Technology of integrating diffractive elements into an ...

Technology of integrating diffractive elements into an image-matrix hologram 1

Andrejs Bulanovs 1, Elena Kirilova 2, Vjacheslav Gerbreder 1 Innovative Microscopy Center, Daugavpils University, Daugavpils, Latvia 2 Department of chemistry, Daugavpils University, Daugavpils, Latvia. e-mail: [email protected] ABSTRACT

The given work investigates a way of integrating Diffractive Optical Elements (DOE) in the structure of a protective image-matrix hologram. An important feature of the suggested method is simplicity of realization by some modification of the frames exposure software and full hardware compatibility with image-matrix technology. Unlike other methods, the suggested one does not require changing the optical scheme between transitions from recording holographic frames area to the DOE ones and vice versa. Any shape of DOE in the hologram approximated to the size of the frame is available. A DOE frame cannot contain any part of a holographic region due to the different method of exposure. The minimal size of one DOE pixel is about 3 micrometer. The method under discussion allows for recording both binary (two-phase) and multilevel (multi-phase) DOE at corresponding calibrations of the employed Spatial Light Modulator (SLM). The method has passed an experimental approbation and is now used for embedding DOE into an image-matrix hologram as an additional security element. PACS: 42.40.Eq; 42.40.Ht; 42.40.My. Keywords: digital holography, diffractive optical elements, protective holograms.

1. INTRODUCTION Maximum complexity of identification elements and development of related technologies can be considered one of the main trends in the development of holographic market. Holograms are used as a protection element; in order to increase protection efficiency, holograms are rapidly integrated with other protective elements. Most interesting in this context is integration of diffractive optical element (DOE) into the structure of hologram, which it can provide an automatic identification of the authenticity of the hologram; this integration allows for identification on the visual level too. Diffractive optical elements are artificial two- or three-dimensional structures which change phase and amplitude of incident electromagnetic wave at every point of its surface and allow for forming beams with predetermined properties. Outwardly, this optical element is a reflecting area with a thin phase micro relief calculated within the diffraction theory. Diffractive optical elements are widely used in the visible spectrum in which well-developed computer methods for calculating the DOE can build a very complex image. This image can be viewed by illuminating the DOE with a collimated laser beam. This will project the image from the surface of the hologram containing the diffractive element. The projected image can then be viewed by placing a diffusion screen or a suitable optical detector such as a CCD chip above the hologram surface. Characteristic dimensions of the DOE structure must comply with the order of wavelength; so when creating the DOE for the optical range, such widespread technology as laser and e-beam raster scanning techniques or mask-based photolithography are usually used [1,2]. But these technologies have not found a wide use in the applied holography for various reasons. Two technologies for optical recording of protective holograms, 'dot-matrix' and 'image-matrix' ones, currently dominate in applied holography [3-5]. Neither of these technologies is directly compatible with the recording of a DOE. But equipment manufacturers could indirectly solve this problem. So, for example, it is possible to record a DOE in the 'dotmatrix' system LightGate using a focused laser beam [6], and the 'image-matrix' system KineMax allows for changing the optical scheme in the transition to recording a DOE [7]. In this paper, we propose a new recording technology of DOE formation, which is fully hardware compatible with the popular 'image-matrix' holographic recording technology.

Holography: Advances and Modern Trends II, edited by Miroslav Hrabovský, Miroslav Miler, John T. Sheridan, Proc. of SPIE Vol. 8074, 80740Y · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.886468

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2. EXPERIMENTAL For recording CGH and DOVID, an image-matrix optical set-up shown in Fig. 1a was used. It utilizes a diode laser 1, whose cleared and expanded beam illuminates a transparent spatial light modulator (SLM) [8-11] 4 located behind the Fourier lens 3. The lens 3 in the focal plane forms the Fourier spatial spectrum of the laser radiation diffracted on the phase image, which is transferred from the PC to the modulator. Intensity peaks of the spectrum that are the farthest from the centre correspond to the higher orders of diffraction. A spatial filter is positioned in the Fourier plane behind the SLM to block a zero order and the orders higher than both first orders. After the reverse Fourier transformation, a hologram with a doubled frequency of diffraction grating is created with the help of the objective 6 at the expense of interference of the I+1 and I-1 orders of the wave diffracted on the modulator. An image is displayed on the SLM and then reduced to a microscopic size (200x150 μm) by the lenses system 6 and recorded on a photoresist plate placed on the XY-stage 7. Hologram recording is performed sequentially by rectangular micron-sized areas (Fig. 1b). The sections tightly fill the entire area of the hologram, and a human eye does not perceive a discrete structure of the hologram consisting of micronsized sections. In order to lessen the perceived discreteness of the hologram structure, the neighbouring rows of its micro-images are shifted regarding each other the length of half-frame. Each separate image from the matrix structure contains a computer-generated hologram; in a simple case, it may be a set of gratings with the grating pitch and orientation adjusted to the requirements.

