Plasma science and technology has enabled advances in very diverse fields: micro-and nanotechnology, chemical synthesis, materials fabrication and, more recently, biotechnology and medicine. While many of the currently employed plasma tools and technologies are very advanced, the types of plasmas used in micro-and nanofabrication pose certain limits, for example, in treating heat-sensitive materials in plasma biotechnology and plasma medicine. Moreover, many physical properties of plasmas encountered in nature, and especially outer space, i.e. very-low-temperature plasmas or plasmas that occur in high-density media, are not very well understood. The present review gives a short account of laboratory plasmas generated under 'extreme' conditions: at cryogenic temperatures and in supercritical fluids. The fundamental characteristics of these cryogenic plasmas and cryoplasmas, and plasmas in supercritical fluids, especially supercritical fluid plasmas, are presented with their main applications. The research on such exotic plasmas is expected to lead to further understanding of plasma physics and, at the same time, enable new applications in various technological fields.

Self-organized patterns appear in many biological, chemical and physical systems, including electric discharges. Under certain conditions, self-organized patterns also form in plasmas generated below room temperature. These so-called cryoplasmas have also shown promise for low-damage materials processing; however, the underlying mechanisms and experimental conditions that lead to either uniform discharges or those containing self-organized patterns are still not understood completely. Here, we investigated the formation and dynamics of self-organized patterns in dielectric barrier cryoplasmas generated at plasma gas temperatures ranging from 264 down to 7 K at a constant gas density ρ = 5 × 10 19 cm −3 . The electrode gap was 0.15 mm and the cryoplasmas were generated at voltages between 0.8 and 1.5 kV, at frequencies ranging from 20 to 30 kHz. The discharges were characterized by time-resolved imaging, optical emission spectroscopy and current-voltage measurements. For temperatures down to 250 K, the discharges are uniform, whereas between 250 and about 140 K, self-organized, bright filamentary patterns form. Below that temperature, the discharge regime changes again to a uniform glow and for temperatures below 20 K, different types of discharges-uniform, but also self-organized dark solitons and bright stripe patterns-are observed. The cryoplasmas show current-voltage characteristics that are similar to atmospheric pressure glow discharges and the different types of uniform or self-organized discharges are suggested to be caused by the disappearance of impurities in the plasma as the temperature is lowered, and changes in the mobilities of ion species and surface charges.

To gain a better understanding of pulsed laser ablation (PLA) processes in high-density fluids, including gases, liquids, and supercritical fluids (SCFs), we have investigated the PLA dynamics in high-density carbon dioxide (CO2) using a time-resolved shadowgraph (SG) observation method. The SG images revealed that the PLA dynamics can be categorized into two domains that are separated by the gas-liquid coexistence curve and the Widom line, which forms a border between the gaslike and liquidlike domains of an SCF. Furthermore, a cavitation bubble observed in liquid CO2 near the critical point exhibited a particular characteristic: the formation of an inner bubble and an outer shell structure. The results indicate that the thermophysical properties of the reaction field generated by PLA can be dynamically tuned by controlling the solvent temperature and pressure, particularly near the critical point.

Plasma-based fabrication of novel nanomaterials and nanostructures is indispensible for the development of next-generation electronic devices and for green energy applications. In particular, controlling the interactions between plasmas and materials interfaces, and the plasma fluctuations, is crucial for further development of plasma-based processes and bottom-up growth of nanomaterials. Electric discharge microplasmas generated in supercritical fluids represent a special class of high-pressure plasmas, where fluctuations on the molecular scale influence the discharge properties and the possible bottom-up growth of nanomaterials. This review discusses an anomaly observed for direct current microplasmas generated near the critical point, a local decrease in the breakdown voltage. This anomalous behavior is suggested to be caused by the concomitant decrease of the ionization potential due to the formation of clusters near the critical point, and the formation of extended electron mean free paths caused by the high-density fluctuation near the critical point. It is also shown that in the case of dielectric barrier microdischarges generated close to the critical point, the high-density fluctuation of the supercritical fluid persists. The final part of the review discusses the application of discharges generated in supercritical fluids to synthesis of nanomaterials, in particular, molecular diamond—so-called diamondoids—by microplasmas generated inside conventional batch-type and continuous flow microreactors.