optical writing mechanism of textured media

Optical writing mechanism of textured media

S. Y. Suh and Harold G. Craighead

Theoretical analysis of the writing mechanism of textured optical storage media revealed a fundamental sen-sitivity advantage due to the absence of an energy barrier other than melting the material. The melting lim-ited optical writing has been experimentally achieved both in textured germanium samples and textured sili-con samples. The analysis also predicted that an instability exists in the fluid motion of the textured col-umn during spot formation, which grows exponentially on a subnanosecond time scale. This instability cou-pled with the excessive Laplace-Young surface pressure ensures the spot formation before the resolidifica-tion of the molten material. This writing process facilitates the optimization of a given medium and greatlysimplifies the materials research.

1. Introduction

There has been a growing concern regarding the rapidincrease in the information generation rate in recentyears, which could surpass the technology limit of thecurrent magnetic storage devices. For example, thedata generation rate could reach as high as 1012 bits/dayby 1990 for NASA's space management programalone."2 The projection may be conservative, consid-ering the fact that weather satellites now generate 1011bits/day.",2 The need for mass information storagesystems initiated large scale research and developmentin many laboratories. While most of the technologicalproblems related to the optical and mechanical aspectsof optical recording using ablative metal thin films havebeen successfully dealt with, the ideal storage mediumremains an area of active research.

The most widely studied media are the write-once-read-only media which rely on thermally induced per-manent physical deformation of a medium on irradia-tion by a highly focused laser beam to produce an op-tically readable mark.3 In many identified applications,the write-once limitation was not considered a problem,since raw data received from satellites, for example, maynecessarily be preserved and redistributed to the wide

S. Y. Suh is with AT&T Bell Laboratories, 555 Union Boulevard,Allentown, Pennsylvania 18103; when this work was done H. G.Craighead was with AT&T Bell Laboratories, Crawfords Corner Road,Holmdel, New Jersey 07733; he is now with Bell CommunicationsResearch, Inc., Crawfords Corner Road, Holmdel, New Jersey07733.

Received 23 July 1984.0003-6935/85/020208-07$02.00/0.© 1985 Optical Society of America.

range of the end users. However, the write-once con-cept understandably limits its application to archivalstorage. This imposes a more stringent long-termstability requirement on the recording media.4 Un-fortunately, more sensitive low melting point metals andsemimetals are prone to environmental degradation.Archival stability combined with low-power writingrequirements is one of the prime concerns in opticalrecording materials research. In a series of papers,Craighead et al. 58 reported the feasibility of a new classof optical information storage techniques which utilizethe irreversible change of the optical properties of amicroscopically textured surface. This storage tech-nique differs from the more publicized ablative holeopening methods in that no material interfaces are in-volved. The surface tension driven forces are sufficientto accomplish the writing with lessmotion of the ma-terial. The proposed storage technique depends on achange in the surface morphology and can be appliedto a wide class of new materials. Since both the opticalproperties and the lateral thermal conductivity of thefilm can be altered to a large extent by the microscopicsurface morphology, the texturing technique broadensthe possible range of useful optical storage media.

The optical writing experiments on films of Ge andSi, textured by reactive ion etching, clearly demon-strated sensitivity advantage of the texturing. One ofthe reasons is that these films are highly absorbing forphotons with energies greater than the band gap energy.Another is that the graded effective refractive index,produced by the texturing, substantially reduces thereflectance of the surface.5 Furthermore, the lateralheat diffusion of the writing energy is greatly reducedby the decoupling inherent in the microcolumnar ge-ometry. However, the aforementioned factors alonecannot account for the observed sensitivity enhance-ment. This led us to study optical writing mechanism

208 APPLIED OPTICS / Vol. 24, No. 2 / 15 January 1985

in textured media. A preliminary theoretical analysisby Suh9 indicated that the surface energy stored on thetextured columns is sufficient to initiate the writingprocess. Here we present a more general analysis whichreveals a fundamental sensitivity advantage of texturedmedia due to the absence of an energy barrier other thanmelting the material itself along with the inherent in-stability of a liquid column.

