1. Introduction

Periodic structures embedded inside transparent materials have a wide range of applications in integrated optics such as Bragg gratings, Fresnel lenses and photonic crystals [1,2]. Femtosecond (fs) laser has been proved to be a powerful tool for fabrication of embedded periodic structures inside transparent materials. When a single fs laser beam is tightly focused inside a transparent material, their high power density of more than 10 TW/cm² and their ultrashort pulse width will lead to high-order nonlinear absorption and induce microscopic modifications within the focal volume [3]. Compared with other traditional fabrication methods of periodic structures, the technique by use of fs laser beam has the following advantages. First, the nonlinear absorption nature confines the pulse energy to the focal volume with the surface unaffected, which makes it possible to fabricate geometrically complex structures in three dimensions. Second, fs laser micromachining can overcome broad bandgap due to the nonlinear absorption, enabling induced structures to be fabricated in a wide range of transparent materials. Moreover, the process is simple and time-saving. Using the fs laser direct writing technique, various periodic structures have been realized in different transparent materials in the past two decades. Usually, the fabrication methods for embedded periodic structures are either employing a multi-beam interference configuration or point-by-point or line-by-line laser scanning (Fig. 1) [4,5].

 

Fig. 1 schematic of the configuration for fabricating periodic structures in transparent materials: (a) point-by-point writing, (b) and (c) using multi-beam interference configuration.

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However, in recent decade, it has been discovered that single femtosecond laser beam can induce periodic structures inside transparent materials without multi-beam interference or point-by-point or line-by-line laser scanning. In this paper, we present recent research developments on single fs laser beam induced periodic structures, including nanovoid array [6,7], tilted grating structures [8] and nanograting [9,10]. We also discuss the formation mechanisms and promising applications of these periodic structures.

2. Femtosecond laser induced nanovoid array

Periodically aligned void structures in transparent materials are important structures for photonic crystals due to large refractive index difference Δn. However, it is difficult to form a periodic void array using the laser interference technique in transparent materials such as glass. In 2005, Kanehira et al. reported a novel method for fabricating periodic nanosized voids inside a glass sample using a single fs laser beam [6]. With this method, periodically aligned voids can be formed spontaneously with a period of micrometer length along the propagation direction of the laser beam.

A regeneratively amplified 800-nm Ti:sapphire laser that emits 120 fs, 1 kHz, mode-locked pulses was applied as the irradiation source. A commercially available borosilicate glass was used and the fs laser beam was focused inside it to a depth of 75 μm from the entrance surface. The formation of an aligned void structure inside a 0.9-mm-thick sample below the focal point along the propagation direction of the fs laser beam was clearly observed in the side-view optical microscope photograph. The formed voids assumed almost a spherical shape, and the successive voids are independent of each other. Moreover, at the focal point or around the voids, there are no microcracks or catastrophic collapses appeared. Interestingly, the resulting aligned void structure contains a periodic part without connected voids or cracks, which are located at a distance of ~90 μm from the bottom surface of the glass sample. The void size and intervoid separation in the periodic part are 380 nm and 1.7 μm, respectively. The length of the periodic part is independent of the pulse energy of the fs laser, whereas the number of voids is pulse dependent and it decreases with an increase in the pulse energy. In addition, the entire length of the periodic void structure gradually increases with increasing pulse numbers and exhibits a constant value of 130 ± 10 μm when the pulse number is increased above 125. This phenomenon is explained as follows. Under the action of fs laser pulse, there occurs induced refractive changes at the focal point, and then the light filament propagates toward the bottom surface of the glass sample. Once the core of the filament line is raised to a sufficiently high temperature, it gets highly absorbing. Then microexplosion takes place firstly around the bottom surface to create a nanosized void in regard to the lower threshold for dielectric breakdown at the surface. When the next fs laser pulse propagates from the focal point, it is trapped in the heated region around the void formed beforehand, resulting in the production of a new high temperature region and the formation of the next void. This process is repeated many times and the consecutive void formation is completed at the focal point. The pulse energy, pulse number, and focal point of the fs laser decide the size of the voids, the period, and the entire length of the void structure inside the glass sample. Thus, the application areas such as optoelectronics, 3D photonic crystals are greatly benefited with this method of easy fabrication of controllable periodic void structures.

