Coupled photonic modes in two regimes [electronic resource] : capturing light on chip and probing mesoscopic thermal radiation
Keywords: Applied Physics.

In 1901, Planck courageously posited that the thermodynamics of radiation can essentially be reduced to a calculation the entropy of an ``irradiated, monochromatic, vibrating resonator'', or photonic mode, as a function of its vibrational energy, and "moreover, it is necessary to interpret the total vibrational energy not as a continuous, infinitely divisible quantity, but as a discrete quantity composed of an integral number of finite equal parts. This insight was pivotal in the development of quantum theory and, it can be argued, initiated the golden age of physics in the 20th century. But in the 21st century the notion of coupled modes is important for more than the thermodynamics of radiation. By examining just a few relevant modes of a system, it becomes possible to effectively and intuitively design novel photonics devices which would be difficult to access with the full Maxwell equations. I will describe one such device for dynamically stopping light, which was inspired by the discovery of electromagnetiically induced transparency phenomenon, first observed in electronic mode transitions in atomic vapours. It turns out that such a system can also be engineered with photonic cavity modes of any scale. In particular I will describe a coupled mode model, applicable to electronic or photonic modes, which theoretically allows one to capture an incident light pulse into a pair of coupled modes. My 2D-FDTD simulations of a prototype photonic crystal waveguide-cavity system demonstrate near complete capture of incident light pulses at the micron/nanosecond scale. The key is time-domain control of the cavity resonance frequencies, which can be implemented with electro-optic modulation or optical free-carrier injection in technologically relevant Si chips. This approach may find application in all-optical photonic circuits, as a means of buffering pulses with 100Thz bandwidth, while preserving phase information of the pulse. The modern theory of coupled modes is also relevant to recent investigations in thermal radiation, the original problem which motivated Planck. His famous blackbody law contains the essential thermodynamics in the form of the thermal occupation of photonic modes, and accurately describes far-field radiation, but when material bodies are very close, their evanescent photonic modes may strongly couple. Consequently, the behavior of thermal radiation from the bodies deviates significantly from Planck's original investigations. The fluctuational electrodynamics formalism, augmented with the tools of scattering theory, have arisen as the natural theoretical approach to electromagnetic heat transfer applicable beyond the far-field limit. First developed over half a century ago, these regime is now accessible experimentally. The first experiments have naturally been performed on dipolar and metallic microspheres at nano-scale separations, at which point these near-field effects dominate and magnify heat transfer by several orders of magnitude. I will present numerically exact fluctuational electrodynamics scattering calculations of radiative heat transfer between a sphere a plate, which agree quantitatively with the earliest measurements, and have since been confirmed in other materials and geometries. Moreover, the results unite previously disparate and uncontrolled approximations. The fluctuational electrodynamics picture, having been confirmed down to the few nanometer scale, has been developed subtantially in recent years, and can be understood generally as the interaction of coupled photonic modes. Even in these simple engineered microscale systems, there are typically hundreds or thousands of modes interacting, however one can imagine systems with carefully engineered modal interactions, which allows for novel thermal devices. In particular, I will discuss a coupled photonic mode scheme for thermal rectifaction though vacuum. This scheme can be implemented with a simple prototype device, consisting of two different crystalline polytpes of silcion carbide with different which temperature dependent surface phonon polariton resonances. The result is an effective thermal diode with directionally dependent heat flow, which can be understood from a purely classical electromagnetic picture. This work has stirred further work in promising thermal devices such as photonic thermal bistability and phase change material thermal transistors.