Optical disk drive and super-resolution reproduction method for optical disk

An optical disk drive for reproducing an optical disk having a recording layer and a super-resolution film provided on a reproduction-beam incident side with respect to the recording layer, the super-resolution film being formed of such a material that an optical aperture formed by irradiation of a reproduction beam remains after the reproduction beam passes. The optical disk drive comprises a reproduction light source for generating the reproduction beam, a reproduction optical system for detecting a reproduction beam having been incident to the recording layer via the optical aperture formed in the super-resolution film, and reflected from the recording layer, and an initialization light source, provided at a back of the reproduction light source in a track direction of the optical disk, for generating an initialization beam for closing the optical aperture remaining on the super-resolution film.

BACKGROUND OF THE INVENTION
 The present invention relates to an optical disk drive and a
 super-resolution reproduction method for an optical disk.
 Optical disk memories, which accomplish information reproduction alone or
 information recording and reproduction by irradiation of a light beam,
 have been put to practical use, as high-capacity, fast-access and portable
 storage media, in various files, such as audio data, image data and
 computer data. It is expected that development of those memories will
 continue. There may be several schemes available to increase the density
 of optical disks, such as shortening the wavelength of a gas laser for
 cutting a master, shortening the wavelength of a semiconductor laser as an
 operational light source, increasing the numerical aperture of an
 objective lens and making optical disks thinner. With regard to recordable
 optical disks, various other approaches are possible: mark length
 recording, and land/grove recording.
 As a scheme having a great effect on improvement on the density of optical
 disks, a super-resolution reproduction technique which uses a medium film
 has been proposed. The super-resolution reproduction was originally
 proposed as a scheme specific to magneto-optical disks. In the
 super-resolution reproduction for magneto-optical disks, a magnetic film
 (super-resolution film) is provided on the reproduction-beam incident side
 with respect to the recording layer so as to cause exchange coupling or
 magnetostatic coupling between them. Then, a reproduction beam is
 irradiated to raise the temperature of the super-resolution film to change
 the exchange force or magnetostatic force, thereby forming an optical
 aperture or optical mask in the super-resolution film to realize
 super-resolution reproduction.
 Later, for ROM disks in addition to MO disk, was reported an attempt to
 provide a super-resolution film whose light transmittance varies with the
 irradiation of a reproduction beam, on the reproduction-beam incident side
 with respect to the recording layer for the purpose of super-resolution
 reproduction. It has become obvious that super-resolution reproduction can
 be adapted to all optical disks like a magneto-optical disk, CD-ROM, CD-R,
 WORM and a phase change optical disk.
 Implementation of super-resolution reproduction of optical disks requires
 that the transmittance of the super-resolution film should change by a
 significant amount with practical reproduction power, an optical aperture
 should be formed fast in as a short period as the pass time of the
 reproduction beam spot, and repeated reproduction should be accomplished
 stably. Prior arts cannot however meet all of those requirements.
 BRIEF SUMMARY OF THE INVENTION
 Accordingly, it is an object of the present invention to ensure
 super-resolution reproduction of an optical disk under practical
 conditions and achieve high density of an optical disk.
 An optical disk drive according to this invention is designed to reproduce
 an optical disk having a recording layer and a super-resolution film
 provided on a reproduction-beam incident side with respect to the
 recording layer, the super-resolution film being formed of such a material
 that an optical aperture formed by irradiation of a reproduction beam
 remains after the reproduction beam passes, and comprises a reproduction
 light source for generating the reproduction beam; a reproduction optical
 system for detecting a reproduction beam having been incident to the
 recording layer via the optical aperture formed in the super-resolution
 film, and reflected from the recording layer; and an initialization light
 source, provided at a back of the reproduction light source in a track
 direction of the optical disk, for generating an initialization beam for
 closing the optical aperture remaining on the super-resolution film.
 A super-resolution reproduction method using this optical disk drive
 comprises a step of irradiating a reproduction beam to form an optical
 aperture in the super-resolution film, a step of detecting a reproduction
 beam having been incident to the recording layer via the optical aperture,
 and reflected from the recording layer, and a step of irradiating an
 initialization beam to close the optical aperture remaining on the
 super-resolution film prior to irradiation of the next reproduction beam.
 A super-resolution reproduction method according to this invention
 reproduces an optical disk having a recording layer and a super-resolution
 film provided on a reproduction-beam incident side with respect to the
 recording layer, the super-resolution film being comprised of a field
 control film which is demagnetized by irradiation of a reproduction beam
 and a magnetization change film whose magnetization direction changes due
 to an influence of a magnetic field of the field control film, and
 comprises the steps of irradiating a polarized reproduction beam to change
 magnetizations of the field control film and the magnetization change
 film, thereby rotating a polarization plane of the polarized reproduction
 beam; and detecting the polarized reproduction beam having been incident
 to the recording layer via the field control film and the magnetization
 change film, and reflected from the recording layer.
 Another optical disk drive according to this invention reproduces an
 optical disk having a recording layer and a super-resolution film provided
 on a reproduction-beam incident side with respect to the recording layer,
 the super-resolution film being comprised of a photoconductive film which
 becomes conductive with irradiation of a reproduction beam, a switching
 film for producing an optical aperture when applied with an electric field
 equal to or greater than threshold strength, and a pair of conductive
 films for applying an electric field to a stack of the photoconductive
 film and the switching film, and comprises a power supply for applying the
 electric field to the stack of the photoconductive film and the switching
 film through the pair of conductive films; a reproduction light source for
 generating the reproduction beam; and a reproduction optical system for
 detecting a reproduction beam having been incident to the recording layer
 via the optical aperture formed in the switching film, and reflected from
 the recording layer.
 A super-resolution reproduction method using this optical disk drive
 comprises a step of irradiating a reproduction beam to form an optical
 aperture in the switching film while applying an electric field to the
 stack of the photoconductive film and the switching film via the pair of
 conductive films, and a step of detecting a reproduction beam having been
 incident to the recording layer via the optical aperture and reflected
 from the recording layer.
 Additional object and advantages of the invention will be set forth in the
 description which follows, and in part will be obvious from the
 description, or may be learned by practice of the invention. The object
 and advantages of the invention may be realized and obtained by means of
 the instrumentalities and combinations particularly pointed out in the
 appended claims.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention will now be described more specifically.
 To begin with, the principle of super-resolution reproduction will be
 discussed. Super-resolution reproduction has two general modes: heat mode
 and photon mode. A super-resolution film which is one constituent of an
 optical disk according to this invention operates in one of the modes. In
 heat mode, a super-resolution film made of a material whose transmittance
 varies with the temperature is used, and an optical aperture with a high
 transmittance is formed in an area whose temperature has risen by the
 irradiation of a reproduction beam. In photon mode, a super-resolution
 film made of a material whose transmittance varies with the number of
 photons to be irradiated is used, and an optical aperture with a high
 transmittance is formed in an area which has a large number of photons in
 a reproduction beam spot.
 Referring to FIGS. 1 and 2, the relationship among marks recorded on the
 recording layer, the reproduction spot and the optical aperture of the
 super-resolution film will be explained. FIG. 1 shows the case of the heat
 mode, while FIG. 2 shows the case of the photon mode. TR.sub.i-1, TR.sub.i
 and TR.sub.i+1 are three adjacent recording tracks. M denotes a recorded
 mark, with the subscription i indicating the track where the mark is
 recorded while the subscriptions j-1, j and j+1 are arbitrary numbers
 indicating the order of the recording marks in an associated track. Sr
 indicates the reproduction spot, and A the aperture of the
 super-resolution film. As the recording beam has a large power level, a
 sufficiently large optical aperture is formed in the super-resolution film
 at the time of irradiation of the recording beam most of which reaches the
 recording layer to form a recorded mark. Those diagrams show the case
 where recording marks are recorded at narrower mark pitches and narrower
 track pitches than those of an ordinary medium which has no
 super-resolution film.
