GB2084786A

GB2084786A - Variable sensitivity optical recording medium and information record

Info

Publication number: GB2084786A
Application number: GB8128447A
Authority: GB
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1980-09-25
Filing date: 1981-09-21
Publication date: 1982-04-15
Also published as: JPS5786139A; FR2490858A1; FR2490858B1; JPH0427614B2; DE3137528C2; DE3137528A1; GB2084786B; NL8104409A

Abstract

A variable sensitivity optical recording medium comprising a substrate (12) having a series of indentations (32) in a major surface (14) and having a covering, including at least an absorptive layer (36), overlying the substrate with the covering filling the indentations thereby forming first and second regions of different respective thicknesses in the optical recording medium where the recording sensitivity of the first regions is greater than the recording sensitivity of the second regions. Various other layers may be included between the substrate (12) and an overcoating (46) to enhance the recording medium. The invention also includes an information record having an information recorded therein as a series of the first regions having different optical properties from the remainder of the first regions and from the optical properties of the second regions. <IMAGE>

Description

SPECIFICATION Variable sensitivity optical recording medium and information record The invention relates to an optical recording medium and an information record having regions of differing recording sensitivity wherein information can be recorded in the regions of higher sensitivity without recording information in regions of lower sensitivity.

Spong, in U.S. Patent 4,097,895 which issued June 27, 1978 and is incorporated herein by reference, has disclosed a bilayer ablative optical recording medium which comprises a reflective layer coated with an absorptive layer, where the thickness of the absorptive layer is chosen so that the reflectivity of the recording medium is reduced. The absorptive layer is melted or ablated during recording, thereby changing the reflectivity of the recording medium. During readout the difference in reflectivity between perturbed and unperturbed portions on the optical recording medium is detected optically and converted into an electrical signal representative of the recorded information.

Bell, in U.S. Patent 4,216,501 issued August 5, 1980 and incorporated herein by reference, has disclosed a trilayer optical recording medium having a transparent spacer layer interposed between the reflective and absorptive layers of the Spong optical recording medium. The thickness of the absorptive layer is so related to the thickness of the spacer layer and to the optical constants (the index of refraction and the extinction coefficient) of the reflective, spacer and absorptive layer so as to reduce the reflectivity of the recording medium. During the recording process the absorptive layer is melted or ablated producing an opening in this layer, thereby changing the reflectivity of the recording medium. Alternatively, the absorptive layer may deform without the formation of an opening or the optical properties of the absorptive layer may change thus changing the reflectivity.

In each of these cases, the optical recording medium has a substantially uniform recording sensitivity from point to point on the recording medium. Thus the shape and dimensions of a change in the absorptive layer, for example an opening in the absorptive layer, depends upon the shape and dimensions of the recording light beam.

Howe, in U.S. Patent No. 4,176,377 issued November 27, 1979, has disclosed a video disc configuration in which the surface is sensitive to recording along a spiral record track is non-sensitive to recording in the regions between adjacent tracks. This may be accomplished by using an unmodulated high power laser to thermally scribe away the unwanted surface areas of a uniformly sensitive record blank. Alternatively, the scribed disc can be used as a master for the photolithographic fabrication of discs having recording sensitive spiral tracks. This additional processing is undesirable since the sensitive surface must have a high degree of perfection in order to faithfully reproduce the information recorded therein.

It would be desirable to have an optical recording medium record having a locally varying recording sensitivity so that the recorded change in the absorptive layer in independent of the shape and dimension of the recording light beam and which also does not require further processing after formation of the absorptive layer.

To this end there is provided according to the present invention an optical recording medium comprising a substrate having a plurality of indentations in a major surface thereof and an absorptive layer overlying the substrate and filling the indentations thereby forming first and second regions in the optical recording medium. The recording sensitivity of the optical recording medium in the first regions is substantially greater than the recording sensitivity of the optical recording medium in the second regions.

The invention is also directed to an information record having an information track recorded therein in the form of a series of areas in the first regions having different optical properties from the remainder of the first regions and from the second regions.

In the accompanying drawings; Figure 1 is a schematic illustration of a perspective view of the optical recording medium of the invention.

Figure 2 is a schematic illustration of a crossectional view of a portion of a first embodiment of the invention.

Figure 3 is a schematic illustration of a crossectional view of a portion of a second embodiment of the invention.

