JPH0534129A - Optical probe - Google Patents

Abstract

PURPOSE:To generate an evanescent wave of a short wavelength arbitrarily and thereby to improve a lateral resolution or the like by using a concentric- circle-shaped diffraction grating (axicongrating) for an optical probe of an optical near field microscope. CONSTITUTION:Probe light 11 falls in the direction of a vertical arrow. The whole probe light 11 is totally reflected by a grating and an evanescent field is generated on the sample side of the diffraction grating. On the occasion, diffracted light 12 of both +first and -first orders exists. A concentric-circle- shaped diffraction grating generating an evanescent wave is provided for an optical probe and a spot of the evanescent wave being smaller than the wavelength of the light is formed at the center of the diffraction grating. By detecting the interaction of this spot and a sample, a microscopic structure of the sample is measured. In this system, the wavelength of the evanescent wave is determined by the grating interval of the diffraction grating. By narrowing the grating interval, a minute spot of an arbitrary diameter can be obtained by an arbitrary wavelength and thus a high resolution in the lateral direction can be realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical probe used for an optical pickup of an optical near field microscope and an optical recording device.

[0002]

2. Description of the Related Art There is an optical near-field microscope that realizes resolution equal to or less than the wavelength of light by using evanescent waves that do not propagate as probe light. This produces an evanescent wave on the optical probe and detects changes in the field caused by interaction with the sample, or
High-resolution sample shape measurement is performed by generating an evanescent wave on the sample surface, and bringing the optical probe close to it to detect the state of the evanescent wave.

The evanescent wave is non-propagating light and is attenuated at a distance of about the wavelength. The optical near field microscope can measure the surface shape of the sample with high resolution by utilizing this property. Further, the evanescent wave is a light wave having a short wavelength that cannot be realized by ordinary propagating light. The shorter the wavelength of the light wave, the higher the resolution in the lateral direction, so that the optical near-field microscope can perform high-resolution measurement.

The evanescent wave is generated when light is incident from the high refractive index side to the low refractive index side at an incident angle of not less than the total reflection angle. Also, when light is incident on a pinhole having a diameter smaller than the wavelength, an evanescent wave is generated on the pinhole. Optical near-field microscopes can be roughly classified into two types according to the method of generating an evanescent wave. One is a photon tunneling near-field microscope and the other is a pinhole near-field microscope.

The photon tunneling type near-field microscope uses an evanescent wave generated when light is incident on a chip or a cover glass having a high refractive index at a total reflection angle or more. The photon tunneling near-field microscope can be realized both as a non-scanning type and a scanning type.

Guerra has been reported to realize a non-scanning type photon tunneling type near field microscope (Guerra: Appl. Opt., 26, 3741, (1990)). This is just a slight improvement on the normal reflection optical microscope,
The sample can be directly observed with high resolution.

The scanning type photon-tunneling near-field microscope is a Courjon (D. Courjon, K Sarayeddin
e, and M. Spajer: Optics Commun., 71, 23 (198
9)), Reddick (RC Reddick, RJ Warmack, and
TL Ferrell: Phys. Rev. B, 39, 767 (1989)), Otsu (Jiang Xiaodong, Naoyuki Tomita, Motoichi Otsu, Optics, Volume 20,
134 (1991)) and others.

The pinhole type near-field microscope uses a pinhole smaller than the wavelength or an evanescent field generated at the tip of the tip by injecting light into an optical waveguide chip whose tip is sharpened to an area smaller than the wavelength.

It is known that the spot spread of the evanescent field at the tip of the chip hardly spreads up to the penetration length of the evanescent wave. Pinhole near field microscopes thus take the form of spot scanning microscopes. This idea was proposed by O'Keefe in 1956 (JA O'Keefe: J. Opt. Soc. Am., 46, 359 (195
6)), and Ash conducted a microwave experiment in 1972 (EA Ash and G. Nicholls: Nature, 237, 510 (19
72)), Issacson's group at Cornell University (E.
Betzig, A. Lewis, A. Harootunian, M. Issacson and
E. Kratschmer: Biophys. J., 49, 269 (1986)) and I
BM Zurich Pohl Group (DWPohl, W. De
nk, and M. Lanz: Appl. Phys. Lett., 44, 651 (198
4)) independently implemented the actual equipment and conducted experiments.

