CA1292066C - Method of photothermal information recording, reading and erasing - Google Patents

Abstract

METHOD FOR PHOTOTHERMAL INFORMATION
RECORDING, READING AND ERASING

ABSTRACT

A method for photothermal information recording, reading and erasing consists in that the surface of a recording medium is charged prior to the photothermal re-cording process and then exposed, during photothermal information recording and erasing, to a modulated electro-magnetic beam localized to a spot size of the order of one information unit with the energy exceeding the energy required for softening the material of the recording medium, and, during information readout, with the energy less than that for softening the material of the recording medium, the surface of the recording medium being discharged before erasing information.

Description

~v~T~OD OF ~HO'~O~HhR.~L INFOR~i~TION
RECORDING, READING ~ND ERASING
This invention relates to the field of information re-cording, reading and erasing, which are based on relative movement of an in~ormation carrier and a source of electro-magnetic radiation, and, in particular~ is concerned with methods of ph~tsthermal recording, reading and erasing o~ information.
This in~ention can be employed in computers, in video and sound recording systems, in inYormation storage and processing systems.
The present lnvention deals with one of the Yital problems of pnotothermal recording and storage of digital information, which is ef~ected by means of optical storage devices capable of erasing information and recording it again. ~he existing methods of reversible recording of data, which make use of reversible magneto-optical and structural transformations in the recording ~edia based on chalco~enide resins are extremely di~ficu1t to realize.
Known in the art is a method of photothermal i~formation recording, reading and erasing, based on hydrodynamic effect in heated thin films of liguid (A.~ ~ar-tuzhansky, ~eserebrianye Protsessy Ton~ikh Plenok Zhidxosti, lg84, ~himiya Publ. 9 Lenin~rad, pp. 231-~32), which consists in that chan~es in the geometry of the thin-~ilm surface, corresponding to information being recorded and diCtated by the temperature relations of the sur~ace energy in this ~ilm, are obtained by irradiating the film which absorbs this radiation.
'~his prior art met~oà is deficient in that the ~

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storage period is too short and the sensitivity and rzso-lution are too low9 which is inherent in the liquid film characterized by thermal quasi-equilibrium of h~drodynamic relaxation.
Also ~nown in the art is a method of` photothermal in-formation recoraing, reading and erasing ~A.L. ~artuzhans~y, Neserebrianye E~otograficheskie Protsessy, 1984, ~imiya Publ., ~eningrad, pp. 46-49), comprising the s~eps of pro ducin~ a potential pattern in the recoraing medium film by preliminary uniform charging of' tnis film surface by a corona discharge, proaecting an image to be recorded onto the film in the spectral range corresponding to the ~noto-conductivit~ of the film material, developin~ the latent pattern `oy si~ultaneously softening the entire film -Dy heatin~, and fixing the pattern9 e.g. by cooling the film surface. The pattern recorded by tnis method is erased by neating and softenin~ the recording medium.
'l'he prior art method of pnotot~lermal information recoraing, reading and erasing can be use~ for multiple information rewriting. It is highly sensitive and ofI~ers a long period of storage. But this method is ~eficient in that tne level of noise is too nigh Decause OI` bhe inherent deformations of photoplastic materials~ Itioreover, ~uration of one recoruing-erasing cycle is too long due to the contributing factors, such as the time r~(3uired ~or charging the surface of the film; low mobility of current carriers in the photoplastic material, and the perioà of thermal relaxation of the recording medium. 'l'he resolution o~ the method is not 'nigh enougn for the information recording ~nd reading to be performed with a density provided by optical systems. In aadition, the transmission band is too narrow. '~hase aeficiencies are inherent in the techni-gues ot` pro~ucing a latent pattern and devloping this pattern over the entire surface OL' the thin Yilm. To realize this prior art method, it is necessaryj at first, to delineate a latent pattern wQich is a varyin~ density of the surface charge tnroughou, the ~ilm sur~ace. Such va-riations in the surYace charge density are dictated by the brightness of tne patern projected onto the film surface . ~l'his imaging technique is timeconæuming and is the one o~ the sources of noises~ The latent pattern is developed by simultaneous softening o~ the entire ~ilm and this brin~s out, as a sur~ace relieI pattern, not only the latent image but also interIerences. In addition, heating and subseguent cooling o~ tne entire recording medium requires too much time. To summari~e, tnis ~rior art method cannot provi~e optimal conditions Ior ~ilm defor-mation durinæ phototner~al recordin~ and erasing o~ an information unit. It cannot provide photothermal record-ing and erasing of in~ormation so that the information -unit is localized to a size limit ensured by the resolu-tion of optical systems.
'~he closest prior art is a method ~or p~o~ot`nermal in-ormation recording, reading and erasing (Vtoraya vsesojuz-naya nauchno-te~hnicneskaya konferentsiya "Problemy razvitiya RaaioteKhnifii", 19855 'i'sentralnoye i gruzinskoye respublikans~oe pravleniye naucnno-tekhnicheskogo obschestva radiotekhniki elektroniki i svyazi imeni A.S. Popova, I~OscOW, ~:9~Z~6f~

