JP2009229386A - Sum frequency generating spectroscopic device and its spectroscopic method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a SFG spectroscopic device and its measuring method by making single channel sum frequency spectroscopic method to have a high function and making wavelength of infrared light and visible light variable. <P>SOLUTION: The sum frequency generating spectroscopic device includes: a pulse laser light source; a long wavelength exciting light generator of wavelength variable for separating the output light and for wavelength converting a part of separation; a short wavelength exciting light generator of wavelength variable for wavelength converting the other part of separation; a first optical system for irradiating the sample with the long wavelength exciting light; a second optical system of irradiating the sample with short wavelength exciting light; a delay device; a third optical system for selecting sum frequency light from light from the sample; and an optical detector. The long wavelength excitation light generator sweeps the wavelength band region, and the short wavelength generator changes output wavelength for every sample. The first optical system, the second optical system and the delay device irradiate the sample so as to synchronously superpose spatially and timely. The third optical system inhibits the light other than the selected sum frequency light by making the sum frequency light incident on the light detector by adjusting the light path of the sum frequency light. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to single-channel SFG spectroscopy, and relates to a sum frequency generation spectroscopic device and a spectroscopic method thereof capable of measuring vibration spectra of surfaces and interfaces, or interface-selective time-resolved measurements.

  Sum frequency generation spectroscopy is well known as a method for selectively analyzing only the surface and interface. Sum frequency generation (SFG) is a phenomenon based on a second-order nonlinear optical effect. When a sample surface is irradiated with two types of laser light (called probe light or excitation light), ω3 = This is a process in which light having a sum frequency of ω1 + ω2 is generated. Since this process occurs only in a medium having no inversion symmetry and not in a medium having inversion symmetry, it is used when selectively analyzing only the surface or the interface.

  For example, an infrared laser beam that can sweep the wavelength and a laser beam in a visible region (for example, 532 nm) with a fixed wavelength are simultaneously irradiated on the sample surface, and the generated SFG light in the ultraviolet or visible region is detected. Since the SFG light increases when the vibration level existing on the surface is equal to the frequency of the incident infrared light, the vibration spectrum of the surface or interface can be obtained by detecting the SFG light while sweeping the wave number of the infrared light. Can do.

  This SFG spectroscopy was first described in 1987 by Y.-S. R. Developed by Shen et al. The feature of their method is that one of the lights uses an infrared pulse whose wavelength is variable. This SFG spectroscopy has interface selectivity in principle as described above, and also uses an ultrashort pulse laser, so it can be applied to interface-resolved time-resolved measurement, and not only the surface but also liquid. / Solid, Gas / Liquid, Gas / Solid, Liquid / Liquid, or a solid buried interface.

  The apparatus used in the original SFG spectroscopy uses a picosecond laser as a light source to emit monochromatic SFG light generated when a measurement sample is simultaneously irradiated with a monochromatic infrared pulse and a monochromatic visible pulse. This is a method of detecting with one detector while changing the wave number of the signal, and is now also referred to as single channel SFG spectroscopy to distinguish it from broadband SFG spectroscopy described later.

  In a normal picosecond SFG spectrometer with a fixed visible light wavelength, the wavelength variable infrared light is oscillated as follows. First, the pulsed light of the Nd: YAG laser is converted into a second harmonic or a third harmonic, and wavelength-visible visible light and near infrared light are extracted by optical parametric oscillation. Further, the wavelength-variable infrared light is continuously generated by the difference frequency generation (DFG) between the near-infrared light and the fundamental wave of Nd: YAG (wavelength = 1064 nm). The laser required for this purpose is a pulse laser having a pulse width of about 20-30 ps and a maximum output of about 50 mJ (wavelength = 1064 nm).

Further, as other SFG spectroscopy apparatuses, the following ones are known: (1) SFG spectroscopy apparatus based on synchronization of an infrared free electron laser and a visible pulse laser.
(2) An SFG spectrometer that demultiplexes the output of the femtosecond laser to generate infrared light and visible light.
(3) An SFG spectrometer in which the visible light intensity is increased in (2) above.
More details are shown below.

(1) SFG spectroscope by synchronization of infrared free electron laser and visible pulse laser.
In 1996, Van de Ham et al. Developed SFG by synchronizing infrared free electron laser and visible pulse laser. This infrared light is broadband light having a wave number width of 80 cm ⁻¹ , and has an advantage that SFG in a certain region can be measured at a time. However, a facility having a large-scale free electron laser must be used, and the entire apparatus becomes large and complicated because the visible laser and the free electron laser are synchronized.

(2) An SFG spectrometer that demultiplexes the output of the femtosecond laser to generate infrared light and visible light.
Meanwhile, a new SFG spectrometer using a femtosecond laser was reported by Richter et al. In 1999 NIST. This method splits pulsed light obtained from a single femtosecond laser to generate broadband infrared light (approximately 280 cm ⁻¹ ) having a wide wavenumber and narrowband visible light having a narrow wavenumber, and broadband from these. This technique is generally called broadband SFG spectroscopy. Similar to the above-described free electron laser, this method has the advantage that it can measure SFG in a constant wavenumber region at a time and can be measured with an S / N ratio superior to that of a normal single channel SFG.