 In the ordinary reproduction method, all the recording marks in the
 reproduction spot Sr contribute to a reproduction signal. Let us consider
 the case where a recorded mark M.sub.i,j is to be reproduced. In this
 case, M.sub.i,j-1, on the same track contributes to the reproduction
 signal, so that an inter-symbol interference occurs. As M.sub.i-1 and
 M.sub.i+1 on adjacent tracks contribute to the reproduction signal, a
 crosstalk occurs.
 As shown in FIGS. 1 and 2, when an area with a high transmittance or the
 aperture A is formed in the super-resolution film, the reproduction beam
 is irradiated onto the recording layer via the aperture A. Because the
 other areas of the super-resolution film than the aperture A have a low
 transmittance, substantially the reproduction beam is not irradiated on
 the recording layer. Therefore, what contributes to the reproduction
 signal is only the recorded mark M.sub.i,j present in the portion where
 the reproduction spot Sr overlaps the aperture A formed in the
 super-resolution film.
 As shown in FIG. 1, an elliptical aperture A (more precisely, one having
 the shape of a droplet with a wide portion lying closer to the
 reproduction spot) is formed in the super-resolution film in heat mode,
 shifted rearward (rightward in FIG. 1) along the track direction with
 respect to the reproduction spot Sr. This occurs due to a time delay to
 the rising of the temperature of the super-resolution film from the
 irradiation of the reproduction spot when the disk is rotated fast. As
 regards the super-resolution film in photon mode, as shown in FIG. 2, the
 number of photons to be irradiated is large near the center portion of the
 reproduction spot Sr (the number of photons indicates a Gaussian
 distribution), so that a circular aperture A is formed.
 FIG. 3 shows the relationship between the temperature (T) and transmittance
 (Tr) of a super-resolution film in heat mode. As indicated by the solid
 line in this figure, the transmittance Tr is, for example, 50% which is
 low, at an ambient temperature Ta (near the room temperature). When the
 reproduction beam is irradiated, the transmittance Tr increases up to, for
 example, 90%, forming an aperture, as the temperature of the
 super-resolution film rises.
 The prior art was premised on, for the super-resolution film, the selection
 of a material whose response speed from the beginning of the irradiation
 of a reproduction beam to the point of transmittance increase is fast and
 whose transmittance promptly returns to a value of the point before the
 irradiation of the reproduction beam after the reproduction beam passes.
 The curves indicated by the broken lines in FIG. 3 show the characteristics
 of a material whose transmittance does not return to a value of the point
 before temperature increase and has a higher value than the former value,
 when the cooling time .DELTA.t after the temperature increase of the
 super-resolution film is short. From the conventional selection criteria,
 the material showing the broken-line characteristics in FIG. 3 is
 inadequate for a super-resolution film. This is because when the linear
 velocity of an optical disk is fast and the cooling time is short, a part
 of an aperture formed in a super-resolution film, made of such a material,
 by the irradiation of a reproduction beam remains open which is considered
 inconvenient for repetitive reproduction. This invention can permit a
 super-resolution film to be formed of a material which has a hysteresis in
 transmittance and extends the freedom of material selection and media
 design.
 Reproduction from an optical disk having such a super-resolution film is
 implemented by using an optical disk drive which comprises an a
 reproduction light source for generating a reproduction beam, a
 reproduction optical system for detecting a reproduction beam having been
 incident to the recording layer via an optical aperture formed in the
 super-resolution film, and reflected from the recording layer, and an
 initialization light source, provided at a back of the reproduction light
 source in a track direction of the optical disk, for generating an
 initialization beam for closing the optical aperture remaining on the
 super-resolution film.
 Super-resolution reproduction by this optical disk drive is carried out as
 follows. First, a reproduction beam is irradiated on the super-resolution
 film to form an optical aperture, a reproduction beam is irradiated on the
 recording layer via the optical aperture and the reproduction beam
 reflected from the recording layer is detected for reproduction. Then,
 before irradiation of the next reproduction beam, the remaining optical
 aperture is closed by an initialization beam. This method can adequately
 accomplish repetitive reproduction.
 Materials which have the characteristics indicated by the broken lines in
 FIG. 3 include a phase change material, an organic material and a liquid
 crystal material. A description will now be given of a case where a phase
 change material which has a long crystallization time is used for a
 super-resolution film and the pass time of a beam spot is shorter than the
 crystallization time.
 FIG. 4A shows the optical state of an optical disk before initialization of
 the super-resolution film, and FIG. 4B depicts an initialization beam spot
 I and the optical state of an optical disk after the initialization of the
 super-resolution film. As shown in FIG. 4A, at least a part of an aperture
 A formed by a reproduction operation remains open before initialization,
 and recording marks are seen through the remaining aperture A. As shown in
 FIG. 4B, the initialization beam spot I has, for example, an elliptical
 shape extending along the track direction, and has enough heating and
 cooling times to crystallize the super-resolution film. After the
 initialization beam spot I passes, the transmittance of the
 super-resolution film drops and returns to the original, low level of the
 solid line in FIG. 3. This can permit adequate super-resolution
 reproduction in the next reproduction operation. Note that even a circular
 spot can be employed if the power of the initialization beam is set
 properly.
 This invention allows a super-resolution film to be formed of a material
 which has such characteristics that the transmittance decreasing speed
 after the passing of the reproduction beam is slow as well as the
 transmittance increasing speed from the beginning of the irradiation of
 the reproduction beam is slow and so the response in forming an optical
 aperture is slow. From the conventional selection criteria, such a
 material is also inadequate for a super-resolution film for the following
 reason. Because it takes time for the transmittance of the
 super-resolution film to increase from the point of the irradiation of a
 reproduction beam, when the linear velocity of an optical disk is fast, an
 aperture is formed after the reproduction beam passes, making reproduction
 itself impossible.
 Materials which show a slow increase in transmittance by the irradiation of
 a light beam and a slow response in forming an optical aperture include a
 chalcogen-based material like As--Se--Ge, and an organic material. Those
 materials have not been used for a super-resolution film though they
 exhibit large changes in transmittance.
 Reproduction from an optical disk having such a super-resolution film is
 carried out by using an optical disk drive which comprises a pre-beam
 light source, provided in front of the reproduction light source in the
 track direction, for generating a pre-beam for forming an optical
 aperture, in addition to the reproduction light source, the reproduction
 optical system and the initialization light source.
 Super-resolution reproduction by this optical disk drive is executed as
 follows. First, a pre-beam is irradiated on the super-resolution film to
 form an optical aperture. Then, a reproduction beam is irradiated on the
 recording layer via the optical aperture and the reproduction beam
 reflected from the recording layer is detected for reproduction. Then,
 before irradiation of the next reproduction beam, the remaining optical
 aperture is closed by the irradiation of an initialization beam.
 In this case, reproduction is conducted with the reproduction beam spot
 overlapping the optical aperture as shown in FIG. 1 by adjusting the time
 for the optical aperture to be formed since the irradiation of the
 pre-beam and the time interval between the irradiation of the pre-beam and
 the irradiation of the reproduction beam. The former response time to the
 formation of an optical aperture can be known by previously testing a
 material to be used for the super-resolution film. The latter time
 interval is obtained by (the distance between the pre-beam light source
 and the reproduction light source)/(the linear velocity of the disk) and
 can thus be properly set. This method permits the super-resolution film to
 be formed of a material which has a slow response to the formation of an
 optical aperture and has conventionally considered inadequate for the
 super-resolution film.
