WO2008050340A1

WO2008050340A1 - Recording medium for use in a three-dimensional optical data carrier, and method and system for data recording/reproducing

Info

Publication number: WO2008050340A1
Application number: PCT/IL2007/001295
Authority: WO
Inventors: Kozo Nakao
Original assignee: Mempile Inc.; Salomon, Yair
Priority date: 2006-10-26
Filing date: 2007-10-25
Publication date: 2008-05-02

Abstract

A recording medium is presented for use in an optical information carrier. The recording medium comprises an organic or metallo-organic substance having a first fluorescent compound convertible into a second fluorescent compound. The fluorescence profiles of the first and second compounds have fluorescence peaks at different wavelengths. This enables data recording in the medium by converting at least a part of the first compound within spaced-apart selected regions of the substance into the second compound, to thereby create a data pattern in the form of an array of the spaced-apart recorded regions, and enables data reproducing by detecting spatial variation of fluorescence from the medium.

Description

RECORDING MEDIUM FOR USE IN A THREE-DIMENSIONAL OPTICAL

DATA CARRIER, AND METHOD AND SYSTEM FOR DATA

RECORDING/REPRODUCING

FIELD OF THE INVENTION

The present invention is in the field of optical storage and relates to an optical information carrier and method and system for recording data to and reproducing data from the carrier.

BACKGROUND OF THE INVENTION

Optical information carriers are widely used in the computer and entertainment industry. Conventional optical information carriers are based on the use of reflective recording media, and accordingly have one or two data layers.

According to an alternative approach, data marks are optically recorded in optical information carriers made of non-linear optical material (recording media) such as polymer materials exhibiting multi-photon absorption. This technique is disclosed for example in WO 01/073779, WO 03/070689, WO 03/077240, WO 06/111972, WO 06/111973, WO 07/060674, WO 07/083308, WO 07/069243, WO 06/075328, WO 06/075326, WO 06/075327, WO 06/075329, all assigned to the assignee of the present application, as well as WO 04/034380. In such carriers, marks are present as local variations of material optical properties. More specifically, the optical material has a fluorescent property variable on occurrence of one- or multi-photon absorption resulted from an interaction with a recording or reading optical beam.

GENERAL DESCRIPTION The present invention provides a novel recording medium for use in an optical information carrier, and a system and method for recording and reproducing data to and from such recording medium. The recording medium is a non-linear medium, in which data recording and reproducing processes are based on multi-photon interaction (e.g. two-photon interaction). It should be noted that the expressions "multi-photon interaction", "multi- photon absorption", or "two-photon interaction", "two-photon absorption" used herein signify involving simultaneous interaction of two, or generally multiple, photons by the medium.

Preferably, the recording medium has substantially no linear absorption (that is linearly proportional to the strength of exciting light) in the wavelength range of multi- photon interaction used in the data reproducing processes. Also, in case the recoding process is based on multi-photon (two-photon) absorption, the recording medium is selected to have substantially zero linear absorption for the multli-photon exciting light.

The recording medium of the present invention comprises an organic or metallo- organic substance having a first fluorescent compound convertible into a second fluorescent compound. The first and second fluorescent compounds have different first and second fluorescence profiles (i.e. fluorescence peaks at different wavelengths), respectively. Thus, during the data recording process, the substance is exposed to light so as to effect said conversion in selected spaced-apart regions of the substance, thereby creating a data pattern (an array of spaced-apart recorded regions) in the form of an array of fluorescent regions and fluorescent spaces between these regions having different ratios of the different fluorescence peaks, respectively. More specifically, a ratio between the fluorescent peaks of the first and second compounds in the recorded region is different from a ratio between the fluorescent peaks of the first and second compounds in the space. Reading of the recorded data is based on detection of spatial variation of the fluorescence from the medium. Thus, according to one broad aspect of the invention, there is provided a recording medium for use in an optical information carrier, the recording medium comprising an organic or metallo-organic substance having a first fluorescent compound convertible into a second fluorescent compound, fluorescence profiles of the first and second compounds having fluorescence peaks at different wavelengths, thereby enabling data recording by converting at least a part of the first compound within spaced-apart selected regions of the substance into the second compound, to thereby create a data pattern in the form of an array of the spaced-apart recorded regions and enabling data reproducing by detecting spatial variation of fluorescence from the medium.

The first and second compounds are capable to fluoresce in response to multi- photon interaction. The first compound may be convertible into the second compound by a first multi-photon interaction, and the second compound is capable to fluoresce in response to a second multi-photon interaction. The first and second multi-photon interactions are different in at least one of the following: a wavelength of the multi- photon interaction, and multi-dimensional combination of parameters of the multi- photon interaction including temporal power profile and duration of the interaction event.

The selected multi-dimensional combination of the above parameters defined operating modes of the recoding and reproducing processes. The operating mode is selected for a given condition of at least one of wavelength, coherence and polarization of the applied radiation. This technique is disclosed in WO 2007/007319 assigned to the assignee of the present application, which is incorporated herein by reference with respect to this specific example.

In some embodiments of the invention, the recording medium is selected such that the fluorescence intensity profiles across wavelength ranges of fluorescence spectra of the first and second compounds are different. An overlap between the fluorescent profiles of the first and second compounds preferably does not exceed 50% of the second compound profile, or more preferably does not exceed 10% of the second compound profile.

The multi-photon absorption spectra of the first and second compounds may be different. As indicated above, the recording medium preferably has substantially no linear absorption in the wavelength range of multi-photon interaction used in the data reproducing processes. More specifically, the wavelength range of the multi-photon interaction causing fluorescence of the first and second compounds (while reading recorded data) has a wavelength for which the recording medium (the first and second compounds) has substantially zero linear absorbance.

In some embodiments of the invention, the first/second compound is selected from trans-2-(2-pyrolylvinyl)-2-pyridine, trans-2-(2-pyrolylvinyl)-2-quinoline, cis-2-(2- pyrolylvinyl)-2 -pyridine, c/5ⁱ-2-(2-pyrolylvinyl)-2-quinoline and substituted derivatives - A -

thereof, having for example at least one halogen, alkyl, aryl, alkoxy, cyano and/or nitro group.

