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Abstract
Polarization sensitive microstructures with different morphologies were induced by irradiating dual lithium niobate crystals with femtosecond laser pulses. An upper lithium niobate crystal served as a mask plate to tailor light field, which led to the formation of crater and arc-shaped structures on the surface of a lower lithium niobate crystal. In single-shot irradiation, the orientation and morphology of resultant microstructures can be tailored by controlling the focusing position, because focus splitting took place when a focused laser light propagated through dual lithium niobate crystals. In scanning, the width and morphology of laser scan lines can be governed using various combinations of focusing position and scanning direction. Furthermore, large-area micro/nanostructures with different topography features were successfully fabricated on the crystal surface and their absorption spectra indicated that the absorptance in the visible wavelength range was strongly dependent on fabricated micro/nanostructures. This new type of structured lithium niobate surfaces can be potentially applied in optical and photonic devices.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Lithium niobate (LiNbO3, LN) is a widely used material in numerous fields due to its outstanding electro-optic, piezoelectric, ferroelectric, pyroelectric, acousto-optic, photorefractive, and nonlinear optical properties [1–3]. Femtosecond laser irradiation has emerged as a powerful tool for micro/nanostructuring because of its capabilities in the ultrahigh precision micromachining and nanomachining of various materials [4–6]. The femtosecond laser structuring of LN is used in optical waveguides [7–9], photonic crystals [10–13], optical modulators [14–16], optical frequency converters [16,17], and 3D data storage devices [18–22]. Studies on surface structuring of LN have found that ripples [23–25], holes [26], and gratings [27] can be produced on LN surfaces through femtosecond laser irradiation. Various surface structures produced on LN surfaces can be used to design structures for a wide range of applications. Intensity distribution of the beam in focal region can be tailored by focusing of a Gaussian beam into a uniaxial birefringent crystal [28–33], such as LN crystal, which can lead to the formation of structures with different morphologies [32].
In this study, we investigated the formation of polarization sensitive microstructures on a lower LN surface by irradiating dual LN crystals with femtosecond laser pulses. An upper LN crystal acted as a mask plate to tailor light field. The focusing position was found to be the key parameter to control not only the orientation and morphology of the surface microstructures produced using a single pulse but also the width and morphology of laser scan lines written by a single scan with provided scanning directions. Based on these results, the morphology of large-area micro/nanostructures was modified using various combinations of the focusing position and scan line spacing, leading to an absorptance change in the visible wavelength range.
2. Experimental setup
Figure 1(a) illustrates the experimental setup. A Ti: sapphire regenerative amplifier laser system (Spectra Physics, Inc.) was employed as a light source, which delivered linearly polarized pulses with 50 fs duration, at a center wavelength of 800 nm and a repetition rate of 1 kHz. The pulse energy was controlled using a half-wave plate, a polarizer, and a neutral density gradient filter. The polarization of a laser beam was controlled using another half-wave plate. After passing through several reflectors and a dichroic mirror, the laser beam was focused at a normal incidence onto/into a sample by using a microscope objective (Olympus MPLFLN 20×, NA = 0.45). The samples used in this work were congruent LN plates (Hefei Kejing Materials Technology Co., Ltd.) with the dimensions of 10 mm × 10 mm × 1 mm in z-cut orientation, and surfaces were polished to obtain an optical grade. Dual LN plates were fabricated in all experiments where a stack of the two same LN plates served as a target [Figs. 1(a) and 1(c)], which was mounted on a computer-controlled 6-axis translation stage (M-840.5DG, PI, Inc.). The space between the two LN plates was approximately 440–460 µm. For the in situ observation of fabrication, a light-emitting diode illuminator and charge-coupled device (CCD) camera were employed. After irradiation, surface morphology was analyzed using a scanning electron microscope (SEM). The optical absorption spectra were recorded using an ultraviolet-visible-near infrared (UV-VIS-NIR) spectrophotometer in the region of 400–800 nm. To understand experimental details and the analysis provided, two key parameters are defined in Fig. 1(b): θ represents an angle between the polarization direction and + x direction, α represents an angle between the scanning and polarization directions. Figure 1(c) illustrates three types of focusing positions, wherein z = 0 indicates that the focusing position was at the surface of the lower LN sample, whereas z < 0 and z > 0 indicate that the focusing position was inside and outside the lower LN sample, respectively.