(3) An SFG spectrometer in which the visible light intensity is increased in (2) above.
Also, Ishibashi et al. Disclosed a multiplex SFG device in which the above broadband SFG spectroscopy is improved in Patent Document 1 (Japanese Patent Laid-Open No. 2002-90293) and Patent Document 2 (Japanese Patent Laid-Open No. 2002-340672). Yes.

  In the disclosure of Patent Document 1, broadband infrared light is created from one of the fundamental split waves output from a single femtosecond laser light source, and narrow-band visible light is created from the other split wave. . Broadband infrared light is obtained by oscillating an optical parametric generation / amplifier and generating a difference frequency between signal light and idler light. Further, the narrow band visible light is generated by dividing the other of the divided waves into two and chirping them in the opposite direction and then combining them to generate a double wave. The broadband infrared light and the narrow-band visible light are condensed on the surface of the sample to generate broadband SFG light, which is then dispersed.

  In addition, in the disclosure of Patent Document 2, the creation of narrow band visible light is different from that of Patent Document 1, and the other split wave is divided into two, chirped in the opposite direction, and then combined to generate a double wave to create narrow band visible light. Further, the wavelength of the visible light that has been created is made variable by optical parametric oscillation. In this way, the wavelength of the narrow-band visible light can be varied substantially continuously, the wavelength that intentionally causes the visible light resonance phenomenon is selected for various measurement objects, and the signal of the SFG light The strength is increased and sensitivity is improved.

  However, in order to generate visible light used in the broadband SFG spectroscopy, it is necessary to cut out light from a pulsed laser light source using a spectroscope or the like from a specific portion of the wavelength band to narrow the band. By this narrowing of the band, the optical pulse becomes longer, for example, the femtosecond pulse becomes a picosecond pulse, and the temporal peak intensity decreases. Furthermore, the loss of laser light cannot be ignored in this narrowing of the band, and the obtained visible light intensity becomes weak. Also in the generation of infrared light, the wavelength converter that generates broadband infrared light oscillates broadband infrared light, so that the peak intensity of the spectrum decreases. In general, SFG requires high-intensity pulsed light, but the broadband SFG spectroscopy described above must use a combination of weak infrared light and weak visible light. For example, a dielectric sample surface or the like generally cannot obtain an SFG intensity sufficient for detection. For this reason, for example, it is necessary to improve the signal-to-noise ratio by accumulating weak SFG light for a long time. Since SFG spectrum measurement can be performed at a time, measurement in a short time is expected. However, as described above, it is difficult to fully utilize the advantages of the broadband SFG method.

  In addition, in the femtosecond broadband SFG spectroscopy, broadband infrared light is used. In order to correct the intensity distribution of the SFG spectrum due to this wavelength distribution, a reference sample such as GaAs that emits strong SFG light is separately measured. An operation to normalize the obtained spectrum is required.

  On the other hand, the single channel SFG spectroscopy requires a longer measurement time than the broadband SFG spectroscopy. This is because SFG generated from monochromatic infrared light and visible light is measured while scanning the wave number of infrared light. However, since the light extracted from the laser as visible light is used as it is, there is no decrease in visible light intensity. In addition, recent advances in laser technology have improved laser stability, allowing long-time measurements, and high-intensity, fast-repeating picosecond lasers on the market. Single channel SFG spectroscopy is still the mainstay. In single channel SFG spectroscopy, since the intensity of SFG light is proportional to the intensity of infrared light and visible light, it is desirable to use an ultrashort pulse laser with a high peak intensity.

In addition, a plurality of known infrared light generation methods are known, but as a commonly used method, first, in a picosecond laser, laser light is converted into LiB ₃ O ₅ crystal, β-BaB ₂ O ₄ crystal, etc. Next, there is a method of generating an optical parametric oscillation, and then generating infrared light of variable wavelength by difference frequency generation (DFG) on the nonlinear optical crystal between the obtained near-infrared light and the fundamental wave of the laser. As this nonlinear optical crystal, an AgGaS ₂ crystal is often used, but the intensity of the generated infrared light has a large wavelength dependency.

Further, when using the Raman shifter described in Non-Patent Document 1 (AL Harris et al., Chem. Phys. Lett., 141 (1987) 350.) or Non-Patent Document 2 (Akihide Wada et al., Surface, 32 (1994) 318.), when infrared light is oscillated with other nonlinear optical crystals such as LiNbO ₃ , CH, OH stretch (2800 to 4000 cm ⁻¹ ) and C═O stretch (1600 to 1800 cm). ^-1 ), it is possible to obtain relatively strong infrared light, but in the lower fingerprint region (1400 to 1000 cm ^-1 ), the infrared light intensity is weak and not sufficient to enable SFG measurement. For this reason, in the measurement in the fingerprint region, in addition to increasing the intensity of infrared light itself, the other visible light intensity is increased to increase the SFG intensity, or the measurement conditions are optimized for the measurement object. Techniques such as increasing the SFG intensity are necessary.

