Curved nanostructures are formed on the lithium niobate surfaces after irradiation with linearly polarized femtosecond laser pulses. It is shown that the curvature of nanostructures critically depends on the overlapping of two successive pulses, which can be controlled by changing the scanning speed or scanning direction of the laser. Electrical field simulation using the finite-difference time-domain (FDTD) method indicates that the electric field is locally enhanced at the crater edge when a focused pulse propagates through an elliptical crater produced by the previous pulse, which is responsible for the formation of the curved nanostructures. From the experimental and simulation results, the formation mechanism of the curved nanostructures is presented.
© 2017 Optical Society of America
Lithium niobate (LiNbO3, LN) is a versatile optical material because of the excellent electro-optic, piezoelectric, acousto-optic, nonlinear optical, and photorefractive properties . LN has been exploited for numerous applications, such as optical waveguides [2–5], modulators [6,7], frequency converters , photonic crystals , and diffraction gratings [5,9]. Therefore, it is substantial to develop a controlled fabrication procedure of micro/nanostructures from this material. Femtosecond (fs) laser has been reported to be an ideal tool for micro/nanomachining [10–12], various controlled micro/nanostructures on LN surfaces generated by fs laser processing have been studied to date [13–16]. Ripples with a width of 100 nm and a period of roughly 200 nm, oriented perpendicular to the incident laser polarization, were created by using 800 nm fs laser pulses with a pulse duration of 50 fs . Recently, ripples oriented parallel to the laser polarization with a period of approximately 200 nm as well as ripples oriented perpendicular to the laser polarization with a period of about 190 nm were generated using a tightly focused sub-15 fs pulsed laser with a high repetition rate . Additionally, nanorod-shaped craters were observed in periphery of the ablated area after irradiation by laser pulses of 80 fs duration . Sub-100 nm surface holes were formed by 150 fs laser pulses . However, the exact mechanism behind the fabrication of various nanostructures via fs laser irradiations has not been clearly ascertained by now.
In this paper, we investigate the formation of curved nanostructures irradiated with fs laser pulses. The curvature of nanostructures can be controlled by varying the laser scanning speed or laser scanning direction and the experimental results are explained by FDTD simulation method and a comprehensive analysis based on the overlapping of two successive pulses affecting the formation of curved nanostructures is provided in order to reveal the mechanism behind the formation of curved nanostructures.
2. Experimental setup
A regeneratively amplified Ti:sapphire laser (Spectra Physics, Inc.) with a central wavelength of 800 nm, a pulse duration of ~50 fs, and a repetition rate of 1 kHz was used for the irradiation experiment. A half wave plate and a polarizer were used to control the laser fluence incident on the samples. After passing a dichroic mirror, the laser beam was focused by a microscope objective (NA = 0.45, 20 × ) onto the sample mounted on a computer-controlled 6-axis translation stage (M-840.5DG, PI, Inc.) and the fabrication process was monitored by a charge-coupled device (CCD) camera placed behind the dichroic mirror. The fabricated structures were studied via using a scanning electron microscope (SEM) and an atomic force microscope (AFM). The samples used in this study were nominally pure (optics-grade) congruent LN plates (Hefei Kejing Materials Technology Co., Ltd.) with the dimensions 10 mm × 10 mm × 1 mm in x-cut orientation.
3. Results and discussion
3.1 Formation of curved nanostructures
Figure 1(a) depicts the impact of scanning speed and laser fluence on the surface morphology of the LN with scanning direction parallel to the polarization. It can be noted that only with suitable combinations of scanning speed and laser fluence can curved nanostructures be formed. When the scanning speed is appropriate, low laser fluence will result in the formation of surface modifications, while ablation is prevented. Moreover, when the laser fluence is proper, the pulse overlap can be controlled by varying the laser scanning speed, leading to the formation of different surface structures. The evolution of surface morphology with increasing scanning speed is demonstrated in Figs. 1(b)‒1(e) for ablation at fixed fluence of 0.84 J/cm2, which is slightly higher than the reported single pulse ablation threshold (~0.63 J/cm2 ). It can be noted that the ripples with perpendicular orientation to the incident laser polarization, are generated with scanning speed of 50 µm/s [Fig. 1(b)]. Moreover, as the scanning speed increases to 100 µm/s, irregular structures are generated [Fig. 1(c)]. Curved nanostructures are formed as the scanning speed further increases to 500 µm/s [Fig. 1(d)] and a series of ablation craters is created with a fast scanning speed of 1000 µm/s [Fig. 1(e)].
