Manipulating plasmonic nanoparticles with tailored optical focal field

Abstract

Since Ashkin and colleagues reported the first stable three-dimensional (3D) optical trapping, or optical tweezers, created using radiation pressure from a single focused laser beam, optical tweezers have become an important tool for research in the fields of biology, physical chemistry and soft matter physics. Plasmonic nanoparticles have attracted increased attentions in recent years due to various applications of plasmon resonance. For example, the local field enhancement offered by the resonant noble metal nanoparticles enable the amplifying of the Raman signal in surface-enhanced Raman scattering spectroscopy (SERS). Due to the noncontact and “holding” nature, optical trapping is well suited to be combined with SERS, potentially enabling ultrasensitive molecular recognition in liquids. However, the plasmonic nanoparticles are generally considered difficult to manipulate due to the large scattering force and severe optical heating effect, especially when the trapping wavelength approaches the resonant wavelength of the plasmonic nanoparticle. We develop a novel strategy to form a stable 3D manipulating of plasmonic nanoparticles even under resonant conditions through careful and purposeful engineering a vectorial optical field as the illumination. The required optical illumination is created by sculpting the amplitude and phase of a cylindrical vector beam. When strongly focused by a high numerical aperture objective lens, this specially engineered vectorial optical field offers the optical pulling feature of the tractor beam, a recently reported technique to generate a scattering force that points against the optical power flow. The main difference is the tractor beam pulls the particles all the way towards to the light source with no equilibrium point, while the particles conveyed by the engineered vectorial optical beam in this work gets stably trapped at an equilibrium position at the beam axis. The additional degree of freedom provided by the spatial distribution of the illumination enables the manipulation of the plasmonic nanoparticle behavior, meeting specific needs of different applications. For example, the particle can be stably trapped at the beam axis when illumination has concentric binary phase at the beam cross section [1]. However, if the illumination carries a spiral phase simultaneously, which is also known as orbital angular momentum (OAM), the particle would orbit around the beam axis due to the conservation of angular momentum. The particle's motion trajectory in terms of rotating radius and direction can be holistically manipulated by adjusting the OAM of the illumination [2, 3]. Besides, the particle can also be stably trapped off the beam axis, which is realized by introducing a sinusoidal varied phase to the illumination [3]. In addition, the high polarizability of these resonant metallic nanoparticles that normally becomes a problem due to the large scattering force can be taken advantage of to reduce the amount of required power, and hence decrease the heating effect and eliminate the ultimate obstacle in 3D stable optical manipulation of resonant plasmonic nanoparticles. This versatile trapping method may open up new avenues for optical manipulation and their applications in various scientific fields.