Optical disk

Disclosed is an optical disk having rows of pits, corresponding to information signals, formed on a transparent substrate, and a reflective film formed on this substrate. The reflective film is formed of amorphous silicon (a-Si) or indium antimonide (InSb), which exhibits a third-order non-linear optical effect. Thus, the reflectance of the reflective film formed of this material increases nearly in proportion to the intensity of the irradiated beam spot. The beam spot irradiated on the optical disk has the highest intensity at the center portion and this intensity becomes weaker as a point in the spot approaches the edge. Therefore, the effective spot size of the beam spot can be reduced, making it possible to reproduce from the reflection type optical disk information signals with a spatial frequency above the cutoff spatial frequency defined by the reproducing optical system. In other words, a row of pits recorded at a high density can be reproduced. As a change in the reflectance of the reflective film does not alter the structure of the material, the reflective film has a fast response to the reflectance change and is hardly deteriorated by the repetitive change in reflectance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a reflection type optical disk. 
2. Description of the Related Art 
A reflection type optical disk has an information recording surface on 
which rows of pits corresponding to information signals are formed as 
tracks. The signals are reproduced by utilizing the phenomenon that when a 
reproduction or read beam spot is irradiated on a row of pits, the 
diffraction by the pits greatly reduces the amount of reflected light. A 
change in the amount of reflected light, which is obtained when the beam 
spot passes through the mirror portion and the target pit portion, is 
converted into an electric signal by a light-receiving unit, thus yielding 
the corresponding information signal. 
For such an optical disk, the signal reproduction resolution is limited 
mostly by the wavelength k of the light source of the reproducing optical 
system and the number of apertures, NA, of the objective lens, and the 
spatial frequency (2NA/.lambda.)) is the cutoff spatial frequency that 
defines the limit of information signals. To achieve high density 
recording of pits on the optical disk, therefore, it is necessary to 
shorten the wavelength .lambda. of the light source of the reproducing 
optical system (e.g., a semiconductor laser) and to increase the number of 
apertures, NA, of the objective lens. Due to technical limitation on the 
improvement of the wavelength of the light source and the number of 
apertures of the objective lens, however, it is difficult to remarkably 
improve the recording density. 
Under such circumstances, Japanese Patent Laid-open No. 3-292632 discloses 
an optical disk whose reflective film is formed of a material of which 
reflectance varies with temperature, such as a thermochromic material or a 
phase-changeable material, to thereby partially change the reflectance 
within the read beam spot. This structure can reduce the effective spot 
size of the read beam spot on the optical disk, allowing the reproduction 
of a spatial frequency above the cutoff spatial frequency defined by the 
reproducing optical system. 
Since the aforementioned material changes its material structure with a 
change of its temperature to vary the reflectance, its response speed is 
generally slow and its repetition characteristic is still poor. 
Furthermore, it is necessary to perform an erasing operation to return the 
changed material structure to the initial status. 
SUMMARY OF THE INVENTION 
With a view to solving the above problems, this invention has been 
accomplished, and it is a primary object of the present invention to 
provide a reflection type optical disk which exhibits a quick response to 
a change in reflectance and an excellent repetition characteristic. 
It is another object of the present invention to provide a reflection type 
optical disk capable of increasing the recording density of pits. 
To achieve the foregoing and other objects and in accordance with the 
purpose of the present invention, a reflection type optical disk according 
to this invention includes a reflection film formed of a material 
exhibiting a non-linear optical effect of the third order. 
When a read beam spot is irradiated on the reflection type optical disk of 
this invention, the status of electrons in the reflective film within that 
beam spot varies in accordance with the light intensity, producing a 
non-linear optical effect of the third order. As the light intensity of 
the beam spot increases, therefore, the reflectance increases. In general, 
as the point of interest gets closer to the center of the beam spot, the 
beam spot has a stronger light intensity. The distribution of the light 
intensity of the beam spot of the reflected light over the optical disk 
becomes greater at the center portion of the beam spot due to a higher 
reflectance of the beam spot at the center portion, whereas this 
distribution of the light intensity becomes smaller at the edge portion of 
the beam spot due to a lower reflectance of the beam spot at the edge 
portion. That is, the effective spot size of the beam spot can be reduced. 
It is therefore possible to reproduce information signals with a spatial 
frequency above the cutoff spatial frequency of the reproducing optical 
system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The features of the present invention that are believed to be novel are set 
forth with particularity in the appended claims. The invention, together 
with objects and advantages thereof, may best be understood by reference 
to the following description of the presently preferred embodiment 
together with the accompanying drawings. 
FIG. 1 illustrates a reflection type optical disk 1 according to the 
present invention. This optical disk 1 has rows of pits, corresponding to 
information signals, formed as tracks on a transparent substrate 2, and a 
reflective film 3 formed on this pits-formed substrate 2. 
The reflective film 3 is formed of a third-order non-linear optical 
material, such as amorphous silicon (a-Si; including the one doped with 
hydrogen (H) or nitrogen (N)) or indium antimonide (InSb), which exhibits 
a third-order non-linear optical effect. This third-order non-linear 
optical effect is one of non-linear optical responses that are observed 
when a laser beam or the like enters in the material. 
The non-linear optical responses of a material are non-linear responses 
that the electric polarization P of the material shows with respect to a 
photoelectric field E of the incident wave. The electric polarization P is 
generally expressed by the following equation. 
