Protective Layer
Reflective Layer (Ag)
/
�
Photochromic Layer (FC-124)
Substrate (Polycarbonate)
Figure 7-21. Illustration of read-only disk structure with a photochromic mask layer.
Sample Disk
Mirror
UV light source (Hg lamp)
I
Pickup NA 0. 55 A-=685
ntnFigure 7-22. Composition of the apparatus for super-resolution readout.
The readout pickup had a laser diode (A-=685 nm) and a objective lens (NA=0.55). The ultraviolet (UV) light for coloring the mask layer was directed to a location on the disk using an optical fiber from a Hg lamp.
11:-3
Figure 7-23 shows a crosstalk comparison for disk substrate 1 without a tracking servo. For the sample without a mask layer, the observed crosstalk was 42 o/o. On the other hand, the crosstalk for the sample with a mask layer was reduced to 18 °1o. This result shows that higher track density can be expected by using a photochromic mask layer. Figure 7-24 shows the comparison of readout eye patterns for disk substrate 1. The amplitude of the shortest (3T) pit was increased from 62 o/o to 81 % by forming the mask layer. Figure 7-25 shows a readout eye pattern comparison for disk substrate 2. For the sample without a mask layer, the amplitude of the shortest (3T) pit was 29 o/o. On the other hand, the amplitude of the 3T pit for the sample with a mask layer was higher at 45 o/o. These results indicate that a higher linear density can also be expected using a photochromic mask layer.
Without a mask With a mask
Figure 7-23. Crosstalk comparison for disk substrate 1 without a tracking servo. The left and right figures correspond to the case without a mask layer and with a mask layer, respectively. The readout laser power was 2.4mW and the relative speed was 1.0 m/s.
Without a mask With a mask
Figure 7-24. Readout eye pattern comparison for disk substrate 1. The left and right figures correspond to the case without a mask layer and with a mask layer, respectively. The readout laser power was 2.4 mW and the relative speed was I. 0 m/s.
Without a mask With a mask
Figure 7-25. Readout eye pattern comparison for disk substrate 2. The left and right figures correspond to the case without a mask layer and with a mask layer, respectively. The readout laser power was 2.4 m W and the relative speed was 0. 7 m/s.
115
7.5 Optical Disk Mastering using Photochromic Super-Resolution 7.5.1 Equations for Super-Resolution Disk Mastering
A super-resolution disk technique that uses a photochromic (or photon-mode bleachable) dye mask layer was proposed in the previous section. Theoretical simulations and experiments revealed that photon-mode mask layers with high optical density are needed for effective super-resolution, that is, high recording density.
Recently, an experimental result of super-resolution using a photo-bleachable dye was reported. [76] The aim of this section is to analyze the photon-mode super-resolution disk mastering process theoretically and to reveal the factors for high-quality pit shapes.
Our previous discussion showed that the transmittance T of a photon-mode dye layer changes according to equation (7 -17) and that the photo-irradiation time dependence ofT is nonlinear when the layer has high optical density. This equation was derived for photochromic super-resolution media having a single photon-mode layer, as shown in Fig. 7-26 a.
Incident light
.---9---, � Photoreactive layer (resist) 'Reflective layer
�Substrate
�---�
a. A single photreactive layer medium
Incident light
Photoreactive dye mask layer
"
, � Photoreactive resist layer
' Reflective layer
� Substrate
b. A two photoreactive layer medium
Figure 7-26. a. Conventional disk mastering. The medium has a single photoreactive layer (resist). b. Super-resolution disk mastering. The medium has two photoreactive layers, that is, a photoreactive bleachable dye mask layer and a resist layer.
116
In the super-resolution mastering (Fig. 1 b), the disk has a bleachable dye mask layer and a photoreactive resist layer. Therefore, equation (7 -17) must be modified for the structure having two photoreactive layers.
For simplicity, we assume that the resist molecules are bleachable and are converted to molecules with high solubility (positive-type resist). [77] Transmittances T,t of the mask layer and TR of the resist layer, which contain photo-bleachable dye molecules (the molar extinction coefficient is
&u)
of concentrationCA.1
and resist molecules (cH.)
ofconcentration C'R, are given by eqs. (7 -31) and (7 -3
2).
where L,,J and LR are the thickness of the mask layer and that of resist layer, respectively.
