Optical Bound States and Non-linearity in Geometrically-Modulated Dielectric Nanowires

Nontechnical description

The interaction of light with spherical or cylindrical particles of microscopic size has frequently been used to understand the general principles of how light interacts with matter of different sizes and shapes. For example, microscopic particles that are comparable to or smaller than the wavelength of light can give rise to surprisingly strong light scattering and absorption. Recently, it has become possible to make microscopic particles with shapes that go beyond simple cylinders to instead have periodic modulations in their size. Under the right conditions, these geometric modulations cause the particles to interact with light in a new way, giving rise to what is termed an 'optical bound state in the continuum,' or BIC. This project uses a combination of theory, computation, synthesis, and measurement to understand how the geometry of microscopic particles can be used to control the properties of BICs. BICs are exciting because they dramatically increase the extent to which light interacts with particles. In some cases, the interaction is sufficiently strong to cause the particles to convert red or infrared light into blue or ultraviolet light, a 'non-linear' effect. The results of this project provide the general principles for controlling BICs in geometrically-modulated particles, thus providing the fundamental principles for controlling light over a broad range of colors at a microscopic scale. The research effort involves undergraduate, graduate, and postdoctoral students in a project that bridges the interface between chemistry, physics, and engineering—providing breadth of experience in nanomaterial synthesis, microfabrication, spectroscopy, microscopy, and modeling.

Technical Description

In cylindrical dielectric particles, Mie resonances can cause surprisingly strong light scattering and light absorption, and the importance and interplay of Mie resonances, leaky-mode resonances, and guided modes in these structures has previously been studied. A relatively new class of resonance, the BIC, has been identified and can theoretically trap light for an infinite time in an ideal system. This project studies the fundamental light scattering, light absorption, and nonlinear properties of BICs in dielectric cylinders with nanoscale lateral size. The silicon structures are synthesized by a bottom-up vapor-liquid-solid process using in situ dopant modulation combined with wet-chemical etching to create precisely-defined morphology. Periodic modulation of the diameter of a dielectric cylinder is shown to introduce a range of BICs of different order, polarization, and symmetry type. Spectroscopic measurements on single NWs of controlled geometry are compared to theoretical models, providing validation of theoretical predictions. The capacity of BICs in silicon NWs to enable nonlinear effects by doubling or tripling the frequency of incoming light is also evaluated. Overall, the proposed study reveals the fundamental characteristics of BICs in dielectric cylinders and highlights important practical applications of these BIC modes.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.