JP3691813B2 - Non-linear Raman spectroscopy method and apparatus - Google Patents

Description

【００３８】
ここで、ω_ｍとｋ_ｍは電場の振動数と波数ベクトルである。ｔ’＝ｔ−ｔ_３、ｔ”＝ｔ_３−ｔ_２、ω’＝ω_１−ω_２＋ω_３でｋ＝ｋ_１−ｋ_２＋ｋ_３である。ｔ_１２はポンプ１とポンプ２の時間差で、χ_ｉｊｋｌ（ｔ’、ｔ”）は４光波混合過程の３次の非線形感受率テンソルのｉｊｋｌ成分である。物質の応答は応答関数Ｒ_ｉｊｋｌ（ｔ）を用いて、線形応答理論から導かれる。ｋ＝ｋ_１−ｋ_２＋ｋ_３の方向に伝播する分極は形式的に次のように書かれる。
【００３９】
【数２】

【００４０】
この分極によって発生する回折されたシグナルのスペクトルは、時間相関関数をフーリエ変換したものであるから、次のようになる。
【００４１】
【数３】

【００４２】
ここで、Δω＝ω−ω_３であり、ωは回折光の振動数である。Ｐ_ｉ（ｔ）をこれに代入し、ｋ＝ｋ_１−ｋ_２＋ｋ_３の方向に進む項だけに着目すると次のような式を得る。
【００４３】
【数４】

【００４４】
ここでＧ^（２） _１ｋ２ｌは二本のポンプ光の２次の相関関数で、チルダはフーリエ変換されていることを示している。実験ではポンプ光同士の時間差はなく、ｔ_１２＝０である。このことから、シグナルのスペクトルを見ることは、物質の周波数応答関数の絶対値の二乗とポンプ光の相互相関関数を見るのと等価である。
【００４５】
非共鳴で、Ｂｏｒｎ−Ｏｐｐｅｎｈｅｉｍｅｒ近似が成り立つ条件では、物質の応答関数Ｒ_ｉｊｋｌ（ｔ）は原子核の応答と電子の即時応答の項に分割されてＲ_ｉｊｋｌ（ｔ）＝σ_ｉｊｋｌδ（ｔ）＋Ｎ_ｉｊｋｌ（ｔ）になる。σの項は電子の即時応答を表し、δ（ｔ）はディラックのδ関数である。Ｎ（ｔ）は原子核の応答関数である。このＲ_ｉｊｋｌを先ほどのスペクトルの式に代入すると次の式を得る。
【００４６】
【数５】

【００４７】
δ関数を積分したので、σにはチルダはつかない。この式から、二本のポンプ光の相関関数Ｇが十分に広がっていれば、物質の周波数応答関数がプローブの広がったスペクトルとして直に観測できることが分かる。見方を変えれば、導入で述べた回折格子的な解釈となる。
【００４８】
５．２．偏光選択
この分光法で得られるシグナルは応答関数の２乗に比例しているので、電子と核の応答が混ざってしまうのは避けられない。しかし、この分光法では４本のビームの偏光を自由に選ぶことができ、適切に設定してやれば片方の応答を消すことができる。例えば、ポンプ光の偏光を共に水平にしておき、それをｘ軸とする。反時計回りを正に取り、プローブはそこから４５°傾けてある。
【００４９】
図６に、ポンプ光、プローブ光及び信号光の偏光についての説明図を示す。
シグナルの偏光をΨとすると、電子σ、核Ｎそれぞれの応答による分極は次のようになる。
【００５０】
【数６】

【００５１】
添え字の意味は左から、シグナル、プローブ、ポンプ２、ポンプ１の偏光のｘ、ｙ成分に比例することを示す。一般的に等方的な物質ではσ_ｙｙｘｘ／σ_ｘｘｘｘ＝１／３、Ｎ_ｙｙｘｘ／Ｎ_ｘｘｘｘ＝−１／２の関係があるので
【００５２】
【数７】