EQU P=X.sup.(1) E+X.sup.(2) E.sup.2 +X.sup.(3) E.sup.3 +. . . (1) 
where X.sup.(1) is a linear susceptibility, X.sup.(1) (i+2,3, . . . ) is a 
non-linear susceptibility. In the equation (1), the first term represents 
a linear polarization, the second term a second-order non-linear 
polarization, and the third term a third-order non-linear polarization. 
The third-order non-linear optical effect utilized in the present 
invention indicates a non-linear refractive index effect that is a change 
in the refractive index of a material in proportion to the intensity of 
the incident wave due to the third-order non-linear polarization of the 
equation (1). Of non-linear optical materials showing the non-linear 
optical effect, those which have large values of X.sup.(3) (esu) as shown 
in Table 1 below are used as the material for the reflective film 3. 
TABLE 1 
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Non-linear Optical Material 
X.sup.(3) (esu) 
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a-Si .about.10.sup.-3 
InSb .about.10.sup.-4 
a-As2S.sub.3 .about.10.sup.-4 
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Given that the intensity of the incident light is I, the refractive index 
is n, the linear refractive index is n.sub.o and non-linear refractive 
index is n.sub.2 (cm.sup.2 /w), the reflectance R when the reflection film 
3 exhibits the non-linear refractive index effect is expressed as follows: 
##EQU1## 
where C(cm/s) is the light speed. Thus, a change in the reflectance R of 
the reflective film 3 with respect to the intensity of the incident wave 
therein becomes as shown in FIG. 2. That is, the reflectance R for the 
above-described reflective film 3 increases nearly in proportion to the 
incident-wave intensity I. 
Information reproduction from the reflection type optical disk of this 
embodiment will be described below. 
As shown in FIG. 3(a), when a reading beam spot 5 is irradiated on the 
optical disk 1 on which a row of pits 4 corresponding to information 
signals are formed, the light intensity I of the beam spot 5 irradiated on 
the optical disk 1 has a distribution as shown in FIG. 3(b). A third-order 
non-linear optical effect appears in a beam spot region 6 on the optical 
disk 1 at this time, and the reflectance R increases in accordance with 
the light intensity I of the irradiated beam spot 5. Accordingly, the 
reflectance R in the beam spot region 6 becomes the highest at the center 
portion of the spot and the lowest at the edge portion of the spot, as 
shown in FIG. 3(c). The reflected spot reflected by the optical disk 1 
therefore has such an intensity distribution that the intensity at the 
edge portion is considerably weaker than the intensity at the center 
portion as shown in FIG. 3(d). That is, the effective spot size .phi. is 
reduced. The dotted line 1 in FIG. 3(d) indicates the light intensity 
distribution of the reflected spot when the reflectance is constant. (It 
should be noted that the light intensity at the center of the spot is the 
same in order to compare the spot sizes.) 
As the effective spot size .phi. according to the present invention is 
smaller than that of the conventional beam spot having a uniform 
reflectance in the beam spot region 6, it is possible to read from the 
optical disk 1 information signals having a spatial frequency above the 
cutoff spatial frequency, which is determined by the number of apertures, 
NA, of the objective lens, and the wavelength .lambda. of the 
semiconductor laser. In other words, the row of pits 4 recorded at a high 
density can be reproduced. 
A change in the reflectance R of the reflection film 3 is caused by a 
change in the status of electrons in the material that occurs due to the 
third-order non-linear optical effect. This phenomenon does not therefore 
change the material structure, so that the response speed with respect to 
the reflectance change becomes several tens of nanoseconds or smaller as 
shown in Table 2 below without carrying out any thermal optimization which 
is normally needed for a phase-changing material. 
TABLE 2 
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Repetition 
Material Response Time 
Characteristic 
______________________________________ 
Conventional 
Thermally 10.sup.-3 -10.sup.-8 sec 
about 10000 
Materials phase-changing times 
material 
Thermochromic 
10.sup.2 -10.sup.-9 sec 
about 20000 
material times 
3rd-order a-Si 10.sup.2 -10.sup.-12 sec 
infinite 
Non-linear 
InSb 10.sup.-8 -10.sup.-9 sec 
infinite 
Materials 
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When the beam spot region 6 on the optical disk 1 moves away, the electron 
status of the reflective film 3 returns to the original status and the 
reflectance R also returns to the initial value. The recovery time is on 
the order of several tens of nanoseconds or below. It is therefore 
unnecessary to perform an erasing operation to recover the reflectance R, 
eliminating the need for any device which performs erasure. This can 
contribute to reducing the required number of components of an optical 
disk player. Further, as the material structure of the reflective film 3 
is unchangeable, this optical disk shows an excellent characteristic of 
the repetitive change in reflectance, thus preventing the reflective film 
from being deteriorated by the repetitive change in reflectance. 
If the a-Si film in the above-described embodiment is doped with hydrogen 
(H) or nitrogen (N), the same advantage would be obtained. 
In short, according to the reflection type optical disk of the present 
invention, the reflective film is formed of a material that shows a 
third-order non-linear optical effect, the reflectance of the reflective 
film increases nearly in proportion to the intensity of the beam spot 
irradiated on the optical disk. The effective spot size of the beam spot 
can therefore be reduced, making it possible to reproduce information 
signals with a spatial frequency above the cutoff spatial frequency 
limited by the reproducing optical system. In other words, a row of pits 
recorded at a high density can be reproduced. As a change in the 
reflectance of the reflective film is caused by a change in the status of 
electrons that occurs due to the third-order non-linear optical effect, 
the response speed can be improved to thereby prevent the reflective film 
from being deteriorated by the repetitive change in reflectance.