Figure 4 is a schematic illustration of a crossectional view of a portion of third embodiment of the invention.

Figure 5 is a schematic illlustration of a crossectional view of a portion of an information record of the invention.

Figure 6 is a schematic illustration of a crossectional view of a portion of a second embodiment of the information record.

Figure 7 is a schematic illustration of a crossectional view of a portion of an optical recording medium having indentations of varying depth.

Referring to Fig. 1, the optical recording medium 10 comprises a substrate 12 having a major surface 14 with a covering 16, including an absorptive layer, overlying the major surface 14 of the substrate 12. The optical recording medium 10 contains first regions 18 which have a higher absorptivity at the wavelength of a recording light beam than second regions 19 which constitute the remainder of the optical recording medium. For illustrative purposes, the first regions 18 are shown arranged in the form of grooves 20 or, alternatively, arranged in the form of a checkerboard pattern 22. As set out below it is evident that the first regions 18 can be arranged in any form in which the user desires to record information in the optical recording medium.Because of the higher absorption in these regions, a recording light beam incident on the optical recording medium 10 will preferentially record information in the form of a change in the optical properties of the recording medium in the first regions 18 as opposed to the second regions 20 which the beam overlaps. This change may take the form of an irreversible recording, for example by forming an opening in the optical absorptive layer, or it may be a reversible recording, for example by reversibly changing the degree of crystallinity of the absorptive layer.

Referring to Fig. 2 a first embodiment 30 of the optical recording medium of the invention comprises a substrate 1 2 having a major surface 14; a plurality of indentations 32 extending a distance into the substrate 12 from the major surface 14 with a plurality of lands 34 between the indentations 32; and an absorptive layer 36 which fills the indentations 32 and may also overlie the lands 34 between the indentations 32. The absorptive layer is then comprised of two portions; a first portion 40 which fills the indentations 32 and is thicker than a second portion 42 which overlies the lands 34 between the indentations. The upper surface 44 of the light absorptive layer 36 is flat and may be coated with a relatively thick overcoat layer 46.

Referring to Fig. 3, the identification of the elements common to the second embodiment 50 and the first embodiment 30 of the invention is the same. The second embodiment 50 of the invention differs from the first embodiment 30 in that a reflective layer 52 overlies the major surface 14 including the indentations 32 and the lands 34 between the indentations; and an absorptive layer 54 which fills the indentations 32 and may also overlie that portion of the reflective layer 52 which overlies the lands 34 between the indentations 32. The absorptive layer is then comprised of two portions, a first portion 56 which fills the indentations and is thicker than a second portion 58 which overlies the lands between the indentations. The flat upper surface 60 of the light absorptive layer 54 may be coated an overcoat layer 46.

Referring to Fig. 4 the identification of the elements common to the third embodiment 70 and the second embodiment 50 of the invention is the same. The third embodiment 70 of the invention differs from the second embodiment 50 in that a spacer layer 72 fills the identations 32 of the major surface 14 and may also overlie the reflective layer 52 above the lands 34. The spacer layer is thus comprised of a first portion 74 in the region of the indentations in the major surface 14 and a second portion 76 which is thinner than the first portion 74 and overlies the lands 34 between the indentations 32. An absorptive layer 78 of substantially uniform thickness overlies the surface 80 of the spacer layer 72.

The third embodiment 70 of the invention is further distinguished from the second embodiment 50 in that a capping layer 82 may overlie the surface 84 of the light absorptive layer 78. An overcoat layer 46 may overlie the capping layer 82 or, in the absence of the capping layer, may overlie the light absorptive layer 78.

The substrate 12 may be formed of a plastics material such as polyvinyl chloride or (poly)methylmethacrylate, typically in the form of a disc. The indentations may be formed in a major surface of the substrate 12 by replication from a master having the indentations thereon by embossing the surface 14 or the indentations may be formed during the compression molding or casting of the disc. The indentations are typically between about 5 nanometers and about 200 nanometers deep.

It is to be understood that the description of the major surface as having indentations therein includes both pits in the surface and features which are raised above the surface of the disc. The function of the pits or the raised features is the same, to provide optically sensitive coatings of varying thickness.