 Note that the initialization light source need not be provided when a
 material which shows a slow response to the formation of an optical
 aperture since the point of the irradiation of a reproduction beam but
 shows a fast response to the closing of the optical aperture after the
 passing of the reproduction beam.
 According to this invention, super-resolution reproduction may be carried
 out by using an optical disk with a super-resolution film comprised of a
 field control film which is demagnetized by irradiation of a reproduction
 beam and a magnetization change film whose magnetization direction changes
 due to the influence of a magnetic field of the field control film.
 A super-resolution reproduction method for this optical disk comprises a
 step of irradiating a polarized reproduction beam to change magnetizations
 of the field control film and the magnetization change film, thereby
 rotating the polarization plane of the polarized reproduction beam, and a
 step of detecting the polarized reproduction beam having been incident to
 the recording layer via the field control film and the magnetization
 change film, and reflected from the recording layer.
 A description will be given of a case where a super-resolution film
 consists of, for example, the stack of a field control film 32 of
 perpendicular magnetization and a magnetization change film 33 of
 longitudinal magnetization as shown in FIG. 5. In this diagram, the
 directions of magnetization are indicated by arrows. Used as the field
 control film 32 is a perpendicular magnetization film such as garnet, rare
 earth-transition metal alloy and Co--Pt-based multilayer film. As the
 magnetization change film 33 is used a longitudinal magnetization film
 such as ferrite and Co-based alloy.
 FIG. 6A is a diagram depicting the temperature (T) dependency of saturation
 magnetization (Ms) of the field control film 32. At the ambient
 temperature Ta (near the room temperature), the field control film 32 has
 a high saturation magnetization. The saturation magnetization of the field
 control film 32 drops as the temperature rises, and the film 32 loses its
 magnetization at the Curie point Tc.
 When the magnetization of the field control film 32 is spatially uniform at
 the temperature Ta, the film 32 is substantially equivalent to an
 evenly-magnetized infinite flat plate and does not generate a magnetic
 field outside. When a reproduction beam is irradiated, the field control
 film 32 absorbs a part of the reproduction beam and increases its
 temperature. The temperature distribution of the field control film 32
 accords to the intensity distribution (approximately a Gaussian
 distribution) of the reproduction beam. Therefore, the saturation
 magnetization Ms of the field control film 32 (represented by the
 perpendicular arrows to the film surface) exhibits a distribution as shown
 in FIG. 6B. Specifically, at the center of the spot, the temperature of
 the field control film 32 is increased to or above the Curie point and the
 film 32 is demagnetized. Because of such a magnetization distribution, the
 field control film 32 generates a magnetic field Hl outside.
 Under the initial state with no reproduction beam irradiated, the
 magnetization of the magnetization change film 33 stacked on the field
 control film 32 is directed in the in-plane direction. However, a leak
 field Hl is produced from the field control film 32 with the irradiation
 of a reproduction beam. When this leak field Hl exceeds the coercive
 force, the magnetization of the magnetization change film 33 is changed
 toward the direction of Hl (nearly perpendicular direction) from the
 in-plane direction. When the reproduction beam passes and the field
 control film 32 is cooled, the magnetization of the magnetization change
 film 33 is directed in the in-plane direction again at the cooled
 position.
 In this case, if linearly polarized light is used as a reproduction beam,
 the plane of polarization of the light is rotated in accordance with the
 direction of the magnetization of the magnetization change film 33 due to
 the Faraday effect. Further, an analyzer is provided in the reproduction
 optical system and the transmission axis of the analyzer is set coincident
 with the polarization plane of a polarized reproduction beam when the
 magnetization of the magnetization change film 33 is directed to the
 perpendicular direction. This is equivalent to an increase in
 transmittance of the super-resolution film with a stacked structure in the
 reproduction detection system.
 FIG. 7 shows the relationship between the magnetic field Hl to be applied
 to the magnetization change film 33 and the transmittance of the
 magnetization change film 33. This relation permits super-resolution
 reproduction to be executed in the situation as shown in FIG. 1.
 According to this invention, a super-resolution film comprised of a
 single-layer perpendicular magnetization film 32 may be used as shown in
 FIG. 8. This perpendicular magnetization film 32 is uniformly magnetized
 upward or downward with respect to the film surface for initialization. In
 the initial state, the polarization plane of the linearly polarized light
 is rotated in accordance with the direction of the magnetization of the
 perpendicular magnetization film 32 due to the Faraday effect. Further, an
 analyzer is provided in the reproduction optical system to set the
 intensity of the transmitted light low when the perpendicular
 magnetization film 32 is in the initial state. When the reproduction beam
 is irradiated, the temperature of the perpendicular magnetization film 32
 rises and the film portion near the center of the spot is heated to or
 above the Curie point, thereby causing demagnetization. As this
 demagnetized area does not exhibit the Faraday effect, the transmittance
 of the polarized reproduction beam is high in the demagnetized area and
 low in the surrounding initialized area. A super-resolution reproduction
 operation can be implemented by using this demagnetized area as an optical
 aperture.
 A longitudinal magnetization film may be used in place of the perpendicular
 magnetization film in FIG. 8. In either case, super-resolution
 reproduction can be accomplished by using a magnetization film whose
 Faraday rotation angle differs between the area where a polarized
 reproduction beam is irradiated and the area where no polarized
 reproduction beam is irradiated.
 According to this invention, a super-resolution film having the stacked
 structure of a field control film 32 of perpendicular magnetization and a
 magnetization change film 33' which coupled to this film 32 by exchange
 coupling, as shown in FIG. 9, may be used. In this case, when the field
 control film 32 is initialized, the magnetization change film 33' receives
 exchange force from the field control film 32 and is magnetized in the
 same direction as the field control film 32. When a reproduction beam is
 irradiated on the super-resolution film with such a stacked structure, the
 temperature of the field control film 32 rises and the film portion near
 the spot center is heated to or above the Curie point, thereby causing
 demagnetization. The demagnetized area of the field control film 32 cannot
 generate exchange force to the magnetization change film 33'. Therefore,
 the direction of magnetization of the magnetization change film 33' in an
 area adjacent to the demagnetized area of the field control film 32 is set
 in the direction of the self-leaked magnetic field from the surrounding
 area (or the direction of an external magnetic field applied as needed).
 As the direction of magnetization differs between the area irradiated with
 a polarized reproduction beam and its surrounding area, super-resolution
 reproduction can be implemented on the same principle as has been
 discussed above.
 In this case, after the reproduction beam passes, the magnetization of the
 field control film 32 is restored and so is the exchange force. Even
 without an initialization field applied, therefore, the direction of
 magnetization of the magnetization change film 33' returns to the original
 state. While the directions of magnetization of the field control film 32
 and the magnetization change film 33' are set the same in FIG. 9, if a
 ferrimagnetic film is used as a magnetization change film, the sub-lattice
 magnetization is aligned so that the net magnetization may be directed in
 the opposite direction.
 According to this invention, super-resolution reproduction may be carried
 out by using an optical disk which has a super-resolution film comprised
 of a photoconductive film which becomes conductive with irradiation of a
 reproduction beam, a switching film for producing an optical aperture when
 applied with an electric field equal to or greater than threshold
 strength, and a pair of conductive films for applying an electric field to
 the stack of the photoconductive film and the switching film.