When a light beam of wavelength A and intensity P is irradiated to the medium, the numbers of photons absorbed by the mask layer and that absorbed by the resist layer during infinitesimal time
dt
are given by eq. (7-33) and eq. (7-34), respectively.n -:�:n
If the photo-bleachable molecules and the resist molecules have the quantum yields ¢,1 and ¢R, respectively, the numbers of reacting molecules
dN,H
and dNH aredNR
=-dnRrpR.
From the relationship between the molarity and the number of molecules, dN,t and dNN can also be expressed as follows:
117
where._)' is the irradiation area and factor 1 o-3 is the correction of units.
From eqs.,
(7-33), (7-34), (7-35), (7-36), (7-37)
and(7-38),
the following differential equations can be derived.Equations
(7-31)
and(7-32)
give relations(7-41)
and(7-42).
oJ;{
=
-2 3
·[,R R R .
L 1'oCR
ot ot
Then, differential equations
(7-43)
and(7-44)
aboutTu
and TR are derived as(7 -1 0)
(7 -11)
(7 -12)
(7 -11)
These equations describe the transmittance changes of the mask layer and resist layer for a uniform light intensity distribution, and correspond to the eq.
(7 -17)
for a single photoreactive layer medium.In order to apply equations
(7 -43)
and(7 -44)
to analyze any spot intensity distribution and resulting pit shape, the expression P S in eqs.(7 -43)
and(7 -44)
are replaced by generalized light flux densityFlux( X, Y,t),
givingiJTH �' Y,t)
�af7ux( X, Y,t)AJ':M rpM TM (X, Y,t)(l- TM (X,
Y,I) xl
+R, 7;, (X,
Y,t)' 7H (X, Y,t) )
(7-1G)
118
(7 -1())
where the coordinates
X
andY
are fixed on the disk andY=O
indicates the center of the track.For a gaussian spot, the flux density
Flux(1(X,Y,t)
of the irradiation is given by eq.(7-47),
where (jj==2
J2
a=.AINA gives the relation between spot diameter l1J and a, and factor 1 o-�is the correction of units. By substituting eq. (7-47) to
rlux(X, Y,t)
of eqs. (7-45) and (7 -46 ), and numerically integrating eqs. (7 -45) and (7 -46) byt,
we get the transmittance distributions1;\t(X, Y,t)
and7R(X, Y, t).
P will be modulated between ON and OFF by the recording signal. By using eq.
(7-32), the number of reacted resist molecules
(
ex: CR(0)-
CR(t))
is given from'l/(.
Theformed pit shape corresponds to the number of reacted resist molecules However, solubility of the resist does not linearly correlate with the number of reacted molecules.
We, therefore, suppose a nonlinear solubility function Q and estimate the pit shape in terms of the function Q.
(7 -!I H)
where the coefficient
K(M-1)
was introduced for correction of unit. Figure 7-27 shows a comparison between CR(O)-CR(t) and Q by log (irradiated light quantity). In this figure, the vertical axes are normalized. Nonlinear response property was enhanced by introducing above function Q.-4 -3 -2 -1 0
Log( irradiated light quantity
(J) )
-4 -3 -2 - 1 0
Log( irradiated light quantity
(J) )
Figure 7-27. Comparison between
CR(O)-CR(t)
and nonlinear solubility function Q vs log(irradiated light quantity). The vertical axis are normalized.7 .5.2 Several Numerical Simulations
In our simulations, several parameters were fixed for simplicity. Table 7-Ill lists the values of these parameters. At first, we compare 3-dimensional pit shapes between the conventional (without mask) method and the super-resolution method using a mask layer Figures 7-28 and 7-29 show shapes of simulated short pits. ln the simulations, we assumed that the sensitivity of the mask layer ( c-l\1
¢1v
1) was equal to that of the resist layer (cR.�), and also assumed that the initial transmittance of the mask layer was 0.01 in order to obtain the super-resolution effect. The recording UV laser power was varied from 1.0 mW to 8.0 mW. Because of the absorption of the mask layer, the light quantity reaching the resist layer with the super-resolution method was smaller than that with the conventional method. To obtain the same magnitude for the top area of a pit with the super-resolution method, a higher recording power is needed. For example, the top area of a pit at the recording power of 2.0 mW with the conventional method was about 0.2 x 0.2 ,um2. To obtain the same top area with the super-resolution method, a power of 4.0 mW was required. Even when we compare pits that have the same top area, appreciable improvement of the pit shape is clearly seen in Figs. 7-28 and 7-29.Table 7-111. Fixed parameters used for the calculations.