In the first embodiment 30 of Fig. 2 the depth of the indentations 32 is chosen such that the thickness of the absorptive layer 36 which fills the indentations 32 is about the thickness required to provide a balance between absorption and reflection to provide higher sensitivity, or is, preferably, slightly less than this thickness. If the depth is less than this thickness then any additional thickness above the indentation will bring the thickness to the desired value. Typically the depth of the indentations 32 is between about 5 and 40 nanometers for the first embodiment 30.

In the second and third embodiments of Figs. 3 and 4, the depth of the indentations is chosen such that the thickness of the layer which fills the indentations is about the thickness required to minimize the local reflectivity of the optical recording medium or preferably slightly less than this thickness. If the depth is less than this thickness, then any additional thickness above the indentation will bring the thickness to the desired value. Typically the depth of the indentations 32 is between about 20 and about 80 nanometers for the bilayer structure of the second embodiment 50 of Fig. 3 and between about 30 and about 200 nanometers for the trilayer structure of the third embodiment 70 of Fig. 4. As will be set forth herein below the preferred thickness of the absorptive or the spacer and absorptive layers depends upon the properties of the materials of the layers.

The reflective layer 52 of Fig. 3 preferably reflects a substantial fraction, preferably at leasti 50%, of the incident light at the wavelengths of the recording and readout light beams and is typically formed of a metal such as aluminum or gold which has high reflectivity at these wavelengths. The reflective layer 52 is typically between about 30 and 60 nanometers thick and may be deposited on the surface 14 of the substrate including the indentations by vacuum evaporation. Alternatively a single or multilayer dielectric reflector may be used.

The absorptive layer 36 of the first embodiment 30 of Fig. 2 and the absorptive layer 54 of the second embodiment 50 of Fig. 3 are formed of a material which absorbs light at the wavelengths of the recording and readout light beams and which can be deposited on the surface 14 or on the reflective layer 52 such that the indentations 32 in the major surface 14 of the substrate are preferentially filled. Once the identations 32 are filled, excess material may form a thin layer over the indéntations 32 and the lands 34 between the indentations 32 to form a smooth, continuous upper surface of the absorptive layer. The absorptive layer in either embodiment may be composed of an organic dye dissolved in a solvent which is then deposited by spinning techniques.A suitable dye contained in a binder is a 4.8% weight to volume solution of 4-phenylazo-1-naphthylamine in Shipley 1350 B photoresist which is diluted 25 to 1 with 2methoxyethyl acetate and which is deposited by spining. This material is useful at a wavelength of a recording or readout light source of about 488 nanometers.

The overcoat layer 46 is composed of a material which is substantially transparent at the wavelengths of the recording and readout light beams. An overcoat layer between about 0.05 and about 1 millimeter thick is useful.

Dust particles which settle on the upper surface of the overcoat layer are then far removed from the focal point of the optical recording and readout system so that the effect on the recording and readout of information is considerably reduced. A useful material for this application is a silicone resin which may be deposited by spinning techniques.

The thickness of the absorptive layer 54 of the second embodiment 50 of Fig. 3 is so related to the optical constants of the reflective and absorptive layers and the overcoat layer, if present, that the reflectivity of the optical recording medium in the first regions is reduced and is preferably minimized corresponding to the anti-reflection condition. The optimum values of the thickness of the absorptive layer can be calculated using, for example, the matrix method as discussed in "Optical Properties of Thin Solid Films" by 0.

S. Heavens, Dover Publications, Inc., New York, 1965 p. 69. The thickness of the absorptive layer having reduced reflectivity typically is between about 20 and about 100 nanometers.

The thickness of the absorptive layer 54 then varies across the surface of the optical recording medium, being thicker in the regions of the indentations 32 than the regions of the lands 34. The first minimum of the reflectivity at the recording and readout wavelengths will occur over the indentations. Once the indentations are filled, the thickness of the second portion 58 above the lands 34 will increase as well as the thickness of the first portion 56 above the indentations 32. As the thickness of the second portion increases, the reflectivity of this portion will decrease while the reflectivity of the first portions after passing through a minimum will begin to increase.

Thus by controlling of the coating thickness above the second portions the regions of lower reflectivity and higher sensitivity can be either over the identations 32 or over the lands 34. Preferably the thickness of the second portion is minimized and the portions over the indentations 32 are the regions of lower reflectivity and higher sensitivity.