II. Media Fabrication

The fabrication of textured media for our experi-ments consisted of film deposition and an etching pro-cess to form the columnar surface features. This gen-eral method has been described in detail elsewhere5 andthe specific case of Ge is discussed in Ref. 8. Briefly,an -1-pum thick germanium film was deposited bythermal evaporation from an electron-beam heatedsource onto glass or fused quartz substrates. Thetexturing of the film was done by reactive ion etching(RIE). During the anisotropic removal of the Ge byRIE an etch resistant material was simultaneously de-posited on the Ge surface to form small islands thatdefined the column size. The etch resistant islandsprotected the underlying Ge while the surroundingmaterial was removed by reactive ions, yielding thecolumnar surface. For Ge the etching was carried outon the cathode of a conventional diode sputtering sys-tem driven at 13.56 MHz with power to the electrodesof the order of 0.5 W/cm2 . An Al seed material wasexposed to the reactive plasma and resulted, apparently,in the formation of <50-nm islands of aluminum com-pound on the surface. The etch gas consisted of amixture of Ar, 02, and CCl2F2. The composition of thegas, rf power, and etch time were controlled to vary thesize, density, and height of the columns as described bySchiavone et al. 10 The diameter of typical columns was<50 nm with -0.3-um height. The films were durableand stable in air with no protection. Dust and abrasionprotective polymer coatings were deposited directly onthe textured surface, by spin coating, with no degra-dation of the writing process.

The etching process described here has proven usefulfor our research and prototype level sample formation,because of the ready control of the sample morphologyand applicability to a range of materials. By intro-ducing the island forming material by sputtering fromthe anode10 it should be possible to form large areadisks. However, other processes such as selectivechemical etching of multicomponent films can producetextured surfaces similar to the reactive ion etchedfilms. These other processes may eventually provemore economical and desirable.

III. Writing Mechanism

We will begin our discussion by briefly reviewing thehole formation process in ablative recording media topoint out the inherent inefficiencies of the ablative holeforming process. We follow this by a theoretical anal-ysis which describes a fundamental sensitivity advan-tage of textured media due to the absence of energybarriers such as interfacial energies, viscous frictions,

and inertial forces other than melting the material it-self.

A. Review of Ablative Hole Opening Process

There have been a number of models describing thehole opening process which could predict the minimumtemperature needed to initiate hole formation. Theliterature on this subject is rather scarce and confusing.For example, the energy required to nucleate holes inan optical recording thin film is claimed to be deter-mined by melting,1"1 2 by vaporization,1 3 -'5 by sub-limation,1 6 and by surface tension.l7 -'9 The experi-mental data and more elaborate theoretical analysis bySuh et al. 20 strongly favor the surface tension gradientdriven writing model. In this model, the laser irradi-ated area develops a temperature gradient due to theGaussian intensity profile of the focused laser beam.This thermal gradient, with the temperature at thecenter being higher than at the edge, produces a surfacetension gradient. This gradient develops a radiallyoutward force in the molten area which must be bal-anced by opposing forces including viscous dissipationforce, inertial force, and interfacial force, etc. It hasbeen shown that the viscous force dominates in a 30-nmTe film over other forces by more than an order ofmagnitude.'8 It should be mentioned here that theseopposing forces are small, but the temperature gradientdriven forces are also very weak.9 Thus for a sensitivewriting, temperature driven forces may not be attrac-tive, since they require excessively high temperature.

The situation is quite different for the surfaces withthe microscale roughness. In this case, the drivingforces originate from the surface free energy rather thantemperature gradient, resulting in a sensitivity en-hancement. We will point out the advantage of thesurface energy driven writing process in texturedmedia.

B. Analysis of the Writing Process in Textured Media

We begin our discussion by considering the completeequations of motion, including viscosity, to show thatviscosity is not important for liquid metals. This canbe seen by observing that for liquid metals the charac-teristic velocity (surface tension divided by viscosity;ou - 5 X 105 cm/sec) is very large. Thus the Reynoldnumber

inertial forceRe viscous force = p( )L/

is large for the liquid metals with high density p andtypical length L, even when L is very small.

Complete equations governing the flow are the con-tinuity equation:

(2)VV = 0;

the momentum equation:

P [d + (v * V)v -VP + ,V 2v,[at VI = c

with the x component

15 January 1985 / Vol. 24, No. 2 / APPLIED OPTICS 209

(3)

(1)

P [au. + VX ax. + v + v, at v. ax d2 4at ax ay atj O x ,ax 2

where v is the kinematic fluid velocity, and P is thepressure. Suppose that the flow is driven by surfacetension so that

Paan aP a L ax L

2

and the pressure gradient 9P/Ox isviscous drag /12V x/OX2 , i.e.,

ap a2 vx

ax $x2

Since

wV2 rVreI aX2 L2

we arrive at

aVx -- .

