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1. Rossby wave second harmonic generation observed in the middle atmosphere

   https://doi.org/10.1038/s41467-022-35142-3  

   He, Maosheng ; Forbes, Jeffrey M. ( December 2022 , Nature Communications)

   Second harmonic generation is the lowest-order wave-wave nonlinear interaction occurring in, e.g., optical, radio, and magnetohydrodynamic systems. As a prototype behavior of waves, second harmonic generation is used broadly, e.g., for doubling Laser frequency. Second harmonic generation of Rossby waves has long been believed to be a mechanism of high-frequency Rossby wave generation via cascade from low-frequency waves. Here, we report the observation of a Rossby wave second harmonic generation event in the atmosphere. We diagnose signatures of two transient waves at periods of 16 and 8 days in the terrestrial middle atmosphere, using meteor-radar wind observations over the European and Asian sectors during winter 2018–2019. Their temporal evolution, frequency and wavenumber relations, and phase couplings revealed by bicoherence and biphase analyses demonstrate that the 16-day signature is an atmospheric manifestation of a Rossby wave normal mode, and its second harmonic generation gives rise to the 8-day signature. Our finding confirms the theoretically-anticipated Rossby wave nonlinearity.

    

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2. Chip-based self-referencing using integrated lithium niobate waveguides

   https://doi.org/10.1364/OPTICA.392363  

   Okawachi, Yoshitomo ; Yu, Mengjie ; Desiatov, Boris ; Kim, Bok Young ; Hansson, Tobias ; Lončar, Marko ; Gaeta, Alexander L. ( June 2020 , Optica)

   The measurement and stabilization of the carrier–envelope offset frequency$f_{\mathrm{C}\mathrm{E}\mathrm{O}}$via self-referencing is paramount for optical frequency comb generation, which has revolutionized precision frequency metrology, spectroscopy, and optical clocks. Over the past decade, the development of chip-scale platforms has enabled compact integrated waveguides for supercontinuum generation. However, there is a critical need for an on-chip self-referencing system that is adaptive to different pump wavelengths, requires low pulse energy, and does not require complicated processing. Here, we demonstrate efficient$f_{\mathrm{C}\mathrm{E}\mathrm{O}}$stabilization of a modelocked laser with only 107 pJ of pulse energy via self-referencing in an integrated lithium niobate waveguide. We realize an$f\text{-}2f$interferometer through second-harmonic generation and subsequent supercontinuum generation in a single dispersion-engineered waveguide with a stabilization performance equivalent to a conventional off-chip module. The$f_{\mathrm{C}\mathrm{E}\mathrm{O}}$beatnote is measured over a pump wavelength range of 70 nm. We theoretically investigate our system using a single nonlinear envelope equation with contributions from both second- and third-order nonlinearities. Our modeling reveals rich ultrabroadband nonlinear dynamics and confirms that the initial second-harmonic generation followed by supercontinuum generation with the remaining pump is responsible for the generation of a strong$f_{\mathrm{C}\mathrm{E}\mathrm{O}}$signal as compared to a traditional$f\text{-}2f$interferometer. Our technology provides a highly simplified system that is robust, low in cost, and adaptable for precision metrology for use outside a research laboratory.

    

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3. Phase-matched nonlinear second-harmonic generation in plasmonic metasurfaces

   https://doi.org/10.1515/nanoph-2018-0181  

   Shams Mousavi, S. Hamed ; Lemasters, Robert ; Wang, Feng ; Dorche, Ali Eshaghian ; Taheri, Hossein ; Eftekhar, Ali A. ; Harutyunyan, Hayk ; Adibi, Ali ( February 2019 , Nanophotonics)

   Abstract The phase matching between the propagating fundamental and nonlinearly generated waves plays an important role in the efficiency of the nonlinear frequency conversion in macroscopic crystals. However, in nanoscale samples, such as nanoplasmonic structures, the phase-matching condition is often ignored due to the sub-wavelength nature of the materials. Here, we first show that the phase matching of the lattice plasmon modes at the fundamental and second-harmonic frequencies in a plasmonic nanoantenna array can effectively enhance the surface-enhanced second-harmonic generation. Additionally, a significant enhancement of the second-harmonic generation is demonstrated using stationary band-edge lattice plasmon modes with zero phase. 