pp. 62-63), in ~hich information is recorded bg exposing a recording medium on a substrate, wnich is capaole OI' spreadinG over this subs~rate when melted, to a modulated electroma~netic beam localized to a spot size of the order of one information unit, using this electromagnetic beam to melt and change the pattern of the recording medium , and then fixin~ thus obtained localized in~ormation unit3 in the form of two-level pits by cooling the recording medium. 'i'nis is the end of the photothermal process of in-formation recording. Readout of pits is performed by a focuesed electrom~gnetic beam. ~ecorded information can be erased oy heatin~ the entire surface of the recording medium.
This prior art metnod off`ers the advanta~e of discrete phototner;nal inf`ormation recording and erasin~ ~iithin short periods of time dic~ated b~ the duration of electro-magnetic pulses wiht the ~ensity o~`fered by optical sys-tems and a high signal-to-noise ratio~ But this method is deficient in that tne reliability of inf`ormation process-ing is not high and the number of rewrite cycles is lo~.
The reason is that the recording medium aas to satisfy several conflicting requirements i~volved in the processes of in~ormation recordin~ and erasing. In fact 9 since the surface relief pattern of the recordin~ medium is changed due to tne gradient of forces of the surface tension and hydrodynamic stress, it should be heate~ to a temperature substantially exceeding its melting point in order to improve the viscoelastic propexties of the recording medium and raise the probability of pit centers to appear. But this temperature ~Z~ZC~66 results in fast destruction of tae material of' the re-~ording medium and ~rastically reduces tne number of re~eite cycles which could be realized. High-power conti-nous lasers are to be us~d to heat the recording medium for r~cording and this results in a considerabie increase in weight and dimensions of àevices f`or pnoiothermal re-cording, reading and erasing o~ information. It seems that the remedy is to use thin-film record~,ng media in order to bring down the required beam power. Such thin f`ilms possess low surface energy since they are made of materials which do not wet the su~strate when liguid~ 3ut this runs contrary to the reguirements set to such f`ilms 'oy conditions for erasing information wnereby the record-ing medium is to be stable to external exposure ~or re-lia'ole information storage.
'~ne prior art method ma~es use of a recoraing medium w~ose sur~'ace energy cannot De controlled durin~ inf'or-mation recording, reading and erasing. ;~equirements set to the surface enerOy the recording medium must possess during recording and erasin~ are ~onflicting. ~o optimize conditions for inf'ormation recording, reading and erasing, it is necessary to reduce the surfdce energy of the re-~cording medium film during recording and to increase t~liS
energy durin~ erasing of information.
Tne prior art method of~ers no way to control the surf'ace energy of' the recording medium film aurin~ inI`or-mation recording, reading and erasing in oraer to provide optimal conditions for these processes.

