WO2014068347A2 - Method and device to produce a holographic element - Google Patents

Abstract

The optical device (110) according to the invention comprises a laser source (113) to provide laser light (112) for preparing a holographic element, an optical reflection element (114) arranged in the propagation path of the laser light, and a beam-splitting element (116) arranged in the propagation path of the laser light behind the optical reflection element. The beam splitting element comprises, in the direction of light propagation, a base surface (116a) and two lateral surfaces (116b1, 116b2) forming an angle with one another, said base surface faces the reflection element and is configured to direct laser light striking on the base surface into the beam splitting element. The lateral surfaces face the photosensitive photoplastic substance and refract a portion of the laser light striking on said lateral surfaces from the beam splitting element towards the photosensitive photoplastic substance, thereby exposing said photosensitive photoplastic substance to an illumination of periodic intensity distribution. The beam-splitting element further comprises a region (116d) that transmits a portion of the laser light entering through the base surface towards the photosensitive photoplastic substance essentially without inducing a change in its propagation direction. Each of the lateral surfaces or the region that transmits without inducing a change in the propagation direction carries or is optically coupled with a polarization-setting element, wherein said polarization-setting element is configured to render the polarization state of the laser light portion exiting through the lateral surfaces and the polarization state of the laser light portion exiting through the region that transmits without inducing a change in the propagation direction orthogonal to one another.

Description

METHOD AND DEVICE TO PRODUCE A HOLOGRAPHIC ELEMENT

The present invention, generally, relates to the preparation of a holographic element. In particular, it relates to a holographic production method of a diffractive optical element, to a device to accomplish said method, as well as to the diffrac- tive optical element obtained by said production method. In particular, the present invention relates to, on the one hand, (surface) relief gratings that can be formed in photosensitive photoplastic substances, especially in their surfaces, by a holographic inscribing technique and, on the other hand, to production methods and devices of the gratings. In general, the relief gratings obtained in this way exhibit extraordinary small surface dimensions.

As it is well-known, a hologram is just a photograph of the spatial interferogram of an object, from which the spatial image of said object can be reconstructed optically. In a simple case, the interferogram is an in-plane interference pattern, for example a periodically (in particular, according to a sine function) modulated/distributed light intensity. If a suitable substance, e.g. a photosensitive plate (that is, a substrate coated with a thin photosensitive layer) is illuminated with light having such an intensity distribution, changes in the optical parameters (i.e. the absorption, the refractive index and, optionally, the thickness) of the photosensitive plate arise in harmony with the light distribution applied. A hologram thus in- scribed (or recorded) into the photosensitive plate, in particular, into its surface, represents a diffractive optical element, specifically an optical grating. In practice, to inscribe optical gratings in this way, apparatuses of high mass (often a mass of several hundreds kilograms) and large inertia, free of vibrations and/or made to be vibration free, as well as a laser equipment are required. Thus, the designation and then the construction of such type of a recording means require high caution and, optionally, significantly larger cost input.

It is also known that optical elements based on light diffraction and, especially diffraction gratings, can be widely used in various functional means, for example in optical analyzers, multiplexers, coupling elements (e.g. to input and to output a light beam into/from optical, opto-electronical means), telecommunication and other (e.g. integrated) optical devices or in the field of information security. Such diffractive optical elements are prepared, generally, in the form of gratings constructed in thin films applied onto light-transmitting (or transparent) substrates. As to their principle of operation, said diffractive optical elements can be divided into amplitude and phase gratings. These types of gratings can be obtained either by a periodic structural change in the substance (through laterally periodic variation of its optical properties) or by forming surface geometrical reliefs.

Amplitude gratings - surface portions varying in accordance with a given periodicity - can be exemplified by layers that are sensitive to X-ray, electron, ion radiations or to light and/or can be modified by photodiffusion: the structure that has changed in the exposed parts induces a variation in the optical properties (refractive index and/or absorption coefficient) of the layer. A light beam striking and propagating through such a periodic structure gets diffracted in various orders, wherein the diffraction parameters (such as the angle of deflection or diffraction, the number and the intensity of the orders) depend on the periodicity of the given structure and the change in the optical parameters of the layer (i.e. the magnitude of the modulation).