We investigated the formation mechanism of the self-organized void arrays further [7]. The femtosecond laser pulses with the energy of 50 μJ were tightly focused into the fused silica glass at a depth of 200 μm beneath the sample surface. The self-assembled void array is formed at a location far away from the sample surface after irradiation with 32 pulses. The result is different from the results presented by Kanehira et al. that the contact of the structure with the bottom surface is the key point to induce the void arrays [6], and this result indicates the role of some other underlying formation mechanism of the void array. Both the nonlinear effects of fs laser pulses propagation and the spherical aberration effect at the interface are given consideration in the investigation of the formation mechanism of the void arrays because of the fact that only the high-NA objective lens could induce such regularly aligned void arrays. A nonparaxial nonlinear Schrodinger equation incorporating the effect of interface spherical aberration was utilized to perform the simulation of fluence distribution of the light field. The simulation result shows that the laser fluence around the Gaussian focal point presents quasi-periodic alternations between maximum and minimum. Since the electron density generated by multiphoton ionization is approximately proportional to |E|⁶, more electrons are generated at the local maximums than at the local minimums. Hence, the occurrence of a serial of microexplosions at the positions with the local maximum laser fluence is quite reasonable. Finally, the ablated materials are ejected to the surrounding volume and then a string of voids are left at the positions with the local maximums. In order to reveal whether nonlinear effects or interface spherical aberration effect is the dominating effect on the formation of the self-organized void arrays, the on-axis fluence distribution is compared between the cases where both the spherical aberration effect and the nonlinear effects are present and only the spherical aberration is present. It turns out that the peak-to-peak contrasts of the fluence in the case with both spherical aberration (SA) and nonlinear effects are much less than that in the case with only SA, as is understandable in regard to the role of multiphoton absorption and plasma absorption in reducing pulse. Based on the comparison above, we consider that the interface spherical aberration is the main reason for the self-formation of void arrays. It is promising to control the formation of the self-assembled void array by adjusting the amount of the spherical aberration.

3. Femtosecond laser induced tilted grating structures

Grating is an important optical element which has various applications in all-optical circuits. Traditionally, one way for fabricating a grating is, to employ a two-beam interference configuration [11], and an alternative way is to scan the focal spot of the laser beam line by line [12]. However, they are either technically complex or very time-consuming. Therefore, a more simple and time-saving method for fabricating grating is in great need. As has been described, self-organized micrograting induced by a near-infrared femtosecond laser has been observed in bulk SrTiO₃ crystal etc [8]. This provides a much more simple promising method for fabricating gratings since we can use laser line scanning to form grating structure.

Single beam fs laser pulses with the features 120-fs pulse width, 800 nm wavelength and 1kHz pulse repetition rate, which were delivered from a Ti: sapphire amplifier, were tightly focused into the bulk SrTiO₃ crystal by a high-numerical-aperture (N.A. = 0.9) microscope objective (100 × ). By translating the laser focus at depth of about 200 μm below the surface, a typical self-organized grating was induced in the plane determined by the translation direction and the light propagation direction. Except for a few nonparallel grooves at the first beginning, a well regular grating was self-formed with the grooves orientated nearly perpendicular to the laser propagation axis with scanning velocity set as 200 μm/s. The angle θ between scanning direction S and the groove orientation g (θ is positive when g situates on the left of S and negative otherwise) turns gradually from negative value to positive value with scanning velocities decreased from 300 μm/s to 50 μm/s. Similarly, by varying the laser repetition rate with scanning velocity unchanged, the induced self-assembled microgratings also manifest very distinct orientation angles θ. That means, by changing the irradiation pulse number per unit scanning length, the groove orientations of those gratings could be well controlled.

Based on two groups of experimental results described above and a typical void string reported in our previous work [13], we predict the appearance of a self-assembled grating formed by connection between voids in the adjacent two void strings. We have developed a void-moving model based on the experimental results presented by Watanabe et al. [14] to explain the evolution of groove orientation. When moving the laser focal point, for the formation of the second void string, new voids will be generated are most likely to occur at the upper corners of the voids in the first string because of the lower damage threshold caused by color center or defects at the interface between the voids and the SrTiO₃ substrate. The second void in the second string can be moved upward by the laser pulses to depart from the second void in the first string, the slower the scanning velocity is, the further the second void in the second string will be driven by more pulses. The movement of latter void strings finally leads to different groove orientations of the formed microgratings, which coincides well with the experimental phenomenon. It is possible to improve the quality of the orientation-controllable self-assembled microgratings by careful adjustment of the combination of the laser parameters and the focusing conditions. This method for fabrication of gratings has the advantages of being mask free, high efficiency, selectivity and localizability, and it has high potential for applications in fabricating large-area gratings inside transparent dielectrics.

