フォトクロミックジアリールエテンを用いた高密度 光メモリの研究

九州大学学術情報リポジトリ

Kyushu University Institutional Repository

フォトクロミックジアリールエテンを用いた高密度 光メモリの研究

辻岡, 強

https://doi.org/10.11501/3132442

出版情報：Kyushu University, 1997, 博士（工学）, 論文博士 バージョン：

権利関係：

CHAPTER 6

Optical Density Dependence of

Write/Read Characteristics in Photon-Mode Photochromic Memory

6.1 Introduction

The quantitative relation among irradiation laser power, material sensitivity and reflectance changes for photochromic media have already been derived , and readout and crosstalk characteristics in multi-wavelength photochromic recording ha ^e been discussed . The results showed that the initial optical density affects various vvrite/read characteristics in the photon-mode photochromic recording. The objective of this chapter is to examine the optical density dependence of secondary harmonic , readout and crosstalk characteristics in multi-wavelength recording to obtain better write/read characteristics.

The equations

( 4-7),

(

4-8)

and (

4-1 0)

were derived in chapter 4. Th ^se equations describe the reflectance change of photochromic media upon photo-irradiation The differential equation (

4-7)

is integrated with the initial reflectance Rnu

R1111 =

[1

^{+ const.}

rl

⁼ exp

(

-

4

_.

606 X()})}

^{({)- 1)}

where ()J) is the optical density of the recording layer. A second equality exi ts when the medium structure is as shown in Fig.

6-1

with a perfect reflective layer. The behavior of the reflectance change depends on Rmr

These equations were expand to analyze the recording mark shapes and secondary harmonics (Sec.

6.2),

readout characteristics (Sec.

6.3)

and the eros talk in multi­

wavelength recording (Sec.

6.4).

In each section, theoretical estimates are compared with the experimental results.

G9

write I read laser beam

substrate

recording layer

� reflective layer

Figure 6-1. Structure of a photochromic optical medium. The recording layer contains photochromic molecules.

6.2 Recording Mark Shapes and Secondary Harmonics

In heat-mode recording, the recording mark ize depend on the writing laser power and the thermal diffusion rate in the recording layer. 1t is well known that there is an optimum writing laser power which gives minimal secondary harm nic in the readout signal. On the other hand, there is no thermal diffusion in photon-mode recording In thi, section, we quantitatively examined what factors control the mark ize Equation ( -l-<

)

was extended to eq. (6-2), which showed the reflectance distribution RC\', }) obtained by scanning the medium with a general laser spot, such as Gaussian spot

R( X , Y) ⁼

[1

^{+ const.x exp}

(

^{-2aF( X ,}

^Y)A.

^·

e;·¢)J'

where .¥ and Y indicate the coordinate fixed on the medium, and are parallel and perpendicular to the recording track direction, respectively. It is assumed that the origin of the Y coordinate lies at the center of the track. Equation (6-2) indicate that the reflectance R..(X, Y) is uniquely determined by the total irradiation F(X,

Y)

at the point

C \',})

The light irradiation distribution F(X,Y) must be know to calculate the reflectance distribution R(X, Y) . The irradiation flux F, which is expressed by eq.

( 4-1 0 )

, should be re-interpret as the local irradiation flux.

F ⁼

p�,w

^{--+ F(X,Y) =}

f�

Flux(X ,Y,t)dt

Flux(X, Y,t) is derived as follows. We consider the Gaussian pot of a recording light beam with a relative velocity ^v (m/s), which is power modulated by recording signal Stx(l) (for example, Sig(t)=8(cosjtl2n), 8: step function,/ recording frequency) lf the variabl ^·

x andy indicate the coordinate fixed on the spot and its origin lie at the center of the pot, Flux(x,y,t) is expressed as follows,

Flux(x, y, t) ⁼ Flux11 ^• 5'ig(l) ^· exp

(

^{-(x� + y2}

⁾

^{I a2}

)

^((i-!J)

where the relation between spot diameter <l> and ^a is given by <t> ⁼ 2

J2

^{a = A I NA , and}

(()-!))

If the origin of y lies at the center of the track, we can set x=X+vt andy= Y, and obtain the

71

following equation:

Fl

^{ux x,y,t} ( )

⁼

^� P.

^· S'.

^1g(t)

^{· exp}

( -((X+ vt)- + y-) I

, , a-,

)

na-

Therefore, we can derive eq.

(6-7)

_{from eqs.}

(6-3)

and

(6-6).

((i-(i)

F(x,y)

�

::'

^{, · exp}

⁽

^-Y'

^I

^a'

⁾ £

^Sig(l)exp

⁽

^-

^{(X+ vt)}

^'

^I

^a'

^}it

^((i-7)

Equations

(6-2), (6-7)

_and

(6-1 )

, which corresponds to the initial condition, determine th reflectance distribution

R(X, Y),

and hence determine the recorded mark hap e.

Figure

6-2

shows the recording marks obtained using the above theoretical calculation for various

R1111

(related to

OD

_{by eq.}

(6-1 ))

. The calculation wa carried out under the following conditions: laser wavelength A,

=633

nm, recording power fJ, , -I ) mW, relative speed

v=l.4

m/s, sensitivity of the medium

8¢ =1400

M-1cm-1, recording signal frequency

.f=600

kHz, and numerical aperture of the objective lens

NA-O

) The contrast levels in each figure are normalized by the difference between initial and maximum reflectance levels. This figure indicates that the mark expan ion i strongly dependent on the initial reflectance, and is suppressed by setting low R1111 (high 0/J) values even under the same recording conditions.

Figures

6-3

and

6-4

show the recording power dependence of the recording marks for

R../11/=0.05

_and

R1111=0.50,

respectively. In both figures, the recording condition�

are the same as those above, except for the recording power. The higher the power, the larger the marks become as shown in Fig.

6-4

_(low

OJJ),

but the mark ize ^· ho\\ n in Fig

6-3

(high

()f))

are negligibly dependent on the recording power in thi range The mark expansion in the low

()f)

medium is due to the laser spot width expansion and the linca1 reflectance change around

R=O. 5

_(Fig.

4-3

₎, and causes a duty ratio deviation of readout signals. These computed results predict that the secondary harmonic in the readout signals for a low ()[) medium are relatively large and increase with recording power On the other hand, the secondary harmonics for a high

()f)

medium are almo t constant with recording power.