For phase gratings, a periodically varying surface relief (or a relief pattern), i.e. a periodic local change of the surface layer gives examples. To achieve the desired shape, periodicity and height of the surface reliefs, in most cases photo- lithographic techniques are utilized. These technological processes include the selective wet or dry etching of the exposed layer, as in a given etching process the exposed and non-exposed surface portions get etched with not the same speed. The light beam striking and propagating through such a periodic structure gets diffracted in various orders, the diffraction parameters (such as the angle of deflec- tion or diffraction, the number and the intensity of the orders) depend on the periodicity of the given structure, as well as the shape and depth of the surface relief.

In practice, both types of grating are widely used, however, the preparation of phase gratings is simpler and cheaper, their efficiency is higher, and a wide range of suitable photosensitive materials is available. The two-step technology (including photolithography and the associated subsequent selective etching) widely used to prepare the gratings, nevertheless, increases significantly the preparation time. Moreover, when it is applied an in situ monitoring of the variation of parameters influencing the properties of the grating to be obtained as a final result can be hardly accomplished. Furthermore, as a consequence of wet etching (especially due to shrinkage and deformation phenomena), the quality of the gratings and thereby the profile of the relief pattern, as well as complex characteristics of the grating get worse.

The holographic preparation technique of diffraction gratings is known. This includes projecting an interference pattern (a periodic sequence of bright and dark fringes) induced by two coherent and monochromatic light waves onto a photosensitive substance. The photosensitive substance is provided by an AS2S3 chalcogenide layer arranged on a silver thin film deposited onto a substrate made of glass. In those regions of the film which are exposed to a larger extent, photodiffusion of silver changes the chalcogenide layer and thus, in a subsequent step, a surface diffraction grating of submicron height can be prepared by selective etching.

As the photodiffusion process is rather lengthy, in the devices used to accomplish said process, a temporally stable interference pattern is generated by a peculiar laser beam transformer. Within the beam transformer the initial beam is splitted into two beams of equal intensities, the total optical paths travelled by which to the plane of interference are equal. Said beam transformer is provided in the form of a two-plane prism, and in this way the periodicity of a grating to be inscribed into the photosensitive layer depends on the cone angle of said prism. Disadvantages of this technique, amongst others, come from the two-step working of the photosensitive material, as well as from the long process time of the photodiffusion process and a simultaneous rapid deterioration of the silver- chalcogenide structure due to spontaneous diffusion. Furthermore, as a consequence of the long exposure, the AS2S3 chalcogenide layers are oxidized which induces As 0₆ microcrystal formation on the surface of the photosensitive layer. This might endamage the quality of the grating thus prepared.

A method is also known, wherein an interference pattern comprised of a pe- riodic sequence of bright and dark fringes arising as a result of the interaction between two coherent, monochromatic and linearly polarized light waves is projected onto a photosensitive substance, while said interference pattern is continuously il- luminated by a linearly polarized auxiliary light generated by a further (auxiliary) light source; the auxiliary illumination is effected by a light beam, the polarization state of which is orthogonal to that of the light waves interfering with each others. Moreover, the polarization vector of the beam that practically writes (from now on, inscribes) the diffraction grating is perpendicular to the lattice vector of the grating to be inscribed.

The method can be accomplished by means of a device that includes the source of the linearly polarized inscribing laser light, and also comprises a laser beam transformer in the form of a two-plane prism that splits the initial laser beam into two. Said prism further ensures that the intensities of the two portions of the light beam splitted, as well as the optical paths travelled by each fractional light beam to the plane of interference (i.e. to the surface of the photosensitive layer provided for being inscribed) are equal. Said device further includes an additional (auxiliary) laser source also capable of emitting linearly polarized laser light. This additional laser source emits polarized light which is not coherent with the laser light that performs inscribing and has a polarization orthogonal to that of the interfering fractional beams (see the paper by J. Teteris, U. Gertners and M. Reinfelde entitled to Photoinduced mass transfer in amorphous AS2S3 films [Physica Status Solidi (C) Volume 8, No. 9, pp. 2780-2784 (2011)]).