The above optical near-field microscope has succeeded in improving the resolution in the depth direction in surface shape measurement. However, it is difficult to improve the lateral resolution, and the result is not so good. This is because the conventional optical probe for an optical near-field microscope has an extremely low efficiency of light intensity that contributes to the interaction of the sample and has a low SN ratio in detection. This is a problem of the prior art.

On the other hand, in the optical recording apparatus, there is a problem to improve the recording density of the optical disk memory or the like. For this purpose, it is necessary to collect a small amount of probe light used for writing and reading on the medium.

An optical pickup using a super-resolution phenomenon for focusing the probe light into a small size has been manufactured as a trial (Nikkei Electronics, vol. 528, 124 (199).
1)). The ultra-high resolution phenomenon utilizes the interference of probe light to reduce the spot diameter. However, the theoretical limit of the spot diameter is up to 1/2 of the wavelength.

From this theoretical limit, it is necessary to shorten the wavelength of the probe light in order to increase the recording density. However, in reality, there is a limit in shortening the wavelength of the probe light, and there is an overhead in increasing the recording density by the conventional technique. The problem with conventional optical pickups is that the spot diameter is
It can only be reduced to about 1/2.

[0014]

SUMMARY OF THE INVENTION The present invention provides a novel optical probe having high resolution and high resolution used in an optical near-field microscope, an optical pickup, etc., and a novel optical probe having high light utilization efficiency. It is an object of the present invention to provide a method for realizing a probe and to provide a novel optical probe that can realize a finer spot diameter.

[0015]

The present invention uses a fine concentric circular diffraction grating (hereinafter referred to as an axicon grating) for an optical probe to generate an evanescent wave having a short wavelength on the probe surface, thereby achieving high resolution, high resolution and Achieve high efficiency of light utilization. By optimizing the shape of the blaze of the diffraction grating and the state of incident light, a spot with a higher energy density is formed and the resolution is further improved.

[0016]

In the present invention, the evanescent wave is generated by the fine diffraction grating.

Now, consider a case where a plane wave is vertically incident on the amplitude type sine wave diffraction grating. If the pitch of the diffraction grating is longer than the wavelength, 0th-order transmitted light and ± 1st-order diffracted light are generated on the transmission side, and the diffraction direction θ is sin θ = λ / d (d = diffraction grating pitch, λ = Incident wavelength) (1) If the wavelength and the pitch of the diffraction grating are the same, the direction of diffraction is orthogonal to the incident light and propagates on the grating surface.

The diffraction grating is assumed to be sufficiently thin. When the lattice spacing is shorter than the wavelength, sin θ becomes larger than 1 according to the equation (1), and cos θ becomes an imaginary number. Letting k be the wave number of the incident light, k cos θ represents the wave number of the diffracted wave propagating in the direction perpendicular to the diffraction grating.

In the axicon grating, when light is incident obliquely or perpendicularly to the grating surface, an evanescent wave is generated on the transmission side of the diffraction grating to form a field. When a sample or a storage medium is present in the evanescent field, they interact with each other, and the effect appears on the return light from the diffraction grating and the scattered light on the sample. If this effect is detected, the sample,
Alternatively, information on the recording medium can be obtained.

The axicon grating can be replaced by a high-refractive-index total reflection prism of a photon tunneling type near-field microscope or a pinhole of a pinhole type near-field microscope. Therefore, the optical near-field microscope using the axicon grating as the optical probe has an action of realizing all the functions of the optical near-field microscope.