 Reproduction from this optical disk is executed by using an optical disk
 drive which comprises a power supply for applying the electric field to
 the stack of the photoconductive film and the switching film through the
 pair of conductive films, a reproduction light source for generating the
 reproduction beam, and a reproduction optical system for detecting a
 reproduction beam having been incident to the recording layer via the
 optical aperture formed in the switching film, and reflected from the
 recording layer.
 A super-resolution reproduction method using this optical disk drive
 comprises a step of irradiating a reproduction beam to form an optical
 aperture in the switching film while applying an electric field to the s
 tack of the pho toconductive film and the switching film through the pair
 of conductive films, and a step of detecting a reproduction beam having
 been incident to the recording layer via the optical aperture and
 reflected from the recording layer.
 A material for the photoconductive film is not particularly restricted,
 and, for example, a-Si may be used. A material for the switching film is
 not particularly restricted as long as i ts light transmittance varies
 with application of a voltage equal to or greater than the threshold
 value; for example, a liquid crystal may be used as the material.
 FIG. 10 depicts the relationship between the conductivity (.sigma.) of the
 photoconductive film and the intensity (I) of light to be irradiated. FIG.
 11 shows the relationship between the light transmittance (Tr) of the
 switching film and the strength (E) of an electric field to be applied.
 The conductivity observed for the photoconductive film is a photon mode.
 That is, while an area where the light intensity is low contains fewer
 photons to excite electrons to the conductive band so that the
 conductivity of the photoconductive film is low, the conductivity
 increases sharply as the light intensity becomes strong to a certain
 degree, and as the light intensity becomes higher, the conductivity is
 saturated. Light irradiation increases the conductivity of the
 photoconductive film such as a-Si at least by a factor of about four
 digits.
 A description will now be given of a case where a liquid crystal is used
 for the switching film. When the electrical field strength is low, the
 liquid crystal molecules are aligned parallel to the film surface,
 obstructing light transmission. When the electrical field strength becomes
 equal to or greater than the threshold value, the liquid crystal molecules
 are aligned perpendicular to the film surface, facilitating light
 transmission. As shown in FIG. 11, therefore, the light transmittance of
 the switching film is low without no electrical field applied, whereas
 when the electrical field strength exceeds the threshold value (E.sub.th),
 the light transmittance rapidly increases. As the electrical field
 strength becomes higher, the light transmittance of the switching film is
 saturated. Apparently, the liquid crystal shows a switching function to
 light transmission.
 In this invention, a predetermined voltage (V) is applied to the
 photoconductive film and the switching film through the conductive films
 which sandwich the former two films. Given that t.sub.p is the thickness
 of the photoconductive film, t.sub.s is the thickness of the switching
 film, .sigma..sub.s is the conductivity of the photoconductive film and
 .sigma..sub.p is the conductivity of the photoconductive film, the voltage
 V.sub.s which is applied to the switching film is expressed by the
 following equation (1).
EQU V.sub.s =(t.sub.s /.sigma..sub.s)/(t.sub.s /.sigma..sub.s +t.sub.p
 /.sigma..sub.p)V (1)
 When no light is irradiated, .sigma..sub.p and .sigma..sub.s both have
 approximately same low values. Even if V is greater than the threshold
 value of the switching film, the voltage V.sub.s to be applied to the
 switching film can be set smaller than the threshold value by properly
 adjusting t.sub.s and t.sub.p. At the time of light irradiation,
 .sigma..sub.p becomes greater by approximately a factor of four digits as
 compared with the case of no light irradiation as mentioned above, and
 V.sub.s is approximated to be nearly equal to V. If V is set equal to or
 greater than the threshold value of the switching film, the switching film
 becomes optically transparent.
 The actual reproduction beam to be irradiated spatially shows a Gaussian
 distribution. In this case, it is possible to set only the near center of
 the reproduction beam spot in an ON state by properly adjusting the
 thickness of the photoconductive film and the reproduction power. That is,
 an optical aperture in the switching film can be made smaller than the
 size of the reproduction beam spot. Thus, super-resolution reproduction
 can be implemented in the state as illustrated in FIG. 2.
 This invention can be adapted to a phase change optical disk like DVD, a
 magneto-optical disk, CD-ROM, CD-R, WORM and the like, and contributes to
 increasing the density of any of the optical disks.
 Preferred embodiments of this invention will now be described with
 reference to the accompanying drawings.
 First Embodiment
 FIG. 12 represents the cross sectional view of an optical disk according to
 this embodiment. A substrate 11 is formed of polycarbonate, 120 mm in
 diameter and 0.6 mm in thickness, and grooves are so formed as to ensure
 land/groove recording at a track pitch of 0.6 .mu.m. This polycarbonate
 substrate 11 is formed by ordinary injection molding. Formed on this
 substrate 11 are a super-resolution film 12 of As--Se--Ge with a thickness
 of 50 nm, a first interference film 13 of ZnS--SiO.sub.2 with a thickness
 of 150 nm, a recording layer 14 of Ge.sub.2 Sb.sub.2 Te.sub.5 with a
 thickness of 20 nm, a second interference film 15 of ZnS--SiO.sub.2 with a
 thickness of 25 nm, and a reflective film 16 of Al with a thickness of 50
 nm. Those films are formed by normal magnetron sputtering.
 The transmission characteristic of the super-resolution film 12 will be
 discussed. The temperature dependency of the transmittance of the
 super-resolution film 12 becomes as indicated by the solid line in FIG. 3.
 This characteristic was measured by forming only the super-resolution film
 12 on a quartz substrate and irradiating a laser beam of a wavelength of
 650 nm. The transmittance of the super-resolution film 12 is low, about
 50%, at an ambient temperature Ta (near the room temperature), rapidly
 increases from about 100.degree. C., goes up to about 90% at about
 150.degree. C., and is saturated at a higher temperature.
 FIG. 14 shows the time response of the transmittance of the
 super-resolution film 12. This characteristic was acquired by checking a
 change in transmittance in situ after irradiating a laser beam of a
 wavelength of 650 nm to the super-resolution film 12 formed alone on a
 quartz substrate. The transmittance of the super-resolution film 12 is
 about 50% when no pulse is irradiated. The transmittance gradually
 increases after irradiation of the pulse, and reaches about 90% after 1 ms
 (tm) after the pulse irradiation. The subsequent attenuation response of
 the transmittance is relatively gentle, and a high transmittance is
 maintained for approximately 5 ms. The reason for the slow time response
 of the transmittance is because a change in transmittance of As--Se--Ge,
 the material for the super-resolution film 12, is caused by atomic
 movement. That is, it takes time for the atoms excited by light
 irradiation to change to another atomic arrangement from the one before
 light irradiation. As the new atomic arrangement is metastable, it
 gradually returns to the original state by thermal disturbance. As this
 response is also very slow, a high transmittance is kept for approximately
 5 ms. Note that those responses can be made faster by, for example,
 heating the super-resolution film.
 FIG. 13 shows an optical disk drive used to reproduce information from the
 optical disk 10 in FIG. 12. The broken line shown on the optical disk 10
 indicates the locus of a reproduction beam. A reproduction light source
 101 and an objective lens 104 for reproduction are provided above the
 optical disk 10. The reproduction light source 101 is also used as a
 recording light source. A pre-beam light source 201 and an objective lens
 202 for a pre-beam are provided in front of the reproduction optical
 system, and an initialization light source 301 and an objective lens 302
 for initialization at the back of the reproduction optical system. The
 initialization light source 301 and the initialization objective lens 302
 do not have to be provided if the transmittance of the super-resolution
 film, after increased, returns to the original low state at a practically
 sufficient speed. The wavelengths of the pre-beam and reproduction beam
 are set to 650 nm, and the wavelength of the initialization beam is set to
 830 nm, with their spots set substantially complete rounds whose full
 widths at half maximum (FWHM) are 0.5 .mu.m. The interval between the
 irradiation positions of the pre-beam spot and the reproduction beam spot
 is set to 2 cm, and the interval between the irradiation positions of the
 reproduction beam spot and the initialization beam spot is set to 5 cm.
