https://scholar.dgist.ac.kr/handle/20.500.11750/73 2026-11-15T15:47:39Z https://scholar.dgist.ac.kr/handle/20.500.11750/61114 Title: Third-Harmonic Generation in Crumpled Silicon Nanomembranes Author(s): Lee, Gwanjin; Katiyar, Ajit K.; Ahn, Jong-Hyun; Kim, Hyunmin; Lee, JaeDong Abstract: Silicon nanomembranes provide mechanically compliant devices for strain engineering. Here, we investigate third-harmonic generation in flat and crumpled silicon nanomembranes of 15 and 35 nm using polarization-resolved third-harmonic generation microscopy with ab initio calculations. In flat silicon nanomembranes, the measured polarization dependence is reproduced by the calculated third-order susceptibility, identifying the intrinsic nonlinear response of silicon. In crumpled silicon nanomembranes, local strain variations between valleys and apexes generate distinct polarization-dependent responses. Uniaxial strain along the [110] direction breaks electronic structure symmetry and induces anisotropy between X- and Y-polarized components, while the real and imaginary parts of the third-order susceptibility exhibit quadratic strain dependence. Morphology-induced redistribution of the local optical field further modifies the response in crumpled geometry, and a finite-difference time-domain-based local-field correction substantially improves agreement with the experiment. Therefore, the third-harmonic response of crumpled silicon nanomembranes is controlled by both strain-induced electronic structure modification and morphology-induced local optical-field redistribution. 2026-09-30T15:00:00Z https://scholar.dgist.ac.kr/handle/20.500.11750/60677 Title: Superradiance and Broadband Emission Driving Fast Electron Dephasing in Open Quantum Systems Author(s): Bae, Gimin; Kim, Youngjae; Lee, Jae Dong Abstract: The physical origin of the ultrashort dephasing time T 2 (approximate to O(1) fs), persistently addressed in the solid-state high-harmonic generation (HHG), remains an unresolved and challenging issue because the electron scattering at such a short timescale within solids is hardly identified. Here we investigate HHG of the 1D Hubbard model in the dissipative quantum environment within a frame of the Lindblad equation. In the study, in the limit of a small interatomic distance, we make the first verification of the solid-state Dicke superradiance, the coherent stimulated emission triggered by the spontaneous emission, as well as the broadband emission mimicking the blackbody radiation due to the electron scattering in each harmonic multiple. Further, we find a strong destructive interference between the Dicke superradiance and the broadband emission, which makes a scale down of the effective electron scattering time and leads to just few-femtosecond dephasing time T 2. This finding explains a long-standing problem of the ultrafast femtosecond electron dephasing in HHG. The present study could also serve as a pertinent platform for understanding the nonequilibrium dissipative dynamics of correlated electron systems. 2026-04-30T15:00:00Z https://scholar.dgist.ac.kr/handle/20.500.11750/60542 Title: Manipulating nonlinear dephasing dynamics of Dirac fermions in nearly degenerate four-wave mixing Author(s): Ha, Seongju; 최영찬; Park, Nam Hun; Choi, Jungseok; Park, Jaedeok; Ahn, Kwang Jun; Park, Hee-Su; Lee, JaeDong; Lee, Sang Min; Yeom, Dong-Il Abstract: Graphene has led the exploration of nonlinear optical responses in two-dimensional materials with exceptionally strong third-order nonlinearity and its electrical controllability. Nonlinear wave mixing with difference-frequency is particularly interesting in graphene because of the divergent nature of third-order susceptibility as the frequency difference approaches zero, but the study on nearly degenerated four-wave mixing (NDFWM) process in graphene is largely unexplored. In this work, we report the giant third-order susceptibility of monolayer graphene, reaching the order of 10-13 m2 V-2 at the optical telecom C-band via the NDFWM process, and its electrical tunability with a high on-off contrast of 23 dB. Moreover, we observed that the NDFWM response under electrical doping exhibits a resonance feature at low pump intensity in ambient conditions, which is substantially altered by varying the pump power. Through non-perturbative quantum master equation calculations, we revealed that our observation is closely related to the dephasing nature of the Dirac fermion of graphene. The decoherence time of photoexcited carriers is estimated up to 70 fs at low pump intensity, which regime is not accessible by other nonlinear means such as high harmonic generation requiring high intensity light. Our findings not only pave an unprecedented route for probing nonlinear dynamics of photoexcited carriers across a wide range but also have a significant impact on ultrafast nonlinear information processing in graphene. 2026-05-31T15:00:00Z https://scholar.dgist.ac.kr/handle/20.500.11750/60357 Title: Pseudospin-selective polarimetric singularities in high-harmonic generation of black phosphorus Author(s): Choi, Youngchan; Bae, Gimin; Lee, J. D. Abstract: Black phosphorus (BP) exhibits net opposite pseudospin polarizations for the electron and hole states. The pseudospin structure of BP causes a selectivity in the optical excitation engaging the symmetry between the optical pump polarization and the pseudospin state, which is confirmed in a simulation of the time-resolved angle-resolved photoemission spectroscopy (tr-ARPES). Further, the pseudospin selectivity is found to drive a unique polarimetric singularity in the high-harmonic generation (HHG). Given the nth-order high-harmonic signal, we reveal that the singularity arises predominantly through the multiphoton interband pathway and thereby becomes markedly substantial at n omega pump >= Eg. omega pump is the pump photon energy and Eg the energy gap of BP. This permits a coherent understanding of the pseudospin selectivity from tr-ARPES to HHG. In particular, the pseudospin-selective polarimetric singularity suggests a potential for the pseudospintronics to be integrated into the intrinsic dynamics due to the light-matter interaction in two-dimensional materials. 2026-02-28T15:00:00Z