The recording medium may be selected such that the first compound has weak or no internal hydrogen bond and the second compound resulted from said conversion has substantial internal hydrogen bond.

According to another broad aspect of the invention, there is provided a recording medium for use in an optical information carrier, the medium comprising a substance having a first compound which is operative based on multi-photon interaction to fluoresce with a first fluorescence profile and which is convertible by exposure to light into a second compound, which is operative based on multi-photon interaction to fluoresce with a second fluorescence profile, the first and second fluorescence profiles having peaks at different wavelengths, each of the first and second compounds having substantially no linear absorption for said multi-photon interactions.

According to yet another broad aspect of the invention, there is provided a recording medium for use in an optical information carrier, the recording medium comprising a substance comprising a first fluorescent compound convertible by exposure to light to a second fluorescent compound, wherein: each of the first and second compounds is an organic or metallo-organic compound; the first compound possesses a first fluorescence of a first wavelength induced by multi-photon interaction; the conversion from the first compound into the second compound occurs as a result of non-linear interaction; the second compound possesses a second fluorescence of a second, different from the first, wavelength induced by multi-photon interaction; the multi-photon interactions causing fluorescence of the compounds and the non-linear interaction causing conversion from the first into the second compound being different in at least one of the following: wavelength and interaction conditions including power profile and duration of interaction event.

According to yet further aspect of the invention, there is provided an optical information carrier comprising the above-described recording medium. The optical information carrier preferably includes at least one reference layer presenting a reflective interface with the recording medium. The reference layer is at least partially transparent for data recording and data reproducing light beams and at least partially reflective for a reference light beam, and has a certain surface relief. The optical information carrier may further include a non-recording layer, which has a fluorescent property different from that of the recording medium, is at least partially transparent for data recording and reproducing and reference beams, and is positioned such that the reference layer is sandwiched between the recording layer and the non-recording layer.

According to yet another broad aspect of the invention, there is provided an optical information carrier comprising a recording medium carrying a three-dimensional data pattern in the form of an array of spaced-apart recorded regions, wherein the recorded regions and the spaces between the recorded regions are responsive to multi- photon interaction by fluorescent profiles having peaks of fluorescence at different wavelengths, respectively. The present invention, in yet a further aspect, provides a method for recording data in the above-described recording medium. The method comprises: focusing a recording light onto selected spaced-apart regions of the recording medium, the recording light having a wavelength suitable for absorption by the first compound so as to convert the first compound into the second compound within said selected regions, thereby creating a pattern of spaced-apart recoded regions presenting a pattern of spatial variation of fluorescence spectra of the recording medium.

The invention also provides a method for reproducing data from the above described optical information carrier. The method comprises: focusing onto a recording layer in the carrier a reproducing light beam having a wavelength suitable for multi- photon absorption by the first and second compounds, thereby inducing first and second fluorescent responses with first and second fluorescent peaks of different wavelengths from the recorded regions and spaces between the recorded regions, respectively, collecting at least one of the first and second fluorescent responses, thereby detecting spatial variation of fluorescence spectra from the medium indicative of the recoded data pattern.

According to yet another broad aspect of the invention there is provided an optical system for use in recording and reproducing data to and from the above- described optical information carrier, the system comprising:

- a light source unit configured and operated for selectively producing a recording light beam and a reproducing light beam of the same or different wavelengths capable to cause non-linear interaction with a recording medium of the carrier; a light directing optics for selectively directing and focusing the recording or reproducing light beam onto a recording layer in the recording medium, and for collecting light coming from the medium; a wavelength selective light detection system configured for detecting florescent light from the data carrier, and generating detection data indicative of spatial variation of the fluorescent light coming from the carrier; and - a control system configured and operable for receiving and analyzing the detection data, and generating output data indicative of a data pattern recorded in the carrier.

The control system may operate for analyzing the detection data by processing intensity of the detected fluorescent components of different first and second wavelengths, e.g. performing differential amplification of the intensity of the detected light of the first wavelength range and that of the second wavelength range.

In some embodiments of the invention, the recording and reproducing light beams are of the same or close wavelengths, both in multi-photon absorption spectra of different compounds in the recording medium having different peaks of fluorescence, respectively, in response to excitation by the reproducing beam. In this case, the recording and reproducing light beams preferably have different intensities (or generally power profiles) and/or the recording and reproducing events are of different durations so as to minimize graying during the data reproducing process.

The light source unit may include a common light source for selectively generating the recording and reproducing beams, or may include two light sources for generating the recording and reproducing light beams of different wavelengths, respectively. In the latter case, the control system is configured and operable to operate the light source unit to produce the recording and reproducing light beams with certain operating modes defined by multi-dimensional combination of the recording and reproducing process parameters. The latter include a temporal power profile and duration of the recording and reproducing event, for a given condition of at least one of wavelength, coherence and polarization of light. To this end, the control system operates to utilize data indicative of a function corresponding to an effect of recording in the medium used and a function corresponding to the effect of reproducing the recorded data, each being a function of at least one of the temporal profile of power and the event duration, to select an operating value for at least one of the temporal power and duration parameters such as to provide a non-degenerate relation between said functions. The light source unit may also produce a reference beam of a wavelength different from the recording and reproducing beams. Accordingly, the detection unit may include a detector for detecting reflections of the reference beam from the information carrier.

BRIEF DESCRIPTION OF THE DRAWINGS hi order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Fig. IA is a block diagram of a data recording/reproducing system according to an example of the present invention.

Fig. IB shows a block diagram of a data recording/reproducing system according to another embodiment of the present invention. Fig. 2 A presents an example of a compound contained in a recording medium convertible from its first fluorescent state into a second fluorescent state, occurring while recording data in the medium.

Fig. 2B shows another example of a compound contained in a recording medium convertible from its first fluorescent state into a second fluorescent state, occurring while recording.

