May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
2049
On the rewriting of ultrashort pulse-induced nanogratings Felix Zimmermann,1,* Anton Plech,2 Sören Richter,1 Andreas Tünnermann,1,3 and Stefan Nolte1,3 1
2
Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany ANKA/Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany 3
Fraunhofer Institute for Applied Optics and Precision Engineering, Albert-Einstein-Straße 7, 07745 Jena, Germany *Corresponding author: Felix.Zimmermann@uni‑jena.de Received March 19, 2015; revised April 8, 2015; accepted April 8, 2015; posted April 8, 2015 (Doc. ID 236212); published April 29, 2015 In this study, we report on the erasure and rewriting of nanogratings by femtosecond laser pulses in the bulk of fused silica. To map the structural processes during rewriting, a combination of optical retardance measurement, small angle X-ray scattering, and scanning electron microscopy was used. The results reveal that already few pulses lead to erasure and formation of anisotropic structures. Repetitive rewriting favors the formation of nanoscopic pores, which increases the optical retardance of nanogratings for large pulse numbers. © 2015 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (220.4241) Nanostructure fabrication; (320.2250) Femtosecond phenomena. http://dx.doi.org/10.1364/OL.40.002049
Ultrashort pulse lasers with pulse durations in the femtosecond range have gained particular interest as universal tools for materials processing of photonic devices in recent years [1,2]. When focusing inside glasses like fused silica, the high intensities trigger nonlinear absorption mechanisms and thus allow to deposit the pulse energy in the otherwise transparent bulk material. Interestingly, local birefringence due to a sub-wavelength nanograting can be induced in a wide laser parameter window [3,4]. Recent studies reveal that these self-assembling nanogratings consist of sheet-like cavities of about 20 nm thickness that arrange and form individual grating planes during the action of several hundreds of laser pulses [5–7]. While the period scales with the laser wavelength λ corresponding to λ∕2n (n-refractive index), the grating planes always arrange perpendicular to the laser polarization [8,9]. This in combination with the threedimensional degree of freedom of the direct writing approach allows for the fabrication of diverse photonic devices ranging from optical vortex converters [10], holograms [11], to microfluidic channels for bio-chip applications [12]. Apart from the initial inscription, it has been found that an existing nanograting can be erased and newly arranged by inscribing again at the same position with a differently oriented polarization direction [13]. This phenomenon is useful for data storage applications since imprinted information in terms of nanograting-based data units can easily be correct or updated as needed. However, up to now no comprehensive picture of this complex physical process exists. In [13], optical polarization contrast measurements and scanning electron microscopy (SEM) of etched samples served to evaluate the rewritten nanograting spots, but detailed and nanoscopic information of the underlying process was lacking. In this regard, the evolution (erasure and formation) from small pores up to full grating planes for single and multiple rewrites remains unexplored. Hence we combined optical retardance (OR) measurements with microscopic techniques, such as small angle X-ray scattering (SAXS) and SEM to analyze these issues. 0146-9592/15/092049-04$15.00/0
For the inscription of nanogratings in the bulk of fused silica, we used the frequency-doubled light (515 nm) of a mode-locked oscillator (Amplitude Systemes, t-pulse 500) emitting pulses at 9.4-MHz repetition rate and a pulse duration of 450 fs. To avoid heat accumulation effects, the repetition rate (R) was decreased to 500 kHz by an external acousto-optical modulator, and the pulse energy was set to 150 nJ by polarization optics. An aspheric lens (New Focus 5722) with a numerical aperture of 0.55 served to focus the pulses in shallow material depths (200 μm) where the aberrations of the objective are minimal. The fused silica samples were translated underneath the laser beam with a 3-axis positioning system, while the effective number of pulses incident on one laser spot (N) was set by changing the velocity of the sample movement (v) corresponding to N 2ω0 R∕v (ω0 0.3 μm—calculated beam waist). To probe the nanogratings by SAXS, fields of (100 × 100) μm2 were inscribed with a hatching line distance of 0.5 μm. For analyzing the evolution of the rewriting process, a reference field (1000 pulses, polarization along x) was rewritten up to 15 times (each time with 