When an axicon grating is used for an optical probe, the evanescent wave is ± 1st order diffracted light or higher order diffracted light. However, 0th-order light also exists in the diffraction grating, and this becomes propagating light. In order to detect the interaction between the evanescent wave and the sample, it is necessary to separate the effect of diffracted light of ± 1st order or more and the effect of 0th order light. Therefore, for example, the separation is performed as follows. When the distance between the sample and the diffraction grating is changed with a sine wave with time, the 0th-order light component in the intensity of the returning light or the scattered light hardly changes, and only the ± 1st-order or more diffracted light components are intensity-modulated. By extracting this modulation component, only the interaction between the evanescent wave and the sample can be detected. Further, the ± 1st-order diffracted light has a higher intensity and a longer penetration length than the higher-order diffracted lights, and therefore the distance between the axicon grating and the sample is reduced to the distance at which the ± 2nd-order or more diffracted light is attenuated. If the measurement is performed with a distance of, the interaction of only ± 1st order diffracted light can be detected.

In terms of resolution, the same thing can be said in principle as that of an ordinary optical near field microscope. In the axicon grating, the smaller the lattice spacing, the shallower the evanescent wave seepage depth and the shorter its wavelength. The resolution can be improved by using a light wave having a short wavelength as the probe light. In the axicon grating optical probe of the present invention, the smaller the lattice spacing is, the more the lateral resolution can be improved. The theoretical limit of resolution is not limited by the wavelength of the probe light, but is determined only by the grating spacing of the diffraction grating. Theoretically, if an axicon grating having a lattice spacing of 1 nm is used for the optical probe, a resolution of 1 nm can be obtained at any wavelength.

By providing the blaze so as to increase the specific diffracted light component, the spot of the evanescent field can be made thin. For example, if the + 1st-order diffracted light toward the center of the axicon grating is strongly diffracted and the -1st-order diffracted light toward the edge does not exist, both ± 1st order diffracted light exist if the blazed axicon grating is used. The center spot is narrower than that of the case, and an evanescent field with low side lobes can be created. Also, by using an optical probe with an axicon grating on a conical prism (hereinafter referred to as an axicon prism), it is possible to further increase the intensity of the central spot and reduce the interaction between the side lobe and the sample due to the effect of the shape. is there. This is because in the axicon grating on the axicon prism, the light enters in the form that the diffraction grating is inclined, so that it goes to the tip of the axicon prism.
This is because the 1st-order diffracted light becomes larger than the -1st-order light and the energy is concentrated on the central spot. Another reason is that the evanescent field due to the axicon grating is generated so as to cling to the axicon prism with a sharp tip, and the position of the side lobe is far from the sample.

[0024]

EXAMPLE FIG. 1 shows an example of a diffraction grating type optical probe using an axicon grating. The grating pitch is shorter than the wavelength of light. The probe light 11 is incident in the direction of the arrow in the figure, and all the incident probe light 11 is totally reflected by the grating to generate an evanescent field on the sample side of the diffraction grating. In this case, the diffracted light 12 exists in both + 1st order and -1st order. The diffraction grating may be turned upside down.

The evanescent field generated in this case is given by a 0th-order Bessel function centered on the central axis of the axicon grating 1, and its intensity distribution is shown by C1 in FIG. The horizontal axis is the distance from the center and is normalized by the wavelength of the evanescent wave, that is, the lattice spacing. The full width at half maximum of the central peak is about 0.3 wavelength.

FIG. 3 shows an example of an axicon grating type optical probe provided with blazes. The blaze is such that the diffracted light from the diffraction grating is only the + 1st-order light directed to the center of the axicon grating 2. When only the + 1st order diffracted light 13 can exist, the evanescent wave made by the light incident on the edge of the grating cannot escape until it reaches the center due to the action of the blaze, and after passing the center, the blaze is reversed. Therefore, it travels as −1st-order light and is emitted from the incident side of the axicon grating.