【００５３】
となる。偏光は１８０°回転対称なので、応答関数は結局、次のように書くことができる。
【００５４】
【数８】

【００５５】
これより、シグナルの偏光を１０８．４°、６３．４°に設定すれば、電子及び核の応答だけを得ることができる。
【００５６】
【発明の効果】
本発明は、例えば、次のような特有の効果を奏する。
▲１▼ 非線形ラマン分光であるため、蛍光のバックグラウンドの除去が可能である。
▲２▼ 広い振動数領域を一度に測定でき、短時間の測定が可能である。（ＣＳ_２のように信号の強いものであると、測定時間は例えば、５−３０秒程度で十分きれいなデータが得られる。）
▲３▼ 周波数可変のレーザーを必要とせず、装置が簡単になる。
▲４▼ 不確定性の制限を受けず、パルスを用いながら、高分解能のラマンスペクトルを測定することが可能である。
【図面の簡単な説明】
【図１】非線形ラマン測定の原理についての説明図。
【図２】非線形ラマン分光方法を実現するための装置の概略構成図。
【図３】非線形ラマン分光装置の構成図。
【図４】ポンプ光、プローブ光、信号光の配置の説明図。
【図５】ＣＳ_２の及びベンゼンのラマンピークの測定データの図。
【図６】ポンプ光、プローブ光及び信号光の偏光についての説明図。
【符号の説明】
１ 単色光源
２ パルス光源
３ 波長変換器
４ 分光器
５ 検出器
６ 分波器
１０ 被測定試料
１１、１２ 第１及び第２の照射偏光調整部
１３ 測定偏光調整部
２１〜２３ 第１〜第３の照射方向調整部
２４ 測定方向調整部[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a nonlinear Raman spectroscopy method and apparatus, and in particular, excitation light and white light or near white light with a narrow time width.(Almost monochromatic)The present invention relates to a nonlinear Raman spectroscopy method and apparatus for detecting a nonlinear Raman scattering peak due to frequency-domain four-wave mixing using excitation light having a wide spectral width.
[0002]
[Prior art]
In general, Raman spectroscopy is a method of irradiating a measured object with monochromatic light such as a laser beam and spectrally diffusing Raman scattered light emitted from a sample, and the emitted Raman scattered light is specific to the substance to be measured. Therefore, it is widely used for material analysis, material evaluation and new material creation. On the other hand, four-wave mixing is a general term for a method of detecting light generated by entering three lights (two pump lights (excitation light) and probe light). For the four-wave mixing, for example, normal degenerate four-wave mixing, non-degenerate four-wave mixing, CARS (coherent anti-Stokes scattering), CSRS (coherent Stokes scattering), photon echo, inverse Raman spectroscopy, Raman gain spectroscopy, optical Kerr effect Many types of nonlinear spectroscopy, such as spectroscopy, fall into this category.
[0003]
Here, CARS (coherent anti-Stokes scattering) or CSRS (coherent Stokes scattering) spectroscopy is a kind of four-wave mixing, but is most closely related to measuring Raman scattering. Basically, these spectroscopic methods measure the intensity of signal light by entering two lights (one of which serves as two light pulses) and changing the frequency of one light. It is. As a variation, there is a method in which three light pulses are inserted and the time difference is changed for measurement. Note: Reference: Levenson "Nonlinear Laser Spectroscopy" Ohm, 1988.
[0004]
Furthermore, the present inventors have proposed an ultrafast time-resolved fluorescence spectroscopy method that can acquire a time-resolved spectrum of weak fluorescence in a wide wavelength region at once using a femtosecond pulse laser and an optical Kerr shutter (specialty). No. 2001-356095).
[0005]
[Problems to be solved by the invention]
However, the conventional Raman scattering spectroscopy as described above has the following problems, for example.
{Circle around (1)} Since normal Raman scattering spectroscopy is linear spectroscopy, fluorescence, which is the same linear process, becomes the background, and in many cases, low-efficiency Raman scattering cannot be measured. Fluorescence is emitted by itself, but also exists as an impurity.
{Circle around (2)} Since CARS is a nonlinear Raman spectroscopy, it is possible to remove the background of fluorescence, but since it is necessary to sweep the frequency, a variable frequency laser is required. In addition, it takes time to sweep, and the signal that can be measured at one time can be measured only with signal light for one frequency, which is not efficient.
[0006]
In view of the above points, the present invention achieves the following objects, for example.
(1) The background of fluorescence can be removed by nonlinear Raman spectroscopy.
(2) A wide frequency range is measured at a time, enabling a short time measurement. For example, CS₂When measuring a signal having a strong signal light, for example, the measurement time is about 5 to 30 seconds, and sufficiently clean data can be obtained. )
(3) The configuration of the apparatus is simplified without requiring a variable frequency laser.
(4) A high-resolution Raman spectrum is measured using pulses without being limited by uncertainty.
[0007]
Moreover, the following measurement is mentioned as an application of this invention, for example.
(1) Measurement of Raman spectra of industrial products containing fluorescent materials
(2) Measurement of Raman scattering spectrum of thin film
(3) Measurement of Raman spectra of biological systems containing fluorescent materials
[0008]
[Means for Solving the Problems]
According to the first solution of the present invention,
Irradiate one of the excitation light with a narrow time width and the other excitation light with a broad spectrum width synchronized with one of the excitation lights, crossing the substance to be measured,
A transient or near-monochromatic probe light is generated in the material to be measured so as to satisfy the Bragg condition.
By spectrally decomposing signal light diffracted by nonlinear Raman scattering in the measured substance and detecting a wide wavelength region simultaneously,
A nonlinear Raman spectroscopy method for measuring a nonlinear Raman peak is provided.
[0009]
According to the second solution of the present invention,
A monochromatic light source that generates a monochromatic or near monochromatic probe light with a narrow spectral width;
A pulse light source that generates one excitation light having a narrow spectrum width and a narrow time width;