(5)

balanced out by

(6) x1

FREE SURFACE

f (, y, Zt )= O

400 - 1000A

Z * 0 SUBSTRATE SURFACE

Fig. 1. Geometry of a molten column. V(t) is the volume definedby plane Z = 0 and by the free surface f(x,y,z,t) = 0.

(7)

(8)

and on the free surface, given by f(x,y,z,t) = 0, we re-quire

Then the inertia terms in Eq. (4) can be written as

p vx + y - + , ax - pv2/L. (9)x ay Oaz

Finally, the magnitude of the inertia terms, relative tothe viscous term, in Eq. (3) can be calculated using Eqs.(7) and (9):

p(v V)v LaoI V2V A -1Ly (10)

which is very large for liquid metals. Thus the as-sumption we started with-that viscosity and surfacetension balance one another-is incorrect and insteadwe neglect viscosity, dropping the terms in ,u,

P da + (v V)v =VP. (1

The resulting equations describe unsteady potentialflow with

v =-V, (12)

where is a scalar velocity potential, and thus bycombining Eqs. (2) and (12) we obtain

V. v= -V 2 = . (13)

Any such flow starting from rest is necessarily irrota-tional, with v the gradient of a scalar velocity potentiali. We use the momentum equation [Eq. (11)] only tocalculate the pressure P. The Bernoulli equation, thefirst integral of the momentum equation, is given by

(P -PO)/P = at-2! [(a-12 (d-/2 + (- (14)at 2 x - O Y) aflz]

for some reference pressure Po. We thus require aharmonic function O(x,y,z,t) with

V24 = 0 (15)

in the liquid volume V = V(t), shown in Fig. 1. On itssolid base, z = 0, we require

Vz = - - = 0,az

(16)

VA * Vf = f,at

(17)

i.e., the surface moves with the liquid. The pressure atthe free surface is given by

P = UK + Po, (18)

where K is the mean surface curvature and Po the am-bient pressure. Thus, Eq. (14) is reduced to

a- 1/2[V]2 = UK/p

at(19)

on the free surface. It may be possible to solve this setof equations numerically, but it is likely to be messy andexpensive because of the necessity of keeping track ofthe free surface at each time step. There is, however,an overriding consideration that is more important thana particular solution; the flow, at least for large aspectratios (height-to-diameter ratio), is unstable to smalldisturbances.

C. Stability of Liquid Column

We can see this by reproducing Rayleigh's 1879analysis of the stability of a stationary, infinite liquidcolumn in equilibrium with the constant pressure Po +a/a for column radius a. We assume that the columnis disturbed from rest by the flow given by the velocitypotential

i = ip(r,0) cos(kz) cos(at + e),

where a, k, and e are arbitrary constants.of Eq. (20) into Eq. (15) yields

02 +i 1 Of/i 1 01 _ =

Or2 r r r2

02

(20)

Substitution

(21)

giving

4/i = In(kr) cos(nO), n = 0,1, . . ., (22)

where In is the modified Bessel function. For the dis-turbed column surface given by

r = r + 4b(0,z,t)

using Eqs. (12) and (20), we find

(23)


4) = -[kIn(kro)/a] cos(nO) cos(kz) sin(at + e),

where ' denotes the derivative with respect to theargument. By definition, the curvature of the disturbedsurface defined in Eq. (23) is given by

1 - 2K=--[kI'n(kro)/ar'](n2 + k2ro -1)

X cos(nO) cos(kz) sin(at + c). (25)

At the surface the excess presure AP, due to the dis-turbance, is given by

P 4/ = -4a(rO) cos(kz) sin(at + e),p at

(26)

and this is equal to the change in curvature multipliedby the surface tension, i.e.,

A ( K-O)0. (27)

By comparing Eqs. (26) and (27), we arrive at our finalresult:

a2 = 1p- [kroIn(kro)/In(kro)](n2 + k2r2 - 1).

