JP2006220983A - Optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element having high transmission efficiency with respect to the optical element wherein intensity of light transmitted through a fine aperture is reinforced. <P>SOLUTION: In the optical element which has first and second surfaces, at least one aperture penetrating from the first surface to the second surface and a conductive film having a regularly changed shape on at least one of the first and the second surfaces and wherein light made incident in at least one surface of the conductive film having the regularly changed shape interacts with a surface plasmon mode on at least one surface of the conductive film to reinforce intensity of light transmitted through the aperture, the conductive film is provided at a side wall part in the aperture. Light transmitted through the aperture is reinforced by forming the conductive film having the same quality as the conductive film formed on the surface also at the side wall part in the aperture. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical element that enhances the intensity transmitted through a minute aperture.

  As a high-density information recording apparatus, various recording media and recording apparatuses such as an optical disk drive, a magneto-optical disk drive, and a hard disk drive (HDD) have been put into practical use. In recent years, coupled with the trend toward digitization of moving images and the trend toward miniaturization of apparatuses, there has been an increasing demand for higher recording density and higher capacity of these recording media.

  In general, the linear recording density of an optical recording medium greatly depends on the wavelength of the laser beam of the reproducing optical system and the numerical aperture NA of the objective lens. That is, since the beam waist diameter is determined by determining the wavelength λ of the laser beam of the reproducing optical system and the numerical aperture NA of the objective lens, the spatial frequency of recording pits capable of signal reproduction is limited to about 2 NA / λ. . Therefore, the density increase in the conventional optical disk is limited by the diffraction limit of the laser beam.

  As a method for overcoming the diffraction limit, so-called near-field light technology using an evanescent wave has attracted attention. An evanescent wave having a short wavelength that passes through an opening having a diameter equal to or smaller than the wavelength opened in the metal film is to be used for optical recording / reproducing.

However, the transmission efficiency of light transmitted through a simple aperture in such a metal film depends on the diameter of the aperture. When the aperture diameter is less than the wavelength of light transmitted through the aperture, the transmittance is (d / λ) ⁴ . Proportional. Therefore, since transmission of light through a minute aperture is severely attenuated, there is a problem that a signal-to-noise ratio that is too low for reading and a sufficient light intensity necessary for writing cannot be obtained. As a result, a practical optical recording / reproducing head using a near-field optical system has not been obtained so far.

  On the other hand, for example, in Patent Documents 1 to 7 and Non-Patent Document 1, the intensity at which light irradiated to a metal film passes through one or more openings having a diameter equal to or smaller than the wavelength provided on the metal film is periodic. By arranging in a simple arrangement, or by providing a periodic surface shape on the metal film in conjunction with at least one opening, it can be significantly increased compared to when there is no periodic opening or surface shape A technique that can be performed is disclosed.

  The enhancement of the transmittance of light transmitted through the aperture array occurs when light incident on the metal film interacts with the surface plasmon mode in a resonant manner. The surface plasmon mode (hereinafter, simply referred to as “surface plasmon”) is an excited state of collective electrons existing at the interface between a metal and an adjacent dielectric medium. The structure in which the aperture array is regularly arranged enables coupling (resonant interaction) between the surface plasmon mode and the incident light. In a conventional optical element with an aperture array, the aperture array has been characterized only as an aperture for transmitting light.

  However, in applications such as optical disk recording / reproduction, it is desirable to achieve higher order transmission as shown in the aperture row with a single aperture or a small set of apertures. It is also desirable to further increase the transmission of the aperture row. However, an aperture device with a diameter less than a wavelength showing sufficient transmission efficiency has not been realized until now. In addition, even in high-efficiency optical transmission using surface plasmons by providing a periodic surface shape on a metal film in association with at least one opening, sufficient optical transmission efficiency is obtained in the conventional manufacturing method. Absent.

  In Patent Document 7, it is considered that the main factor of the decrease in the light transmission efficiency is “surface plasmons are scattered by the surface irregularities present at random”, and in order to improve the light transmission efficiency due to the plasmon effect, There has been proposed a method of reducing surface irregularities present at random other than the periodic surface shape. That is, in an optical element having a conductive film having an opening of a wavelength less than that and a periodic surface shape, and transmitting light passing through the opening by interaction with the surface plasmon, there is an effect of improving surface unevenness. An optical element is disclosed that improves the reduction in light transmission efficiency due to the plasmon effect by inserting an intermediate layer.