Figure 6-2. Calculated recording marks for various initial reflectances Rnll The calculation was carried out using eqs. (6-2) and (6-7) under the following conditions: laser wavelength A =633 nm, recording power Prec=1.5 mW, relative speed v=1.4 m/s, sensitivity of medium t:¢ =1400 M-1cm-1, recording signal frequency .f=600 kHz, and numerical aperture of objective len NA=O 5

We carried out an experiment to confirm the above theoretical re ult The photochromic media were prepared as follows. The recording layer (thicknes 0 5 _�-tm) was prepared by spin-coating a cyclohexanone solution containing poly tyrene and photochromic diarylethene, 2-( 1-octyl-2-methyl-3-indolyl)-3-(2,3,5-trimethyl-.3-thyenyl) maleic anhydride, as shown in Fig. 6-5, onto a glass disc substrate The compound converts from the open-ring form to the closed-ring form (from an uncolored tate ( olid line) to a colored state (broken line) in Fig. 6-5) upon irradiation with 450-500 _{nm light} The photogenerated colored form converts to the open-ring form upon irradiation w1th 550-700 nm light. Vacuum-evaporated Ag films were deposited as a reflective lay r ^on the recording layer to obtain media having the structure shown in Fig 6-1. In order to investigate the initial reflectance (R1111) dependence (or ()f) dependence), we prepared two samples: one had Rrnr=0.6 (()f)= 0.1) and the other had Rmr=0.15 ₍ ()/) = 0.4) _{at 633 nm} by regulating the concentration of the photochromic compound The reflectance spectra of these samples in the initial state are shown in Fig. 6-6. We carried out ^{v ..} riting by photobleaching and readout by detecting reflectance changes with a 633 nm He e laser

Free 1.5111 W

J>,.ec-2.5n1W

Figure 6-3. Calculated recording power dependence of marks for a low initial reflectance (high OIJ) medium. The calculation was carried out using eqs. (6-2) _and (6- 7) under the following conditions: initial reflectance Rlm=0.05, laser wavelength A 633 nm, relative speed v=1.4 m/s, sensitivity of medium c-¢=1400 M-1cm-1, recording ignal frequencyf=600 kHz and numerical aperture of objective lens NA=0.5.

1),.(!( =0.5tn W

Figure 6-4. Calculated recording power dependence of marks for a low initial reflectance (high

OD)

medium. The calculation was carried out using eqs (6-2) and (6-

7)

under the following conditions: initial reflectance R11u=0.50, laser wavelength A. �633 nm, relative speed v=1.4 m/s, sensitivity of medium c:¢=1400 M�1cm-1, recording ignal frequency.f=600 kHz and numerical aperture of objective lens NA=O 5.

..c (/)

<(

400

\

\

\

\

\

\

\

M

�

Me

--<

_{S �},

Y�

_{Me N} ��

Y

500

(�ct

Photochrom1c compound

600 700

Wavelength (nm)

Figure

6-5.

Absorption spectra of 2-(l-octyl-2methyl-1-indolyl )-3-(2 ^__ ), 5- trimetyl) maleic anhydride dispersed in polystyrene films

Figure

6-7

shows the apparatus employed for the experiment. It should be noted that a focal servo system using a near-infrared laser diode was adopted in thi� apparatus to avoid undesirable additional photoreactions during focusing, and avalanche photodiodes (APD� Hamamatsu Photonics

S5343),

which have high ensitivity at a short light wavelength, were also adopted. Two laser beams

(633

_{nm and}

780

_{nm) were}

superposed using a dichroic mirror (DM) and focused onto the same spot using a ingle objective lens (OL). The

633

nm laser was modulated to a certain frequency u ing an acoustooptical modulator (A OM) during the recording proce s and introduced into a pickup optical system through an optical fiber which conserved the light polarization. A neutral density

(ND)

filter regulated the laser power. In the reading proces , the beam reflected from the medium passed through the objective lens and was detected by the APD. The apparatus also had an analogous optical system for a

515

nm Ar laser, which will be discussed in a later section. The write/read conditions are summarized in Table

6-

I.

77

/

I

I

I

I

-... y

I/

I

..._ _,1

�

c

50

ro

11 Low 00 i

+-'

I

(.) Q) '+= Q) 0::

/

I

...__/

High 00

o�

^{__ ._ __ �--�--�--�--�}

400 500 600

Wavelength (nm)

700

Figure 6-6. Initial reflectance spectra of the samples used in the experiment

optical fiber

Ar laser

ND AOM t ^{PBS QWP}

J 515nm J-+)J ₀ � 8

f1 -{)t- APD1

HeNe laser

ND AOM

J633nm 1-+rtJ�

f2

ND: neutral density filter

Photochromic Disc

OL �

DM1

�

T /

^�

I

,( 7V _DM2

OWP PBS -t>!-APD2

AOM: acousto-optical modulator

f1 ,f2: recording signal FE sensor

QWP

PBS: polarizing beam splitter

PBS

QWP: quarter-wave plate DM1 ,DM2: dichroic mirror OL: objective lens LD: laser diode

APD1 ,APD2: avalanche photo diode FE sensor: focal error sensor

controller

Figure 6-7. Apparatus for the experiment.

78

780nm LD

Table 6-l. Recording conditions in the experimental for measurements of secondary harmonics

(C2/C)

and carrier-to-noise ratio

(C/N)

Laser wavelength (nm) 633

Writing power (mW) 0 I -2.4

Recording signal frequency (Hz) 300

Relative speed (rn/s) 1.4

Figure 6-8 shows the recording power dependence of the carrier-to-noi e rati and the secondary harmonics. When an R1111=0.6 sample was used, the econdary harmonic monotonically increased in magnitude from -30 dB to -20 dB with incr a ing recording power Prec· This is attributed to the mark expansion shown in Fig. 6-4 On the other hand, when the R1111=0 .15 sample was used, the secondary harmonic was as low as -40 dB. This low value indicates that the duty ratio of readout signal is about 0 5 because of the smaller mark size, and agrees with the result shown in Fig. 6-3 A high ()f) in the recording layer must be set to obtain low secondary harmonic

co � z 0

60 60

R 1=0.6

40

m en

co ::£ 20 ::£

� z 0

u 0 0J

N 20 0

20 u

4

r--

₄₀

••

0 0

Prec [mW] Prec [mW]

Figure 6-8. Recording power dependence of the carrier -to-noise ratio and of the secondary harmonics. Recording conditions are summarized in Table 6-1.

6.3 Repeatedly Readout Characteristics

In this section, we discuss the output level changes after many times readout operations. In the previous chapter we reported the changes of the reflectance and of the signal current

I�

_after m readout operations with a laser power P"'JI ^. ( eqs.

