1. Introduction

Ultrafast laser processing of transparent materials has attracted considerable interest over the past two decades due to its unique versatility to realize submicron three-dimensional (3D) modifications [1–3]. Among all femtosecond laser induced modifications, self-assembled periodic micro-nano structures (PMNSs) formed by laser direct writing in transparent materials, including nanogratings (NGs) and void array [4,5], are especially fascinating for their great potential in optics. In particular, the NGs whose periods are smaller than the laser wavelengths, exhibiting form birefringence and rewritable feature [6,7], have been harnessed for various applications including micro/nanofluidic devices [8,9], 5D optical data storage [10,11], polarization sensitive optical elements [12–14], operation of optomechanical devices [15], and so forth. During the recent decade, extensive efforts have been carried out in the investigation of NGs formation in various materials by different research groups [16–23]. However, in-volume polarization dependent NGs have only been observed in several kinds of glass and a few indirect band gap semiconductors up to now [4,16–22], which are unlike the formation of surface NGs that were observed virtually on any type of materials including amorphous [24], crystalline dielectrics [25], semiconductors [26], and metals [27].

As is known, optical dielectric crystals are one of the two types of widely used optical materials in modern optics other than glasses. To name a few, electrooptic crystals are widely used for the modulation of light phase, energy and polarizations [28], nonlinear crystals can extend tunable range of laser wavelength by optical frequency conversion [29], and laser crystals are the favorite gain media for solid-state laser systems due to their lower lasing thresholds than glasses [30]. Therefore, numerous nanograting-based applications can be anticipated if ultrafast direct writing of self-assembled NGs can be realized in crystals. Nevertheless, except the observations of periodic nanoplanes in sapphire [31] and periodic nanovoids in TeO₂ crystal [32], few investigations on NGs formation inside transparent crystals have been conducted.

Quartz crystal is a type of silica materials with crystal structure composed of a continuous framework of silicon–oxygen tetrahedral [SiO₄], the composition of which is identical to fused silica glass—the reported best material for in-volume NGs formation. In this letter, we demonstrate that it is feasible to induce self-organized in-volume NGs in quartz crystal with femtosecond pulses operating at 500 kHz. The transition from nanovoids and nanogrooves to NGs with increasing pulse energy is observed. More interestingly, besides the periodicity in the laser polarization direction, PMNSs with equal spacing close to the laser wavelength in the laser scan direction and laser propagation direction are also observed. The three-dimensional periodic micro-nano structures are distinctive to the reported structures usually formed in glass and other crystals.

2. Experiments

The quartz crystal used in this work is a commercial α-SiO₂ crystal sample with the size of 10 × 10 × 2 mm³. The two 10 mm × 10 mm faces were cut along the C plane (0001) and well-polished before laser processing. Two laser systems, including a commercial femtosecond Yb-fiber laser system (FLCPA-02USCT11, Calmar Laser, Inc.) that delivers laser pulses of 370 fs, 1030 nm at a repetition rate of 500 kHz, and a regeneratively amplified mode-locked Ti:sapphire laser system that delivers 800 nm, ∼150 fs laser pulses of 1 kHz repetition rate, were employed for processing. The sample was mounted onto a computer-controlled XYZ translation stage with XY resolution of 100 nm (the writing direction is as along the X axis, and the laser propagation direction is along the Z axis). The laser pulse energy is controlled by a combination of a Glan polarizer and a half-wave plate, and the laser polarization can be changed by another half-wave plate. The consequent linearly polarized laser beam was focused into the samples at normal incidence at a depth of 80 μm by a 50 × microscope objective with a numerical aperture (NA) of 0.55 (beam waist radius ω₀ ≈1.14 μm). In this configuration, the optical axis of the quartz sample was always along the Z axis, i.e. parallel to the laser propagation direction.

A series of lines were written inside the sample with div erse laser parameters by translating the sample perpendicularly to the Z axis (in XY plane). Optical microscope images were captured with a CCD attached to a computer using back illumination after the entire laser writing process. Raman spectra were obtained with a confocal Raman microscope (Renishaw inVia) after mechanically polishing the sample to the approximate depth of the beam waist location in XY plane. For scanning electron microscopy (SEM) observation, the samples were further etched in 1 mol/L hydrofluoric (HF) solution for 6 hours at room temperature, which is much longer than the etching process in fused silica with the same HF concentration [7], to obtain high-contrast imaging.