 Recording and reproduction are carried out as follows by using this drive.
 The optical disk is rotated at a linear velocity of 10 m/s, and the
 reproduction (recording) light source 101 is driven with power of a high
 recording level to form a sequence of marks on the recording layer at mark
 pitches of 0.2 .mu.m. Then, a pre-beam is irradiated as a series of
 high-frequency pulses on the track where the sequence of marks is formed,
 forming optical apertures in the super-resolution film at the proper
 intervals. Under this situation, a reproduction beam spot is irradiated
 for reproduction. At this time, the atoms of the super-resolution film are
 rearranged by the irradiation of the pre-beam spot, so that an optical
 aperture smaller in size than the spot is gradually formed in the
 super-resolution film. Because the interval between the irradiation
 positions of the pre-beam spot and the reproduction beam spot is adjusted
 in accordance with the linear velocity of the disk as mentioned above, the
 time from the irradiation of the pre-beam spot to the irradiation of the
 reproduction beam spot becomes 1 ms (the optimal time acquired from FIG.
 14). When an optical aperture reaches the irradiation position of the
 reproduction beam spot, therefore, the transmittance of the
 super-resolution film 12 becomes a high value, about 90%, at which
 reproduction can be done at the most efficient timing. Therefore, marks
 with mark pitches of 0.2 .mu.m, which cannot identified in the normal
 reproduction operation, can be reproduced at a high resolution.
 Next, the optical aperture is closed by the irradiation of the
 initialization beam, so that repetitive reproduction can be carried out
 continuously. If the super-resolution reproduction operation is performed
 continuously on the same track without using the initialization beam, an
 optical aperture is not closed completely and is partially open for the
 time (about 10 ms) from the first reproduction to the next reproduction.
 In this case, repetitive reproduction gradually increases the size of the
 optical aperture from a predetermined value, thus lowering the resolution.
 The foregoing description has been given with reference to the case where a
 material with such a property that the response of atomic rearrangement by
 light irradiation is slow is used for the super-resolution film. If a
 material with such a property that the response of temperature increase by
 light irradiation is slow is used for the super-resolution film, by
 contrast, the heating time should be elongated by using a beam extending
 longer in the track direction as the pre-beam spot. In this case, although
 the mark pitch in the track direction cannot be narrowed, an effect of
 narrowing the track pitch can be obtained.
 Second Embodiment
 FIG. 15 represents the cross sectional view of an optical disk according to
 this embodiment. A substrate 21 is formed of polycarbonate, 120 mm in
 diameter and 0.6 mm in thickness, and grooves are so formed as to ensure
 land/groove recording at a track pitch of 0.6 .mu.m. This polycarbonate
 substrate 21 is formed by ordinary injection molding. Formed on this
 substrate 21 are an SiO.sub.2 film 22, a super-resolution film 23 of
 Ge.sub.2 Sb.sub.2 Te.sub.5 +5 at % Sb containing an element for reducing
 the melting point and crystallization temperature, a first interference
 film 24 of ZnS--SiO.sub.2, a recording layer 25 of Ge.sub.2 Sb.sub.2
 Te.sub.5, a second interference film 26 of ZnS--SiO.sub.2, and a
 reflective film 27 of Al. Those films are formed by normal magnetron
 sputtering. The SiO.sub.2 film 22 is provided to prevent thermal damage on
 the substrate 21. The material for the super-resolution film 23 is a phase
 change material whose crystallization time is about 70 ns. A substrate
 (not shown) identical to the substrate 21 is adhered to the top of the
 reflective film 27. After an optical disk with the above structure is
 prepared, the recording layer is initialized to be crystalline by using an
 initialization device.
 The temperature dependency of the transmittance of this super-resolution
 film becomes as indicated by the solid line in FIG. 3. This is because the
 transmittance in the initialized crystalline state is lower than that in
 an amorphous state. The evaluation of the transmittance characteristic
 when the super-resolution film is gradually heated up and cooled down does
 not show the behavior of the broken lines in FIG. 3. It is however
 predicted that in view of the crystallization time of the super-resolution
 film, the transmittance shows the characteristic as indicated by the
 broken lines in FIG. 3 if the linear velocity of the optical disk is fast
 and the cooling time is shorter than 70 ns.
 FIG. 16 shows an optical disk drive used to reproduce information from an
 optical disk 20 in FIG. 15. Provided above the optical disk 20 are a
 reproduction light source 101, a half mirror 103, an objective lens 104
 for a reproduction beam, and a reproduction signal processing system 105.
 An initialization light source 301 and an objective lens 302 for
 initialization are provided at the back of the reproduction optical
 system. The reproduction beam in use has a wavelength of 685 nm, and the
 objective lens 104 in use has the numerical aperture NA of 0.6. A beam
 spot on the film surface is a complete round whose FWHM is approximately
 0.5 .mu.m. The linear velocity of the optical disk is set to 10 m/s. In
 this case, the time for the reproduction beam to pass the film surface is
 about 50 ns, shorter than the crystallization time of the super-resolution
 film 22. The wavelength of the initialization beam is set to 720 nm, and
 the objective lens 302 in use is an aspherical lens. The initialization
 beam spot on the film surface has an elliptical shape of about 2 .mu.m in
 the track direction and about 1 .mu.m in the track width direction. In
 this case, the time for the initialization beam to pass the film surface
 is about 200 ns.
 Recording and reproduction are carried out as follows by using this drive.
 First, a series of recording marks on the recording layer are formed on
 the recording layer at mark pitches of 0.2 .mu.m. Then, a reproduction
 beam is irradiated on the track where the recording marks are formed,
 while changing the power.
 A description will now be given of a reproduction behavior when the
 initialization power is set to the optimal level and the reproduction
 power is gradually increased from 0.3 mW. Until the reproduction power
 becomes 0.6 mW, a reproduction signal is hardly detected. When the
 reproduction power becomes equal to or greater than 0.6 mW, CNR rapidly
 rises and shows the maximum value at the power of about 1 mW. As the
 reproduction power is increased further, CNR gradually falls down to a low
 CNR value equal to the one in the case of no super-resolution film. This
 phenomenon can be explained as follows. When the reproduction power is too
 low, an optical aperture is not formed in the super-resolution film, so
 that the amount of light reaching any recorded mark is too small to obtain
 a reproduction signal. When the proper reproduction power is used, by
 contrast, an optical aperture is formed. As a result, as shown in FIG. 1,
 only one of two recording marks existing in the spot can be reproduced
 efficiently. If the reproduction power is too high, a large optical
 aperture is formed so that two recording marks existing in the spot can
 not be identified separately.
 If the same track is continuously reproduced without irradiation of the
 initialization beam, as a comparative example, the reproduction CNR is
 immediately attenuated to the level in the case where there is no
 super-resolution film. This is because the crystallization time of the
 phase change film used as a super-resolution film is longer than the pass
 time of the reproduction beam spot, leaving the aperture open.
 For the purpose of comparison, a description will be given of reproduction
 of an optical disk which uses a phase change film of Ge.sub.1 Sb.sub.2
 Te.sub.4 with a short crystallization time, as a super-resolution film.
