JP2014215214A - Measuring method of physical property of sample - Google Patents

Abstract

A method for measuring a physical property of a sample capable of detecting a signal of an arbitrary frequency using a pump probe method is provided. A pump optical pulse train having a repetition frequency of fHz is amplitude-modulated at a frequency FHz to generate a pump optical pulse train including a frequency component of fn? FHz (n is an arbitrary integer). The sample is irradiated with the amplitude-modulated pump light pulse train through a delay optical path. The probe irradiated with the pump light pulse train is irradiated with the probe light pulse train having a repetition frequency of FHz, and a signal containing a frequency component of fn? FHz resulting from the irradiation of the pump light pulse train is detected from the sample. . The FHz is in the range of 0 to f / 2 Hz. [Selection] Figure 1

Description

  The present invention relates to a method for measuring physical properties of a sample using a pump probe method.

  When a material is irradiated with an ultrashort laser pulse having a time width of picoseconds to subpicoseconds, an acoustic wave in a frequency range of GHz to THz is generated in the material due to a local and steep temperature rise due to light absorption. . The state of propagation of the acoustic wave can be detected by a transient change in light reflectance on the sample surface caused by the propagation (Non-Patent Document 1). This experimental technique is called “picosecond laser acoustic method”. The picosecond laser acoustic method is a pump pulse train (for example, an optical pulse train or an electric pulse train) for causing a specific phenomenon in a sample, and a probe pulse train (for example, an optical pulse train or the like for detecting a change in the sample due to this phenomenon). Belongs to the “pump probe method”. The picosecond laser acoustic method is widely applied to non-destructive investigation of a nanometer-scale structure of a substance (Non-Patent Document 2), and examination of ultrafast dynamics of excited electrons in a substance (Non-Patent Document 2). Reference 3).

  The present inventors have developed a highly sensitive and highly stable optical interferometer for detecting a change in the complex light reflection coefficient in the picosecond laser acoustic method (Non-patent Document 4). By combining this optical interferometer and an optical scanner, the present inventors measure the propagation of surface acoustic waves in the GHz frequency domain as a time-resolved two-dimensional image with micron-scale spatial resolution and picosecond time resolution. The technology which can be developed was also developed (nonpatent literature 5, nonpatent literature 6). This technology has achieved great results in direct measurement of acoustic wave dispersion relations in phononic crystals, verification of phononic band gaps, and the like (Non-Patent Document 7).

C. Thomsen, J. Strait, Z. Vardeny, H. J. Maris, and J. Tauc, "Coherent Phonon Generation and Detection by Picosecond Light Pulses", Phys. Rev. Lett., Vol. 53, pp. 989-992. G. Andrew Antonelli, Humphrey J. Maris, Sandra G. Malhotra, and James M. E. Harper, "Picosecond ultrasonics study of the vibrational modes of a nanostructure", J. Appl. Phys., Vol. 91, pp. 3261-3267. Guray Tas and Humphrey J. Maris, "Electron diffusion in metals studied by picosecond ultrasonics", Phys. Rev. B, Vol. 49, pp. 15046-15054. David H. Hurley and Oliver B. Wright, "Detection of ultrafast phenomena by use of a modified Sagnac interferometer", Optics Letters, Vol. 24, pp. 1305-1307. Y. Sugawara, OB Wright, O. Matsuda, M. Takigahira, Y. Tanaka, S. Tamura, and VE Gusev, "Watching Ripples on Crystals", Phys. Rev. Lett., Vol. 88, pp.185504-1 -185504-4. Takehiro Tachizaki, Toshihiro Muroya, Osamu Matsuda, Yoshihiro Sugawara, David H. Hurley, and Oliver B. Wright, "Scanning ultrafast Sagnac interferometry for imaging two-dimensional surface wave propagation", Rev. Sci. Instrum., Vol. 77, pp 043713-1-043713-12. Dieter M. Profunser, Oliver B. Wright, and Osamu Matsuda, "Imaging Ripples on Phononic Crystals Reveals Acoustic Band Structure and Bloch Harmonics", Phys. Rev. Lett., Vol. 97, pp. 055502-1-055502-4.

