JP2007207394A - Optical memory head and optical recording and reproducing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical memory head or the like which can perform high speed recording and reproducing and has sufficiently large WD (Working Distance). <P>SOLUTION: The optical memory head has: a light source held between an upper part electrode 101 supplying light of the prescribed wavelength λ, a substrate 102, two distribution Bragg reflection mirrors 103, an active layer 104, and an lower part electrode 105; a minute opening 108 having a smaller diameter than the prescribed wavelength λ; and a negative refraction lens 109 which is constituted of a material indicating negative refraction and converges light from the minute opening 108 to the surface of an information layer 11 being the prescribed plane and/or converges light from the surface of the information layer 112 to a position of the minute opening 108. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an apparatus for recording and reproducing digital information by optical means, and a method thereof.

  In recent years, so-called optical recording media such as Compact Disc (CD) and Digital Versatile Disk (DVD) have become widespread. These optical recording media are used as information media for storing and distributing digital data such as personal computers and digital contents such as movies.

  Furthermore, Blu-ray Disk (BD) and High-Definition Digital Versatile Disk (HD-DVD) are entering the practical stage as next-generation optical recording media. The next-generation optical recording medium can store large-capacity data of 20 to 30 GB in the early stage of development and 100 GB in combination with multilayer recording.

  On the other hand, the capacity of Hard Disk Drive (HDD) is rapidly increasing. High-definition video is also popular. Considering this, next generation optical recording technology having a recording capacity of several hundred GB to 1 TB is expected. For this reason, holographic memory (for example, see Non-Patent Document 1), near-field optical recording, and the like are in the development stage.

  Further, for example, Non-Patent Document 2 describes a technique for producing and reproducing a ROM disk equivalent to 150 GB using a BD and a solid lens. Hereinafter, the optical recording medium is appropriately referred to as “media”.

  A technique for improving the recording capacity by optical recording using near-field recording or a solid immersion lens employs a method of associating physical recording pits with bits of digital information through signal encoding.

  Here, let L be the linear size of a physical recording pit. Then, the recording capacity increases in proportion to the square of the linear size L. On the other hand, the data rate at the time of recording or the data rate at the time of reproduction increases in proportion to the linear size L. For this reason, there arises a problem that the recording or reproducing time per medium increases in proportion to the linear time L.

  As a solution to the problem of an increase in recording time or reproduction time, for example, Patent Document 1 and Non-Patent Document 3 disclose a large amount of data in a vertical cavity surface emitting laser diode (hereinafter referred to as “VCSEL” as appropriate) array. A method for recording and reproducing tracks simultaneously and in parallel has been proposed. This method has problems such as the efficiency of conversion from radiated light to near-field light and the accuracy of tracking a large number of VCSELs. If these problems are solved, it can be considered an effective technique for the above-described problems of recording time and reproduction time.

  Massively parallel near-field optical recording using a VCSEL array will be described with reference to FIGS. FIG. 2 is a schematic view of the VCSEL array 10 as viewed from the front. A large number, for example, N × N VCSELs 11 are installed on the laser substrate 12 in a lattice pattern. Laser light is emitted from the opening of each VCSEL (indicated by a black dot in FIG. 2) in a direction perpendicular to the paper surface.

  FIG. 3 shows a cross-sectional configuration of the VCSEL array 10 in the vertical direction. Three adjacent VCSELs 11 are included. The VCSELs 11 arranged on the laser substrate 12 emit laser light (indicated by dotted lines in the figure). The emitted laser light travels at a certain angle. The microlens 21 provided for each VCSEL converges the traveling laser light. The converged laser beam is condensed near the opening 22.

  The diameter of the opening 22 is set smaller than the wavelength of the laser beam. For this reason, most of the laser light cannot pass through the opening 22 in the state of radiated light. Here, near-field light having a wavelength similar to the diameter of the opening 202 oozes out from the opening 22. Then, recording and reproduction on the optical disc 30 are performed using near-field light.

  The optical disk 30 has a structure in which an information recording layer 32 is provided on a disk substrate 31. Further, a protective layer or the like may be further coated. In order to perform near-field recording, the thickness of the coating needs to be as thin as the diameter of the opening 22, usually several tens of nm.

  For example, as described in Non-Patent Document 4, the region where the near-field light that has oozed out from the opening 22 is relatively condensed is a region that is at most about the diameter of the opening 22. Therefore, in order to obtain the effect of near-field optical recording, the distance between the opening 22 and the information recording layer 32 (working distance, hereinafter referred to as “WD” as appropriate) needs to be several tens of nm. is there.