90° rotated polarization), while the number of pulses was varied from 20 up to 3000 [see Fig. 1(a)]. The nanograting fields were measured by SAXS at beamline cSAXS at the Swiss Light Source (PSI Villigen, CH). A detailed description of the experimental setup and data analysis can be found in [7]. Due to the experimental setup used and the high beam brilliance, maximum feature sizes up to 400 nm could directly be measured by SAXS. The fields were illuminated by an 11.2-keV X-ray beam parallel to the propagation direction of the inscribing laser (z). After the direct X-ray beam is blocked, the scattered radiation is recorded by a pixellated direct-converting detector (Pilatus 2M). After standard image corrections were applied and the background from the pristine material was subtracted, the recorded two-dimensional scattering signal was integrated in a narrow cone along the main axis x; y to observe the scattered intensity Iq (q—scattering vector) [14]. As useful quantity, the so-called Porod invariant P, © 2015 Optical Society of America
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further illumination leads to a continuous increase of birefringence. Further studies revealed that only 5 pulses per spot suffice to delete the directional information of optical birefringence. Similar behavior has been observed in [13] where at about 100 pulses incident on one spot, the polarization contrast in both grating directions is the same. The higher pulse number is presumably due to the different laser parameters used. To illustrate the corresponding nanograting structure, SEM images after polishing and etching (1% hydrofluoric acid for 90 s at room temperature) were acquired. A lower repetition rate of 10 kHz was used in this case to achieve even lower pulse numbers while avoiding complications due to high speeds of the sample movement. Figure 2(a) shows a sequence of increasing number of pulses with perpendicular polarization relative to the prior irradiation. Already for 5 pulses, grating planes corresponding to both the old and new polarization direction can be found. The new planes are not yet well ordered, but quite large. Thus the anisotropy resulting
Fig. 1. (a) Sketch of the nanograting sample. The reference field (“Ref.,” 1000 pulses per spot, E⃗ x ) is up to 15 times rewritten, every time with 90° rotated polarization, while the number of pulses varies from 20 up to 3000 pulses. (b) Color-coded image of the optical retardance measurement (probe wavelength 587 nm) of the corresponding nanograting fields. The sign of the retardance accounts for the two distinct fast axis orientations (“±” − Ey ∕E x ) of the induced nanogratings.
R P Iq · q2 dq ∼ ϕ · 1 − ϕ was determined. For a diluted system, as empty pores in an isotropic glass matrix, P is directly proportional to the volume-filling fraction ϕ. It hence serves as a crucial quantity for the evolution of the total pore volume of nanogratings [6,15]. Since this analysis can be done for different azimuthal orientations relative to the laser polarization, the directions x and y (see Fig. 1) can be studied independently. For mapping the rewrite process, this means that features corresponding to the original and the newly formed grating can be mapped independently due to the anisotropic nature of nanogratings. Furthermore, due to the arrangement of small pores in periodic grating planes, distinct correlations show up, and the scattered intensity is modulated by the structure factor Sq corresponding to Iq Fq2 · Sq [16]. By removing the fitted form factor Fq of the small pores (Beaucage fit [7,17]), the structure factor (including characteristic features as, e.g., diffraction peaks) of the particular nanograting can be analyzed. To macroscopically evaluate fields of rewritten nanogratings, we performed optical retardance measurements [see Fig. 1(b)] using a commercial strain analyzer (Ilis StrainMatic M4/60.13, wavelength 587 nm). The colorcoded image indicates a strong change in retardance from about −50 nm to 40 nm after (single) rewriting the reference field with only 20 pulses per spot, while
Fig. 2. (a) SEM images of rewritten nanograting traces (top view) after polishing and etching. (b) Recorded SAXS pattern of nanogratings after single rewriting the reference field with 0, 20, and 3000 pulses per spot. (c) Porod invariant for single (1) and multiple (7, 15) rewritten nanogratings. The formation of the new grating (full symbol) as well as the erasure of the old grating (open symbols) are plotted. The gray shaded area marks the region of the reference field (“Ref.,” 1000 pulses per spot, E⃗ x ). The dashed line marks the difference in invariant for odd rewrites while a direct comparison with even (Ex ) rewrites is hindered.