The evanescent field generated at this time changes depending on the propagation distance of the minus first-order light after passing through the center.
The propagation distance is determined by the blazed shape of the diffraction grating. The calculation result of the intensity distribution of the evanescent field when the propagation distance is ½ wavelength is shown in C2 of FIG. As a result when there is no blaze, the central peak is narrower and the side lobe height is smaller than that of C1. The width of the central peak and the shape of the side lobes change depending on the propagation distance of the −1st order light, and the shorter the propagation distance of the −1st order light, the narrower the width of the central peak and the lower the sidelobe height.

The full width at half maximum of the spot of the evanescent field generated when the axicon grating is used is 0.3 times or less the wavelength of the evanescent wave. Since the wavelength of the evanescent wave is the same as the lattice spacing, it is possible to create a spot with an arbitrary diameter at an arbitrary wavelength by choosing the lattice spacing of the axicon grating. The axicon grating is created by electron beam processing, photo etching, diamond grinding, or the like.

Next, an embodiment of an objective chip using a conical prism (hereinafter, axicon prism) is shown in FIG. The axicon grating 2 is formed on the conical surface of the axicon prism 3. For forming the diffraction grating, electron beam processing, diamond grinding, photo etching or the like is used as in the above-described embodiment. The lattice spacing of the axicon grating 2 on the conical surface of the axicon prism 3 is made thin so that the diffracted light becomes an evanescent wave traveling on the conical surface. The condition is that the refractive index of the axicon prism 3 is n2, the surrounding refractive index is n1, and the axicon prism 3 is
Letting ψ be the total apex angle of, the lattice spacing of the axicon grating should be λ / {n1-n2cos (ψ / 2)} or less.

In the vicinity of the center of the axicon prism 3, the evanescent waves 14 propagating from the surroundings strengthen each other to form a very strong field. The spot size is determined by the grating spacing of the diffraction grating on the conical surface, regardless of the wavelength of the incident light. Further, due to the effect of the shape of the prism, the intensity of the central spot is concentrated on the tip of the axicon prism 3, and a spot with high energy density can be formed.

Further, the intensity of the spot center can be enhanced by making the incident probe light 11 a Gaussian beam.

It is also possible to enhance the intensity of the center by using incident light that converges on the center of the axicon prism 3 or the axicon gratings 1 and 2.

By blocking the incident probe light 11 by half, the height of the second peak can be reduced and the relative intensity of the center can be increased.

By depositing a metal thin film on the surfaces of the axicon gratings 1 and 2 to excite photoexcited surface plasmons, it is possible to enhance the field intensity and improve the light utilization efficiency.

An optical near-field microscope constructed by using these optical probes can observe a sample in all modes that can be realized by an ordinary optical microscope.

FIG. 5 shows an embodiment of the reflection microscope. In the reflective type, the surface shape can be measured at the same time as it is disassembled in the lateral direction. The light from the light source 26 is reflected by the beam splitter 23, focused by the lens 22, and then irradiated on the optical probe 4 to form a spot of an evanescent field near the center axis of the probe. When this spot hits the surface of the sample 25 and light is reflected, the reflected light passes through the optical probe 4 and the beam splitter 23 and reaches the detector 27. The detector 27 measures the intensity of the returning light from the optical probe 4. If the distance between the sample 25 and the optical probe 4 is changed in a sinusoidal manner and only the component that changes in synchronization with it is extracted, only the sample information at the spot can be extracted. In this example, the local reflectance and absorptance of the sample can be measured.
Furthermore, if the detector 27 is used for spectral analysis, it is possible to obtain spectral information. A laser, a white light source, or the like is used as the light source.

FIG. 6 shows an embodiment in which a transmission microscope is constructed. The scattered light from the sample 25 is collected on the detector 27 and measured to obtain local information of the sample. Similar to the embodiment of the reflection microscope, the distance between the sample 25 and the optical probe 4 is changed to extract only the sample information near the spot. The local absorption rate of the sample can be measured. A laser or a white light source is used as the light source 26.

