JPH08101120A - Wavelength dispersion correcting optical device for evanescent wave intrusion length - Google Patents

Abstract

PURPOSE: To correct the evanescent wave intrusion length varying according to the wavelength changing the incident angle to the total reflection surface of a prism in accordance with the wavelength by the use of a dispersing element. CONSTITUTION: A diffraction light originating from an incident white light at a certain angle on a diffraction lattice 4 is collimated by a lens 1 having a focal distance f, and upon image inversion by two lenses 3, is converged by a lens 2 having a focal distance f. The optical axis of this lens system is laid on the line tying the center of the lattice 4 with a prism 5 which is formed into a semi-columnar shape and whose angle change due to refraction is ignorable. Because therein the desired incident angle can not be obtained so long as the dispersion angle of the lattice 4 remains as it is, the total reflection surface of the prism 5 is inclined from the optical axis so that an offset angle is generated, and thereby, the desired incident angle is established. This permits obtaining an evanescent wave intrusion length which is constant in the visible ray region. That is, it is possible to segregate beams of light having different, wavelengths using dispersing element of the lattice 4 and change the incident angle according to the wavelength by converging onto the total reflection surface by the lenses, and the evanescent wave intrusion length can be made constant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical device using an evanescent wave such as an ATR spectroscopic device and a near field microscope.

[0002]

2. Description of the Related Art Since an evanescent wave is a light localized within a range of penetration length equal to or shorter than a wavelength, it is used for investigating properties near the interface and obtaining information in the depth direction. Conventionally, as a device using this, an ATR spectroscopic device, a near-field microscope, etc. are generally used. For ATR spectroscopy,
New Experimental Chemistry Course 4 Basic Technology 3 Light [II] (edited by the Chemical Society of Japan, Maruzen, published March 20, 1984), pages 408 to 416. Regarding the near field microscope, for example, S.M. Jiang et al., The Japanese Journal of Applied Physics, Vol. 30 (1991), pp. 2107 to 2111 (Japanese Journal of Appli).
ed Physics 30, 1991, pp. 210
7-2111).

However, since the penetration length of the evanescent wave is a function of wavelength, when light with a wide spectrum width such as white light is used for the purpose of spectroscopy, the penetration length of the evanescent wave varies depending on the color, that is, the wavelength. . Hereinafter, for convenience, this is referred to as wavelength dispersion of the penetration length of the evanescent wave. For example, in the visible light region, the penetration length is almost doubled. This means that in ATR spectroscopy and the like, the effective depth varies depending on the wavelength, and therefore the measurement is performed under different conditions. In addition, the near-field microscope brings about a problem that the resolution varies depending on the wavelength.

Conventionally, a method of correcting this by calculation or appropriately adjusting conditions such as an incident angle while sweeping its wavelength using monochromatic light has been used. However, these methods have problems that conditions such as uniform sample are required and that measurement takes a lot of time.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical system in which the penetration length of an evanescent wave is not wavelength-dispersed even when light having a wide spectrum width such as white light is used.

[0006]

Since the penetration length of the evanescent wave depends on the incident angle, the incident angle may be changed for each wavelength to automatically correct the chromatic dispersion of the penetration length. Changing the incident angle for each wavelength can be realized by using a dispersive element such as a diffraction grating.

[0007]

FUNCTION The penetration length of the evanescent wave is given by (Equation 1).

[0008]

[Equation 1]

Where tin is the angle of incidence of light on the total reflection surface, n
p is the refractive index of the prism, ns is the refractive index of the medium in contact with the prism total reflection surface, and lambda is the wavelength of light.

Therefore, it is understood that the longer the wavelength of light, the larger the incident angle. By using a dispersive element such as a diffraction grating to spatially separate light having different wavelengths and condensing the light on a total reflection surface with a lens or the like, the incident angle can be changed depending on the wavelength. The dispersion angle of the diffraction grating, etc. does not reach the desired incident angle as it is, so an offset angle is created by tilting the total reflection surface from the optical axis, and the angle is a constant multiple using a lens system with an appropriate magnification. By enlarging or reducing the size, it is possible to obtain an incident angle with which the penetration length is constant by selecting appropriate conditions.

[0011]

【Example】

Example 1 An example of the present invention will be described with reference to FIG. The collimated white light is made incident on the diffraction grating 4 at an angle a, the diffracted light is collimated by the lens 1 having the focal length f1, the image is inverted by the two lenses 3, and the light is condensed by the lens 2 having the focal length f2. To do. The glass prism 5 is processed into a semi-cylindrical shape so that an angle change due to refraction can be ignored. It may be spherical. The optical axis of the lens is arranged on the line connecting the centers of the diffraction grating 4 and the prism 5. The angle between this line and the normal to the diffraction grating 4 is c. The angle at which the prism 5 is further tilted, that is, the above-mentioned offset angle is t. Two lenses 3 are used because the shorter the wavelength of light, the smaller the diffraction angle,
This is because it is necessary to invert the image in the arrangement of FIG. 1 in order to reduce the angle of incidence on the prism 5.