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
from both gratings compensate each other, and the optical retardance vanishes. For ongoing illumination, the old grating planes become blurred, while the new planes come up. A subsequent ordering mechanism can be observed. A nanoscopic picture of the erasure and formation of pores and cracks is gained by the SAXS measurements of the rewritten fields. The corresponding false-color plots of the scattering distribution in the detector plane are shown in Fig. 2(b). An elongated streak is visible, which marks the direction of the short axis of the elongated pores (laser polarization direction). A number of spots can be found superimposed on this streak for the initial pattern and the rewriting with larger pulse numbers. These spots mark the diffraction orders of the linear grating formed by the ordered nanopores. The scattering pattern strongly changes after already 20 pulses [see Fig. 2(b)]. First, the streak along the initial polarization direction shrinks, and the diffraction peaks close to the center disappear. This points to the erasure of both the shape and alignment of the nanoscopic features after a few laser pulses. This is confirmed by the strongly decreased invariant in Fig. 2(c). Furthermore, a streak along the new polarization direction shows up. It gets more distinctive with increasing number of pulses. The Porod invariant along the direction of the new grating (y) considerably increases in pace with the strong change in birefringence. Indeed, the invariant reflects the total number of scattering objects and correlates with the optical retardance [18]. However, the erasure of pores is not a complete process since some scattering features, illustrated by the invariant along the initial grating direction, remain and can be observed even up to 3000 pulses [see Figs. 2(a) and 2(c)]. The same holds true for multiple rewrites, as can be compared in Fig. 2(c) for the first and 7th (or 15th) rewriting process. After repeated rewriting, both the Porod invariant and the retardance tend to even become stronger. A direct comparison between even (E x ) and odd rewrite (E y ) is nevertheless hindered due to slight differences in optical retardance [see Fig. 3(a)] [19] and correspondingly a small shift (invariant) but with otherwise similar behavior. Looking closer, two regimes with a transition at about 100 pulses can be distinguished [Figs. 2(c) and 3(a)]. In the beginning of the nanograting evolution (for smaller number of pulses), the optical retardance is still small, in particular after multiple (14/15) rewrites as Fig. 3(a) shows. For larger pulse numbers, a strong increase in pores [see Fig. 2(c)] and correspondingly higher retardance is measured in particular after multiple rewrites. The SEM images give an explanation for this behavior, where during the first shots, the pronounced anisotropy of the old grating prevails, and newly formed pores are still irregularly shaped with less defined elongation. These early pores are nevertheless prominent and are seeding the further evolution. After long irradiation (≳100 pulses), the pores become elongated and better aligned. The relative enhancement of this process for repeated rewriting is caused by the repetitive annealing and quenching, which leads to a porous glass matrix promoting the formation of new inhomogeneities.
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Fig. 3. (a) Retardance, (b) grating period, and (c) intensity of the first- and second-order diffraction peak of rewritten nanograting fields. Please note that the error bars in (b) account for the diffraction peak width (1∕e of the maximum intensity). The gray shaded area marks the region of the reference field.