3.2 Formation mechanism of curved nanostructures
To find out the physical origin for the formation of curved nanostructures, the corresponding electric field distribution was obtained by simulating the propagation of a focused pulse through a crater, which was pre-defined by the previous pulse. A commercial-grade simulator (Lumerical) based on the FDTD method was used to perform the numerical calculations of the electric field distribution. In the numerical simulations, the laser source was estimated as a linearly polarized Gaussian beam with a wavelength of 800 nm. LN is a negatively birefringent uniaxial crystal with an ordinary, no, and extraordinary, ne, refractive index for 800 nm wavelength being no = 2.25 and ne = 2.17, respectively [15,18]. Light propagating in LN experiences a refractive index which depends on its polarization and propagation directions . In calculation the light was incoming along the x-axis to the LN surface with a polarisation in the yz plane, so a refractive index of no was used for the component of polarisation projected onto the y-axis and a refractive index of ne was used for the component of polarisation projected onto the z-axis. It is worth noting that the theoretical analysis was only a semi-quantitative one, without considering the changes in the refractive index induced by an applied external electric field. In the experiments, a series of ablation craters were formed at different laser fluence ranging from 0.68 J/cm2 to 0.95 J/cm2 with a scanning speed of 2000 µm/s. The AFM studies reveal that these craters have elliptical shapes with a maximum depth at the center [Fig. 2(b), inset]. Therefore, the eccentricity () was f the ellipse from circular. The eccentricity of the craters shows weak dependence on the laser fluence, but the depth of the craters increases with increasing the laser fluence, as shown in Fig. 2(b). Thus, the crater generated by the previous pulse is modeled as a hemi-ellipsoidal in the numerical simulations. The geometry of the hemi-ellipsoidal crater can be described by three parameters a, b, and h, which denote the semi-major axis, semi-minor axis, and depth, respectively, as depicted in Fig. 2(a). Based on experimental observations, we fix a, b, and h to be 730, 610 and 70 nm. The pulse-to-pulse spacing d (distance between two successive pulses) is found to be a crucial parameter for the formation of curved nanostructures. At a laser scanning speed of 1000 µm/s, which nominally corresponds to 1000 nm pulse-to-pulse spacing at a 1 kHz repetition rate, the electric field is less affected by the existing crater due to the wider spacing [Fig. 2(c)]. Accordingly, a series of ablation craters are produced in this case [Fig. 2(d)]. As the pulse-to-pulse spacing is decreased to 700 nm, the electric field would be enhanced at the crater edge [Fig. 2(e)], producing the slightly curved nanostructure [Fig. 2(f)]. With further decrease of pulse-to-pulse spacing, the strongest electric field appears at the crater edge [Fig. 2(g)], producing the highly curved nanostructure [Fig. 2(h)]. Therefore, it can be concluded that the curvature of curved nanostructures is strongly dependent on the degree of electric field enhancement at the crater edge that can be determined by the pulse-to-pulse spacing (or equivalently, the scanning speed). In addition, the period of the curved nanostructures decreases from 670 to 490 nm as the pulse-to-pulse spacing decreases from 700 to 500 nm, which indicates to a very strong dependence on pulse-to-pulse spacing.
3.3 Dependence of curved nanostructures on the angle (α) between scanning direction and the polarization direction and potential mechanism
The angle between scanning direction and the polarization direction (that is, α) was modified by varying the scanning direction, while the polarization direction was kept constant in all of the experiments, as shown in Fig. 3(a). Figure 3(b) illustrates that the curvature radius of the curved nanostructures depends most strongly on α. The edge of curved nanostructure is best approximated by a circular arc, indicated by white dashed line in Fig. 3(c). Since the circular arc is part of a circle, it has a constant value for the curvature. Therefore, the curvature radius can be taken as the radius of the circular arc. The nanostructures with various shapes are generated at α ranging from 0° to 90°, as shown in Figs. 3(c)-3(i). The curved nanostructures are formed at α ranging from 0° to 75°, with the curvature radius which is in direct relation with α, as shown in Fig. 3(b). The curve of nanostructures tends to gentle with the increase of α and becomes almost straight at α = 75°. The highly curved nanostructures are formed at α in 0° to 30° range. At α = 45°, 60° and 75°, the slightly curved nanostructures are formed and irregularly shaped nanocraters are generated at α = 90°. It is important to note that the nanostructures are sharply curved when the scanning direction is parallel to the laser polarization. Figures 3(j)–3(p) display the results of numerical simulations accompanied by images of electric field distributions calculated by the FDTD method. In this part of the simulation, the focus position of the laser light is varied by using different α and input parameters for the simulation are as follows: a = 600 nm, b = 475 nm, h = 40 nm, and d = 400 nm. The strongest electric field appears at the edge of elliptical crater and expands along the scanning direction gradually with the increase of α, thus producing the nanostructures of varying shapes. It should be emphasized that the electric field is significantly enhanced only along the edge of elliptical crater at α = 0° [Fig. 3(j)], and in the enhanced field region, the curvature of the elliptical crater edge is minimal in this case, leading to the formation of highly curved nanostructures. At α = 90°, the strongest electric field not only appears at the edge of elliptical crater, but also appears near the edge along the scanning direction [Fig. 3(p)], and the curvature of the elliptical crater edge is maximal in the enhanced field region, leading to the formation of irregularly shaped nanocraters.