  As for the visible light used as the excitation light in the single channel SFG, when a picosecond Nd: YAG laser is used as the laser light source, the double wave 532 nm of the light extracted from the laser is most common. When this excitation light is used, molecules present at the interface absorb infrared light and visible light, and transition to a virtual level.

  Here, when the wavelength of visible excitation light is made variable and adjusted to the wavelength of electronic transition of molecules existing at the interface or the absorption wavelength of the substrate or the like, SFG light resonates with not only infrared light but also visible light, and the obtained SFG It is known that the intensity of light increases. SFG spectroscopy uses the resonance between the infrared light and the vibrational state of the molecules present at the interface, but by utilizing the above resonance with visible light (double resonance), the molecules at the interface It becomes possible to discriminate more detailed information at the interface such as what orientation is taken and whether the association state between molecules is the same or different between the bulk and the interface. In addition, if the double resonance condition is used, it is possible to generate a signal that can be sufficiently detected even with a smaller number of molecules. Therefore, analysis of slight impurities at the interface and efficient vibration mode with weak SFG intensity are possible. Great effect can be expected for detection.

  Further, when the target sample has fluorescence in the wavelength region of the SFG light to be measured, it is difficult to detect weak SFG light due to the fluorescence from the sample. However, when the wavelength of visible excitation light is variable, the influence of fluorescence can be removed by changing the wavelength of incident visible light and changing the wavelength of SFG light.

JP 2002-90293 A JP 2002-340672 A A.L.Harris et al., Chem. Phys. Lett., 141 (1987) 350. Akihide Wada et al., Surface, 32 (1994) 318. A.A. Mani, et al., Surf. Sci., 502 (2002) 261.

  This paper proposes a two-color variable SFG spectrometer and a method for measuring the same, in which single-channel SFG spectroscopy is enhanced and the wavelengths of infrared light and visible light are made variable.

Although it is single channel SFG spectroscopy, double resonance conditions can be realized, and not only the CH stretch, OH stretch, and C = O stretch regions that have been widely used in SFG, but also a wide red range from 4000 to 1000 cm ^−1. It becomes possible to measure in the external wave number region.

  The present invention is a sum frequency generation spectroscopic device for performing spectroscopic analysis by sum frequency generation at an interface of a sample to be measured. This sum frequency generation spectroscopic device includes a pulsed laser light source and a wavelength-tunable long-wavelength excitation light that branches the output light of the pulsed laser light source, converts the wavelength of a part of the branched light, and outputs long-wavelength excitation light. A generator, a wavelength-tunable short-wavelength excitation light generator that outputs a short-wavelength excitation light by wavelength-converting another part of the output light, and a first that irradiates the sample to be measured with the long-wavelength excitation light An optical system, a second optical system that irradiates the sample to be measured with the short wavelength excitation light, and one or a plurality of delay devices that delay either one or both of the long wavelength excitation light and the short wavelength excitation light, A third optical system that selects the sum frequency light of the long wavelength excitation light and the short wavelength excitation light from the light from the sample to be measured; and a photodetector that detects the selected sum frequency light. However, the long wavelength excitation light generator can sweep a predetermined wavelength band, and the short wavelength excitation light generator can change the output wavelength for each sample to be measured. Further, the first optical system, the second optical system, and the delay device are configured to irradiate the long-wavelength excitation light and the short-wavelength excitation light so that they are spatially and temporally overlapped with the sample to be measured. The three optical system has a function of adjusting the optical path of the sum frequency light so as to be incident on the photodetector, and suppresses light other than the selected sum frequency light.

  The long-wavelength pump light generator and the short-wavelength pump light generator each have a parametric optical oscillator, and the long-wavelength pump light and the short-wavelength pump light are respectively emitted from a pulse laser light source by parametric light oscillation. Wavelength converted.

  In addition, a first double wave generator is further provided that generates a double wave by branching a part of the output light of the pulse laser light source. In order to use this output light as the short wavelength excitation light, the output light of the short wavelength excitation light generator or the output light of the first second harmonic generator can be selected.

  In order to expand the coverage of the short wavelength pumping light, (1) a second second harmonic generator for generating a second harmonic of the output light of the parametric optical oscillator, and (2) an output of the parametric optical oscillator A light selector for selecting one of light and output light of the second harmonic generator and irradiating the sample to be measured.

In addition, in order to prevent the sample to be measured from being damaged by the long wavelength excitation light or the short wavelength excitation light, the first intensity adjuster for adjusting the intensity of the long wavelength excitation light and the intensity of the short wavelength excitation light are adjusted. And a second strength adjuster.

  The first and second intensity adjusters can be realized by the long wavelength excitation light generator or a wave plate provided on the incident side of the short wavelength excitation light generator, respectively. By rotating this, the intensity of each incident light is adjusted.

  The output light of the first second harmonic generator and the short wavelength excitation light are irradiated onto the sample to be measured through the same optical path. At this time, since the short wavelength excitation light interferes with the detection of the sum frequency light, the third optical system selects the sum frequency light by changing the filtering band according to the wavelength of the short wavelength excitation light. Provide a filter.