This is accompanied by the decrease in grating period and increase of the diffraction intensity of the grating period, as shown in Figs. 3(b) and 3(c) (exemplary for 1 and 15 rewrites). However, the diffraction intensity for repeated rewriting does not develop in the same way as the retardance. This is a consequence of a more strongly perturbed glass with more disordered pores preventing the grating regularity to improve. The optical retardance on the other side is more related to the absolute number and elongation of pores, which is promoted by repeated rewriting and directly mapped by the Porod invariant. The analysis of the rewrite process also provides details of the grating formation mechanism in general. Here, we observe that the initial steps consist of forming pores, which start to be elongated early, but not well aligned [20]. Average distances are probably more related to localization of heat deposition and strain interaction between pores. Later on, more (narrow) pores develop by splitting of initial pores and intrusion of new pores. This is the origin of period shrinkage toward a new equilibrium value, the latter of which might be determined stronger by nanoplasmonic excitations [8]. For applications as nanograting based optical encoding (as e.g., in some kind of storage application [21]), rewriting is a promising feature. However, care has to be taken to set the illumination parameters to update single data units, i.e., the usage of an intermediate regime of pulses incident can be beneficial to avoid a drift in optical retardance during ongoing rewrites. Moreover, the
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degradation of grating quality might influence the decoding of single data units. Considering numerous rewrites can lead to ambiguity of data bits. In summary, the phenomena of erasing and rewriting of ultrashort pulse-induced nanogratings were analyzed by using microscopic techniques as SAXS and SEM as well as macroscopic characterization tools as optical retardance measurements. We could show that, once a nanograting is formed, the rewriting process starts with few large and aligned pores, which subsequently increase in number. The increased overall porosity facilitates the reformation process, which is mapped by the strong retardance and small nanograting period in the beginning of the rewrite process. During ongoing rewrites, the glass matrix is repetitively heated and quenched, which promotes the formation of inhomogeneities. Thus, the performance in terms of the optical retardance of nanogratings improves for high pulse numbers. It is observed that with ongoing rewrites, the number of old remaining nanostructures, i.e., induced by the old polarization state, also increase which perturbs the grating quality but does not affect the overall retardance. Therefore, nanaograting voxels inscribed by optimized laser parameters exhibit great potential for data storage applications with multiple writing cycles. The work is supported by Deutsche Forschungsgemeinschaft (DFG) via priority program SPP 1327 (NO 462/5-2) and by the program “Research with Photons, Neutrons and Ions” by the Helmholtz Society. A. P. acknowledges the Heisenberg grant of the DFG. We thank N. Sergeev for recording the SEM images. We acknowledge beamtime at the Swiss Light Source (PSI, Villigen Ch) and excellent support by M. Guizar-Sicairos. We also appreciated discussions with A. Menzel. References 1. K. Itoh, W. Watanabe, S. Nolte, and C. B. Schaffer, MRS Bull. 31, 620 (2006).
2. R. R. Gattass and E. Mazur, Nat. Photonics 2, 219 (2008). 3. P. G. Kazansky, H. Inouye, T. Mitsuyu, K. Miura, J. Qiu, K. Hirao, and F. Starrost, Phys. Rev. Lett. 82, 2199 (1999). 4. Y. Shimotsuma, P. G. Kazansky, J. Qiu, and K. Hirao, Phys. Rev. Lett. 91, 247405 (2003). 5. M. Lancry, F. Brisset, and B. Poumellec, “In the heart of nanogratings made up during femtosecond laser irradiation,” in Proceeding of Bragg Gratings, Photosensitivity, and Poling in Glass Waveguides (BGPP), Karlsruhe, Germany, 2010 (The Optical Society, 2010), paper BWC3. 6. S. Richter, A. Plech, M. Steinert, M. Heinrich, S. Döring, F. Zimmermann, U. Peschel, E. B. Kley, A. Tünnermann, and S. Nolte, Laser Photon. Rev. 6, 787 (2012). 7. F. Zimmermann, A. Plech, S. Richter, S. Döring, A. Tünnermann, and S. Nolte, Appl. Phys. A 114, 75 (2014). 