As far as the photosensitive layer is concerned, the above discussed method and device can be most preferably used with layers of As-Se systems belonging to the group of amorphous chalcogenides. Inscribing a diffraction grating takes place in a single step by utilizing the polarization dependent giant mass-transport observed in glass-like chalcogenide semiconductors (for further details, see e.g. the paper by M. L. Trunov, P. M. Lytvyn, S. N. Yannopoulos, I. A. Szabo and S. Kokenyesi entitled to Photoinduced mass-transport based on holographic recording of surface relief gratings in amorphous selenium films [Appl. Phys. Letters 99, 051906 (2011)] with no subsequent additional finishing steps.

The above process, however, suffers from a huge disadvantage. In particu- lar, to inscribe said grating, usage of an auxiliary laser source is a requisite. This results in a recording device that is more complicated and extensive, and furthermore can be constructed in a more costly manner. In light of the above, the object of the present invention is to provide a holographic method and device to accomplish the method which allow a single step preparation of an optical diffraction element (particularly, a relief diffraction grating) of excellent quality (i.e. devoid of physical degradations) in a photosensitive photoplastic substance. A further object of the invention is to work out a holographic recording/inscribing method and device that can be used to form the optical diffraction element by the laser source providing the illumination with periodical intensity distribution alone without a need to apply further auxiliary light sources of any type. A yet further object of the invention is to provide a compact record- ing/inscribing device for preparing optical diffraction gratings that is also simple and thus requires relatively low productions costs.

It was recognized in our studies that the requirement to provide auxiliary illumination in the form of a separate light source can be eliminated through a suitable change in the path of laser light emitted by the laser source that provides the illumination with periodical intensity distribution as an interference image within the framework of the known holographic preparation technique of relief diffraction gratings, and thus formation of a relief diffraction grating in photosensitive photoplastic substances can be simplified to a significant extent. In particular, said relief diffraction grating is prepared by directing the laser light emitted by the laser source for intensity modulated illumination through the combination of an optical element ground to a given shape, similar to a Fresnel biprism, and polarization layers with appropriate polarizing effects that are preferably arranged on those exit surfaces of said biprism which lie in the direction of light propagation.

In what follows, the present invention will be discussed in more detail with reference to the attached drawings, wherein

- Figure 1 illustrates schematically a compact recording device to accomplish a prior art two-beam holographic inscribing (recording) process;

- Figure 2 is a schematic diagram of an improved three-beam holographic recording method according to the invention;

- Figure 3 shows the improved three-beam holographic recording method according to the invention for an in situ application performed by an atomic force microscope (AFM); - Figures 4A and 4B show the AFM image of a holographic surface relief diffraction grating prepared by the inventive three-beam holographic recording method and device and the XZ-plane section (profile) of said relief diffraction grating, respectively;

- Figure 5 illustrates the geometrical relations of the optical element similar to a Fresnel biprism used in the holographic recording device according to the invention;

- Figure 6 shows a possible further embodiment of the compact holographic recording device according to the present invention; and

- Figure 7 is the AFM image of an essentially perfect surface relief diffraction grating according to the invention prepared in the amorphous As20Se80 chalcogenide.