3. Results and discussion

Firstly, the influence of pulse energy on the structural evolution of laser modified regions was investigated with 500 kHz laser system. A series lines were written with pulse energy ranging from 0.8 μJ to 1.6 μJ for three polarization orientations relative to the laser scan direction (0°, 45° and 90°). The writing speed was set as 50 μm/s, equal to a pulse density 10⁴ pulses per micrometer. After polishing the sample to the approximate depth of the beam waist location, the morphologies of the modified regions in XY plane were inspected by SEM before etching. However, the modified regions were nearly undistinguishable to the unirradiated area under SEM observation, which indicates that no obvious holes or cracks were formed. To analyze the features of the underlying modifications, the polished sample was etched in HF solution for 6 h. Figure 1 shows the top view SEM images of the modified regions after etching. For pulse energy of 0.8 μJ, the region modified with 0° polarization is mainly covered by isolated nanovoids with an average size of ~150 nm diameter, as shown in Fig. 1(a), while the regions modified with polarization angled 45° and 90° to scan direction are covered by both nanovoids and nanogrooves. The average width of the nanogrooves is less than 100 nm. The observed nanovoids and nanogrooves indicate selective etching in laser modified region, which reflects the modified material density and distribution of defects [3]. It can be seen that the nanogrooves tend to orient perpendicularly to polarization direction, which is similar to the behavior of NGs in fused silica [3]. When pulse energy is increased to 1.2 μJ, the area covered by nanovoids and nanogrooves considerably extended, as depicted in Fig. 1(b). However, the dependence of the structures on laser polarization direction is still not explicit. When the pulse energy was further increased to 1.6 μJ, more new features appeared in the modified regions, as demonstrated in Fig. 1(c). Most notably, the areas between the nanogrooves have developed into periodic nanoripples i.e. NGs whose orientations are perpendicular to laser polarization direction for all the three polarizations. The period of the NGs (Λ_E) is around 400 nm (~λ/2.6), which is obviously smaller than the central wavelength (λ = 1030 nm) of the laser. The incomplete structures and big cracks in Fig. 1(c) might be due to higher pulse energy induced material dissociation and stress concentration that are more vulnerable to HF solution.

 

Fig. 1 SEM images (top view) of lines inscribed by femtosecond pulses with various pulse energies: (a) 0.8 μJ, (b) 1.2 μJ, (c) 1.6 μJ after being etched with HF acid. The polarization direction of incident laser beam is indicated by blue arrows on the top of each column while the red arrow indicates the writing direction. Processing conditions: the scan speed is 50 μm/s; the pulse duration is 370 fs, the repetition rate is 500 kHz. The optical axis of the crystal sample is parallel to the laser propagation direction.

Download Full Size | PDF

To further analyze the morphology of the lines written with 1.6 μJ at different depth, the SEM images of different layers in the XY plane separated by 3 μm along laser propagation direction were obtained, as shown in Fig. 2. Clearly, critical features of laser induced NGs, orienting perpendicularly to laser polarization direction, are revealed after etching in several top layers. The regularity of the microstructures is better when polarization is perpendicular to writing direction, which is in accordance with previous observations in fused silica [33]. The bottom layers show similar orientation of nanogrooves as observed in the layers modified by 1.2 μJ (see Fig. 1(b)), which reveals a decreased light intensity at the bottom. More interestingly, there is another periodic structure with period Λ_S appearing along the scan direction on top layers of modified regions for the three polarizations, which has not been observed in the volume of transparent materials previously. The value of Λ_S is around 1 μm, which is very close to the λ of laser and does not change with different scan speeds (no difference between 50 μm/s and 100 μm/s in this work). In addition, we note that the period Λ_S did not appear when pulse energy is less than 1.6 μJ, which indicates a relatively high energy threshold on the formation of this periodicity.

 

Fig. 2 SEM images (top view) of 5 rows of lines inscribed by different polarized pulses with pulse energy fixed at 1.6 μJ, the adjacent rows are separated by 3 μm along laser propagation direction. The schematic on the left illustrates the cross section of laser modified zone. The wave vector k indicates the laser propagation direction. The incident laser polarization direction for each column is indicated (black arrows). The images in each column show the features of laser modified regions on different horizontal planes after polishing followed by 1 vol. % HF etching for 6 h.