  However, the conventional picosecond laser acoustic method has a problem that, in principle, the frequency of the detectable acoustic wave is limited to an integral multiple of the repetition frequency of the pump light pulse train. Therefore, the conventional picosecond laser acoustic method is unsuitable for applications in which, for example, the band edge frequency of a phononic band gap or the resonance frequency of a resonator having a high Q value is accurately checked.

  The present invention has been made in view of such a point, and an object of the present invention is to provide a method for measuring a physical property of a sample, which can detect a signal having an arbitrary frequency using a pump probe method. .

  In the pump probe method, the inventor applies a pump pulse train including a frequency component of fn ± FHz (n is an arbitrary integer) generated by modulation to a sample, and a signal obtained by applying the probe pulse train to the sample. The fn + FHz frequency component and the fn−FHz frequency component contained in the above are found to be separated and detected, and the present invention has been completed.

That is, the present invention relates to a method for measuring physical properties of the following samples.
[1] A method for measuring physical properties of a sample using the pump probe method, wherein a pump pulse train having a repetition frequency of fHz is amplitude-modulated with a frequency FHz, and a frequency component of fn ± FHz (n is an arbitrary integer) Generating a pump pulse train including: applying the amplitude-modulated pump pulse train to the sample; and causing the probe pulse train to reach the sample t ′ seconds after the pump pulse train reaches the sample. Applying a probe pulse train having a repetition frequency of fHz to the sample to which the pump pulse train is applied, and detecting a signal including a frequency component of fn ± FHz resulting from the application of the pump pulse train from the sample; A method for measuring physical properties of a sample.
[2] The pump pulse train undergoes amplitude modulation and then passes through a delay circuit, and the step of detecting the signal includes a step of Fourier transforming the in-phase signal X detected using a lock-in amplifier. [1] The measuring method as described in.
[3] Instead of applying the probe pulse train having a repetition frequency of fHz, the probe pulse train having a repetition frequency of fHz is generated by amplitude modulation at a frequency F′Hz (F ′ is a value different from F). The measurement method according to [1], wherein an amplitude-modulated probe pulse train is applied.
[4] The pump pulse train or the probe pulse train passes through a delay circuit after amplitude modulation, and the step of detecting the signal includes a step of Fourier transforming the in-phase signal X detected using a lock-in amplifier. The measurement method according to [3].
[5] The step of detecting the signal includes a step of generating a complex signal X + iY from the in-phase signal X and the 90 ° phase signal Y using a two-phase lock-in amplifier, and a step of Fourier transforming the complex signal X + iY. And the measurement method according to [1] or [3].
[6] The t ′ seconds are in the range of 0 to 1 / f seconds, and the step of generating the complex signal X + iY is based on the complex signal X + iY with respect to 0 ≦ t ′ ≦ 1 / f. The measurement method according to [5], including a step of generating a complex signal X + iY for> 1 / f.
[7] The measurement method according to any one of [1] to [6], wherein the FHz is in a range of 0 to f / 2 Hz.
[8] The measurement method according to any one of [1] to [7], wherein at least one of the pump pulse train and the probe pulse train is an optical pulse train.
[9] The measurement method according to [8], wherein the signal is a surface displacement of the sample due to propagation of a surface acoustic wave caused by irradiation with a pump light pulse train.

  According to the present invention, a signal having an arbitrary frequency can be detected in the pump probe method, and the physical properties of the sample can be measured more accurately.

It is a schematic diagram which shows the structure of the measuring apparatus which concerns on Embodiment 1 of this invention. It is a schematic diagram which shows the structure of the measuring apparatus which concerns on Embodiment 2 of this invention. It is a time-resolved two-dimensional image of the surface acoustic wave measured in the Example. It is a graph which shows the dispersion | distribution relationship of the surface acoustic wave measured in the Example.

  Hereinafter, embodiments according to the present invention will be described in detail with reference to the drawings. In the following description, as a representative example of the physical property measurement method for a sample according to the present invention, an acoustic wave of an arbitrary frequency is generated by irradiating a pump light pulse train, and the acoustic wave is propagated by irradiating a probe light pulse train. A time-resolved two-dimensional imaging method of the surface acoustic wave to be detected will be described. According to the time-resolved two-dimensional imaging method of surface acoustic waves according to the following embodiments, for example, the elastic constant and internal structure of a sample can be measured.

