single femtosecond pulse holography using polymethyl methacrylate

Single femtosecond pulse holography usingpolymethyl methacrylate

Yan LiVenture Business Laboratory, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

[email protected]

Kazuhiro Yamada, Tomohiko Ishizuka, Wataru Watanabe and Kazuyoshi ItohDepartment of Material and Life Science, Graduate School of Engineering, Osaka University,

2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

Zhongxiang ZhouDepartment of Applied Physics, Harbin Institute of Technology, Harbin 150001, China

Abstract: Holographic gratings have been written on the surface and insidetransparent polymethyl methacrylate (PMMA) with individual 130 fs laserpulses at 800 nm. A surface-relief grating is fabricated by ablation and thediffraction efficiency is measured to be about 20%. A volume grating insidePMMA is formed by the change in the refractive index induced by thetwo-beam interference fringes. Holographic data storage on the surface isrealized when one beam carries information. The stored information can benondestructively reconstructed when the fluence of the read beam is reducedbelow the threshold.

2002 Optical Society of America

OCIS codes: (320.2250) Femtosecond phenomena; (090.0090) Holography; (160.4890) Organicmaterials

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2. S. M. Kirkpatrick, J. W. Baur, C. M. Clark, L. R. Denny, D. W. Tomlin, B. R. Reinhardt, R. Kannan and M. O.Stone, “Holographic recording using two-photo-induced photopolymerization,” Appl. Phys. A 69, 461-464(1999).

3. D. J. Pikas, S. M. Kirkpatrick, D. W. Tomlin, L. Natarajan, V. Tondiglia and T. J. Bunning, “Electricallyswitchable reflection holograms formed using two-photon photopolymerization,” Appl. Phys. A 74, 767-772(2002).

4. C. Diamond, Y. Boiko and S. Esener, “Two-photon holography in 3-D photopolymer host-guest matrix,” Opt.Express 6, 64-68 (2000), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-6-3-64.

5. J. Si, J. Qiu, J. Zhai, Y. Shen and K. Hirao, “Photoinduced permanent gratings inside bulk azodye-dopedpolymers by the coherent field of a femtosecond laser,” Appl. Phys. Lett. 80, 359-361 (2002).

6. Th. Schneider and J. Reif, “Influence of an ultrafast transient refractive-index grating on nonlinear opticalphenomena,” Phys. Rev. A 65, 023801-1-10 (2002).

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10. K. Kawamura, T. Ogawa, N. Sarukura, M. Hirano and H. Hosono, “Fabrication of surface relief gratings ontransparent dielectric materials by two-beam holographic method using infrared femtosecond laser pulses,”Appl. Phys. B: Lasers Opt. 71, 119-121 (2000).

11. K. Kawamura, N. Sarukura, M. Hirano and H. Hosono, “Holographic encoding of permanent gratingsembedded in diamond by two beam interference of a single femtosecond near-infrared laser pulse,” Jpn. J.Appl. Phys. Part 2, 39, L767-L769 (2000).

(C) 2002 OSA 21 October 2002 / Vol. 10, No. 21 / OPTICS EXPRESS 1173#1508 - $15.00 US Received July 30, 2002; Revised October 11, 2002

mailto:[email protected]

http://www.opticsexpress.org/abstract.cfm?URI=OPEX-6-3-64

12. K. Kawamura, N. Sarukura, M. Hirano and H. Hosono, “Holographic encoding of fine-pitched microgratingstructures in amorphous SiO2 thin films on silicon by a single femtosecond laser pulse,” Appl. Phys. Lett. 78,1038-1040 (2001).

13. K. Kawamura, N. Sarukura, M. Hirano, N. Ito and H. Hosono, “Periodic nanostructure array in crossedholographic gratings on silica glass by two interfered infrared-femtosecond laser pulses,” Appl. Phys. Lett. 79,1228-1130 (2001).

14. Y. Li, W. Watanabe, K. Yamada, T. Shinagawa, K. Itoh, J. Nishii and Y. Jiang, “Holographic fabrication ofmultiple layers of grating inside soda-lime glass with femtosecond laser pulses,” Appl. Phys. Lett. 80,1508-1510 (2002).

15. Y. Li, W. Watanabe, K. Itoh and X. Sun, “Holographic data storage on nonphotosensitive glass with a singlefemtosecond laser pulse,” Appl. Phys. Lett. 81, 1952-1954 (2002).

