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Ultrafast Mode-locked Lasers
Coherent light sources emitting ultrashort pulse and high peak power are essential for investigation on diverse nonlinear and ultrafast phenomena. We are developing ultrafast solid-state lasers operating in various spectral ranges using Ti:sapphire, Yb:KYW, Yb:KLuW, Cr:forsterite, Cr:YAG, Cr:ZnS, etc. By using saturable absorbers such as SWCNTs, SESAM, and graphene, stable continuous-wave mode-locked operation with femtosecond pulses can be achieved.
Related publications
J. W. Kim, S. Y. Choi, W. T. Kim, B. J. Kang, W. B. Cho, G.-H. Kim, F. Rotermund*, "550-MHz carbon nanotube mode-locked femtosecond Cr:YAG laser," Chin. Opt. Lett. 16, 061404 (2018). (highlights & cover story)
W. B. Cho, J. E. Bae, S. C. Lee, N. S. Myoung, F. Rotermund*, "Femtosecond Mid-IR Cr:ZnS laser with transmitting graphene-ZnSe saturable absorber," Curr. Opt. Photon. 7, 738 (2023).
D. W. Kim, J.-H. Park, S.-E. Lim, F. Rotermund*, "Repetition-rate-tunable femtosecond mode-locked Yb:GYSO laser in the sub-GHz regime," Opt. Express 33, 44323 (2025).
Compact Waveguide Lasers
Recently, waveguide lasers which simplify a solid-state laser cavity into on-chip scale are in great interests. In particular, waveguide lasers can generate high-repetition-rate (GHz-level) mode-locked pulses to be utilized in fast sampling and high precision metrology. In addition, novel types of saturable absorbers necessary to achieve stable and self-starting pulse formation are developed based on low-dimensional carbon nanostructures.
Related publications
J. E. Bae, T. Calmano, C. Kränkel, F. Rotermund*, “Controllable dynamic single- and dual-channel graphene Q-switching in a beam-splitter-type channel waveguide laser," Laser Photon. Rev. 16, 2100501 (2022). (inside front cover)
J. E. Bae, M. Hyun, D. W. Kim, X. Mateos, J. G. Ajates, C. Romero, J. R. V. D. Aldana, J. Kim*, F. Rotermund*, "Characterization of noise spectra in low-jitter GHz mode-locked fs-laser-inscribed waveguide lasers," Opt. Laser Tech. 179, 111412 (2024).
D. W. Kim†, S. Won†, J.-H. Park, Y.-J. Kim*, F. Rotermund*, "Sub-picosecond mode-locked laser operation in a femtosecond-lased-inscribed Yb:CaF2 channel waveguide," Opt. Express 32, 42801 (2024).
Nanocarbon Photonics
Nanocarbons such as carbon nanotubes (CNTs) and graphene are one of the most attractive materials for applications in photonics and optoelectronics. We are working on developing novel ultrafast switching devices based on nanocarbons for laser mode-locking and Q-switching. The broadband absorption and nonlinear optical properties of CNTs can be optimized through precise control of synthesis and fabrication processes. We have demonstrated CNT-based saturable absorbers applicable for broadband femtosecond laser mode-locking in a spectral range between 0.8 and 2.4 µm. We are also developing graphene saturable absorbers with almost 'unlimited' operation wavelength range due to unique point bandgap structure.
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THz wave generation and spectroscopy
Terahertz (THz) wave has recently attracted attention as a future communication platform, non-destructive imaging, and spectroscopy methods. THz pulses can be typically generated by transient current and optical rectification in photo-conductive antennas and nonlinear crystals, respectively. Based on Ti:sapphire oscillator and regenerative amplifier system, we establish various THz setups for different measurements, such as linear/nonlinear THz time-domain spectroscopy, optical-pump/THz-probe, and THz emission spectroscopy, etc. We study characteristics of various materials, including 2D materials, magnetic materials, and nonlinear materials, through the above THz setups.
