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1. Explicit formula for high-order sideband polarization by extreme tailoring of Feynman path integrals

   https://doi.org/10.1103/PhysRevB.107.174308  

   Wu, Qile; Sherwin, Mark S. (May 2023, Physical Review B)

   High-order sideband generation (HSG), as an analog of the interband processes in high-harmonic generation (HHG) in solids, is a nonperturbative nonlinear optical phenomenon in semiconductors that are simultaneously driven by a relatively weak near-infrared (NIR) laser and a sufficiently strong terahertz (THz) field. We derive an explicit formula for sideband polarization vectors in a prototypical two-band model based on the saddle-point method. Our formula connects the sideband amplitudes with the laser-field parameters, electronic structures, and nonequilibrium dephasing rates in a highly nontrivial manner. Our results indicate the possibility of extracting information on band structures and dephasing rates from high-order sideband generation experiments with simple algebraic calculations. We also expect our approach to be useful on the quantitative understanding of the interband HHG. 

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2. Continuum Reverberation Mapping of Mrk 876 over Three Years with Remote Robotic Observatories

   https://doi.org/10.3847/1538-4357/ace342  

   Miller, Jake A.; Cackett, Edward M.; Goad, Michael R.; Horne, Keith; Barth, Aaron J.; Romero-Colmenero, Encarni; Fausnaugh, Michael; Gelbord, Jonathan; Korista, Kirk T.; Landt, Hermine; et al (August 2023, The Astrophysical Journal)

   Abstract Continuum reverberation mapping probes the size scale of the optical continuum-emitting region in active galactic nuclei (AGN). Through 3 yr of multiwavelength photometric monitoring in the optical with robotic observatories, we perform continuum reverberation mapping on Mrk 876. All wave bands show large-amplitude variability and are well correlated. Slow variations in the light curves broaden the cross-correlation function (CCF) significantly, requiring detrending in order to robustly recover interband lags. We measure consistent interband lags using three techniques (CCF, JAVELIN, and PyROA), with a lag of around 13 days from u to z . These lags are longer than the expected radius of 12 days for the self-gravitating radius of the disk. The lags increase with wavelength roughly following λ 4/3 , as would be expected from thin disk theory, but the lag normalization is approximately a factor of 3 longer than expected, as has also been observed in other AGN. The lag in the i band shows an excess that we attribute to variable H α broad-line emission. A flux–flux analysis shows a variable spectrum that follows f ν ∝ λ −1/3 , as expected for a disk, and an excess in the i band that also points to strong variable H α emission in that band. 

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3. High-temperature and low-threshold interband cascade lasers at wavelengths longer than 6  μm

   https://doi.org/10.1117/1.OE.57.1.011021  

   Rassel, S.M.Shazzad; Li, Lu; Li, Yiyun; Yang, Rui Q.; Gupta, James A; Wu, Xiaohua; Aers, Geof C. (January 2018, Optical Engineering)

   InAs-based interband cascade (IC) lasers with improved optical confinement have achieved high-temperature operation with a threshold current density as low as 333 A/cm2 at 300 K for emission at 6003 nm. The threshold current density is the lowest ever reported among semiconductor mid-infrared lasers at similar wavelengths. These InAs-based IC devices lased in pulsed mode at temperatures up to 357 K near 6.28 μm. A narrow-ridge device was able to operate in continuous-wave mode at temperatures up to 293 K at 6.01 μm. 

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4. Generalized Wilson loop method for nonlinear light-matter interaction

   https://doi.org/10.1038/s41535-022-00472-4  

   Wang, Hua; Tang, Xiuyu; Xu, Haowei; Li, Ju; Qian, Xiaofeng (December 2022, npj Quantum Materials)

   Abstract Nonlinear light–matter interaction, as the core of ultrafast optics, bulk photovoltaics, nonlinear optical sensing and imaging, and efficient generation of entangled photons, has been traditionally studied by first-principles theoretical methods with the sum-over-states approach. However, this indirect method often suffers from the divergence at band degeneracy and optical zeros as well as convergence issues and high computation costs when summing over the states. Here, using shift vector and shift current conductivity tensor as an example, we present a gauge-invariant generalized approach for efficient and direct calculations of nonlinear optical responses by representing interband Berry curvature, quantum metric, and shift vector in a generalized Wilson loop. This generalized Wilson loop method avoids the above cumbersome challenges and allows for easy implementation and efficient calculations. More importantly, the Wilson loop representation provides a succinct geometric interpretation of nonlinear optical processes and responses based on quantum geometric tensors and quantum geometric potentials and can be readily applied to studying other excited-state responses. 

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5. On the Su–Schrieffer–Heeger model of electron transport: Low‐temperature optical conductivity by the Mellin transform

   https://doi.org/10.1111/sapm.12604  

   Margetis, Dionisios; Watson, Alexander B.; Luskin, Mitchell (August 2023, Studies in Applied Mathematics)

   Abstract We describe the low‐temperature optical conductivity as a function of frequency for a quantum‐mechanical system of electrons that hop along a polymer chain. To this end, we invoke the Su–Schrieffer–Heegertight‐bindingHamiltonian for noninteracting spinless electrons on a one‐dimensional (1D) lattice. Our goal is to show via asymptotics how the interband conductivity of this system behaves as the smallest energy bandgap tends to close. Our analytical approach includes: (i) the Kubo‐type formulation for the optical conductivity with a nonzero damping due to microscopic collisions, (ii) reduction of this formulation to a 1D momentum integral over the Brillouin zone, and (iii) evaluation of this integral in terms of elementary functions via the three‐dimensional Mellin transform with respect to key physical parameters and subsequent inversion in a region of the respective complex space. Our approach reveals an intimate connection of the behavior of the conductivity to particular singularities of its Mellin transform. The analytical results are found in good agreement with direct numerical computations. 

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