Fig. 3A shows the absorption and fluorescence spectra of the compound in its first, non-recorded and second, recorded states.

Fig. 3B shows a variation of the intensity of fluorescence of the non-recorded and recorded states. Fig. 3C shows the absorption and fluorescence spectra of the non-recorded and recorded states of the compound contained in the recording medium.

Fig. 4 shows yet another example of a data recording/reproducing system of the present invention. DETAILED DESCRIPTION OF EMBODIMENTS

Referring to Fig. IA, there is illustrated, by way of a block diagram, an example of the configuration of a recording/reproducing system, generally designated 100, suitable for recording/reproducing data in an optical data carrier 10 of the present invention.

The optical data carrier (disc) 10 comprises a recording medium 1 configured for enabling recording therein a three-dimensional pattern of spaced-apart recorded regions arranged in multiple virtual layers. The carrier may include a stack of at least two plates, each including a recording medium in which data can be arranged in multiple virtual layers. In the present example, a single recording layer is shown solely to simplify the illustration.

According to the invention, the recording medium 1 contains an organic or metallo-organic substance which is capable of being in the form of a first compound A, which responds to a multi-photon (e.g. two-photon) interaction by a first fluorescent signal in a first wavelength range, and which is convertible by exposure to light into a second compound B, which in turn responds to a multi-photon interaction by a second fluorescent signal in a second wavelength range, such that the first and second fluorescent signals have peaks at different wavelengths. It should be understood that the expressions "multi-photon interaction", "multi-photon absorption", or "two-photon interaction", "two-photon absorption" used herein signifies involving simultaneous interaction of two, or generally, multiple photons by the medium. Also, the selected substance of the recording medium is such that each of the first and second compounds has substantially no linear absorption (that is proportional to the strength of the light) in the wavelength range(s) of non-linear processes used in the data reproducing (and also no linear absorption for light used in the data recording in case the latter is based on multi-photon interaction).

Within the context of the present invention, the term "organic compound" refers to any compound containing at least one carbon atom. The "metallo-organic compound" is an organic compound, as defined, containing at least one metal element, being preferably bonded to at least one atom of the organic compound through, e.g., ionic or covalent bonding. The organic or metallo-organic substance is preferably conjugated in at least a part thereof, or fully conjugated, so as to increase the susceptibility to the multi-photon interaction and accordingly increase a fluorescent signal in response to said interaction. Some not limiting examples of the recording medium will be described more specifically further below with reference to Figs. 2 A and 2B.

In the present not limiting example, the recording medium is carried by a substrate 4, and the data carrier also includes a reference layer 2 presenting a reflective interface with the recording layer 1. Generally, the same reference layer may be associated with more than one recording layer. In the present example, the reference layer 2 is located on top of an adhesive layer 3.

The reference layer 2 is at least partially transmitting for recording/reproducing wavelength(s) and at least partially reflective for a reference beam wavelength. The reference layer 2 serves for controlling the focusing of a recording/reproducing light beam on a desired position in the recording layer 1.

The adhesive layer 3 is non-recording layer and serves for adhering the reference layer 2 to the substrate 4. The non-recording layer 3 is highly transmitting for wavelength(s) of the reference and recording/reproducing beams, while its material composition differs in a fluorescent property from the material of the recording medium 1 used in the data carrier. The non-recording layer 3 may be composed of a material having no fluorescent property at all or a material differing in fluorescence emission efficiency or emission wavelength from the recording medium 1.

The data recording/reproducing system 100 includes a light source system including a first light (laser) source 11 operative to produce a recording/reproducing beam L₁, and a second reference light (laser) source 21 operative to emit a reference beam L₂. The system 100 further includes a light detection system configured for detecting fluorescent light from the data carrier and distinguishing between different wavelengths of fluorescent peaks of compounds A and B (or different ratios of fluorescence in recorded regions and spaces between them). Thus, the detection system includes a detection unit (detector 17A and 17B) for detecting fluorescence coming from (e.g. transmitted through) the data carrier 10, and in the present example also includes a detection unit 27 for detecting reflection of the reference beam from the data carrier. Further provided is a light directing system, generally at 17, configured for directing and focusing the recording/reproducing beams onto a desired location in the medium and for directing light returned from the medium (reflection of the reference beam and the fluorescent response of the medium) towards the detection system. The detectors 17A and 17B are associated with a collection optics 15 (formed by two lenses in the present example), and serve for detecting the light response of the medium to the reading beam, as will be described more specifically further below. The detection unit 27 is associated with its imaging optics 26 (e.g. two lenses) and serves for detecting reflection of the reference beam from the reference layer 2. Also provided in the system 100 is a control system 30, connectable to the light source system and to the detection system (via wires or wireless signal transmission as the case may be), and operating to adjust the operational mode of the light source system and receive and analyze the output of the detection system.

The recording/reproducing laser source unit 11 includes a light source capable of generating light for data recording to and reproducing from the recording medium based on two-photon absorption. This may be a laser light with a wavelength λj of about 660 nm, for example. In this connection, it should be noted that the interactions (non-linear interactions) used in the data recording and data reading (multi-photon) processes may utilize the same or different wavelengths of the recording and reproducing beams. In any case and especially in case the same wavelength is used for recording and reading, the non-linear interactions of the recording and reading processes preferably have different operating modes or regimes defined by multi-dimensional combination of the process parameters. The latter include temporal power profile (or shape) and duration of the respective irradiation event (recording event or reading event). The operating regime is selected for a given condition of at least one of wavelength, coherence and polarization of the applied radiation. This technique is disclosed in WO 2007/007319 assigned to the assignee of the present application, which is incorporated herein by reference with respect to this specific example. More specifically, data indicative of a function corresponding to an effect of recording in the medium used and a function corresponding to the effect of reading the recorded data (i.e. the fluorescent response to reading radiation) is utilized. Each of these functions is a function of at least one of such parameters as a temporal profile of power of exciting radiation (or generally, interacting radiation) within the respective one of the recording and reading events, and the event duration. The data indicative of said functions is utilized to select a certain operating regime or mode, namely an operating value (or range of values) for at least one of the above parameters during the recording and/or reading process, where the selected operating regime is such as to provide a non-degenerate relation between said functions. This allows for controlling the effect of recording during the reading process (termed "graying").

