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THz air photonics using laser-induced air plasma is one of the leading frontiers in the THz community. Ambient air, when excited with an intense femtosecond laser beam, exhibits the remarkable ability to generate and detect pulsed THz waves through a nonlinear optical process. Significant advances in the use of air plasma for emitting, controlling, enhancing and measuring broadband THz waves have opened up a range of research opportunities. However, one of the major challenges for the research community and in real world applications is that plasma formation requires the use of an intense laser (mJ pulse energy), but most femtosecond laser oscillators only have pulse energies in the range of pJ to tens of nJ. The investigation of THz photonics, specifically the exploration of laserinduced plasmas at the micro-nano scale and beyond, is a frontier. Microplasmas generated by tightly focused optical excitation beams with controlled polarization serve as a new THz source with its unique radiation pattern and easy operation. The laser energy threshold for THz wave generation, the power scaling and the generation efficiency from microplasmas are significantly different from those of elongated plasmas. Our estimation indicates that the micronano plasma approach could reduce the necessary optical pulse energy by five orders of magnitude, while still obtaining a comparable or better signal-to-noise ratio for THz time-domain spectroscopy. This would be made possible by the high electron density (1019 cm–3 or more) achievable with a tight focus laser excitation, which correlates with the THz generation efficiency, and the use of laser oscillators with a much higher pulse repetition rate, as compared to the currently employed amplified laser systems (100 MHz vs 1 kHz). The THz micro-nano plasma is expected to lead to key technologies that will enable further interdisciplinary research and continued advancements of numerous THz wave sensing and spectroscopy developments. It serves as a vehicle for studying the extreme THz science.
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