CN1155948C - Method for recording, erasing and reading imformation of near-field dual-photon storage - Google Patents

Abstract

The present invention belongs to the technique of optical information storage. The present invention discloses a novel method for near-field dual-photon storage by using a solid immersion lens to record, erase and read data information on a low threshold value dual-photon storage medium of azoic liquid crystal polymer and combining the principles and the advantages of near-field storage and dual-photon storage. An information bit far less than a Rayleigh limit can be obtained, and the smallest information bit is 120 nm at present. A miniaturized laser can be used as a light source so as to easily realize the practical application of the present invention.

Description

Near-field dual-photon canned data record, the method for wiping and reading

One. technical field

The invention belongs to the optical information memory technology, be specifically related to two kinds of technology of near field optic storage and two-photon storage.

Two. background technology

The optical information memory technology of super-high density has tempting application prospect.At present, the main path of realization ultrahigh density storage has two kinds: a kind of is exploitation short wavelength's LASER Light Source; Another kind is exactly to realize ultraphotic diffraction limit technology.In the latter, the near field optic storage is the focus of studying at present with the two-photon storage.

The diffraction optics principle shows, the size of the beam focus after lens focus can not be less than 0.61 λ/(nx sin θ) (being referred to as Rayleigh limit) usually, wherein λ is a light wavelength in a vacuum, and n is the refractive index (n=1 in air) of medium, and θ is the sail angle of light.Near field optic storage be distance between control optical memory head and the storage medium in 100nm (about λ/4), make full use of the storage of the effect realization of the light field that suddenly the dies information bit littler than Rayleigh limit.Utilize the near field optic storage person of being studied that solid immersion lens carries out to think the scheme of approaching reality.Solid immersion lens is a kind ofly to equal or be slightly larger than hemispheric lens, light beam is focused on its planar end surface, its refractive index n＞1 (adopted at present solid immersion lens refractive index usually all between 2～3) is so thereby utilize solid immersion lens can break through Rayleigh limit, obtain the size that less focus point hot spot reduces information bit.For example, people such as B.D.Terris once utilized solid immersion lens to realize that the storage of the information bit of 350nm (sees: B.D.Terris etc. " Near-field optical data storage using solid immersionlens ", Appl.Phy.Lett.Vol.65,388-390,1994).(the material refractive index is big more, more little with the distance of storage medium but because its storage density depends on the size of material refractive index of solid immersion lens and the size of the distance between solid immersion lens and the storage medium, then information bit is more little), (λ is about 500～800nm for used usually visible light or LASER Light Source, utilize the size of the resulting information bit of solid immersion lens to be difficult to break through 200nm down together).

Thereby two-photon excitation is meant medium and absorbs the process that two lower energy photons are excited to high-energy state; Two-photon storage then is meant the local physical property (as: absorptivity, refractive index, fluorescence etc.) of utilizing two-photon absorption process change storage medium, thus realize optical information record, wipe and read.Because two-photon is a kind of third-order nonlinear optical effect, its absorption point only is confined to the intersection point place of enough big focus of light intensity or two-beam usually, thereby the size of information bit is reduced, increased the storage density of unit area/volume.Under the situation of linear absorption, because the absorptivity of medium is proportional to light intensity, promptly $I=I_0{e}^{-\frac{r^2}{w^2}},$ So the size of information bit is proportional to the radius w (spot radius w just equals the Rayleigh radius under limiting case) of hot spot; And under the situation of two-photon absorption, the absorptivity of medium be proportional to light intensity square, promptly $I^2=I_0^2{e}^{-\frac{r^2}{{(w/\sqrt{2})}^2}},$ Thereby the size of information bit is proportional to

That is the size of two-photon canned data position also can break through Rayleigh limit and (sees: James H.S., Watt W.W " Three-dimensional optical data storage in refractive media by two-photonpoint excitation " Opt.Lett.Vol.16, No.22, P1780,1991).But for used usually visible light or LASER Light Source, can not break through 200nm equally.