A known compact optical recording device 10 shown in Figure 1 comprises a laser source 13 connectable to a suitable electric supply (not shown) and emit- ting linearly polarized laser light 12, a reflection element, i.e. a reflective surface - preferentially, in the form of a mirror 14 - reflecting at the wavelength of said laser light 12, and a prism 16 that splits the laser light 12 into two partial beams 12a and 12b. An interference pattern generated by the inscribing partial beams 12a and 12b of the laser source 13 strikes a photosensitive sample 18 in which, preferably in its surface layer, it forms the optical diffraction grating. Said prism 16 comprises a base surface 16a and lateral surfaces 16b1 and 16b2, each forming a predefined angle with said base surface 16a, wherein the line of intersection of said lateral surfaces 16b1 and 16b2 defines an apex line 16c. The prism 16 ensures that, on the one hand, the intensities of said partial beams 12a, 12b are equal and, on the other hand, the optical paths travelled by said partial beams 12a, 12b each to the plane of the interference pattern (that is, to the photosensitive sample 18, in particular to its surface, to be inscribed into) are equal as well. To achieve a compact design, the laser source 13, the mirror 14 and the prism 16 are combined into a single unit in a suitable manner, i.e. in an arrangement wherein the base surface 16a faces said mirror 14, and the thus obtained single unit serves as propagation medium for the laser light 12. In the one-step inscribing process of a diffraction grating or grating matrix forming an optical diffraction element with extra-small dimensions (that is, when a relief pattern is created in the photosensitive layer mechanically) according to the present invention, an interference image (that is, an intensity modulated illumina- tion generated in the form of a periodically alternating sequence of dark and bright fringes or an illumination of periodic intensity distribution) is projected onto a layer of a photosensitive substance arranged on a substrate, as illustrated in Figure 2; here, the interference image forms due to the interaction of two coherent and monochromatic linearly polarized light waves that was obtained by amplitude- division of a laser beam emitted by a single illuminating laser source and then travelled equal optical paths. Simultaneously with exposing the photosensitive layer to the illumination of periodic intensity distribution, the portion of said layer illuminated in this way is exposed to an illumination by a third beam as well that is also obtained by amplitude-division of the laser beam emitted by the laser source. This third beam is linearly polarized as well, however, its polarization state is orthogonal to that of the two partial beams resulting in said intensity modulated illumination. As the photosensitive layer, an athermically softening substance is applied, i.e. a material, the viscosity of which varies merely upon being effected by light (illumination) and without heating. Thus, a mass transport, i.e. the formation of a surface relief becomes possible in the surface of the layer illuminated by the intensity modulated light. Numerous inorganic and organic (e.g. azobenzene) polymers belong to this class of so-called photosensitive photoplastic materials as will be discussed later on in more detail, for example, amorphous chalcogenides among others. It should be here noted that the construction shown in Figure 2 also allows to monitor and thus to control the recording process of the relief diffraction grating through detecting various diffraction orders of a yet further independent illuminating laser light (in this case with a wavelength of 0.44 μιτι, in particular) that is incident upon and gets diffracted from said grating.

As it can be seen in Figure 2, the holographic three-beam recording method according to the invention is performed by a recording device 110 with a very compact optical construction. In this embodiment, the optical recording device 110 comprises a laser source 113 emitting laser light 12 (being optionally linearly or circularly polarized), as well as a beam transformer (a beam-splitting element 1 16; see in Figure 5 in enlarged view) that is provided by a two-plane biprism that has got a base surface 116a and two refractive lateral faces 1 16b1 and 1 16b2 forming a predefined apex angle at an apex 116c. Said beam-splitting element 116 is cut off by a plane at a given distance from its apex 1 16c; said plane is essentially parallel with the base surface 1 16a and lies a distance h (see Fig. 5) apart from it. That is, a portion of the beam-splitting element 1 16 in the vicinity of its apex 1 16c is separated and thus its cross-section in a plane perpendicular to the base surface 116a forms an isosceles trapezoid. Preferably, the beam-splitting element 116 is made of amorphous S1O2 with optical quality ground limiting walls. It is also apparent to a person skilled in the art that said beam-splitting element 1 16 can be made of other optical materials as well, e.g. from sapphire.