In some embodiments of the invention, the recording/reproducing laser source unit 11 is configured to have a switchable output such as to irradiate the recording medium with light at an average output of 1 W with a pulse width of about several 10 ps on recording, and with light at an average output of 0.5 W with a pulse width of about several 10 ps on reproducing.

The recording/reproducing beam Li is directed by a beam splitter 12 and a mirror 13 onto a focusing optical system 14 which focuses this light beam on a desired region in the recording medium 1 as a fine spotlight. The focused position of the beam Li in a direction of the disc thickness is controlled by driving the focusing optical system 14 with a focusing servomechanism. Focusing of the recording or reproducing beam onto the recording medium results in two-photon interaction of the respective beam with the recording medium in the focused position, causing respectively, the medium conversion from the compound A to compound B, or fluorescence response of the medium. More specifically, when during the recording process, the beam Li is focused on a desired position in the recording layer 1 with a certain beam intensity and during a certain short time, the compound A at the focused position is modified and converted into the compound B. As a result, the fluorescent property (peak of fluorescent response) of the recording medium at the focused position is changed, thereby executing data recording. When during the data reproducing process, the beam Li is focused on the previously recorded position, a fluorescent light is emitted in accordance with the existing compound in the medium at the recorded position (the concentration of the compound A or B), where the fluorescent responses of the different compounds are of different wavelengths (those of the peaks of fluorescence). As indicated above, in order to eliminate or at least significantly reduce an effect of graying (effect of recording during reading), the operating modes of the recording and reading processes (beam parameters) are appropriately selected taking into account functions describing the recording and reading events. Considering the data reading process, a part of the fluorescent light (response or read signal) within a certain solid angle is led through a lens system 15 and a wavelength selective element (dichroic mirror or a filter) 16 to light detectors 17A, 17B, which provide detected signals to allow data reproduction based thereon. More specifically, in the present not limiting example, the two detectors 17A and 17B serve for receiving the wavelengths of the fluorescent responses from compounds B and A, respectively, where the concentration of compound B is relatively high in the recorded regions compared to that in the spaces between the recorded regions, and the concentration of compound A is relatively high in the spaces between the recorded regions as compared to that of the recorded regions. In this connection, it should be noted that the recording medium may be such that in its initial, non-recorded, state it contains a mixture of compounds A and B with greater concentration of compound A than the compound B, and therefore the recording medium in its non-recorded state is referred to as being in the form of a compound of structure A. During the recording procedure, spaced-apart regions (recorded regions) are created, having higher concentration of compound B as compared to the spaces between them and lower concentration of compound A as compared to the spaces between the recorded regions. The detection of the two fluorescent signals with peaks at different wavelengths enables reconstruction of a three-dimensional pattern of the spaced-apart recorded regions, and thus identifying the recorded data.

It should however be noted that only one of the detector 17A and 17B can be used receiving either one of the fluorescent signals, i.e. that coming from the recorded regions (having higher concentration of compound B) or that coming from non-recorded regions (containing higher concentration of compound A). In this case the detected data is in the form of a sequence of timely separated fluorescence coming from the recording medium, indicative of the data pattern.

In general, the detected signal derived from total of the light of the first wavelength range and the second wavelength range or anyone of the signal derived from the first or the second wavelength range can be used for data reproducing. One embodiment of the present invention uses the signal derived from the separated light of the second wavelength range (compound B). Another embodiment of the present invention uses a processed result from both of the separated signals. For example, a difference between both detected signals can be computed at a processing unit 40, which provides a processed result to allow data reproduction based thereon. The processing unit may be an arithmetic operator such as operational amplifier. By differential amplification of the signals, the signal can be amplified and noise related to both signals can relatively be decreased. The processing unit 40 may be a constructional part of the computer system 30 or a separate unit e.g. connectable to the control unit 30.

The dichroic mirror 16 may have a property to transmit light with shorter wavelengths and reflect light with longer wavelengths than 500 run. In the present example, two more wavelength selective filters 18 and 19 are provided, the filter 18 being located in an optical path of light transmitted through the dichroic mirror 16 and propagating towards the detector 17 A, and the filter 19 being located in an optical path of light reflected from the dichroic mirror 16 and propagating towards the detector 17B.

For example, the wavelength selective filter 18 may have a property to transmit light with wavelengths longer than 400 nm and shorter than 500 nm and block other light. This allows the recording/reproducing beam Li or any other light outside this range (ambient light) to be prevented from entering the detector 17 A. The wavelength selective filter 19 may have a property to transmit light with wavelengths longer than 500 nm and shorter than 650 nm and block other light. In order to precisely form a beam spot of the recording/reproducing beam Li in a desired depth of the recording medium, the light directing optics is preferably configured with a corrected spherical aberration. In addition, the focusing optical system 14 is also designed not to cause any spherical aberration out of a certain tolerance. As for the reference beam L₂, the spherical aberration can be tolerated to higher extent.