The principle of storing from the storage of the near field of above summary and two-photon as can be seen, they are to manage to obtain higher storage density from beam quality and storage medium two aspects respectively.If both are combined, reduce simultaneously the spot size of focus point and the spot size of two-photon absorption point, then should obtain better effect, for used usually visible light or LASER Light Source, be expected to make the size of information bit to break through 200nm.But, owing to its excitation threshold of material that can carry out two-photon absorption is all very high (usually 10 ⁴Mw/cm ²), that is existing two-photon storage medium is to the energy requirement higher (only in this way could produce effective two-photon absorption) of storage light source; And the efficient intensity transmitance near field when storage is very low, can not satisfy the energy requirement of two-photon absorption: in the process of carrying out the near field storage with solid immersion lens, be focused the decay rapidly after leaving its planar end surface of light beam on the solid immersion lens planar end surface, thus the distance between storage medium and the solid immersion lens must be enough closely (being less than λ/6 usually) just can guarantee to have enough light transmission rates.Therefore, both of near field storage and two-photon storage are in conjunction with existing very big obstacle, and being considered to can not realization.

Three. summary of the invention

The object of the present invention is to provide a kind of size of information bit that can make less than the near-field dual-photon canned data record of 200nm, the method for wiping and reading.

Solution of the present invention is as follows:

The recording method of a kind of near-field dual-photon canned data, its process is: the storage disks that will adopt azobenzene liquid crystal polymer to make as storage medium places on the workbench, to shine, make light wave to focus on the film to the azobenzene liquid crystal polymer film on the storage disks as the light path that the light wave that laser instrument produced of recording light source is formed by diaphotoscope, polarizing plate, convergent lens and solid immersion lens, the position of the mobile adjustment film by workbench; Recording light wave polarization direction maintains static or changes by physical method; Described storage disks is made into two-way translucent construction, and its top-down basic layer structure is: thin dielectric film/storage medium film/thin dielectric film/substrate.

A kind of near-field dual-photon canned data method for deleting, its process is: will place on the workbench according to the storage disks that information has been write down in above-mentioned recording method, to shine, make light wave to focus on the film to the azo liquid crystal polymer film as the light path that the light wave that laser instrument produced of wiping light source is formed by diaphotoscope, polarizing plate, λ/4 wave plates, convergent lens and solid immersion lens, the position of the mobile adjustment film by workbench.

A kind of near-field dual-photon canned data read method, its process is: will place on the workbench according to the storage disks that information has been write down in above-mentioned recording method, to pass through catoptron as the light wave that laser instrument produced that reads light source, polarizing plate, the light path that convergent lens and solid immersion lens are formed is shone the azo liquid crystal polymer film, light wave is focused on the film, and then another group light path of forming by second convergent lens and analyser plate focuses on the photodetector light wave, the direction of shaking thoroughly of analyser plate is vertical with the direction of shaking thoroughly of polarizing plate, the position of the mobile adjustment film by workbench, the polarization direction of the reading light wave light wave polarization direction during with record differs to be 45 ° of definite values or to maintain static.

For above-mentioned information record or method for deleting, the neodymium-doped yttrium vanadate solid frequency double laser that adopts the green light wavelength is as light source; For above-mentioned information-reading method, the semiconductor laser that adopts the red light wavelength is as light source.

The present invention be by adopt solid immersion lens to a kind of new-found, have low two-photon excitation threshold value material---azobenzene liquid crystal polymer carries out information stores, thereby realized the near-field dual-photon storage.Azobenzene liquid crystal polymer is a kind of photic material of selling off, and can produce birefringence effect under the inducing of light.But only found its single photon absorption characteristic in the past, the birefringence effect of single photon due to absorbing promptly near first absorption peak of azobenzene liquid crystal polymer, can occur.The present invention then points out, can produce photic birefringent phenomenon owing to two-photon excitation at the second absorption peak place of azobenzene liquid crystal polymer, and the threshold value of this photic birefringence excitation process is very low (is generally less than 40mW/cm ²), therefore, make it possess possibility with combining of near field storage means.Simultaneously, this two-photon excitation also has the character that can wipe rewriting: by the recording light source or the irradiation of wiping light source, can make the information bit place that birefringent generation or disappearance take place, thereby realize the record of data and wipe; By reading the irradiation of light source, then can make the information of information bit place record obtain reflection.Because required excitation threshold is very low, can also be achieved by the irradiation of miniature laser, and the volume of memory device is reduced greatly, helps the practicability of this scheme.

