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We generalize the concept of photonic gauge potential in real space, by introducing an additional "synthetic" frequency dimension in addition to the real space dimensions. As an illustration we consider a one-dimensional array of ring resonators, each supporting a set of resonant modes having a frequency comb with spacing Ω, undergoing a refractive index modulation at the modulation frequency equal to Ω. We show that the modulation phase provides a gauge potential in the synthetic two-dimensional space with the dimensions being the frequency and the spatial axes.Such gauge potential can create a topologically protected one-way edge state in the synthetic space that is useful for high-efficiency generation of higher-order side bands.

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The heating of a solar cell has significant adverse consequences on both its efficiency and its reliability. Here to fully exploit the cooling potential of solar cells, we experimentally characterized the thermal radiation and solar absorption properties of current silicon solar cells and, on the basis of such experimental characterization, propose a comprehensive photonic approach by simultaneously performing radiative cooling while also selectively utilizing sunlight. In particular, we design a photonic cooler made of one-dimensional photonic films that can strongly radiate heat through its thermal emission while also significantly reflecting the solar spectrum in the sub-band-gap and ultraviolet regimes. We show that applying this photonic cooler to a solar panel can lower the cell temperature by over 5.7 °C. We also show that this photonic cooler can be used in a concentrated photovoltaic system to significantly reduce the solar cell temperature or required cooling power. This photonic cooler can be readily implemented in current photovoltaic modules as a retrofit to improve both efficiency and lifetime. Our approach points to an optimal photonic approach for thermal management of solar cells.

The colours of outdoor structures, such as automobiles, buildings and clothing, are typically chosen for functional or aesthetic reasons. With a chosen colour, however, one must control the radiative thermal load for heating or cooling purposes. Here we provide a comprehensive calculation of the tunable range of radiative thermal load for all colours. The range exceeds 680 Wm−2 for all colours, and can be as high as 866 Wm−2, resulting from effects of metamerism, infrared solar absorption and radiative cooling. We experimentally demonstrate that two photonic structures with the same pink colour can have their temperatures differ by 47.6 °C under sunlight. These structures are over 20 °C either cooler or hotter than a commercial paint with a comparable colour. Furthermore, the hotter pink structure is 10 °C hotter than a commercial black paint. These results elucidate the fundamental potentials of photonic thermal management for coloured objects.

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We study the chemohydrodynamic pattern formation during interfacial mass transfer accompanied by a neutralization reaction. The system, which is placed in a Hele-Shaw cell, is a configuration of two immiscible liquid phases in contact along a plane interface. In the upper, organic layer a carboxylic acid is dissolved, the concentration of which is far beyond the equilibrium partition ratio. Interfacial acid transfer initiates the neutralization with an organic base dissolved in the lower, aqueous layer. Focus is on the exploration of a novel instability consisting of a regular cellular structure penetrating into the aqueous bulk solution. By several complementary experimental methods, including shadowgraph visualization with different magnifications, particle image velocimetry, differential interferometry, and detailed measurements of relevant material properties, the driving mechanism of the instability is identified. Synthesis of the experimental results suggests that lateral differences in buoyancy are responsible for the convection.