This invention relates to high density optical memories and, more particularly, to an optical recording/reproducing method utilizing near-field light and recording medium fluorescent light, an optical recording/reproducing recording medium and an optical recording/reproducing apparatus.
In recent years, in order to make [the] optical memories with higher density, there has been a shift in recording/reproducing methods from compact disks (CD) to digital video disks (DVD). For example, the CD in its surface is recorded with pits as concave/convex structures having a size of nearly a wavelength of laser light to be used upon reproducing and a depth of approximately a quarter of the wavelength. For reproducing the information, interference of light is utilized, When a laser light spot is illuminated to the pit, because the pit depth is almost a quarter wavelength, the difference in optical path between the reflection light reflected upon a pit bottom and the reflection light on a surface outside the pit is given a half of the wavelength of the illuminated laser light. Thus, the reflection light available is weak as compared to the case of illuminating a laser light spot to the surface outside the pit. In this manner, the presence or absence of a pit is determined by detecting an intensity of reflection light, thus reproducing information recorded on the CD. Although DVD recording/reproducing is conducted by illuminating a laser light spot to a recording medium, the pit size can be made smaller than the CD by shortening a laser wavelength to be used thus enabling higher density recording/reproducing.
Meanwhile, there is known a near-field optical microscope for observing a microscopic surface texture of a sample by using a probe having a microscopic aperture with a diameter of an illumination light wavelength or smaller, e.g. approximately {fraction (1/10)}th of the wavelength, and through utilizing a near field (evanescent field). In this near-field optical microscope, the probe microscopic aperture and the sample surface are approached in distance to nearly the diameter of the probe microscopic aperture so that a near field caused on the sample surface due to illumination of propagation light from a sample backside is detected by the probe. In this case, the near field caused on the sample surface involves an intensity and phase reflecting a sample surface microscopic texture. This near field is scattered through the probe microscopic aperture and extracted as propagation light, and received by a photodetector, thereby achieving a resolution unrealized in the conventional optical microscope.
Consequently, the utilization of the near-field optical microscope technology as stated above makes possible recording/reproducing exceeding the recording density on the conventional information recording medium such as a CD or a DVD.
For high density recording/reproducing, there is a reduction of bit size and multi-valued bit information. It is possible for the DVD to conduct recording/reproducing with higher density than the CD by shortening the illumination light wavelength and thereby reducing the bit size. However, because of using means to reduce a spot diameter through a lens, the illumination light spot diameter onto the recording medium cannot be reduced to a half of the wavelength or smaller due to a light diffraction limit. Due to this, if the bit size becomes a half wavelength of illumination light or smaller, it is impossible for the conventional optical system to record/reproduce information. Consequently, for recording/reproducing with higher density than the DVD, there is a need for shortening a wavelength to be used. Also, the CD and DVD obtain information only of 0, 1 from each bit without being multi-valued.
Meanwhile, if utilizing a near field that can reduce the spot diameter to a half of an illumination wavelength or smaller exceeding the light diffraction limit, high density recording/reproducing is feasible due to reducing the bit size. However, there has been no proposal to render the information from each bit multi-valued in order to provide higher density. Therefore, it is an object of the present invention to reduce the bit size by using a near field and at the same-time to provide a scheme for recording/reproducing multi-valued information for each bit, in order to realize high density recording/reproducing. Also, another object is to obtain a recording medium and apparatus for recording/reproducing.
In order to achieve the above objects, an optical recording method according to the present invention illuminates a near-field light onto a recording medium having at least one or more types of phosphors, to fade fluorescence of a particular phosphor in a microscopic region on the recording medium. The use of near-field light can reduce the size of one recording bit as compared to the conventional optical system using a lens. Also, although the conventional method was recording of binary values of 0, 1 for one recording bit, among a plurality of kinds of phosphors, fluorescence of a particular phosphor only fades to record the presence or absence of fluorescence, rendering the information amount on one recording bit multi-valued. From these, higher density recording than the conventional method is feasible.
