Uniform deep-subwavelength ripples produced on temperature controlled LiNbO3:Fe crystal surface via femtosecond laser ablation

Applied Surface Science 478 (2019) 779–783

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Uniform deep-subwavelength ripples produced on temperature controlled LiNbO3:Fe crystal surface via femtosecond laser ablation ⁎

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Yanan Lia, Qiang Wua,d, , Ming Yangb, , Qiang Lia, Zhandong Chenc, Chunling Zhanga, Jun Suna, Jianghong Yaoa, Jingjun Xua,d a Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education, TEDA Institute of Applied Physics and School of physics, Nankai University, Tianjin 300457, China b National Key Laboratory of Science and Technology on Power Sources, Tianjin Institute of Power Sources, Tianjin 300384, China c College of Science, Guangxi University for Nationalities, Nanning 530006, China d Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Ultrafast laser process Lithium niobate Transparent dielectrics Deep-subwavelength ripples

In this paper, we studied the femtosecond laser induced periodic surface structures (fs-LIPSS) formation on LiNbO3:Fe crystal with the assistance of elevated temperature. Uniform deep-subwavelength fs-LIPSS with the period of 190 ± 10 nm was successfully fabricated on crystal surface as the sample temperature was at 1000 °C. And the measurement of Transmission Electron Microscopy showed that fs-laser treated samples remained crystalline. We believed that thermal excitation plays a critical role in the formation of fs-LIPSS. The initial carrier density of LiNbO3:Fe are effectively increased during this process, and high concentration surface carriers are effectively excited by femtosecond laser irradiation, which make great contribution to formation of fs-LIPSS. Moreover, we also found that the doped Fe2+ ions are conducive to the fs-LIPSS formation. The iron doped LiNbO3 crystal with uniform surface structure has great application prospects in related fields, such as on-chip photonic-integrated platforms, nanophotonics, and nanostructures for micro-fluidics.

1. Introduction Femtosecond laser induced periodic surface structures (fs-LIPSS), also known as ripples, have been extensively studied over decades [1–5]. A general explanation for the formation of fs-LIPSS is that the interference between incident light and surface wave modulates the deposition of laser energy, consequently induces the periodic ablation [3,6]. Fs-LIPSS are categorized into near-subwavelength ripples (NSRs) and deep-subwavelength ripples (DSRs), according to the ratio between the period of fs-LIPSS and incident wavelength. The period of fs-LIPSS is mainly related to excited carrier density [3], the bandgap structure of material [7], incident wavelength and laser treatment environment [8]. By optimizing these parameters, fs-LIPSS have already been fabricated on metal and semiconductor surface, which significantly modify material properties, such as optical absorption [9,10], photoemission [11], hydrophilicity and hydrophobicity [12]. LiNbO3 crystal is a transparent dielectric with wide bandgap around 3.7 eV. Due to its excellent acousto-optic, electro-optic, pyroelectric,

piezoelectric, ferroelectric, and nonlinear optical properties, LiNbO3 is widely employed in numerous technical applications such as optical waveguides, photonic crystals, and gratings [13–15]. Conventionally, the LiNbO3 devices with micro-/nano-scale structures are usually made by wet etching or dry etching process which are poor resolution and high time-cost. One of promising improvements is to introduce femtosecond laser machining to fabricate micro-/nano-structures on LiNbO3 crystals surface. Due to the advantages like programmable control, fast speed, and ultra-high resolution, femtosecond laser machining has been already applied in industry manufacturing [16]. By exposure of 800 nm fs laser pulses with a pulse duration of 50 fs, small ablation craters were induced and periodic ripples were found on the bottom [17]. With the irradiation of 800 nm fs laser pulses with a pulse duration of 80 fs duration and a repetition rate of 1 kHz, the randomly distributed nanorod-shaped craters were found in the ablated area [18]. On exposure to 85 MHz sub-15 fs pulsed 800 nm Ti:sapphire laser light, shallow ripples were obtained that originated from melting and rapid resolidification [19]. Therefore, new methods are urgently expected,

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Correspondence to: Q. Wu, Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education, TEDA Institute of Applied Physics and School of physics, Nankai University, Tianjin 300457, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (Q. Wu), [email protected] (M. Yang). https://doi.org/10.1016/j.apsusc.2019.02.037 Received 26 July 2018; Received in revised form 3 February 2019; Accepted 5 February 2019 Available online 06 February 2019 0169-4332/ © 2019 Published by Elsevier B.V.