Japanese Patent Laid-Open No. 11-72607 JP 2000-1111851 A JP 2000-171763 A JP 2001-133618 A US Pat. No. 6,236,033 US Pat. No. 6,285,020 JP 2003-287656 A “Extraordinary optical transmission through sub-wavelength hole arrays”, Nature, Vol. 391, pp. 667-669 (February 12, 1998))

  As described above, in the optical element disclosed in Patent Document 7 (Japanese Patent Laid-Open No. 2003-287656), the transmission efficiency of transmitted light is improved by having an intermediate layer that has an effect of improving the surface unevenness. However, in the structure of the conventional example, since the intermediate layer 67 is exposed on the inner wall portion 66 of the opening as shown in FIG. 6, the present inventors have a novel problem of adversely affecting the light transmission efficiency. I found Further, considering high-speed recording / reproduction on an optical disc, higher transmission efficiency is required.

  The present invention has been made in view of the above-described conventional problems, and an object thereof is to provide an optical element with high transmission efficiency.

  In order to achieve the above object, the present invention has a first surface and a second surface, has at least one opening extending from the first surface to the second surface, and the first surface and the second surface. A conductive film having a regularly changing shape on at least one of the two surfaces, and light incident on at least one surface of the conductive film having the regularly changing shape is the conductive film. A conductive film on the side wall in the opening as an optical element that interacts with the surface plasmon mode on at least one surface of the conductive film, thereby enhancing the intensity of light transmitted through the opening It is characterized by.

  According to the present invention, the light transmitted through the opening is enhanced by forming a conductive film of the same quality as the conductive film formed on the surface of the side wall in the opening.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

<Embodiment 1>
1A is a schematic configuration diagram of an optical element according to the present invention, and FIG. 1B is a schematic cross-sectional view of an opening. As shown in FIG. 1A, the optical element of the present invention has a first surface 1 and a second surface 2, and at least one opening 3 penetrating from the first surface 1 to the second surface 2. The first surface and the second surface have a conductive film 4 on at least one surface, and the at least one conductive film 4 has a regularly changing shape 5 and is adjacent to the conductive film. Thus, the intermediate layer 6 made of a material different from that of the conductive film is provided, and the conductive film 4 is provided on the inner wall 7 of the opening. The present invention is characterized by having a conductive film on the opening side wall 7 of the opening 3 that penetrates. As a result, the light transmitted through the opening 3 is enhanced. The material of the conductive film constituting at least one of the first surface and the second surface is like all metals or doped semiconductors. Further, a conductive material such as Al, Ag, Au, Pt, Cr, or an alloy containing these is desirable.

The material constituting the intermediate layer 6 is preferably a material having an effect of improving surface properties other than the regularly changing shape 5. Specifically, a material that inhibits crystallization when laminated or alloyed is desirable. More specifically, Cu, W, Mo, Nd, Y, Pt, Ti, Ta, Si, Cr, Zr, Nb, C, Ge, CoZr alloy, SiO ₂ , Al ₂ O ₃ , Ta ₂ O ₅ , NiO , TiO ₂ , ZrO ₂ , NbO ₃ , or the like, or a multilayer structure of a material constituting the conductive film 4 and a material having an effect of improving the surface property, or a material constituting the conductive film 4 and a surface property are improved. An alloy with an effective material is desirable.

  The total thickness of the conductive film 4 and the intermediate layer 6 constituting at least one of the first surface and the second surface is desirably optically opaque. That is, it is desirable to be larger than the skin thickness of incident light (thickness such that the intensity of the skin depth electromagnetic field is 1 / e). However, the thickness of the conductive film 4 constituting at least one of the first surface and the second surface is not limited. As the thickness of the conductive film 4 constituting at least one of the first surface and the second surface, it is desirable to select a film thickness that provides optimum characteristics. More desirably, the thickness is not less than the skin thickness.

  As the conductive film 4 formed on the inner wall portion 7 of the opening, it is desirable to use the same material as the conductive film 4 constituting at least one of the first surface and the second surface. The thickness of the conductive film 4 formed on the inner wall 7 of the opening is preferably equal to or greater than the skin thickness of incident light. More preferably, it is more than twice the skin thickness of the incident light.