Based on the observations in Fig. 3, the formation of curved nanostructures can be understood using the following mechanism: firstly, a femtosecond pulse is absorbed, resulting in an elliptical crater formation. As the next pulse comes, the electric field is enhanced at the crater edge, due to the appropriate pulse-to-pulse spacing, leading to a curved nanostructure. The curvature of nanostructure depends on the degree of electric field enhancement and the profile of the elliptical crater in the enhanced field region. Subsequent laser pulses strengthen the nanostructure, producing the clear nanostructure.
3.4 Dependence of structures on α in double scanning mode
Double scanning mode was used, in order to elucidate the effect of the scanning direction on the fabricated nanostructures. Laser scanning was performed two times along the same isolation line with a fast scanning speed and with the same polarization direction. The elliptical ablation craters were formed on the crystal surface at the first scanning procedure. The second time scanning was carried out when re-positioning the beginning of the first time scanning. However, the laser spots of both scanning were not completely overlapped, due to parameters such as the backlash or vibration of the translation stage. For one laser spot site, the first time pulse forms the elliptical crater, the spot of second time pulse, then overlaps at different locations of the crater, leading to the formation of nanostructures. The shape of nanostructures depends on the overlapping location, i.e., the crater edge [Fig. 4] or the central of crater [Fig. 5]. In both cases, the surface structures were written at laser fluences ranging from 0.73 J/cm2 to 0.89 J/cm2 and with scanning direction either perpendicular or parallel to laser polarization. It is worthwhile remarking that the polarization direction was kept constant. At the laser scanning speed of 2000 µm/s, the curved nanostructures were formed when the second time irradiation was performed at the edge of craters, as shown in Fig. 4. Figure 4(a) shows the curvature radius of curved nanostructure edges for two scanning directions. The generated curved nanostructures for two scanning directions at laser fluence of 0.84 J/cm2 are shown in Fig. 4(b) and Fig. 4(d), respectively. One of the curved nanostructure edges is profiled with white dashed lines in Figs. 4(c) and 4(e). Using suitable combinations of laser fluence and scanning speed results in the overlap of the second time pulse spots almost with the central of craters previously produced by the spots of first time pulse, generating symmetric curved structures, as shown in Fig. 5. Figure 5(a) shows the curvature radius of symmetric curved structure edges for two scanning direction. The generated symmetric curved structures for two scanning directions at laser fluence of 0.84 J/cm2 are shown in Fig. 5(b) and Fig. 5(d), respectively. One of the symmetric curved structure edges is profiled with white dashed lines in Figs. 5(c) and 5(e). The edges of both structures have smaller curvature radiuses when the scanning direction is parallel to the laser polarization compared to the case where the motion is perpendicular to the laser polarization, as shown in Fig. 4 and Fig. 5. Accordingly, highly curved nanostructures are produced with the scanning direction parallel to the laser polarization, which is consistent with experimental results in Fig. 3.
3.5 Dependence of curved structures on scanning speed and scanning direction
Scanning speed and scanning direction both affect the formation of curved nanostructures. The evolution of the surface nanostructures with various scanning speeds and scanning directions at laser fluence of 0.73 J/cm2 is showed in Fig. 6. Due to the relative displacements between the laser spot and the sample, the scanning speed gradually increases from 400 to about 750 µm/s and then decreases to 400 µm/s, while keeping the transverse speed constant at 400 µm/s. Subsequently, the sample moves in the opposite direction with the same scanning speed. The transverse speed is controlled by moving the translation stage in the horizontal direction and the longitudinal speed is ascribed to the vibration of the translation stage in the vertical direction. Therefore, the scanning track is a sine curve in this experiment. For segment a in the sine curve, the change of scanning speed is much larger than scanning direction, so scanning speed plays a significant role in surface nanostructures formation. The slightly curved nanostructure transforms into ablation crater, then into slightly curved nanostructure due to the oscillation of scanning speed. For segment b, however, scanning speed varies slightly in the range where the highly curved nanostructures is created, and thus, scanning direction dominates the shape of highly curved nanostructures, that is the shape of the crater edge formed after irradiation by the former pulse of two successive pulses. Consequently, the effect of scanning speed and scanning direction on the curved nanostructures is consistent with the previous experiments [Fig. 1 and Fig. 3].
In summary, the irradiation of lithium niobate surface with femtosecond laser pulses leads to the formation of curved nanostructures. The curvature of nanostructures is shown to be critically dependent on the overlapping of two successive pulses. The FDTD simulation results reveal that electric field enhancement is generated at the crater edge by simulating the propagation of a focused pulse through an elliptical crater produced by the previous pulse, which is responsible for the formation of curved nanostructures. Through varying the scanning speed, the degree of electric field enhancement is modified, leading to the modification of the curvature of nanostructures. Through varying the scanning direction, the enhanced field region is shifted along the edge of elliptical crater, resulting in the modification of the curvature of nanostructures.
National Natural Science Foundation of China (NSFC) (grants 91323301, 51675048 and 51675049).
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