Workshop: Plasma Photonics, Metamaterials, Strongly Coupled Plasmas

Low-temperature plasma is formed in various spatial configurations, and its spatial design is controllable when we tune parameters of its boundary conditions. Plasma photonic crystals (PPCs) and plasma metamaterials (PMs) have their own spatial configurations to manipulate electromagnetic waves at microwaves and milli-meter waves. PPCs are one-, two- or three-dimensional microplasma assemblies in which parameters of each microplasma are similar [1]. Such spatial periodic structure yields the corresponding periodic change of refractive index for propagating electromagnetic waves, and extraordinary phenomena such as bandgap formation are observed. PMs are composed of plasma and solids, with spatial periodic structure to possess macroscopic homogeneity. The refractive index can be negative [2], and elaborate controls of electromagnetic waves are observed, like microwave cloaking [3].

Then, the next question arises, on their plasma capability as electromagnetic-wave controllers, as to how and why such plasma shapes work functionally for such purposes. Energy consumption is localized as discharge electrodes are installed to achieve favorable spatial profiles, and neutral gas pressure determines diffusive profiles of plasma density on its edge. As a thermodynamic media, while its energy localization is linked to low configurational entropy, part of its thermalized (high-entropy) fraction can be removed by collisions with flowing neutral particles, with the thermodynamic entropy kept at a low level. Such a state is clearly exemplified in PPCs and PMs, where both have two- or three-dimensional designed profiles. At a sufficiently low level of wave energy, the structures can perform thermodynamic work such as wave reflection in bandgaps of PPCs. One advantage in these devices is rapid tunability after turning on the plasma generator, and its feasibility of short-time building-up and erasing is also reasonable when we follow comprehension of their entropy states.

Funding acknowledgement

This work was partly supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT/JSPS KAKENHI) with Grant Nos. JP24H00036 and JP24K21198.