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Fig. 1. (a) SEM image of LiNbO3: Fe surface morphology processed at 1000 °C with 1000 pulses and 6.0 kJ/m2 fluence. The black double arrow in the upper-right corner indicates the laser polarization. (b) the magnified picture of the black square frame. (c) the 2D-FFT of fs-LIPSS generated on LiNbO3:Fe sample. The diameter of the black ring corresponds to the distance of the 2D-FFT intensity of the two peak positions. (d) TEM image of fs-LIPSS. (e, f) the High-resolution Transmission Electron Microscopy and selected area electron diffraction patterns of the microstructure, respectively.

2. Experiment setup

which could be used to efficiently fabricate uniform ripples on LiNbO3 surface. In this work, we heated the samples to mitigate this problem. Distinct from our previous work [20], in this work, the experiment is carried out on irons doped LiNbO3, which has been rarely studied in this research field. Benefit from the doped Fe2+ ions, the interaction of femtosecond laser with LiNbO3:Fe crystal has been enhanced, thus leading to the formation of uniform deep-subwavelength fs-LIPSS. The proposed methods can efficiently fabricate the uniform fs-LIPSS, which will facilitate the development and application of fs-LIPSS on LiNbO3:Fe surfaces in related fields of photonic integrated devices and nanophotonics. In this work, we report the fabrication of deep sub-wavelength ripples at the surface of iron-doped lithium niobate (LiNbO3:Fe) crystals via femtosecond laser pulses irradiation at elevated temperature. Uniform ripples with a period of 190 ± 10 nm and direction of perpendicular to laser polarization were formed at the surface of LiNbO3:Fe crystals. We found that in the fabrication process, the sample temperature is an important parameter for controlling the surface morphology. In the scenario of high temperature treatment, plenty of hot carriers are effectively excited in this system, which greatly enhance laser-material interaction and significantly influence the formation of fs-LIPSS. What's more, compared with undoped LiNbO3 crystal, excited electrons concentration in LiNbO3:Fe crystal is increased due to impurity energy levels formed by Fe2+ ions and the interaction of femtosecond laser pulses with LiNbO3:Fe crystal is promoted. In addition, the result of Transmission Electron Microscopy measurement show that the fs-laser treatment area remains crystalline, which is beneficial for further application.

Experiments were carried out using a Ti:sapphire laser system, which delivers 120 fs duration pulses at the central wavelength of 800 nm with the repetition of 1 kHz. An electromechanical shutter was adopted to control the pulse numbers. Both laser fluence and polarization orientation were controlled by the combination of half wave plate and Glan-Taylor Prism. The spatial profile of the laser pulse was nearly Gaussian, and the laser beam was loosely focused by a lens with 500 mm focal length, with a laser spot radius (1/e2) of 39 μm on the sample surface. The average laser power was measured using a power meter. A Z-cut LiNbO3:Fe (0.1 mol%) wafer with the dimension of 10 mm × 10 mm × 1 mm was polished on both sides before the experiments. It was then cleaned by acetone and isopropanol in an ultrasonic bath and blow-dried with nitrogen gas. The cleaned sample was mounted on a heating stage which can control the temperature of the sample from 20 °C to 1000 °C. A series of ablation spots were prepared by varying laser fluence, pulse number (50–1000) and heating temperature, where the laser fluence refers to average fluence (F = F0/ πw02, F is the average fluence, F0 is the pulse energy and πw02 is laser spot area). Surface morphology and structural phases of these spots were obtained by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). 3. Results and discussion Fig. 1(a) shows the SEM image of LiNbO3:Fe crystal surface irradiated by 1000 pulses with laser fluence of 6.0 kJ/m2 at 1000 °C. A small area (shown in the insert picture (b)) is selected and enlarged for analyzing the details of morphology. The uniform fs-LIPSS are clearly 780