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Figure 1. a) Optical scheme of a image-matrix holographic recording device. b) SEM image of a frame from the structure of an image-matrix hologram.

The maximum frequency of diffraction grating that can be obtained by this technology is estimated according to the following formula: Fmax =2 n.a./λ , where n.a. is a numerical aperture of objective 6, and λ -is a laser wave length. The minimal frequency of diffraction grating is determined by the size of the central diaphragm (L ~ 1mm) in the mask 5, which blocks the zero order of diffraction and can be calculated by the formula: Fmin=L/(λ f ), where λ is a laser wave length, f - focal length of objective 6, and L - size of the central diaphragm. In our case, the range of spatial frequencies obtained by the formulas is equal to f = 300-1500 mm-1. This range is sufficient for recording security holograms consisting of a set of diffraction gratings. For recording DOE or arbitrary relief microstructures, a technological possibility of optical recording with spatial frequencies is necessary, that form the amplitude image from the range fDOE =0-S-1, where S is a minimum size of the cells that make up the required topology of microstructures. Low spatial frequencies required for recording DOE can be obtained if the Fourier filter 5 is removed from the optical scheme (Fig. 1a) and SLM is used in the wavefront amplitude modulation mode. In this case, it becomes possible to construct an image on the photoresist surface with a spatial frequency spectrum from 0 to fmax = n.a./λ , that is f~0-700 mm-1. But such an optical scheme is not suitable for holographic recording. The described changes in the optical scheme during the transition from recording holograms to recording DOE and other microstructures are used in some models of the equipment, but this technological solution greatly increases the cost of the device. We propose a new recording technology of DOE, which at the hardware level is fully compatible with the popular image-matrix holographic recording. The basic idea is to create the structure of DOE by shifting the interference field which is filling the DOE topology form. The principle of forming one pixel of DOE is shown in Fig. 2. With a triple exposure and a shift of interference pattern that is used to record the diffraction grating with period d, at a distance d / 3

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perpendicular to the interference lines in theory, we obtain a uniform distribution of exposure in this area with small edge effects. The shift of the interference field with such a high precision is easily achieved under the displacement of the current image on the optical modulator (SLM) used in the 'image-matrix' recording technology. The technological sequence of integrating the diffraction element in the structure of an image-matrix hologram looks as shown in Fig. 3. At the first step, the graphical image that should be restored by means of laser beam reflected from the DOE surface (Fig.3a) is created. At the second step, the topology of the DOE (Fig.3b) with taking into account its optical resolution is calculated by means of a conventional iterative Fourier transform algorithm; for this purpose, for example, we use the software 3Lith from Raith. The DOE phase distribution is encoded as a grey level image file, in which the grey level simply represents a relief level. At the next step, calculation of image frames for optical recording is performed; at this point each pixel of the DOE in frame is filled by a simple diffraction grating with a certain period (Fig.3c). At the last step, a general topology of the hologram composed of holographic and DOE frames is created; in this case, the form of the diffractive element in the hologram can be arbitrary in discreteness of one frame.

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I x d Figure 2. The principle of forming a DOE cell by means of shifting the interference field.

The program for hologram calculation generates an image file for each frame with a name containing information about the location of the frame in a hologram, as well as its content (DOE or hologram). In the process of hologram recording, the control program reads the appropriate image file and displays its content on the optical modulator. If the file corresponds to the holographic region, then optical recording with a given exposure time is performed and afterwards transition to writing the next frame occurs. In case if the content of the file corresponds to the DOE area, the optical recording algorithm is changed. After the first exposure, the control program shifts the image displayed on the SLM a few pixels and then this cycle of optical recording is repeated several times. The computer programs used for calculating image frames of the hologram and subsequent optical recording are of own development. It is experimentally determined that the best result is achieved at a triple exposure with a shift of 1/3 period, which corresponds to the theoretical grounding. It is worth noting that it is possible to combine holographic and DOE areas in a single frame; in this case, two graphics files will correspond to this frame. Recording of all areas from the first file takes place during the first exposure; and during subsequent exposures with the corresponding image shift, only DOE areas from the second file are recorded.