Using the above sum frequency generation spectrometer,
(1) The wavelength of the short-wavelength excitation light is set to any resonance level of the interface to be measured of the sample to be measured,
(2) Adjusting the third optical system so that the sum frequency light is incident on the photodetector with respect to the short wavelength excitation light having a set wavelength,
(3) Spectral analysis by generating the sum frequency by sweeping the wavelength of the long wavelength excitation light.

In particular, (1) the procedure of setting the wavelength of the short-wavelength excitation light to any resonance level of the interface to be measured of the sample to be measured,
(1-1) Sweeping the wavelength of the short-wavelength excitation light to acquire the absorption spectrum of the sample to be measured (1-2) Searching for the optimum peak from the acquired absorption spectrum,
(1-3) The procedure of setting the wavelength of the short-wavelength excitation light to the wavelength of the optimum peak can also be performed.

  Embodiments of the present invention will be described below in detail with reference to the drawings. In the following description, devices having the same function or similar functions are denoted by the same reference numerals unless there is a special reason.

  The present invention relates to a sum-frequency generation spectroscopic device in which a picosecond single channel SFG is enhanced and the wavelength of visible light is variable to change two colors, and a spectroscopic method thereof. FIG. 1 shows a block diagram thereof.

  The light source 1 is, for example, an Nd: YAG laser having a pulse width of about 20 ps and a maximum output of 130 mJ (wavelength = 1064 nm). The pulsed light from this laser is split into three, (1) for generating wavelength-tunable infrared pulsed light as pumping light A, and (2) wavelength-tunable that can select the wavelength as pumping light B within a predetermined range. Conventionally, it is used for the generation of light and (3) the excitation light C for the second harmonic wave (wavelength = 532 nm) used as the visible excitation light in the sum frequency measurement. Further, (4) a double wave is further generated from the excitation light B, and is used as the excitation light D.

  In order to generate an infrared pulse that is the excitation light A, light of up to about 50 mJ (wavelength = 1064 nm) is incident on the wavelength converter 2. A part of the infrared light output from the wavelength converter 2 can be divided and incident on the infrared light intensity monitoring Joule meter 3 to monitor the infrared light intensity.

The visible light that is the excitation light B is visible in which a branched laser beam (wavelength = 1064 nm) is incident on the wavelength converter 4 to select a wavelength in a wide range from the visible range to the near infrared range. Produce light. Further, visible light that is the output of the wavelength converter 4 is further incident on the second harmonic wave generator 6 to perform wavelength conversion to generate excitation light D. By using the second harmonic generator 6, it is possible to extend the usable wavelength band to the ultraviolet region. One of the excitation light B and the excitation light D can be selected by the light switching mirror 16. Here, the wavelength converter 4 is an optical parametric oscillation device using, for example, a LiB ₃ O ₅ crystal.

  As the excitation light C, the above-mentioned second harmonic (wavelength = 532 nm) is incident on the second harmonic generator 5 to generate the visible light of 532 nm (maximum 2 mJ at the wavelength = 532 nm). This visible light is used for single channel SFG measurement in which the wavelength of visible light is fixed.

More specifically, for infrared pulse generation, light of a maximum of about 50 mJ (wavelength = 1064 nm) is incident on the wavelength converter 2. In general, the intensity of infrared pulse light used for SFG spectroscopy is about 100 to 200 μJ (wave number = 3000 cm ⁻¹ ) in the intensity of infrared light using a laser with an output of about 30 to 40 mJ in the case of a picosecond laser. Is sufficient. In this case, it is also possible to divide excess laser light and introduce it into a visible light wavelength converter to create visible light with variable wavelength.

When SFG measurement can be performed in the vicinity of wave number = 3000 cm ⁻¹ and using double resonance conditions, there is no need to use high-power visible light. This is because, as described above, the visible light resonates with the electronic state of the molecular species at the interface, and the signal intensity of the SFG increases.

However, when an infrared region having a longer wavelength (for example, 1200 cm ⁻¹ to 1000 cm ⁻¹ ) is to be measured, a laser pulse light source with a higher output is required. This is because the oscillation efficiency of the infrared light of the nonlinear optical crystal is lowered. When there is no margin in the output of the laser light source, in order to perform SFG measurement in a short time, it is necessary to increase the laser output to near the maximum value. At this time, since there is almost no surplus laser light, it is difficult to perform two-wavelength variable SFG spectroscopy in which the wavelength of visible light is variable. For this reason, it is desirable to introduce incident light having an intensity sufficient to obtain an output of about 100 μJ even in the vicinity of 1000 cm ⁻¹ to the infrared generator.

  In addition, when the target non-measurement sample has fluorescence in the wavelength region of SFG, a laser pulse light source having a sufficiently large output is required. This is because weak SFG light is disturbed by strong fluorescence and it is difficult to perform SFG measurement. In such a case, the SFG measurement can be performed in a state where the wavelength of the SFG light to be used is sufficiently separated from the fluorescence wavelength region by using visible light having a longer wavelength. However, even in this case, since the resonance condition of visible light is not used, the wavelength-tunable visible light having sufficient intensity is required for the generation of SFG light. For example, visible light having an intensity comparable to or higher than the double wave (wavelength = 532 nm) branched from the light source 1 is required.