( 4-12)-( 4-17))

Until now, it had been considered that the output monotonically decrea e with repetition of readout operations. However, our theoretical calculation shows that not only monotonic decrease but also temporary increase of the output are pos ible depending on the recording conditions and initial reflectance.

The following experiment, in which the same photochromic media and apparatu as used in the experiment described in Sec. 6.2 were employed, was carried out The write/read conditions are shown in Table 6-II. The readout power was set at

2

_{1-1 W, about}

one hundredfold higher than the super-low-power readout level, in order to accelerate th experiment and to obtain the results rapidly. Figure 6-9 shows the experimental result ^. A temporary increase in the output was observed under the conditions of low writing power and high optical density medium. The readout repeatability was improved about one order of magnitude by using a high OD sample. We can therefore conclude that it i desirable to set a high optical density and to use a suitable writing power to obtain stable readout.

Table 6-II. Write/Read conditions of repeatedly readout characteri tics experiment.

Laser wavelength (nm) 633

Writing power (mW) 2.5

Reading power

(!-l

_W) 20

Recording signal frequency (kHz) 300

Relative speed (m/s)

1.4

� ::J 0...

� ::J 0 -1 0

o o a ^o

0 8

-- ---�_{• LJ} -�_{• •} ·-.__• _.__. __ .___._._

• •

---•• ^---�^{---- -}

• Rini=0.6 Prec=2.5 mW

o Rini=0.15 Prec=1.1 mW

• Rini=0.15 Prec=2.5 mW

•

Readout Cycles

•

• q.

•

Figure 6-9. Readout cycle dependence of carrier levels (experimental) Write/read conditions are summarized in Table 6-II.

6.4 Crosstalk in Multi-Wavelength Recording

The crosstalk property in multi-wavelength recording have already been analyzed and a crosstalk reduction method was proposed . In that paper, the expressions of the crosstalk were derived under the condition of a low optical density ( ()JJ � 0.2) In order to know the optical density dependence of crosstalk, it is necessary to derive equation

We consider, for simplicity, two-wavelength recording using two photochromic materials, A and B, with different absorption maxima at wavelengths A1 and A�, as h wn in Fig. 6-10. At A1, both the compounds A and B have absorption; the ab orption intensity of the compound B is weaker than that of the compound A. At A.7, the compound B has stronger absorption intensity than the compound A The coe. istence of the two absorption bands at the writing/reading wavelength causes cro stalk.

..0 (/)

<(

compound A compound 8

Wavelength

Figure 6-10. Schematic illustration of the spectra of two photochromic compounds with different absorption bands.

Now, we consider a medium has the structure shown in Fig. 6-1, and assume that its recording layer comprises the compounds A and B and that its reflective layer has perfect reflectance. The reflectance Ri at wavelength A1 (i= I ,2) is expressed by

(()-H)

82

where c.:¥1 and Cr are the molar extinction coefficient and concentration of the compound .. _Y (..¥=A, B), respectively. The number of photons dnx 1 absorbed by molecule .\'during infinitesimal time interval dt is

A

C'x,Cx

dn_\.1 ^{= -�} _h ^· P _I ^· (1 - R _I ) ₍' ₍' _dt '

C c,· ^{l1 I} +(:,'Hz H ^{( ( i-!))}

The total number of reacting molecules dNx in terms of light irradiation at wavelennth ).1 and A-2 is

((i-^{1 ())}

The relationship between molarity and molecular number is

dN_\ ⁼ -CxSLNa X 1 _{o-J, ((i-ll)}

Combining eqs. (6-8), (6-9) and (6-10), and transforming Nr to(\- by eq. (6-11), we obtain the following differential equation:

(()-I�)

On the other hand, from the relation of R1 ^(eq. (6-8)), the following equation i derived.

2

_{6' -- =}

()('x

---^{1 1 OR1}

• Xt

ot 4.6L R ot

_\ = . .J .H I

(( i-^{I : l)}

From eqs. (6-12) and (6-13), the following differential equation, which describe R1 and therefore the signal and the crosstalk, can be derived.

() R,

�

^P1A1

^{� x�A.B}

¢x 1 c,·x�6·x� ('x

- =2a --_(' R(1-R )-=-=---

I 1 "" • c

Of J= .:! J..) LJ).'=A.H ^{(:,X 1 X}

(( i-^I 11)

Equation (6-14) corresponds to the extensions of eq. (4-7) for multi-wavelength recording. The solution R1 can be obtained if simultaneous nonlinear eqs. ( 6-14) and ( 6-8)

are solved. However, we could not solve them explicitly. In tead, in order to obtain the solution Rh numerical calculations were carried out, and then, the ignal component (

( J.s)

and crosstalk components

( O,Lc , O,Nc)

were derived by using R., and u ing the equations

(5-11)-(5-14).

(see Chap.

5)

Figure

6-11

shows examples of numerical calculation results. In the calculation , we set the wavelength of the write/read laser beam to

A.1=515

nm for CH 1 and A._.,-63 3 _nm for CH2, and assumed that only crosstalk at CH2 from CHI in the writing proce ^· could exist and that only compound B could absorb at 633 nm, in order to compare the results to those of the experiment. Calculations were made under two condition of the initial reflectance (i.e.,

Rl/11=0.6

(low

OD)

and Rm,=O.l (high

OIJ))

at A._.,=633 nm The irradiation energy at CH2 was used for the horizontal axis in place of the writing laser power and the irradiation energy at CHI was set to a constant value

(50

mJ/cm2). Figure 6

-

^{1 I} show the results for readout channel CH2. A considerable difference is that the nonlinear component

O.,·c

^I

Os

of crosstalk for the high

OD

medium had a minimal value and that for the low

OJ)

medium increased monotonically with increasing irradiation energy

en 2.

1-�

Low OD

---

H1gh OD

0

05 (normalized)

/--�-

g:

^{-20 ONc/Os}

� 0

0.1

Irradiation Energy (J/cm 2)

\

\ \ I I I

/ /

I

01 Irradiation En erg: ( J/cm2)

Figure

6-11.

Multi-wavelength crosstalk behavior.