The laser beam 2 emitted by the laser source 13 and, optionally, changed as to its propagation direction by 90° via an optical reflection element 1 14 (preferably a metallic mirror) in order to facilitate said very compact design, enters the beam-splitting element 116 through its base surface 1 6a and is divided into three parts by said element 1 16. Two of the partial beams obtained by division, which are linearly polarized and equal in intensity, leave the beam-splitting element 116 through the oblique refractive lateral faces 116b1 and 1 16b2 of said biprism (for symmetry considerations, Fig. 5 illustrates only one of said lateral faces) and then travel toward the layer of a photosensitive substance 1 18a, defining the plane of an interference image 119 to be inscribed into it, arranged on a substrate 1 18. The third partial beam leaves the beam-splitting element 1 16 through its top surface 116d constructed by cutting off said biprism; preferably, this third partial beam's intensity is twice as large as that of the interfering partial beams exiting through the refractive lateral faces 116b1 and 116b2. Said third partial beam leaving the beam-splitting element 1 16 is linearly polarized as well, however, its polarization plane is orthogonal to that of the interfering partial beams. In the exemplary embodiment, polarization-setting elements are arranged on/over the lat- eral surfaces 116b1 , 1 16b2 or the cut-off top surface 116d of the biprism (preferably, if the laser light 112 is linearly polarized, said polarization-setting elements are provided by polarizing layers formed of λ/2-plates, not shown in the drawings) that ensures mutually orthogonal linear polarization of the inscribing illumination (i.e. which carries the interference image 119) and the auxiliary illumination that, in prior art solutions, is generated by a separate light source. Thus, in this case, exposure of the interference image 119 to an illumination provided by the third partial beam is accomplished by a linearly polarized light beam which is coherent to the two partial beams producing the intensity modulated illumination (i.e. the interference image 119 itself) in said photosensitive substance 118a.

It should be here noted that if the initial laser light 112 emitted by the laser source 113 is unpolarized, to induce the desired polarization of said laser light 2, a liquid crystal cell 120 can be arranged in the light path between the laser source 113 and the reflection element 114, as takes place in case of the exemplary embodiment shown in Figure 2.

Depending on the photosensitive substance used, polarization of the partial beams used to inscribe can be either parallel with (p-p polarization scheme) or or- thogonal to (s-s polarization scheme) the lattice vector of the inscribed relief diffraction grating. Accordingly, the polarization state of the third partial beam used as the auxiliary illumination (and thus to accelerate/enhance mass transport resulting in the relief diffraction grating) can be either s-polarized or p-polarized.

Spectral sensitivity of the photosensitive substance utilized in the method according to the invention corresponds to the wavelength of the laser light used. In our studies, in case of laser diodes emitting in the range of red, a photosensitive substance, or rather a layer made of said photosensitive substance, in the form an As-Se chalcogenide glass rich in selenium, or to be more precise a substance comprising selenium in the amount of 70 to 90 atomic% found to be highly advan- tageous.

Figure 3 shows a further variant of the holographic recording method according to the invention adapted to be performed in-situ in an atomic force microscope (AFM). This variant of the inventive recording method also enables to monitor and thus to control the inscribing process of said relief diffraction grating. Fig- ures 4A and 4B illustrate an AFM scan image of a holographic diffraction grating produced in a layer of a suitable photosensitive substance by means of the above discussed holographic recording method and device and the sectional profile of said grating taken in the XZ-plane, respectively.

Figure 5 depicts a beam-splitting element 116 (a biprism without its apex) used in a compact optical recording device 1 10 according to the invention along with a beam path that exits from said beam-splitting element 116 just at the boundary line between the top surface 116d and the refractive lateral face 1 16b2. For the sake of simplicity and because of the existing axial symmetry, in Figure 5, only half of the beam-splitting element 1 16 is shown. It should be, nevertheless, here noted that the beam-splitting element 1 16 can be prepared by two mechani- cally separated half-elements as well that form a rectangular triangle in plane section each and when combined (that is, arranged apart from one another by a given distance) they take the form of the beam-splitting element 116 shown in Figure 2 differing from it in that a gap between the half-members is filled with air instead of the material of the beam-splitting element 116. In this embodiment of the beam- splitting means 1 16, the two half-elements can be displaced in a transverse direction relative to each others in parallel with the base surface of the beam-splitting element 116. Thus, a linear dimension (for example, along the x axis, see e.g. Figure 4A) of the relief diffraction grating to be recorded into the photosensitive substance can be enlarged. Relative displacement of the half-elements can be ef- fected by various displacing means suitably attached to said half-elements and commonly used in the field of optical measurements (e.g. by micrometric screws, etc.), as it is known by a skilled person in the art.