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4. Boosting the efficiency of high harmonic amplification by modulating a plasma-based X-ray laser with the second harmonic of the fundamental frequency laser field

   Khairulin, I.R. ; Antonov, I.R. ; Kocharovskaya, O. ( October 2021 , International conference on Ultrafast Optical Science)

   Attosecond pulses formed by high order harmonics (HHs) of an infrared (IR) laser field is a powerful tool for studying and controlling ultrafast dynamics of electrons in atoms, molecules and solids at its intrinsic time-scale. However, in the X-ray range the energy of attosecond pulses is rather limited. Their amplification is an important but very challenging problem since none of the existing amplifiers can support the corresponding PHz bandwidth. In our previous work [1] we proposed a method for the attosecond pulse amplification in hydrogen-like active medium of a recombination plasma-based X-ray laser dressed by a replica of the fundamental frequency IR field used for the HH generation. Due to the IRfield-induced sub-laser-cycle Stark shift and splitting of the lasing energy levels the gain of the active medium is redistributed over the combination frequencies, separated from the resonance by even multiples of the frequency of the IR field. If the incident HHs forming an attosecond pulse train are tuned in resonance with the induced gain lines and the active plasma medium is strongly dispersive for the modulating IR field, then during the amplification the relative phases of harmonics and (under the optimal choice of the IR field strength) the shape of the amplified pulses will be preserved. In the present work we show the possibility of boosting the efficiency of HH amplification by modulating the active medium of an X-ray laser with the second harmonic of the fundamental frequency IR field. We show that under the action of a laser field (with arbitrary frequency) the gain redistribution occurs not only over the even combination frequencies discussed in [1], but also over the odd frequencies separated from the resonance by odd multiples of the laser frequency. Besides, nearly half of the medium gain is contained in the even induced gain lines, and nearly half in the odd. If the modulating field is the second harmonic of the IR field, used for the generation the HHs and attosecond pulses, then the seeding HHs can be tuned in resonance with both even and odd gain lines simultaneously, which will make the overall gain much higher as compared to the previously considered case of the fundamental frequency modulating field (when only the even gain lines play the role). By the example of the C5+ X-ray laser with 3.38 nm wavelength of the inverted transition we show the possibility of increasing the efficiency of 430 as pulse amplification by 8.5 times when the active medium is modulated with the second harmonic of the fundamental frequency IR field with wavelength 2.1 µm. 

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5. A Model for Studying the Biomechanical Effects of Varying Ratios of Collagen Types I and III on Cardiomyocytes

   https://doi.org/https://doi.org/10.1007/s13239-020-00514-7  

   Roman, Brian ; Kumar, Shweta. A ; Allen, Shane C ; Delgado, Monica ; Moncayo, Sabastian ; Reyes, Andres M. ; Suggs, Laura J ; Chintalapalle, Ramana. ; Li, Chunqiang Q ; Joddar, Binata. ( June 2021 , Cardiovascular engineering and technology)

   null (Ed.)

   Purpose To develop a novel model composed solely of Col I and Col III with the lower and upper limits set to include the ratios of Col I and Col III at 3:1 and 9:1 in which the structural and mechanical behavior of the resident CM can be studied. Further, the progression of fibrosis due to change in ratios of Col I:Col III was tested. Methods Collagen gels with varying Col I:Col III ratios to represent a healthy (3:1) and diseased myocardial tissue were prepared by manually casting them in wells. Absorbance assay was performed to confirm the gelation of the gels. Rheometric analysis was performed on each of the collagen gels prepared to determine the varying stiffnesses and rheological parameters of the gels made with varying ratios of Col I:Col III. Second Harmonic Generation (SHG) was performed to observe the 3D characterization of the collagen samples. Scanning Electron microscopy was used for acquiring cross sectional images of the lyophilized collagen gels. AC16 CM (human) cell lines were cultured in the prepared gels to study cell morphology and behavior as a result of the varying collagen ratios. Cellular proliferation was studied by performing a Cell Trace Violet Assay and the applied force on each cell was measured by means of Finite Element Analysis (FEA) on CM from each sample. Results Second harmonic generation microscopy used to image Col I, displayed a decrease in acquired image intensity with an increase in the non-second harmonic Col III in 3:1 gels. SEM showed a fiber-rich structure in the 3:1 gels with well-distributed pores unlike the 9:1 gels or the 1:0 controls. Rheological analysis showed a decrease in substrate stiffness with an increase of Col III, in comparison with other cases. CM cultured within 3:1 gels exhibited an elongated rod-like morphology with an average end-to-end length of 86 ± 28.8 µm characteristic of healthy CM, accompanied by higher cell growth in comparison with other cases. Finite element analysis used to estimate the forces exerted on CM cultured in the 3:1 gels, showed that the forces were well dispersed, and not concentrated within the center of cells, in comparison with other cases. Conclusion This study model can be adopted to simulate various biomechanical environments in which cells crosstalk with the Collagen-matrix in diseased pathologies to generate insights on strategies for prevention of fibrosis. 

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