Fig. 1. (a) Experimental setup. HWP: half-wave plate; P: polarizer; GF: neutral density gradient filter; S: shutter; BS: beam splitter; DM: dichroic mirror; MO: microscope objective; CCD: charge-coupled device camera. (b) Polarization angle θ and relative angle α between the scanning and polarization directions. K: laser beam propagation direction; E: laser polarization direction; S: scanning direction. (c) Three types of focusing positions with respect to the surface of the lower LN sample. The focusing position is denoted using green arrow for the three cases.
3. Results and discussion
3.1 Dependence of the orientation and morphology of surface microstructures on the focusing position
Figure 2(a) illustrates the effect of the focusing position and laser fluence on the surface morphology of LN. From bottom to top, the surface morphology evolved on increasing z from region 1 to region 9 for a given laser fluence. All irradiations presented in Fig. 2 were produced on the surface of lower LN by using single femtosecond laser pulses. Figures 2(b)–2(q) illustrate the evolution of surface morphology with increasing z for ablation at a fixed fluence of 43.3 J/cm2. When z = −28 µm, an elliptical crater with a long axis oriented perpendicular to laser polarization was obtained [Fig. 2(b)]. When z was in the range of −26 to −14 µm, the irradiated area featured a central crater accompanied with a pair of arc-shaped structures (ASSs) on the left and right sides [Figs. 2(c)–2(e)]. The ASSs, which emerged as the nearest neighbor of the central crater, were called type I ASSs. At z = −12 µm, a second pair of ASSs that emerged from outside was called type II ASSs [Fig. 2(f)]. Increasing z to −10 µm led to a decrease in the size of type I ASSs and increase in the size of type II ASSs [Fig. 2(g)]. Only the central crater and a pair of type II ASSs were observed at z in the range of −8 to −6 µm [Fig. 2(h)], whereas a pair of type I ASSs was observed at z in the range of −4 to −2 µm [Fig. 2(i)]. The orientation of type I and type II ASSs was perpendicular to laser polarization when z < 0. The elliptical crater with the long axis oriented parallel to laser polarization occurred at z in a range of 0 to 4 µm [Figs. 2(j) and 2(k)]. The type I ASSs, that were oriented parallel to laser polarization, were observed on both the sides of the central crater at z in a range of 6 to 16 µm [Figs. 2(l)–2(n)]. Subsequently, the type II ASSs with the same orientation were observed from outside the type I ASSs at z = 18 µm [Fig. 2(o)]. A further increase in z to 22 µm considerably reduced the size of type I ASSs, whereas it increased the size of type II ASSs [Fig. 2(p)]. The crater occurred again at z in a range of 26 to 28 µm [Fig. 2(q)]. The resulting surface microstructures can be categorized as follows: crater; a pair of ASSs; and central crater with one or two pairs of ASSs on both the sides, thus the orientation of induced surface microstructures can be defined as follows: a long axis orientation of the crater; an orientation of ASSs; and an orientation that is the long axis orientation of the central crater and is also the orientation of ASSs, accordingly.
Fig. 2. (a) Surface microstructures formed on LN through single pulse irradiation at various combinations of the focusing position and laser fluence. (b)–(q) SEM images of surface microstructures created with different focusing positions at a laser fluence of 43.3 J/cm2. E: laser polarization direction. Orientation: ⊥ and // denote that the orientation of induced surface microstructures is perpendicular and parallel to the laser polarization direction, respectively. Letters in (a) link parameters with SEM images.
3.2 Dependence of the orientation of the surface microstructures on polarization direction
Figure 3 illustrates microstructures induced on the surface of lower LN by using single pulse with different states of polarization at focusing position z = −28, −22, −10, 4, 12, and 22 µm, respectively. The laser fluence was fixed at 43.3 J/cm2. At the polarization angles of θ = 0°, 45°, 90°, and 135°, the orientation of induced surface microstructures was always perpendicular to the polarization of the laser light when z < 0 [Figs. 3(a)–3(l)], whereas that of induced surface microstructures was always parallel to the polarization of the laser light when z > 0 [Figs. 3(m)–3(x)]. The formation of induced surface microstructures was strongly dependent on laser polarization.