A wavelength converter that converts one excitation light from the pulsed light source into the other excitation light having a wide spectral width synchronized with the one excitation light;
A spectrometer that simultaneously detects a wide wavelength range
With
One excitation light from the pulse light source and the other excitation light from the wavelength converter are irradiated in a crossing manner in the substance to be measured, and the probe light from the monochromatic light source is applied to the transient grating generated in the substance to be measured. Is provided so as to satisfy the Bragg condition, thereby providing a nonlinear Raman spectroscopic apparatus that performs spectral decomposition on the signal light diffracted by nonlinear Raman scattering by the substance to be measured by the spectrometer.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
1. principle
FIG. 1 is an explanatory diagram showing the principle of nonlinear Raman measurement.
(1) Generally, two frequencies f₁And f₂(F₁> F₂) Light (pump light, excitation light) is incident on a substance for spectroscopic measurement, the frequency f₁-F₂The vibration in the substance that exactly coincides with this is excited (induced Raman effect).
(2) Here, the frequency f₃When the third light (probe light) is incident on the substance, the light scattering process causes f₃+ F₁-F₂(Anti-Stokes scattering) and f₃-F₁+ F₂Each signal light which is light having a frequency of (Stokes scattering) is generated (Raman scattering).
(3) Actually, these processes (1) and (2) do not occur separately but occur as a series of processes. In the process of (2) only, the incident light has a frequency f.₃It is linear because it is only light with, but if both processes (1) and (2) occur, the frequency f₁, F₂, F₃As a result, a non-linear effect occurs.
(4) Since Raman scattering in which signal light to be measured is generated is non-linear, the direction of signal light is determined by the directions of pump light and probe light. Therefore, fluorescence can be spatially removed from the signal light. The direction of the predetermined signal light is determined by the momentum conservation law established between these lights. That is, in general, the momentum of light is represented by hk / 2π (where h is the Planck constant and k is the magnitude of the wave vector). The direction in which the signal light is emitted from the substance to be measured has the following phase matching condition depending on the direction in which the probe light is incident.
k₄= K₃+ K₁-K₂  (K₂And k₃Are incident side by side)
Or
k₄= -K₁+ K₂+ K₃  (K₁And k₃Are incident side by side)
(Where k₁~ K₄Are pump light 1 and 2, wave vectors of probe light and signal light, respectively)
In short, pump light k₁And k₂The probe light k so that the Bragg condition is satisfied in the transient grating made by₃, Signal light is emitted in the direction of almost regular reflection. Here, almost means k₁And k₂This is because when the energy of is different, the Bragg condition deviates slightly.
(5) Further, one of the incident lights (for example, the frequency f₂) Is not monochromatic and contains many frequencies of light, various beats occur simultaneously, and various vibrations in the material are excited simultaneously. In this case, the frequency f₃When the probe light is incident, light of various frequencies is emitted due to the process of light scattering. If this is detected at once using a measuring device equipped with a spectroscope, a CCD camera, etc., vibration in the substance will occur. Can be measured in a wide range at once.
[0011]
2. Comparison with conventional methods
Next, the main features of nonlinear Raman spectroscopy based on the above-described principle will be described in comparison with the prior art.
[0012]
(1) Normal Raman scattering
In normal Raman scattering, vibrations in a thermally excited material are examined in the process (2) of “1. Principle”. Since this normal Raman scattering is basically linear, the fluorescence generated with the same linear is often in the way, and measurement is often difficult for many samples such as industrial products and biological materials.
On the other hand, since the Raman scattering of the present invention is nonlinear, the direction of emitted light is determined. Therefore, it can be spatially distinguished from linear fluorescence whose direction is not determined. Since pulsed light is used to excite vibrations, for example, if a CCD camera with a gate is used as a detector, it can be distinguished in terms of time, and is particularly advantageous for measuring a Raman spectrum from which fluorescence has been removed. It is.
[0013]
(2) Four-wave mixing
As described above, four-wave mixing is a general term for a method of detecting light generated by entering three lights (two pump lights and probe lights). In normal four-wave mixing, pulse light is used for all three lights, and measurement data is obtained by changing the arrival time of pulses or changing the frequency.
In contrast, the features of the present invention can be summarized in the following three points.
1) The third light (probe light) uses a continuous light or a Q switch pulse close to continuous light whose spectrum is close to a single color.
2) Only the spectrum of the generated signal light is measured without changing the arrival time of the pulse or changing the frequency.
3) Light having a broad spectrum is used as one of pump lights.
[0014]
(3) CARS (Coherent anti-Stokes scattering) or CSRS (Coherent Stokes scattering)