As the diffraction grating 4, one having 1200 lines / mm is used, and first-order diffraction is used. The penetration length of the evanescent wave was measured using a near field microscope. As a light source, a light emitted from a tungsten lamp was passed through a pinhole and then collimated by a lens.

A = 0.58 degrees, c = 36.8 degrees, t = 0.
FIG. 2 shows the calculation result and the measurement result of the penetration length of the evanescent wave when 6 degrees and f1 / f2 = 0.57. From this, it can be seen that an almost constant penetration length was realized in the visible region. However, the calculation and measurement in this case were performed in the case where the sample 11 was not present in FIG. 1, that is, the case where the total reflection surface was the interface between air and the prism.

Although a total of four lenses are used here, an equivalent optical system can be realized by combining appropriate lenses, and the number can be easily reduced. Further, although the experiment was carried out for the penetration depth of 120 nm, similar results can be obtained for various penetration lengths by selecting appropriate conditions. Further, in this embodiment, an achromatic lens is used in order to eliminate chromatic aberration of the lens.

Further, in the present embodiment, absorption measurement is made possible by assuming ATR spectroscopy. The totally reflected light is reflected at different angles for each wavelength. That is, since the light has already been spectrally dispersed, a spectrum distribution can be obtained by measuring the intensity as it is using the one-dimensional photodiode array. Here, the two-dimensional photodiode array 10 is arranged. The sample 11 is in close contact with a part of the total reflection surface of the prism 5, but the reflected light from the part without the sample can be used as the reference light as shown in FIG. If this is measured at the same time with the light that hits the sample by using a two-dimensional photodiode array, it is possible to measure two light fluxes. Instead of the two-dimensional photodiode array 10, two or more one-dimensional photodiode arrays may be used. The lens 12
Is for collimating the reflected light.

In the case of application to spectroscopic measurement as described above, since the diffraction grating was originally required, the present invention does not cause an increase in the number of expensive components such as the diffraction grating.

Embodiment 2 Another embodiment of the present invention will be described with reference to FIG. Generally,
This is the same as the first embodiment, but in this case, the arrangement is such that it is not necessary to invert the image. As a result, the number of lenses can be reduced, and adjustment and the like can be simplified. Furthermore, in consideration of fixing the prism, we decided to adjust the offset angle on the diffraction grating side. A penetration depth of 120 nm is also possible in the same manner as in Example 1, but a penetration depth of 150 is used here.
It was adjusted to be nm. The parameter at this time is a
= 1.56 degrees, c = 39.8 degrees, t2 = 0.6 degrees, f1 /
f2 was 0.34.

[0018]

According to the present invention, since the penetration length of the evanescent wave can be prevented from changing depending on the wavelength, measurement using the evanescent wave, for example, ATR spectroscopy, fluorescence spectroscopy in the depth direction of the interface, near-field microscope, etc. It is possible to suppress the change in the effective depth due to the wavelength and the change in the resolution due to the wavelength. Furthermore, for samples that pose a problem of aging,
It is necessary to simultaneously measure the necessary wavelength region by using a light source such as white light having an extremely wide spectrum width as a light source. In this case, it is important to use the present invention.

In this embodiment, the diffraction grating is used as the dispersion element, but it may be a prism. Further, the study was conducted on visible light, but the same can be realized for ultraviolet light and infrared light. Although white light is used as the light source, a wavelength tunable light source such as a dye laser, a titanium sapphire laser, or a parametric oscillator may be used. This time,
Although it is necessary to sweep the wavelength of the laser to measure each wavelength, a strong light source can be obtained. Even in this case, if the optical device of the present invention is used, it is not necessary to change the incident angle for each wavelength, and a fixed optical system can be used.

Usually, when a thin sample is used,
The wavelength dispersion of the refractive index of the sample can be ignored. Even if the wavelength dispersion of the refractive index of the sample cannot be ignored, or even if a material with a large wavelength dispersion of the refractive index is used as the prism, by considering these, the condition that the wavelength dispersion of the penetration length can be ignored can be considered. Can be found.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention.

FIG. 2 shows wavelength dependence of penetration length of evanescent wave.

FIG. 3 is a diagram showing a sample arrangement in an ATR spectroscope.

FIG. 4 is a configuration diagram of another embodiment of the present invention.

[Explanation of symbols]

1-3: lens, 4: diffraction grating, 5: prism, 10:
Two-dimensional photodiode array, 11: sample, 12: lens.

Claims (3)

[Claims] 1. A device having a mechanism for generating an evanescent wave using a light source having a spectral width, in order to correct a change in penetration length of the evanescent wave with wavelength,
A wavelength dispersion correcting optical device for evanescent wave penetration length, characterized in that an incident angle to a total reflection interface of a prism is changed by a wavelength by using a dispersive element. 2. An optical device according to claim 1, wherein a diffraction grating is used as the dispersion element, and a wavelength dispersion correction optical device for evanescent wave penetration length. 3. An optical device according to claim 1 or 2, wherein a prism having a semi-cylindrical shape or a hemispherical shape is used, and an evanescent wave penetration length chromatic dispersion correction optical device is used.