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1. Ellipticity dependence of anticorrelation in the nonsequential double ionization of Ar

   https://doi.org/10.1364/OE.475497  

   Chen, Zhangjin ; Li, Shuqi ; Kang, Huipeng ; Morishita, Toru ; Bartschat, Klaus ( November 2022 , Optics Express)

   Within the framework of the improved quantitative rescattering (QRS) model, we simulate the correlated two-electron momentum distributions (CMDs) for nonsequential double ionization (NSDI) of Ar by elliptically polarized laser pulses with a wavelength of 788 nm at an intensity of 0.7 × 10¹⁴W/cm²for the ellipticities ranging from 0 to 0.3. Only the CMDs for recollision excitation with subsequent ionization (RESI) are calculated and the contribution from recollision direct ionization is neglected. According to the QRS model, the CMD for RESI can be factorized as a product of the parallel momentum distribution (PMD) for the first released electron after recollision and the PMD for the second electron ionized from an excited state of the parent ion. The PMD for the first electron is obtained from the laser-free differential cross sections for electron impact excitation of Ar⁺calculated using state-of-the-art many-electronR-matrix theory while that for the second electron is evaluated by solving the time-dependent Schrödinger equation. The results show that the CMDs for all the ellipticities considered here exhibit distinct anticorrelated back-to-back emission of the electrons along the major polarization direction, and the anticorrelation is more pronounced with increasing ellipticity. It is found that anticorrelation is attributed to the pattern of the PMD for the second electron ionized from the excited state that, in turn, is caused by the delayed recollision time with respect to the instant of the external field crossing. Our work shows that both the ionization potential of the excited parent ion and the laser intensity play important roles in the process.

    

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2. Nonsequential double ionization of Ar in near-single-cycle laser pulses

   https://doi.org/10.1364/OE.398035  

   Chen, Zhangjin ; Liu, Fang ; Wen, Hua ; Morishita, Toru ; Zatsarinny, Oleg ; Bartschat, Klaus ( July 2020 , Optics Express)

   Using the improved quantitative rescattering (QRS) model, we simulate the correlated two-electron momentum distributions (CMD) for nonsequential double ionization (NSDI) of Ar by near-single-cycle laser pulses with a wavelength of 750 nm at an intensity of 2.8 × 10¹⁴W/cm². With the accurate cross sections obtained from fully quantum mechanical calculations for both electron impact excitation and electron impact ionization of Ar⁺, we unambiguously identify the contributions from recollision direct ionization (RDI) and recollision excitation with subsequent ionization (RESI). Our analysis reveals that RESI constitutes the main contribution to NSDI of Ar under the conditions considered here. The simulated results are directly compared with experimental measurements [Bergueset al.,Nat. Commun.3,813(2012)10.1038/ncomms1807] in which each NSDI event is tagged with the carrier-envelope phase (CEP). It is found that the overall pattern of both the CEP-resolved and the CEP-averaged CMDs measured in experiment are well reproduced by the QRS model, and the cross-shaped structure in the CEP-averaged CMD is attributed to the strong forward scattering of the recolliding electron as well as the depletion effect in tunneling ionization of the electron from an excited state of the parent ion.

    

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3. All-optical sampling of few-cycle infrared pulses using tunneling in a solid

   https://doi.org/10.1364/PRJ.420916  

   Liu, Yangyang ; Gholam-Mirzaei, Shima ; Beetar, John E. ; Nesper, Jonathan ; Yousif, Ahmed ; Nrisimhamurty, M. ; Chini, Michael ( May 2021 , Photonics Research)

   Recent developments in ultrafast laser technology have resulted in novel few-cycle sources in the mid-infrared. Accurately characterizing the time-dependent intensities and electric field waveforms of such laser pulses is essential to their applications in strong-field physics and attosecond pulse generation, but this remains a challenge. Recently, it was shown that tunnel ionization can provide an ultrafast temporal “gate” for characterizing high-energy few-cycle laser waveforms capable of ionizing air. Here, we show that tunneling and multiphoton excitation in a dielectric solid can provide a means to measure lower-energy and longer-wavelength pulses, and we apply the technique to characterize microjoule-level near- and mid-infrared pulses. The method lends itself to both all-optical and on-chip detection of laser waveforms, as well as single-shot detection geometries.

    

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4. Coherent modulation of the electron temperature and electron–phonon couplings in a 2D material

   https://doi.org/10.1073/pnas.1917341117  

   Zhang, Yingchao ; Shi, Xun ; You, Wenjing ; Tao, Zhensheng ; Zhong, Yigui ; Cheenicode Kabeer, Fairoja ; Maldonado, Pablo ; Oppeneer, Peter M. ; Bauer, Michael ; Rossnagel, Kai ; et al ( April 2020 , Proceedings of the National Academy of Sciences)

   Ultrashort light pulses can selectively excite charges, spins, and phonons in materials, providing a powerful approach for manipulating their properties. Here we use femtosecond laser pulses to coherently manipulate the electron and phonon distributions, and their couplings, in the charge-density wave (CDW) material 1T-TaSe₂. After exciting the material with a femtosecond pulse, fast spatial smearing of the laser-excited electrons launches a coherent lattice breathing mode, which in turn modulates the electron temperature. This finding is in contrast to all previous observations in multiple materials to date, where the electron temperature decreases monotonically via electron–phonon scattering. By tuning the laser fluence, the magnitude of the electron temperature modulation changes from ∼200 K in the case of weak excitation, to ∼1,000 K for strong laser excitation. We also observe a phase change of π in the electron temperature modulation at a critical fluence of 0.7 mJ/cm², which suggests a switching of the dominant coupling mechanism between the coherent phonon and electrons. Our approach opens up routes for coherently manipulating the interactions and properties of two-dimensional and other quantum materials using light.

    

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5. Progress, challenges, and opportunities of terahertz emission from liquids

   https://doi.org/10.1364/JOSAB.446095  

   E, Yiwen ; Zhang, Liangliang ; Tsypkin, Anton ; Kozlov, Sergey ; Zhang, Cunlin ; Zhang, X. -C. ( January 2022 , Journal of the Optical Society of America B)

   Broadband terahertz (THz) wave emission from flowing liquid targets has been demonstrated under short optical pulse excitation. Observations have been reported by using liquid THz sources, including optimal angle of incidence, preference of subpicosecond pulse excitation, and strong sideway emission. Compared with solid targets, the fluidity of liquid allows each laser pulse to interact with a fresh area, which makes it possible to use a table-top laser with a high repetition rate for excitation. Liquids with a comparable material density to solids make them promising candidates for the study of high-density plasma and bright THz sources. In this paper, we review recent progress, challenges, and opportunities of THz emission from liquids. This topic may offer new possibilities in the exploration of THz liquid photonics and may play an indispensable role in the study of laser-liquid interaction.

    

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