Image storage through gray scale encoding of second harmonic signals in azo-dye copolymers Denis Gindre,a) Ibrahima Ka, Alex Boeglin, Alain Fort, and Kokou D. Dorkenoob) Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504 CNRS-ULP, 23 rue du Lœss, BP 43, F-67034 Strasbourg cedex 2, France. We have investigated optical data and image storage using polymeric materials functionalized with azo-dye molecules. Azobenzenze copolymers are well suited for two and three-dimensional storage due to their ability to undergo two-photon isomerization. The sample is initially poled to generate a spatially homogeneous second harmonic signal because of the uniform orientation of the push-pull molecules. The writing of the information is created locally through the disorientation of the azo-chromophores by successive isomerization cycles induced through a two photon absorption process. The resulting micron-scale controlled decrease of the second harmonic signal can be exploited to perform optical storage. As a demonstration of the feasibility of the method, we store an image in a polymer thin film with an 8-bit gray-scale encoding. The 256 levels of gray are defined by the amounts of local irradiation focused on the sample and they can only be retrieved through second harmonic detection, and not by linear optical imaging techniques. © 2006 American Institute of Physics

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Permanent address: Laboratoire des Propriétés Optiques des Matériaux et Applications, UMR CNRS 6136, Université d’Angers, 2 bd Lavoisier, F-49045 Angers cedex, France. b) Electronic mail: [email protected].

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Polymers doped with optically active molecules have been intensively studied this last decade in the search for superior materials for optical storage systems. In the race to obtain a high capacity storage device, poly(methyl methacrylate) (PMMA) based holographic materials of high optical quality and mechanical stability have been successfully used to demonstrate the recording of images in large volumes1-2. To overcome the limitations in fabrication rate of such holographic devices, an original recording architecture using a bit-oriented photopolymeric material presenting a sharp illumination threshold has been tested by Wang and Esener3. In a pioneering work, Parthenopoulos and Rentzepis4 had already thought about using holographic media to record binary data in 3D by spatially addressing localized volumes situated at the coincidence of two focused laser beams which are used to induce two-color two-photon absorption in photochromic dyes. Since then, a simpler and more convenient solution to take advantage of the spatial resolution offered by quadratic optical phenomena in designs using a single highly focused infrared (IR) source has been broadly pursued, notably by the Prasad group5. With respect to the molecular processes involved, we should mention that the stability of the two states in spiro-based photochromes has recently been considerably improved6. However, this class of active molecules is by no means the only one being actively developed. Indeed, photo-bleachable dyes7 and non linear optical chromophores conferring photorefractive8 or second harmonic generation9 properties to polymeric materials have also been proposed for the fabrication of optical data storage devices. In the latter work, the cis-trans isomerization of the azo-dye Disperse Red 1 (DR1) is used to manipulate the molecular orientation in order to modulate locally the quadratic susceptibility in the recording medium. The same photo-isomerization process has been employed to induce orientational hole-burning to create local birefringence as a means towards data storage10.

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In this study, we extend a novel approach we recently developed11 to encode information into DR1grafted PMMA through the graded reduction in an initial uniform second harmonic generation (SHG) faculty. In our method, the push-pull azo-chromophores have first to be oriented and then frozen in a given orientation through conventional poling techniques. Thus, starting with a second harmonic emitting “bright” material (all bits set to “1”), the two-photon absorption (TPA) process can be used to locally disorient the chromophores in desired microscopic volumes via cis-trans isomerization cycles. The resulting decrease in second harmonic intensity is directly related to the amount of induced disorientation. In this work, we will restrict ourselves to quasi-2D samples to demonstrate data storage in pixels carrying 8-bits of information each, a technique well suited to encode images. In addition, the low threshold of irradiation used in the process does not alter the surface relief of the sample and no changes can be observed in conventional microscopy. The stored picture has to be retrieved by scanning the sample while detecting the second harmonic signal levels with an appropriate setup.

A 20% wt solution in 1,1,2-trichloroethan of PMMA grafted at 10% with DR1 was spin coated on transparent microscope slides. The resulting samples were 400 nm thick films as measured by a Dektak profilometer. In the corona poling device, under nitrogen atmosphere, a high voltage (5kV) is applied to two 30 μm diameter tungsten wires placed xx cm above the samples. They are then placed on X, Y, Z, θ stages under a x50 microscope objective (numerical aperture = 0.45) and above a scanning SHG imaging microscope collecting the light through an identical objective, as shown in Fig. 1. The angle θ between the normal to the surface of the sample and the optical axis is adjusted to optimize the level of SHG signals, while the X, Y, Z motorized displacement stages are computed controlled in order to keep the focal point in the plane of the sample. A Tsunami Ti:saphire tunable laser (670-1100 nm) with 120 fs pulse duration and 80 MHz repetition rate provides the optical

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excitation. The polarization and the power of the incident beam are adjusted with a half-wave plate and a Glan-Taylor polarizer. The light exiting the sample is sent through a Schott BG 39 filter to eliminate the fundamental frequency and then directed to the input slit of a spectrometer. The spectrum is recorded by a fast cooled CCD camera. At the wavelength of 800 nm, with the power set to about 2 mW, the corona poled samples yield nearly uniform SHG signals when scanning their areas. The spectral widths of these harmonic signals correlate well with the characteristics of the fundamental femtosecond pulses and present a signal to noise ratio greater than 10 to 1.