Download Full Size | PDF

To obtain a 3D view of the modified region, cross sections of the lines were also examined by SEM after etching. Figure 3 shows the SEM images of the cross sections of the lines inscribed with pulse energy ranging from 0.8 μJ to 1.6 μJ in two cases: polarization parallel (E∥S) and perpendicular (E⊥S) to scan direction. In the case of E∥S (Fig. 3(a)), no NGs related features could be identified because they orient perpendicularly to scan direction and thus the periodicity is closed in the exposed plane. However, sub-wavelength structures with period Λ_E can be observed in the case of E⊥S (Fig. 3(b)), which indicates the transition of random nanogrooves to polarization dependent NGs with increasing pulse energy. More notably, there is another periodic structure with explicit period Λ_k along laser propagation direction appearing on the cross section when pulse energy is increased to 1.2 μJ and above. The spacing of Λ_k is a constant of ~1 μm, which is also closes to the λ and stays unchanged under the pulse energy between 1.2 and 1.6 μJ. Moreover, the structure nearly covers the whole cross section without variation on the period along the laser propagation direction. Note that the periodicity is several orders larger than the adjacent interplanar spacings (less than 1 nm) of quartz crystal, and it appears only when the pulse energy is higher the threshold (1.2 uJ), thus the observed structure change does not reflect the effect of the periodicity of crystal structures.

 

Fig. 3 SEM images of cross sections of the lines inscribed with different pulse energy in two polarization cases: (a) polarization parallel (E∥S) and (b) perpendicular (E⊥S) to the scan direction. The period Λ_k along laser propagation direction is observed with pulse energy ≥1.2 μJ when polarization direction is perpendicular to scan direction.

Download Full Size | PDF

The structural change in the laser modified regions was characterized by Raman spectroscopy. The Raman spectra of the SiO₂ crystal matrix and the surface of modified regions by 500 kHz pulses after polishing (before etching) were shown in Fig. 4. The Raman bands centered at 208.5 cm⁻¹, 354 cm⁻¹ and 465 cm⁻¹ can be assigned to symmetric A₁ vibrations; the bands centered at 128 cm⁻¹, 695 cm⁻¹ and 1160 cm⁻¹ are due to the degenerate E modes; and the band centered at 393 cm⁻¹ is originated from the nondegenerate E modes of ɑ-quartz [34]. Although the Raman spectra of the matrix and modified regions exhibit Raman peaks at the same position, two notable changes are identified. Firstly, the intensity of the peaks in modified regions decreased by roughly 33% in comparison with that of the matrix, and the relative Raman intensities of the Raman peaks have been obviously changed after irradiation (see the inset in Fig. 4(a)). Secondly, the widths of the Raman peaks have all broadened after irradiation (Figs. 4(b)-4(e)). These changes in the Raman spectra indicate distortion of crystalline structure in the modified region by laser irradiation. The distortion could be due to defect formation, bond breaking, amorphization or material density variation, which guides us to further achieve selective etching by HF acid so that the features of the microstructures induced by laser would be exposed, as shown in Fig. 1 and Fig. 2, though SEM examination cannot distinguish the difference between irradiated and unirradiated areas before etching.

 

Fig. 4 (a) Raman spectra of the quartz sample on unmodified region (matrix) and exposed region irradiated by 500 kHz pulses with polarization parallel (0 deg) and perpendicular (90 deg) to scan direction (Inset shows the relative intensity of the two typical peaks 128.5 cm⁻¹,208.5 cm⁻¹ normalized at 465 cm⁻¹), (b) the FWHMs of the three Raman peaks centered at 128.5 cm⁻¹,208.5 cm⁻¹, and 465 cm⁻¹ at different position, and (c, d, e) the experimental data and Voigt fitting of three Raman peaks in the irradiated area to determine the FWHM. The pulse energy and scanning speed are 1.6 μJ and 50 μm/s, respectively.