Related publications
S.-H. Lee†, B. J. Kang†, B.-W. Yoo, S.-C. Lee, S.-J. Lee, M. Jazbinsek, H. Yun, F. Rotermund*, O-P. Kwon*, “Terahertz phonon mode engineering of highly efficient organic terahertz generators,” Adv. Funct. Mater. 27, 1605583 (2017). (journal cover)
B. J. Kang, S.-H. Lee, W. T. Kim, S.-C. Lee, K. Lee, G. Benacchio, G. Montemezzani, M. Jazbinsek*, O-P. Kwon*, F. Rotermund*, "New class of efficient terahertz generators: Effective terahertz spectral filling by complementary tandem configuration of nonlinear organic crystals," Adv. Funct. Mater. 28, 1707195 (2018). (inside front cover)
S.-J. Kim†, I. C. Yu†, D.-J. Kim, M. Jazbinsek, W. Yoon, H. Yun, D. Kim, F. Rotermund*, O-P. Kwon*, “A New Class of Organic Crystals with Extremely Large Hyperpolarizability: Efficient THz wave generation with wide flatpspectral-band,” Adv. Funct. Mater. 33, 2209915 (2023).
C. Lee†, E.-T. Oh†, T. G. Park, S. Lee, S. J. Kim, Y.-S. Choi*, F. Rotermund*, D. K. Yoon*, "Terahertz-polarizing Effect Based on Geometric Anisotropy of Ti3C2Tx MXene Nanosheets," Carbon 225, 119156 (2024).
Terahertz nonlinear photonics
We focus on THz nonlinear & time-resolved spectroscopy and THz nonlinear phenomena in various materials including highly nonlinear organic crystals, low-dimensional nanomaterials, metamaterials, hybrid nanostructures and their applications. In addition, we develop ultrafast high-power (> 10 mW), high-field(~1 MV/cm) broadband THz source to investigate THz nonlinearities and their enhancement. Different THz setups based on Ti:sapphire oscillator and regenerative amplifier are established for different applications.
Related publications
Y.-M. Bahk†, B. J. Kang†, Y. S. Kim, J.-Y. Kim, W. T. Kim, T. Y. Kim, T. Kang, J. Rhie, S. Han, C.-H. Park, F. Rotermund*, D.-S. Kim*, “Electromagnetic saturation of angstrom-sized quantum barriers at terahertz frequencies,” Phys. Rev. Lett. 115, 125501 (2015). (editor's suggestion & cover article)
K. Miyamoto†, B. J. Kang†, W. T. Kim, Y. Sasaki, H. Niinomi, K. Suizu, F. Rotermund*, T. Omatsu*, “Highly intense monocycle terahertz vortex generation by utilizing a Tsurupica spiral phase plate,” Sci. Rep. 6, 38880 (2016).
I. H. Baek†, J. M. Hamm†, K. J. Ahn†, B. J. Kang, S. S. Oh, S. Bae, S. Y. Choi, B. H. Hong, D.-I. Yeom, B. Min, O. Hess, Y. U. Jeong, F. Rotermund*, “Boosting the terahertz nonlinearity of graphene by orientation disorder,” 2D Mater. 4, 025035 (2017).
C. Kim†, S. W. Jin†, S. C. Lee, D. Oh, S. Baek, B. Min, H. J. Yang*, J.-H. Son*, F. Rotermund*, "Frequency-dependent demethylation of cancer DNAs by high-power terahertz radiation," Adv. Photon. Res. 6, 2400231 (2025).
UV-to-THz Time-resolved & Nonlinear Spectroscopy
In order to understand ultrafast carrier dynamics and nonlinear responses, we investigate degenerate and non-degenerate pump-probe & nonlinear spectroscopy, whereas different femtosecond excitation sources (Ti:sapphire laser, Yb-doped, Cr-doped and Er-doped lasers, OPO, OPA and THz sources) are used. We are able to measure both transmission- and reflection-type samples in a broad spectral range from UV to THz.
Related publications
J. J. Yoo, G. Seo, M. R. Chua, T. G. Park, Y. Lu, F. Rotermund, Y.-K. Kim, C. S. Moon, N. J. Jeon, J.-P. Correa-Baena, V. Bulovic, S. S. Shin, M. G. Bawendi, J. Seo, “Efficient perovskite solar cells via improved carrier management,” Nature 590, 587 (2021). (journal cover)
J. Park†, B. J. Kang†, G. Seo, J. Kim, T. G. Park, S. Nam, J. J. Yoo, K. Lee, Y. U. Jeong, M. Park, S. S. Shin*, F. Rotermund*, "Cross-correlation between crystallinity and optoelectronic properties of mixed perovskite thin films through multiple time-resolved spectroscopy," Adv. Energy Mater. 15, 2400225 (2025).