For recording process, its principle causes birefringence to be led to a phenomenon based on the two-photon excitation of linearly polarized light: will be at room temperature (far below the glass temperature of material) evenly be coated on transparent substrate the formed isotropic solid film of azobenzene liquid crystal polymer as storage medium layer, need not make preorientation and handle; Substrate material can adopt glass or plastics; The substrate that has applied storage medium is called storage disks, is supported on can do on the mobile workbench of machinery.Can make storage medium generation two-photon absorption and excite birefringent light wave to become linearly polarized light what recording light source produced by polarizing plate, focus on the storage medium layer by convergent lens and solid immersion lens, tracer signal is carried out the near field by solid immersion lens and is focused on again.Storage medium produces birefringence by two-photon excitation at the focus place, forms the optically anisotropic spot of optical axis perpendicular to recording light wave polarization direction, as the information bit of record.Machinery by workbench moves and can rotate or mobile disc, to adjust distance between film and the solid immersion lens, also the focal position changed data are recorded in the different parts of storage medium layer.If during record the polarization direction of light wave is maintained static, then the angle α that forms between the tangential direction of the optical axis direction of information bit and focussing movement track remains unchanged, and at this moment realizes binary bit storage by having or not of information bit; If change the polarization direction of light wave during record, this can realize by some physical methods, as: change polarizing plate shake the thoroughly method of direction or the method for magneto-optic modulation, then respective change is α to the angle α that forms between the tangential direction of the optical axis direction of information bit and focussing movement track along with the variation of light wave polarization direction _i(i=1,2 ... that is can realize the GTG storage of N system by the polarization direction (being angular multiplexed method) that changes light wave N).The wavelength of recording light source must be able to make the storage medium azobenzene liquid crystal polymer produce the two-photon excitation birefringence, is generally 500～560nm, to be good (being convenient to select laser instrument) in the wavelength coverage of green light; Solid immersion lens is that the high glass (as barium fluoride, phosphatization potassium etc.) of employing refractive index is made, hemispherical or thickness is the super dome lens (wherein n is the refractive index of material, and r is the radius of a ball) of r+ (r/n).

In order to realize the near field storage, the present invention has also specially designed a kind of compact disk structure of two-way printing opacity, be that the top-down basic layer structure of storage disks is: thin dielectric film/storage medium film/thin dielectric film/substrate (as shown in drawings), and cancelled the metal film layer that plays the reinforcing reflex effect in the existing CD.Wherein, thin dielectric film plays protective seam, can adopt silicon nitride (SiN), silicon dioxide (SiO ₂) etc. material.By the storage disks of above structure fabrication can make record, wipe and read light source by lower floor substrate and dielectric protective seam etc. or directly the dielectric protective seam by the upper strata shine on the storage medium; rather than resemble substrate etc. that existing CD must pass through lower floor and shine on the storage medium (this structure both had been unfavorable for solid immersion lens near storage medium, again can not two-way printing opacity) again.When carrying out information stores; solid immersion lens will place the side of leaving substrate; be that solid immersion lens is positioned at the side near the upper strata thin dielectric film; like this solid immersion lens just can be very near or directly touch the thin dielectric film protective seam; so that satisfy the requirement of near field storage better, it can scratch storage medium film and needn't worry.The thickness that is in that layer thin dielectric film of the superiors' (promptly outer) is less than 100nm, can be at 20nm between the 100nm, so that the distance between solid immersion lens and the storage medium is enough little.Thin dielectric film can prepare by magnetron sputtering method, and the preparation of storage medium then can be adopted spin-coating method.