[Embodiment 1]
FIG. 1 is a schematic diagram showing a configuration of a measurement apparatus (a surface acoustic wave time-resolved two-dimensional imaging apparatus) for carrying out a physical property measurement method for a sample according to Embodiment 1 of the present invention.

  As shown in FIG. 1, the measuring apparatus 100 includes a pulsed laser light source 110, a nonlinear optical crystal 120, a first dichroic mirror 130, an acousto-optic modulator 140, a delay optical path (delay circuit) 150, a mirror 160, and an interferometer 180. , An optical scanner 190, a second dichroic mirror 200, an objective lens 210, a photodetector 220, a function generator 230, and a lock-in amplifier 260. A sample 300 is installed at a predetermined position of the measuring apparatus 100. The sample 300 is irradiated with a pump light pulse train 320 and a probe light pulse train 330.

  The pulse laser light source 110 emits an optical pulse train 310 having a repetition frequency of fHz and a pulse time width of 10 fs to 1 ns. The repetition frequency fHz of the optical pulse train 310 is not particularly limited, but is, for example, in the range of 100 kHz to 100 MHz. The pulse laser light source 110 is, for example, a mode-locked Ti-sapphire laser.

  The optical pulse train 310 emitted from the pulse laser light source 110 is converted into an optical pulse train 310 ′ including the second harmonic by the nonlinear optical crystal 120. The optical pulse train 310 ′ is divided by the first dichroic mirror 130 into a pump optical pulse train 320 having the same center wavelength as the second harmonic of the optical pulse train 310 and a probe optical pulse train 330 having the same center wavelength as the optical pulse train 310. . The wavelengths of the pump light pulse train 320 and the probe light pulse train 330 may be reversed.

  The pump light pulse train 320 is amplitude-modulated by the acousto-optic modulator 140 at the frequency FHz. Here, F (Hz) is not more than half of the repetition frequency f (Hz) of the optical pulse train 310, that is, within a range of 0 to f / 2 (Hz). As is well known, when a wave having a frequency of fHz is amplitude-modulated at a frequency of FHz, a frequency component of f ± FHz is generated. Therefore, the amplitude-modulated pump light pulse train 320 includes frequency components of fn ± FHz (n is an arbitrary integer).

  The amplitude-modulated pump light pulse train 320 is guided to the second dichroic mirror 200 via the delay optical path 150 and the first mirror 160. The delay optical path 150 includes two mirrors 151 and 152 and a corner cube 153, and the optical path length of the pump light pulse train 320 can be adjusted by adjusting the distance between the mirrors 151 and 152 and the corner cube 153.

  On the other hand, the probe light pulse train 330 enters the interferometer 180. The type of interferometer 180 is not particularly limited as long as a surface displacement of about 1 pm of the sample can be detected. The interferometer 180 is a two-arm Sagnac interferometer described in, for example, Japanese Patent Application Laid-Open No. 2001-228121.

  The probe light pulse train 330 that has entered the interferometer 180 is guided to the second dichroic mirror 200 via the optical scanner 190. The optical scanner 190 includes a mirror 191, a movable mirror 192, and two lenses 193 and 194, and the position of the condensing point of the probe light pulse train 330 can be moved by rotating the movable mirror 192. In the optical scanner 190, each optical element is arranged so as to constitute a 4f optical system.

  The pump light pulse train 320 and the probe light pulse train 330 that have reached the second dichroic mirror 200 are irradiated onto the sample 300 via the objective lens 210, respectively. At this time, the optical path length of the delay optical path 150 is adjusted so that the probe light pulse arrives after a desired delay time t ′ (second) has elapsed since the arrival of the pump light pulse. When the pump light pulse train 320 reaches the sample 300, an acoustic wave is generated inside and on the surface of the sample 300. The frequency of the generated acoustic wave is spread over a wide band, and the upper limit is about GHz. Corresponding to the frequency component of fn ± FHz (n is an arbitrary integer) included in the pump light pulse train 320, the generated acoustic wave includes a frequency component of fn ± FHz.