In the third embodiment 70 of the invention as shown in Fig. 4, it is the thickness of the spacer layer 72 which is varied. The spacer layer is comprised of a material which is substantially transparent at the wavelengths of the recording and readout light beams and which can be deposited on the light reflective layer 52 such that the indentations 32 in the surface of the substrate are preferentially filled, thereby forming a first portion 74 of the spacer layer. After the indentations are filled, the thickness of the first portion 74 and a second portion 76 above the lands 34 will increase. A suitable material for the spacer layer is poly-#u-methylstyrene dissolved in toluene and deposited using spinning techniques.

The light absorptive layer 78 which overlies the spacer layer 72 is comprised of a material which is absorptive of light at the wavelengths of the recording and readout light beams.

Suitable materials for this layer include titanium, rhodium, tellurium, selenium, telluriumor selenium-based alloys, arsenic trisulfide and arsenic triselenide. These materials may be deposited by vacuum evaporation. After exposure to the atmosphere, some of these materials will oxidize leaving an absorptive layer which is effectively thinner than the layer originally deposited. This effect may be compensated for by depositing a layer which is thicker than that desired with the subsequent oxidation reducing the effective of thickness to the desired value.

The thicknesses of the spacer and absorp tive layers 72 and 78 respectively are chosen such that the reflectivity of the first or second portions of the optical recording medium are reduced and are calculated taking into consideration the optical properties of the reflective, spacer, and absorptive layers and the capping and overcoat layers, if present, and the wavelengths of the recording and readout light beams. Typically the thickness of the spacer layer to produce a reduced reflectivity is between about 20 and about 200 nanometers, and the thickness of the absorptive layer is between about 3 and about 50 nanometers.

The variation in the sensitivity across the surface of the information record is then obtained by variations in the thickness of the spacer layer; the spacer layer being thicker in the region of the indentations 32 than in the regions of the lands 34. As in the case of the absorptive layer of the second embodiment 50 of Fig. 3, the first minimum in the reflectivity may occur over the indentations. Once the indentations are filled by the spacer layer the thickness of the second portion 76 of the spacer layer above the lands 34 will increase, as well as the thickness of the first portion 74 in the indentations. As the thickness of the second portion increases, the local reflectivity of the optical recording medium in this region will decrease while the reflectivity of the first portion, after reaching the minimum, will begin to increase.Thus, by controlling the thickness of the spacer layer above the second portion the regions of lower reflectivity and higher sensitivity can either be over the indentations 32 or over the lands 34. It is preferred that the thickness of the second portion of the spacer layer be minimized, thus making the regions of the first portions 74 above the indentations 32 the regions of lower reflectivity and higher sensitivity.

The capping layer 82 may overlie the upper surface 84 of the light absorptive layer 78.

The function of the capping layer is to prevent thermal damage to the overcoat layer 46 if a high melting temperature material, such as titanium or rhodium, is used for the absorptive layer 78. If, however, a low melting temperature material such as tellurium, selenium, or an alloy containing one of these materials is used as the absorptive layer 78, the capping layer is not needed to prevent thermal damage of the overcoat layer 46.

However, if the capping layer 82 and the spacer layer 72 are formed of materials which are effective for inhibiting the formation of openings or other permanent deformations of the absorptive layer 78, a reversible recording, that is, a recording which may be erased and rerecorded upon, may be formed in the absorptive layer 78. Suitable materials for the absorptive layer 78 in this embodiment include tellurium, selenium, tellurium or selenium based alloys, arsenic trisulfide and arsenic triselenide. Suitable materials for use as the capping layer which are effective to inhibit the formation of a reversible recording include silicon dioxide, silicon monoxide, titanium dioxide, and tellurium dioxide. These materials may deposited using electron beam evaporation techniques.Typically the thickness of a capping layer suitable for inhibiting the formation of openings or other deformations in the absorptive layer is between about 50 and about 500 nanometers thick.

It is to be noted that if it is desired to record information reversibly, the overcoat and capping layers 46 and 82 must also be substantially transparent at the wavelength of an erasing light beam and the absorptive layer 78 must be absorptive at the wavelength of the erasing light beam.

The indentations in the surface of the disc can be arranged in a variety of different formats such as those illustrated in Fig. 1.

The indentations may be in the form of a continuously indented circular or spiral groove in the disc surface such that the optical recording medium is thicker over the groove than in the lands between the grooves. If the optical recording medium is then more sensitive in the region over the groove than in the region over the lands, the light beam which overlaps both the groove and the lands surrounding the groove will preferentially record information in the groove as opposed the lands surrounding it.