Referring now to Figure 5, the interference beam angle Θ that defines the period of the relief diffraction grating to be produced can be easily obtained based on some basic considerations with making use of the geometrical data specified in Figure 5, that is

Θ = a + 90° - arcsin (n cosa) ,

wherein a is the half of the apex angle of the biprism at its apex 1 16c, and n stands for the relative refractive index of the substance of the biprism. Based on Figure 5, distance x between the interference fringe and the biprism can be obtained through

x = Δ ctg6 , wherein Δ represents the half-width of the top surface 116d of the biprism obtained by cut off. Based on the above relations, the total height of the recording device that can be used in the method according to the invention, that is the distance between the top surface of the beam-splitting element 116 and the surface (i.e. the place of the interference image) of the photosensitive photoplastic substance to be arranged in the light path after said element 116. In case of carrying out the recording in an AFM apparatus, this distance can be at most 5 to 10 mm.

A compact optical recording device 210 can be seen in sectional view (upper diagram) and in top view (lower diagram) in Figure 6 that corresponds to a possible embodiment of the holographic recording device according to the invention; the top view representation of said device 210 is taken in such a case, wherein the photoplastic substance is removed. In this embodiment, laser light 212 emitted by a laser source 213 strikes, in case of need after directing through a polarizer 220, a reflective surface 214 arranged in a holder 224. Having reflected from the surface 214, said laser light 212 passes through an opening formed in the holder 224 to ensure light transmission. In the propagation path of the laser light 212 subsequent to the opening of the holder 224, a beam-splitting element 216 is arranged that transforms said laser light 212. In this exemplary embodiment of said device 210, the beam-splitting element 216 is preferably provided in the form of a biprism that exhibits light transmitting base and top surfaces that are essentially parallel with one another and light refractive lateral surfaces (for further details, see Figure 5). The beam-splitting element 216 is located, preferably with its entire height, within the opening of the holder 224; preferentially, said element 216 is wedged in said opening. The beam-splitting element 216 splits the laser light 212 striking essentially at right angle to its base surface into three partial beams, similarly to the other embodiments thereof discussed previously in detail. The partial beams leave the beam-splitting element 216 through its lateral surfaces and top surface. Then, by passing through one or more polarization-setting elements 226 arranged on the lateral surfaces and the top surface or being in optical cou- pling with these surfaces, wherein said elements 226 are preferably provided in the form of half-wave plates (λ/2-plates) or liquid crystal cells of appropriate thickness, said partial beams get into polarization states, on the one hand, suitable for inscribing a relief pattern 219 into a photosensitive substance 218 and, on the other hand, required to accelerate the inscription process of said relief pattern 219. The polarization states concerned are shown in plots I and II of Figure 6 (similarly to Figures 2 and 3) by indicating the oscillation planes of the electric field vectors of the partial beams leaving the beam-splitting element 216 and the polarization- setting element(s); here, the polarization states of the individual partial beams - relative to the axes of a rectangular Cartesian coordinate system (Χ,Υ,Ζ) shown in Figure 6 as well - are represented by symbols <-> (p-polarized) and Θ (s-polarized) added to the graphical illustrations of said partial beams.

As it was mentioned previously, the polarization states used to perform/accelerate inscribing depend on the initial polarization state of the laser light 212. In the arrangement illustrated in Figure 6, the laser light 212 emitted by the laser source 213 is linearly polarized (in the vertical or horizontal plane) by the polarizer 220, and thus said polarization-setting element(s) 226 is(are) preferably provided in the form of half-wave plate(s) and can be positioned e.g. on the two lateral surfaces or the top surface of the beam-splitting element 216 (as it is shown in Figure 6).

The photosensitive substance 218 is located at a given distance apart from the top surface of the beam-splitting element 216, wherein the distance is defined by the geometrical relations shown e.g. in Figure 5. Here, said distance is achieved by inserting a spacer 227 that rests on the holder 224 and is provided with a light-transmitting opening basically covering the opening of said holder 224. Dimensions of the opening of the spacer 227 in a plane perpendicular to the propagation of light determine the in-plane dimension of the relief pattern 219 that can be written into the photosensitive substance 218. Thus, the dimensions concerned are chosen to be as large as possible. It is also noted that, optionally, the laser source 213 can be integrated into the holder 224 by means of suitable clamping means. Such a measure enhances compactness of the recording device 210 according to the invention.

