Using Optical Vortex To Control the Chirality of Twisted Metal Nanostructures

Abstract

We discovered for the first time that light can twist metal to control the chirality of metal nanostructures (hereafter, chiral metal nanoneedles). The helicity of optical vortices is transferred to the constituent elements of the irradiated material (mostly melted material), resulting in the formation of chiral metal nanoneedles. The chirality of these nanoneedles could be controlled by just changing the sign of the helicity of the optical vortex. The tip curvature of these chiral nanoneedles was measured to be <40 nm, which is less than 1/25th of the laser wavelength (1064 nm). Such chiral metal nanoneedles will enable us to selectively distinguish the chirality and optical activity of molecules and chemical composites on a nanoscale and they will provide chiral selectivity for nanoscale imaging systems (e.g., atomic force microscopes), chemical reactions on plasmonic nanostructures, and planar metamaterials.

Transfer of Light Helicity to Nanostructures

“…These results demonstrate that the chirality of the nanoneedle is determined only by the sign of L. S can increase (decrease) the spiral frequency of a nanoneedle when the sign of L is the same as (opposite to) that of S. . At pulse energies higher than the order of millijoules, the spiral structure of the fabricated nanoneedle disappeared, as reported concerning our previous study [18].…”

“…1(b) and 1(c)]. As we found in previous studies [17,18], this phenomenon indicates that the optical gradient force collects the melted metal, and the optical scattering force directs it efficiently toward the on-axis core of the optical vortex, thereby confining the melted metal in the core to form the chiral nanoneedle. In Gaussian beams that do not have an on-axis core, the collection of the melted metal does not occur efficiently.…”

“…There are few reports on the use of structured light itself to form chiral structures on the nanoscale. Recently, we discovered that the helicity of a circularly polarized optical vortex can be directly transferred to an irradiated metal sample, resulting in the formation of chiral nanoneedles [16][17][18]. This is the first demonstration, to the best of our knowledge, of nanostructures created by structured light with angular momenta, and it clearly represents a new scientific phenomenon.…”

“…5, the simulated angular momentum densities also indicate that two optical vortices with J ¼ 2 are degenerate. The laser energy level used in the present experiments was near the ablation threshold, as described in our previous publication [18]. This low energy permits the metal to melt only in the bright ring of the optical vortex.…”

“…0 % 33 m. The pulse energy was varied in the range of 0.2-0.8 mJ. As in our previous studies [17,18], four vortex pulses were overlaid so as to maximize the nanoneedle height. All experiments were performed at atmospheric pressure and room temperature.…”