The reference light source unit 21 (e.g. laser source) serves for tracking and focusing servo control of the recording medium aside from the recording/reproducing light source 11 and is operative to emit the reference beam L₂, of a different wavelength λ_2> of about 780 nm, for example. The reference beam L₂ is directed to the beam splitter 12 and mirror 13 towards the focusing optics 14 which focuses this beam onto the reference layer concentrically with the recording/reproducing beam Li propagation. Preferably, a polarization control arrangement is provided in the optical path of the reference beam from its light source towards the beam splitter 12: the reference beam L₂ passes through a polarizing beam splitter 22, and a polarization rotator (1/4- wavelength plate) 23. Also provided in the path of the reference beam L₂ is a focusing optical system 24, which focuses the polarized reference beam onto the beam splitter 12. In order to control the focused position of the recording/reproducing beam Li in a desired focusing position, the reference beam L₂ is focused on the reference layer 2 by the action of the focusing optical systems 14 and 24. The reference beam L₂ reflected from the reference layer 2 travels back on the same optical path, and is then reflected by the polarizing beam splitter 22 and enters the lens system 26 and detector 27. Based on an output signal from the detector 27, the focusing optical systems 14, 24 are controlled such that the focused position of the reference beam L₂ always coincides with the reference layer 2. Retaining the reference beam L₂ focused on the reference layer enables control of the focused position of the recording/reproducing beam Li on the basis of the reference beam. The tracking control can for example be executed using a well-known push-pull technology or the like, in which case the detection unit 27 comprises a four-part split detector or the like. It should be noted that preferably the reference beam and the reproducing beam should be separated. One of the ways to do it is to separate wavelengths: one being of a first wavelength and the second being of a second wavelength.

Referring to Fig. IB, there is illustrated a data recording/reproducing system 200 according to another example of the invention for recording/reading information in an optical information carrier 10. The information carrier utilizes a recording medium configured as described above, namely including a substance capable of being substantially in the form of a first compound A (e.g. having greater concentration of the first compound as compared to that of a second compound) which is excitable by multi- photon interaction to emit fluorescence of a first wavelength, and is convertible by nonlinear interaction into the second compound B which is in turn excitable by multi- photon interaction to emit fluorescence of a second different wavelength. In the present example, the information carrier is also of a kind having reference layer(s).

To facilitate understanding, the same reference numbers are used for identifying components that are common in systems 100 and 200. The system 200 is configured generally similar to the above-described system 100, but includes an additional light source unit 50 producing a heating light beam L₃ of a wavelength different from the reading or recording wavelength(s). The heating beam L₃ is directed to the carrier 10 through the same focusing/collecting optics 14 as the recording/reproducing and reference beams. To this end, in the present example, the mirror 13 is replaced by a beam splitter/combiner. The heating beam L₃ is applied to a record position during recording together with the recording/reproducing beam Li. The heating beam L₃ is focused almost on the same position as the recording/reproducing beam Li through the focusing optical system 14. The tracking and focusing servo control can be executed at the control system 30 based on the detected signals from the detectors 27 and 17A and/or 17B.

As indicated above, the optical information carrier of the present invention preferably includes one or more reference layers 2. The reference layer 2 has a reflecting surface formed of a film having a low reflectance (around 2-50 %), and may be vapor-deposited on a patterned surface preformatted on the lower surface of each recording layer 1 by a well-known stamper. Alternatively, this reflecting surface may be formed with a difference in refractive indices between (i) the enclosing (above and below) first recording layer 1 and a second recording layer or substrate (or non- recording layer 3), and (ii) the reference layer structure, the adhesive layer 3 and optional coatings. The reflecting surface is provided with a certain pattern (surface relief) in the form of pits having a certain width and depth providing response pattern to the different wavelengths. Pits are employed for tracking of the reference beam L₂ and for calibration of the reference beam L₂ and the recording/reproducing beam Li in the tracking direction and in the focusing direction. The pits are therefore formed to allow detection of the recording/reproducing beam Li focused on the reference layer 2 and detection of the reference beam L₂ focused on the reference layer 2. The principles of using such a patterned reference layer and its configurations are described in WO07069243, WO07083308, WO06111972 , and WO06111973, all assigned to the assignee of the present application, and incorporated herein by reference.

The adhesive layer 3 is sufficient if it has a high transmissivity for the recording/reproducing beam Li. As indicated above, a material different in fluorescent property from the material of the recording layer 1 may be used. For example, as the material of the non-recording layer 3, a polycarbonate, a methyl methacrylate copolymer (PMMA), an acrylic photopolymeric adhesive optically cured, or an epoxy resin or the like may be used. Reference is made to Figs. 2A and 2B showing two specific but not limiting examples of compounds suitable to be used in the recording medium of the present invention and their conversion from compound A to compound B by two-photon interaction (constituting non-linear interaction). In the example of Fig. 2 A, a compound of structure Al is shown, being trans-2- (2-pyrolylvinyl)-2-pyridine, which has a trans configuration of the central double bond and is fluorescent in response to multi-photon interaction. To prepare an optical carrier with such recording medium, a substance containing a substantial concentration of compound Al may be dispersed in a substrate material such as a plastic medium, e.g. may be contained as a component of the plastic material. When such a compound of structure Al is exposed to exciting light that causes two-photon absorption, the compound Al being in the trans configuration is transformed into the cis configuration shown by compound Bl. It should be noted that instead of a compound of structure Al, any other organic or metallo-organic compound, generally of structure A, can be used, which satisfies the following conditions: it exhibits fluorescence of a certain wavelength (at a fluorescence peak) in response to multi-photon interaction, and when exposed to irradiation capable of inducing non-linear interaction with the medium, converts into another compound of structure B (for example the trans or the cis compound of structure A converts into the other compound B), which exhibits fluorescence of a different wavelength (peak of fluorescence) in response to multi-photon interaction. As indicated above, each of the compounds of structures A and B is preferably at least partially conjugated.

As indicated above, the substance in its initial state, namely before being excited by light to cause the conversion, may also contain a compound B, but of a smaller concentration compared to the concentration of compound A.