Figure 7 shows the AFM image of an almost perfect surface relief diffraction grating prepared in an amorphous As20Se80 chalcogenide layer that is 2 μΐη in thickness by the compact recording device 210 of Figure 6 in accordance with the inventive method by means of a solid state diode laser emitting at a wavelength of λ=650 nm with an intensity of 30 mW. As it can be seen from Figure 7, the peak- to-valley height is about 1 μιη in the inscribed relief grating.

An important feature of the present invention is the tiny, but monolith design that allows to inscribe the holographic relief gratings under simple laboratory circumstances without making use of the so-called conventional holographic table with a mass of several hundred kilograms and equipped with suitable vibration damping.

Another important feature of the present invention is that if the choice of material for the biprism used is combined with the application of one or more polarization layers, holographic relief diffraction gratings can be equally recorded in the p-p or the s-s polarization schemes due to the rotation of polarization planes of the laser beam that crosses said biprism and the laser beam that is splitted by said biprism (see plots I and II in Figure 2).

It is also apparent for a skilled person in the art that recording of relief diffraction gratings by the method and device according to the present invention can be performed significantly simpler and more precisely compared to the known solutions, and due to the photosensitive photoplastic substances utilized in the method, the actual time required to prepare the holographic relief diffraction grat- ings also decreases to a significant extent.

The present invention is suitable for inscribing holographic relief gratings of small dimensions (that is, which are about 1 to 3 mm in diameter). Said gratings can equally be individual optical elements or grating matrices that can be used e.g. in the filed of optoelectronics, in optical coupling means, sensors, light sources, etc.. By integrating the available small-sized laser modules and the optical element that can be relatively simply fabricated, a not too expensive holographic recording device according to the invention can be obtained that can be applied anywhere (equally in experimental and development laboratories, factories, schools) to prepare e.g. holographic relief grating prototypes. As a consequence of simplicity and small size of the device, it can be remarkably used in in situ AFM measurements, as well as for in situ fabrication of prototypes on the microscopic scale which has not been possible so far. After suitable surface treatment (e.g. hardening, providing with a metallic coat, etc.) the holographic relief diffraction grating prepared by the method and the device according to the invention can be also used as negative printing plate for duplication purposes. Furthermore, if a non-transparent mask is arranged in at least a portion of the path of the inscribing beams and/or the beam serving for the auxiliary illumination, the holographic relief diffraction grating inscribed in this way can be provided with a pre-defined marking in a certain region thereof as the mask pattern gets superimposed on the intensity modulated illumination. Thus an arbitrary micro-/nanoimprint can be created in a simple manner without an additional etching step. It is apparent to a person skilled in the art that the thus generated micro-/nanoimprints can be used as security elements as well.

The present invention also performs well under low temperature circumstances until plasticity of the photoplastic substance is affected to a significant extent by the decrease in temperature. Moreover, the holographic recording method and device are also capable of in situ modifying and/or deleting a relief diffraction grating inscribed in the photosensitive photoplastic substance; by interchanging the polarization state of the partial beams providing the intensity modulated illumination and the polarization state of the third partial beam used for the auxiliary illumination, the relief pattern induced earlier by mass transport can be ..smoothed out", and then a new relief pattern (of the opposite phase) can be created in the photosensitive substance.

Claims

1. An optical device (110; 210) to produce a holographic optical element, especially a relief diffraction grating, in a photosensitive photoplastic substance (118; 218), the optical device comprising a laser source (113; 213) to generate laser light (1 2; 212) with propagation path for preparing the holographic element, an optical reflection element (114; 214) arranged in the propagation path of the laser light, and a beam-splitting element (116; 216) arranged in the propagation path of the laser light behind the optical reflection element, said beam splitting element comprising, in the direction of light propagation, a base surface (116a) and two lateral surfaces (116b1 , 116b2) forming an angle with one another, said base surface facing the reflection element and configured to direct laser light striking on the base surface into the beam splitting element, said lateral surfaces facing the photosensitive photoplastic substance and refracting a portion of the laser light striking on the lateral surfaces from the beam splitting element towards the photosensitive photoplastic substance, thereby exposing said photosensitive photoplastic substance to an illumination of periodic intensity distribution, said beam-splitting element (116; 216) further comprising a region (116d) that transmits a portion of the laser light entering through the base surface towards the photosensitive photoplastic substance essentially without inducing a change in the propagation direction of the laser light, said lateral surfaces or the region that transmits without inducing a change in the propagation direction carrying or being optically coupled with a polarization-setting element (226), said polarization-setting element being configured to render the polarization state of the laser light portion exiting through the lateral surfaces and the polarization state of the laser light portion exiting through the region that transmits without inducing a change in the propagation direction orthogonal to one another.