Os, Ou.fO_,.

and 0 ·('10. ^· indicate signal, linear crosstalk component and nonlinear eros talk component, respectively.

o-/\=0

QYB:>

^{s le s}

compound A

400 500 600

Wavelength (nrn)

400 500

I Oc1

compound B

600

Wavelength (11111)

700

Figure 6-12. Absorption spectra of 2,3-bis(2-methylbenzo[b ]thiophen-3-yJ) maleic anhydride (compound

A)

and 2-( 1-octyl-2-methyl-3-indolyl )-3 -(2,3 ,5- trimethyl-3-thyenyl) maleic anhydride (compound

B)

dispersed in poly tyrene films.

An experiment to confirm the optical density dependence of crosstalk was carried out as follows. Two kinds of diarylethenes, 2)-bis(2-methylbenzo [b] thioph n-3-yl) maleic anhydride (compound

A)

and 2-(1-octyl-2-methyl-3-indolyl)-3-(2, ,5-trimethyl-3- thyenyl) maleic anhydride (compound B), as shown in Fig. 6-12, were di per ·ed in polystyrene and used for the recording layer (thickness: 0. 7 !lm). The concentration of compound

A

was about 15 wt.o/o. The concentration of compound B was adjusted to vary the OD at the wavelength of 633 nm. Consequently, two amples having initial reflectances, 0.1 (high

OD)

_{and 0.6 (low}

OD),

_{at 633} nm, were prepared for the experiment.

The Ag reflective layer was formed by vacuum evaporation and we obtained media having the structure shown in Fig. 6-1. Compound

A

converts from the open-ring form to the closed-ring form upon irradiation with 400-460 nm light. Bleaching of the longer­

wavelength absorption is induced by irradiation with 500-600 nm light. We therefore carried out recording by photo bleaching of the colored state (broken line in upper figure of Fig. 6-12) and reading by detecting reflectance changes with a 515 nm Ar Ia er The photoreaction of compound B was already described in the previous section, and the write/read operations were carried out in the same manner. The apparatu for two­

wavelength multiplexed recording is illustrated in Fig. 6-7. In the reading proces ^·, the readout beams reflected from the medium were divided by the DMs and detected by

81)

corresponding avalanche photodiodes (APDs). We anal zed the output using a spectrum analyzer.

The write/read conditions are summarized in Table 6-Ill ln order to clarify the influence of multi-wavelength crosstalk, we recorded two different frequencie 240 kHz in the Ar laser (A.J=515 nm) channel (CHI) and 300 kHz in the HeNe la er

(A

² 631

nm)

channel (CH2).

0

Table 6-Ill. Two-wavelength write/read condition .

Laser wavelength ( nm)

Writing power (mW) Reading power ( n W) Recording signal frequency (kHz)

Relative speed (m/s)

C H 1

1 OdB/div

515

^nm RBW 10kHz

CHI 515 2.0

30 240

CH2 633

^nm

1.4

CH2 633

0.4-2 4 20 300

10dB/diV RBW 1OkHz

300

frequency (kHz)

600 0 300

frequency (kHz)

Figure 6-13. Output power spectra of CH 1 and CH2.

HG

600

Figure 6-13 shows the output power spectra of CH l and CH2 for the high 00 sample with writing powers of 2.0 mW inCH I and 1.7 mW in CH2 The cro. talk (JOO kHz) in CHI was a small value of -30 dB reflecting the lower ab orption at 63J nm and the small absorption change at 515 nm for compound A. On the other hand, th linear component of crosstalk in CH2 was observed at 240 kHz, and the n nlinear compon nts were observed at 60 kHz and 540 kHz. We measured the writing Ia er pO\\ er dependence of the linear component (240 kHz) a 01 ^r· I()\. and that of the nonlinear component ('::;40 kHz) as () ^\'(' I o,\' by varying the HeNe laser power and keeping the Ar la�er pO\\Cl constant.

Figure 6-14 shows the crosstalk behavior. When the 1<1111 ^0.6 (lovv 00) �o,amplc was used, the linear cro stalk component gradually decreased and the nonlinear crosstalk component gradually increased with increasing writing Ia er powe1 1), c· In contrast both the linear and nonlinear crosstalk components decreased with increasing laser pO\\ cr for the R/1.,=0. 15 (high

00)

sample Such crosstalk behavior can be qualitatively understood on the basis of the results shown in Fig. 6-11, in which both cro talk componenh decrease in a lower irradiation energy range than that of the minimal value of 0, 1 0, in Fig. 6-1 I . An important result is that the nonlinear crosstalk component can be reduced by using a high optical density medium and by adjusting the recording condition· A len\

nonlinear component is desirable, because the linear crosstalk component can be removed by a crosstalk reduction operation� however the nonlinear crosstalk component cannot always be reduced completely.

10 _I 10

I< I () I 'i

0- ^•

� -

⁰

co

/

^{co lmear}

� ^-10 � I mea r - � ^-10

�

1i) cu (/) 0

I....

0

� IC

·---·

� ^-20

-20 r-

�

^{(/) (/) 0}

-30 r-

\

^{0 I.... -30 0}

-40 _{r- nonlinear} ^·- -40

/'

⁰

nonlinear

I 1 -50 0

-50 0 1 2 3 2

!) _ru ( m W) ^fJ," (m W)

Figure 6-14. Linear and nonlinear crosstalk behaviors of CH2 output. The horizontal axis indicates CH2 (HeNe) writing laser power. Write/read conditions are summarized in Table 3-III

R7

3

6.5 Conclusion

The optical density dependence of photochromic optical memories was examined It was found that the initial optical density in the recording layer plays an important role in various write/read characteristics. Secondary harmonics, repeated readouts and multi­

wavelength crosstalk characteristics were improved by using high optical density media

CHAPTER 7

Super-Resolution Optical Disks

7.1. Introduction

One of the approaches to the high-density memory i super-re olution di ^k technologies such as MSR(Magnetically induced Super Resolution) [68][69] The up ^r­

resolution is based on the nonlinear response of the recording media to the readout lase I

light intensity. [70] The nonlinear response of the mask layer is ba ed on either a thermal threshold property[71] [72] or nonlinear optical propertie uch a the ·aturabl absorption of organic dyes.[73][74][75] The aim of this chapter i to derive an equation which can estimate the transmittance change of the photobleachable dye, such as a saturable absorption dye or a photochromic dye mask layer, and to estimate the nece ary conditions for efficient super resolution.