4. Femtosecond laser induced polarization-dependent nanograting

Nanograting has a wide range of applications in micro- and nano-fluidics, porous capillaries for biofiltering and rewritable data storage etc. However, it is very difficult to engrave such a small structure in transparent materials. In 1999, Kazansky et al observed fs laser induced polarization-dependent light scattering in Ge-doped silica glass [15]. Soon after, Qiu et al. observed so-called memorized polarization-dependent emission in various glasses and crystals, and predicted permanent polarization-dependent micro-structure induced by fs laser [16]. In 2003, Shimotsuma et al. first demonstrated that nanograting structures can be induced inside fused silica by fs laser, which provides a promising technique for fabricating nanograting inside transparent materials [9].

The laser radiation produced by regenerative amplified mode-locked Ti:sapphire laser (150 fs pulse duration, 200 kHz repetition rate) in Gaussian mode, operating at a wavelength of 800 nm was focused via 100 × (N.A. = 0.95) microscope objective into the silica glass samples at ~100 μm below the surface . The Secondary electron (SE) images of the polished silica sample expose that the morphology of an irradiated sample in the examined cross section almost does not change, namely, a void does not exist. On the other hand, the BE images reveal a periodic structure of stripelike dark regions with low density of material and of ~20 nm width which are aligned perpendicular to the writing laser polarization direction. Auger spectra mapping of silicon and oxygen was carried out to test whether oxygen defects were formed in the regions corresponding to dark domains of the BE image. The results indicate that the periodic structure observed in the BE image consists of periodically distributed oxygen-deficient regions (SiO_2-x). The Auger signal intensity is proportional to the concentration of element constituting the surface, which gives an estimate to the value x ~0.4. They further observed the decrease of the grating period with an increase of the exposure time. The grating periods were about 240, 180, and 140 nm for the number of light pulses of 5 × 10⁴, 20 × 10⁴, and 80 × 10⁴, respectively, and for the pulse energy of 1 μJ. The role of pulse energy on the features of periodic nanostructures produced for a fixed exposure time was also investigated and an increase of the period with the pulse energy was observed. Grating periods of 180, 240, and 320 nm were measured at pulse energies of 1, 2, and 2.8 μJ, respectively, and for the number of light pulses of 20 × 10⁴. The explanation of the observed phenomenon is proposed as follows. Once a high free electron density is produced by multiphoton ionization, the material has the properties of plasma and absorbs the laser energy via one-photon absorption mechanism of inverse bremsstrahlung (joule) heating. The light absorption in the electron plasma causes the excitation of bulk electron plasma density waves. Such an electron plasma wave could couple with the incident light wave only if it propagates in the plane of light polarization. Initial coupling is produced by inhomogeneities induced by electrons moving in the plane of light polarization [15]. The periodic structure, created via a pattern of interference between the incident light field and the electric field of the bulk electron plasma wave, enhances the coupling. Consequently, there occurs a periodic modulation of the electron plasma concentration and the structural changes in glass. A positive gain coefficient for the plasma wave will lead to an exponential growth of the periodic structures oriented perpendicular to the light polarization, which become frozen within the material.

Recently, many groups have investigated the formation mechanisms of the self-organized nanograting. Bhardwaj et al. suggested a so-called nanoplasmonic model [17] to explain the nanograting-formation process, which considered that the order naturally evolves from a random distribution of nanoplasmas over many shots due to the memory mechanism and mode selection. Inspired by the idea proposed by Bhardwaj et al., Liang et al. proposed a model to explain the dependence of the width and spacing of grooves on the number of overlapped shots in both stationary and scanning cases, based on both the so-called nanoplasmonic model and the incubation effect [18]. They simulated the local intensity distribution with different shot number and considered that the precise shape of local intensity distribution (i.e. nanoplasmonic effect) together with the reduction of the ablation threshold (i.e. incubation effect) is responsible for nanograting formation. In particular, they exposed the key role played by local field side-maxima appearing along the laser polarization axis in triggering the nanograting formation.