As described above, CARS or CSRS spectroscopy is a kind of four-wave mixing. On the other hand, in the present invention, one of pump lights uses light having a wide spectrum, and monochromatic probe light is used. It is different from CARS and CSRS in that it is used and the spectrum of signal light is measured. One of the features is that the measurement time is overwhelmingly fast.
[0015]
(4) Removal of fluorescence in Raman scattering
In general, if the background of fluorescence in the measurement of Raman scattering interferes, for example,
A) When the bulk is measured and the bulk itself emits fluorescence
B) When the bulk is measured and the impurity emits fluorescence
C) When measuring small or trace amounts
are categorized.
To remove such fluorescent background in linear Raman scattering spectroscopy, for example:
1) Time-gating using a time-correlated single-photon counting method with a pulsed excitation light source
2) Using an optical car shutter with a pulsed excitation light source
3) Use polarization difference
4) Use infrared excitation light
5) Use strong light to fade the fluorescent material
Etc. are known. In both cases A) and B), a considerable part can be solved by using the infrared excitation light of 4). However, in the case of C), none of the methods 1) to 5) is sufficiently removed. Originally, Raman scattering and fluorescence are the same secondary optical processes that absorb and emit light. What distinguishes the two depends on how long the polarization produced by the incident light continues to oscillate without changing the phase, and light emission after the phase shift is called fluorescence. Therefore, Raman scattering can be seen when measuring an early part. However, due to uncertainties, if a measurement is performed with a gate only at a part that is too early, the spectrum will spread and the Raman peak will not be understood. In that sense, the above method 2) is naturally limited, and the above 1) is insufficient in time resolution. The above 3) generally cannot be separated with polarized light. The above 4) is not suitable for the measurement of a small amount of contents, and the above 5) is not general because not all of them are discolored and the sample is damaged. In such a case, nonlinear Raman spectroscopy provides an ideal spectroscopy method.
[0016]
3. Schematic configuration for nonlinear Raman spectroscopy
FIG. 2 shows a schematic configuration diagram of an apparatus for realizing the nonlinear Raman spectroscopy method.
This apparatus includes a monochromatic light source 1, a pulse light source 2, a wavelength converter 3, a spectrometer 4, a detector 5, and a branching filter 6, and performs spectroscopic measurement of the sample 10 to be measured.
[0017]
In this example, the sample 10 may be anything that transmits light. In addition, when performing a reflection type spectroscopic measurement, you may be other than a transmissive | pervious thing.
The monochromatic light source 1 is a laser having a narrow spectrum width for generating probe light. The monochromatic light source 1 may be a continuous wave laser such as an argon laser, a helium neon laser, or a YAG laser, but it is more efficient to use a Q switch laser in synchronization with a pulse light source. Examples of such a laser include a Q switch YAG laser and a Q switch YLF laser.
[0018]
The pulse light source 2 is a pulse laser for generating excitation light (pump light). The pulse light source 2 is preferably a laser with a high peak value, such as femtosecond or picosecond pulses. Examples of such a laser include a mode-locked titanium sapphire laser, a regenerated and amplified mode-locked titanium sapphire laser, and a mode-locked YAG laser. The demultiplexer 6 makes the laser beam emitted from the pulse light source 2 a predetermined polarization and branches it toward the wavelength converter 3.
[0019]
The wavelength converter 3 converts the light emitted from the pulse light source 2 into pulse light having a different wavelength and a wide spectrum width and synchronized in time. The excitation light by the wavelength converter 3 is white light or light close to white light, and it is preferable that the frequency difference from the original pulse light source is just about the Raman scattering frequency to be measured. Examples of such a wavelength converter 3 include white light having a wide spectral width generated by applying pulsed light to a water / heavy water mixture or sapphire, or stimulated Raman generated by applying light to a substance such as benzene or acetone. Examples include using scattered light and optical parametric oscillation light.
[0020]
The spectroscope 4 and the detector 5 simultaneously detect signal light to be measured from the sample 10 in a wide wavelength region. The detector 5 can be appropriately used as long as it is a two-dimensional detector other than the CCD. When a CCD detector provided with a time gate is used as the detector 5, separation can be performed in terms of time, which is advantageous in removing fluorescence. Fluorescence can be removed or reduced by adjusting the relationship between the irradiation timing of pump light and the measurement timing and / or measurement time by the time gate.
[0021]
Next, the operation of the nonlinear Raman spectroscopic measurement method will be described.
The light from the pulse light source 2 is demultiplexed by the demultiplexer 6 and becomes one excitation light and the other excitation light converted by the wavelength converter 3. One excitation light from the pulse light source 2 and the other excitation light from the wavelength converter 3 are crossed and irradiated in the substance to be measured 10. Then, the probe light from the monochromatic light source 1 is incident on the transient grating generated in the material to be measured 10 so as to satisfy the Bragg condition, so that the signal light diffracted by the material to be measured by the spectroscope 4 is spectrally resolved. A wide wavelength region is simultaneously measured by the detector 5.