  For example, Non-Patent Document 1 describes a configuration in which a slice pad 23 is provided or a floating slider technique used in an HDD or the like in order to accurately control WD.

JP 2004-288246 A L. Hesselink et al. , Optical and Quantum Electronics Vol. 25 (9), S611 (1993) Kimihiro Saito et al., Optics Japan 2005, Lecture Proceedings, p. 572 Goya Akiya et al., O plus E Vol. 24 (1), p. 60-71 (2002) Toshiharu Saiki et al., “Nanoscale optical properties” (Ohm, 2004) T. T. et al. W. Ebesen et al. , Nature Vol. 391, p. 667 (1998) J. et al. A. Matteo et al. , Optics Express Vol. 13, p. 636 (2005) J. et al. B. Pendry, Physical Review Letters Vol. 85, p. 3966 (2000) L. Liu et al. , Optics Express Vol. 12, p. 4835 (2004)

  However, in the prior art configuration, the WD is too small relative to the size of the VCSEL array 10. The interval between the VCSELs constituting the VCSEL array 10 is usually 20 μm. For example, the length of one side of a 100 × 100 array reaches about 2 mm, that is, tens of thousands of times of WD.

  For this reason, even if the slice pad 23 is provided, the VCSEL array 10 and the optical disc 30 may collide. As a result, the VCSEL array 10 and the optical disc 30 are damaged. In addition, the tracking control becomes unavoidable. Here, if a flying slider is used, stable tracking control is possible. However, as can be easily imagined from the HDD technology, when a flying slider is used, slight vibrations of air greatly affect the flying operation. Therefore, it is necessary to integrate the head and the recording medium in a strong housing. There is. Therefore, it is necessary to increase the number of parts, to increase the accuracy of processing and assembly, and to integrate the head and the recording medium, and the optical recording characteristics such as low cost and removable are impaired.

  The present invention has been made in view of the above, and an object of the present invention is to provide an optical memory head and an optical recording / reproducing method capable of high-speed recording and reproduction and having a sufficiently large WD.

  In order to solve the above-described problems and achieve the object, according to the present invention, a light source that supplies light having a predetermined wavelength, a minute aperture having a diameter smaller than the predetermined wavelength, and a material exhibiting negative refraction are configured. And an optical element for performing at least one of condensing the light from the minute aperture on the predetermined surface and condensing the light from the predetermined surface to the position of the minute aperture. An optical memory head can be provided.

  According to a preferred aspect of the present invention, it is desirable that the optical element made of a material exhibiting negative refraction is disposed in the vicinity of the minute aperture.

  Moreover, according to the preferable aspect of this invention, it is desirable that the optical element comprised with the material which shows negative refraction is a slab-like lens.

  According to a preferred aspect of the present invention, it is desirable that the thickness of the optical element made of a material exhibiting negative refraction is larger than 20 nm and smaller than 1 mm.

  According to a preferred aspect of the present invention, when the predetermined wavelength is λ, it is desirable that the thickness of the optical element made of a material exhibiting negative refraction is larger than λ / 30.

  According to a preferred aspect of the present invention, when the predetermined wavelength is λ, it is desirable that the maximum value of the width of the minute aperture is smaller than λ / 2.

  According to a preferred aspect of the present invention, it is desirable that the light source has a plurality of light emitting elements arranged in an array.

  According to another aspect of the invention, a light supplying step for supplying light having a predetermined wavelength, a disposing step for disposing a minute aperture having a diameter smaller than the predetermined wavelength, and an optical element made of a material exhibiting negative refraction. And a condensing step for condensing the light from the minute aperture onto the predetermined surface and at least one of condensing the light from the predetermined surface onto the position of the minute aperture. An optical recording / reproducing method can be provided.

  According to the present invention, it is possible to provide an optical memory head capable of high-speed recording and reproduction and having a sufficiently large WD.

  Embodiments of an optical memory head and an optical recording / reproducing method according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

  FIG. 1 shows a cross-sectional configuration of an optical memory head 100 according to Embodiment 1 of the present invention. Between the upper electrode 101 and the lower electrode 105, there are a substrate 102, a distributed Bragg reflector (DBR) 103, an active layer 104, and a distributed Bragg reflector 103 in order from the top in the figure. Has been placed.

  A voltage is applied between the upper electrode 101 and the lower electrode 105. Thereby, a current equal to or higher than the threshold current flows. Then, laser oscillation occurs in the active layer 104 sandwiched between the distributed Bragg reflectors 103. As a result, laser light is emitted from the opening of the lower electrode 105. A VCSEL is configured from the upper electrode 101 to the lower electrode 105 and corresponds to a light source.