That is, in order to oscillate a tunable visible light capable of performing SFG measurement in a wide infrared wavelength region (4000 cm ⁻¹ to 1000 cm ⁻¹ ) and capable of SFG measurement at any wavelength, the original laser is used. It is necessary to use a laser that can oscillate with higher intensity than the conventional one.

  Therefore, the sample 7 includes infrared light (excitation light A) output from the wavelength converter 2, visible light (excitation light B) output from the wavelength converter 4, and a double wave branched from the light source 1. (Wavelength = 532 nm) (excitation light C) or output light (excitation light D) of the second harmonic generator 6 can be irradiated.

  In the detection of SFG light, SFG can be measured with various optical arrangements depending on the phase matching condition. For example, measurement is possible even in an arrangement in which infrared light of excitation light A and visible light of excitation light B are incident on sample 7 from opposite directions. For example, this arrangement is used in the two-color variable SFG spectrometer described in Non-Patent Document 3 (A.A. Mani, et al., Surf. Sci., 502 (2002) 261.). In this case, as shown in FIG. 2A, the emitted SFG light is significantly different from the direction in which the visible light is reflected. Therefore, it is relatively easy to separate the SFG light from the reflected light of the visible light. .

  However, this arrangement has the disadvantage that the detection system must be moved each time the wavelength is changed. This is because changing the visible light wavelength of the excitation light B greatly changes the direction of the emitted SFG light.

  On the other hand, in the arrangement shown in FIG. 2B, the visible light of the excitation light B and the infrared light of the excitation light A are incident on the sample 7 from the same direction. This arrangement has the disadvantage that it becomes difficult to separate the excitation light B of the SFG light from the reflected light of the visible light, but the SFG light has an advantage of having directivity. For example, in FIG. 1, the spectroscope 9 is separated from the sample 7 position by 1 m or more, a plurality of slits between the sample 7 and a long wavelength cut filter 8 having an optical density (OD) of 6 or more in the wavelength region of excitation light. By disposing, reflected light and scattered light from the visible light sample 7 are removed.

  Another advantage of this arrangement is that the visible light of the excitation light C (double wave 532 nm of the YAG laser) can be used in the alignment of the visible light of the excitation light B. That is, infrared light cannot be observed with the naked eye, but in order to specify the irradiation position on the sample, first, normal SFG light consisting of visible light of excitation light C and infrared light of excitation light A is detected. For example, a He—Ne laser beam serving as a guide for the irradiation position is aligned with the irradiation position of the light of 532 nm. Thereafter, the light switching mirror 13 shown in FIG. Here, FIG. 3 is a diagram showing the vicinity of the sample to be measured in FIG. 1 in more detail. FIG. 3B shows a case where the light switching mirror 13 is not set. By adjusting the irradiation position of the tunable visible light sample taken out from the visible light generator 4 to the irradiation position of the He-Ne laser, it is possible to obtain a spatial superposition on the sample with the infrared light. Is possible. After spatially superimposing, the optical delay circuit 11 of FIG. 1 is scanned, and finally the temporal superposition of infrared light and wavelength-tunable visible light is obtained. As described above, it is possible to efficiently capture the SFG light from the sample 7 without a large change in the optical path.

  Depending on the object to be measured, it is necessary to suppress the intensity of the excitation light because the sample is easily damaged by infrared light or visible light as excitation light. Conversely, it is necessary to increase the intensity of infrared light as in the measurement at the solid-liquid interface. For this reason, it is desirable that the intensity of the excitation light can be adjusted as appropriate. For this reason, in the configuration of FIG. 1, a half-wave plate 15 is provided in each incident portion of the infrared generator 2 and the visible light generator 4. By rotating each of these, the intensity of each incident light can be controlled independently.

  In addition, the visible light generator 4 for generating the wavelength-tunable visible light has substantially the same configuration as that of the infrared light generator 2 using optical parametric oscillation, and therefore is substantially the same as the visible light of the excitation light C. Oscillation of near-infrared light having a certain intensity is also possible. Many substances are transparent in the near-infrared light region, and it is possible to reduce sample damage due to the use of near-infrared light in two-color variable SFG spectroscopy. For example, there are black materials such as nanocarbon materials and pigment materials that have strong absorption in the visible region, so depending on the sample, if the excitation light is visible or ultraviolet light, it will be absorbed and damaged In some cases, it is desirable to measure in the near-infrared light band.