[0022]
The position where the signal light is measured is determined by setting the magnitudes of the wave number vectors of the excitation light 1 and 2, the probe light and the signal light, respectively,₁, K₂, K₃, K₄When the phase matching condition k₄= K₁-K₂+ K₃Or k₄= -K₁+ K₂+ K₃Can be adjusted to meet. In addition, the excitation lights 1 and 2 are synchronized in time and can be specified in polarization, and the probe light is adjusted to be polarized at a predetermined angle from the polarization of the excitation lights 1 and / or 2. be able to. Further, since the polarization of the signal light is determined by the polarization of the excitation light and the probe light, while the fluorescence is determined by the polarization of only the probe light, the fluorescence can be reduced by adjusting the polarization of the probe light. Furthermore, the fluorescence can be removed or reduced by adjusting the relationship between the irradiation timing of the excitation light and the measurement timing and / or measurement time.
[0023]
4). Configuration of nonlinear Raman spectrometer
FIG. 3 shows a configuration diagram of the nonlinear Raman spectroscopic apparatus.
This apparatus performs spectroscopic measurement of the sample 10 to be measured, and similarly to FIG. 2 and the explanation thereof, the monochromatic light source 1, the pulse light source 2, the wavelength converter 3, the spectroscope 4, the detector 5, and the duplexer. 6, and further includes first and second irradiation polarization adjustment units 11 and 12, measurement polarization adjustment unit 13, first to third irradiation direction adjustment units 21 to 23, measurement direction adjustment unit 24, and computer 30. .
[0024]
In the apparatus of this embodiment, as an example, a Q-switched YLF laser is used as the monochromatic light source 1 that generates probe light. The YLF laser has, for example, characteristics with a wavelength of 523 nm and a pulse width of 6 ns. In addition, a mode-locked titanium sapphire laser obtained by amplifying a Ti: S laser (titanium sapphire laser) with a regenerative amplifier (Regen) is used as the pulsed light source 2 that generates pump light. This characteristic is, for example, a pulse laser with a wavelength of 800 nm, a pulse width of 130 fs, 160 mW, and 1 kHz. The beam may be divided immediately at the exit, and the pulse width may be monitored with an autocorrelator. As the wavelength converter 3, white light generation using a water / heavy water mixture is used. A CCD detector is used as the detector 5. Further, the computer 30 inputs the output from the detector 5 and analyzes the measurement result.
[0025]
The first and second irradiation polarization adjustment units 11 and 12 adjust the polarization of the other excitation light and probe light, respectively. The measurement polarization adjustment unit 13 adjusts the polarization of the signal light. The 1st thru | or 3rd irradiation direction adjustment parts 21-23 each adjust the irradiation direction to the to-be-measured substance of one and the other excitation light and probe light. The measurement direction adjustment unit 24 adjusts the measurement direction of the signal light from the substance 10 to be measured. For example, the measurement direction adjusting unit 24 sets the magnitudes of the wave number vectors of the excitation light 1 and 2, the probe light, and the signal light to k, respectively.₁, K₂, K₃, K₄When the phase matching condition k₄= K₁-K₂+ K₃It is adjusted to meet.
[0026]
Each adjustment of the first and second irradiation polarization adjustment units 11 and 12, the measurement polarization adjustment unit 13, the first to third irradiation direction adjustment units 21 to 23, and the measurement direction adjustment unit 24 is controlled by the computer 30. You may make it adjust manually.
[0027]
Next, the measurement of the Raman peak using such an apparatus will be described.
The pulse laser from the pulse light source 2 including the regenerative amplifier is lowered to about 20 mW with a variable ND filter. If it is too strong, the cell is prevented from being burnt or pierced. Next, this light is divided into two using a duplexer 6 having a λ / 4 plate and a polarizer. On the white light side, the polarization can be made vertical or horizontal through a first irradiation polarization adjustment unit having a λ / 2 plate. The white light side is squeezed with a lens through a wavelength converter 3 made of a 1: 1 mixture of water and heavy water to generate white light. Since white light spreads and is generated, it is returned to parallel light using a lens having a short focal length. In this example, a red filter is placed immediately after this. This is to prevent the detection of what is considered to be the scattering of the short wavelength component of the white light when measuring without a filter, so that it is not possible to know whether the signal is a signal. The other is left as it is, and the two pump lights are squeezed by the lens so that the focal point overlaps at the position of the sample 10. It is possible to adjust the angle and timing of the mirror so that SHG comes out most strongly by placing a BBO crystal at the sample position with some visual observation.
[0028]
Next, the BBO crystal is replaced with a pinhole (50 μm) and pump light is passed through. After both pass through the pinhole, the YLF laser of the monochromatic light source 1 is further squeezed with a lens and passed. The light from the YLF laser of the monochromatic light source 1 can also change the polarization direction by inserting the second irradiation polarization adjustment unit 12 having a λ / 2 plate in the middle.