Recording laser wavelength (m) 351 X I 0-0 Numerical aperture of the objective lens 0.9
Relative speed (m/s) 1. 75
Reflectance of the reflective layer 1.0
Sensitivity of the resist layer
HR�
(M-1cm-1) 1.0X 104Initial transmittance of the resist layer
TR(X,Y,t=O)
0.1Recording frequency (Hz) 3. 0 X 1 06 (for short pits) 1.0 X 106 (for long pits)
Duty ratio of the recording laser pulse 0.1 (for short pits) 0.5 (for long pits)
P=1.0 mW
P=2.0 mW
P=4.0 mW
P=8.0 mW
Figure 7-28. Three-dimensional shapes of simulated short pits formed without the dye mask layer. The recording power was varied from 1. 0 mW to 8.0 mW. The calculation unit (mesh) was 0.05 f1Il1 square.
122
P=1.0 mW
P=2.0 mW
P=4.0 mW
P=8.0 mW
Figure 7-29. Three-dimensional shapes of simulated short pits formed with the dye mask layer. The recording power was varied from 1. 0 m W to 8 0 mW. The calculation unit (mesh) was 0.05 J1ll1 square.
12�-3
Figure 7-3 0 shows the detailed comparison between the sections of the pits in the radial direction. The calculation unit (mesh) was 0.025 J1Il1 square. The pit formed by the conventional method (P= 1. 5 m W) has a wall with a low angle slope. On the other hand, the pit formed by the super-resolution method (P=4.0 mW) has a wall with a steep slope and, therefore, has a sharp shape. Since ideal pits should have a vertical wall, the pit formed by the super-resolution method has a higher quality than that formed by the conventional method.
Without mask P=1.5 mW
With a mask (T)\r=0.01) P=4mW
0.2 !J.m
� _..
Figure 7-30. Detailed comparison between the sections in the radial direction of the pits with and without the dye mask layer. The calculation unit (mesh) was 0.025 J1Il1 square.
124
Figure 7-31 shows a comparison of long pits formed by the conventional method and by the super-resolution method. The same tendency as for the short pits was observed. In the calculations, the recording frequency was 1.0 x 106 Hz, the duty ratio of the recording laser pulse was 0.5, the recording laser power was 2.0 mW for the conventional method and 4.0 mW for the super-resolution method, and the other conditions were the same as those listed in Table 7-III.
To know the parameters affecting the pit shape, we carried out several simulations by changing the parameters. The parameters that were varied were the sensitivity and the transmittance of the mask; other parameters were fixed as in Table 7-III.
Figure 7-32 shows the mask sensitivity dependence of the pit shape, the sections in the radial direction. TM was fixed at 0. 01 , and the recording power was adjusted so that the top area of each pit has a magnitude of 0.2 x 0.2 ;m? Under three different sensitivities ( tr-.1¢t.1= 1.0 x 103, 1.0 x 104 and 4.0 x 104 ), the sections of the pits have approximately the same shapes, but the edge of the top area for a high sensitivity mask is rounded slightly.
Figure 7-3 3 shows the mask transmittance dependence of the pit shape, the sections in the radial direction. The recording power was also adjusted in this case so that the top area of each pit has a magnitude of 0.2 x 0.2 JLI112. Under four different transmittances (T=1 .0 (without a mask), 0.2, 0.01 and 0.002), the sections of the pits have different shapes. Apparently, the pit formed by utilizing a low transmittance mask has a wall with a steep slope, and has a sharp shape. We can, therefore, conclude that the transmittance of the mask directly affects the quality of the pit shape, and that a high
quality pit shape can be obtained by using a low transmittance mask.
Figure 7-31. Three-dimensional shapes of simulated long pits. The upper figure shows pits formed without the dye mask layer and the lower figure shows pits formed with the dye mask layer. The recording power wa 2. 0 mW and 4.0mW, respectively. The calculation unit (mesh) was 0.05 pm square.
12G
0.2 �m
� .
l:M
¢M
= 1 000 l/(mol· em) P = 19 mW.s·M
¢M
= 10000 1/(mol· ctn) P = 4.0 mW&M
¢M
= 40000 1/(mol· em)P = 2.2 mW
Figure 7-32. Mask sensitivity dependence of a pit shape. The section in the radial direction are displayed. The calculation unit (mesh) was 0.025 ;m1 square.
127
Tr-.1 = 0.2 P = 2.5 mW
T':-.1 = 0.01
= 4.0 mW
T I= 0.002
= 4.7 mW
Figure 7-33. Mask transmittance dependence of a pit shape. The sections in the radial direction are displayed. The calculation unit (mesh) was 0.025 Jlil1 square.