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Fig. 2. (a–d) show the SEM image of Fe doped LiNbO3 surface morphology processed by 50 pulses with fluence of 6.0 kJ/m , 8.5 kJ/m , 11.0 kJ/m2, and 13.5 kJ/m2, at 500 °C, 750 °C, 900 °C, and 1000 °C, respectively. The black double arrow in the lower-left corner indicates the laser polarization. 2

semiconductor, the orientation of fs-LIPSS is perpendicular to the laser polarizations. The surface patterns were further analyzed by two dimensional Fast Fourier Transform (2D-FFT) in Fig. 1(c). As shown in Fig. 1(c), the fs-LIPSS is with a spatial period around 190 ± 10 nm. TEM measurement was carried out to identify structural changes within the laser irradiated regions. Fig. 1(d) presents the TEM image of LiNbO3: Fe surface processed at 1000 °C. Fig. 1(e, f) show the Highresolution Transmission Electron Microscopy and selected area electron diffraction patterns of the microstructure surface, respectively. As shown in Fig. 1(d–f), the fs-laser induced microstructure remains crystalline. Generally, crystalline materials usually become polycrystalline or amorphous phase after a complex resolidification process due to ultrafast laser irradiation [21,22]. In these experiments, the samples undergo a high temperature annealing, which is beneficial for recrystallizing process [22]. The formation of fs-LIPSS is considered to be related to incident laser fluence, sample temperature. To evaluate the effect of temperature and laser fluence on fs-LIPSS generation, the surface morphologies ablated at different temperatures and different laser fluences were studied in this experiment. Fig. 2(a–d) illustrate surface morphologies irradiated with 50 pulses with fluence of 6.0 kJ/m2, 8.5 kJ/m2, 11.0 kJ/ m2, and 13.5 kJ/m2 at 500 °C, 750 °C, 900 °C, and 1000 °C, respectively. At 500 °C, there is no modification on the surface when irradiated with fluence below 11.0 kJ/m2. And with fluence increasing to 13.5 kJ/m2, a bowl-like crater without fs-LIPSS formed at the surface. At 750 °C, a circular heat affected area is found in the area irradiated with fluence of 6.0 kJ/m2, 8.5 kJ/m2, and 11.0 kJ/m2. As the temperature raised to 900 °C, fs-LIPSS forms with the period of 190 ± 10 nm found in the area that irradiated with fluence of below 11.0 kJ/m2, as illustrated in Fig. 2 (a3–a5) and (b3–b5). Fig. 2 indicates that the sample temperature is an important

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Fig. 3. (a–d) show the SEM images of LiNbO3:Fe surface morphology irradiated by fluence of 6.0 kJ/m2with N = 50, 100, 200, 500 pulses at the temperature of 1000 °C, respectively. The insets in (a–d) are the 2D-Fast Fourier Transform of (a–d), respectively.

observed in this area, whose period is much smaller than the incident laser wavelength (800 nm). Like the fs-LIPSS formed in metal or 781

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Fig. 4. shows the morphologies on LiNbO3:Fe crystal surface (a) and pure LiNbO3 crystal surface (b) irradiated by 200 pulses with a constant fluence of 6.0 kJ/m2 under 1000 °C. The black double arrow indicates the direction of polarization of the incident laser.

role in the interaction process of femtosecond laser with LiNbO3:Fe, in which Fe2+ ions act as donors of electrons can immediately be ionized via absorption photons and then release free electrons [34]. Then the released electrons serve as seed electrons for impact ionization, which facilitate the excitation of high density electrons. Besides, the dopant of Fe ions introduces impurity energy level with an energy gap around 1.1 eV above the valance band [35], so that it can assist electrons transition. Therefore, the dopant of Fe ions is conducive to the formation of uniform ripples.