Download Full Size | PDF

Further investigation on the formation of PMNSs in quartz crystal was carried out by 1 kHz femtosecond laser (800 nm @ 150 fs) for comparison. However, no PMNSs can be induced with reasonable combination of experimental conditions (pulse energy ranges from 0.1 to 2 μJ, and scan speed ranges from 0.5 to 50 μm/s) until big cracks appeared, as shown in Fig. 5(a). In addition, pulse density as high as 10000 pulses/μm was also applied for 1 kHz laser by irradiating one spot for 10 seconds (static irradiation), but only more random big cracks appeared in the irradiated spot and no periodic structures could be observed. Generally speaking, the 500 kHz pulses with longer wavelength and pulse duration are more in favor of heat accumulation [35], which is harmful to the integrity and properties of NGs formed in glass [36], compared to the 1 kHz pulses. However, here in the quartz crystal, the heat accumulation effect seems helpful to the formation of NGs, which is significantly different to that in fused silica. In addition, the optical microscope images in Figs. 5(b) and 5(c) clearly show that the edges of the lines inscribed by 1 kHz pulses are covered by small cracks while those inscribed by 500 kHz pulses are much smoother. The difference in morphology also reflects the influence of the heat accumulation effect. According to the previous publication [35], 500 kHz pulses (520 nJ, 0.65 NA focusing lens) can create a high temperature field with average temperature reaching to 3000 °C around the laser focus (diameter of ~4 μm) after several tens of pulses in borosilicate glass. Such a high temperature field cannot be created with 1 kHz femtosecond pulses providing other conditions unchanged. Here in this work, higher pulse energy (~1.6 μJ) and higher pulse density (10⁴ pulse/μm) were employed under similar focusing conditions (0.55 NA objective lens) for 500 kHz laser pulses compared to the previous publication. Therefore, a similar high temperature field is likely created in quartz during irradiation, which may lead to a melting process around the focal region as the melting temperature of quartz is only 1750 °C. A tentative explanation could be that the heat accumulation is conducive to the increase of freedom and amplitude of thermal motion of atoms in crystals, which is beneficial for the ions migration or rearrangement of the lattice structure in the focal region towards the formation of NGs. In other words, the heat accumulation effect plays a positive role at the early stage during the formation of NGs in crystals. However, whether the heat accumulation effect will further break the structure of the as-formed NGs, as observed in glass, requires further verification with more systematic experiments.

 

Fig. 5 (a) SEM images (top view) of the lines inscribed by 1 kHz femtosecond pulses with pulse energy of 1 μJ (the left three images) and 1.8 μJ (the right image) after being etched by hydrofluoric acid. (b) and (c) are optical microscope images (top view) of lines inscribed by 1 kHz femtosecond pulses and 500 kHz femtosecond pulses, respectively. The laser pulse energy is 1 μJ, and the scan speed is 5μm/s for 1 kHz and 50 μm/s for 500 kHz. The laser polarization direction is indicated by blue arrows.

Download Full Size | PDF

For the explanation of the formation mechanism of NGs in glass, physical models such as interference of bulk electron plasma waves with the incident light [4], nanoplasmonics [37], and interference and self-trapping of exciton-polaritons [38,39], have been proposed. Here in the quartz crystal, the formation of the NGs with period of Λ_E may originate from the same mechanism because they show the same behavior as their counterparts in glass. As for the interpretation of the formation of the other two PMNSs with periods of Λ_S and Λ_k, further systematic experiments and calculations are required. In fused silica, a second periodicity in the propagation direction (Λ_k) was also observed previously [40], which is explained as a result of interference of propagating exciton-polaritons [39]. Cao et al. also observed a quasi-periodic structure with equal period around λ/n along the laser propagation direction in lithium niobium silicate glass [41]. In this work, however, there are some new features of the periodic structure with period Λ_k observed in quartz as compared to the previous observations. Firstly, the value of Λ_k in this work is a constant which is close to λ, while the one in fused silica initiates from λ/n (n is the refractive index) and increases along laser propagation direction. Secondly, the periodic structure almost covers the whole modified region in quartz, while the counterpart in fused silica and lithium niobium silicate glass mainly located in the head of the laser track. Thirdly, the periodic structure with period Λ_k has only been observed when laser polarization is perpendicular to writing direction. Therefore, it is not enough to explain the formation of such peculiar PMNSs by simply adopting the model of interference of propagating exciton-polaritons. Further efforts will be carried out on the interpretation and manipulation of the as observed PMNSs in different directions for potential optical applications.

4. Conclusions

In conclusion, in-volume self-assembled three-dimensional PMNSs can be induced in quartz crystal by tightly focused femtosecond laser pulses operating at 500 kHz. With effective pulse energy, three different types of periodic structures, the first one with indicative period (Λ_E) in the direction of the laser beam polarization, the second period (Λ_S) in the scan direction, and the third period (Λ_k) in the laser propagation direction can be formed in modified regions. The PMNSs are better organized at ~1.6 μJ. The two periods, Λ_S and Λ_k, are a constant of ~1 μm which is close to λ. The PMNS with period Λ_k covers the whole cross section of modified region. The results may be helpful for the understanding of formation mechanism of PMNSs inside crystals and lead to new applications of crystalline materials based on the manipulation of the self-assembled PMNSs.