  The probe light pulse train 330 is reflected from the surface of the sample 300 to become a reflected light pulse train 340. When the surface of the sample 300 is displaced due to the propagation of the surface acoustic wave, the reflected light pulse train 340 includes a signal related to the surface displacement (for example, phase change). The reflected light pulse train 340 enters the interferometer 180 via the objective lens 210 and the optical scanner 190. Interferometer 180 and photodetector 220 detect a signal (for example, phase change) related to the surface displacement included in reflected light pulse train 340. In order to ensure a sufficient signal-to-noise ratio, only the signal synchronized with the modulation frequency FHz of the pump light pulse train 320 is detected from the signals detected by the photodetector 220 using the lock-in amplifier 260.

  The condensing position of the pump light pulse train 320 is fixed at a specific position on the surface of the sample 300. On the other hand, the condensing position of the probe light pulse train 330 is moved by rotating the movable mirror 192. Thus, by scanning the condensing position of the probe light pulse train 330 with respect to the condensing position of the pump light pulse train 320, a two-dimensional distribution image of the surface displacement can be obtained. Also, a time-resolved two-dimensional image of an acoustic wave can be obtained by obtaining a two-dimensional distribution image of surface displacement while changing the delay time t ′ of the probe light pulse with respect to the pump light pulse. The spatial resolution in the horizontal direction is usually about 1 μm, and the scanning range is about 500 μm × 500 μm at the maximum.

  The function generator 230 is connected to the acousto-optic modulator 140 and transmits a signal for modulating the pump light pulse train 320. The function generator 230 is also connected to the lock-in amplifier 260.

  In the method for measuring physical properties of a sample according to the present embodiment, the propagation of acoustic waves on the surface of the sample 300 is detected by the following procedure using the measuring apparatus 100 having the above configuration.

(1) A pump light pulse train 320 having a repetition frequency of fHz is amplitude-modulated at a frequency FHz to generate a pump light pulse train 320 including a frequency component of fn ± FHz (n is an arbitrary integer).
(2) The sample 300 is irradiated with the amplitude-modulated pump light pulse train 320 through the delay optical path 150.
(3) The sample 300 irradiated with the pump light pulse train 320 is irradiated with the probe light pulse train 330 whose repetition frequency is fHz, and the frequency of fn ± FHz (n is an arbitrary integer) resulting from the irradiation of the pump light pulse train 320 A signal including a component is detected from the sample 300.

  In the measurement method according to the present embodiment, in step (1), the pump light pulse train 320 having a repetition frequency of fHz is amplitude-modulated at the frequency FHz. As described above, by performing this, the pump light pulse train 320 including the frequency component of fn ± FHz (n is an arbitrary integer) can be generated. In the step (2), when the sample 300 is irradiated with the amplitude-modulated pump light pulse train 320, an acoustic wave of fn ± FHz is generated. Therefore, by selecting a frequency within the range of 0 to f / 2 (Hz) as F (Hz), an acoustic wave having an arbitrary frequency can be generated. In the step (3), the surface displacement of the sample 300 caused by the acoustic wave thus generated is detected by lock-in using the probe pulse train 330 having a repetition frequency of fHz.

  The output signal of the lock-in amplifier 260 is a function of the irradiation position of the probe light pulse train 330 on the sample 300 and the delay time t ′. Therefore, when the irradiation position of the probe light pulse train 330 is fixed, the output signal of the lock-in amplifier 260 is a function of the delay time t ′. As shown in FIG. 1, when the pump light pulse train 320 is amplitude-modulated and passes through the delay optical path 150, an acoustic wave having a frequency of fn ± FHz exhibits a vibration of fn ± FHz on the delay time t ′ axis. . Therefore, when the output signal (in-phase signal X) of the lock-in amplifier 260 that is a function of the delay time t ′ is Fourier-transformed with respect to the delay time, information on the amplitude and phase of the frequency component (acoustic wave) of fn ± FHz is obtained. be able to. That is, a signal including a frequency component of fn ± FHz resulting from irradiation of the pump light pulse train 320 can be detected from the sample 300 by separating the frequency component of fn + FHz and the frequency component of fn−FHz.

