Spiral relief formation in an azo-polymer film by the irradiation of a circularly polarized optical vortex beam

Abstract

Summary form only given. Optical vortices with a helical wavefront due to a phase singularity, and circularly polarized light with a helical electric field have an orbital angular momentum (Lħ) and a spin angular momentum (Sħ), respectively. Consequently, a circularly polarized optical vortex will carry the total angular momentum (Jħ=Lħ+Sħ), defined by the vector sum of the orbital and spin angular momenta. In recent years, we and our-coworkers discovered an interesting phenomenon, in which the angular momentum of the optical vortex is transferred to an irradiated metal target through an ablation process, providing twisted nanoneedles [1,2].In this paper, we present the first demonstration of the formation of a spiral surface relief in an azobenzenecontaining polymer (azopolymer) film through a light-induced mass migration (a trans-cis phase transition) process [3] by the irradiation of a circularly polarized optical vortex with non-zero total angular momentum. These spiral reliefs in the azopolymer film will potentially be applied to planar chiral metamaterials and plasmonic nanostructures. They can also provide a new materials science, for instance, optical nanolithography and selective identification of the chirality of chemical composites in nanoscale imaging systems. A CW frequency-doubled Nd:YVO4 laser with a wavelength of 532 nm was used, and its output was converted to be a circularly polarized first-order optical vortex (L=1) by a spiral phase plate (SPP) (providing azimuthally 2ȧphase shift) and a quarter-wave plate (QWP). The sign of the orbital angular momentum was then the same (or opposite) as that of the spin angular momentum. The resulting total angular momentum of the optical vortex was characterized by J=2 (or J=0). To reverse the sign of J, the SPP and the QWP were also inverted. The vortex beam was focused to be a ψ3 μm annular spot by an objective lens (NA~0.44) system onto an azopolymer film, and its intensity was measured to be 810 W/cm2. The morphology of the azo-polymer surface during vortex beam exposure was observed by a CCD camera. The surface relief was formed within a few seconds, and subsequently, its tip started to orbit the on-axis core of the optical vortex at a frequency of 1.5 Hz, forming the spiral relief. AFM images of the surface reliefs formed by the irradiation of optical vortices with J = 2, 0, and 1 are shown in Fig. 1. The exposure time was 1.5 seconds. The formed relief (it looks like a dump) at J=2 has a spiral conical surface with 2 turns, although the image quality is limited by the spatial resolution of the AFM. Its height at J = 2 was measured to be ~350 nm. When the sign of S was reversed, i.e., J = 0, the relief (its height was ~250 nm) had no spiral structure. We also mention that such spiral reliefs were not formed by irradiation of the linearly polarized optical vortex beam (J = 1). In conclusion, we demonstrated spiral surface relief formation in an azo-polymer surface by the irradiation of a circuratly polarized optical vortex beam. The spiral direction of the relief can be controlled only by varing the sign of J of the optical vortex. The spiral relief formation is originated by the transfer of the angular momentum of the optical vortex to the azo-polymer through a light-induced mass migration process.