8. R. Taylor, C. Hnatovsky, and E. Simova, Laser Photon. Rev. 2, 26 (2008). 9. L. P. R. Ramirez, M. Heinrich, S. Richter, F. Dreisow, R. Keil, A. V. Korovin, U. Peschel, S. Nolte, and A. Tünnermann, Appl. Phys. A 100, 1 (2010). 10. M. Beresna, M. Gecevicius, P. G. Kazansky, and T. Gertus, Appl. Phys. Lett. 98, 201101 (2011). 11. W. Cai, A. R. Libertun, and R. Piestun, Opt. Express 14, 3785 (2006). 12. K. Sugioka and Y. Cheng, Light Sci. Appl. 3, e149 (2014). 13. R. S. Taylor, C. Hnatovsky, E. Simova, P. P. Rajeev, D. M. Rayner, and P. B. Corkum, Opt. Lett. 32, 2888 (2007). 14. A. Plech, P. Leiderer, and J. Boneberg, Laser Photon. Rev. 3, 435 (2009). 15. H. K. Kammler, G. Beaucage, D. J. Kohls, N. Agashe, and J. Ilavsky, J. Appl. Phys. 97, 054309 (2005). 16. A. Plech, V. Kotaidis, A. Siems, and M. Sztucki, Phys. Chem. Chem. Phys. 10, 3888 (2008). 17. G. Beaucage, J. Appl. Crystallograph. 28, 717 (1995). 18. F. Zimmermann, A. Plech, S. Richter, A. Tünnermann, and S. Nolte, Appl. Phys. Lett. 104, 211107 (2014). 19. M. Gecevičius, M. Beresna, J. Zhang, W. Yang, H. Takebe, and P. G. Kazansky, Opt. Express 21, 3959 (2013). 20. S. Richter, M. Heinrich, S. Döring, A. Tünnermann, and S. Nolte, Appl. Phys. A 104, 503 (2011). 21. J. Zhang, M. Gecevičius, M. Beresna, and P. G. Kazansky, Phys. Rev. Lett. 112, 033901 (2014).
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On the rewriting of ultrashort pulse-induced nanogratings Felix Zimmermann,1,* Anton Plech,2 Sören Richter,1 Andreas Tünnermann,1,3 and Stefan Nolte1,3 1
2
Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany ANKA/Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany 3
Fraunhofer Institute for Applied Optics and Precision Engineering, Albert-Einstein-Straße 7, 07745 Jena, Germany *Corresponding author: Felix.Zimmermann@uni‑jena.de Received March 19, 2015; revised April 8, 2015; accepted April 8, 2015; posted April 8, 2015 (Doc. ID 236212); published April 29, 2015 In this study, we report on the erasure and rewriting of nanogratings by femtosecond laser pulses in the bulk of fused silica. To map the structural processes during rewriting, a combination of optical retardance measurement, small angle X-ray scattering, and scanning electron microscopy was used. The results reveal that already few pulses lead to erasure and formation of anisotropic structures. Repetitive rewriting favors the formation of nanoscopic pores, which increases the optical retardance of nanogratings for large pulse numbers. © 2015 Optical Society of America OCIS codes: (140.3390) Laser materials processing; (220.4241) Nanostructure fabrication; (320.2250) Femtosecond phenomena. http://dx.doi.org/10.1364/OL.40.002049
Ultrashort pulse lasers with pulse durations in the femtosecond range have gained particular interest as universal tools for materials processing of photonic devices in recent years [1,2]. When focusing inside glasses like fused silica, the high intensities trigger nonlinear absorption mechanisms and thus allow to deposit the pulse energy in the otherwise transparent bulk material. Interestingly, local birefringence due to a sub-wavelength nanograting can be induced in a wide laser parameter window [3,4]. Recent studies reveal that these self-assembling nanogratings consist of sheet-like cavities of about 20 nm thickness that arrange and form individual grating planes during the action of several hundreds of laser pulses [5–7]. While the period scales with the laser wavelength λ corresponding to λ∕2n (n-refractive index), the grating planes always arrange perpendicular to the laser polarization [8,9]. This in combination with the threedimensional degree of freedom of the direct writing approach allows for the fabrication of diverse photonic devices ranging from optical vortex converters [10], holograms [11], to microfluidic channels for bio-chip applications [12]. Apart from the initial inscription, it has been found that an existing nanograting can be erased and newly arranged by inscribing again at the same position with a differently oriented polarization direction [13]. This phenomenon is useful for data storage applications since imprinted information in terms of nanograting-based data units can easily be correct or updated as needed. However, up to now no comprehensive picture of this complex physical process exists. In [13], optical polarization contrast measurements and scanning electron microscopy (SEM) of etched samples served to evaluate the rewritten nanograting spots, but detailed and nanoscopic information of the underlying process was lacking. In this regard, the evolution (erasure and formation) from small pores up to full grating planes for single and multiple rewrites remains unexplored. Hence we combined optical retardance (OR) measurements with microscopic techniques, such as small angle X-ray scattering (SAXS) and SEM to analyze these issues. 0146-9592/15/092049-04$15.00/0