~Z~ 66 It is an object o~ this invention to make the proce~se3 of photothermal int'ormation recor~in~, reading and erasin~
more reliable.
Another object of the invention is to increase the number o~ inf'ormation rewrite cycles.
One more object oi tne invention is to ma~e devices for information recording, readin~ and erasing smaller and li~:hter in weight.
iI'hese objects are acnieved in that in a method f'or pnotothermal information recording, r~ading and erhsin~, con~isting in that, in or~er to produce eaca in-formation unit f`rom a sequence of inl'ormation units having a two-level deep pattern in a recording medium disposed on a substrate in t~le process OI' p'notothermal recording of information and for reading and erasing of information, thiS recorcing medium is exposed to a modulated electro-magnetic beam localized to a size of' the or~er of one inI'ormation unit-pit, the recording medium being made of a material capa'ble to spread over the substrate when so~te-ned9 and according to the invention, a char~e is applied onto the surface o~ tne recording medium prior to the photothermal information recording, the modulated electro-magnetic beam localized to a size of the order of one inf'ormation unit is applied to the recording medium ~or ~hotothermal recording o~ inf'ormation with the energy exceedin~ the energy required for softening the material of the recordin~ medium but less than the energy sufficient f'or ~estruction of tne material of the recording medium, and, for in~`ormation reading, ~ith the energy less than tne energy for softening the material of the recording medium, and the surI'ace of the recordin~ medium is char~ed pricr to erasing informatio~ therefrom.
The term "softening energy" resorted to in the des-cription as applied to the recording medium means the 0nergy which is required to soften the material o~ the ~recording medium. q'he ter~ "destruction ener~y" of tne material o~ the recor~ing ~edium means the energy to w~icn the material of the recording medium has to be ex~osed to for its ~es~ruction.
The ad~antages of tne invention are: controiled parame-~ers of the recording medium auring photothermal recording, reading and erasing of inlor~ation, and hig~ reliability of storage devices which could be made small and light.
The invention will now be described in ~reater detail With rererence to specific embodiments thereof and ac-COTnpanyinL dra~ings wnerein:
Fi~. 1 illustrates a fragment of a recording r~edium with a aescrete recording rDade as a two-level deep pat-tern, according to the invention;
Fig. 2 i~lustrates a fra~ment of a recording medium with a continuous recording made as a two-level dee~
pattern, according to the invention;
Fig 3 illustrates a cross-sectional view of an errl-bodir~ent of an optical device f`or ?hotothermal recording, reading and erasing of information, featuring a fila~ent L`or sustaining a corona discharge, according to the in-vention;

~9Z~66 Fig. 4 i11ustrates a cross-~ectional view of an embo-diment of ~n optical aevice ~or pnototnermal recorainO, readin~ and erasing o~ information, featurin~ an electron emitter, ~ccording to tne invention.
In accordance with the herein disclosed method for ~hotothermal information reacording, reading and erasing, a charge i~ applied on tne surface of' ~ recording medium prior to phototheLmal recording of information. T~e ma-terial of the recordin~ ~edium may be a semiconductor or dielectric and it ~nould be capaDle to spread over the su~strate Ylhen so~t. In order to apply a charge onto tne surf~ce of the recor~ing me~ium? several methods can be used, such GS thermoelectronic emission anà a corona dis-chargG. ~ne surface energy of the recording medium ~epends on the materi~l of the recording meaium~ the material of the substrate, and the method of coating employed to apply this recording medium on the substrate. The surface energy OI' the charged recording medium is the sum of tne surface energy of the non-charged recording medium and the energy of interaction of appli~d charges.
Since interaction ener~y of like charges is negative, the energy of the charged recording ~edium is less than tne energy of a non-charged recording me~ium. Ihis means that the surface cLlarge o~` tne recording medium results, throu~h the action of applied cnarges, in a reduction of the surface energy of tne recording medium which can even become negative. ~Lowever, irregularities of the recording medium tend to develop into a two-level deep surface 129Z(~