  In this embodiment, the configuration of a VCSEL array including a large number of VCSEL groups is used as the light source. For convenience of explanation, FIG. 1 shows one VSCL.

  A microlens 106 and a semiconductor probe 107 are installed further below the lower electrode 105 in the drawing. The semiconductor probe 107 has a minute opening 108 at its lower end. The diameter of the minute aperture 108 depends on the size of a recording pit (mark) to be recorded or reproduced, but is usually about 10 to 100 nm.

  Laser light emitted from the opening of the lower electrode 105 is condensed near the minute opening 108 by the microlens 106. Here, the laser light is visible light having a wavelength of about 400 to 780 nm.

  The diameter of the minute aperture 108 is an order of magnitude smaller than the wavelength. For this reason, the laser beam cannot pass through the minute opening 108. Then, the laser light becomes near-field light and oozes out downward of the microscopic aperture 108 in the drawing.

  Compared with the laser light applied to the minute aperture 108, the amount of the near-field light that has oozed out is weak. For this reason, a configuration in which a fine periodic structure is provided around the microscopic aperture 108 or a configuration in which the shape of the microscopic aperture 108 itself is a fractal is desirable. Thereby, it is possible to further amplify the amount of the near-field light oozing out.

  A negative refraction lens 109 made of a material exhibiting negative refraction is disposed below the semiconductor probe 107 in the drawing. The negative refraction lens 109 has a slab shape.

  Here, the material exhibiting negative refraction refers to a material in which at least one of dielectric constant, magnetic permeability, and refractive index with respect to the wavelength of laser light has a negative value. For example, in the visible light region, there are metal thin films such as gold and silver.

  It is also known that structural materials such as photonic crystals and metamaterials exhibit negative refraction. In this case, the refractive index of the substance constituting the structural material may take a negative value, or the effective refractive index as the structure may take a negative value.

  Here, the “material exhibiting negative refraction” will be further described. The resolution capability of conventional optical systems is mainly limited by the diffraction limit of visible light. Here, an optical material having a negative refractive index (hereinafter referred to as “negative refractive material” as appropriate) is realized. If a negative refraction material is used, ultrahigh resolution imaging (hereinafter referred to as “complete imaging” as appropriate) exceeding the diffraction limit is possible.

  Even when the refractive index takes a negative value, if the real part of the dielectric constant or permeability is a negative value, a negative refraction phenomenon is observed with respect to an electromagnetic wave in a specific polarization state. Further, in the periodic structure such as the photonic crystal described above, the photonic band is folded in the reciprocal lattice space. As a result, although the refractive index, the dielectric constant, and the magnetic permeability are all positive materials, a specific material is used. A negative refraction phenomenon is observed with respect to the electromagnetic wave of a specific polarization state at a wavelength.

  In view of the above circumstances, in this specification, a material that exhibits a negative refraction response to a specific electromagnetic wave is referred to as a “material exhibiting negative refraction”. It goes without saying that the expression “material exhibiting negative refraction” is a broader concept than negative refraction materials.

  In addition to the above-mentioned photonic crystals and metamaterials, materials exhibiting negative refraction include metal thin films, chiral substances, left-handed materials, backward wave materials (Backward Wave Materials), negative phase velocity media (Negative Phase Velocity Material (Medium)) and the like are known.

  As described above, the slab-shaped negative refraction lens 109 using a material exhibiting negative refraction not only has a geometric optical lens action, but also can realize complete image formation without the spread of a point image due to aberration or diffraction. Is known (see, for example, Non-Patent Document 7).

  Also, ideal complete imaging is realized only when the sum of the refractive indexes of the negative refraction lens 109 and the surrounding medium becomes zero. In this embodiment, the laser beam is directly incident on the negative refraction lens 109 from the atmosphere.

  It can be considered that the refractive index of the atmosphere is about 1.0. For this reason, the refractive index of the negative refraction lens 109 is preferably about −1.0. In this specification, the expression “perfect imaging effect” represents the case where the above-described ideal perfect imaging is realized and the case where the imaging performance is higher than the diffraction limit due to the effect of the negative refraction lens 109. .

