Near-Field Scanning Optical Microscopy Characterization of Metastructures
AbstractMetamaterials and metasurfaces, defined by their ability to achieve functionalities above and beyond normal materials and surfaces, have found a myriad of applications in the fields of electronics, condensed matter, and most notably (for the subject of this thesis), optics. These metastructures typically consist of materials intelligently patterned or structured in a way to realize some traditionally difficult or previously impossible to achieve material property or function. Simply put, metastructures allow for not only the creation of new optical elements, but also the expansion and improvement of existing optics. On the more fundamental research side, just by virtue of their novelty, metastructures inherently open up new research paths. Thus, the focus of this thesis is twofold: the creation of optical elements with exotic wavefront control and the characterization of these new optical elements.
The optical elements studied here are all devices based on the control of surface plasmon polaritons (SPPs). While the advantages and disadvantages of using SPPs will be discussed later, the use of SPPs as the electromagnetic modes of study and manipulation suggests, and in some cases necessitates, the use of a specific characterization technique: near-field scanning optical microscopy (NSOM). NSOM is able to probe the electromagnetic near-field of the surface plasmons, enabling direct optical detection of the otherwise non-radiative SPPs.
First, we describe the creation and measurement of a metastructure that acts as a switchable and tunable SPP lens. The metalens has four operating wavelengths in the visible and exhibits on/off switching based on the polarization of the incident light. The performance of the lens is characterized by an effective numerical aperture of NA≈.7 and a concentration of optical energy of an order of magnitude higher than an isotropic source of SPPs. This corresponds to an efficiency of roughly 25%. Beyond the performance of the actual lens, the design principle itself is also noteworthy. The creation methodology of the metalens is based on holographic principles and is the driving force behind the metalens’ ability to switchably focus the operating wavelengths. We note that the design principle is not inherent to surface plasmons, and can act as a blueprint for free space lenses as well.
Next, we demonstrate a metastructure that is capable of creating and steering surface plasmon wavefronts based on the angle of incidence and the spin angular momentum of the photons impinging on the metastructure. An interesting and elegant pedagogical analogy to wakes is made, in that, the metastructure creates surface plasmon wakes. Wakes are a general wave phenomenon whereby a disturbance propagates through a medium faster than the phase velocity of the waves it creates. Examples of this in nature include boat wakes, sonic booms, and Cherenkov radiation. Our experiment was the first observation of this effect with surface plasmons. We achieved this by creating a ‘running wave of polarization’ along a one-dimensional metastructure. This running wave of polarization has a specifically tailored phase velocity, allowing for the creation of surface plasmon wakes. By using a suitably designed metastructure, we demonstrated the steering of surface plasmon wakes. Furthermore, the sign of the wavevector of the wake is dependent on the spin-angular-momentum of the incident light, and an analogy to Cherenkov and reversed Cherenkov radiation is discussed.
The next chapter of this thesis expands, experimentally and theoretically, on the previous section regarding Cherenkov surface plasmon wake generation. In designing a new metastructure, we demonstrate two anisotropies in surface plasmon wake generation. The first anisotropy is an asymmetric wake angle—wakes that propagate away from the metastructure at different angles. We note that, in general, this is not possible with just a single medium with a single refractive index. The second anisotropy is a focusing/defocusing effect for the two wakes created. These asymmetries are made possible by the use of a more complex metastructure—a single line of sequentially rotated bimodal V-shaped antennas. The crux of the metastructure performance is that the second mode of the V-antenna breaks the symmetry of the system and can be used to create anisotropies in surface plasmon wavefront generation.
Lastly, we discuss a work-in-progress experiment utilizing techniques developed during the course of this thesis. The motivation for this work hinges on the fact that measuring the refractive index of small-area (<100 μm^2), thin (<100 nm) films is notoriously difficult using common techniques such as ellipsometry. We develop a methodology using surface waves to measure two unknowns: the in-plane n_(||) and out-of-plane n_⊥ refractive index of exfoliated hexagonal boron nitride (h-BN). Elegantly, surface waves are well-suited as measurement tools for two-dimensional materials as they are able to interact along the length of the structure, rather than the thickness. Two experiments are performed to determine the two unknown refractive indices. The first experiment utilizes a standing wave mode in the h-BN to probe the in-plane refractive index only, and the second experiment employs SPPs, which are able to probe both the in-plane and out-of-plane refractive indices. Given the results of both experiments, both the in-plane and out-of-plane refractive indices can be uniquely determined for a range of visible frequencies. In the spirit of science, we note that this is an ongoing experiment and our analysis has not fully concluded as of yet, but promising preliminary results are contained herein.
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