A photo-bleachable organic dye layer shows a nonlinear re pan e, when used a� a mask layer (Fig. 7-1). The transmittance of the mask layer increases with increa -ing readout light intensity. The effective transmittance at the central area is higher than the transmittance of the edge area because of the gaussian intensity distribution of the laser light. An effective super-resolution spot can be formed by taking the product of the readout laser intensity and the transmittance of the mask layer as shown in Fig. 7-2

mask layer

m�:m,�,,�- reflective layer protective layer

Figure 7-1. Structure of a super-resolution medium having an organic dye mask layer. The mask layer contains saturable dye molecules. A readout laser beam is reflected by the reflective layer after passing through the mask layer

Power profile of a readout laser

Transmittance profile of a mask layer

Figure 7-2. Laser power profile of a readout spot and transmittance profile of a mask layer under the effect of the readout laser beam.

�)()

7.2. Nonlinear Transmittance Change in Photoreactive Mask Layer and Effective Super-Resolution Spot

7 ^.2^.1 General Analysis of the Transmittance of a Photoreactive Mask Layer

Photoreactions, in general, proceed in proportion to the number of photons absorbed by the compound. Therefore, the photoreactive rna k Ia er show a lineat response to the light intensity. However, the discussion in chapter 4 hovved that the reflectance R of a photochromic layer changes according to equation ( 4-7), ( 4-8) ^and ( 4-1 0), and that the photo-irradiation time dependence of R is nonlinear wh n the ab orption of the layer is high. These equations were derived for photochromic media having the structure shown in Fig 4-l . When we discu s the uper-re olution effect equations ( 4-7), ( 4-8) and ( 4-1 0) must be modified for the structure hown in Fig 7-1.

The major difference between these two structures is whether the reflectance of th (reflective or recording) layer which lies under the photoreactive (recording or mask) layer is regarded as 1 OOo/o or not. We modified the equations (7-1 ), (7-2) and (7-.3) fot the

structure of Fig. 7-1, in which the reflectance of the recording layer i R,.LL.

We assumed the following simple conditions.

(i)A molecule in the ground state (molecular absorption coefficient ^t:: M"1cm-1) is converted to the excited state (or a isomer for photochromic reaction) by absorbing a photon.

(ii)The molecule in the excited state cannot absorb the photon.

(iii)The excited state has a lifetime of ^r s. (infinity lifetime for ph t chromtc isomer)

The readout beam is assumed to pass through the mask layer (forward), be reflected by the recording layer with reflectance Rrec and then be passed through the rna k layer again (backward) as shown in Fig. 7-1. According to the above conditions, the tran mittance t of the mask layer, which contains molecules in the ground state of concentration ( ^·�

,,

^is

given by the following equation,

T = exp

(

^{-2.3c-Cc; L}

)

^{, (7 -1)}

where Cc; has the time dependence and c;( Tc;L is equal to the optical density The total reflectance R of the medium is

R ⁼ R T2. _rec (7-'2)

The total number of molecules in a unit volume ( 1 liter) Nr is constant and given by

9]

where Nc; and NE are the number of molecules in the ground and in the excited state in a unit volume.

During light irradiation, the photo-exciting and deactivation proces e coexi t.

Upon irradiation with light of wavelength A and intensity P, the number of photon absorbed by the mask layer during infinitesimal time

dt

in the forward light pass and backward light pass are given by

forward:

and

backward:

dn1

⁼

-(1-

PA.

T)dt,

he

dn b

^{= -}PA. ^he

T

^· R ^rec

(1 - T)dt

^·

Therefore, the total number of photons absorbed by the mask layer is

dn,

⁺

dnb

⁼ PA. ^he

(1- r)(l

⁺ R,e�

r)dt.

( 7- 1)

(7-G)

(7 -li)

Thi is equal to the number of molecules that was in the ground state and is reduced by the photo-excitation. On the other hand, the number of deactivated molecules during infinitesimal time

dt

is proportional to the number of excited molecules with proportional constant 1 I r,

(7-7)

Therefore, the change in the number of molecules in the ground state is given by

(7-H)

The number of molecules in unit volume is expressed by the molarity as shown below for the molecules in the ground state and for all molecules:

Nc. _r ⁼ Cc.LSN _{, a} x 10-� ,

92

{7-1())

where ( ^'r is the total concentration of dye and the factor 1 _o-� is for the correction of unit From eq. (7-I ), the following equations are derived,

IJT /iC.

IJt

^{= 2.3} _c:LT

/l/' ,

1

('c ' ⁼ 2.3 _{c: L} ^ln T.

(7-¹1)

(7-12)

Initial transmittance T0 (before irradiation) is given using total dye concentration ( ^·,,

Cr ⁼ I

1 I_n

7;,.

2.3 {;' - c7-1 :n

Using eqs. (7-9), (7-IO), (7-II), (7-I2) _and (7-13), _eq. (7-8) is transformed as follow ,

/iT 1 _{T P}

--;---

^=-Tln-" + _a-5,

A,c:¢

^{T(1- T)}

(

l +

R,.ecr).

ut ^r

T

^._

(7 -1 !J)

Note that there are many independent parameters in eq (7 -14) that affect the transmittance change of the mask layer, e.g., ^r, P

5', A., t; Rrec

^and

1;).

This equation determines the transmittance of the mask layer when irradiated with light of a uniform intensity.

Under the condition of low optical density approximation

(

Ahs ⁼

tf

C(i ^�

0.2 ),

the exponential of eq. (7 -1) may be expanded as follows,

T

^{=I- 2.3 ·}

t1.Cu,

^{(7-1 [))}

Moreover, by considering the stationary state

( IJT /

^{IJt =}

⁰⁾

and a single pass through the mask layer

(Rrec=O),

^{eq. (7 -14)} can be simplified as follows,

-- I+ M

(

^hcN,S '

)

All - 2.3 X lOi _PAC:T

9:-3

-I

(7- ]())

where Ao is the initial absorbance (c.LC10,) and M ⁼ A"- A . This approximation, however, is not practical, because the initial transmittance of the rna k layer mu� t be low in order to avoid crosstalk from neighboring recording marks. Instead, we have to olve eq. (7 -17) numerically.

For the photochromic mask layer, the differential equation (7-17) about J' i derived by introducing infinity lifetime ^T,

(7- 17)

7.2.2. Several Numerical Simulations for a Saturable Dye Mask Layer

We calculated the irradiation time dependence of T by eq. (7 -14) for seve raJ conditions of P, &, rand initial transmittance T0. The values of -1=680 X I o-') m, }(, ^-0 and S=l.O X 1 o-8 cm2 were kept constant. P=4.0 X 1 o-3 W, ^t: =20000 M-1cm-1, ^r I 00

1 o-9 s and 7�1=0. 2 were employed as standard condition .

