Transversal optical singularity induced precision measurement of step-nanostructures.

Optical singularities indicate zero-intensity points in space where parameters, such as phase, polarization, are undetermined. Vortex beams such as the Laguerre-Gaussian modes are characterized by a phase factor eilθ, and contain a phase singularity in the middle of its beam. In the case of a transversal optical singularity (TOS), it occurs perpendicular to the propagation, and its phase integral is 2π in nature. Since it emerges within a nano-size range, one expects that TOSs could be sensitive in the light-matter interaction process and could provide a great possibility for accurate determination of certain parameters of nanostructure. Here, we propose to use TOSs generated by a three-wave interference to illuminate a step nanostructure. After interaction with the nanostructure, the TOS is scattered into the far field. The scattering direction can have a relation with the physical parameters of the nanostructure. We show that by monitoring the spatial coordinates of the scattered TOS, its propagation direction can be determined, and as consequence, certain physical parameters of the step nanostructure can be retrieved with high precision.

Optical singularity assisted method for accurate parameter detection of step-shaped nanostructure in coherent Fourier scatterometry.

Accurate determination of the physical parameters of nanostructures from optical far-field scattering is an important and challenging topic in the semiconductor industry. Here, we propose a novel metrology method to determine simultaneously the height and side-wall angle of a step-shaped silicon nanostructure. By employing an optical singular beam into a typical coherent Fourier scatterometry system, both parameters can be retrieved through analyzing the intensity profile of the far-field scattering pattern. The use of singular beam is shown to be sensitive to slight changes of the parameters of the step. By changing the relative direction between the singularity and structure, the height and side-wall angle can both be retrieved with high precision. This new method is robust, simple, and can provide valuable means for micro-and-nano- metrologies.

Superresolution effect due to a thin dielectric slab for imaging with radially polarized light.

Improving the image quality of small particles is a classic problem and especially challenging when the distance between particles are below the optical diffraction limit. We propose a imaging system illuminated with radially polarized light combined with a suitable substrate that contains a thin dielectric layer to demonstrate that the imaging quality can be enhanced. The coupling between the evanescent wave produced in a designed thin dielectric layer, the small particles and the propagating wave forms a mechanism to transfer sub-wavelength information about the particles to the far field. The smallest distinguished distance reaches to 0.634λ, when the imaging system is composed of a high numerical aperture (NA=0.9) lens and the illumination wavelength λ = 632nm, beyond the diffraction limit 0.678λ. The lateral resolution can be further improved by combining the proposed structure with superresolution microscopy techniques.

Pancharatnam-Berry phase shaping for control of the transverse enhancement of focusing.

We show that elongating a tightly focused field in the direction perpendicular to the optical axis is possible. We demonstrate our approach by specially shaping the Pancharatnam-Berry (PB) phase. Moreover, the analytical formulae required to calculate the strength vectors and energy flux of the three-dimensional electromagnetic fields near the focus of an aplanatic optical system are derived using the Richards and Wolf vectorial diffraction methods. Calculations reveal that the transverse enhancement is controllable and depend on the phase index in the PB phase, thereby giving rise to a focus with tunable length and subwavelength width in the focal plane.

Electronic spill-out induced spectral broadening in quantum hydrodynamic nanoplasmonics.

The hydrodynamic theory is a powerful tool to study the nonlocal effects in metallic nanostructures that are too small to obey classical electrodynamics while still too large to be handled with a full quantum-mechanical theory. The existing hydrodynamic model can give accurate quantitative predictions for the plasmonic resonance shifts in metallic nanoplasmonics, yet is not able to predict the spectral width which is usually taken as a pre-set value instead. By taking account the fact that due to electron density spill-out from a surface, the Coulomb interaction screening is less efficient close the surface thus leads to a higher electron-electron scattering rate in this paper, we study how the electron-density-related damping rate induced by such Coulomb interaction will affect the plasmonic spectral broadening. We perform the simulation on a Na nanowire, which shows that the absorption spectra width is wider when the size of the nanowire becomes smaller. This result is consistent well with the reported experiment. Therefore, our theoretical model extends the existing hydrodynamic model and can provide much more quantum insight about nonlocal effects in metallic nanostructures.

Plasmonic tweezers: for nanoscale optical trapping and beyond.

Optical tweezers and associated manipulation tools in the far field have had a major impact on scientific and engineering research by offering precise manipulation of small objects. More recently, the possibility of performing manipulation with surface plasmons has opened opportunities not feasible with conventional far-field optical methods. The use of surface plasmon techniques enables excitation of hotspots much smaller than the free-space wavelength; with this confinement, the plasmonic field facilitates trapping of various nanostructures and materials with higher precision. The successful manipulation of small particles has fostered numerous and expanding applications. In this paper, we review the principles of and developments in plasmonic tweezers techniques, including both nanostructure-assisted platforms and structureless systems. Construction methods and evaluation criteria of the techniques are presented, aiming to provide a guide for the design and optimization of the systems. The most common novel applications of plasmonic tweezers, namely, sorting and transport, sensing and imaging, and especially those in a biological context, are critically discussed. Finally, we consider the future of the development and new potential applications of this technique and discuss prospects for its impact on science.

Dynamic plasmonic nano-traps for single molecule surface-enhanced Raman scattering.

Intense electric fields at the nanoscale are essential for single molecule surface-enhanced Raman scattering (SERS) detection. Such fields can be achieved in plasmonic nano-gaps between nanoparticles and metal films through hybridization of surface plasmons. The nano-gaps could be formed and dynamically controlled by using plasmonic tweezers; however, the aggregation of particles in the plasmonic field degrades each particle's enhancement and spoils the nanosized-spatial resolution. Here, dual-plasmonic tweezers are proposed and demonstrated to accurately control the number of nano-gaps and enhancement by tailoring a crater-shaped potential well in the nano-trap system. As the electric field in the nano-gap is intense, SERS spectral signatures of a single molecular level are probed simultaneously. These advantages point towards the implementation of enhanced Raman spectra, and broad applications in optical molecular detection.

In-plane trapping and manipulation of ZnO nanowires by a hybrid plasmonic field.

In general, when a semiconductor nanowire is trapped by conventional laser beam tweezers, it tends to be aligned with the trapping beam axis rather than confined in the horizontal plane, and this limits the application of these nanowires in many in-plane nanoscale optoelectronic devices. In this work, we achieve the in-plane trapping and manipulation of a single ZnO nanowire by a hybrid plasmonic tweezer system on a flat metal surface. The gap between the nanowire and the metallic substrate leads to an enhanced gradient force caused by deep subwavelength optical energy confinement. As a result, the nanowire can be securely trapped in-plane at the center of the excited surface plasmon polariton field, and can also be dynamically moved and rotated by varying the position and polarization direction of the incident laser beam, which cannot be performed using conventional optical tweezers. The theoretical results show that the focused plasmonic field induces a strong in-plane trapping force and a high rotational torque on the nanowire, while the focused optical field produces a vertical trapping force to produce the upright alignment of the nanowire; this is in good agreement with the experimental results. Finally, some typical ZnO nanowire structures are built based on this technique, which thus further confirms the potential of this method for precise manipulation of components during the production of nanoelectronic and nanophotonic devices.