-L --I1e2r.h -

reO

n f ( D )where 0 S5f S 2^T

2r,,

AEs T-7rD 2 foa

Fig. 2. Surface energy of textured media: h, height of the column;ro, radius of the column; D, diameter of the laser beam spot; f, projectarea fraction occupied by the column; n, number of columns insidethe laser beam spot; a, aspect ratio; U, surface tension; E8, surface

energy.

The surface energy gain, AE T, by coalescence of thecolumns during writing in textured media can be ob-tained using the geometry shown in Fig. 2:

(28)

This is always positive for n = 1,2,. . ., the flow beingoscillatory and neutrally stable, even for k = 0, corre-sponding to plane disturbances. For n = 0, however,a 2 is negative, and the disturbance grows exponentiallywhenever

AET = -7rD2 fa, (33)

where a is the aspect ratio, f is the area fraction occupiedby the columns at the column-film interface, and D isthe diameter of the written spot. For the ablativefilms,' 7 the surface energy change during optical writingis given by

kro < 1. (29)

Define a wavelength X as

2irk= X (30)

Then the column is unstable when

X > 27rro, (31)

i.e. when the wavelength is greater than the circumfer-ence. Furthermore growth is very rapid, as HUH.

This simple analysis is strictly valid only for an infi-nite column. For a finite column the end condition z*VA = 0 at z = 0 is satisfied, but this is not the correctcondition at the free end, z = L. Moreover, a flat-endedcolumn is not even in equilibrium. Such a columnwould be expected to be unstable for aspect ratio L/ro>2-7r.

Note that this is an axially symmetric instability, withgrowth time

T - - 0.3 nsec, (32)

for a 400-A diam Ge column, the column tending tobreak up into beads, but we cannot tell what happensonce the motion becomes this large. For a dense forestof etched columns this probably means that the meltedcolumns coalesce very early in the process. For a thinforest this instability may blur the edges of a meltedspot.

D. Ablative Recording Media vs Textured Media

We will consider highly idealized situations to makedirect comparison between the two recording modes.

E~s =-7D2 [1/2-1 -( D-) ] U, (34)

where AE' is the energy gain by writing, h is thethickness of the film, and a, is the surface tension of thesubstrate. The surface energy gain by fluid motion forthe textured media [Eq. (33)] is always negative, but forthe ablative mode the energy change [Eq. (34)] is posi-tive at the beginning of hole formation, i.e., for small D.Thus the surface energy acts as a barrier for the ablativerecording, but it acts as a driving force for the texturedmedia. Moreover, the amount of the energy gain iseasily controllable for the textured media through theaspect ratio and column density.

Viscous dissipation for the film structures shown inFig. 3 can be compared by the ratio of the velocity gra-dient

[Vv _ 4hr = (10-2)IIV 9A h'

(35)

where superscripts T and A denote textured and abla-tive media, respectively, and h' is the textured columnheight. Similarly, for the inertial dissipation, wehave

CO -:tD= (10 1), (36)

where co is the acceleration. Since the viscous force isalmost an order of magnitude greater than the inertialforce for the ablative media, the oppositing force of thetextured media is almost negligible compared with thatof the ablative recording media. This points out sen-sitivity advantage of the textured media over ablativerecording media.


(24) D

I.I

Another major difference can be seen when we com-pare the force required to overcome the energy barrier.For a textured medium, in order for the surface tensiondriven pressure, AP,, to overcome the inertial and vis-cous energy barriers, we require

APa > pX2 h2- (37)

where r is the hole formation time, and X1,X2 are themaximum travel distances of the material in the direc-tions parallel to the fluid motion and stagnant portionof the column, respectively, during the spot formation,i.e.,

(38)

(39)

Xi - (1-f)h',I3r2h' 1/32 4.

The right-hand side of Eq. (37) is estimated to be <0.1atm for the molten metal columns of the dimensionshown in Fig. 1. On the other hand, AP, predicted bythe Laplace-Young equation,21

(A)TEXTURED MEDIA

hI D

hi -DI I' 'II I

(B)ABLATIVE MEDIA

VISCOUS DISSIPATION

AVT hro 0 0 (1.-2)AVA hD

INERTIAL DISSIPATION

WT .. h_ i )-A 2D

Fig. 3. Viscous and inertial dissipation energy: h', thickness of theablative thin film; Vv, velocity gradient; w, acceleration.

