioned adjacent to the sample; a controller that applies a periodically varying voltage to the cantilever and outputs a change amount signal that is a voltage or current corresponding to a change in the resonance frequency of the cantilever; The lock-in amplifier measures the change amount signal based on the reference signal.

2. 2. The measurement system of claim 1, The trigger generation and delay time controller switches the variable delay value between a measurement delay time and a reference delay time, and the reference delay time is longer than the time it takes for the sample to relax from an excited state excited by the first laser.

3. 3. The measurement system according to claim 2, The lock-in amplifier is the trigger generation and delay time controller sets a first value to the measurement delay time and a predetermined value to the reference delay time, and measures the first change amount signal; The measurement system further comprises a trigger generation and delay time controller that sets the measurement delay time to a second value and measures the second change amount signal with the reference delay time set to the predetermined value.

4. 2. The measurement system of claim 1, The measurement system further comprises a tunneling current measurement unit that measures a tunneling current flowing through the probe tip.

5. 2. The measurement system of claim 1, A measurement system, wherein the cantilever is made of a quartz crystal oscillator and has a tuning fork shape.

6. 2. The measurement system of claim 1, The measurement system further comprises an indicator that causes the trigger generation and delay time controller to output the first signal and the second signal based on the movement of the cantilever.

7. 7. The measurement system according to claim 6, A measurement system in which the indicating device outputs the first signal and the second signal at a frequency that is an integer multiple or an integer fraction of the vibration frequency of the cantilever.

8. 8. The measurement system according to claim 7, A measurement system in which the indicating device adjusts the phases of the first signal and the second signal so that the pump light and the probe light are irradiated onto the sample when the tip of the cantilever is closest to the sample.

9. A measurement method performed by a measurement system including a first laser that outputs pump light, which is a pulsed laser, in response to an input first signal, a second laser that outputs probe light, which is a pulsed laser, in response to an input second signal, an auxiliary optical system that guides the pump light and the probe light to a sample, a cantilever having a probe tip disposed close to the sample, and a lock-in amplifier, inputting the first signal and the second signal to the first laser and the second laser; a variable delay value, which is a difference between a timing at which a signal is input to the first laser and a timing at which the first signal and the second signal are input to the second laser, is switched among a plurality of ways, and the first signal and the second signal are repeatedly input; outputting a reference signal to the lock-in amplifier when switching the variable delay value; applying a periodically varying voltage to the cantilever and outputting a change amount signal which is a voltage or current according to a change in the resonance frequency of the cantilever; the lock-in amplifier measuring the change amount signal by phase-sensitive detection using the reference signal.

10. 10. The measurement method according to claim 9, The measurement method further includes an input step of inputting the first signal and the second signal to the first laser and the second laser based on the movement of the cantilever.

11. The measurement method according to claim 10, A measuring method, wherein the input step inputs the first signal and the second signal at a frequency that is an integer multiple of the vibration frequency of the cantilever or a frequency that is an integer fraction of the vibration frequency of the cantilever.

12. The measurement method according to claim 11, In the input step, the phases of the first signal and the second signal are adjusted so that the pump light and the probe light are irradiated onto the sample when the tip of the cantilever is closest to the sample.

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