1. Introduction

With the rapid development of high-pulse-energy and high-peak-power lasers, particularly atUV wavelengths and in the femtosecond range, interest in laser direct writing of holographicgratings has increased. In order to replicate two-beam interference fringes as a change in itsrefractive index, absorption, or thickness, a recording material usually needs to bephotosensitive or have high absorption at the wavelength of the writing beams. The resultingsurface-relief or refractive-index gratings can be applied to diffractive optics, opticalcommunication, holographic data storage, and optical information processing. Because of itshigh peak power, a femtosecond laser is able to fabricate holographic gratings in transparentmaterials by two-photon-absorption or multiphoton processes. With multiple femtosecondpulses, holographic gratings have been recorded in doped materials, such as photorefractivepolymers 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-photonfluorophore 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 inbarium fluoride [6], charge-transfer polymers sensitized with varying concentrations of C60 [7],and solid-state films of close-packed semiconductor nanocrystals [8]. The induced transientgratings are useful for ultrafast optical information processing and the study of surfacedynamics. It has been shown that permanent gratings are induced on glass when the irradiationintensity is above the damage threshold. Below this threshold, the grating relaxes when therecording beams are blocked [9]. With individual femtosecond laser pulses, permanent gratingshave been encoded on the surface of glasses, crystals, and SiO2 thin films [10-13]. We haverecently demonstrated the holographic fabrication of multiple layers of gratings insidesoda-lime glass [14] and holographic data storage on the surface of silica, soda-lime and leadglasses [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 singlefemtosecond pulse holography in a commercial PMMA sheet (Shinkolite-A, Mitsubishi Rayon).The sample is almost completely transparent for the laser wavelength employed in ourexperiments. Both relief gratings on the surface and refractive-index gratings inside PMMAhave 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 thatdescribed 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 intotwo beams that are focused by two identical lenses of 500 mm focal length and thensymmetrically incident on the sample at an angle θ of ~17° to the normal. When the setup isadjusted to give a perfect spatial and temporal overlap of the two beams, we can observe a highcontrast interference pattern with the aid of an optical microscope. The clearest fringe patternlies 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 thefollowing of this paper.

Both relief gratings on the surface and refractive index gratings inside PMMA can beinduced with only one femtosecond pulse. When we record a surface-relief grating, we movethe sample so that the recording plane is on the front surface of the sample. After exposure, thebright 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. Thisoptical microscope image was taken under the illumination of a halogen lamp across thesample. The incident energy of each beam was ~80 µJ or the total energy was ~160 µJ. Thegrating size was ~ 50 µm.

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

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

10µm

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ght(

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The grating profile was analyzed by an atomic force microscope (AFM). Figure 2 shows anAFM 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 easilychanged by varying the angle θ according to the formula θλ sin2/=d , where λ is wavelength ofthe writing beam. The groove depth is ~ 300 nm. Because the grating is formed through themultiphoton process with a material-dependent threshold, the profile of the grating is notsinusoidal. 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 eachbeam is over 100 µJ, visible distortion appears in the central part where the intensity is muchhigher. 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 diffractionefficiency of the second order was less than 1%. When we wrote a grating, we used a singlepulse. On the other hand, when we read the grating by either recording beam, we irradiated thegrating with multiple pulses at a repetition rate of 1kHz. When the power of the read beam wasbelow 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% neutraldensity filter.

3. Refractive-index gratings inside PMMA

In our experiments, the pulse energy of each beam should be above 50 µJ to ablate a surfacerelief grating. However, if the pulse energy of each beam is over 100 µJ, distortion will appearas mentioned above. When only one recording beam is used, ablation will not happen if thepulse energy is lower than 180 µJ. Therefore, when the recording plane is deep inside thesample and the pulse energy of each recording beam is between 60 and 100 µJ, no surfaceablation will occur because neither beam can cause surface damage. Nevertheless the intensitiesof 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

(b)

zero-order first-order (×10)

(a)

10µm


written inside PMMA. Due to the threshold, the grating is localized within the focal volume andmultiple layers of grating can be recorded inside PMMA with tight focusing as done insoda-lime glass [14].

Figure 3 presents the readout image of a grating formed by a single femtosecond pulsewhen the recording plane was at a depth of ~200 µm. The pulse energy of each beam was ~80µJ. Unlike a surface relief grating, the recorded grating inside PMMA cannot be readout fromthe normal direction. But we can observe a weak image of the grating when the halogen lampilluminated at Bragg angle. When we read the grating by either recording beam, we can obtaina clear image as shown in Fig. 3(a) which was taken through a 50× objective focusing onto therecording plane. These suggested that the recorded grating was a volume grating. From the topand side views, we estimated the thickness of the grating to be about 100 µm. The diffractionpattern is given in Fig. 3(b) in which the zero-order beam was attenuated by a 10% neutraldensity filter. The diffraction efficiency of the first order was about 0.8%. With the model of asinusoidal 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 presentthe experimental results of the recording and reconstruction of a data image. One beam is usedas the reference beam and focused using a 500 mm focal-length lens. The other beam is changedinto the object beam. It is expanded by a Galileo telescope and modulated by a data mask. Anaperture with a diameter of ~15 mm is placed in front of the mask to ensure a uniformillumination. Then the data-bearing object beam is focused onto the sample surface by a lens of50 mm focal length. The data mask, which consists of 9 spots in 3×3 array at a spacing of ~3mm as shown in Fig. 4(a), is placed in the front focal plane. After exposure to the interferencefringes of the reference and object beams, a Fourier transformed hologram is recorded. Onlyone pulse was used to write a hologram. The stored information can be reconstructed by thereference beam. An example of the recorded holograms is presented in Fig. 4(b). The energiesof the reference beam and object beam are both ~80 µJ per pulse. The energy of the object beamis measured after the aperture in front of the data mask. The reconstructed data image is shownin Fig. 4(c). The recorded holograms cannot be erased during reconstruction when the power ofthe reference beam is reduced below the ablation threshold.

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

(a) (b)

(c)

3mm

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5. Conclusion

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

Acknowledgement

The authors are grateful to Hong-Bo Sun of Department of Applied Physics, Graduate School ofEngineering, Osaka University, for useful discussion and technical help.


single femtosecond pulse holography using polymethyl methacrylate

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