In both the transmission type and the reflection type microscopes, the scattered light of the laser is cut in front of the detector 27 and the fluorescence from the sample is measured to operate as a fluorescence microscope. Similarly, if Raman scattered light is detected on the detector side, it operates as a Raman microscope.

FIG. 7 shows an embodiment of a microscopic Fourier spectroscopy apparatus in which a near field microscope is connected to an infrared spectroscope. Even if the wavelength of infrared light is 10 μm, the absorption distribution of the sample can be observed with a spatial resolution of 1 μm. In this way, the resolution below the wavelength of the probe light cannot be realized by the existing device. This is because the spot size is determined by the diffraction limit as long as the objective lens is used.

An embodiment applied to an optical pickup is shown in FIG. Laser light emitted from the semiconductor laser 21 is incident on the optical probe 4 to generate an evanescent field spot on the probe surface. The recording medium 31 is brought close to the optical probe 4 to the position where the evanescent field exists.
Information is recorded in the recording medium 31 as a two-dimensional absorptance, reflectance or phase distribution. These are read by the evanescent field on the surface of the optical probe 3.

It is also possible to change the local absorption rate or reflectance by changing the substance in the vicinity of the spot by injecting light having sufficiently high intensity into the optical chip to generate an evanescent wave. Information can be written in the recording medium by changing the intensity of incident light to create a local absorption or reflectance distribution.

Since the spot diameter of the evanescent wave on the surface of the optical probe is very small and the energy density is high,
Information can be recorded with high density and high SN ratio. For example, if an optical probe with a spot diameter of 1 nm is used, the recording density will be 1 bit / nm2, that is, 1 Tbit / mm2, and 2400 Tbit per 3.5 inch disc.
Can be recorded.

[0043]

By using an axicon grating for the optical probe of the optical near-field microscope, it is possible to generate an evanescent wave having an arbitrarily short wavelength. Therefore, there is an effect of improving the lateral resolution and height resolution of the microscope.

The spot diameter of the evanescent field is
It can be arbitrarily reduced, the energy efficiency of the interaction with the sample is improved, and the sample with a high SN ratio can be measured.

Further, depending on whether the axicon grating is formed on the axicon prism or the grating is provided with a blaze, the central spot can be made thin and the side lobe can be lowered.

By using the optical probe of the present invention in the optical pickup, the spot can be made smaller and the energy density of the central spot can be increased. This has the effects of increasing the recording density and enabling reading and writing with a high SN ratio.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an example of an optical probe using an axicon grating.

FIG. 2 is a field intensity distribution on an optical probe by an axicon grating.

FIG. 3 is an explanatory view showing an example of an optical probe in which an axicon grating is provided with a blazing.

FIG. 4 is an explanatory diagram showing an example of an optical probe in which an axicon grating is provided on an axicon prism.

FIG. 5 is an explanatory diagram showing an application example to a reflection type near field microscope.

FIG. 6 is an explanatory diagram showing an application example to a transmission type near field microscope.

FIG. 7 is an explanatory diagram showing an application example to an infrared Fourier spectroscopy apparatus.

FIG. 8 is an explanatory diagram showing an application example to an optical pickup.

[Explanation of symbols]

1, 2 axicon grating 3 axicon prism 12, 13, 14 Evanescent wave 4 Optical probe 26 light source 28 Lock-in amplifier 29 Stage drive 30 stages 31 Cassegrain objective

Claims (4)

[Claims] 1. An optical probe having a diffraction grating that generates an evanescent wave. 2. The optical probe according to claim 1, wherein the diffraction grating is provided on a conical surface of a conical prism. 3. The optical probe according to claim 1, wherein the diffraction grating is provided with a blaze. 4. The optical probe according to claim 1, wherein the light incident on the diffraction grating is a convergent light or a Gaussian beam.