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1. Broadband THz Sources from Gases to Liquids

   https://doi.org/10.34133/2021/9892763  

   E, Yiwen; Zhang, Liangliang; Tcypkin, Anton; Kozlov, Sergey; Zhang, Cunlin; Zhang, X.-C. (July 2021, Ultrafast Science)

   Matters are generally classified within four states: solid, liquid, gas, and plasma. Three of the four states of matter (solid, gas, and plasma) have been used for THz wave generation with short laser pulse excitation for decades, including the recent vigorous development of THz photonics in gases (air plasma). However, the demonstration of THz generation from liquids was conspicuously absent. It is well known that water, the most common liquid, is a strong absorber in the far infrared range. Therefore, liquid water has historically been sworn off as a source for THz radiation. Recently, broadband THz wave generation from a flowing liquid target has been experimentally demonstrated through laser-induced microplasma. The liquid target as the THz source presents unique properties. Specifically, liquids have the comparable material density to that of solids, meaning that laser pulses over a certain area will interact with three orders more molecules than an equivalent cross-section of gases. In contrast with solid targets, the fluidity of liquid allows every laser pulse to interact with a fresh area on the target, meaning that material damage or degradation is not an issue with the high-repetition rate intense laser pulses. These make liquids very promising candidates for the investigation of high-energy-density plasma, as well as the possibility of being the next generation of THz sources. 

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2. Varying pre-plasma properties to boost terahertz wave generation in liquids

   https://doi.org/10.1038/s42005-020-00511-1  

   Ponomareva, Evgenia A.; Ismagilov, Azat O.; Putilin, Sergey E.; Tsypkin, Anton N.; Kozlov, Sergei A.; Zhang, Xi-Cheng (December 2021, Communications Physics)

   null (Ed.)

   Abstract Laser-driven nonlinear phenomena can both reveal the structural features of materials and become the basis for the development of various translated technologies, including highly intense terahertz sources. Here we realize a modified single-color double-pulse excitation scheme for enhancing the terahertz wave generation in flat liquid jets, and we show that the pre-ionization effect is crucial for finding the optimal input conditions. The experimental results, being supported by numerical simulations, reveal the preference for longer pre-pulses to induce the effective ionization process and shorter signals for the strong laser-plasma interaction. In addition to the identified features of the terahertz wave energy enhancement with respect to the duration change for both pulses and their ratio variation, we state the possibility of achieving the optical-to-THz conversion efficiency value up to 0.1% in the case of double-pulse excitation of an α -pinene jet. 

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3. Room temperature terahertz quantum cascade laser sources with 215  μ W output power through epilayer-down mounting

   https://doi.org/10.1063/1.4812814  

   Lu, Q Y; Bandyopadhyay, N; Slivken, S; Bai, Y; Razeghi, M (July 2013, Applied Physics Letters)

   We report room temperature terahertz (THz) quantum cascade laser sources with high power based on difference frequency generation. The device is Čerenkov phase matched and spectrally purified with an integrated dual-period distributed-feedback grating. Symmetric current injection and epilayer-down mounting of the device onto a patterned submount are used to improve the electrical uniformity and heat removal, respectively. The epilayer-down mounting also allows for THz anti-reflective coating to enhance the THz outcoupling efficiency. Single mode emission at 3.5 THz with a side-mode suppression ratio and output power up to 30 dB and 215 μW are obtained, respectively. 

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4. Coherent control of asymmetric spintronic terahertz emission from two-dimensional hybrid metal halides

   https://doi.org/10.1038/s41467-021-26011-6  

   Cong, Kankan; Vetter, Eric; Yan, Liang; Li, Yi; Zhang, Qi; Xiong, Yuzan; Qu, Hongwei; Schaller, Richard D.; Hoffmann, Axel; Kemper, Alexander F.; et al (December 2021, Nature Communications)

   Abstract Next-generation terahertz (THz) sources demand lightweight, low-cost, defect-tolerant, and robust components with synergistic, tunable capabilities. However, a paucity of materials systems simultaneously possessing these desirable attributes and functionalities has made device realization difficult. Here we report the observation of asymmetric spintronic-THz radiation in Two-Dimensional Hybrid Metal Halides (2D-HMH) interfaced with a ferromagnetic metal, produced by ultrafast spin current under femtosecond laser excitation. The generated THz radiation exhibits an asymmetric intensity toward forward and backward emission direction whose directionality can be mutually controlled by the direction of applied magnetic field and linear polarization of the laser pulse. Our work demonstrates the capability for the coherent control of THz emission from 2D-HMHs, enabling their promising applications on the ultrafast timescale as solution-processed material candidates for future THz emitters. 

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5. Ultrafast THz emission spectroscopy of spin currents in the metamagnet FeRh

   https://doi.org/10.1063/5.0201789  

   Lv, Yinchuan; Shim, Soho; Gibbons, Jonathan; Hoffmann, Axel; Mason, Nadya; Mahmood, Fahad (April 2024, APL Materials)

   Heterostructures of ferromagnetic (FM) and noble metal (NM) thin films have recently attracted considerable interest as viable platforms for the ultrafast generation, control, and transduction of light-induced spin currents. In such systems, an ultrafast laser can generate a transient spin current in the FM layer, which is then converted to a charge current at the FM/NM interface due to strong spin–orbit coupling in the NM layer. Whether such conversion can happen in a single material and how the resulting spin current can be quantified are open questions under active study. Here, we report ultrafast THz emission from spin–charge conversion in a bare FeRh thin film without any NM layer. Our results highlight that the magnetic material by itself can enable spin–charge conversion in the same order as that in a FM/NM heterostructure. We further propose a simple model to estimate the light-induced spin current in FeRh across its metamagnetic phase transition temperature. Our findings have implications for the study of the ultrafast dynamics of magnetic order in quantum materials using THz emission spectroscopy. 

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