Multiscale micro-/nanostructures on single crystalline SiC fabricated by hybridly polarized femtosecond laser
Introduction
States of light with spatially homogeneous state of polarization (SoP), such as linearly or circularly polarized light, are often referred to as scalar light field. A vector light field with inhomogeneous distribution of SoP in space often behaves differently from scalar light field, and shows unique advantages in some specific fields [1,2]. For example, a radially polarized beam can be applied to particle acceleration [3] because of its longitudinal field components after being tightly focused [4,5]. Moreover, its gradient force also has great advantages in the process of particle capture [6]. When vector beams are applied to material processing, such as laser cutting and drilling, different absorption of light with different polarization states leads to different processing efficiency [7]. In addition, due to the polarization dependence of laser-induced surface periodic surface structures (LIPSSs), some new surface patterns fabricated by cylindrical vector beam have also been reported with potential applications in the fields of bionics and medicine [8,9].
High-order Poincaré sphere (HoPS) as shown in Fig. 1(a) is an effective geometric representation of conventional cylindrical vector beam [10,11]. The radially polarized light can be described by a point on the equator of HoPS and the poles of HoPS represent two mutually orthogonal circular polarizations with opposite topological charges. However, whether it is scalar light beam or conventional cylindrical vector beam, the local polarizations on transverse profile along the beam axes are the same type with the same ellipticities at different positions in space. For example, the localized polarizations of radially polarized light on the transverse profile are all linearly polarized, although their directions are different at different positions. Hybrid vector beam with hybridly polarized light field is a special kind of vector beam. On the transverse profile along the beam axes, the polarizations of different positions cover linear, elliptical and circular polarization. It has spatially varying degree of polarization ellipticity, that is, the ellipticity of the polarization changes continuously with position spanning a complete meridian closed circle on the surface of the Poincaré sphere [12] as shown in Fig. 1(b).
Generation of hybrid vector beam has been demonstrated [13,14], and it also has unique properties under tightly focus conditions [12]. However, to our knowledge, there is no literature reported on the application of hybrid vector beam to the fabrication of surface multiscale structures. Femtosecond laser-induced periodic surface structures, also called ripples, have been extensively studied, and the orientation of LIPSSs usually perpendicular or parallel to polarization [15], [16], [17]. The surface structures induced by conventional cylindrical vector beams often have spatially variant characteristics similar to the SoP of beam. The long-axis orientation and ellipticity of the hybrid polarization are inconsistent in different regions, therefore, micro-/nanostructures different from that induced by linearly polarized or conventional cylindrical vector beam can be fabricated theoretically.
In our work, we demonstrated a simple and effective method for generating femtosecond arbitrary hybrid vector beams by a cascade of a superstructure polarization converters (S-waveplate) [18] and a quarter-wave plate (QWP). The ellipticity at any azimuth angle on the transverse profile along the beam axes is derived by analyzing the Jones matrix. By rotating the linear polarization of incident laser and the fast axis direction of QWP, the polarization distribution of hybrid vector beam on the transverse profile rotates, and the induced structures change accordingly. We predict that the periodic surface structures fabricated by hybrid vector beam can be divided into four regions according to the degree of polarization ellipticity, which also shows good agreement with our experimental results. A variety of different composite structures with multiscale features have been prepared on the surface of monocrystalline 4H-SiC, which are expected to be applied in nano-/micro-electromechanical systems (NEMS/MEMS) [19], thermal photovoltaics [20], thermal management [21] and other fields.
Section snippets
Theoretical analysis
Fig. 2 shows the schematic diagram of the system for generating hybrid vector beams.
The Jones matrix of S-waveplate can be expressed as [22]where m is the order of S-waveplate, which can be viewed as a combination of a certain number of half-wave plates [18]. Since the number of half-wave plate (HWP) is finite, the value of the azimuth cannot be continuously changed. Only some discrete values can be taken as
M in Eq. (2) is the number of half-wave
Experiment and results
A femtosecond laser (SpOne-8-F2P-SHG, Spectra Physics) used and processing system are shown in Fig. 4. The center wavelength λ of laser is 520 nm and the pulse width is about 300 fs. A HWP and a Glan-Laser polarizer (GLP) are combined to control the power of laser. The laser is beam-expanded and collimated by a beam expander, which consists of two lenses and then passes through the S-waveplate and QWP. The laser beam (M² ≈ 1.08) was expanded and collimated and then passed through the
Conclusions
- (1)
A simple and efficient method for generating femtosecond arbitrary hybrid vector beams by a cascade of a super-structured polarization converter and a quarter-wave plate was demonstrated. Based on the Jones matrix, the degree of polarization ellipticity at arbitrary azimuth angle on the transverse profile along the beam axes is deduced.
- (2)
The surface structures fabricated by hybrid polarized femtosecond laser are rotational symmetric, and the rotation angle is 90° The orientations of structures
Declaration of Competing Interest
None.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (grant no. 51705233) and the Science and Technology Planning Project of Shenzhen City(grant no. GRCK2017082316213662).
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