Alternatively, a series of indentations may be arranged in circular or spiral tracks spaced apart from one another with alternating indentations and lands along the direction of the track. The indentations can, in this case, be of equal length on each track or alternatively be of equal angular length thus being longer at greater distances from the center of the disc.

Thus, information recorded in the disc will be recorded and readout in the first case at a varying frequency but at a constant recorded element size while in the second case the information may be recorded and read out at a constant rate but with a varying recorded element size. If the optical recording medium is more sensitive over the indentations than over the lands then a light beam which overlaps both an indentation and the lands along the track will preferentially record information only in the region of the indentation.

Another alternative is a checkerboard pattern where an indentation along a track has lands next to it both along the track and perpendicular to the track.

Referring to Fig. 5, a bilayer information record 90 having an information track recorded therein is shown. The identification of the elements common to the information record 90 and the recording medium 50 shown in Fig. 3 is the same. The information record 90 is shown with the indentations arranged as a series of indentations along a track with lands between the indentations. Information is recorded in the form of a series of openings 92 in the absorptive layer 54 with the presence or absence of an opening 92 and the spacing between openings being indicative of the recorded information. Alternatively the openings can be formed over the lands 34.

Referring to Fig. 6, the trilayer information record 100 having an information track recorded therein is shown. The identification of the elements of the information record 100 and the trilayer recording medium 70 shown in Fig. 4 is the same. Information is recorded in the form of a series of areas 102 in the absorptive layer 78 which have different optical properties from the remainder of the absorptive layer. These areas may take the form of an irreversible deformation of the absorptive layer or a reversible change in the optical properties the absorptive layer, such as results from a change in the degree of crystallinity of the absorptive layer 78.The presence or absence of a change in the optical properties of the absorptive layer results in a change in the reflectivity of the recording medium, with the length and the spacing between these areas being indicative of the recorded information.

It is to be noted that, if the recording light beam overlaps the regions of lower sensitivity and the recording beam power density is sufficiently large, the change in the absorptive layer which is indicative of the recorded information can also occur in the second regions of lower sensitivity.

The optical recording medium of the invention has regions of different recording sensitivity because of the varying thickness of the layers. These thickness differences are caused by the presence of indentations in the substrate surface. At the time of fabrication of the substrate, information may be prerecorded into the optical recording medium by the formation of indentations in a particular spatial distribution or by varying the depth of the indentations. For example, the depth of a portion of a spiral or circular groove may be modulated thereby forming a third region having a reflectivity different from the reflectivities of the first and second regions. This information can be used, for example, for track identification, to indicate the beginning or end of a track or to provide synchronization data.Similar information encoding schemes can be used for other arrangements of the indentations. Referring to Fig. 7 the identification of the elements common to the optical recording medium 110 and the optical recording medium 50 illustrated in Fig. 3 is the same. The indentations 112 and 114 extend different distances into the substrate 12 thereby forming the third region.

In the operation of an optical recording, reading or erasing system the light beam is centered on the information track. For the optical recording medium disclosed herein the light beam is also centered on the first regions of higher sensitivity during the recording process. If the reflectivity is different between the first and second regions, this different can be used to provide a signal proportional to the displacement of the light beam transverse to the track. This signal can be used to correct the position of the light beam on the track.

Claims

1. An optical recording medium comprising: a substrate having a major surface with a plurality of indentations therein; and a covering comprising at least an absorptive layer, which covering overlies the major surface and fills the indentations, and is absorptive of light at the wavelength of a recording and a readout light beam; thereby forming first and second regions in the recording medium such that the recording sensitivity of the recording medium in the first regions is greater than the recording sensitivity of the recording medium in the second regions.

2. An optical recording medium according to claim 1 wherein the indentations extend a distance between about 5 nanometers and about 200 nanometers into the substrate from the major surface.

3. An optical recording medium according to claim 1 or 2 wherein the substrate is a plastics material.

4. An optical recording medium according to any of claims 1-3 wherein said covering comprises a reflective layer which is interposed between the substrate and the absorptive layer and which reflects a substantial portion of the light incident thereon at the wavelengths of the recording and readout light beams; wherein the thickness of the absorptive layer is chosen such that the reflectivity of the recording medium in the first regions is reduced.