Fig. 3. (a)–(x) SEM images of LN surface morphologies processed using single pulse with the polarization angles of θ = 0°, 45°, 90°, and 135°, at focusing position z = −28, −22, −10, 4, 12, and 22 µm, respectively. The red double arrow denotes the laser polarization direction.
3.3 Formation mechanism of polarization sensitive microstructures
Focus splitting can take place when a focused Gaussian beam propagated through a uniaxial birefringent crystal [28–33]. LN is a negatively birefringent uniaxial crystal [1–3]. An incident linearly polarized laser beam splits into two beams after entering the LN crystal along optical axis and two foci are formed [31,32], as illustrated in Fig. 4. Front focus is mainly formed by the e-beam and back focus by the o-beam [32,33]. The calculated intensity distributions of the beam in whole focal region showed that the front focus and back focus were elongated perpendicular and parallel to laser polarization in the xy plane, respectively [32]. The lower LN surface was located in front focal region when z < 0 [Fig. 4(a)], whereas it was located in back focal region when z > 0 [Fig. 4(b)]. Therefore, the orientation of the surface microstructures produced at z < 0 was perpendicular to that of the surface microstructures produced at z > 0 because of the two orthogonally oriented foci. The simulated field distributions of the beam at different focusing positions showed that the orientation of arc-shaped patterns formed in front focal region was perpendicular to laser polarization, whereas that of arc-shaped patterns formed in back focal region was parallel to laser polarization [33], which was in good agreement with our experimental results.
Fig. 4. Formation of two foci in focal region by focusing of a linearly polarized laser beam into dual LN crystals along optical axis at (a) z < 0 and (b) z > 0. The incident laser light was polarized along the x-axis. Front focal plane and back focal plane denote the location of the front focus and back focus, respectively. The lower LN surface is denoted using black arrow for the two cases.
3.4 Dependence of the width and morphology of laser scan lines on the focusing position and angle between the scanning and polarization directions
The different focusing position caused the line width to vary, as illustrated in Fig. 5(a), wherein the scanning direction was either perpendicular or parallel to laser polarization (i.e., α = 90° or 0°) at a laser scanning speed and the laser fluence of 2000 µm/s and 40.6 J/cm2, respectively. A few of the laser scan lines presented in Figs. 5 and 6 curve slightly due to slight vibrations of the translation stage. For α = 90°, the line widths of surface microstructures produced at z in the range of −24 to −16 µm exhibited a substantial increase compared with those produced at z in the range of −34 to −26 µm, because the type I ASSs occurred and oriented perpendicular to laser polarization [Figs. 5(b) and 5(c)]. However, for α = 0°, line widths varied slightly at the z range between −34 and −16 µm, mainly because the type I ASSs in this case oriented perpendicular to laser polarization [Figs. 5(h) and 5(i)]. For single pulse irradiation, the widths of induced surface microstructures formed in region 2, where the type I ASSs occurred, were also close to the length of the crater formed in region 1 [Figs. 2(a)–2(e)]. For α = 90° or 0°, the width of laser scan lines exhibited a sharp increase at z = −14 µm, which was attributed to the formation of the type II ASSs that oriented perpendicular to laser polarization [Figs. 5(d) and 5(j)]. The type II and type I ASSs exhibited the same orientation, however, the generation of type II ASSs can increase line width at α = 0°. For single pulse irradiation, the widths of induced surface microstructures formed in a part of region 3 and entire region 4 [Figs. 2(g) and 2(h)], where the type II ASSs occurred, were also considerably larger than those formed in region 2 [Figs. 2(b)–2(e)]. For α = 90°, the width of laser scan lines decreased sharply at z = −4 µm. For α = 0°, a sharp decrease in the width of laser scan lines was observed at z = −6 µm. The disappearance of the type II ASSs caused the sharp decrease in the line width [Figs. 5(e) and 5(k)]. The absence of the type I ASSs in both cases further reduced the line width at z = 0 µm. For single-shot ablation, the type II ASSs disappeared, and only the type I ASSs appeared in region 5, and the type I ASSs disappeared and only the crater appeared in region 6 [Figs. 2(i)–2(k)]. For α = 0°, the formation of the type I ASSs oriented parallel to laser polarization at z in a range of 6 to 14 µm resulted in larger line widths than at z in a range of 0 to 4 µm [Fig. 5(l)]. A high increase in the width of the laser scan lines was observed again at z ranging between 16 and 18 µm because of the appearance of the type II ASSs [Fig. 5(m)]. For α = 90°, line widths did not vary substantially at z in the range of 0 to 20 µm [Figs. 5(f) and 5(g)], that is, the presence of the type I and type II ASSs in this case did not significantly influenced on the line width, mainly because the ASSs were oriented parallel to the laser polarization. For single-shot ablation, induced surface microstructures formed in region 7 and 8 exhibited similar widths, which were also not highly influenced by the presence of the type I and type II ASSs [Figs. 2(l)–2(p)]. For α = 90°, the formation of a series of craters led to a sharp decrease in the line width at z = 22 µm. For α = 0°, the decrease in the line width was observed at z = 20 µm.