2. The optical device (110; 210) according to Claim 1 , wherein the polarization- setting element (226) is provided by a half-wave plate or a liquid crystal cell of suitable thickness.

3. The optical device (110; 210) according to Claim 1 or 2, wherein the region that transmits without inducing a change in the propagation direction is provided by a planar region that is essentially parallel with the base surface and is located between the lateral surfaces and joined continuously to said lateral surfaces.

4. The optical device (110; 210) according to Claim 3, wherein the section of said beam-splitting element taken in a plane crossing the region that transmits without inducing a change in the propagation direction and perpendicular to the base surface exhibits the shape of an isosceles trapezoid.

5. The optical device (110; 210) according to any of Claims 1 to 4, wherein the laser source (113; 213) is provided by a laser source that emits unpolarized laser light (112; 212) and a polarizer is inserted into between said laser source and said reflection element (114; 214).

6. The optical device (110; 210) according to any of Claims 1 to 5, wherein said polarizer is provided by a liquid crystal cell (120; 220).

7. The optical device (110; 210) according to any of Claims 1 to 6, wherein said optical device (110; 210) is provided in the form of a single compact integrated unit.

8. A method to produce a holographic optical element, especially a relief diffraction grating, in a photosensitive photoplastic substance, said method comprising the steps of generating laser light by a single laser source, directing the laser light through a beam-splitting element and thereby splitting said laser light into partial beams having a first polarization state and a further partial beam having a second polarization state, said second polarization state being orthogonal to said first polarization state, the beam-splitting element comprising, in the direction of light propagation, a transparent base surface with a first surface area and a transparent top surface with a second surface area, said second surface area being smaller than said first surface area, said base surface and said top surface being essentially parallel with one another, the beam-splitting element further comprising refractive lateral surfaces forming an angle with each of said base surface and said top surface, directing the partial beams having the first polarization state to the photosensitive photoplastic substance and exposing a given portion of said photo- sensitive photoplastic substance to an illumination with a periodic intensity distribu- tion generated by said partial beams having the first polarization state, thereby inducing a mass transport in a surface layer of the thus illuminated portion of said photosensitive photoplastic substance leading to the formation of the relief grating, wherein by exposing said portion of the photosensitive photoplastic substance to a simultaneous illumination by said further partial beam having the second polarization state influencing the mass transport induced in the photosensitive photoplastic substance by the illumination with a periodic intensity distribution.

9. The method according to Claim 8, wherein a difference between the polarization state of the partial beams having the first polarization state and the polarization state of the further partial beam having the second polarization state is created by a polarization-setting element that is either arranged on the lateral surfaces of said beam-splitting element or on the top surface of said beam-splitting element or is optically coupled with the respective surface of said beam-splitting element.

10. The method according to Claim 9, wherein the beam-splitting element is pro- vided by a half-wave plate or a liquid crystal cell of suitable thickness.

11. The method according to any of Claims 8 to 10, wherein during influencing the mass transport, an acceleration and/or an increase in efficiency of said mass transport is performed.

12. The method according to any of Claims 8 to 10, wherein as the photosensitive photoplastic substance amorphous chalcogenide semiconducting substances, preferably As-Se systems are used.

13. Use of an illumination with a periodic intensity distribution generated through an interaction of partial beams having a first polarization state obtained from laser light emitted by a laser source simultaneously with a partial beam having a second polarization state obtained from said laser light emitted by the laser source, said first polarization state and said second polarization state being orthogonal to one another, to produce a holographic optical element, preferably a relief diffraction grating, in a photosensitive photoplastic substance.