 Repetitive super-resolution reproduction can be performed on this optical
 disk without irradiating an initialization beam. If a super-resolution
 film of Ge.sub.2 Sb.sub.2 Te.sub.5 +5% Sb is used and super-resolution
 reproduction is carried out with irradiation of the initialization beam,
 by contrast, CNR of the reproduction signal is advantageously very large
 to ensure stable reproduction, as compared with the case of using the
 super-resolution film of Ge.sub.1 Sb.sub.2 Te.sub.4. The reason for this
 advantage is that the super-resolution film of Ge.sub.2 Sb.sub.2 Te.sub.5
 +5% Sb has a large change in transmittance between the crystalline state
 and the melting state.
 According to this invention, as apparent from the above, the range of
 selectable materials for the super-resolution film is significantly
 widened, thus facilitating the optimization of parameters necessary for
 the super-resolution reproduction operation, such as the range of the
 transmittance change, the temperature range where the transmittance
 varies, and the number of repetition of reproduction operations. This can
 ensure super-resolution reproduction with a high signal quality and high
 reliability.
 Third Embodiment
 FIG. 17 shows the cross sectional view of an optical disk according to this
 embodiment. A glass substrate 31 with grooves is manufactured by a method
 of spin-coating a resist on a glass substrate, developing it with spiral
 exposure, etching the portion uncovered with the resist by reactive ion
 etching, thus forming grooves, then removing the resist. Formed on this
 substrate 31 are a field control film 32 of Bi-substituted garnet with a
 thickness of 100 nm, a magnetization change film 33 of Ba ferrite with a
 thickness of 100 nm, a first interference film 34 of ZnS--SiO.sub.2 with a
 thickness of 150 nm, a recording layer 35 of Ge.sub.2 Sb.sub.2 Te.sub.5
 with a thickness of 20 nm, a second interference film 36 of ZnS--SiO.sub.2
 with a thickness of 25 nm, and a reflective film 37 of Al--Mo with a
 thickness of 50 nm. Those films are formed by normal magnetron sputtering.
 Because acquisition of a field control film and a magnetization change
 film which show predetermined characteristics requires that the substrate
 temperature at the time of forming the films should be set as high as
 200.degree. C., a glass substrate having a high heat resistance is used in
 this embodiment. It is to be noted that should the film forming technology
 be improved in the future to be able to form a field control film and a
 magnetization change film with predetermined characteristics at a lower
 temperature, an ordinary plastic substrate may be used.
 The single-layer field control film shows the same temperature dependency
 of saturation magnetization Ms as illustrated in FIG. 6A. The saturation
 magnetization Ms at near the room temperature and the Curie point Tc are
 typically about 200 emu/cc and about 150.degree. C., though they slightly
 depend on the film composition and the film forming conditions. The leak
 magnetic field Hl generated outside, when this field control film shows
 the magnetic distribution with demagnetization occurring at the center
 portion of the reproduction spot as shown in FIG. 6B, is calculated to be
 about 350 Oe at a maximum.
 The single-layer magnetization change film shows the following light
 transmission characteristic that was acquired by checking the intensity of
 the transmission light by means of the optical system equipped with an
 analyzer with respect to the incident linearly polarized light. In this
 embodiment, the transmission axis of the analyzer is set substantially
 perpendicular to the oscillation plane of the incident polarized light to
 make the transmittance lower when the magnetization of the magnetization
 change film is directed in-plane direction. When the magnetization of the
 magnetization change film is directed in-plane direction, the Faraday
 rotational angle is about 0.1.degree., and the transmission light
 intensity is low. When a magnetic field of 300 oe (nearly the coercive
 force of the magnetization change film) or greater is applied
 perpendicularly to the magnetization change film, the magnetization of the
 magnetization change film is directed perpendicular to the film surface.
 As a result, the Faraday rotation angle becomes larger, and the
 transmission light intensity increases rapidly. By adjusting the angle of
 the transmission axis of the analyzer and the sensitivity of the
 differential detection system at this time, the transmission light
 intensity can be adjusted. For example, it is possible to acquire the
 characteristic as shown in FIG. 7 by setting the transmittance with the
 magnetization of the magnetization change film directed in-plane direction
 to 50% and setting the transmittance with the magnetization of the
 magnetization change film directed perpendicular direction to 80%.
 Before setting the optical disk of FIG. 17 on the disk drive, an
 initialization magnetic field equal to or greater than the coercive force
 of the field control film (approximately 1.5 kOe) is applied
 perpendicularly to the optical disk to set the magnetization of the field
 control film uniform. As the easy axis of magnetization of the field
 control film is perpendicular to the film surface, perpendicular
 magnetization is maintained even after removing the magnetic field. When
 the initialization magnetic field is applied, the magnetization of the
 magnetization change film also becomes perpendicular to the film surface.
 Because the easy axis of magnetization of the magnetization change film is
 in the in-plane direction, however, the magnetization after removal of the
 magnetic field is randomly aligned in the in-plane direction.
 FIG. 18 shows the essential structure of the optical disk drive used in
 this embodiment. Referring to FIG. 18, an optical disk 30 of FIG. 17 is
 set on the rotational shaft of a spindle motor 71. For information
 recording and reproduction, a laser 101 is driven by a light source
 control system 110 to irradiate a laser beam on the optical disk 30 via a
 first lens 102, a polarized beam splitter 103 and an objective lens 104.
 In the reproduction operation, reflected light from the optical disk 30,
 after having passed the objective lens 104 and the polarized beam splitter
 103, is processed in the reproduction signal processing system 105 to read
 out recorded information.
 First, the spindle motor 71 is activated to rotate the optical disk 30 at a
 linear velocity of 10 m/s, and a laser beam from the laser 101 is
 irradiated on the optical disk 30 to record information there.
 Specifically, the semiconductor laser is operated with a pulse train of
 such a frequency as to set the mark pitches of 1 .mu.m over a
 predetermined track to form a series of recording marks there. Then, the
 semiconductor laser is moved over another track and is operated with a
 pulse train of such a frequency as to set the mark pitches of 0.9 .mu.m
 over a predetermined track to form a series of recording marks there. A
 sequence of recording marks are formed in this manner by changing the
 recording frequency in such a way as to make the mark pitches shorter by
 0.1 .mu.m from 1 .mu.m to 0.1 .mu.m while shifting a track to be recorded.
 At this time, overwrite recording is performed on both lands and grooves.
 As higher power is applied in recording operation than in reproduction
 operation, the temperature of the field control film in the area of a size
 equal to or greater than FWHM of the beam spot becomes equal to or higher
 than Tc. Accordingly, the area of the magnetization change film which has
 a size equal to or greater than FWHM of the beam spot is also magnetized
 perpendicularly. As a result, the super-resolution film with the stacked
 structure becomes transparent, and recording marks equivalent to those
 formed in the case of no super-resolution film used can be formed. The
 size of marks becomes about 0.5 .mu.m, approximately the same as FWHM of
 the spot, in the track width direction, and has a length in the track
 direction which is determined by the FWHM of the spot and the recording
 pulse length. To make the mark pitches shorter than FWHM of the spot,
 pen-tip recording should be made so that the size in the track width
 direction should become smaller than 0.5 .mu.m.