Fig. 5. (a) Width of laser scan lines dependence on focusing positions for two orthogonal scanning directions. Each scan line was fabricated at the laser fluence and scanning speed of 40.6 J/cm2 and 2000 µm/s, respectively. (b)–(m) SEM images of laser scan lines produced on the surface of lower LN through ablation at different focusing positions for two orthogonal scanning directions. Letters in (a) link parameters with SEM images. The red double arrow and blue arrow denote the laser polarization direction and the scanning direction, respectively.
Fig. 6. (a) Absorption spectra of LN crystals with polarization sensitive structures. (b)‒(g) SEM images of lower LN surface morphology produced through femtosecond laser ablation with the different scan line spacing (d) for two different focusing positions (z). The red double arrow denotes the laser polarization direction.
3.5 Dependence of the absorptance of processed LN crystals on the morphology of large-area surface micro/nanostructures
The optical property of surface structures induced on lower LN was characterized using a UV-VIS-NIR spectrophotometer, and absorption spectra for different surface micro/nanostructures are presented in Fig. 6(a). All samples were fabricated with the fluence and scanning speed of 40.6 J/cm2 and 2000 µm/s, respectively. For comparison, the absorptance of a mechanically polished sample before laser irradiation is illustrated in Fig. 6(a). The absorptance of surface micro/nanostructures depended on the scan line spacing (d) and focusing position (z). At z = −10 µm, smaller was the scan line spacing, higher was the absorptance of surface micro/nanostructures [Figs. 6(b)–6(e)]. The absorptance of surface microstructures produced at z = −2 µm was lower than that produced at z = −10 µm when d = 8 µm [Figs. 6(c) and 6(f)], whereas the absorptance of surface micro/nanostructures produced at z = −2 µm was higher than that produced at z = −10 µm when d = 4 µm [Figs. 6(d) and 6(g)]. Moreover, regular micro/nanostructures, featuring periodic microstructures and nanostructures aligned perpendicular and parallel, respectively, to laser polarization, were produced at z = −2 µm with d = 4 µm [Fig. 6(g)], and their absorptance in a certain visible wavelength was even higher than that of the random micro/nanostructures formed at z = −10 µm with d = 2 µm [Fig. 6(e)].
4. Summary
We explored the femtosecond laser ablation of dual LN crystals. The role of upper LN was light field tailoring. The orientation and morphology of polarization sensitive microstructures, produced on the surface of lower LN by using the single pulse irradiation, can be tuned by controlling the focusing position because focus splitting took place when a laser light focused into dual LN crystals along optical axis. Various orientations of induced surface microstructures were obtained by adjusting the focusing position and polarization direction. Moreover, in the fast scanning case, the focusing position and scanning direction collaboratively dominated the width and morphology of laser scan lines. Additionally, the morphology of large-area micro/nanostructures can be controlled by tuning the focusing position and scan line spacing, which led to the different absorptance of the LN surface in the visible wavelength range. Hence, a one-step simple method for the surface structuring of LN can be used for tailored designs in various optical and photonic applications.
Funding
National Natural Science Foundation of China (51675048); National Key Research and Development Program of China (2018YFB1107200).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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