1. Introduction

With the rapid development of high-pulse-energy and high-peak-power lasers, particularly at UV wavelengths and in the femtosecond range, interest in laser direct writing of holographic gratings has increased. In order to replicate two-beam interference fringes as a change in its refractive index, absorption, or thickness, a recording material usually needs to be photosensitive or have high absorption at the wavelength of the writing beams. The resulting surface-relief or refractive-index gratings can be applied to diffractive optics, optical communication, holographic data storage, and optical information processing. Because of its high peak power, a femtosecond laser is able to fabricate holographic gratings in transparent materials by two-photon-absorption or multiphoton processes. With multiple femtosecond pulses, holographic gratings have been recorded in doped materials, such as photorefractive polymers sensitized by two-photon absorption [1], optical resin or syrup with a large two-photon cross-section dye [2,3], photopolymeric cubes containing a highly efficient two-photon fluorophore encapsulated in a host epoxy [4], and azo-dye-doped bulk PMMA [5]. Time-resolved studies show that holography can be induced on the sub-picosecond time scale in barium fluoride [6], charge-transfer polymers sensitized with varying concentrations of C₆₀ [7], and solid-state films of close-packed semiconductor nanocrystals [8]. The induced transient gratings are useful for ultrafast optical information processing and the study of surface dynamics. It has been shown that permanent gratings are induced on glass when the irradiation intensity is above the damage threshold. Below this threshold, the grating relaxes when the recording beams are blocked [9]. With individual femtosecond laser pulses, permanent gratings have been encoded on the surface of glasses, crystals, and SiO₂ thin films [10–13]. We have recently demonstrated the holographic fabrication of multiple layers of gratings inside soda-lime glass [14] and holographic data storage on the surface of silica, soda-lime and lead glasses [15].

Compared with inorganic glass materials, PMMA has good properties such as lightness, flexibility, and easy formability. In this paper we present experimental results of single femtosecond pulse holography in a commercial PMMA sheet (Shinkolite-A, Mitsubishi Rayon). The sample is almost completely transparent for the laser wavelength employed in our experiments. Both relief gratings on the surface and refractive-index gratings inside PMMA have been written. The application to holographic data storage is also demonstrated.

2. Relief gratings on the surface

The experiments were carried out in air at room temperature. The setup was similar to that described in Ref.14. A Ti:sapphire femtosecond laser system with regenerative amplification (Spitfire, Spectra-Physics) generates a 130 fs pulse centered at 800 nm. The pulse is split into two beams that are focused by two identical lenses of 500 mm focal length and then symmetrically incident on the sample at an angle θ of ∼17° to the normal. When the setup is adjusted to give a perfect spatial and temporal overlap of the two beams, we can observe a high contrast interference pattern with the aid of an optical microscope. The clearest fringe pattern lies in a plane that is through the two overlapping focal points and normal to the perpendicular bisector of the two recording beams, which will be referred to as the recording plane in the following of this paper.

Both relief gratings on the surface and refractive index gratings inside PMMA can be induced with only one femtosecond pulse. When we record a surface-relief grating, we move the sample so that the recording plane is on the front surface of the sample. After exposure, the bright interference fringes induce surface ablation when the fluence is above the threshold. Figure 1 shows a typical grating encoded on a PMMA sheet with thickness of 1 mm. This optical microscope image was taken under the illumination of a halogen lamp across the sample. The incident energy of each beam was ∼80 μJ or the total energy was ∼160 μJ. The grating size was ∼50 μm.

 

Fig.1. Optical microscope image of a surface-relief grating on PMMA written with pulse energy of each beam of ∼80 μJ.

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Fig.2. (Top) AFM image of the grating structure and (bottom) the cross-sectional profile.

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The grating profile was analyzed by an atomic force microscope (AFM). Figure 2 shows an AFM image of the central portion of a grating written under the same conditions. A cross-sectional profile is demonstrated at the bottom. The grating period d is ∼1.5 μm. It can be easily changed by varying the angle θ according to the formula d = λ/2sinθ, where λ, is wavelength of the writing beam. The groove depth is ∼ 300 nm. Because the grating is formed through the multiphoton process with a material-dependent threshold, the profile of the grating is not sinusoidal. Additionally, the profile changes with the incident energy of the recording beam. The higher the pulse energy, the deeper the grooves and the sharper the ridges between grooves. The grating size will correspondingly become larger. However, when the pulse energy of each beam is over 100 μJ, visible distortion appears in the central part where the intensity is much higher. The thin ridges between the deep grooves cannot stand alone and tend to topple.