  The present invention belongs to single channel SFG spectroscopy, and its advantages and disadvantages are the same as in the conventional case. For example, there is a drawback in that it takes time to continuously sweep the wavelength of infrared light during measurement as described above. On the other hand, the fluctuation of the time width of the pulse of the wavelength-converted visible excitation light is negligibly small as compared with the femtosecond broadband SFG, and the light irradiation amount per unit area of the wavelength-tunable visible excitation light is the usual 532 nm. There is an advantage that it is easy to ensure the same or more than the SFG used as the excitation light. In general, the obtained infrared light intensity has a wave number dependency, but there is an advantage that, in principle, a reference sample is not necessary for correcting the intensity of the emitted SFG light. That is, since the SFG light intensity is proportional to the infrared light intensity, it is only necessary to monitor the infrared light intensity with the Joule meter 3 and to normalize with the obtained infrared light intensity. The wavelength-variable excitation light as the excitation light B or the excitation light D is used with a fixed wavelength in the SFG measurement at each wavelength. In addition, the visible light intensity takes a constant value excluding fluctuations due to laser fluctuations, so it is standardized in the same way as in the case of infrared light by monitoring the laser light intensity at individual wavelengths. Therefore, it is not necessary to measure the reference sample separately and normalize the spectrum.

  Next, a method for reducing the size of the apparatus will be described. When the laser for the light source, infrared generator, variable wavelength visible light generator, sample measurement unit, and SFG light detection unit are arranged in a plane on the optical platen, a fairly large optical platen is used. There is a need to. In an apparatus using ultrashort pulse light, if the surface plate is distorted due to changes in ambient temperature, the optical path, pulse width, etc. change, so accurate temperature management at room temperature is required for accurate measurement. Become. In this respect, a large optical surface plate is disadvantageous. Moreover, when it arrange | positions in planar shape, there exists a fault that adjustment work of each unit is difficult to advance. In order to eliminate these disadvantages and disadvantages, as shown in FIG. 4, the optical surface plate has a two-story structure to reduce the size. As a result, small-scale temperature control is required, and workability is improved. Further, the spectroscope 9 and the photodetector 10 using a photomultiplier tube are arranged at the lower part of the surface plate of the second floor structure, and can cover the four sides to efficiently remove stray light and scattered light from the surroundings. Arranged.

  The infrared light of the excitation light A and the tunable visible light of the excitation light B or the ultraviolet light of the excitation light D are spatially superimposed on the sample 7. The SFG light emitted from the sample has directivity and is light having a shorter wavelength than the excitation light used. For this reason, stray light due to scattering from the surface of the sample 7 can be removed by disposing the long wavelength cut filter 8 in front of the spectroscope 9 and the photodetector 10 for detecting SFG light. The optical delay circuit 11 realizes temporal superimposition of infrared light and wavelength-variable visible light. This uses a stepping motor to change the delay time and can be remotely operated. Further, the angle of the generated SFG light with the selected excitation wavelength varies slightly depending on the phase matching condition. By making it possible to finely adjust the position of the optical path adjuster 12 composed of slits and mirrors installed in front of the photodetector 10, SFG measurement can be continuously performed without moving the arrangement of the spectrometer and the entire detector. Can be done. As a result, SFG measurement capable of selecting an excitation wavelength in a wide wavelength range from the ultraviolet region to the near infrared region is possible.

  Further, as shown in FIG. 1 and more specifically in FIG. 3A, the incident light path with respect to the sample of the visible light of the excitation light B or the ultraviolet light of D is arranged coaxially with the excitation light C. Further, as shown in FIG. 3A, a removable light switching mirror 13 is arranged, and when this mirror 13 is removed as shown in FIG. 3B, the excitation light C is used. Similar to SFG measurement, single-channel SFG spectroscopy with fixed wavelength using excitation light having a wavelength of 532 nm can be performed.

FIG. 5 shows an SFG spectrum obtained by measuring a sample in which octadecanethiol (C ₁₈ H ₃₇ SH, ODT) is adsorbed on a gold substrate using the apparatus shown in FIG. . The three peaks that can be clearly seen in this spectrum are derived from the vibration of the terminal methyl group of the ODT molecule. It can be seen that the three peak shapes appearing in the SFG spectrum change by sequentially changing the wavelength of the visible excitation light. The shape changes from a downwardly convex shape on the long wavelength side to a dispersed shape on the short wavelength side. This is convex downward when excited with light having a wavelength of 532 nm, but when the wavelength is changed, non-resonant SFG light generated at the gold substrate interface and SFG light that resonates with molecular vibration of the terminal methyl group of the ODT. The phase difference changes due to the interference, and the spectrum shape changes.

FIG. 6 shows an SFG spectrum when carbon nanotubes, which are black samples, are coated on a silver substrate. The peak observed in the vicinity of 1600 cm ⁻¹ is a vibration mode called G band characteristic of the graphite structure. Thus, it can be seen that a good SFG spectrum can be obtained even in the case of a sample that easily absorbs light.