  The regularly changing shape 5 has raised areas and / or depressed areas, and these areas are arranged with regularity. A region which is lifted and / or pushed down without penetrating the total thickness of the first surface and the conductive film 4 and the intermediate layer 6 constituting at least one surface of the second surface is regularly arranged. It is defined by the term shape that changes. The conductive film 4 and the surface plasmon mode can be strongly coupled to the incident light by the regularly changing shape 5. As a result, the aperture transmits light with high efficiency according to a wave number conservation law that depends on the period of the regularly changing shape 5. Details are disclosed in JP-A No. 2000-111181, JP-A No. 2000-171663, JP-A No. 2003-287656, US Pat. No. 6,236,033 and US Pat. No. 6,285,020.

  Compared with the transmission of light through the same size and the same number of apertures when there is no regularly changing shape 5, the degree of light transmission is much greater. The above exemplary regularly changing shape 5 is merely an example and does not limit the invention. More precisely, the regularly changing shape 5 can have other configurations, and a shape that provides optimum characteristics can be selected. For example, FIG. 2 shows an embodiment in which the regularly changing shape is a concentric periodic array of grooves, and FIG. 3A shows an embodiment in which the regularly changing shape is a periodic array of depressions or protrusions 8. FIG. 3B shows an embodiment in which the regularly changing shape is a one-dimensional periodic array of grooves 38b, and FIG. 3C shows an embodiment in which the regularly changing shape is a two-dimensional periodic array of grooves 38c. Respectively. The regularly varying shape 5 can be chosen to be any shape that allows the surface plasmon mode to be strongly coupled to incident light without departing from the scope of the present invention.

  The surface shape other than the regularly changing shape is preferably 1/10 or less of the wavelength of the incident light. More desirably, it is 1/100 or less of the wavelength of incident light.

  The aperture may be circular, rectangular, elliptical, or any other shape that fits the track pitch and minimum pit length of the optical storage medium, or a slit shape. The aperture size is desirably equal to or less than the wavelength of light incident on the aperture. In the case of the slit shape, it is desirable that the slit width (the width of the shortest dimension of the slit) is equal to or less than the wavelength of the light incident on the aperture. The shape of the opening described above is merely an example, and does not limit the present invention, and a shape that provides optimum characteristics can be selected. Further, there may be a plurality of openings.

  Next, embodiments of the present invention will be described in detail with specific examples, but the present invention is not limited to the following examples without departing from the gist thereof.

  FIG. 2A is a plan view showing a regularly changing shape 25 on the first surface 1 side of the present embodiment. The regularly changing shape 25 of the present embodiment has an opening 23 having a diameter a, and a concentric groove having a period b and a width c. The diameter a is preferably equal to or less than the wavelength of light incident on the opening.

  In the embodiment, the diameter of the opening is defined as an opening-side opening diameter a including a conductive film, as shown in FIG. FIG. 2B is a schematic cross-sectional view showing the configuration of this embodiment. In the present embodiment, the first surface 21 and the second surface 22 are provided with a first conductive film 24a and a second conductive film 24b having shapes that regularly change in phase with each other. However, the present invention is not limited to this configuration, and a conductive film having a regularly changing shape 25 may be provided on only one of the first surface and the second surface. In addition, the first surface and the second surface may have regularly changing shapes different from each other. As a layer configuration, a first conductive film 24 a, an intermediate layer 26, and a second conductive film 24 b are sequentially stacked on the quartz substrate 20. Further, a conductive film 29 is formed on the inner wall portion of the opening.

  In Example 1, the period of the concentric groove structure was 340 nm, the groove width was 170 nm, the groove depth was 50 nm, and the central opening was 50 nm. 20 nm of Al was formed as the conductive film layer 24a, 100 nm of C was formed as the intermediate layer 26, and 20 nm of Al was formed as the conductive film 24b. 3 nm of Al was formed as the conductive film 29 on the inner wall of the opening.

  Next, a creation method of the first embodiment will be described.

A quartz substrate is set in a focused ion beam (FIB) apparatus, and a concentric groove structure is processed using a minimum beam diameter under a vacuum condition of 1 × 10 ⁻⁵ Pa or less. Thereafter, the quartz substrate is set in a molecular beam deposition film forming apparatus, and a first conductive film and an intermediate layer are stacked and formed under a vacuum condition of 1 × 10 ⁻⁸ Pa or less. Again, it is set in the FIB apparatus, and an opening is processed at the center of the concentric groove structure using the minimum beam diameter under a vacuum condition of 1 × 10 ⁻⁵ Pa or less.