The diffraction efficiency of the first order was measured to be ∼20% while the diffraction efficiency of the second order was less than 1%. When we wrote a grating, we used a single pulse. On the other hand, when we read the grating by either recording beam, we irradiated the grating with multiple pulses at a repetition rate of 1kHz. When the power of the read beam was below 8 mW or the pulse energy was below 8μJ, the surface-relief grating cannot be erased.

 

Fig.3. (a) A refractive index grating inside PMMA. The pulse energy of each recording beam was ∼80 μJ. (b) The diffraction pattern. The zero-order beam was attenuated by a 10% neutral density filter.

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3. Refractive-index gratings inside PMMA

In our experiments, the pulse energy of each beam should be above 50 μJ to ablate a surface relief grating. However, if the pulse energy of each beam is over 100 μJ, distortion will appear as mentioned above. When only one recording beam is used, ablation will not happen if the pulse energy is lower than 180 μJ. Therefore, when the recording plane is deep inside the sample and the pulse energy of each recording beam is between 60 and 100 μJ, no surface ablation will occur because neither beam can cause surface damage. Nevertheless the intensities of bright interference fringes inside the sample are high enough to induce densification, resulting in the change in refractive index. Then, a refractive index grating is consequently written inside PMMA. Due to the threshold, the grating is localized within the focal volume and multiple layers of grating can be recorded inside PMMA with tight focusing as done in soda-lime glass [14].

Figure 3 presents the readout image of a grating formed by a single femtosecond pulse when the recording plane was at a depth of ∼200 μm. The pulse energy of each beam was ∼80 μJ. Unlike a surface relief grating, the recorded grating inside PMMA cannot be readout from the normal direction. But we can observe a weak image of the grating when the halogen lamp illuminated at Bragg angle. When we read the grating by either recording beam, we can obtain a clear image as shown in Fig. 3(a) which was taken through a 50× objective focusing onto the recording plane. These suggested that the recorded grating was a volume grating. From the top and side views, we estimated the thickness of the grating to be about 100 μm. The diffraction pattern is given in Fig. 3(b) in which the zero-order beam was attenuated by a 10% neutral density filter. The diffraction efficiency of the first order was about 0.8%. With the model of a sinusoidal volume phase grating, the maximum refractive-index change was calculated to be ∼2×10^-4 using Kogelnik’s coupled wave theory.

4. Holographic data storage on PMMA

This setup can be easily adapted for holographic data storage [15]. In the following we present the experimental results of the recording and reconstruction of a data image. One beam is used as the reference beam and focused using a 500 mm focal-length lens. The other beam is changed into the object beam. It is expanded by a Galileo telescope and modulated by a data mask. An aperture with a diameter of ∼15 mm is placed in front of the mask to ensure a uniform illumination. Then the data-bearing object beam is focused onto the sample surface by a lens of 50 mm focal length. The data mask, which consists of 9 spots in 3×3 array at a spacing of ∼3 mm as shown in Fig. 4(a), is placed in the front focal plane. After exposure to the interference fringes of the reference and object beams, a Fourier transformed hologram is recorded. Only one pulse was used to write a hologram. The stored information can be reconstructed by the reference beam. An example of the recorded holograms is presented in Fig. 4(b). The energies of the reference beam and object beam are both ∼80 μJ per pulse. The energy of the object beam is measured after the aperture in front of the data mask. The reconstructed data image is shown in Fig. 4(c). The recorded holograms cannot be erased during reconstruction when the power of the reference beam is reduced below the ablation threshold.

 

Fig.4. Holographic data storage on the surface of PMMA. (a) A data mask. (b) The corresponding hologram recorded with the pulse energy of each beam of ∼80 μJ. (c) The reconstruction image of the data mask.

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5. Conclusion

In conclusion, both relief gratings on the surface and refractive index gratings inside PMMA have been written by the two-beam interference of individual 130 fs laser pulses at 800 nm. A surface-relief grating is fabricated by ablation. The higher the incident pulse energy, the larger the grating size and the deeper the groove depth. The diffraction efficiency of the first order can reach 20%. When one beam carries information, holographic data storage on the surface can be realized. A volume grating inside PMMA is formed by the change in the refractive index. Due to the threshold, the grating is localized within the focal volume and multiple layers of gratings can be recorded inside PMMA with tight focusing.