The tunable visible light generator 4 can be the same as the infrared generator. In this case, light from visible to near infrared generated by optical parametric oscillation can be introduced into the built-in AgGaS ₂ crystal, and infrared light can be produced by difference frequency generation (DFG). In addition to the above, by using this configuration, it is possible to take out light whose wavelength is arbitrarily selected in a wide wavelength range from ultraviolet to visible, near infrared, and infrared. Thus, it is possible to measure the interface-selective photoexcitation time-resolved vibration spectrum by utilizing the fact that an arbitrary wavelength can be selected. At this time, a dichroic filter that transmits light of 532 nm and reflects light of other wavelengths is used as the mirror 14 in FIG. Further, by controlling and scanning the optical delay circuit 11 in FIG. 1, it is possible to perform time-resolved SFG measurement of the interface excited at a specific wavelength without greatly changing the optical arrangement. Here, the time-resolved SFG measurement of the interface is a condition for measuring the sum frequency of the second harmonic and the infrared light, and further irradiates the pumping light with the wavelength variable pulse light having an output value higher than the second harmonic. Measurement. The timing at which the sample is irradiated with the pump light pulse is controlled by the optical delay circuit 11, that is, gradually shifted from the timing at which the sum frequency is generated. It measures (probes) the sum frequency at a precursor state of a reaction that moves or deforms. Changes in the time domain longer than the pulse width of the laser appear as intensity changes and shape changes on the spectrum. Such measurement is usually called time-resolved measurement (or pump-and-probe method).

When measurement is performed under the double resonance condition using the above sum frequency generation spectroscopic device, the following is performed.
(1) First, the wavelength of the short wavelength excitation light is set to any resonance level of the interface to be measured of the sample to be measured.
(2) Next, the third optical system is adjusted so that the sum frequency light is incident on the photodetector with respect to the short wavelength excitation light having a set wavelength.
(3) Spectral analysis by generating the sum frequency by sweeping the wavelength of the long wavelength excitation light.
As described above, when SFG measurement can be performed using double resonance conditions, there is no need to use high-power visible light. Therefore, it is possible to suppress damage to the sample to be measured by irradiation with visible light.

For example, when the absorption or reflection spectrum of the sample to be measured is obtained by sweeping the wavelength of the short-wavelength excitation light using the optical system shown in FIG. 8, the resonance level used for visible light is measured and determined. can do. In this case, the procedure of (1) setting the wavelength of the short-wavelength excitation light to any resonance level of the interface to be measured of the sample to be measured is as follows.
(1-1) The absorption or reflection spectrum of the sample to be measured is acquired by sweeping the wavelength of the short wavelength excitation light. At this time, it is easy to configure the optical path so that the photodetector 10 is used as the photodetector.
(1-2) Search the optimum peak from the acquired absorption spectrum,
(1-3) The wavelength of the short wavelength excitation light is set to the wavelength of the optimum peak.
However, when performing SFG measurement, for example, an optical system shown in FIG. 9 is used.

  SFG spectroscopy is an interface-selective vibrational spectroscopy, and is a highly versatile spectroscopy because there are few restrictions on the object to be measured. However, although a high-intensity pulse laser is used, the light to be detected is weak and a highly sensitive detector is required, and since it is vulnerable to scattered light, stray light, and fluorescence, there are many things that are practically impossible to measure in actual systems. . In the present invention, in consideration of these points, (1) it is possible to measure at multiple wavelengths to make it less susceptible to light emission, fluorescence, and damage of the sample, and (2) an ultraviolet region that can resonate with the electronic state of the functional organic material. (3) It can be applied to non-smooth samples to strengthen the analysis of real materials, (4) Time-resolved SFG measurement can be performed only by exchanging mirrors, (5) Not only the vibration state of the interface due to the second-order nonlinear effect but also various nonlinear spectroscopy can be measured, and (6) SFG measurement is possible in a wide infrared wavelength region. This provided a powerful interface spectroscopic analyzer.

  The fields to which the present invention can be applied include fields such as liquid crystal displays, organic EL displays, and organic field effect transistors as devices using functional organics. In any of these fields, the molecular level control of the interface between organic materials or between an organic material and a metal affects the performance of the device. The organic EL material is a material that originally emits fluorescence in the visible range, and it is difficult to measure the interface with a conventional visible wavelength fixed SFG spectrometer. Furthermore, phenomena such as adhesion, delamination and interface segregation at the bonding interface of dissimilar materials are phenomena at the buried interface, which makes it difficult to analyze with other surface analysis techniques. . However, since SFG spectroscopy is an analysis method using light, if light reaches the target interface, it is also possible to target the buried interface.

  Since the SFG spectroscopy uses an ultrashort pulse laser as a light source, time-resolved SFG measurement utilizing high time resolution is possible even by applying the present invention. For example, it can be applied to molecular dynamics analysis by time-resolved measurement at the interface.

  Furthermore, the conventional visible light wavelength fixed SFG spectroscopy is applied to the structural analysis of the interface with black samples and pigment materials including nanocarbon materials, and the interface with water in biomaterials and electrochemistry. The present invention can also be applied to systems that have been difficult to solve, and structural analysis of interfaces that have not yet been elucidated.