  Near-field light that oozes out from the minute aperture 108 is imaged on the information layer 112 of the optical disk 110 by the negative refraction lens 109. Under the condition that a complete imaging effect can be obtained, a near-field light spot having the same intensity distribution as that immediately below the vicinity of the minute aperture 108 is reproduced in the information layer 112. Therefore, a minute near-field light spot that can originally be formed only in the vicinity of the minute aperture 108 can be formed at a position away from the minute aperture 108 and the negative refraction lens 109.

  Next, an optical information recording / reproducing method using the optical memory head 100 will be described. First, in a light supply step, light having a predetermined wavelength is supplied. Next, in the arranging step, the minute openings 108 having a diameter smaller than the predetermined wavelength are arranged. In the condensing step, the negative refraction lens 109 condenses the light from the minute aperture 108 onto the surface of the information layer 112 which is a predetermined surface, and the light from the surface of the information layer 112 to the position of the minute aperture 108. At least one of focusing is performed.

  Here, as is apparent from FIG. 1, the geometrical optical imaging condition of the negative refraction lens 109 is that the distance 2d between the minute aperture 108 and the information layer 112 is exactly twice the thickness d of the negative refraction lens 109. It is to become.

  That is, by disposing the negative refraction lens 109 in the vicinity of the minute aperture 108, the distance WD between the surface of the negative refraction lens 109 on the optical disc 110 side and the information layer 112 can be increased.

  The thickness of the negative refraction lens 109 in the direction along the optical axis is desirably larger than 20 nm. Furthermore, preferably this thickness is greater than 50 nm. In addition, this thickness is desirably greater than 100 nm. More preferably, the thickness is greater than 1 μm.

  The thickness of the negative refraction lens 109 in the direction along the optical axis is preferably smaller than 1 mm. Furthermore, when the wavelength of the laser beam is λ, the thickness of the negative refraction lens 109 is preferably larger than λ / 30. In addition, it is desirable that the maximum value of the width of the minute opening 108 is smaller than λ / 2.

  In the near-field optical recording of the prior art, the minute opening 108 provided at the tip of the semiconductor probe 107 and the information layer 112 of the optical disk 110 have to be brought close to a distance of 20 to 50 nm. In this embodiment, when the thickness of the negative refraction lens 109 is larger than 50 nm, a WD comparable to this thickness can be obtained. Thus, this example has a clear advantage over the prior art.

  The thickness of the negative refraction lens 109 can be further increased to, for example, 100 nm or more. Thereby, WD can be further increased. Therefore, the degree of freedom for setting the distance between the minute aperture 108 and the negative refraction lens 109 is increased, which is more preferable.

  In this way, by increasing the thickness of the negative refraction lens 109 within a range that does not impair the complete imaging effect, the WD during recording and reproduction can be increased. For example, when the refractive lens 109 is thicker than 1 μm, massively parallel near-field optical recording can be realized without using a slice pad or a slider mechanism. Since the slice pad is not used, the information layer and the optical pickup can be prevented from being damaged. Further, since the slider mechanism is not used, the portability of the information medium can be improved.

  As long as only the WD at the time of recording and reproduction is considered, it is better that the negative refraction lens 109 is thicker. If the thickness of the negative refraction lens 109 is greater than 1 mm, it is not preferable because the complete imaging effect is impaired by light absorption and scattering by the lens itself.

  In a reproduction system that reproduces information recorded by the recording optical memory head 100, the voltage between the electrodes of the light source is monitored by the intensity of light returned to the optical memory head 100. The information recorded on the optical disk 110 can be read by making the change in the voltage between the electrodes correspond to the binary information.

  Specifically, depending on the presence or absence of information bits recorded in the information layer 112, for example, when information bits are present, the information bits are set in a crystalline state having a high reflectance. The light reflected by the information bit enters the light source through the micro aperture 108 and the micro lens 106. And the voltage between electrodes is monitored by the strength of the light returned to the light source.

  In contrast, when there is no information bit, the information bit is set in an amorphous state having a low reflectance. In the region where no information bit exists, the impedance change of the laser element as the light source is small. For this reason, even if the light reflected by the information bit enters the light source, the change in the electrode voltage of the light source is reduced. Therefore, the voltage between the electrodes of the light source can be monitored and these can be made to correspond to the binary information. As a result, information recorded by the recording optical memory head can be read.

  At the time of reproduction, each VCSEL is constantly supplied with an injection current to monitor the voltage change between the terminals of the electrode caused by the intensity of light returning to the VCSEL. With such a mechanism, it is possible to read information bits recorded on the optical recording medium. Note that either a recording optical memory head and a reproduction optical memory head can be individually configured, or a recording and reproduction can be shared by one optical memory head.

