Light twists a material on the nanoscale
‘Chiral’ or ‘handed’ structures are non-superimposable mirror images of each other, in the same way that a left hand differs from a right hand. Chiral molecules may be identical in their chemical behavior but differ in their optical and biological activity. Chiral nanostructures could enable us to distinguish and select between such molecules and chemical composites on the nanoscale. They may also enable nanoscale imaging systems (e.g., atomic force microscopes and scanning tunneling microscopes) and permit chemical reactions on plasmonic nanostructures and planar metamaterials to be chirally selective. One of our goals is to fabricate artificial devices with such unique optical properties.
Light beams can have orbital as well as spin angular momentum, and an ‘optical vortex’ is a beam of light with a helical wavefront (due to a phase singularity) that has orbital angular momentum mℏ, where ℏ is Planck's constant divided by 2π and m is an integer known as the topological charge. Circularly polarized light with a helical electric field also carries spin angular momentum, sℏ.1 Such angular momenta have been widely investigated for applications including optical trapping2 and guiding, as well as super-resolution microscopy.3
Recently, we discovered that light angular momenta can directly force an irradiated metal sample to form tiny spiral conical surface structures we term ‘chiral nanoneedles.’4–6 To the best of our knowledge, this is the first time it has been demonstrated that light angular momenta can create nanoscale structures.
A schematic diagram of our experimental setup is shown in Figure 1. We used a conventional flash-lamp-pumped Q-switched neodymium-doped yttrium aluminum garnet laser (with 1064nm wavelength, 30ns pulse duration, and 10Hz pulse repetition frequency). The light beam acquires orbital angular momentum by traveling through a spiral phase plate (SPP), a quartz disk divided by electron beam etching into 16 azimuthal regions of varying thickness for an nπ/8 phase shift (where n is an integer between 0 and 15). This shaped the beam output as a first-order optical vortex with m=1.
A second-order optical vortex with m=2 was also produced by two overlaid SPPs. A quarter-wave plate (QWP), placed in the optical path between the SPPs and a focusing lens, increased the spin angular momentum of the optical vortex by retarding light polarized in one direction by π/4 compared to light polarized in the perpendicular direction. We reversed the sign of the optical vortex helicity by inverting the SPPs and the QWP. The target in the present study was a 2mm-thick polished plate of tantalum, which has a relatively low ablation threshold compared to other metals.
An objective lens focused the optical vortex onto the target to a spot 65μm in diameter. The pulse energy was in the range 0.2–0.8mJ. To maximize the nanoneedle height, we fired four vortex pulses at the same place on the target. All experiments were performed at atmospheric pressure and room temperature. We used a JSM-6010LA scanning electron microscope to observe the ablated surface of the tantalum plate with a spatial resolution of 8nm at 3kV.
We found that the optical vortex with a total angular momentum of j=2 produced a nanoneedle with a spiral conical surface at the center of the ablation zone: see Figure 2(a, b). Additionally, the spiral direction (chirality) of the nanoneedle was inverted by reversing the sign of the total angular momentum: compare Figure 2(c, e) with Figure 2(d, f). The spiral frequency of the nanoneedle (defined as the number of turns divided by the height of the chiral nanoneedle) was also found to be determined by the magnitude of the total angular momentum j.7 The height of the chiral nanoneedles was measured as typically 10μm. The tip curvature of the nanoneedle was inversely proportional to the numerical aperture of the objective lens, and its minimum value was measured as ∼36nm, corresponding to less than 1/25th of the laser wavelength (1064nm).
The chiral nanoneedle formation can be well understood by the following model. The metal melted on irradiation by the optical vortex pulse is collected in the ring of the optical vortex by the optical gradient force. The angular momenta (orbital and spin) of the optical vortex are then transferred to the molten metal, yielding orbital motion around the dark core of the optical vortex. The shape is also directed by the optical scattering force, resulting in chiral needle formation.
We also used energy-dispersive x-ray (EDX) spectroscopy to estimate the relative amounts of the chemical components of the fabricated nanoneedle. The EDX spectrum is almost identical to that of the substrate surface, indicating that the needle is perfectly metallic.
In summary, a chiral nanoneedle, formed by transfer of angular momenta from helical light to a material surface, could be used in novel devices, such as planar chiral metamaterials,8 and plasmonic nanostructures, that will also help us in our research to selectively identify chiral chemical composites. For instance, we are investigating how to use a 2D chiral nanoneedle array as an optical polarization rotator in the mid-IR or terahertz region, for which optical devices are limited. We are also working to use the transfer of angular momenta from light to physical objects to biomicroelectromechanical devices to activate a living cell, and also to screw nanoelements to a metal substrate.
The authors acknowledge financial support from the Japan Science and Technology Agency and from the Japan Society for the Promotion of Science (grant 21360026).