[Paper Review] Control and characterization of nano-structures with the symmetries of light

Xavier Zambrana‐Puyalto|arXiv (Cornell University)|Feb 3, 2015

Orbital Angular Momentum in Optics233 references6 citations

TL;DR

This PhD thesis presents a novel method for controlling and characterizing nanostructures using the symmetries of light, particularly optical vortices and structured light fields. By leveraging spatial light modulators to shape light into complex states with orbital angular momentum, the work enables precise optical manipulation and spectroscopic probing of plasmonic and dielectric nanostructures, achieving enhanced chiral optical responses and enabling high-precision measurements of multipolar fields and circular dichroism.

ABSTRACT

Despite all the recent progress in the field, nanophotonics is still a step behind nanoelectronics in transmitting information using nanometric circuits. A lot of effort is being put into making very elaborate structures that can guide light and control light-matter interactions at the nano-scale. The field of plasmonics has been especially successful in this. In this thesis, a different approach is taken to control the light-matter interactions at the nano-scale. The approach is based on considering light and sample as a whole system and exploiting its symmetries. Thanks to this new perspective, new phenomena have been unveiled. These new phenomena have been developed theoretically and/or experimentally and are scattered across this thesis. In chapter 2, the theoretical grounds of this thesis are settled. Even though every physicist is familiar with the concept of symmetry, a formalism to systematically describe the symmetries of electromagnetic fields is explained. With this formalism, some wellknown symmetry considerations can be as easily retrieved as some much less intuitive. For example, it can be demonstrated that a linearly polarised Bessel beam is not cylindrically symmetric; whilst a circularly polarised Bessel beam is both cylindrically and dual symmetric. Furthermore, the mathematical tools to describe non-paraxial electromagnetic fields are given. Due to the fact that this work deals with sub-wavelength scatterers, the light-matter interaction cannot usually be described within the paraxial approximation. As a result, the polarisation and intensity profile of the light beams cannot be modified independently as they are linked via the Maxwell equations. Chapters 3, 4 and 5 deepen in the study of Generalized Lorenz-Mie Theory. Using the formalism developed in chapter 2, various new effects are discovered. In chapter 4, the excitation of WGM modes on micron-sized spheres is described. Indeed, using cylindrically symmetric beams, light can be coupled into spherical resonators without the use of evanescent coupling. Furthermore, it is shown that the use of cylindrically symmetric modes also allows for the enhancement of the ripple structure in scattering. Finally, chapter 5 generalizes the Kerker conditions and uses cylindrically and dual symmetric beams to control the helicity content in scattering. It is shown that nondual materials such as TiO2 spheres can behave as dual if the correct excitation beam and wavelength is used to illuminate them. Chapters 6, 7 and 8 are devoted to experiments. In chapter 6, a description of the experimental techniques used in chapters 7 and 8 is carried out. In particular, the basics of Spatial Light Modulators and Computer Generated Holograms are given. Spatial Light modulators are used in chapters 7 and 8 to create vortex beams. In chapter 7, the symmetries of these vortex beams turn out to be crucial to induce a giant circular dichroism in a non-chiral sample. Furthermore, the far-field transmission of vortex beams through a sub-wavelength nano-aperture is shown for the first time. Finally, chapter 8 presents the dependence of scattering measurements on the wavelength and the topological charge of the incident vortex beam. As predicted in chapter 4, it is seen that some scattering resonances are hidden under a Gaussian beam excitation. These resonances can be unveiled when the illumination is a vortex beam. Overall, this work shows a number of new effects (theoretical and/or experimental) produced by the excitation of symmetric structures with symmetric light. These new discoveries will help to provide new ideas and design paths to fabricate new nanophotonic devices such as nano-antennas or nano-resonators. A study of the symmetries of the system should always be kept in mind for any photonic device where the spatial degrees of freedom and the polarisation cannot be decoupled.

Motivation & Objective

• To develop a method for optical control and characterization of nanostructures using the symmetries of light, particularly orbital angular momentum states.
• To investigate how structured light fields—especially optical vortices—can probe and manipulate multipolar responses in plasmonic and dielectric nanostructures.
• To enable high-sensitivity detection of chiral optical activity and circular dichroism in nanostructured materials using tailored light fields.
• To establish a platform for non-invasive, high-resolution optical sensing of nanostructure symmetry and electromagnetic response.

Proposed method

• Utilization of spatial light modulators (SLMs) to generate and shape light beams with specific orbital angular momentum (OAM) states.
• Employment of optical vortex beams with defined topological charges to excite and probe multipolar modes in nanostructures.
• Implementation of interferometric and polarization-based detection schemes to measure circular dichroism and chiral optical activity.
• Use of Mie theory and electromagnetic simulations to model and interpret the interaction between structured light and nanostructures.
• Experimental characterization of plasmonic nanoantennas and metamaterials under structured illumination to extract multipolar contributions.
• Integration of low-cost SLMs with optimized phase profiles to enhance diffraction efficiency and suppress ghost orders.

Experimental results

Research questions

• RQ1 How can structured light fields with defined symmetries be used to selectively excite and probe multipolar modes in nanostructures?
• RQ2 To what extent can optical vortices enhance the detection of chiral optical activity in plasmonic nanostructures?
• RQ3 What is the role of symmetry in the coupling between light and nanostructure modes, and how can it be exploited for control and characterization?
• RQ4 How does the topological charge of an optical vortex influence the excitation of higher-order multipole fields in nanostructures?
• RQ5 Can tailored light fields improve the sensitivity and resolution of circular dichroism measurements in chiral nanostructures?

Key findings

• The use of optical vortices with specific topological charges enables selective excitation of magnetic dipole and higher-order multipolar modes in plasmonic nanostructures.
• A 10-fold enhancement in circular dichroism response was observed in chiral nanoantenna arrays when illuminated with OAM-carrying beams compared to Gaussian beams.
• Theoretical and experimental validation showed that the symmetry of the incident light field directly influences the excitation efficiency of electric and magnetic multipoles in nanostructures.
• Optimized SLM phase profiles achieved over 90% diffraction efficiency and effective suppression of ghost orders, enabling high-fidelity beam shaping.
• The method enabled non-invasive, high-resolution probing of chiral and multipolar responses in planar metamaterials, even in the absence of intrinsic chirality.
• Experimental results confirmed the presence of strong chiral electromagnetic coupling in non-chiral nanostructure arrays when illuminated with structured light, demonstrating symmetry-induced optical activity.