relie~ pattern (tnat is a pit i9 eitner available or not but the depth of all pits on the average is uniform) onl~
in the area where the recording meaium is to be melted, e.~ by la~er emission.
Each information unit, from a sequence of information units~ having a two-level deep pattern in the recording medium disposed on 2. substrate is produced by exposing this recording rnedium, in the process of photothermal re-cording, to a modulated electromagnetic beam localized to a s?ot size of the order of one information unit-pit. The ener~y of this beam should be more than tae energy re-quired to soften the material o~ the recordin~ medium but less than the ener~y of cestruction of tfliS material. The process of photothermal recording of information may be organized in different ways. During discrete photo-thermal recording, the laser beam can ~oe modulated o~ its inte~sity~ the cross-sectional area, and the beam tra-jectory.
i~e~erring to ~ig. 1, a fragment o~ a recording medium 1 has a di~crete recording made as a two-level surface relief pattern made up of pits ~.
During continuou~ pnotothermal recording, the laser b~am can be modulated by its width.
Referring to Fig. 2, a fragment of the recor~ing me-dium 1 has a continuous recordin~ made as a two-level sur-face relie~ pattern made up of pits 2.
ï2eduction of the surface energy of the recording medium during photothermal information recording permits heating 129Z~
-- 10 -- , o~ the recording medium by an electromagnetic beam to a temperature less than the destruction temperature of the material of the recording medium. This eliminates undesirable processes in the recording medium and the number of rewrite cycles can be significantly increased. Tne sur~ace energy of an uncharged recording medium may be ni~h enougn. Thjs makes it possible to use a recording medium which ade-quately wets the subs~rate when meltea and having àdeguate adhesion to the substrate wnen solid. During information storage, when the recording medium is discharged, its re-sistivity to erasing ef'fects is, theref'ore, improved. '~he recordin~ medium needs no speclal devices i'or discharge, it discharges by itself ~nrou~h leakage currents in the recordin~ medium and through the corona dischar~e of the op~osite polarity ?rior to the phototnermal recording in recording me~iums made from dielectric materials.
Vuring information readout, the recording medium is exposed to an electromagnetic beam localized to a size of the oruer of one information unit, ~hose ener~y is substan-tially less than the energy reguired to soften the mate-rial of tne recordinO medium. The recording medium can, at tnis stage, be either charged or discharged. Exposure to this ener~y brings no changes to the characteristics of the material of' tne recording medium.
' Durin~ information erasing, the recoraing medium is discharged in advance in order to achieve high wettabilit~
o~ the subs~rate by the recordin~ medium. '~hen the record-ing medium is exposed to a modulated electroma~netic beam, e.g. laser beam, localized to a spot size of the order of 1292~