Thereafter, the film is returned to the film formation apparatus, and a second conductive film is formed. At this time, the conductive film 29 on the inner wall portion of the opening is also formed at the same time. Since the conductive film is stacked on the first surface side opening quartz, it is set again in the FIB apparatus, and the concentric circle is used with the minimum beam diameter under the vacuum condition of 1 × 10 ⁻⁵ Pa or less. An opening is machined in the center of the groove structure. The film thickness of the conductive film 29 on the inner wall part of the opening is determined by the angle of the side wall part of the opening formed in the first conductive film and the intermediate layer, the film forming condition of the second conductive film, and the second conductive film again. It can be adjusted by changing the processing conditions when the opening is processed after film formation. An appropriate protective film may be deposited on the second conductive film.

  In Example 2, the configuration is the same as that of the sample of Example 1 except that the thickness of the conductive film on the side wall is set to 6 nm.

Example 3 has the same configuration as the sample of Example 1 except that the thickness of the conductive film on the side wall is 2 nm.
[Comparative Example 1]
In Comparative Example 1, the film configuration is the same as that of the sample of Example 1 except that the thickness of the conductive film on the side wall is set to 0 nm. FIG. 4 is a schematic cross-sectional view showing the configuration of the first comparative example. The intermediate layer 6 is exposed on the inner wall portion 47 of the opening.

Next, a production method of this comparative example 1 will be shown. A quartz substrate is set in a focused ion beam (FIB) apparatus, and a concentric groove structure is processed using a minimum beam diameter under a vacuum condition of 1 × 10 ⁻⁵ Pa or less. Thereafter, the quartz substrate is set in a molecular beam deposition film forming apparatus, and a first conductive film, an intermediate layer, and a second conductive film are stacked and formed under a vacuum condition of 1 × 10 ⁻⁸ Pa or less. . At this time, an appropriate protective film may be deposited on the conductive material. Again, it is set in the FIB apparatus, and an opening is processed at the center of the concentric groove structure using the minimum beam diameter under a vacuum condition of 1 × 10 ⁻⁵ Pa or less.

表１に、実施例１、実施例２、実施例３、比較例１の各サンプルにおける波長４００ｎｍ近傍の光に対する透過率を測定し、比較例１の透過効率を基準にした光透過効率の増幅率を示す。光透過効率は以下の式により算出した。

Table 1 shows the transmittance of light in the vicinity of a wavelength of 400 nm in each sample of Example 1, Example 2, Example 3, and Comparative Example 1, and amplification of light transmission efficiency based on the transmission efficiency of Comparative Example 1 Indicates the rate. The light transmission efficiency was calculated by the following formula.

(Amplification factor of light transmission efficiency) = (light intensity emitted from the opening of each sample) / (light intensity emitted from the sample of Comparative Example 1) × 100
From this result, it can be seen that the light transmittance can be increased by forming a conductive film on the side wall of the opening. The Al skin thickness for light having a wavelength of about 405 nm is about 3 nm. From this, it can be seen that the degree of increase in light transmittance is particularly strong when the thickness of the conductive film on the side wall of the opening is equal to or greater than the skin thickness.

  The period of the concentric groove structure was 600 nm, the groove width was 300 nm, the groove depth was 200 nm, and the central opening was 100 nm. As the first conductive film, Ag was formed to 100 nm, as the intermediate layer, C was formed to 100 nm, and as the second conductive film, Ag was formed to 100 nm. The thickness of the conductive film on the side wall was 3 nm. The creation method is the same as in Example 1.

Example 5 has the same configuration as that of the sample of Example 4 except that the thickness of the conductive film on the side wall is set to 5 nm.
[Comparative Example 2]
The film configuration is the same as that of the sample of Comparative Example 1 except that the thickness of the conductive film on the side wall is set to 0 nm. The creation method is the same as in Comparative Example 1.

表２に、波長６２０ｎｍ近傍の光に対する透過率を測定し、比較例２の光透過率を１００としたときの光透過率の増減率を示す。波長６２０ｎｍにおいても、開口の側壁部に導電性膜を形成することで、光透過率を増加させることが可能となることが分かる。又、波長６２０ｎｍ程度の光に対するＡｇの表皮厚さは約２. ５ｎｍである。このことから、波長６２０ｎｍにおいても、開口の側壁部に導電性膜を形成することで光透過率を増加させることが可能となることが分かる。

Table 2 shows the increase / decrease rate of the light transmittance when the transmittance for light in the vicinity of a wavelength of 620 nm is measured and the light transmittance of Comparative Example 2 is 100. It can be seen that even at a wavelength of 620 nm, the light transmittance can be increased by forming a conductive film on the side wall of the opening. The skin thickness of Ag for light having a wavelength of about 620 nm is about 2.5 nm. From this, it can be seen that even at a wavelength of 620 nm, the light transmittance can be increased by forming a conductive film on the side wall of the opening.