 Reproduction is carried out as follows. The following discusses
 reproduction in the case where a series of recording marks are formed at
 pitches of 0.2 .mu.m. CNR of a reproduction signal obtained by
 continuously oscillating the laser to gradually increase the reproduction
 power from 0.5 mW by 0.1 mW varies as follows. CNR gradually increases
 from the point where the reproduction power is about 0.5 mW, sharply rises
 and reaches a practical value at about 1 mW, and keeps its value until
 approximately 1.5 mW. CNR gradually falls when the reproduction power
 exceeds about 1.5 mW, and is hardly obtained at about 2.5 mW. The reason
 for this behavior can be explained as follows. When the reproduction power
 becomes about 0.5 mW, the temperature of the field control film near the
 center of the spot becomes equal to or greater than the Curie point and an
 optical aperture is formed in the magnetization change film at the center
 of the spot, so that a reproduction signal is obtained. When the
 reproduction power is in the range of about 1 mW to about 1.5 mW, an
 optical aperture of the proper size is formed in the magnetization change
 film, ensuring efficient super-resolution reproduction of a series of
 recording marks at pitches of 0.2 .mu.m. When the reproduction power
 exceeds about 1.5 mW, however, an optical aperture becomes too large so
 that signals are picked up from adjacent marks and CNR starts falling.
 When the reproduction power further increases and becomes about 2.5 mW, an
 aperture of a size of about FWHM of the laser spot is formed, making it
 impossible to separately reproduce two recording marks formed at a pitch
 of 0.2 .mu.m.
 When the mark pitch is wider than 0.2 .mu.m, CNR does not drop so much even
 if the reproduction power is increased above 1.5 mW. The slight decrease
 in CNR occurs because when reproduction is done with high power, an
 optical aperture becomes large so that recording marks on adjacent tracks
 are picked up.
 FIG. 19 illustrates the relationship between the mark pitch (MP) and CNR
 when reproduction is carried out with the reproduction power fixed to 1.2
 mW. In FIG. 19, the broken line shows the relationship for the
 conventional optical disk without a super-resolution film, while the solid
 line shows the relationship for the optical disk of this embodiment. With
 regard to the conventional optical disk, for the mark pitch of 0.4 .mu.m
 or smaller, CNR drops drastically due to the influences of inter-symbol
 interference and crosstalk. By contrast, the optical disk of this
 embodiment shows a high CNR even when the mark pitch is reduced to 0.2
 .mu.m. When the mark pitch is large, CNR of the conventional optical disk
 is slightly higher than that of the optical disk of this embodiment
 because the conventional optical disk has no super-resolution film and a
 high efficiency of using the reproduction beam.
 Although the foregoing description has been of the case where a
 perpendicular magnetization film is used as a field control film and a
 longitudinal magnetization film is used as a magnetization change film,
 this invention is not limited to this particular case.
 For example, a longitudinal magnetization film may be used as a field
 control film in which case, a ring magnet or a ring recording magnetic
 pole used for a magnetic disk is used to uniformly initialize the field
 control film along the tracks. When the field control film formed of the
 longitudinal magnetization film is heated to be demagnetized with
 irradiation of the reproduction beam, it is possible to generate a greater
 magnetic field than the one produced by the field control film which is
 comprised of a perpendicular magnetization film. Even if the coercive
 force of the magnetization change film is large, therefore,
 super-resolution reproduction is still possible.
 Further, a perpendicular magnetization film may be used as a magnetization
 change film. In this case, the magnetization of the magnetization change
 film is initialized upward or downward to the film surface. Furthermore,
 the transmission axis of the analyzer is so set that the transmittance of
 the polarized reproduction beam in the initial state becomes low. When the
 field control film is heated to be demagnetized with irradiation of the
 reproduction beam, this field control film generates a magnetic field in
 the opposite direction to the initial magnetization direction of the
 magnetization change film. Super-resolution reproduction can be
 accomplished by inverting the direction of the magnetization of the
 magnetization change film in the reproduction operation in this manner. In
 this case, the direction of the magnetization of the magnetization change
 film, which has been inverted by the reproduction operation, is held
 unchanged. To repeatedly execute the super-resolution reproduction
 operation, therefore, an initialization magnet is provided at the back of
 the reproduction light source to restore the direction of the
 magnetization of the magnetization change film.
 If an optical aperture smaller in size than FWHM of the beam spot is formed
 in the magnetization change film at the time of irradiating a high-power
 recording beam as well as in the reproduction operation, it is possible to
 form a series of recording marks smaller than the recording beam spot,
 thus ensuring super-resolution recording. This method can further improve
 the recording density as compared with the case where only
 super-resolution reproduction is conducted. In this case, however, an
 optical aperture in the reproduction operation becomes very small and
 efficiency light usage decreases, which reduces the intensity of a
 reproduced signal.
 Fourth Embodiment
 FIG. 20 shows the cross sectional view of an optical disk according to this
 embodiment. A substrate 41 is formed of polycarbonate, 120 mm in diameter
 and 0.6 mm in thickness, with grooves so formed as to ensure land/groove
 recording. Formed on this substrate 41 are a first electrode film 42 of
 ITO, a switching film 43 of a liquid crystal, a photoconductive film 44 of
 a-Si, a second electrode film of ITO, a first interference film 46 of
 ZnS-SiO.sub.2, a recording layer 47 of GeSbTe, a second interference film
 48 of ZnS--SiO.sub.2, and a reflective film 49 of Al--Mo. A glass
 substrate 50 of the same size as the polycarbonate substrate 41 is
 provided on the reflective film 49. The materials for the upper and lower
 substrates may be reversed.
 This optical disk can be prepared by the following method. The first
 electrode film 42 is formed on the polycarbonate substrate 41 by
 sputtering. To lead out the first electrode, a mask is provided at the
 innermost periphery of the disk to expose an electrode leading portion,
 and a lead is formed by sputtering Au there. After the reflective film 49,
 the second interference film 48, the recording layer 47, the first
 interference film 46 and the second electrode film 45 are formed on the
 glass substrate 50 by sputtering, the photoconductive film 44 is formed by
 CVD. To lead out the second electrode, a mask is provided at the innermost
 periphery of the disk to expose an electrode leading portion, and a lead
 is formed by sputtering Au there. Then, the polycarbonate substrate 41 and
 the glass substrate 50 are placed against each other so that the first
 electrode film 42 faces the photoconductive film 44, a liquid crystal is
 injected between both substrates, and both substrates are then adhered by
 using the inner peripheral portion and outer peripheral portion where no
 films are formed. At the time of adhering the substrates, a care should be
 taken not to short-circuit the lead of the first electrode film 42 with
 the lead of the second electrode film 45. When the disk is set on the
 drive, the individual leads should be connected to terminals provided at a
 disk holder.
 The terminals provided at the disk holder are supplied via sliding contacts
 with the voltage from a power supply provided in the drive, thereby
 applying a voltage between the two electrode films. The applied voltage
 typically lies in the range of several volts to several scores of volts,
 though it differs depending on the type and thickness of the liquid
 crystal.
 When the drive can withstand a high voltage, a voltage of an order of
 several kilovolts may be applied from outside the disk. When the external
 voltage is applied, the electrode films, the leads and the associated
 contacts can be omitted. For example, a relatively thick ITO film is
 formed on the glass substrate and is placed on the reproduction-beam
 incident side, an ordinary metal electrode is provided on the opposite
 side to the disk, and a voltage of several kilovolts is applied between
 both films. As a predetermined electrical field has only to be applied to
 the photoconductive film and the switching film within the reproduction
 beam spot, the electrode may be made considerably smaller.
 It is preferable to make the liquid crystal as thin as 1 .mu.m though its
 thickness is not particularly restricted. It is to be noted however that
 even when the liquid crystal is thicker than the depth of focus of the
 reproduction beam, only the liquid crystal in the portion where the
 photoconductive film is enabled can be made transparent as long as the
 focal point lies on the photoconductive film. The thickness of the liquid
 crystal to that of the photoconductive film are set optimally based on the
 equation (1). When the liquid crystal in use is of an STN type and a
 lightly-doped a-Si film is used as the photoconductive film, for example,
 the ratio of the thickness of the liquid crystal to that of the
 photoconductive film should be set to approximately 10:1. With the liquid
 crystal having a thickness of 1 .mu.m, for example, the photoconductive
 film should be formed as thin as 100 nm.