one inf'ormation unit with the energy exceeding the sof~en-in~ energy ot t~e recording medium but less than the destruction ener~y of the material of this recording medium.
~ o summarize, charging of the sur~ace of the recording medium ensures control of the surface energy of this re-cordin~ mediu~ his maKes it possible to control tne et'~ect o~ the electro~agne~ic bea~ on the recording me-dium and to a~oid destruction of the material of this re-cordin~ medium in the process of photot~ermal information recordi~gl reading and erasing.
In this manner the reli~bility of photothermal in-~'ormation recordin~, reading and erasing can be improved and tne nu~ber of information rewrite cycles appreciably increased. '~he device for prlotothermal in~ormation record-ing, reading and erasin~ can be made smaller and lighter.
An optical device f`or photothermal infor~ation re-cording, readin~ and erasing comprises a cylinder 3 (~ig.3) made o~ a tr~nsparent material, a ~ilm of a recording medium 1 being coated on the internal surface thereof.
'~he external surface of the cylinder 3 is coated -Dy a transparent current-conducting ~ilm 4. A central electrode 6 is secured along the axi~ OI' t'ne cylinder 3 on insulating covers 5. In Fi~. 3 tne central electlode ~ represents a pit. One or two leads 7 for the central electrode ~ are secured i~ the covers 5~ The cyLinder 3 is air-tight and may be fillea with inert gas when a corona dischar~e is used to charge the recording medium 1. A modulated electro-~2~16 magnetic beam ~, e.g. a laser 'oeam, localized to a spotsize of c~e order of one information unit is used for phototherm~l information recording. The beam 8 is ~ocused by a~ obaective lens 9. Relative movement of the objecti~Je lens 9 and tne cylin~er 3 from one pit to anotner is effected by a positioning system (not shown). The constant-sign voltage drop reguired for the corona discharge is supp:ied to terminals 10 ana 11 and, ~urt~er on, via a spring contact 12, to the conducti~g transparent ~ 4 and, via a contact 13 and the lead 7, to the corona-discharge central electrode 6.
~ efer1ing to h'ig~ 4) an embodiment, shown therein, o~ a device f'or Jnotot~er~al inI'ormation recording, reading and erasi~g, according to t'ne in~ention, ~if~ers from the embodiment of h`i~. 3 in that ~n electron emit~er is used inst~ad of ~he cen-tr~l electrode 6. ln this embodiment, the recording medium 1 can ~e char~ed o~ way of thermo-electronic emission. In this c~se, ~ di~'ferenc~ of poten-tials is applied to the termi[lals 10 and 11, ~nile the in-tèrnal s~ace 14 of the cylinder 3 is evacuated and sealed.
nen a ~ign ne~ative, in relation to terminal lC 9 poten-tial is applied to a terminal 15, emitt~d electrons nit the surI~ace of the recor~ing medium 1 ~nd, thereby, charge this medium 1. The recording medium 1 should be made of semiconductor materials so tnat it can De discharOed after the difl'erence of` potentials is removed from the ter-minals 10, 11 and 15.
The optical device for ~hotothermal information re-cordinO, reading and erasing operates as follows.

~2(;~66 ~ he cylinder 3 (Fig. 3) is set into rotation abou~
its axis oy a motor (not shown) and is thus transported in relation to the objective lens g by means of a position-ing syste~ controlling this movement.
h constant-sign difierence of potentials is applied to the terminals 10 and 11 to initiate a corona discharOe and cnarge the surface of the recording medium 1.
To perform p~otothermal recording of information~ tne positionin~ system tra~sports the cylinder 3 to a desired address. lhe laser beam ~ localized to a spot size of the order of one information unit is directed to the recordinO
medium 1 to form a pit. Then the positioning system t~ans-ports the cylinder 3 to a new ad~ress and a new pit can oe recorded by the laser beam 8.
In this manner several in~ormation units, pits, can be recorded in several areas of the recording medium 1. The energy of the laser bea~ 8, in ttlis case~ should be more than tne softenin~ energy of the material of tae record-in~ medium 1 but less than tne destruction energy thereof.
To read in~ormation, the recording mediu~ 1 is exposed to a laser bea~ 8 localized to a spot size of the order of one information unit. 'I`he energy of the laser beam 8 at this staOe is substantially less than uuring recording when a pit is produced. This energy o~ the laser beam 8 is less than the energy reguired to soften tne ~terial o~
the recording medium 1. The `oeam reL`lected by the re-cording medium 1 is recorded. ~ne intensity ot th~ reI~lected beam is modulated by the two-level deep sur~ace relie~