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Figure 3. Technological sequence of DOE formation. a) Development of the original image. b) Calculation of the DOE. c) Preparation of image frames for optical recording.

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3. RESULTS AND DISCUSSION The interference field created by the calculated image on the modulator may to have an arbitrary configuration within a single frame. Figure 4a,b shows the case where the interference field wraps around the micro text with the height of 30 and 50μm (Fig. 4a) and fills the interior region of the micro text of the same size (Fig. 4b). The intensity distribution of the optical interference field was registered on the positive organic photoresist of the AZ1800 series. Figure 4c,d shows the SEM images of micro text, which were obtained by optical recording with the shift of interference pattern shown in Fig.4a,b. At the displacement of the image by 1 pixel on the modulator with a resolution of 1024x768, the interference field on the surface of the photoresist is shifted by 0.195μm in case if the frame size has a standard dimension of 200x150μm. For recording DOE, we used shifting the image by 2 pixels, which corresponds to a 0.39μm shift of the interference pattern. The period of the interference is three times greater than the displacement and composes 1.17μm or is 850mm-1in the frequency terms. It is possible to improve the efficiency of the proposed method by using the minimum image shift of 1 pixel on the SLM and a corresponding increase of the interference field frequency, which fills the topology of the DOE, up to 1700 mm-1. In our optical system, it is necessary for this purpose to reduce the wavelength of the used laser from 445nm to 405nm, and increase the numerical aperture of objective from n.a. = 0.3 to 0.4.

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d) Figure 4. a,b) SEM image of interference field distribution recorderd on photoresist. c,d) SEM image of photoresist surface after recording with shifting of interference field.

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Figure 5. a,b) Images of boundary distortions inherent to interference field shifting technology: line with width 10μm, square of size 10x10μm , c) Part of diffractive optical element.

A characteristic disadvantage of this technology is presence of edge distortions of microstructures. Two typical kinds of microstructure boundary distortion are shown in Fig. 5a,b. In one kind of distortion the boundaries of microstructure acquire a jagged shape as shown in Fig. 5a. The second kind of distortion is associated with the height unevenness of the surface relief of microstructures in the boundary areas as can be seen in Fig. 5b. Linear dimensions of these distortions in size are comparable to the period of the interference field and therefore may be partially reduced by using higher spatial frequencies. If the size of microstructures is several times higher than the size of boundary distortion, the effectiveness of technology can be considered acceptable for producing an additional element of protection for holograms. The image in Fig. 5c, showing a section of the hologram containing DOE, was recorded using the described technology. The latter makes it possible to record not only binary but also multi-level DOE. By varying the energy dose at the surface of a certain photoresist, different relief levels can be obtained. This can be achieved by filling each pixel of DOE with a diffraction grating of different efficiency. For the SLM, the diffraction efficiency can be modulated within a wide range by changing colour of the grating lines. The resolution of this method of recording DOE is about 3 micrometers, that is, for recording a DOE frame of the size 200x150 microns, calculation is performed in the mode of 64x48 pixels per frame. A portion of the hologram area composed of holographic and a DOE frame is shown in Fig. 6a. In figure Fig. 6b, an image reconstructed by a laser beam from the hologram area containing the diffraction element can be seen. The technology considered in the present article is attractive for the formation of microstructures of relief and relief-phase diffractive elements, which significantly improve the protective properties of the hologram. The relief microstructure of the photoresist surface is copied, after coating with a conductive layer, onto a hard metallic material, usually nickel, by the galvanic method. Once a hologram pattern has been created, a traditional hologram manufacturing process, including electroforming, mechanical recombining, and embossing, can be used in the mass production of holograms.

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Figure 6. а) Part of the image-matrix hologram containing holographic and DOE frames. b) Image reconstruction from DOE with the help of laser illumination.

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CONCLUSION The proposed method allows for integration of a diffractive element into the structure of an 'image-matrix' hologram without any hardware modification of the recording device only by a slight change in the algorithm of a control program. This method of recording DOE can be considered a cheaper alternative to the existing methods in terms of competitive efficiency. The presented technology has been tested and can be readily used as an additional degree of protection in the production of 'image-matrix' holograms.

ACKNOWLEDGMENTS This research was partly supported by the ESF project “Starpdisciplinārās zinātniskās grupas izveidošana jaunu fluorescentu materiālu un metožu izstrādei un ieviešanai” Nr. 2009/0205/1DP/1.1.1.2.0/09/APIA/VIAA/152

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