Figures 7-3 and 7-4 show the plots of normalized total reflectance H. R,n with varying readout power P and molar extinction coefficient ^c.:, respectively When J> and ^�:­

are high, the stationary state with higher transmittance is readily reached The re pon� to the irradiation should be nonlinear in order to obtain the super-resolution effect Ideally, at the low laser power range (12 mW) the increasing speed of R Rn·t should initially be very small, and the saturated value of R Rrec after more than 100 ns should be small. The curves in Fig. 7-3 show such a tendency.

Figure 7-5 shows the excited state lifetime dependence of R R,.eL Thi figur shows that a short lifetime causes an insufficient transmittance increase. A long lifetime is therefore desirable for effective readout through the mask layer.

Figure 7-6 shows the initial transmittance dependence of the normalized total reflectance. As mentioned, a nonlinear transmittance change of the rna k layer i required to achieve efficient super-resolution. This figure indicates that such a nonlinear re pon e to the light irradiation is performed by employing a lower initial tran mittance 1;, (higher optical density) of the mask layer.

P=6mW P=4mW

0

Time (s)

Figure 7-3. Dependence of normalized total reflectance HI H.,L _on readout power. (A.=680X 10-9m, Rrec=0.6, S=l.OX 10-s cm2, c.=20000 M-1cm-1, r=1 00 X 1 o-9 _s, To=0.2.)

0

& =20000M- 1 em- 1

& =1 OOOOM- 1 em- 1

Time

(s)

[x1

^o-

7]

2

Figure 7-4. Dependence of R!Rrec on molecular absorption coefficient (.-1=680 X 1o-9_m, Rrec=0.6, S=1.0 X 1 o-8 _cm2, P=4.0 X 1 o--' W, r=l 00 X 10-9 _s, To=0.2.)

9!)

0

r=1000ns r=100ns

r=SOns

r=20ns

r =10ns

5 Time (s)

Figure 7-5. Dependence of normalized total reflectance R/Rrec on readout power. (.A-=680 X 10-9 m, Rrec=0.6, S=1.0 X 1 o-8 cm2, P=4.0 1 o--' W, c=20000 M-1cm-1, T0=0.2.)

0 2 4

Time (s)

Figure 7-6. Dependence of normalized total reflectance R R,e� on readout power. (.A-=680 ^X 10-9 m, Rrec=0.6, S=l.O X 10-8 cm2, P=4.0 ^X

Io--' W, z=lOO X 10-9 ^s, c=20000 M-1cm-1.)

9G

7.3 Theoretical Analysis of Photochromic Super-Resolution 7 .3.1 Super Resolution Spot

Equation (7-17) determines the transmittance for a uniform light inten ity distribution. Figures 7-7 and 7-8 show the numerical calculation of the total reflectance change in the photochromic mask layer for various values of initial transmittance r- 1;) and of recording layer reflectance Rm_, respectively. The vertical axis of Fig 7-7 represents the relative reflectance when the difference between the initial reflectance and the perfectly bleached reflectance is normalized to 1. Nonlinearity is induced for the lower value of initial transmittance 7() (higher optical density). Figure 7-8 show that the relative reflectance change is scarcely affected by Rrec· This mean the method can be applied to various types of recording layers (magneto-optical, phase change, read only and so on which have a variety of reflectance).

0 1 2

time (s)

3 [x1 0-6 ]

Figure 7-7. Relative reflectance changes by photo-irradiation. The reflectance of the recording layer Rrec was fixed as 0. 8.

Q) u c co

t5

it= Q)

�

0.5

"'0 .� Q) co

E

0 c

0 1

²

time

(s)

3

[x1 0-6 ]

Figure 7-8. Relative reflectance changes by photo-irradiation. The initial transmittance

T0

was fixed as

0.

_1.

In order to apply equation (7 -17) to analyze any spot mtensity distribution and resulting transmittance distribution, the expression p sy in eq. (7-17) i replaced by generalized flux density I<'lux(X,

Y,

t), giving

d?( �/ ^,1)

^{� �a Flux(}

^X,

^{Y, t)}

^()(�I

¹

)At:¢ T( X,

_Y,t)(l �

T(X,

_{Y,t))(l +} R"'

'!'(X,

Y,

I))

(7- 1 H)

where the coordinates

X

_{m and Y} m are fixed on the media and Y

= 0

indicate the center of the recording track. The step function

B (

-t1)

( B

(t)

=

_{1 for}

I

� 1 ;

B (I)= 0

_for

I

< I _{) i} ·

introduced to allow us to know the state of the moment (t

=

ti). (The readout pot reaches the origin of the coordinates

X

_and Y at

11=0.)

For a gaussian spot, the flux density FILtxc;(X, Y,t) of the irradiation is given by eq. (7 -19),

, p

-+

( (. ' l)

^"

)

I� luxe;

(X,

_{Y, t)} =

--, _na-

^{x 1 o- exp}

^{- \ (X-}

^{vt)- + y- I}

^a-

^{(7-] �))}

where " m/s is the relative speed, <I> =

2..fia

= A, I

NA (NA:

numerical aperture of objective lens) gives the relation between spot diameter <I> _{m and}

a

m, and factor I o--+ _is

98

the correction of units. By substituting eq.

(7-19)

into Flux(.X,Y.t) _{of eq. (7-1}

)

and numerically integrating eq.

(7 -18)

by t, we are able to get the tran mittance di. tribution

l{X, YJ1 ).

A super-resolution spot corresponds to the overlapped area of a readout pot and a higher transmittance region in the mask layer, as illustrated in Fig.

7-9

The pn duct of

Rrec

ltX. Y,t1)2 _and Fluxc;(X,Y,t=t1) gives the effective super-resolution spot

Flux':m.C\", }',11)

for a reflected light at t=/1,

Figure

7-10

shows the shape of a calculated effective super-resolution spot on the .)(coordinate under the conditions of

A-=780 X 1

o-9 m,

r;¢ =1400

M-1 • cm-1,

,,= 1 4

m/ , P=2.5 X 1 o-3

W, NA=0.5, Rrec=0.5

and the initial transmittance of the rna k layer is 0 I Figure

7-11

shows the 3-dimensional expressions of a readout spot and an effective super-resolution one. In the figure, the X _and Y coordinates indicate the spot moving direction along the recording track and the direction across the track, respectively The vertical axis represents the normalized intensity of the spots. The super-re olution sp t i smaller than the readout gaussian spot.

Bleached Mask Area

Super-Resolution Spot

Figure

7-9.