  In this embodiment, the microlens 106 and the semiconductor probe 107 are used to irradiate the minute aperture 108 with laser light. However, the effect of the present invention is not limited to this configuration. For example, laser light may be propagated using a waveguide or a reflection optical system.

  In addition, the configuration may be such that the microlens 106 is not included as long as the near-field light amount necessary for recording or reproduction can be obtained. Furthermore, if the electrode and the distributed Bragg reflector can be designed to have a higher light collecting property, a configuration in which the opening of the lower electrode 105 also serves as the minute opening 108 becomes possible. As a result, a very simple and low-cost VCSEL array can be realized.

  In this embodiment, the configuration is a VCSEL array composed of a large number of VCSEL groups. Here, it goes without saying that the effect of increasing the WD by disposing the negative refraction lens 109 does not depend on the number of VCSELs. In particular, even when there is only one VCSEL, it is clear that a WD of 20 to 50 nm brings about practical effects of the present invention. Therefore, even when there is one VCSEL, the effect of the present invention is sufficiently recognized. Of course, the greater the number of VCSELs, the greater the utility of taking WD.

  In this embodiment, a surface recording type optical disc comprising a substrate 111 and an information layer 112 is used. However, the surface protective layer may be coated as long as the advantage of near-field optical recording, that is, the advantage of a small recording or reproducing beam spot is not impaired.

  The optical disk 110 may be any of a ROM type in which information is engraved by injection molding, a WO (Write Once) type in which a recording layer contains a dye or an alloy, and a rewritable disk type by phase change recording or magneto-optical recording. But it ’s okay. As described above, the present invention can be applied to all information recording media in which the recording density is improved by reducing the recording beam spot and the reproducing beam spot.

  In this embodiment, near-field light is generated independently from laser light oscillated by a plurality of different VCSELs, but the present invention is not limited to this. For example, light from the same light source may be spatially branched, and near-field light may be generated independently from the branched laser light.

  Further, in the above embodiment, the expression “light” is used for the radiation responsible for imaging, but the effect of the present invention is not limited to visible light. Specifically, it is expected for electromagnetic waves in general including radio waves, radio waves, microwaves, terahertz waves, infrared rays, ultraviolet rays, X-rays, γ rays and the like. Also in the embodiment, there is no restriction on the wavelength of the radiation responsible for imaging. Thus, the present invention can take various modifications without departing from the spirit of the present invention.

  As described above, the optical memory head according to the present invention is suitable for an optical system including an optical element made of a material exhibiting negative refraction.

It is a figure which shows schematic structure of the optical memory head based on Example 1 of this invention. It is a figure which shows the structure which looked at the VCSEL array of a prior art from the front. It is a figure which shows the cross-sectional structure of the VCSEL array of a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Optical memory head 101 Upper electrode 102 Substrate 103 Distributed Bragg reflector 104 Active layer 105 Lower electrode 106 Micro lens 107 Semiconductor probe 108 Micro opening 109 Negative refraction lens 110 Optical disk 111 Substrate 112 Information layer

Claims (8)

A light source that supplies light of a predetermined wavelength;
A minute aperture having a diameter smaller than the predetermined wavelength;
It is made of a material exhibiting negative refraction, and performs at least one of condensing light from the minute aperture onto a predetermined surface and condensing light from the predetermined surface onto the position of the minute aperture. And an optical element.   The optical memory head according to claim 1, wherein the optical element made of a material exhibiting negative refraction is disposed in the vicinity of the minute aperture.   The optical memory head according to claim 1, wherein the optical element made of a material exhibiting negative refraction is a slab lens.   The optical memory head according to any one of claims 1 to 3, wherein a thickness of the optical element made of a material exhibiting negative refraction is larger than 20 nm and smaller than 1 mm.   5. The optical memory head according to claim 1, wherein when the predetermined wavelength is λ, the thickness of the optical element made of a material exhibiting negative refraction is larger than λ / 30. .   6. The optical memory head according to claim 1, wherein when the predetermined wavelength is λ, the maximum value of the width of the minute aperture is smaller than λ / 2.   The optical memory head according to claim 1, wherein the light source includes a plurality of light emitting elements arranged in an array. A light supply step for supplying light of a predetermined wavelength;
An arrangement step of arranging a microscopic aperture having a diameter smaller than the predetermined wavelength;
At least one of condensing light from the minute aperture onto a predetermined surface and condensing light from the predetermined surface onto the position of the minute aperture by an optical element made of a material exhibiting negative refraction And a light collecting step for performing the optical recording and reproducing method.

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