 The following will discuss the results of previously having examined the
 characteristic in FIG. 10 with respect to a single-layer photoconductive
 film sandwiched by a pair of ITO electrodes. Specifically, with a voltage
 applied between the ITO electrodes, a beam in 50 .mu.m in diameter with a
 uniform intensity was irradiated from an He--Ne laser and a change in
 current or conductivity was checked as the power was gradually increased
 while monitoring the circuit current. The conductivity when no light is
 irradiated is 10.sup.-5 S/cm, which gradually increases in accordance with
 an increase in laser power, sharply rises at about 10 W, and reaches a
 saturation value of 10.sup.-1 S/cm at about 15 W. The spot size of the
 laser beam in actual use is about 0.5 .mu.m in terms of FWHM of the spot.
 The aforementioned value of 10 W with 50 .mu.m in diameter is equivalent
 to 1 mW in terms of the actual reproduction power.
 FIG. 11 shows a variation in the light transmittance of a single layer of a
 liquid crystal (thickness of 1 .mu.m) sandwiched by a pair of ITO
 electrodes while gradually increasing the applied voltage. The
 transmittance when no voltage is applied is approximately 40%. The
 threshold voltage at which the transmittance starts increasing is about 5
 V (the electrical field strength is 50 kV/cm). when the applied voltage
 becomes about 7 V, the transmittance reaches a saturation value of about
 80%.
 FIG. 21 shows the essential structure of the optical disk drive used in
 this embodiment. Referring to FIG. 21, an optical disk 40 in FIG. 20 is
 attached to the disk holder of the rotational shaft of the spindle motor
 71. The disk holder is provided with terminals to be connected to the
 leads of the electrode films of the disk. The leads from the first and
 second electrode films of the disk are respectively connected to the
 terminals provided on the disk holder, and further connected to a voltage
 source 72 via sliding contacts. For information recording and
 reproduction, as in FIG. 18, the laser 101 is driven by the light source
 control system 110 to irradiate a laser beam on the optical disk 40 via
 the first lens 102, the polarized beam splitter 103 and the objective lens
 104. With regard to reproduction, reflected light from the optical disk
 40, after having passed the objective lens 104 and the polarized beam
 splitter 103, is processed in the reproduction signal processing system
 105 to read out recorded information.
 First, the optical disk is set in the initialization device to crystallize
 the GeSbTe film of the recording layer. The spindle motor 71 is activated
 to rotate the optical disk 40 at a linear velocity of 10 m/s, and a laser
 beam from the laser 101 is irradiated on the optical disk 40 to record
 information there. Specifically, the semiconductor laser is operated with
 a pulse train of such a frequency as to set the mark pitches of 1 .mu.m
 over a predetermined track to form a series of recording marks there.
 Then, the semiconductor laser is moved over another track and is operated
 with a pulse train of such a frequency as to set the mark pitches of 0.9
 .mu.m over a predetermined track to form a series of recording marks
 there. A sequence of recording marks are formed in this manner by changing
 the recording frequency in such a way as to make the mark pitches shorter
 by 0.1 .mu.m from 1 .mu.m to 0.1 .mu.m while shifting a track to be
 recorded. At this time, overwrite recording is performed on both lands and
 grooves. As higher power is applied in recording operation than in
 reproduction operation, the photoconductive film has a high conductivity
 in the area of a size equal to or greater than FWHM of the beam spot.
 Accordingly, the area of the switching film which has a size equal to or
 greater than FWHM of the beam spot likewise becomes transparent, allowing
 the formation of recording marks identical to those formed in the case
 where no super-resolution film is used. The size of recording marks
 becomes about 0.5 .mu.m, approximately the same as FWHM of the spot, in
 the track width direction, and has a length in the track direction which
 is determined by FWHM of the spot and the recording pulse length. To make
 the mark pitches shorter than FWHM of the spot, pen-tip recording should
 be made so that the size in the track width direction should become
 smaller than 0.5 .mu.m.
 Reproduction is carried out as follows. The following discusses
 reproduction in the case where a series of recording marks are formed at
 pitches of 0.2 .mu.m. FIG. 22 shows a change in CNR of a reproduction
 signal acquired by continuously oscillating the laser to gradually
 increase the reproduction power from 0.5 mW by 0.1 mW. CNR gradually
 increases from the point where the reproduction power is about 0.5 mW,
 sharply rises and reaches a practical value at about 1 mW, and keeps its
 value until approximately 1.5 mW. CNR gradually falls when the
 reproduction power exceeds about 1.5 mW, and is hardly obtained at about
 2.5 mW. The reason for this behavior can be explained as follows. When the
 reproduction power becomes about 0.5 mW, the conductivity of the
 photoconductive film starts increasing. When the reproduction power lies
 in the range of about 1 mW to about 1.5 mW, an optical aperture of the
 proper size is formed, ensuring efficient super-resolution reproduction of
 a series of recording marks at pitches of 0.2 .mu.m. When the reproduction
 power exceeds about 1.5 mW, however, an optical aperture becomes too large
 so that signals are picked up from adjacent marks and CNR starts falling.
 When the reproduction power further increases and becomes about 2.5 mW, an
 aperture of a size of about FWHM of the laser spot is formed, making it
 impossible to separately reproduce two recording marks formed at a pitch
 of 0.2 .mu.m.
 When the mark pitch is wider than 0.2 .mu.m, CNR does not drop so much even
 if the reproduction power is increased above 1.5 mW. The slight decrease
 in CNR occurs because when reproduction is done with high power, an
 optical aperture becomes large so that recording marks on adjacent tracks
 are picked up.
 FIG. 23 illustrates the relationship between the recorded mark pitch (MP)
 and CNR when the reproduction power is fixed to 1.2 mW. In FIG. 23, the
 broken line shows the relationship for the conventional optical disk
 without a super-resolution film, while the solid line shows the
 relationship for the optical disk of this embodiment. With regard to the
 conventional optical disk, for the mark pitch of 0.4 .mu.m or smaller, CNR
 drops drastically due to the influences of inter-symbol interference and
 crosstalk. By contrast, the optical disk of this embodiment shows a high
 CNR even when the mark pitch is reduced to 0.2 .mu.m. When the mark pitch
 is large, CNR of the conventional optical disk is slightly higher than
 that of the optical disk of this embodiment because the conventional
 optical disk has no super-resolution film and a high efficiency of using
 the reproduction beam. However, this embodiment can also acquire
 practically sufficiently high CNR by optimizing the film structure.
 When the time response of the photoconductive film or the switching film is
 too slow to close an optical aperture after the passing of the
 reproduction beam, the film should be initialized by providing, at the
 back of the reproduction light source, some means for applying an
 electrical field in the opposite direction. This design can implement
 stable repetitive reproduction.
 If an optical aperture smaller in size than FWHM of the beam spot is formed
 even when irradiating a high-power recording beam, it is possible to form
 a series of recording marks smaller than the recording beam spot, thus
 ensuring super-resolution recording. This method can improve the recording
 density more than is done in the case where only super-resolution
 reproduction is performed.
 Additional advantages and modifications will readily occur to those skilled
 in the art. Therefore, the invention in its broader aspects is not limited
 to the specific details and representative embodiments shown and described
 herein. Accordingly, various modifications may be made without departing
 from the spirit or scope of the general inventive concept as defined by
 the appended claims and their equivalent.