[0029]
Once all three lights pass through the pinhole, sample 10 (eg CS₂: Because the signal is strong), set the timing of the pump and the probe. The timing of the YLF laser of the monochromatic light source 1 is adjusted with a delay time from the timing of the pulse laser from the regenerative amplifier of the pulse light source 2 by an external trigger. The top of the mountain was adjusted so that one of the pump light and the probe light could be monitored with an oscilloscope. The sample emits light when the three beams overlap spatially and temporally, and was used as a landmark. However, when the signal on the high frequency side is weak and cannot be confirmed from the beginning, after making the low frequency region visible, the incident angle of the probe should be reduced and shifted to the high frequency side.
[0030]
FIG. 4 is an explanatory diagram of the arrangement of pump light, probe light, and signal light.
First, in order to see the low frequency region, the incident angles of the pump light and the probe light are set to about 800: 523. This is because the grating spacing of the diffraction grating is given by Λ = λ / (2 sin θ)._in: Tanθ_pr≒ sinθ_in: Sinθ_pr= Λ_in: Λ_prBecause it becomes. The two pump lights are considered to be incident symmetrically. In this arrangement, the signal appears in a spot shape in a direction symmetrical to the probe.
[0031]
If you see swaying diffracted light behind the sample, cut off the extra scattered light with an aperture. However, note that the signal emission direction moves slightly when moving to the high frequency side, so it is impossible to detect the signal if the aperture is reduced too much. The signal is collimated with a lens having a focal length of about 150 mm, and is put into a double monochromator with a focal length of 1 m through a polarizing plate. The slit width is about 150 μm and the center wavelength is 19102 cm at the center of the probe light.^-1Keep it close. While the signal is monitored by the computer 30, the last polarizing plate is turned so as to reduce Rayleigh scattering. If there is a signal, the spectrum of the probe light spreads. Therefore, the signal is maximized by adjusting the incident angle of the probe light, the timing, and the position of the cell.
[0032]
To detect the Raman peak on the high frequency side, the incident angle θ of the probe light_prShould be reduced. Ideally, the high frequency side can be seen only with the angle and timing of the probe, but the pump light spreads in time due to the effect of group velocity dispersion when white light is used. You also have to adjust the timing. Furthermore, since the center wavelength of the double monochromator must also be changed, the following three steps were repeated to move to a position where there was a peak.
(1) Center wavelength of double monochromator is 200cm^-1Increase the rank.
(2) Confirm that the signal is output.
(3) Adjust the incident angle and timing of the probe light and the timing of the pump light so that the high frequency side becomes stronger.
1000cm with this method^-1CS₂And the Raman line of benzene could be detected. Actually, even if the angle of the probe is not moved much, it may be possible to see the high frequency side only by the timing of the pump light. This is considered to be because components with various incident angles already exist because the widened probe is narrowed by the lens.
[0033]
Figure 5 shows the CS₂An example of the measurement data of the Raman peak of benzene and benzene is shown. Although the measurement time is as short as about 5 seconds, for example, data with a good S / N ratio was obtained.
[0034]
5. Measurement theory
Next, the theory of measurement of nonlinear Raman spectroscopy will be described in detail.
5.1. Four-wave mixing
In the frequency domain four-wave mixing spectroscopy, two pump lights (excitation light) having a short pulse width are condensed on a sample, and probe light is incident on another pulse having a wide pulse width. At this time, the electronic state of the molecule (atom) in the sample and the energy state of the rotation / vibration motion of the molecule are excited by the energy difference between the two pump lights in the sample. At this time, if the energy difference and the vibration frequency are the same, a Raman shifted peak appears in the spectrum separately from the center wavelength of the probe.
[0035]
According to another interpretation, a diffraction grating that disappears in a short time due to the interaction between the interference fringes of the two pump lights and the vibration of the substance is formed on the sample, and the signal light is in phase matching condition k = k.₁-K₂+ K₃Strongly diffracted in the direction satisfying At this time, the frequency of the signal light is the Doppler effect caused by the collision with the moving diffraction grating, and ω = ω₁−ω₂+ Ω₃It has become. This shows that the original frequency is added to the frequency corresponding to the vibration of the molecule of the substance, which is equivalent to measuring Raman scattering.
[0036]
Consider a case in which two pump lights with short pulses and probe light that can be regarded as approximately CW are incident on the sample. Two pump lights E_m(R, t) = E_m(T) e^iω _m ^t-ik _m ^{・ R}(M = 1, 2), probe light is E₃(R, t) = E₃e^iω ₃ ^t-ik ₃ ^{・ R}And E_m(R, t) is time t_mIs incident on the sample. The i-th component P of the third-order nonlinear polarization at this time_i(R, t) can be written as:
[0037]
[Expression 1]