In the non-limiting example of Fig. 2B, another (conjugated) compound, trans- 2-(2-pyrolylvinyl)-2-quinoline, satisfying the above conditions is presented. In Fig. 2B, compound of structure A2 being in the trans configuration and having a fluorescence of a certain wavelength in response to the non-linear interaction is converted into the cis compound B2, e.g. upon exposure to exciting light that causes two-photon absorption. The cis compound B2 also exhibits fluorescence, but of a different wavelength, in response to the multi-photon interaction. The above exemplified compounds may be substituted by halogen, alkyl, aryl, alkoxy, cyano or nitro. The substance for the recording medium is selected such that it is capable of being in the form of first and second compounds, where each of these compounds in the process of two-photon absorption provides for simultaneous absorption of the two photons. Therefore, the compound has substantially no 1 -photon (linear) absorbance at the wavelength of the two-photon interaction. So, when recording or reproducing beam passes through the recording medium, there is substantially no or little extinction by absorption or scattering of the light by the recording medium. The excited first compound yields fluorescence and transforms into a second compound in some definite proportion through control of the irradiation profile. The second compound also absorbs two photons simultaneously. So, when recording or reproducing beam passes through the material containing the second compound, there is also substantially no or little extinction by absorption or scattering of the light by the recording medium. The excited second compound yields fluorescence and can be transformed into the first compound in some definite proportion. So, if the recording medium is irradiated by light capable of converting compound A to compound B, some dynamic equilibrium defined by the wavelength of said light will be reached after long time irradiation. The wavelength of fluorescence (at the fluorescent peak) of the first and second compounds is different and can be separated by wavelength selective mirror or filter. For the recording medium of the present invention to have higher performance, it is preferable that there is substantially no or little linear absorption of anyone of its first and second compounds in the wavelength range causing the fluorescence of the second compound as well as in the wavelength range of said fluorescence. This is because when there is significant absorbance in the wavelength range of the fluorescence, generated (emitted) fluorescent light is absorbed while it passes through the medium itself. It is preferable that the wavelength of fluorescence peak of the second compound differs from the wavelength of absorbance peak of anyone of the first and second compounds for more than lOOnm. It is more preferable that the difference is more than 150nm. It is further preferable that the difference is more than 200nm. It should be noted that the recording medium substance can be selected such that the wavelength ranges of the fluorescence of the first and second compounds are slightly overlapping, or significantly overlapping up to a full overlap. However, the fluorescence property (fluorescence intensity profile) of the medium may change upon a change of the state of the medium (higher concentration of compound A or of compound B) if the fluorescence quantum yield is different for these compounds and if the fluorescence intensity profile across the wavelength ranges is significantly different. This is true even for the case the fluorescence wavelength ranges of the first and second compounds are substantially overlapping. Having a significantly different fluorescence intensity profile across the wavelength ranges means that wavelength selective filter(s) can differentiate between the fluorescence of the first and second states of the medium (i.e. those of the higher concentration of compound A or compound B as compared to the other). The technique of this invention provides for increasing the signal performance when reading the recorded data, by using either detection of the fluorescence of the second compound (created during the recording process) or detection of both the fluorescence of compound A and fluorescence of compound B. The recording medium (containing fluorescent compound A convertible into fluorescent compound B) is preferably selected such that an overlapping ratio in the fluorescence spectra (profiles) of the medium is preferably less than 50%, or more preferably less than 20%, and even more preferably less than 10%. The overlap between the fluorescence spectra can for example be expressed by:

overlap = where F_A and F_B are the respective

fluorescent spectra.

The preferred selection of the recording medium material is a well defined wavelength difference (shift) between the maximal value (peaks) of fluorescence of the first and second compounds. The difference is preferable to be larger than 50nm, more preferably larger than lOOnm, further more preferable to be larger than 150nm. Thus, a difference in the fluorescence spectra (or fluorescence profiles) may be referred to as having peaks of fluorescence occurring at different wavelengths, for the recorded regions (containing higher concentration of compound B) and the non- recorded regions (containing higher concentration of compound A). Having fluorescence peaks well separated in wavelengths (irrespective of whether the wavelengths ranges of fluorescent spectra are overlapping or not) provides for easy detection of recorded positions, with higher contrast (better SNR) which benefits, inter alia, from the reduction of background signal from the unrecorded volume of the data carrier.

In the embodiment of the present invention, the substance of the recording medium comprises a first fluorescent compound that can be converted into the second compound by trans/cis isomerization. The first compound may have no intramolecular hydrogen bond while when converted into the second compound it has an intramolecular hydrogen bond. The difference in the presence of intramolecular hydrogen bond between the first and second compounds may bring a wavelength difference in the fluorescence peaks of the first and second compounds. Reference is made to Fig. 3A showing absorption and fluorescence spectra of the substance exemplified in Fig. 2A when in the form of the first and second compounds Al and Bl. In the figure, the reference symbols A_t and F_t respectively indicate the absorption spectrum and the fluorescence spectra of the compound Al, while the reference symbols A_c and F_c respectively indicate the absorption and the fluorescence spectrum of the compound B 1.

When a light beam Li with a wavelength shown in the figure (670nm) is applied as the recording light to cause multi-photon (e.g. two-photon) absorption, the compound Al is converted into compound Bl in those locations of the medium which interacted with the focused light beam. The wavelength difference of the peaks of F_c and A₀ spectra is around 210nm and an overlap ratio of the spectra F_t in F_c is less than 10%. As a result, the absorption spectrum of the compound at the focused position of the exciting beam is varied from the curve A_t to A_c, and the fluorescence spectrum varies from the curve F_t to F_c.

In practice, as shown in Fig. 3B, at a non-recorded region of the recording medium (predominantly comprising active compound in the form of compound state Al), only a fluorescence wavelength almost associated with the curve F_t can be obtained (position (a)), namely the fluorescence wavelength associated with the curve F_c is almost zero. To the contrary, when a certain exposure of the interacting light (e.g. multi-photon absorption) is applied, the peak of the curve F_t lowers while the curve F_c slightly appears from almost zero (position (b)). Further, when exposure is increased, the peak of the curve F_t lowers further while the peak of the curve F_c becomes large in contrast (position (c)). In this way, the substance is modified from its one compound- structure to the other to change the fluorescence spectrum, data is recorded at the focused position. Fig. 3C shows absorption and fluorescence spectra for the compounds of Fig.