For the inscription of nanogratings in the bulk of fused silica, we used the frequency-doubled light (515 nm) of a mode-locked oscillator (Amplitude Systemes, t-pulse 500) emitting pulses at 9.4-MHz repetition rate and a pulse duration of 450 fs. To avoid heat accumulation effects, the repetition rate (R) was decreased to 500 kHz by an external acousto-optical modulator, and the pulse energy was set to 150 nJ by polarization optics. An aspheric lens (New Focus 5722) with a numerical aperture of 0.55 served to focus the pulses in shallow material depths (200 μm) where the aberrations of the objective are minimal. The fused silica samples were translated underneath the laser beam with a 3-axis positioning system, while the effective number of pulses incident on one laser spot (N) was set by changing the velocity of the sample movement (v) corresponding to N 2ω0 R∕v (ω0 0.3 μm—calculated beam waist). To probe the nanogratings by SAXS, fields of (100 × 100) μm2 were inscribed with a hatching line distance of 0.5 μm. For analyzing the evolution of the rewriting process, a reference field (1000 pulses, polarization along x) was rewritten up to 15 times (each time with 90° rotated polarization), while the number of pulses was varied from 20 up to 3000 [see Fig. 1(a)]. The nanograting fields were measured by SAXS at beamline cSAXS at the Swiss Light Source (PSI Villigen, CH). A detailed description of the experimental setup and data analysis can be found in [7]. Due to the experimental setup used and the high beam brilliance, maximum feature sizes up to 400 nm could directly be measured by SAXS. The fields were illuminated by an 11.2-keV X-ray beam parallel to the propagation direction of the inscribing laser (z). After the direct X-ray beam is blocked, the scattered radiation is recorded by a pixellated direct-converting detector (Pilatus 2M). After standard image corrections were applied and the background from the pristine material was subtracted, the recorded two-dimensional scattering signal was integrated in a narrow cone along the main axis x; y to observe the scattered intensity Iq (q—scattering vector) [14]. As useful quantity, the so-called Porod invariant P, © 2015 Optical Society of America
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further illumination leads to a continuous increase of birefringence. Further studies revealed that only 5 pulses per spot suffice to delete the directional information of optical birefringence. Similar behavior has been observed in [13] where at about 100 pulses incident on one spot, the polarization contrast in both grating directions is the same. The higher pulse number is presumably due to the different laser parameters used. To illustrate the corresponding nanograting structure, SEM images after polishing and etching (1% hydrofluoric acid for 90 s at room temperature) were acquired. A lower repetition rate of 10 kHz was used in this case to achieve even lower pulse numbers while avoiding complications due to high speeds of the sample movement. Figure 2(a) shows a sequence of increasing number of pulses with perpendicular polarization relative to the prior irradiation. Already for 5 pulses, grating planes corresponding to both the old and new polarization direction can be found. The new planes are not yet well ordered, but quite large. Thus the anisotropy resulting
Fig. 1. (a) Sketch of the nanograting sample. The reference field (“Ref.,” 1000 pulses per spot, E⃗ x ) is up to 15 times rewritten, every time with 90° rotated polarization, while the number of pulses varies from 20 up to 3000 pulses. (b) Color-coded image of the optical retardance measurement (probe wavelength 587 nm) of the corresponding nanograting fields. The sign of the retardance accounts for the two distinct fast axis orientations (“±” − Ey ∕E x ) of the induced nanogratings.