5. An optical recording medium according to claim 4 wherein the thickness of the absorptive layer in the first regions is such that the reflectivity is minimized.

6. An optical recording medium according to any preceding claim wherein the absorptive layer is composed of an organic material.

7. An optical recording medium according to claim 4 wherein said covering further comprises a spacer layer interposed between the reflective layer and the absorptive layer and filling the indentations; wherein the thicknesses of the spacer and absorptive layers are such that the reflectivity of the recording medium in the first regions is reduced.

8. An optical recording medium according to claim 7 wherein the thicknesses of the spacer and absorptive layers in the first regions are such that the reflectivity is minimized.

9. An optical recording medium according to any preceding claim further comprising a relatively thick overcoat layer overlying the absorptive layer.

10. An optical recording medium according to claim 7 or 8 further comprising a capping layer overlying an absorptive layer and a relatively thick overcoat layer overlying the capper layer.

11. An optical recording medium according to claim 7, 8 or 10 wherein the absorptive layer is composed of a material selected from the group consisting of titanium, rhodium, tellurium, selenium, tellurium based alloys, selenium based alloys, arsenic trisulfide, and arsenic triselenide.

12. An optical recording medium according to any preceding claim wherein the indentations are arranged in the form of a groove in said major surface of the substrate.

13. An optical recording medium according to any of claims 1-11 wherein the indentations are arranged as an alternating series of first and second regions.

14. An optical recording medium according to any preceding claim wherein a portion of the indentations have depths extending a distance into the substrate different from the remaining indentations thereby forming a third region having a reflectivity different from the reflectivities of the first and second regions.

15. An information record having information recorded therein comprising: a substrate having a major surface with a plurality of indentations therein and a covering comprising at least an absorptive layer, which covering overlies the major surface and fills the indentations and is absorptive of light at the wavelength of a recording and a readout light beam; thereby forming first and second regions in the record such that the recording sensitivity in the first regions is greater than the recording sensitivity in the second regions; and wherein the recorded information is represented by a plurality of the first regions having different optical properties from the remainder of the first regions and from the second regions, with the length and spacing of the first regions having different optical properties being indicative of the recorded information.

16. An information record according to claim 15, wherein the indentations extend a distance between about 5 nanometers and about 200 nanometers into the substrate from the major surface.

17. An information record according to claim 15 wherein the substrate is composed of a plastics material.

18. An information record according to claim 15, 16 or 17 wherein said covering further comprises a reflective layer interposed between the substrate and the absorptive layer and which reflects a substantial portion of the light incident thereon at the wavelengths of the recording and readout light beams; and wherein the thickness of the absorptive layer is chosen such that the reflectivity of the record in the first regions is reduced.

19. An information record according to any of claims 15-18 wherein the thickness of the absorptive layer in the first regions is such that the reflectivity is minimised.

20. An information record according to claim 18 wherein said covering further comprises a spacer layer interposed between the reflective layer and the absorptive layer and filling the indentations; wherein the thicknesses of the spacer and absorptive layers are such that the reflectivity of the record in the first regions is reduced.

21. An information record according to claim 20 wherein the thicknesses of the spacer and absorptive layers in the first regions are such that the reflectivity is minimised.

22. An information record according to any of claims 15-21 further comprising a relatively thick overcoat layer overlying the absorptive layer.

23. An information record according to claim 20 or 21 further comprising a capping layer overlying the absorptive layer and a thick overcoat layer overlying the capping layer.

24. An information record according to claim 20, 21 or 23 wherein the absorptive layer is composed of a material selected from the group consisting of titanium, rhodium, tellurium, selenium, tellurium based alloys, selenium based alloys, arsenic trisulfide, and arsenic triselenide.

25. An information record according to any of claims 15-24 wherein the indentations are arranged in the form of a groove in the major surface of the substrate.

26. An information record according to any of claims 15-24 wherein the indentations are arranged as an alternating series of first and second regions.

27. An information record according to any of claims 15-24 wherein a portion of the indentations have depths extending a distance into the substrate different from the remaining indentations thereby forming a third region having a different optical properties from the optical properties of the first and second regions.

28. An optical recording medium substantially as hereinbefore described with reference to Figs. 1-4 of the accompanying drawings.

29. An information record substantially as hereinbefore described with reference to Figs.

1-4 of the accompanying drawings in conjunction with Fig. 5 or 6 thereof.