Also, in an optical recording method according to the invention a near-field light having a corresponding wavelength to a particular phosphor is illuminated onto a recording medium having at least one or more types of phosphors, to fade fluorescence of the particular phosphor in a microscopic region on the recording medium. The use of near-field light can reduce the size of one recording bit as compared to the conventional optical system using a lens. Where fading fluorescence of one kind of a particular phosphor, the fluorescence is faded by illuminating a light having a corresponding wavelength onto this phosphor. At this time, fluorescence of other phosphors does not fade. Also, where fading fluorescence of a plurality of kinds of phosphors, a corresponding wavelength of light to each phosphor is illuminated separately or simultaneously, thereby fading fluorescence of them. At this time, fluorescence of the phosphor not corresponding to the wavelength dose not fade. In this manner, arbitrary phosphors are changed in fluorescence whereby recording to one bit can be rendered multi-valued. From this, higher density recording than the conventional method is feasible.
Also, in an optical recording method according to the invention a near-field light is illuminated onto a recording medium having at least one or more types of phosphors and the near-field light is changed in light amount, to fade fluorescence of the particular phosphor in a microscopic region on the recording medium. The use of near-field light can reduce the size of one recording bit as compared to the conventional optical system using a lens.
Also, fluorescence is faded in the order of fluorescence readier to be photooxidized due to an illumination amount of near field light among a plurality of kinds of phosphors contained in a thin film, whereby the information amount of one recording bit can be rendered multi-valued. Also, where two kinds of phosphors are the same in light wavelength for fading fluorescence, it is impossible to vary the wavelength to fade fluorescence of either one of them. However, because the method of changing the illumination amount is not dependent upon the wavelength, one of the phosphors can be faded in fluorescence even where using phosphors the same in light wavelength to fade fluorescence. From these, higher density recording is feasible than in the conventional.
Also, in an optical reproducing method according to the invention a near-field light is illuminated onto a recording medium having at least one or more types of phosphors and recorded with data microscopically by the presence or absence of fluorescence on a particular one of the phosphors, to specify a fluorescent one of the phosphors from a spectrum obtained. By detecting from a spectrum the presence or absence of fluorescence on each phosphor contained in a recorded bit, it becomes feasible to reproduce multi-valued information recorded on a microscopic bit.
Also, in a recording medium according to the invention a thin film containing at least one or more types of phosphors is formed on a substrate. By doing so, a recording medium for optical recording/reproducing is obtained which is increased in density by rendering information recording/reproducing multi-valued.
Also, in a recording medium according to the invention thin films each having only one type of phosphor are formed on a substrate depending on the kind of phosphor. Because each kind of phosphor is isolated, energy transfer between different phosphors is suppressed so that fluorescent light can be obtained with efficiency. By doing so, a recording medium for optical recording/reproducing is obtained which is increased in density by rendering information recording/reproducing multi-valued.
Also, a recording medium according to the invention forms an insulation film between the thin films formed. This causes energy transfer only between the phosphors to thereby increase excitation efficiency, increasing fluorescent efficiency, obtaining intense fluorescent light and improving the S/N ratio during reproduction. By doing so, a recording medium for optical recording/reproducing [could be] obtained which is increased in density by rendering information recording/reproducing multi-valued.
Also, a recording medium according to the invention has on the substrate a metal film on a side formed with the thin films. By doing so, the excitation light illuminated to a recording medium surface transmits through each thin film, and is thereafter reflected on the metal film and again, illuminates the thin film, improving excitation efficiency and increasing fluorescent intensity. By doing so, a recording medium for optical recording/reproducing is obtained which is increased in density by rendering information recording/reproducing multi-valued.
Also, an optical recording apparatus according to the present invention comprises: a recording medium having phosphors, a head for producing a near-field light, an adjusting mechanism for adjusting a wavelength and/or a light intensity of the near-field light, and an approaching mechanism to cause the recording medium and fluorescence of the head to approach. Due to this, a near-field light can be illuminated only to a microscopic region on the recording medium, and fluorescence of the phosphors contained in the microscopic region of the recording medium can be faded in fluorescence by using a near-field light, obtaining a high density optical recording apparatus.
Also, an optical reproducing apparatus according to the invention comprises a recording medium having phosphors, a head for producing a near-field light to excite the phosphors, a mechanism for specifying a fluorescent one of the phosphors from a spectrum of the recording medium, and an approaching mechanism to cause the recording medium and the head to approach. Due to this, a high density optical reproducing apparatus is obtained which illuminates a near-field light to a recording medium having at least one kind or more of phosphors and microscopically faded in particular fluorescence of the phosphors to specify a fluorescent phosphor from an obtained spectrum.