R P Iq · q2 dq ∼ ϕ · 1 − ϕ was determined. For a diluted system, as empty pores in an isotropic glass matrix, P is directly proportional to the volume-filling fraction ϕ. It hence serves as a crucial quantity for the evolution of the total pore volume of nanogratings [6,15]. Since this analysis can be done for different azimuthal orientations relative to the laser polarization, the directions x and y (see Fig. 1) can be studied independently. For mapping the rewrite process, this means that features corresponding to the original and the newly formed grating can be mapped independently due to the anisotropic nature of nanogratings. Furthermore, due to the arrangement of small pores in periodic grating planes, distinct correlations show up, and the scattered intensity is modulated by the structure factor Sq corresponding to Iq Fq2 · Sq [16]. By removing the fitted form factor Fq of the small pores (Beaucage fit [7,17]), the structure factor (including characteristic features as, e.g., diffraction peaks) of the particular nanograting can be analyzed. To macroscopically evaluate fields of rewritten nanogratings, we performed optical retardance measurements [see Fig. 1(b)] using a commercial strain analyzer (Ilis StrainMatic M4/60.13, wavelength 587 nm). The colorcoded image indicates a strong change in retardance from about −50 nm to 40 nm after (single) rewriting the reference field with only 20 pulses per spot, while
Fig. 2. (a) SEM images of rewritten nanograting traces (top view) after polishing and etching. (b) Recorded SAXS pattern of nanogratings after single rewriting the reference field with 0, 20, and 3000 pulses per spot. (c) Porod invariant for single (1) and multiple (7, 15) rewritten nanogratings. The formation of the new grating (full symbol) as well as the erasure of the old grating (open symbols) are plotted. The gray shaded area marks the region of the reference field (“Ref.,” 1000 pulses per spot, E⃗ x ). The dashed line marks the difference in invariant for odd rewrites while a direct comparison with even (Ex ) rewrites is hindered.
May 1, 2015 / Vol. 40, No. 9 / OPTICS LETTERS
from both gratings compensate each other, and the optical retardance vanishes. For ongoing illumination, the old grating planes become blurred, while the new planes come up. A subsequent ordering mechanism can be observed. A nanoscopic picture of the erasure and formation of pores and cracks is gained by the SAXS measurements of the rewritten fields. The corresponding false-color plots of the scattering distribution in the detector plane are shown in Fig. 2(b). An elongated streak is visible, which marks the direction of the short axis of the elongated pores (laser polarization direction). A number of spots can be found superimposed on this streak for the initial pattern and the rewriting with larger pulse numbers. These spots mark the diffraction orders of the linear grating formed by the ordered nanopores. The scattering pattern strongly changes after already 20 pulses [see Fig. 2(b)]. First, the streak along the initial polarization direction shrinks, and the diffraction peaks close to the center disappear. This points to the erasure of both the shape and alignment of the nanoscopic features after a few laser pulses. This is confirmed by the strongly decreased invariant in Fig. 2(c). Furthermore, a streak along the new polarization direction shows up. It gets more distinctive with increasing number of pulses. The Porod invariant along the direction of the new grating (y) considerably increases in pace with the strong change in birefringence. Indeed, the invariant reflects the total number of scattering objects and correlates with the optical retardance [18]. However, the erasure of pores is not a complete process since some scattering features, illustrated by the invariant along the initial grating direction, remain and can be observed even up to 3000 pulses [see Figs. 2(a) and 2(c)]. The same holds true for multiple rewrites, as can be compared in Fig. 2(c) for the first and 7th (or 15th) rewriting process. After repeated rewriting, both the Porod invariant and the retardance tend to even become stronger. A direct comparison between even (E x ) and odd rewrite (E y ) is nevertheless hindered due to slight differences in optical retardance [see Fig. 3(a)] [19] and correspondingly a small shift (invariant) but with otherwise similar behavior. Looking closer, two regimes with a transition at about 100 pulses can be distinguished [Figs. 2(c) and 3(a)]. In the beginning of the nanograting evolution (for smaller number of pulses), the optical retardance is still small, in particular after multiple (14/15) rewrites as Fig. 3(a) shows. For larger pulse numbers, a strong increase in pores [see Fig. 2(c)] and correspondingly higher retardance is measured in particular after multiple rewrites. The SEM images give an explanation for this behavior, where during the first shots, the pronounced anisotropy of the old grating prevails, and newly formed pores are still irregularly shaped with less defined elongation. These early pores are nevertheless prominent and are seeding the further evolution. After long irradiation (≳100 pulses), the pores become elongated and better aligned. The relative enhancement of this process for repeated rewriting is caused by the repetitive annealing and quenching, which leads to a porous glass matrix promoting the formation of new inhomogeneities.