  On the other hand, unlike the configuration shown in FIG. 1, when the delay optical path 150 is arranged before the acousto-optic modulator 140 or when the delay optical path 150 is arranged in the optical path of the probe light pulse train 330. The acoustic wave having a frequency of fn ± FHz exhibits fnHz vibration on the delay time t ′ axis. Therefore, even if the Fourier transform is performed on the output signal (in-phase signal X) of the lock-in amplifier 260 as described above, the frequency component of fn + FHz and the frequency component of fn−FHz cannot be separated. In this case, a complex signal X + iY may be generated from the in-phase signal X and the 90 ° phase signal Y using a two-phase lock-in amplifier as the lock-in amplifier 260, and the obtained complex signal X + iY may be Fourier transformed. In the Fourier transform of a real function, the difference between positive and negative frequencies is meaningless because the Fourier amplitude for positive and negative frequencies is always in a complex conjugate relationship. On the other hand, in the Fourier transform of a complex function, the Fourier amplitudes of positive and negative frequencies are always in an independent relationship. By utilizing this property, it is possible to separate the frequency component of fn + FHz and the frequency component of fn−FHz.

Table 1 shows how the complex signal X + iY changes with respect to the delay time t ′ (delay time dependency of X + iY) when the pump light pulse train 320 is modulated. In Table 1, “pump optical path” means an optical path from the first dichroic mirror 130 to the second dichroic mirror 200 through which the pump light pulse train 320 passes. The “probe optical path” means an optical path from the first dichroic mirror 130 to the second dichroic mirror 200 through which the probe light pulse train 330 passes. The delay time dependent duplex is in the same order as the acoustic wave duplex of frequency fn ± FHz.

In all the modes shown in Table 1, when Fourier transform is performed as in the following formula (1), the complex amplitude of the component of fn ± FHz can be obtained separately for ±.

  As described above, the method for measuring physical properties of a sample according to the present embodiment applies a pump light pulse train including a frequency component of fn ± FHz (n is an arbitrary integer) generated by modulation to the sample. The signal of arbitrary frequency is detected by separating and detecting the frequency component of fn + FHz and the frequency component of fn−FHz contained in the signal obtained by applying the probe light pulse train to the sample. Properties can be measured more accurately.

[Embodiment 2]
In the second embodiment, an example in which the physical property measurement method for a sample according to the first embodiment and the heterodyne method are combined will be described.

  FIG. 2 is a schematic diagram showing a configuration of a measurement apparatus (a surface acoustic wave time-resolved two-dimensional imaging apparatus) for carrying out a physical property measurement method for a sample according to Embodiment 2 of the present invention.

  As shown in FIG. 2, the measuring apparatus 100 ′ includes a pulsed laser light source 110, a nonlinear optical crystal 120, a first dichroic mirror 130, a first acousto-optic modulator 140, a delay optical path 150, a mirror 160, a second Acousto-optic modulator 170, interferometer 180, optical scanner 190, second dichroic mirror 200, objective lens 210, photodetector 220, first function generator 230, second function generator 240, mixer 250, and lock-in amplifier 260. The measurement apparatus 100 ′ according to the second embodiment is different from the measurement apparatus 100 according to the first embodiment in that both the pump light pulse train 320 and the probe light pulse train 330 are modulated. Therefore, the same components as those of the measurement apparatus 100 of the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  In the present embodiment, the probe light pulse train 330 enters the interferometer 180 after passing through the second acousto-optic modulator 170. The probe light pulse train 330 is amplitude-modulated at a frequency F ′ Hz by the second acousto-optic modulator 170. Here, F ′ (Hz) is a value different from the modulation frequency F (Hz) of the pump light pulse train 320.

  The first function generator 230 is connected to the first acousto-optic modulator 140 and transmits a signal for modulating the pump light pulse train 320. The second function generator 240 is connected to the second acousto-optic modulator 170 and transmits a signal for modulating the probe light pulse train 330. The first function generator 230 and the second function generator 240 are also connected to the lock-in amplifier 260 via the mixer 250. The lock-in amplifier 260 is configured to detect a frequency component of | F−F ′ | Hz.