[0038]
Where ω_mAnd k_mIs the electric field frequency and wave vector. t '= t-t₃, T ″ = t₃-T₂, Ω ’= ω₁−ω₂+ Ω₃K = k₁-K₂+ K₃It is. t₁₂Is the time difference between pump 1 and pump 2, χ_ijkl(T ′, t ″) is the ijkl component of the third-order nonlinear susceptibility tensor of the four-wave mixing process. The response of the substance is the response function R_ijklDerived from linear response theory using (t). k = k₁-K₂+ K₃The polarization propagating in the direction of is written formally as follows.
[0039]
[Expression 2]

[0040]
Since the spectrum of the diffracted signal generated by this polarization is a Fourier transform of the time correlation function, it is as follows.
[0041]
[Equation 3]

[0042]
Where Δω = ω−ω₃Ω is the frequency of the diffracted light. P_iSubstituting (t) into this, k = k₁-K₂+ K₃Focusing only on the terms that proceed in the direction of
[0043]
[Expression 4]

[0044]
Where G⁽²⁾ _1k2lIs a second-order correlation function of the two pump lights, and indicates that the tilde is Fourier transformed. In the experiment, there is no time difference between the pump lights, and t₁₂= 0. Thus, looking at the spectrum of the signal is equivalent to looking at the square of the absolute value of the frequency response function of the material and the cross-correlation function of the pump light.
[0045]
Under the condition of non-resonance and the Born-Openheimer approximation, the substance response function R_ijkl(T) is divided into terms of nuclear response and electron immediate response_ijkl(T) = σ_ijklδ (t) + N_ijkl(T). The term σ represents the immediate response of electrons, and δ (t) is Dirac's δ function. N (t) is a nuclear response function. This R_ijklSubstituting into the previous spectral equation, we get
[0046]
[Equation 5]

[0047]
Since the δ function is integrated, there is no tilde in σ. From this equation, it can be seen that if the correlation function G of the two pump lights is sufficiently widened, the frequency response function of the substance can be observed directly as a broadened spectrum of the probe. From a different perspective, the interpretation is similar to the diffraction grating described in the introduction.
[0048]
5.2. Polarization selection
Since the signal obtained by this spectroscopic method is proportional to the square of the response function, it is inevitable that the response of electrons and nuclei will be mixed. However, in this spectroscopic method, the polarization of the four beams can be freely selected, and if one is set appropriately, one of the responses can be eliminated. For example, both the polarizations of the pump light are kept horizontal, and this is taken as the x-axis. The probe is tilted 45 ° counterclockwise.
[0049]
FIG. 6 is an explanatory diagram for the polarization of pump light, probe light, and signal light.
If the polarization of the signal is ψ, the polarization due to the response of each of the electron σ and the nucleus N is as follows.
[0050]
[Formula 6]

[0051]
The meaning of the subscript is proportional to the x and y components of the polarization of the signal, probe, pump 2 and pump 1 from the left. For generally isotropic materials, σ_yyxx/ Σ_xxx= 1/3, N_yyxx/ N_xxxBecause there is a relationship
[0052]
[Expression 7]

[0053]
It becomes. Since the polarization is 180 ° rotationally symmetric, the response function can eventually be written as:
[0054]
[Equation 8]