2051
Fig. 3. (a) Retardance, (b) grating period, and (c) intensity of the first- and second-order diffraction peak of rewritten nanograting fields. Please note that the error bars in (b) account for the diffraction peak width (1∕e of the maximum intensity). The gray shaded area marks the region of the reference field.
This is accompanied by the decrease in grating period and increase of the diffraction intensity of the grating period, as shown in Figs. 3(b) and 3(c) (exemplary for 1 and 15 rewrites). However, the diffraction intensity for repeated rewriting does not develop in the same way as the retardance. This is a consequence of a more strongly perturbed glass with more disordered pores preventing the grating regularity to improve. The optical retardance on the other side is more related to the absolute number and elongation of pores, which is promoted by repeated rewriting and directly mapped by the Porod invariant. The analysis of the rewrite process also provides details of the grating formation mechanism in general. Here, we observe that the initial steps consist of forming pores, which start to be elongated early, but not well aligned [20]. Average distances are probably more related to localization of heat deposition and strain interaction between pores. Later on, more (narrow) pores develop by splitting of initial pores and intrusion of new pores. This is the origin of period shrinkage toward a new equilibrium value, the latter of which might be determined stronger by nanoplasmonic excitations [8]. For applications as nanograting based optical encoding (as e.g., in some kind of storage application [21]), rewriting is a promising feature. However, care has to be taken to set the illumination parameters to update single data units, i.e., the usage of an intermediate regime of pulses incident can be beneficial to avoid a drift in optical retardance during ongoing rewrites. Moreover, the
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degradation of grating quality might influence the decoding of single data units. Considering numerous rewrites can lead to ambiguity of data bits. In summary, the phenomena of erasing and rewriting of ultrashort pulse-induced nanogratings were analyzed by using microscopic techniques as SAXS and SEM as well as macroscopic characterization tools as optical retardance measurements. We could show that, once a nanograting is formed, the rewriting process starts with few large and aligned pores, which subsequently increase in number. The increased overall porosity facilitates the reformation process, which is mapped by the strong retardance and small nanograting period in the beginning of the rewrite process. During ongoing rewrites, the glass matrix is repetitively heated and quenched, which promotes the formation of inhomogeneities. Thus, the performance in terms of the optical retardance of nanogratings improves for high pulse numbers. It is observed that with ongoing rewrites, the number of old remaining nanostructures, i.e., induced by the old polarization state, also increase which perturbs the grating quality but does not affect the overall retardance. Therefore, nanaograting voxels inscribed by optimized laser parameters exhibit great potential for data storage applications with multiple writing cycles. The work is supported by Deutsche Forschungsgemeinschaft (DFG) via priority program SPP 1327 (NO 462/5-2) and by the program “Research with Photons, Neutrons and Ions” by the Helmholtz Society. A. P. acknowledges the Heisenberg grant of the DFG. We thank N. Sergeev for recording the SEM images. We acknowledge beamtime at the Swiss Light Source (PSI, Villigen Ch) and excellent support by M. Guizar-Sicairos. We also appreciated discussions with A. Menzel. References 1. K. Itoh, W. Watanabe, S. Nolte, and C. B. Schaffer, MRS Bull. 31, 620 (2006).
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