AP,,= U + ][1" + 1

(40)

is in excess of 100 atm for the same geometry, where r,and r2 are the principal radii. Thus the energy barrieris practically nonexistent for the textured media andmelting will ensure the spot formation.

The situation is quite different for the ablative moderecording. The required surface tension differential,Au, across the radius of the hole to overcome the energybarriers is given by18-2 0

A > PD 2 + Ph'D 2 (41)

8hr 4T2

for the film geometry shown in Fig. 3. For a typical Tefilm, Eq. (41) implies that the required temperature toform a hole is in excess of 1200 K, which is greater thanits boiling point. The calculations presented here aretoo approximate to justify any quantitative statement,but they are adequate to reveal the fundamental dif-ference between the two recording techniques.

E. Sensitivity

The foregoing analysis revealed that melting is suf-ficient to form an optically readable spot in the texturedmedia. Furthermore, the hole formation time is veryrapid, typically <1 nsec for most metallic columns aspredicted by Eq. (32). The calculation of the thresholdenergy for textured media is straightforward and re-quires the solution of the heat diffusion equation,

- T(r,t) = V D VT(r,t) + Q(r,t),at

(42)

where T(r,t), D, and Q are the temperature at point rat time t, the heat diffusion coefficient, and the rate ofheat generation, respectively. Approximate solutionsof Eq. (42) in all degrees of sophistication have beenwidely reported in the literature.22-2 5 Our approachis to show qualitative relationship between sensitivityand material parameters using a 1-D solutions obtainedby Kivits et al.'8 :

AT = [1+ 2CsP(Dst)l/ - iAIot)1 7rpfCfh v Cfh)

(43)

where AT, C, A, and Io are the temperature rise in thefilm above the ambience, heat capacity, absorptivity,and power density, respectively. The subscripts f ands denote film and substrate.

Before applying Eq. (43) to derive a qualitative ex-pression for the sensitivity of the textured media, wemake the following simplifying assumptions: (1) thetextured columns are in direct contact with the sub-strate; (2) no radial heat diffusion takes place; (3) notemperature gradients develop across the film thickness;(4) the recording laser beam has a Gaussian intensityprofile with a beam waist of 2rW; and (5) to make therecorded spot readable, the film temperature should beraised to the melting point within the radius rm from thespot center. Under these assumptions, the thresholdlaser writing power Pt can be obtained from Eq. (43)as

Pt = [1 +2Ptj ( At f ) ATm exp _ 21

(44)

where ATm and f are the melting temperature above theroom temperature and the filling factor, i.e., the volumefraction of material in the textured layer. As will beshown in Sec. IV, the experimental results for opticalwriting are in support of the theoretical analysis de-veloped in this section.

IV. Experimental Results

The result of a numerical calculation of the thresholdpower using Eq. (44) is plotted in Fig. 4 and is comparedwith published data.26 In this calculation we estimatethe filling factor to be -0.5 and the textured layerthickness to be 0.2 /Am. Determination of the minimumreadable spot size rm is somewhat arbitrary since itdepends on the optical readout system. For this cal-culation, three different values of rm are used: rm = 0


I I I I I I I I I120

15

o10

5

50 100PULSE TIME ns)

0.8

C)

- 0.4

0.6

0O

0

I-

0.2

0

500 1000

Fig. 4. Threshold power as a function of pulse time for texturedamorphous germanium. Dots denote experimental data obtained

from Ref. 26: - -- , rm = rw/i./; -, rm = rw/2; - - -, rm = 0.

represents an absolute minimum threshold power, rm= rw/2 allows a very small volume of melt pool aroundthe center of the writing beam due to high latent heatof fusion of Ge samples used in our experiments, andfinally rm = rw/xf2to produce a large written spot. Theradius of the laser beam ro was taken to be 0.34 Am, andthe optical absorptance was assumed to be 0.95. Wealso assumed that the heat capacity per unit volume ofamorphous Ge is fairly close to the crystalline value.Considering the approximation in the temperaturecalculation, the experimental results of optical writingon textured Ge samples are in good agreement with thecalculated threshold power.