For erase process, its principle causes birefringence to eliminate phenomenon based on the two-photon excitation of circularly polarized light: will place on the workbench according to the storage disks that information has been write down in above-mentioned recording method, can make storage medium produce two-photon absorption and excite birefringent light wave to become linearly polarized light what wipe that light source produces by polarizing plate, become circularly polarized light by λ/4 wave plates again, focus on the storage medium layer that has write down information by convergent lens and solid immersion lens then, erase signal is carried out the near field by solid immersion lens and is focused on; Machinery by workbench moves and can rotate or mobile disc, to adjust the distance between film and the solid immersion lens and the focal position is changed; When light wave focuses on the information bit, circularly polarized light will make its anisotropic hot spot disappear, thus the data of wiping different parts on the disc; The wavelength of wiping light source must be able to make medium produce two-photon absorption, generally also be 500～560nm, to be all good (can and wipe light source and adopt same laser instrument recording light source like this, wipe effect also better) mutually to reduce equipment cost with the wavelength of recording light source.

For reading process, its principle is based on the polarization interference effect of birefringent material: will place on the workbench according to the storage disks that information has been write down in above-mentioned recording method, become read the light wave beyond storage medium two-photon absorption band that light source produces into linearly polarized light by polarizing plate, focus on the storage medium layer that has write down information by convergent lens and solid immersion lens again, read signal and carry out near field focusing by solid immersion lens; Machinery by workbench moves and can rotate or mobile disc, adjusting the distance between film and the solid immersion lens and the focal position is changed, thereby reads the data of different parts on the disc; The divergent beams that see through storage medium are collected and are focused on the photodetector by second convergent lens that is positioned at the storage disks opposite side, and its light intensity will change electric signal into to be for further processing; Between second convergent lens and photodetector, need to place analyser plate, make the direction of shaking thoroughly of shake thoroughly direction and polarizing plate of analyser plate orthogonal.Owing to arrive the light intensity I and the sin of photodetector surfaces ²2 α are directly proportional, therefore, under the bit storage situation, it is 45 ° of definite values that the light wave polarization direction in light wave polarization direction and when record when reading is differed, this can realize by some physical methods, as: the method for the method of the direction of shaking thoroughly of change polarizing plate or magneto-optic modulation, the angle α that forms between the tangential direction of the optical axis direction of information bit and focussing movement track is 45 ° at this moment, the light intensity I that arrives photodetector surfaces through information bit is for maximum, to obtain maximum contrast, improve signal to noise ratio (S/N ratio); Under the GTG storage condition, the light wave polarization direction when reading is maintained static, α=α is promptly arranged this moment ₁, α ₂... α _N, the light intensity of photodetector surfaces promptly has N different value, to demonstrate the information bit of different gray-scale values, promptly realizes the data read of GTG; The wavelength that reads light source should be away from the absorption band of medium, and the unlikely anisotropy that obviously weakens information bit is generally more than the 630nm, to be good (being convenient to select laser instrument) in the wavelength coverage of red light.

In addition, for reading process, but also reverse design (resulting effect is same) of its light path: and promptly the below photodetector that is positioned at storage disks as the laser instrument that reads light source then is positioned at the top (otherwise with light channel structure shown in the drawings) of storage disks.Read light wave and become linearly polarized light, focus on the storage medium layer that has write down information by second convergent lens again by analyser plate (this moment, it played the effect of polarizing plate); The divergent beams that see through storage medium are collected by solid immersion lens that is positioned at the storage disks opposite side and convergent lens and are focused on the photodetector through the reflection of catoptron, and the polarizing plate of placing between convergent lens and catoptron uses as analyser plate this moment.

For above-mentioned data recording or method for deleting, the neodymium-doped yttrium vanadate solid frequency double laser (its wavelength is 532nm) that can adopt the green light wavelength is as light source, the two-Photon Absorption Properties at the second absorption peak place of this photon energy and azobenzene liquid crystal polymer is complementary, and can make medium produce two-photon excitation birefringence efficiently.For above-mentioned method for reading data, the semiconductor laser (its wavelength is 650nm) that then can adopt the red light wavelength is as light source, because this wavelength away from the absorption band of recording medium, makes the unlikely information that writes down on the information bit is produced of the process that reads destroy.The laser instrument of above-mentioned green light and red light wavelength has had the commercial product of miniaturization.