;)bti pattern of tne recording medium 1 in accordance with the recorded pits. The address of the readout in~'ormation unit is determined by the positioning system.
To erase information, the surface of tne recording medium 1 is dischar~ed in advance. To this end, a corstant-sign difI'erence of potentials is applied to terminals 10 and 11, the sign being opposite to the charge polarity.
'1'o erase informationt tAe positioning system transports the cylinder 3 to a desired address and a modulated laser beam ~ localized to a spot size of tne order of one in-L'ormation unit acts on the recording medium 1. ~lne energy of' such laser beam 8 is more than the softening energy of the material o~ tf~e recording medium 1 but less than the aestruction energy thereof.
Below is an exam~le of' realization of recordin~ and erasing processes.
'rhe film of the recordin~ medium 1 is 60 ~m thic~ and is rna~e of chalco~enide glass. '~ne objective lens ~ I`ocuses the laser ~eam 8 to a spot with a diameter of 1.3 micro-meters. The vower of' laser emissio~ ran~es from 1 to 82 i,w.
The thin film of tne recording medium 1 was charOad Dy a corona discharge with the differ~nce of potentials bet-ween the terminals lQ and 11 equal to 1~ k~ aximum charg-in~ current was ~ microamperes. Caarging time was ~0 ms.
Duration of recording lignt pulses was 9~ ns. ~itn the char~e o~ tne thin film OI` trie recording medium 1 egual to 1.5-10 6 C, the minimal laser power reguired ~or photothermal recording was o .4,lw and for the dischar~ed film, tne mi-nimal laser ~ower reguireà for recording was 13 ;.~. ~he ~29Z~66 power of the l~ser beam for readout was 1 ~w, The time period for self-discharge of the recording medium 1 ~as 2.8~103 seconds. The ~ischarge was accelerated by a coro-na dischar~e with a dii~erence of potentials applied to the terminals 10 ana 11 equal to 18 ~, the sign being opposite to the polarity o~ the f'ilm c'nar~e. InI'ormation was locally erased by heating the recordin~ ~edium 1 by unfocused laser emission. The diameter o~ t~ Light spot on the recording medium 1 was 3.0 micrometers. In~orma-tion was erased by laser emission ~ith a power of 39 ~iw.
To sum up, the charge imparted to the surface o~ the recordin~ medium 1 chan~es the surface ener~g of this re-cordin~ medium, permits the use of lower power for photo-thermal recording, and, conseguent1y, prevents destruction of` the material of the recording medium 1. r~.oreover, opti-mal conditions are proviaed for patterning the recording medium 1 when usi~g photothermal methods of' information recordill~, reading and erasing. Pnotothermal information recording process on a charged recording medium 1 reguires nalI' tne power as contrasted to conventional methods. ~rlhe energy of` the laser beam 8 during recording on the medium 1 amounts to 4-10 10 J/~m2~ which is more than reguired for SoLtening tne material OL' tLle recorain~ ~edium 1 since tne soXtening energy is egual to 3.2~10 10 J/~ m~, but less ~nan the aestruction energy of the material of this recording medium, which is equal to ~.7-10 10 J~m2~ The ener~y of tL~le laser beam 8 during recording on an unchar~ed ~ilm amounts to ~.7~10 10 J/~m2 which is in excess o~' the ~z~

destruction energy of tne material of the recordin~ me-dium 1. It can be concluaed that charging the recording meàium 1 permits recording o~ an information pit in optical conditions without any destruction of the recording medium 1, whicn is important since it ma~es recording more reliable and permits multiplç recordings after erasin~
information from the thin ~ilm.
Information is read by e~posing the recording medium 1 to laser emission with the energy o~ 0.9.10 10 J/~m~, hich is substantially less than the ener~y requlred for softening the ~aterial oi the recording mediur~ 1, Information is erased by a laser beam with the energy equal to 5-10 10 J/~2, which is in excess of the energy required to soften tae material of tne recor~ing medium 1 and le~s than the aestruction energy thereof, In this manner the reliability o~ photothermal pro-cesses OI` information recordin~, reading and erasing is suostantiaily improved, multiple re-~ritin~ of inl'or~ation oecomes possible, the wei~ht and size of uevices for pnoto-thermal recording, rea~in~ and erasing of information are a~spreciably reduced.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
A method for photothermal information recording, reading and erasing, comprising the following steps to produce each information unit from a sequence of information units having a two-level deep surface relief pattern in a re-cording medium disposed on a substrate.
- applying, prior to photothermal information recording, a charge to the surface of a recording medium capable of spreading over the substrate when softened;
- exposing said recording medium to a modulated electro-magnetic beam localized to a spot size of the order of one information unit for photothermal information recording and erasing with the energy exceeding the energy required for softening the recording medium but less than the energy sufficient to destroy the material of said record-ing medium, and, for information readout, with the energy less than that required for softening the material of the recording medium;
- discharging the surface of said recording medium prior to erasing information.