https://scholar.dgist.ac.kr/handle/20.500.11750/73 2026-08-03T11:17:58Z 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; Choi, Youngchan; 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 https://scholar.dgist.ac.kr/handle/20.500.11750/58574 Title: Floquet engineering of excitons in monolayer MoS2 Author(s): Park, Hyosub; Lee, JaeDong Abstract: Floquet dynamics temporally deforms the band structure of the system, called Floquet engineering, under a strong optical field. Although a single resonant pulse simultaneously drives both exciton and Floquet dynamics, the Floquet engineering during exciton generation remains unexplored. We examine the Floquet engineering of excitons by a theoretical simulation of the time-resolved and angle-resolved photoemission spectroscopy in monolayer MoS2. Our finding reveals that the exciton experiences nontrivial engineering inducing a distinct asymmetry in the development of spectral splitting with the time delay and pump intensity, which are underlain by the electron-hole interaction inherent in the exciton binding. © 2025 American Physical Society. 2025-04-30T15:00:00Z https://scholar.dgist.ac.kr/handle/20.500.11750/58295 Title: Strain-Induced Bandgap Narrowing in Crumpled TMDs for NIR Light Detection Author(s): Katiyar, Ajit Kumar; Kim, Youngjae; Kim, Beom Jin; Choi, Jonggyu; Hoang, Anh Tuan; Lee, JaeDong; Ahn, Jong-Hyun Abstract: Transition metal dichalcogenides (TMDs) such as MoS2 and WS2 emerge as promising materials in optoelectronics, especially for flexible photo- /image-sensors due to their direct bandgap nature. However, the intrinsic bandgaps of these semiconductor monolayers (e.g., MoS2 ≈1.86 eV and WS2 ≈2.0 eV) restrict the operational wavelength range of developed photosensors in the visible spectrum. In addition, their ultrathin nature provides a limited optical absorption cross-section that restricts the device's performance. Exploiting the strong impact of strain on the electronic band structure, strain engineering has emerged as a promising approach for adjusting the electrical and optical characteristics of layered semiconductors. In particular, the application of tensile strain in MoS2 and WS2 can decrease their bandgaps, which potentially can extend the optical absorption toward the near-infrared (NIR) wavelength. Herein, a non-conventional crumpling approach is employed to incorporate uniaxial tensile strain into a graphene/TMD/graphene metal-semiconductor-metal photodetector (PD) array. The utilized crumpled geometry provides exclusive photon management with enhanced light scattering and trapping at the sinusoidal surface that results in increased light absorption in NIR wavelength range. © 2025 Wiley-VCH GmbH. 2025-04-30T15:00:00Z