Multiscale micro-/nanostructures on single crystalline SiC fabricated by hybridly polarized femtosecond laser

Highlights

• Multiscale structures were fabricated by hybrid vector beams for the first time.

• Shapes of multiscale patterns can be controlled by designing state of polarization.

• Polarization ellipticity at arbitrary azimuth angle of hybrid vector beam is deduced.

• LIPSS fabricated by hybrid vector beams are rotational symmetric with 90° rotation angle.

• Circular polarized laser induces the formation of nanoparticles.

Abstract

Hybridly polarized beam is a new type of vector beam different from conventional cylindrical vector beam. In our work, we present a simple and effective method to generate hybridly polarized beams by using a cascade of a super structured polarization converter and a quarter-wave plate. The transformation of light field was analyzed by using Jones vector and Jones matrix, and the Stokes parameters of the generated vector beams were measured to verify the states of polarization. Various novel multiscale micro-/nanostructures were fabricated on surfaces of 4H-SiC by using the hybrid vector beams, which are different from that induced by linearly polarized beam or conventional cylindrical vector beam. Experimental results show that the multiscale characteristics can be adjusted with a high degree of freedom by designing states of polarization. This research provides a new idea for preparation of multiscale micro-/nanostructures that may be useful in the fields of nano-/micro-electromechanical systems, thermal photovoltaics, and thermal management.

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.