Phase is a fundamental resource for optical imaging but cannot be directly observed with intensity measurements. The existing methods to quantify a phase distribution rely on complex devices and structures and lead to difficulties of optical alignment and adjustment. We experimentally demonstrate a phase mining method based on the so-called adjustable spatial differentiation, by analyzing the polarization of light reflection from a single planar dielectric interface. Introducing an adjustable bias, we create a virtual light source to render the measured images with a shadow-cast effect. From the virtual shadowed images, we can further recover the phase distribution of a transparent object with the accuracy of 0.05λ RMS. Without any dependence on wavelength or material dispersion, this method directly stems from the intrinsic properties of light and can be generally extended to a broad frequency range.
We observe from simulations that a doubly resonant structure can exhibit spectral behavior analogous to
electromagnetically induced transparency, as well as superscattering, depending on the excitation. We develop a
coupled-mode theory that explains this behavior in terms of the orthogonality of the radiation patterns of the
eigenmodes. These results provide insight in the general electromagnetic properties of photonic nanostructures and
A metallic slot waveguide, with a dielectric strip embedded within, is investigated for the purpose of enhancing
the optics-to-THz conversion efficiency using the difference-frequency generation (DFG) process. To describe
the frequency conversion process in such lossy waveguides, a fully-vectorial coupled-mode theory is developed.
Using the coupled-mode theory, we outline the basic theoretical requirements for efficient frequency conversion,
which include the needs to achieve large coupling coefficients, phase matching, and low propagation loss for both
the optical and THz waves. Following these requirements, a metallic waveguide is designed by considering the
trade-off between modal confinement and propagation loss. Our numerical calculation shows that the conversion
efficiency in these waveguide structures can be more than one order of magnitude larger than what has been
achieved using dielectric waveguides. Based on the distinct impact of the slot width on the optical and THz
modal dispersion, we propose a two-step method to realize the phase matching for general pump wavelengths.