<Embodiment 2>
Next, an embodiment of an optical head that uses the optical element of the present invention and floats on a rotating optical storage medium to write / read information at high density will be described.

  The “optical storage medium” used in the description of the present embodiment means any medium on which data is written and / or read using light, such as an optical disk such as a DVD and a CD-ROM and an optical tape, or Includes phase change media as used in other types of optical media such as magneto-optical materials, but is not limited to phase change media (in the case of magneto-optical materials, only writing is optical And reading is performed magnetically).

  Furthermore, the “optical head” used in the description of the present embodiment is a device that accumulates data on an optical storage medium (“write”) and / or retrieves data stored on an optical storage medium (“read”). means. The optical head in the present invention includes one that can perform only reading, only writing, or both reading and writing.

  FIG. 5 shows a schematic view of an optical head manufactured using the optical element of the present invention.

  The optical head 57 is formed on a slider shape for causing the optical head to fly to a predetermined height by the rotation of the optical storage medium 56. Laser light emitted from the laser 55 was introduced through an optical fiber 50 and collimated by arranging a collimator lens 51 made of a microlens. Further, the collimated light changes its optical path in the direction perpendicular to the total reflection mirror 54 and is guided to the optical element 53 according to the present invention by the focus lens 52 placed therebelow. At this time, the light irradiation range centered on the opening on the optical element is a region of 3 to 10 as a period in a regularly changing shape, and is designed in consideration of light use efficiency. It is desirable. At this time, the aperture diameter in the optical element was set to 20 nm to 200 nm.

  In order to reproduce the information recorded on the optical storage medium, the reflected light from the medium can be read out by forming a photodetector near the opening on the second surface of the optical element 53 in the optical head of FIG. it can. Further, a magneto-optical recording medium can be used as the optical storage medium, and the leakage magnetic flux from the medium can be reproduced by a head using a magnetoresistive effect. From this optical head, a recording pattern having the same size as the aperture diameter is formed on the optical storage medium, and it is possible to obtain a reproduced waveform with a good S / N ratio for both light reflection and magnetic reproduction. It was.

  Further, by using the optical element of the present invention, a condenser or a scanning near-field microscope apparatus can be configured. In addition, since the embodiment of the optical element described above is intended for use in recording / reproducing on an optical disk, the case where there is one aperture has been described. However, if a plurality of apertures are arranged periodically, a wavelength selective optical filter (particularly, Other optical components such as UV, visible light, near infrared wavelengths) and photolithography masks can also be constructed.

It is a typical block diagram which shows the optical element of this invention. It is a typical block diagram which shows the optical element of the Example of this invention. It is a figure which shows the example of the shape which changes regularly. It is a typical block diagram which shows the optical element of the comparative example 1 and the comparative example 2 of this invention. It is the schematic of the optical head produced using the optical element of this invention. It is typical sectional drawing which shows the optical element of a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st surface 2 2nd surface 3 Opening 4 Conductive film 5 Shape which changes regularly 6 Intermediate layer 7 Opening inner wall part

Claims (4)

Having a first surface and a second surface, having at least one opening from the first surface through the second surface, and regularly on at least one of the first surface and the second surface A surface plasmon on the at least one surface of the conductive film, wherein the light incident on the at least one surface of the conductive film having the regularly changing shape has a conductive film having a changing shape. An optical element that interacts with a mode and thereby enhances the intensity of light transmitted through the aperture,
An optical element comprising a conductive film on a side wall portion in the opening.   Having a first surface and a second surface, having at least one opening from the first surface through the second surface, and regularly on at least one of the first surface and the second surface A conductive film having a changing shape, and having an intermediate layer made of a material different from the conductive film adjacent to the conductive film, and having the regularly changing shape. An optical element in which light incident on at least one surface of the film interacts with a surface plasmon mode on at least one surface of the conductive film, thereby enhancing the intensity of light transmitted through the aperture. 2. The optical element according to claim 1, further comprising a conductive film on a side wall portion in the opening.   The optical element according to claim 1, wherein the conductive film on the side wall in the opening has a film thickness equal to or greater than a skin thickness of incident light.   2. The optical element according to claim 1, wherein the conductive film on the side wall in the opening has a film thickness that is at least twice the skin thickness of incident light.

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