  Also in the measuring apparatus 100 ′ of this embodiment, when the output signal (in-phase signal X) of the lock-in amplifier 260, which is a function of the delay time t ′, is Fourier-transformed with respect to the delay time, the frequency component (acoustic wave) of fn ± FHz is obtained. ) Information about the amplitude and phase. That is, a signal including a frequency component of fn ± FHz resulting from irradiation of the pump light pulse train 320 can be detected from the sample 300 by separating the frequency component of fn + FHz and the frequency component of fn−FHz. This is the same even when the delay optical path 150 is arranged not in the optical path of the pump optical pulse train 320 but in the optical path of the probe optical pulse train 330 (however, the delay optical path 150 is not in the second acoustic wave). (Only when it is arranged after the optical modulator 170). In the present embodiment, an acoustic wave having a frequency fn ± FHz (n is an arbitrary integer) is observed as an acoustic wave having a frequency fn ± FHz or fn ± F′Hz.

  On the other hand, unlike the configuration shown in FIG. 2, when the delay optical path 150 is arranged before the first acousto-optic modulator 140, or when the delay optical path 150 is in the optical path of the probe optical pulse train 330, When arranged before the acousto-optic modulator 170, even if the Fourier transform is performed on the output signal (in-phase signal X) of the lock-in amplifier 260 as described above, the frequency component of fn + FHz and the frequency component of fn−FHz. The frequency component cannot be separated. As in the first embodiment, in this case, a complex signal X + iY is generated from the in-phase signal X and the 90 ° phase signal Y by using a two-phase lock-in amplifier as the lock-in amplifier 260, and the obtained complex signal X + iY may be Fourier transformed.

Table 2 shows how the complex signal X + iY changes with respect to the delay time t ′ (delay time dependency of X + iY) when both the pump light pulse train 320 and the probe light pulse train 330 are modulated. The delay time dependent duplex shown in Table 2 is in the same order as the acoustic wave duplex of the frequency fn ± FHz.

As in the first embodiment, in all aspects shown in Table 2, when Fourier transform is performed as in the following formula (1), the complex amplitudes of the components of fn ± FHz are obtained separately for ±. Can do.

In both Embodiments 1 and 2, it is preferable to acquire data of X (t ′) + iY (t ′) for t ′ as wide as possible from the viewpoint of improving the accuracy of Fourier transform. However, the measurable range of t ′ is limited because it is limited by the length of the delay optical path. However, if data of X (t ′) + iY (t ′) is acquired within the range of t′∈ [0, T], X (t ′) + iY (t ′) can be expressed by the following formula (2 ). T is the repetition period (1 / f second) of the optical pulse train 310 emitted from the pulse laser light source 110. That is, if the following equation (2) is used, the complex signal X + iY for t ′> 1 / f can be generated based on the complex signal X + iY for 0 ≦ t ′ ≦ 1 / f.

Here, as the phase factor Δ, the one shown in Table 3 is used according to the modulation method. Note that in an embodiment not shown in Table 3, it is not necessary to expand X (t ′) + iY (t ′).

  In the measuring apparatus 100 according to the first embodiment, the photodetector 220 is required to have a sufficient band capable of detecting the light intensity modulation at the frequency FHz. On the other hand, in the measuring apparatus 100 ′ according to the second embodiment, the photodetector 220 is required to have a band capable of detecting the light intensity modulation with the frequency | F−F ′ | Hz. Therefore, the measurement method according to the second embodiment has an effect that the bandwidth required for the photodetector can be reduced in addition to the effect of the measurement method according to the first embodiment.

  In each of the above embodiments, an example has been described in which an acoustic wave having an arbitrary frequency is generated by irradiating the pump light pulse train, and propagation of the acoustic wave is detected by irradiating the probe light pulse train. However, the physical property measurement method is not limited to this. For example, in the method for measuring physical properties of a sample according to the present invention, an electric pulse train may be applied to the sample as a pump pulse train and / or a probe pulse train. Further, in the method for measuring physical properties of a sample according to the present invention, precession or relaxation of electron spin, a signal due to plasmon propagation, or the like may be detected.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail with reference to an Example, this invention is not limited by these Examples.

  In this example, the results of measuring the propagation of acoustic waves on the sample surface by combining the pump probe method and the heterodyne method as in the second embodiment are shown. In this embodiment, an optical pulse train is used as the pump pulse train and the probe pulse train.

  A gold thin film (film thickness 40 nm) was formed on one surface of a crown glass substrate (thickness 1 mm) by vacuum deposition. The obtained glass substrate with a gold thin film was used as a sample.