1 is a block diagram showing a sum frequency generation spectroscopic device of the present invention. FIG. It is a block diagram which shows the structure which isolate | separates SFG light from excitation light. It is a schematic diagram which shows the structure for making it easy to superimpose infrared excitation light and visible excitation light spatially. It is a schematic diagram showing a sum frequency generation spectroscopic device in which the optical surface plate has a two-story structure and is miniaturized. Octadecanethiol the gold substrate _{(octadecanethiol (C 18 H 37 SH} , ODT)) which is a diagram illustrating an SFG spectrum of when the sample was measured obtained by monolayer adsorption. It is a figure which shows a SFG spectrum when a carbon nanotube is apply | coated on the silver substrate. It is a schematic diagram which shows the optical system for measuring an interface selective time-resolved vibration spectrum. It is a figure which shows the example of the optical system for sweeping the wavelength of the said short wavelength excitation light, and obtaining the absorption or reflection spectrum of the said to-be-measured sample. It is a figure which shows the example of the optical system for sweeping the wavelength of the said short wavelength excitation light, obtaining the absorption or reflection spectrum of the said to-be-measured sample, and performing a double resonance SFG measurement.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Wavelength converter 3 Joule meter 4 Wavelength converter 5, 6 2nd harmonic wave generator 7 Sample 8 Long wavelength cut filter 9 Spectrometer 10 Optical detector 11 Optical delay circuit 12 Optical path adjuster 13, 14 Mirror 15 Half Wave plate 16 Optical switching mirror

Claims (9)

A pulsed laser light source, a wavelength-tunable long-wavelength pumping light generator for branching the output light of the pulsed laser light source, wavelength-converting part of the output light of the branching to output a long-wavelength pumping light, A wavelength-tunable short wavelength excitation light generator for converting a part of output light to output short wavelength excitation light, a first optical system for irradiating the sample to be measured with the long wavelength excitation light, and the short wavelength excitation A second optical system for irradiating the sample to be measured, one or a plurality of delay devices for delaying one or both of the long wavelength excitation light and the short wavelength excitation light, and light from the sample to be measured A third optical system for selecting the sum frequency light of the long wavelength excitation light and the short wavelength excitation light, and a photodetector for detecting the selected sum frequency light,
The long wavelength excitation light generator is capable of sweeping a predetermined wavelength band,
The short wavelength excitation light generator can change the output wavelength for each sample to be measured.
The first optical system, the second optical system, and the delay device are configured to irradiate the long-wavelength excitation light and the short-wavelength excitation light so as to be spatially and temporally overlapped with the sample to be measured.
The third optical system has a function of adjusting the optical path of the sum frequency light so as to be incident on the photodetector, and suppresses light other than the selected sum frequency light,
A sum frequency generation spectroscopic apparatus for use in spectroscopic analysis by sum frequency generation at an interface of the sample to be measured.   The sum frequency generation spectroscopic apparatus according to claim 1, wherein each of the long wavelength excitation light generator and the short wavelength excitation light generator includes a parametric optical oscillator. A first double wave generator for branching a part of the output light of the pulse laser light source to generate a double wave;
The output light of the short wavelength excitation light generator or the output light of the first second harmonic wave generator can be selected as the short wavelength excitation light, according to any one of claims 1 and 2. The sum-frequency generation spectroscopic apparatus described. A second harmonic generator for generating a second harmonic of the output light of the parametric optical oscillator;
A light selector for selecting one of the output light of the parametric light oscillator and the output light of the second harmonic generator and irradiating the sample to be measured;
The sum frequency generation spectroscopic device according to claim 1, further comprising: A first intensity adjuster for adjusting the intensity of the long wavelength excitation light;
A second intensity adjuster for adjusting the intensity of the short wavelength excitation light;
The sum frequency generation spectroscopic device according to claim 4, further comprising:   Each of the first and second intensity adjusters is a wavelength plate provided on the incident side of the long wavelength excitation light generator or the short wavelength excitation light generator. 6. The sum frequency generation spectroscopic device according to claim 5, wherein the light intensity is adjusted.   The third optical system includes a filter that changes the filter band and selects the sum frequency light, and the output light of the first second harmonic generator and the short wavelength excitation light are transmitted in the same optical path. 4. The sum frequency generation spectroscopic apparatus according to claim 3, wherein the sum frequency generation spectroscopic apparatus is configured to irradiate a measurement sample. A spectroscopic method using the sum frequency generation spectroscopic device according to any one of claims 1 to 7,
(1) a procedure for setting the wavelength of the short-wavelength excitation light to any resonance level of the interface to be measured of the sample to be measured;
(2) a procedure for adjusting the third optical system so that the sum frequency light is incident on the photodetector with respect to the short wavelength excitation light having a set wavelength;
(3) a procedure for performing spectral analysis by generating the sum frequency by sweeping the wavelength of the long-wavelength excitation light;
A spectroscopic method comprising: (1) The procedure for setting the wavelength of the short-wavelength excitation light to any resonance level of the interface to be measured of the sample to be measured,
(1-1) a procedure for acquiring the absorption spectrum of the sample to be measured by sweeping the wavelength of the short-wavelength excitation light;
(1-2) a procedure for searching for an optimum peak from the acquired absorption spectrum;
(1-3) a procedure for setting the wavelength of the short-wavelength excitation light to the wavelength of the optimum peak;
The spectroscopic method according to claim 8, comprising:

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