In sum, feature instantiation of the present invention is in the following areas: the principle that near field storage and two-photon are stored combines, used a kind of two-photon storage medium of low threshold value, and carry out the record near field, wipe and read desiring data information stored by solid immersion lens, light source uses the laser instrument of miniaturization, and storage disks is designed to two-way translucent construction.

Combined with near field storage of the present invention and two-photon are stored both advantages, adopt azobenzene liquid crystal polymer as storage medium, and write down, wipe and read signal and all carry out near field focusing by solid immersion lens, realized a kind of storage means of brand-new near-field dual-photon, to obtain higher storage density.Thereby its crucial part is to utilize two-photon excitation to produce photic orientation phenomenon at the second absorption peak place of azobenzene liquid crystal polymer.(be generally less than 40mW/cm and the threshold value of this photic birefringence excitation process is very low ²), under the situation that the efficient intensity transmitance is very low in the time of not only can storing in the near field, satisfy the energy requirement of two-photon absorption, thereby the combination that makes near field storage and two-photon store both is achieved; And reduced the power of laser instrument significantly, can make full use of the storage that business-like miniature laser is realized data message.

According to above narration as can be known, the present invention has following advantage:

1) utilize solid immersion lens to carry out the near-field dual-photon storage on azobenzene liquid crystal polymer, the size that can reduce to store hot spot simultaneously from two aspects is independent near field storage of a kind of ratio or two-photon storage more efficient methods.It can obtain the information bit much smaller than Rayleigh limit, and minimum has been accomplished 120nm at present.

2) owing to adopt business-like miniature laser to realize the storage of data message, reduced significantly laser instrument power, reduced equipment volume, thereby greatly reduce carrying cost, make that practical application of the present invention is easy to realize.

The size of focus point depends on the material refractive index of solid immersion lens greatly during 3) owing to the use solid immersion lens, so if use big refractive index materials to do solid immersion lens, can also further raising storage density.

Four. description of drawings

Accompanying drawing 1 is main light channel structure synoptic diagram.It realizes writing down, wiping, read three kinds of functions by a cover system.Wherein the solid line of light wave trend is represented record and is wiped light wave, and dotted line represents to read light wave; And the λ that dot-and-dash line is represented/4 wave plates 4 only insert light path when wiping, and then withdraw when writing down and reading.

Accompanying drawing 2 is the basic structure synoptic diagram of storage disks.

Five. embodiment

The embodiment of concrete operation steps and information stores process following (with reference to accompanying drawing):

When carrying out data recording, the 532nm green glow that laser instrument 6 sends sees through selects catoptron 5, polarizing plate 11, convergent lens 3 and solid immersion lens 2 to focus on the storage medium film of storage disks 1, the medium at focus center place produces the spot with birefringent characteristic through two-photon absorption, becomes a recording information bits.For making focus point as far as possible little, the distance between solid immersion lens and the storage medium film will remain on below the 100nm.By adjusting mechanical mobile device with rotation or mobile disc, thereby the direction of shaking thoroughly again by adjusting polarizing plate is to change the GTG storage that the light beam polarization direction realizes bit storage or N system; When wiping, insert λ/4 wave plates 4 in the back of polarizing plate, make the light wave that incides on the film become circularly polarized light, at this moment information bit becomes isotropy under the irradiation of circularly polarized light, and birefringent characteristic disappears, thereby reaches the purpose of wiping.During reading of data, the 650nm ruddiness that another laser instrument 7 sends is through selecting catoptron 5, polarizing plate 11, convergent lens 3 and solid immersion lens 2 shine on the film, its transmitted light is focused on the photodetector 9 by convergent lens 8 and analyser plate 10, analyser plate and the polarizing plate direction of shaking thoroughly is orthogonal, the light intensity I of " sandwich " structure that sees through this polarizing plate/storage disks/analyser plate relevant with α (α is the angle between the tangential direction of the optical axis direction of information bit and focussing movement track), utilize the power of photodetector detection transmitted light, get final product the data of reading and recording.Above-mentioned used selection catoptron is reflection red light only, but can see through green light, that is its dual-use material: not only made catoptron, but also made diaphotoscope; In the used storage disks, azobenzene liquid crystal polymer film 13 is isotropic " polymethylacrylic acid (right-cyano group azo phenoxy group) ethyl ester " film, thickness is 30nm, thin dielectric film 12 is the SiN film, the thickness outer and middle layer is respectively 20nm and 150nm, the material of substrate 14 is a glass, and thickness is 1.2mm; The dome-type (radius 1mm, refractive index be 1.687) of used solid immersion lens for adopting BaF7 to make; Convergent lens is 100 times a microcobjective (numerical aperture 0.8); Photodetector is a semiconductor photocell; Recording light source and wipe the neodymium-doped yttrium vanadate (Nd:YVO that light source is power 15mw ₅) solid frequency double laser (wavelength 532nm), reading light source is that power is the semiconductor laser (wavelength is 650nm) of 10mw.