parameter for affecting the formation of femtosecond laser induced surface structures. Generally, in order to excite considerable number of electrons from valance band to conduction band, high fluence of laser pulse was released to material surface at low temperature, resulting in electrons emission and Coulomb explosion inevitably, which has been clearly observed in various materials [23–26]. Then LiNbO3 crystal surface will be damaged, as shown in Fig. 2(d1–d2). While irradiated by the pulse with fluence of below the damage threshold, because of its naturally large bandgap and fairly low carrier density, there is no any modification could be found in the irradiated area, as shown in (a1–c1). However, under the treatment with high temperature T > 900 °C, the results are obviously different from the cases of temperature T < 750 °C. One of the reasons is that shallow traps will be thermally ionized and release electrons, so the initial electron density will increase [27]. Then impact ionization will become more intense, resulting in the density of excited electrons increase in LiNbO3 crystal dramatically [28,29]. The other reason is that the optical absorption property of LiNbO3 highly depends on the number of acoustic phonons, which is a function of temperature. At elevated temperature, there are more acoustic phonons, consequently it is more likely that electrons can absorb phonons and photons simultaneously to realize indirect transition [30]. Overall, the increased initial electron concentration and more frequent indirect transition will strengthen the interaction between femtosecond laser and LiNbO3 crystal. Therefore, high concentration of carriers can be excited, which make the surface exhibit metallic behavior and support Surface Plasmon Polaritons (SPPs). Researchers believed that the stimulation of SPPs are crucial for the formation of fsLIPSS [3,6,31,32]. As shown in Fig. 2, uniform ripples will form on the surface irradiated by femtosecond laser at high temperature. All of these indicate that the heat treatment on LiNbO3:Fe crystal has a great impact on the formation of ripples. The influence of different number pulse on uniform ripples formation has been also studied in experiment. Fig. 3 shows the SEM images of the areas on LiNbO3:Fe irradiated with different laser pulse number (N). As shown in Fig. 3, with the pulse number increasing, both the period and direction of LIPSS haven't changed. Comparing Fig. 3(d) with Fig. 3(a), the uniformity and clearness of ripples have been distinctly improved, and the phenomenon is similar to the that observed on silicon, which is associated with grating-assisted ablation [6,33]. Further we found that the doped Fe2+ ions in LiNbO3:Fe crystal also inspire the formation of uniform fs-LIPSS. Fig. 4(a) and (b) exhibit the SEM images of LiNbO3:Fe and pure LiNbO3 surface irradiated by 200 pulses with a constant fluence of 6.0 kJ/m2 under 1000 °C, respectively. At LiNbO3:Fe surface, uniform fs-LIPSS was found in whole of the exposure area. In contrast, the fs-LIPSS with bifurcation is partially formed and randomly distributed at pure LiNbO3 surface, as shown in Fig. 4(b). This comparison shows that the dopant of Fe ions play a key

4. Conclusion In this paper, we explore the formation of fs-LIPSS with the assistance of elevated temperature in LiNbO3:Fe. The uniform deep subwavelength ripples with the period of 190 ± 10 nm and direction of perpendicular to laser polarization were successfully fabricated at 1000 °C. And the lattice remains crystalline after strong laser fluence irradiation, which indicates that annealing effect is involved as well at the elevated temperature. We believe that thermal excitation which enhances the initial carrier density plays a critical role for laser-matter interaction, high concentration surface carriers are effectively excited. Besides, laser shots will affect the fs-LIPSS morphology by grating-assisted ablation. Furthermore, comparing with pure LiNbO3 crystal, the doped Fe2+ ions in LiNbO3:Fe crystal also provides additional contributions through photoexcitation. This simple method that regulated the carrier density by thermal excitation gives us insight for understanding laser-matter interaction and can be applied for other transparent dielectrics to improve the fabrication of photonics integrated device. Acknowledgements This work was financially supported by the National Natural Science Foundation of China, China (11574158), the Fundamental Research Funds for the Central Universities, the 111 Project (B07013), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT_13R29). The authors would like to acknowledge Qinglian Li of the Nankai University Lab of Photoelectric Materials and Devices for providing the LiNbO3 samples. References [1] M. Birnbaum, Semiconductor surface damage produced by ruby lasers, J. Appl. Phys. 36 (1965) 3688–3689. [2] H.M. van Driel, I.E. Sipe, J.F. Young, Laser-induced periodic surface structure on solids: a universal phenomenon, Phys. Rev. Lett. 49 (1982) 1955–1958. [3] M. Yang, Q. Wu, Z. Chen, B. Zhang, B. Tang, J. Yao, I.D. Olenik, J. Xu, Generation and erasure of femtosecond laser-induced periodic surface structures on

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