Illustration of the proposed photochromic super-resolution readout method. Then the readout spot scans the precolored (initialized) mask layer of a super-resolution disk, only the restricted mask area corresponding to the backward portion of the spot is bleached by photoreaction. A smaller effective uper­

resolution spot is formed in the overlapping area of the readout spot and the bleached mask area.

99

,-...

� (/)

·-c:

::::s .� ..0 _�

...._, ro

� 0.5

>"( C/)

;::s

k:

(j

;...- ...:

�

f-^•. ;

0

�---1----

Transmittance of Mask Layer

I \

I \

'-Jt Readout Spot

I I I I I

Effective

\-+

_I

Mo�ing Direction

\ofaSpot

I

\

\

... ^\

--�---

-1 0 1

RELATIVE COORDINATE(f.l.m)

Figure 7-10. The intensity distribution of a super-resolution pot along the X axis.

Light Intensity

Readout Spot (Gaussian) Super-Resolution Spot

Figure 7-11. Calculated intensity distributions of an actual readout spot (Gaussian distribution) (left) and an effective super-resolution spot (right).

(,1

=

_{780 nm,}

NA=O.S,

v=1.4 m/s,

P=2.5

mW, c·¢=1400 M-1cm-',

1;1=0.1(01)=1

₀₎₎

100

7.3.2.

Crosstalk between Adjacent Tracks

In this section, we calculate the crosstalk between adjacent recording track for a conventional spot and a super-resolution readout spot. The time dependent readout photocurrent

fsJc;(/1)

is obtained by taking the integral of the product of

l<fux(

^...

'()'J1)

(light intensity distribution of a spot) and

Rrec(X,Y)

(the reflectance di tribution) in general,

where the integral is carried out over the area of the spot, y is the pickup efficienc defined by the ratio of the light intensity arriving at the photodiode to that reflected from the media and '7 is the photoelectric conversion efficiency of the photodiode. For simplicity, the photocurrents obtained from the areas of mark and land

Im,uk

and

l,.�nd,

respectively) are defined by the following equations,

Rnwrk(X,Y)

indicates the reflectance distribution of a recording layer in which a long recording mark exists, and

RtaniX,Y)

indicates the reflectance of the land area. The integrals (7-22) and (7-23) correspond to the upper and lower figures of Fig 7-12, respectively. The (peak-to-peak) signal current intensity is defined by

I ^SJG

=II -I I - mark

^{lund ' (7-�11)}

The crosstalk current I

rRos

is obtained by replacing

Flux< ;(X, Y,t=O)

with

F

l

ux

^c;(

X

,

Y

^�

D,t=O)

in eqs. (7-22) and (7-23), where

D

m corresponds to the track pitch, and by taking the absolute value of their difference.

I

^CROS

=II' - mark ^-!'

^{lund '}

I

^(7-�G)

!'murk= ff dXdY Rmork(X,Y)Fiuxc,(X,Y

^{+ D,t}

= ^0),

101

(7-'27)

The crosstalk power ratio OcRos is defined by the square of the ratio of the signal current intensity Is1u to the crosstalk current intensity IrRos, and is expressed in dB by eq (7-28).

I C'ROS ()C'ROS =: 20Joglfi _-- I ^SJCi

(7-:!.H)

The expressions for a super-resolution spot can be obtained in the same way by repla ing Fluxc; in eqs. (7 -22), (7 -23 ), (7 -26) and (7 -27) with FluxsR.

y

Figure 7-12. Illustration of the mask and the land used in the calculation of equations (7-22) (upper) and (7-23) (lower), respectively.

Figures 7-13 and 7-14 show track pitch dependence of the era talk for a readout spot and a super-resolution spot. The conditions were the same as tho e in Fig. 7-l 0 and Fig. 7-11, and the reflectances of the mark and land were 0.8 and 1.0, respectively. Figure 7-13 shows the dependence of initial transmittance 10. When the initial optical den ity i higher, the crosstalk becomes lower. Figure 7-14 shows the readout power dependence.

There is an optimum readout power for obtaining lower crosstalk. The super-re olution method can increase the track density in comparison with the conventional method

0

� _J -20

<{

1--C/) (/) 0 rr: 0

-40

P=2.5

mW

0.5 1.0

TRACK PITCH

(�m)

Figure 7-13. The crosstalk between adjacent recording tracks at vanous initial transmittance values. The readout power Prep was fixed as 2.5 mW.

0 T0 =0.1

-

(1)

""0

...._

� -20

_J <{

(f) 1-- (f) 0 0 cr:

-40

0.5 1.0

TRACK PITCH ( �m)

Figure 7-14. Readout power dependence of the crosstalk. The transmittance of the mask layer was fixed to T0=0.1.

7 .3.3 Linear Recording Density

The super-resolution method increases the linear recording density because the diameter along the track direction of the effective spot is narrower than the conventional readout spot, as shown in Figs. 7-9, 7-10 and 7-11. On the other hand, the readout ignal is distorted because the super-resolution spot has an asymmetric shape. In order to know the distortion effect, we calculated the readout waveform and frequency characteristics The conventional readout signal current

!Hf;(t)

can be obtained by the following integral.

The super-resolution readout signal can also be obtained as follows, fslgSR

( 1)

⁼ Y'l

JJ dXdY Rrec ( X, Y)

F/ux,\'R

(X, Y, I),

where

Rrec(X,

Y) is the reflectance distribution of the recording layer.

(7-2�))

Figure 7-15 compares the readout output waveforms obtained from a

104

conventional tnethod and a super-resolution readout method for various recording mark lengths. Note that 7'0=1.0 corresponds to the conventional readout method The relative speed of v= _1.4 m/s was adopted and other conditions were the same as before In general, the output signal level of the super-resolution readout method i maller than that of the conventional one because of light absorption of the mask layer. However, it hould be noted that this is not essential from the viewpoint of modulation tran fer function characteri tics. Therefore, the vertical lines of Fig. 7-15 are normalized prop rly. For the short mark length (900 kHz), we can see that a higher output level is obtained by the super-resolution readout method in comparison with the conventional method Figure 7-

15 also shows that the waveform obtained by super-resolution readout method contain· a distortion and a phase-shift. These are caused from the asymmetric hape of the ·uper­

resolution spot. Figure 7-16 shows 70 dependence of the frequency characteri tic. The higher optical density (lower transmittance) of the mask layer cau es a higher linear recording density.