The nanograting in silica glass induced by femtosecond laser irradiation resulted in uniaxial birefringence phenomenon, which has been referred as self-assembled form birefringence. Recently, Shimotsuma et al. demonstrated rewritable five-dimensional (5D) optical data storage using such structures [19]. For the recording, a single laser beam with the polarization direction controlled with a λ /2 wave plate is split into 100 beams of equal pulse energy (1.0 μJ) with the help of spatial light modulator based on liquid crystal- on-silicon technology. The “world map” within a 3.4 × 1.8 mm square was prepared inside fused silica while each data dots were irradiated with two femtosecond beams simultaneously. Reading of the recorded information was realized with the polarization optical microscope. Adjusting the time delay between the orthogonally polarized double pulses can result in the difference in the direction of the slow axis of birefringence, which is always perpendicular to the writing light polarization, and also result in the difference in the retardance. As a result, with a nearly monochromatic circularly polarized light illuminating the sample with the “world map”, the “pointillism” composed of multi-colored dots corresponding to different slow axis orientation of the form birefringence was clearly observed. Simultaneously the phase retardation can be also read out. Therefore, the rewritable optical data storage in “five dimensions” (3 coordinates, the slow axis direction, and retardance) with a 300 Gbit cm⁻³ capacity corresponding to 10 times of the usual 12 cm BlueRay disk capacity was realized. This technique, being simple and easily available, has the advantage over other types of optical memory, e.g., holographic memory, fluorescence 3D memory, and spectra hole-burning (SHB) memory.

Besides the rotated nanograting structures in the plane perpendicular to the incident light propagation direction, we also observed rotated self-organized nanograting structures along the light propagation direction [10]. A series of lines were written with different laser polarization directions in fused silica glass with 250 kHz femtosecond laser irradiation. After irradiation, the glass sample was mechanically polished to expose the transversal cross section of each line, and soaked in 1 mol% hydrofluoric (HF) aqueous solution to be etched for 2 min. In the classical image of birefringence of fs laser written lines in fused silica, we observed an intensity variation between two lines with opposite scanning directions in each group. We attribute it to the non-reciprocal writing effect of fs pulse laser in isotropic medium, which is also called “Quill Pen Effect” caused by the correlation between the beam scanning and the pulse front tilt (PFT) of the fs laser. In SEM images, we observed etched nanograting patterns in both directions, perpendicular to and along the incident light propagation direction. Meanwhile, the direction of the nanograting patterns rotated with changing the laser polarization direction. In fact, as far as the two rotations are concerned, the etched pattern can be taken as the projection on two mutual orthogonal planes for a rotated 3D periodic nanostructure. We think an additional electric oscillation along the light propagation direction possibly worked on the trapping electron plasma. As the laser beam used here was close to being Gaussian spatial profile and focused into the glass with the normal direction, the most possible reason causing an electric field vector along the light propagation direction is the PFT, which commonly suffers from spatiotemporal distortions due to group-velocity dispersion when fs pulses propagate in dispersive medium, e.g., transparent dielectrics or microscope objective, and moreover, the PFT magnitude will be significantly increased in the focus area [20]. Therefore, we can resolve the incident electric vector E into two orthogonal sub-vectors, E// and E⊥. Thus, the rotation of the formed nanograting in x-z plane will depend mainly on E//, while the other rotation in x-y plane is dominated by E⊥. We can simultaneously control two orthogonal rotations by varying polarization direction of single laser beam. This process implies that a 3D interference mechanism between the electron plasma and the incident electric field plays a substantial role in writing and rotating the self-organized nanograting in fused silica.

Moreover, we found that moving the laser focal point along the laser propagation direction, polarization-dependent nanogratings can also be induced in fused silica by an 800 nm, 1 kHz fs laser. The 150 fs pulses were focused into a fused silica sample at ~200 μm below the surface, and then the laser focal point was pulled toward the surface along the laser propagation direction at a constant speed of 5 μm/s, and stopped at 50 μm from the surface. Optical image obtained between crossed polarizers in Fig. 2(b) shows that regions irradiated with different laser polarizations exhibit varied birefringence signals. And SEM images in Fig. 2(c) show signs of the formation of polarization-dependent nanogratings. The result may provide some new information on the investigation of nanograting formation mechanism.

 

Fig. 2 Optical images of the regions irradiated with different laser polarizations in bright-field (a) and between crossed polarizers (b), SEM images of the same irradiated regions as in (a) after polishing and etching (c).

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5. Conclusion

In this article, we have reviewed several kinds of single fs laser beam induced periodic structures in different transparent materials, including nanovoid array, tilted grating structures and polarization-dependent nanograting. All of these induced periodic structures with three-dimensional controllability and high processing precision can be achieved in a simple and time-saving way. Based on these research results, we are convinced that fs laser induced periodic structures will find more promising applications in integrated optics. Fs laser can induce various nonlinear effects and cause composite reactions, finally resulting in complicated phenomena. By using pulse shaping, we expect to realize precise control of the fs laser induced micro-structures, and find important applications of such composite structures.