Claims (7)

1. An information recording method for near-field two-photon storage, in which the light waves produced by the laser are irradiated to the storage medium, and it is characterized in that the specific process is: the storage disc (1 ) is placed on the working platform, and the light wave generated by the laser (6) as the recording light source passes through the optical path formed by the transmission mirror (5), polarizing plate (11), converging lens (3) and solid immersion lens (2) The azo liquid crystal polymer film on the storage disc is irradiated to focus the light wave on the film, and the position of the film is adjusted by moving the working platform; the polarization direction of the recording light wave is fixed; the storage disc is made into a two-way transparent Optical structure, its basic hierarchical structure from top to bottom is: dielectric film (12)/storage medium film (13)/dielectric film/substrate (14); described azo liquid crystal polymer is polymethacrylic acid ( p-Cyanoazophenoxy)ethyl ester. 2. The information recording method of near-field two-photon storage as claimed in claim 1, characterized in that the polarization direction of the recording light wave is changed by a physical method; the physical method is to change the vibration transmission direction of the polarizing plate or methods of magneto-optic modulation. 3. An information erasing method for near-field two-photon storage, in which the light wave generated by the laser is irradiated to the storage medium on which the information has been recorded, and it is characterized in that the specific process is: the storage disc (1) on which the information has been recorded Put it on the working platform, pass the light wave generated by the laser (6) as the erasing light source through the transmission mirror (5), polarizing plate (11), λ/4 wave plate (4), converging lens (3) and solid The optical path formed by the immersion lens (2) irradiates the azo liquid crystal polymer film on the storage disk, focuses the light wave on the film, and adjusts the position of the film through the movement of the working platform. 4. An information reading method for near-field two-photon storage, in which the light wave generated by the laser is irradiated to the storage medium on which the information has been recorded, and it is characterized in that the specific process is: the storage disc (1) on which the information has been recorded Put it on the working platform, pass the light wave generated by the laser (7) as the reading light source through the optical path composed of the reflector (5), polarizing plate (11), converging lens (3) and solid immersion lens (2) Irradiate the azo liquid crystal polymer film on the storage disc to focus the light wave on the film, and then focus the light wave on the photoelectric On the detector (9), the vibration transmission direction of the polarizing plate is perpendicular to the vibration transmission direction of the polarizing plate. By moving the working platform to adjust the position of the film, the difference between the polarization direction of the read light wave and the light wave polarization direction during recording is one The fixed value is 45°. 5. The information reading method of near-field two-photon storage according to claim 4, characterized in that the polarization direction of the read light wave is fixed. 6. The information recording or erasing method of near-field two-photon storage according to claims 1 to 3, characterized in that the recording light source or erasing light source is a neodymium-doped yttrium aluminate solid frequency-doubled green laser with a wavelength of 532nm . 7. The information reading method for near-field two-photon storage according to claims 4-5, characterized in that the reading light source is a semiconductor red laser with a wavelength of 650 nm.

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