Liquid targets are capable of offering rapid refreshment for per shot, which makes it viable of using high repetition rate lasers. Here, we push the liquid targets to a low temperature condition. By generating a stable flowing line of liquid nitrogen in ambient environment, we successfully observe broadband terahertz wave generation under the excitation of subpicosecond laser pulses. This demonstration provides more possibilities in choosing potential materials for studying the THz wave generation process, and in understanding the laser-induced ionization from a new perspective.
Nanoparticles are a favorable way to enhance ionization for ultrashort laser pulse focusing on liquid targets. We experimentally investigate how nanoparticles affect the THz generation from deionized water for different concentrations. However, no obvious enhancement is observed comparing with the signal from deionized water under the same excitation condition. Thus, the concentration of nanoparticles is not a crucial factor to enhance THz wave generation within the order of 10^8~10^11 particles/ml. We believe our results provide useful information on enhancing the liquid THz emission source by nanoparticles
THz liquid photonics is a new research frontier in laser-matter interaction community. We have successfully demonstrated THz wave generation from ionized liquids, including from liquid water, liquid nitrogen, and liquid gallium. Preferable to general targets, a flowing liquid line provides a fresh area for each excitation pulse, so the chaos and debris caused by the previous pulse will not influence the next one. This makes it possible of using a kHz repetition rate laser for excitation. THz wave generation from ionized liquids presents photoionization processes that are different from those in gases.
In general, the terahertz metamaterial filter based on the complementary structure mainly has the fixed resonant frequency and asymmetric resonant frequency edges. In this paper, a thermally control terahertz narrow bandpass filter consisting of a periodic array of complementary wires embedded with thermosensitive semiconductor indium antimonide (InSb) has been proposed. Due to the structure of the filter is relatively simple, it can be more easily fabricated by lithograph technology compared with traditional terahertz metamaterial filter structure. Furthermore, its performance has also been analyzed based on the effective medium theory and the Drude model. The results show that the resonant frequency shifts from 1.16THz to 2.11THz with the increasing of the temperature from 160K to 360K, and the blueshift of resonance frequency as large as 81% can be implemented. Meanwhile, the reflectivity at the resonant frequency almost tends to be zero and the transmissivity spectrum at the resonant frequency exhibits sharp and symmetric edges. The filter can be applied to THz imaging system, and can effectively improve the imaging quality as a result of well characteristics of dynamic tuned filer and relatively simple structure.
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