  Using the apparatus shown in FIG. 2, the gold thin film of the sample was irradiated with the pump light pulse train and the probe light pulse train, and the surface displacement due to the surface acoustic wave was measured.

  As the pulse laser light source 110, a mode-locked Ti-sapphire laser (repetition frequency: 76 MHz, pulse time width: 100 fs) was used. The optical pulse train 310 (center wavelength 830 nm) emitted from the pulse laser light source 110 is pumped with a pump light pulse train 320 (second harmonic; center wavelength 415 nm) and probe light using the nonlinear optical crystal 120 and the first dichroic mirror 130. It was divided into a pulse train 330 (center wavelength 830 nm).

  The pump light pulse train 320 was amplitude-modulated by the first acousto-optic modulator 140 at a frequency of 7.7 MHz. The amplitude-modulated pump light pulse train 320 contained frequency components of 76n ± 7.7 MHz (n is an arbitrary integer). The amplitude-modulated pump light pulse train 320 was irradiated perpendicularly to a predetermined position of the sample 300 via the delay optical path 150 and the objective lens 210, and surface acoustic waves were repeatedly generated on the surface of the sample 300. The focal position of the pump light pulse train 320 during measurement was unchanged.

  On the other hand, the probe light pulse train 330 was amplitude-modulated at a frequency of 9.4 MHz by the second acousto-optic modulator 170. The sample 300 is irradiated with the amplitude-modulated probe light pulse train 330 via the same objective lens 210, and the interferometer 180, the photodetector 220, and the lock-in amplifier 260 are used to propagate the sample 300 due to the propagation of the surface acoustic wave. The surface displacement was measured. At this time, the delay optical path 150 for the pump light pulse train is set so that the probe light pulse arrives after a predetermined delay time (34 steps within a range of 0 to 13.2 ns) has elapsed since the arrival of the pump light pulse. The optical path length was adjusted. Further, the two-dimensional distribution (200 μm × 200 μm) of the surface displacement for the same delay time was measured by scanning the converging position of the probe light pulse train 330 using the movable mirror 192. As the interferometer 180, a two-arm Sagnac interferometer described in Japanese Patent Application Laid-Open No. 2001-228121 was used. This interferometer irradiates two probe light pulses at different timings with respect to one pump light pulse, and detects a phase difference between reflected light of the two probe light pulses. Thereby, the surface displacement speed of the sample 300 is measured. The lock-in amplifier 260 is configured to detect a frequency component of 1.7 MHz (= 9.4 MHz-7.7 MHz).

  FIG. 3 shows an example of the time-resolved two-dimensional image of the obtained surface acoustic wave. This figure is a plot of the in-phase output X of the lock-in amplifier, and shows the distribution of the surface displacement rate 11.2 nanoseconds after the pump light pulse is irradiated to the center of the screen. A plurality of concentric wave packets can be seen around the irradiation position of the pump light pulse. From this result, it can be seen that the pump light pulse is periodically emitted.

  As described above, the in-phase output X and 90 ° phase output Y of the lock-in amplifier are obtained while changing the delay time in 34 stages, and the time-resolved two-dimensional image as shown in FIG. 34 sheets were acquired. Subsequently, the complex amplitude X (t ′) + iY (t ′) was obtained in the range of 0 ≦ t ′ ≦ 1 / f. Further, by using the relationship of X (t ′ + 1 / f) + iY (t ′ + 1 / f) = exp (−2πiF / f) {X (t ′) + iY (t ′)}, the complex amplitude X (t ') + IY (t') was expanded to the range of t '> 1 / f. The obtained results were Fourier transformed along the delay time axis and the two-dimensional space axis. The Fourier amplitude obtained is a function of the frequency fn ± FHz and the (two-dimensional) wave number. This function takes a finite value only when the set of the frequency fn ± FHz and the wave number satisfies the dispersion relation of the acoustic wave generated in the sample.