Figure 5 shows the experimental results of opticalwriting on textured Ge samples on glass substrate. Thethreshold energy for 100-nsec pulses is -1.0 nJ for a4-min etched sample, while the energy required to meltthe spot is estimated to be -0.93 nJ using the exactnumerical solution of heat diffusion equation [Eq. (42)]developed by Suh and Anderson.24 This stronglysupports the melting limited writing. On the otherhand, for a 2-min etched sample the threshold energydecreased to -0.5 nJ for the same pulse length but at theexpense of lower reflection contrast. This may simplyimply that for smaller sized columns the amount of thematerial to be melted is less (thus decreased thresholdenergy) and reflectivity change is smaller because ofhigher initial reflectance. As shown in the same figure,these results represent a considerable improvement overthe nontextured thin Ge film. It should be mentionedhere that the ablative recording in a Ge film is ratherefficient due to its exceptionally high temperaturecoefficient of the surface tension. Therefore, the sen-sitivity enhancement by the texturing would be morepronounced for the other ablative recording films.

Figure 6 shows similar results obtained for texturedSi samples. For 6-min etched samples, the thresholdenergy for 100-nsec pulses is -2.7 nJ. Considering thethermal loss to the substrate, we may have achieved

3 10 20 30POWER (mW)

40 50

Fig. 5. Textured germanium. Contrast ratio is defined by I (Ra -

Rb)/(Ra + Rb) , where Ra and Rb are reflection coefficients beforeand after the optical writing. Optical writing pulse length is 100 nsecfor all three cases: -A-A-, 1-min etched sample; -0-0-,

2-min etched sample; -0-0-, 4-min etched sample.

I .- R 2.7

0.8 -

z 0.6

W04

, 0.4

A:

0.2 _

- 4 MINUTE ETCHED SAMPLE---- 6 MINUTE ETCHED SAMPLE

^ IIn R.RP _o Rb

1 II

I v-P 1C 0 6.25 12.50 18.75 25.00 31.25

POWER (mW)

R-1.3

I I I

37.50 43.75 50.00

Fig. 6. Textured silicon. All the experimental conditions are thesame as those of Fig. 5 except the sample preparation parameters:

-, 4-min etched sample; - - -, 6-min etched sample.

melting limited writing again. For a finer column sizedmedium (4-min etched films), we have also observed thesensitivity increase (factor of 2) and the decrease in thereflection contrast. This is consistent with the trendof the Ge data, indicating that the results obtained fromthese two samples are a general property of the texturedmedia rather than the specific materials used in theseexperiments.

V. Conclusion

Theoretical analysis reveals a fundamental sensitivityadvantage of textured media due to the insignificantenergy barrier to the writing process and the develop-ment of a surface tension driven pressure because of thetextured surface curvature. This allows a meltinglimited optical writing on the textured media. Thesensitivity is not affected by the substrate other thanby the conductive thermal loss, as opposed to the ab-lative writing in which the interfacial energy betweenthe film and substrate plays a vital role in determiningthe sensitivity of the medium. The experimental data


rm r/2rm r/2rmno

I I. . . . . . . . . . .

O' ' ' '1 1

Lo0

I I

.. v,

presented in this paper are in agreement with the the-oretical analysis.

The benefits of the texturing are significant, specif-ically: (1) It facilitates the material selection due tosimple criteria used to select a material. (2) The sen-sitivity is not greatly affected by the substrate otherthan the thermal loss consideration. (3) Increasedsensitivity allows the use of a more stable material. Insummary, an ideal recording medium consists oftextured stable but low melting point film on a substratewith low thermal conductivity and good adhesionproperties. The specific media discussed in this paperare demonstrative and are not the ideal media as de-scribed by these criteria.

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24. S. Y. Suh and D. L. Anderson, "Latent Heat Effects of PulsedLaser Beam Induced Temperature Profiles in Optical RecordingThin Films," Appl. Opt. 23, 3965 (1984).

25. U. C. Paek and A. Kestenbaum, "Thermal Analysis of Thin-FilmMicromachining with Lasers," J. Appl. Phys. 44, 2260 (1973).

26. G. A. Niklasson and H. G. Craighead, "Threshold Laser Powersof Textured Optical Storage Media," Opt. Eng. 23,443 (1984).

We acknowledge help from J. A. Lewis in mathe-matical analysis. We thank W. J. Tomlinson, D. A.Snyder, and M. F. Dautartas for helpful discussions andE. J. Alexander and L. K. Anderson for encouragement.We thank L. M. Schiavone for help with sample pro-duction and D. E. Tamburino for assistance in the op-tical writing experiment.


optical writing mechanism of textured media

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