128
7.6 Conclusion
The application of a saturable or photochromic dye rna k layer for uper
resolution in an optical memory w.as examined. Super-resolution readout for high recording density read-only disks was demonstrated using a photochromic diarylethene mask layer . A nonlinear transmittance change was achieved when the initial optical density in the mask layer was high.
Chapter 8
Summary and Conclusion
The aim of the present research was to study the application of photochromic diarylethenes for high-density optical memory.
The important results of each chapter are summarized as follow
Chapter 2 refers to light sources for photochromic reactions. Single GaN ba ed LED's were found to emit two wavelengths of light, that is, UV and visible or tv.o vi ible light, depending on the driving modes, pulsed or constant current modes Photochromic coloring and bleaching reactions of photochromic diarylethenes are performed u tng the single GaN-based LED's. GaN-based light-emitting LED's are promi ing light out c s
for application of photochromic devices.
Chapter 3 deals with a photochromic reactivity of 2-(1 ,2-dimethyl-3-indolyl)-3-(2,4,5-trimethyl-3-thienyl)-maleic anhydride in polymer matrices. The sensitivity (product of molar extinction coefficient and quantum yield) was found to depend on it, concentration in poly(vinyl butyral) film. Energy transfer from the open-ring to the closed-ring form was suggested as a possible mechanism for the time dependence of the sensitivity in the polymer film containing high chromophore concentration by irradiation with 515 nm light.
In chapter 4, a theoretical relation between the sensitivity of phot chromic compounds and possible data transfer rate was derived and a new nondestructive readout method, namely a super! ow-power readout, was proposed. 1 06 readout cycle were demonstrated for the photochromic diarylethene media.
In chapter 5, is concerned with multi-wavelength recording. Theoretical analysi of crosstalks between multiplexed channels showed a new possibility of cro talk reduction. The proposed method was demonstrated for two-wavelength recording using diarylethene derivatives having broad absorption bands. The crosstalk wa dramatically reduced from -2dB to -25dB by the method. Super-low power readout method described in the previous chapter was also uccessfully applied to the multi
wa elength recording media.
1:10
Chapter 6 deals with the optical density dependence of write/read characteristic of the photochromic memory. Low secondary harmonics, many times readout operation with a superlow-power laser and low nonlinear crosstalk of multi-wavelength recording were demonstrated for high optical density media.
Chapter 7 is concerned with super-resolution optical disks with a photochromic mask layer. Theory of super-resolution with a photon-mode ( aturable dye and photochromic dye) mask layer was formulated. Super-resolution readout wa ucce fully performed for high-density read-only disks. Super-resolution mastering method wa al o analyzed. It has become apparent that high quality pits for a read-only di k were able to be formed by using the method.
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Acknowledgements
The author wishes to express to his sincerest gratitude to Profes r Ma ahiro lne, Kyushu University, for his guidance and hearty encouragement through thi work The author would like to acknowledge the helpful suggestions of Professor Tisato Kajiyama, Professor Junichi Hojo and Professor Jun Yamada of Kyushu University with the compilation of this dissertation.
The author would like to thank Dr. Yukinori Kuwano, managing director of the R&D headquarters, SANYO Electric Co. Ltd., and Dr. Shoichi Nakano, general manager of the New Materials Research Center, SANYO Electric Co., Ltd , for their encouragement.
The author's grateful thanks are especially due to Dr. Naonori Hirata, Mr Kimi Yasuda, Mr. Meguru Ohara and Mr. Katsutoshi Hirose, Kobe Natural Product &
Chemicals Co. Ltd., and Mr. Atsushi Ishikawa and Mr. Yukio Horikawa, Development Lab., Kanebo Ltd., for synthesizing the photochromic diarylethene and helpful discussions.
The author is deeply grateful to Mr. Toshio Harada and Mr. Fumio Tatezono, SANYO Electric Co. Ltd., for their occasional discussions and profound intere t
The author is also thankful to Dr. Ken'ichi Shibata, Dr. Minoru Kume, Mr.
Yasuhiko Itou, Mr. Kazuhiko Kuroki, Mr. Yoshiaki Shimizu. Mr. Sakuro Kuroda, Mr Koutaro Matsuura, Mr. Seiji Murata and Dr. Kenji Torazawa , SANYO Electric o. Ltd , for their support and encouragement.
Finally, the author is particularly grateful to his wife, Akiko T ujioka for her understanding and encouragement.
September 1997 Tsuyoshi Tsujioka