[0055]
Therefore, if the polarization of the signal is set to 108.4 ° or 63.4 °, only the response of electrons and nuclei can be obtained.
[0056]
【The invention's effect】
The present invention has, for example, the following specific effects.
(1) Since it is nonlinear Raman spectroscopy, the background of fluorescence can be removed.
(2) A wide frequency range can be measured at a time, and a short-time measurement is possible. (CS₂If the signal is strong like this, the measurement time is about 5-30 seconds, for example, and sufficiently clean data can be obtained. )
(3) No need for a variable frequency laser makes the device simple.
(4) It is possible to measure a high-resolution Raman spectrum using pulses without being limited by uncertainty.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of the principle of nonlinear Raman measurement.
FIG. 2 is a schematic configuration diagram of an apparatus for realizing a nonlinear Raman spectroscopy method.
FIG. 3 is a configuration diagram of a nonlinear Raman spectroscopic apparatus.
FIG. 4 is an explanatory diagram of the arrangement of pump light, probe light, and signal light.
FIG. 5 CS₂The figure of the measurement data of the Raman peak of benzene and benzene.
FIG. 6 is an explanatory diagram of polarization of pump light, probe light, and signal light.
[Explanation of symbols]
1 Monochromatic light source
2 Pulse light source
3 Wavelength converter
4 Spectrometer
5 Detector
6 duplexer
10 Sample to be measured
11, 12 First and second irradiation polarization adjustment units
13 Measurement polarization adjuster
21-23 1st-3rd irradiation direction adjustment part
24 Measuring direction adjuster

Claims (13)

And hand pump light synchronized one same excitation light said, and broad other excitation light spectral width than said one of the excitation light irradiated by crossing the substance to be measured,
The transient grating generated by the measurement substance, a monochromatic or nearly monochromatic probe light incident to satisfy the Bragg condition,
By analyzing the spectrum of the signal light diffracted by the nonlinear Raman scattering of the measured substance and detecting the signal light to be measured ,
A nonlinear Raman spectroscopy method for measuring a nonlinear Raman peak. The other excitation light, nonlinear Raman spectroscopy method of claim 1, characterized in that the light of the white light or nearly white light. The position where the signal light is measured is that the phase matching condition k ₄ = k when the magnitudes of the wave number vectors of one and the other of the excitation light, the probe light, and the signal light are k ₁ , k ₂ , k ₃ , and k _{4 3.} The nonlinear Raman spectroscopic method according to claim 1, wherein the nonlinear Raman spectroscopic method is adjusted to satisfy ₁ −k ₂ + k ₃ or k ₄ = −k ₁ + k ₂ + k ₃ . One excitation light and the other excitation light are synchronized in time, and
4. The nonlinear Raman spectroscopic method according to claim 1, wherein various components of the nonlinear susceptibility tensor can be measured by adjusting the polarization directions of the probe light and the one and other excitation lights.   The nonlinear Raman spectroscopic method according to claim 1, wherein fluorescence is removed or reduced by adjusting the polarization of the probe light.   6. The nonlinear Raman spectroscopic method according to claim 1, wherein fluorescence is removed or reduced by adjusting a relationship between an irradiation timing of excitation light and a measurement timing and / or a measurement time. Monochromatic or monochromatic light source for generating substantially monochromatic probe light,
A pulse light source for generating a hand of the excitation light,
Based on one excitation light from the pulsed light source, a wavelength converter that synchronizes with the one excitation light and converts the other excitation light having a wider spectrum width than the one excitation light;
And a spectrometer for spectral decomposition of the signal beam diffracted by the substance to be measured,
One excitation light from the pulsed light source and the other excitation light from the wavelength converter are irradiated in a crossing manner in the material to be measured, and probe light from the monochromatic light source is applied to the transient grating generated in the material to be measured. Is a nonlinear Raman spectroscopic apparatus that spectrally decomposes and measures the signal light diffracted by nonlinear Raman scattering by the substance to be measured by the spectrometer.   The nonlinear Raman spectroscopic apparatus according to claim 7, further comprising: a demultiplexer that makes one excitation light from the pulse light source have a predetermined polarization and branches toward the wavelength converter.   The nonlinear Raman spectroscopic apparatus according to claim 7 or 8, further comprising first and second irradiation polarization adjustment units for adjusting the polarization of the other excitation light and probe light, respectively.   The nonlinear Raman spectroscopic apparatus according to claim 7, further comprising a measurement polarization adjustment unit for adjusting the polarization of the signal light. First to third irradiation direction adjusting units for adjusting the irradiation directions of the one and other excitation light and probe light to the measurement target material,
The nonlinear Raman spectroscopic apparatus according to claim 7, further comprising a measurement direction adjustment unit for adjusting a measurement direction of the signal light from the substance to be measured. When the magnitudes of the wave number vectors of one and the other of the excitation light, the probe light, and the signal light are k ₁ , k ₂ , k ₃ , and k ₄ , respectively, the measurement direction adjusting unit is phase matching condition k ₄ = k ₁ The nonlinear Raman spectroscopic apparatus according to claim 11, wherein the nonlinear Raman spectroscopic apparatus is adjusted so as to satisfy −k ₂ + k ₃ or k ₄ = −k ₁ + k ₂ + k ₃ .   The nonlinear Raman spectroscopic apparatus according to claim 7, wherein the wavelength converter includes any one of a water / heavy water mixture, sapphire, benzene, acetone, or a nonlinear crystal.

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