In order to modulate the rate of disorientation of the chromophores and, consequently, to check the corresponding amplitude of the SHG signals, we have induced local TPA processes on successive areas of the sample, exposing them to increasing levels of irradiations. Notice that using the XYZ stages, the sample can be repositioned to execute at regularly spaced intervals a series of exposures for either varying illumination durations while the IR power is kept constant or for fixed shutter times while the irradiation intensity is incremented, for essentially the same effect. This stage corresponds to writing an array of dots with an increasing disorder among the chromophores. Fig. 2a shows the SHG signals obtained by scanning the sample with illumination intensities ranging from 0.4 mW to 64 mW for a duration of 25 ms. At the lowest power setting, no detectable level of disorientation among the chromophores is observed (position “0” μm in Fig. 2a), while at the highest level the preferential order of the chromophores has apparently been completely annealed, extinguishing the SHG signal (position “1000” μm in Fig. 2a). It is worthwhile to notice that the disorientation of the chromophores (or equivalently the decrease of the SHG intensity) begins only above the 300 μm position, i.e. above an illumination power of c.a. 4.2 mW. In the inset, we have plotted an enlargement of the transient second harmonic responses of a few dots excited with an intermediate light intensity (27 mW) to show

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the typical shape of the signals. Another representation of this behavior is shown in Fig. 2b where we have plotted the evolution of the normalized contrast, defined by [SHG(I=0) –SHG(I)]/SHG(0), as a function of the amount of TPA-inducing light intensity I. Examining the resulting curve, we can identify three different regimes, a quadratic regime between 0 mW and 10 mW, a quasi-linear regime between 10 mW and 25 mW, and a saturation regime above 25 mW. The second regime is especially interesting: first, the contrast measurements show that the noise is weak and, second, classical microscopy techniques indicate that no physical damages affect the sample. Therefore, we can divide the corresponding normalized SHG contrast between 0.2 to 0.7 into 256 levels as shown on the grayscale on the right-hand side in Fig. 2b. It is thus possible to define a correspondence between the value of the normalized contrast (or the 28 levels of the gray-scale) and the intensity (or the duration) of the TPA-inducing light.

In order to demonstrate the validity of this approach, we have chosen to pixelize the black/white picture shown in Fig. 3a. We construct a 2D matrix of 100x100 pixels in which, for each pixel of 2 μm × 2 μm, the measured gray level value is converted to a corresponding illumination time ranging from 0 ms to 255 ms. This 100x100 matrix of time durations has been used with a fixed incident power of 10 mW to promote disorientation among the chromophores in a sample presenting an initial uniform SHG signal. Following this, the sample has been scanned using the SHG setup with a low IR intensity (2.2 mW) to avoid further isomerization of the azo-push-pull molecules. The corresponding reconstruction of the initial image is shown in Fig. 3b. After this procedure, using a conventional microscope, we checked that no trace due to this illumination can be detected, as can be seen in Fig. 3c. Therefore, Fig. 3 demonstrates that a photograph can be encoded into a polymeric film in such a way that it can only be retrieved through SHG imaging, while remaining invisible in linear optics.

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In conclusion, we have presented a new approach to encode optical information in azo-dye polymeric materials. The principle is based on the control of the disorientation of azo-dye molecules through successive cis-trans isomerization cycles induced through two-photon absorption in pre-poled films. Indeed, one would think that it is both simpler, as well as more permanent, to promote disorder instead of order in a given medium. This local control of the dis-alignment of the chromophores leads to a corresponding modulation of the second harmonic generation in microscopic volumes as small as 1 μm3 due to the quadratic behavior of both the two-photon absorption and the second harmonic generation processes. Moreover, the relatively high sensitivity of this technique allows the storage not only of binary bits but also of gray-scale encoded information (here on 8 bits) on microscopic areas. An example of application is the storage of pictures undetectable by usual optical means. Thus, our approach can be used to carry hidden information in the storage medium. References 1

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List of Figure Captions

Fig. 1. Experimental set-up based on two-photon isomerization for the inscription and on second harmonic generation for the read out of the information.

Fig. 2. SHG response after marking a narrow strip on the sample with separated dots through TPA depoling with increasing intensities. a) Continuous scan of the strip at 3mW: the position 0 corresponds to an irradiation at 0.4mW and 1000 to 64 mW. b) Normalized SHG contrast of the individual dots as a function of the irradiation power; the linear portion of the plot can be used to define a gray-scale as shown on the right-hand side.

Fig. 3. Demonstration of 8 bit coded data retrieval through SHG imaging of. a) a photograph of Greta Garbo; b) SHG image obtained through TPA depoling after encoding the digitalized picture a) into 100 by 100 pixels, each 2 μm in diameter; c) optical microscope view of the polymeric film showing no detectable changes to the sample area containing the image as indicated by the four angle marks set 200 μm apart.

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Fig. 1

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Fig. 2.

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Fig. 3.

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