2B. The difference of peak wavelength of F_c and A₀ is around 180nm and overlap ratio of F_t in F_c is less than 10%. Absorption spectra may be different between that of the trans compound A2 and that of the cis compound B2. Especially, when two-photon absoφtion is applied, the difference becomes greater. For example, the compound A2 may be excited by two-photon absorption using a laser light of 670 ran, and the compound B2 may be excited by two-photon absorption using a laser light of 780 nm wavelength. Both compounds emit fluorescent light of different wavelengths (i.e. peaks of fluorescence occurring at different wavelengths).

When the light beam Li is applied on the focused position as the reproducing light, according to the state of the compound (or the fluorescence spectrum), at that position the amounts of light received at the detectors 17A, 17B vary. If information has been recorded at the focused position and the compound Bl has been formed partly, fluorescent light is emitted in accordance with the fluorescence spectrum F_c. Therefore, the amount of light received at the detector 17B becomes larger while the amount of light received at the detector 17A becomes smaller. If no information is recorded at the focused position and only the compound Al is present, fluorescent light is emitted in accordance with the fluorescence spectrum F_t. The amounts of light received at the detectors 17A and 17B can be compared with each other, thereby reproducing the information at the focused position.

The present invention is not limited to the above-described embodiment but rather can be modified variously without departing from the scope of the invention. For example, two detectors 17 A, 17B are provided in the above embodiment such that the former detects the fluorescent light based on multi-photon absorption in the compound before writing while the latter detects the fluorescent light based on multi-photon absorption in the compound after writing. As an alternative to this example, only one detector, e.g. the detector 17B, may be used to detect the fluorescent light based on multi-photon absorption in the compound resulting from the conversion (Bl, B2 in the examples of Figs. 2A and 2B), while the fluorescent light based on multi-photon absorption in the initial compound (Al, A2 in Figs. 2 A and 2B) may be removed or split. Such a configuration can exert the similar effect. Fluorescent light according to Al or A2 can also be used instead of Bl or B2, but fluorescent light according to Bl or B2 that increases according to recording is preferable because of better S/N ratio. Referring to Fig. 4, there is shown yet another example of a recording/reading system 300 suitable to be used in the present invention. Here, the recording/reproducing light source unit 11 is configured for producing two light beams (e.g. by using two laser light sources HA, HB) of two different wavelengths, e.g. of 660 nm and 780 nm. Thus, two light beams Ll and Ll' are produced by light source unit 11. The laser source HA operative to emit a light with a wavelength of 660 nm is used on data recording, while the laser light source HB operative to emit a light with a wavelength of 780 nm is used on data reproducing. The light beams with different wavelengths from the light sources HA, HB are allowed to be transmitted through or reflected by a dichroic prism 41 to arrange optical axes thereof, and the polarizing directions thereof are arranged at a polarizing plate 42. The beam Ll' is converted into a parallel light through a collimator leans 43 and further directed to an information carrier 10 via a beam splitter/combiner 12, mirror 13 and focusing lens system 14 as described above with reference to Fig. IA. hi this way, in the case the absorption spectrum of the compound of the recording medium before writing and that after writing are different, the use of two different wavelengths that are adequately selected can prevent the signal quality from deteriorating due to writing to a non-recorded portion on data reproducing (the so-called "graying"). Thus, the present invention provides a novel recording medium for use in an optical information carrier, and system and method for recording/reading data in such information carrier. Those skilled in the art would readily appreciate that various modifications and changes can be applied to the embodiments of the invention as described above, without departing from its scope defined in and by the appended claims.

Claims

CLAIMS:

1. A recording medium for use in an optical information carrier, the recording medium comprising an organic or metallo-organic substance having a first fluorescent compound convertible into a second fluorescent compound, fluorescence profiles of the first and second compounds having fluorescence peaks at different wavelengths, thereby enabling data recording by converting at least a part of the first compound within spaced-apart selected regions of the substance into the second compound, to thereby create a data pattern in the form of an array of the spaced-apart recorded regions and enabling data reproducing by detecting spatial variation of fluorescence from the medium.

2. The recording medium of Claim 1, wherein the first and second compounds are capable to fluoresce in response to multi-photon interaction.

3. The recording medium of Claim 1, wherein the first compound is convertible into the second compound by non-linear interaction.

4. The recording medium of Claim 3, wherein said non-linear interaction comprises multi-photon interaction.

5. The recording medium of Claim 4, wherein the first compound is convertible into the second compound by a first multi-photon interaction, and the second compound is capable to fluoresce in response to a second multi-photon interaction different from the first multi-photon interaction in at least one of the following: a wavelength of the multi-photon interaction, and multi-dimensional combination of parameters of the multi-photon interaction including temporal power profile and duration of the interaction event.

6. The recording medium of any one of Claims 1 to 5, wherein the fluorescence intensity profiles across wavelength ranges of fluorescence spectra of the first and second compounds are different.

7. The recording medium of any one of Claims 1 to 6, wherein an overlap between the fluorescent profiles of the first and second compounds does not exceed 50% of the profile of the second compound.

8. The recording medium of any one of Claims 1 to 7, wherein overlap between the fluorescent profile of the first and second compounds does not exceed 10% of the profile of the second compound.

9. The recording medium of any one of Claims 1 to 8, wherein multi-photon absorption spectra of the first and second compounds are different.

10. The recording medium of any one of Claims 4 to 9, wherein the substance has a substantially zero linear absorbance for one or more wavelengths in the wavelength range causing the multi-photon interaction and for the wavelengths of fluorescent peaks of the first and second compounds.

11. The recording medium of any one of Claims 1 to 10, wherein the first compound is selected from trans-2-(2-pyrolylvinyl)-2-pyridine, trans-2-(2- pyrolylvinyl)-2-quinoline, cis-2-(2-pyrolylvinyl)-2-pyridine, cis-2-(2-pyrolylvinyl)-2- quinoline and substituted derivatives thereof having at least one halogen, alkyl, aryl, alkoxy, cyano and/or nitro group.