1'a

^=1.0

�--r-�����--��

1

Normalized output level

300kHz

500kHz

700kHz

900kHz

1'0 ⁼0.1

Figure 7-15. Readout waveform at various recorded mark lengths. The left figure indicates the waveform of the conventional readout method ( l(1= _1. 0 is optically equivalent to no mask layer) and the right figure the waveform of the super-re olution readout method.

l()f)

0

-co

'"0

._

1-:::>

a...

1-:::> T0 ⁼

0

.

6

0

w -10

> T0 ⁼

1

.

0

1-<!:

_J w 0::

400 600 800

FREQUENCY (kHz)

Figure 7-16. Recording frequency dependence of the output level ^m conventional and super-resolution readout methods at variou initial transmittance values.

7.4 Photochromic Super Resolution Readout

7.4.1 Nonlinear Transmittance Change in a Photochromic Mask Layer

As shown in Fig. 7-9, when the readout spot scans the precolored (initialized) mask layer of a super-resolution disk, only the restricted mask area corre ponding to the backward portion of the spot is bleached by the photo-reaction, because the irradiated light quantity is integrated in this area. Therefore, a smaller effective uper-re oluti n spot can be formed by overlapping the area of the readout spot and the bleached mask area.

To obtain effective super-resolution, it is necessary that the tran mittance of the mask layer vary nonlinearly with the readout light quantity. We theoretically analyzed the relationship between the photon-mode reaction of photochromic molecule and the transmittance change of the photochromic mask layer, and concluded that the nonlinear transmittance change was obtained by setting the optical density (OD) of the rna k layer to a high level in Sec. 7.3. The nonlinear change of the mask layer is expected when

OD ^� 0.5 in the initial state.

In order to examine the above prediction, we performed the following experiment Figure 7-17 shows the molecular structure and absorption spectrum of the photochromic material (FC-124) used in our experiment. The open ring state converts to the clo ed ring state by irradiation of ultraviolet (300-350 nm) light and the absorption in the r d wavelength region (600-700 nm) increases. On the other hand, the closed ring state converts to the open ring state by irradiation of red light and the absorption in the red wavelength region decreases. Therefore, a red laser beam can be used for super­

resolution readout. This material has a high isomerization ratio, enabling a high optical density to be achieved in the thin film state.

-... en

Q) (.) c co ...0

L...

0 en ...0

<!: \

\

\_

A

400 600 800

Wavelength (nm)

Figure 7-17. Molecular structure and absorption spectrum of the photochromic material (FC-124) used in the experiment.

At first, we investigated the transmittance change of photochromic film tatically Figure 7-18 shows the sample structure and the experimental apparatus. Three sample were prepared by vacuum evaporation of Ag as a reflective layer, FC -124 as a photochromic layer and CaF2 as a protective layer on a glass substrate. They included a reference sample which had no photochromic layer, a low ()f) sample and a high ()f) sample. The initial OD values were set to be higher than 0.5 for the high()]) ample and lower than 0. 5 for the low OD sample by calibrating the thickness. The thickness and OJ J of the photochromic layer for each sample are given in Table 7-1.

Table 7-I. OD and thickness of the photochromic layer of the sample use d m t e static mvest1gat10n. . h

OD at A. =633 nm (single pass) thickness ( �m)

reference 0 0

Low OD sample 0.3 0.2

High OD sample 0.6 0.4

Beam Splitter

Pulsed Light

(A.

^=633nm)

PIN Photodiode

�

Objective Lens

Digital

Oscilloscope

.--------=---.

�

Protective Layer ( CaF2)

I�

Photochromic Layer (FC-124)

L___ ^________

__l.,

Reflective Layer (Ag)

Glass Substrate SAMPLE

Figure 7-18. Sample structure and experimental apparatus for static investigation of the transmittance change of photochromic film usmg a

single laser pulse.

Pulse Width _{50 J.l. s}

< >

\

Nonlinear Change

Reference

Low 00 sample (00=0.3)

High 00 sample (00=0.6)

Figure 7-19. Relative reflectance change obtained by static investigation of photochromic films.

These samples were initialized by irradiation of UV light (Hg-lamp). A sing! light pul e from a HeNe laser

(A

=633 nm) was used to irradiate the samples and the corresponding reflected light was detected by a pin photodiode. Relative reflectance change were measured by observing a waveform displayed on a digital oscilloscope. The pul e width was 50 �s and the peak power was 1 mW. Figure 7-19 shows the waveform di, played on the digital oscilloscope. The reference sample has a rectangular wa eform that corresponds to no reflectance changes. An increase in reflectance was ob erved for the photochromic film samples according to photo-bleaching of the photochromic layer. An apparent nonlinear change is observed for only the high ()f) sample

7 ^.4^.2^. Application to Read-Only Disks

Super-resolution readout was examined for read-only optical disks Figure 7-20 hows the SEM photographs of disk substrates. The specification of the di k ub trat used in the experiment are summarized in Table 7-II. Disk substrate 1 ha a recording density similar to the conventional compact disk and disk substrate 2 ha a higher recording density (shortest (3T) pit: 0.48 �m, track pitch· 0.85 �m) An eight to foUI1een modulation (EFM) signal was recorded on each disk. Figure 7-21 ho\\. the disk structure. The photochromic mask layer (FC-124) was prepared by a vacuum evaporation method and the reflective layer was overcoated on it by the same method. The ()}) of the mask layer was set at 0. 5 in the colored state at a wavelength

A

of 685 nm, by calibrating the thickness. Figure 7-22 shows the schematic diagram of the apparatu for the super-resolution readout. The readout pickup had a laser diode

(A.

=685 nm) and an objective lens (NA=0.55). In order to initialize the mask layer, ultraviolet (

V)

light wa, directed to a location on the disk using an optical fiber from a Hg lamp. The other readout conditions were as follows: the readout laser power wa 1 0-2.4 mW, the

1V

light power was about 10 mW/cm2 and the relative speed was 0.7-1.4 m/ Since the areal density of the coloring light intensity was low, the mask layer was initialized after everal rotations of the disk. The reflectance of the initialized disk was about 1 0 o/o

Substrate 1 Substrate 2

Figure 7-20. SEM photographs of substrates 1 and 2.

Table 7-II. Specifications of the disk substrates used in the experiment.

Disk substrate 1 Disk substrate 2

Material polycarbonate

Diameter (mm) 120

Thickness ( mm) 1.2

Signal eight to fourteen

3T pit length (�m) 0.84 0.48

Track pitch (�m) 1.6 0.85

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

_ntn

Figure 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