  FIG. 4 is a graph showing the dispersion relation of the surface acoustic wave in the sample. In this graph, the peak of the Fourier amplitude is plotted on a plane having the wave number as the x-axis and the frequency as the y-axis. The fact that each point extends in the horizontal direction is due to the accuracy of the wave number determined by the size of the measurement region. From this graph, it can be seen that each point is located on one of the two curves. A curve with a small inclination corresponds to a Rayleigh wave, and a curve with a large inclination corresponds to a longitudinal wave type pseudo surface acoustic wave.

  In FIG. 4, the points are arranged so as to be paired around 76 nMHz (n is an arbitrary integer). These two points corresponding to the pair correspond to positive and negative detuning frequencies (76n ± 7.7 MHz). That is, in this experiment, it can be seen that a signal of 76n + 7.7 MHz is distinguished from a signal of 76n−7.7 MHz. Furthermore, it should be noted that, corresponding to the acoustic wave dispersion relationship, the point on the high frequency side of the two points to be paired is located on the high wave number side. This is consistent with the expected dispersion relationship.

  By performing the same measurement as in the present embodiment while arbitrarily changing the modulation frequency F of the pump light pulse train and the modulation frequency F ′ of the probe light pulse train, it is possible to precisely measure the acoustic wave dispersion relationship in the sample. . This is very useful for measuring complex dispersion relationships of phononic crystals and measuring the frequency response of resonators.

  The method for measuring physical properties of a sample according to the present invention is useful, for example, for inspection of a wireless communication device and evaluation of characteristics of a resonator.

100, 100 'Measuring device 110 Pulse laser light source 120 Nonlinear optical crystal 130 First dichroic mirror 140 (first) acousto-optic modulator 150 Delay optical path 151, 152, 160, 191 Mirror 153 Corner cube 170 Second acousto-optic Modulator 180 Interferometer 190 Optical scanner 192 Movable mirror 193, 194 Lens 200 Second dichroic mirror 210 Objective lens 220 Photo detector 230 (First) function generator 240 Second function generator 250 Mixer 260 Lock-in amplifier 300 Sample 310, 310 ′ Light pulse train 320 Pump light pulse train 330 Probe light pulse train 340 Reflected light pulse train

Claims (9)

A method for measuring physical properties of a sample using a pump probe method,
Amplitude-modulating a pump pulse train having a repetition frequency of fHz with a frequency FHz to generate a pump pulse train including a frequency component of fn ± FHz (n is an arbitrary integer);
Applying the amplitude-modulated pump pulse train to a sample;
A probe pulse train having a repetition frequency of fHz is applied to the sample to which the pump pulse train is applied so that the probe pulse train reaches the sample t ′ seconds after the pump pulse train reaches the sample, Detecting a signal containing a frequency component of fn ± FHz resulting from application of a pump pulse train from the sample;
A method for measuring physical properties of a sample. The pump pulse train passes through a delay circuit after being amplitude-modulated,
The step of detecting the signal includes a step of Fourier transforming the in-phase signal X detected using a lock-in amplifier.
The measurement method according to claim 1.   Instead of applying the probe pulse train having a repetition frequency of fHz, the probe pulse train having a repetition frequency of fHz is generated by amplitude modulation at a frequency F′Hz (F ′ is a value different from F). The measurement method according to claim 1, wherein a probe pulse train is applied. The pump pulse train or the probe pulse train passes through a delay circuit after being amplitude-modulated,
The step of detecting the signal includes a step of Fourier transforming the in-phase signal X detected using a lock-in amplifier.
The measurement method according to claim 3. Detecting the signal comprises:
Generating a complex signal X + iY from the in-phase signal X and the 90 ° phase signal Y using a two-phase lock-in amplifier;
Fourier transforming the complex signal X + iY;
The measuring method of Claim 1 or Claim 3 containing this. The t ′ seconds are in the range of 0 to 1 / f seconds,
The step of generating the complex signal X + iY includes the step of generating the complex signal X + iY for t ′> 1 / f based on the complex signal X + iY for 0 ≦ t ′ ≦ 1 / f.
The measuring method according to claim 5.   The measurement method according to claim 1, wherein the FHz is in a range of 0 to f / 2 Hz.   The measurement method according to claim 1, wherein at least one of the pump pulse train and the probe pulse train is an optical pulse train.   The measurement method according to claim 8, wherein the signal is a surface displacement of the sample due to propagation of a surface acoustic wave caused by irradiation of a pump light pulse train.