12. The recording medium of any one of Claims 1 to 10, wherein the second compound is selected from polymer covalently bound to trans-2-(2-pyrolylvinyl)-2- pyridine, trans-2-(2-pyrolylvinyl)-2-quinoline, cis-2-(2-pyrolylvinyl)-2-pyridine, cis-2- (2-pyrolylvinyl)-2-quinoline and substituted derivatives thereof having at least one halogen, alkyl, aryl, alkoxy, cyano and/or nitro group.

13. The recording medium of any one of Claims 1 to 10, wherein the first compound has a weak or no internal hydrogen bond and the second compound resulted from said conversion has a substantial internal hydrogen bond.

14. A recording medium for use in an optical information carrier, the medium comprising a substance having a first compound which is operative based on multi- photon interaction to fluoresce with a first fluorescence profile and which is convertible by exposure to light into a second compound, which is operative based on multi-photon interaction to fluoresce with a second fluorescence profile, the first and second fluorescence profiles having peaks at different wavelengths, each of the first and second compounds having substantially no linear absorption for said multi-photon interactions.

15. A recording medium for use in an optical information carrier, the recording medium comprising a substance comprising a first fluorescent compound convertible by exposure to light to a second fluorescent compound, wherein: each of the first and second compounds is an organic or metallo-organic compound; the first compound possesses a first fluorescence of a first wavelength induced by multi-photon interaction; the conversion from the first compound into the second compound occurs as a result of non-linear interaction; the second compound possesses a second fluorescence of a second, different from the first, wavelength induced by multi-photon interaction; the multi-photon interactions causing fluorescence of the compounds and the non-linear interaction causing conversion from the first into the second compound being different in at least one of the following: wavelength and interaction conditions including power profile and duration of interaction event.

16. An optical information carrier comprising the recording medium of any one of Claims 1 to 15.

17. The optical information carrier of Claim 16, comprising at least one reference layer presenting a reflective interface with the recording medium, the reference layer being at least partially transparent for data recording and data reproducing light beams and at least partially reflective for a reference light beam and being formed with a certain surface relief.

18. The optical information carrier of Claim 17, comprising a non-recording layer, which has a fluorescent property different from that of the recording medium, is at least partially transparent for data recording and reproducing and reference beams, and is positioned such that the reference layer is sandwiched between the recording layer and the non-recording layer.

19. An optical information carrier comprising a recording medium carrying a three- dimensional data pattern in the form of an array of spaced-apart recorded regions, wherein the recorded regions and the spaces between the recorded regions are responsive to multi-photon interaction by fluorescent profiles having peaks of fluorescence at different wavelengths, respectively.

20. The optical information carrier of Claim 17, wherein the recording medium is configured as defined by any one of Claims 1 to 15.

21. A method for recording data in the recording medium of any one of Claims 1 to 13, the method comprising: focusing a recording light onto selected spaced-apart regions of the recording medium, the recording light having a wavelength suitable for absorption by the first compound so as to convert the first compound into the second compound within said selected regions, thereby creating a pattern of spaced-apart recoded regions presenting a pattern of spatial variation of fluorescence spectra of the recording medium.

22. A method for reproducing data from the optical information carrier of Claim 19 or 20, the method comprising: focusing onto a recording layer in the carrier a reproducing light beam having a wavelength suitable for multi-photon absorption by the first and second compounds, thereby inducing first and second fluorescent responses with first and second fluorescent peaks of different wavelengths from the recorded regions and spaces between the recorded regions, respectively, collecting at least one of the first and second fluorescent responses, thereby detecting spatial variation of fluorescence spectra from the medium indicative of the recoded data pattern.

23. An optical system for use in recording and reproducing data to and from an optical information carrier of any one of Claims 16 to 20, the system comprising:

- a light source unit configured and operated for selectively producing a recording light beam and a reproducing light beam of the same or different wavelengths capable to cause non-linear interaction with a recording medium of the carrier;

- a light directing optics for selectively directing and focusing the recording or reproducing light beam onto a recording layer in the recording medium, and for collecting light coming from the medium; - a wavelength selective light detection unit configured for detecting fluorescent light from the data carrier, and generating detection data indicative of spatial variation of the fluorescent light coming from the carrier; and a control system configured and operable for receiving and analyzing the detection data, and generating output data indicative of a data pattern recorded in the carrier.

24. The system of claim 23, wherein the control system is configured and operable for analyzing the detection data by processing intensity of the detected fluorescent components of different first and second wavelengths.

25. The system of claims 23, wherein said processing is a differential amplification of the intensity of the detected light of the first wavelength range and that of the second wavelength range.

26. The system of any one of claims 23 to 25, wherein said recording and reproducing light beams are of the same or close wavelengths, both in multi-photon absorption spectra of different compounds in the recording medium having different peaks of fluorescence, respectively, in response to excitation by the reproducing beam, the recording and reproducing light beams having different intensities so as to minimize graying during the data reproducing process.

27. The system of any one of Claims 23 to 26, wherein the light source unit comprises a common light source for selectively generating the recording and reproducing beams.

28. The system of any one of Claims 23 to 26, wherein the light source unit comprises two light sources for generating the recording and reproducing light beams of different wavelengths, respectively.

29. The system of Claim 27 or 28, wherein the control system is configured and operable to operate the light source unit to produce the recording and reproducing light beams with operating modes defined by multi-dimensional combination of the recording and reproducing process parameters.

30. The system of Claim 29, wherein the multi-dimensional combination of the recording and reproducing process parameters includes a temporal power profile and duration of the recording and reproducing event, for a given condition of at least one of wavelength, coherence and polarization of light.

31. The system of Claim 29 or 30, wherein the control system operates to utilize data indicative of a function corresponding to an effect of recording in the medium used and a function corresponding to the effect of reproducing the recorded data, each being a function of at least one of the temporal profile of power and the event duration, to select an operating value for at least one of the temporal power and duration parameters such as to provide a non-degenerate relation between said functions.

32. The system of any one of Claims 23 to 31, wherein the light source unit is configured and operable to produce a reference beam of a wavelength different from the recording and reproducing beams, the detection unit comprising a detector for detecting reflections of the reference beam from the information carrier.