J. Lee†, C. Sun†, J. Park†, C. Kim, S. Lee, D. Lee, M.-H. Lee, B. Kim, Y. H. Kim, J. Kim, S. Lee, S. Jeong, W. B. Ying, X. Song, S. Cho, F. Rotermund*, Y.-H. Kim*, J.-Y. Lee*, "High Efficiency (>10%) AgBiS2 Colloidal Nanocrystal Solar Cells with Diketopyrrolopyrrole-Based Polymer Hole Transport Layer," Adv. Mater. 37, 2413081 (2025).
G. Seo, J. J. Yoo, S. Nam, D. S. Lee, S. Gao, B. K. Kim, S. J. Sung, B. J. Kang, D. W. de Quilettes, J. Park, J.-S. Park, I. S. Cho, F. Rotermund, S. I. Seok*, S. S. Shin*, "Efficient and luminescent perovskite solar cells using defect-suppressed SnO2 via excess ligand strategy," Nat. Energy 10, 774 (2025). (journal cover)
J. Lee, C. S. Moon, Y. Kim, Y. Y. Kim, E. Y. Park, J. J. Yoo, G. Kim, H. Lee, H. Na, J. Park, D. Yoon, F. Rotermund, E. S. Oh, T.-S. Kim, S. Song, T. J. Shin, J. H. Noh, N. J. Jeon, J. Seo, "Tailored n value engineering of Dion-Jacobson 2D layers enables efficient and stable perovskite solar cells." Joule 10, 102301 (2026).
Y.-H. Seo, E. Y. Park, C. S. Moon, D. G. Lee, J. Park, F. Rotermund, G. S. Han, T. K. Lee, N. J. Jeon, B. J. Kang, Y. Y. Kim, "Alcohol-adduct mediated precursor stabilization strategies for conformal SnO2 layers in highly efficient and stable perovskite solar modules." Adv. Energy Mater. e06764 (2026).
M.-H. Lee†, M. S. Kim†, J. Park†, Y. Cho, S.-J. Hong, N. Kwon, T. Ki, J. Lee, B. Kim, K. Lee, J. Seo, S. K. Kwak*, F. Rotermund*, J.-Y. Lee*, "Hole-transfer cascade-engineered donor polymer for unencapsulated perovskite solar cells with improved moisture stability," Nat. Energy (2026) in press.
Nonlinear optical characterization of novel materials
One of our research interests is the nonlinear optical characterization of various materials, including 2D materials, with high resolution. We are able to investigate second/third-order nonlinearity such as second harmonic generation and nonlinear absorption, which provide quantitative information for ultrafast photonics. The transmission change can be resolved with a resolution of less than 0.1%.
Related publications
T. G. Park, B. K. Choi, J. Park, J. Kim, Y. J. Chang*, F. Rotermund*, "Interlayer coupling and ultrafast hot electron transfer dynamics in metallic VSe2/graphene van der Waals heterostructures", ACS Nano 15, 7756 (2021).
T. G. Park, H. R. Na, S.-H. Chun, W. B. Cho, S. Lee, F. Rotermund*, “Coherent control of interlayer vibrations in Bi2Se3 van der Waals thin-films,” Nanoscale 13, 19264 (2021).
T. G. Park, J. H. Jeon, S.-H. Chun, S. Lee, F. Rotermund*, "Ultrafast interfacial carrier dynamics and persistent topological surface states of Bi2Se3 in heterojunctions with VSe2," Commun. Phys. 5, 182 (2022).
T. G. Park, E.-T. Oh, S. Kim, Y. Ou, J. Moodera, H. Kim, F. Rotermund*, "Ultrafast formation of quantized interlayer vibrations in Bi2Se3 by photoinduced strain waves," Opt. Express 30, 35988 (2022).
T. G. Park, C. Kim, E.-T. Oh, H. R. Na, S.-H. Chun, Y.-H. Kim, S. Lee, F. Rotermund*, "Ultrafast acousto-optic modulation at the near-infrared spectral range by interlayer vibrations," Nanophoton. 13, 1059 (2024).
T. G. Park, S. Baek, J. Park, E.-C. Shin, H. R. Na, E.-T. Oh, S.-H. Chun, Y.-H. Kim*, S. Lee*, F. Rotermund*, "Spectroscopic Evidence of Ultrafast Topological Phase Transition by Light-Driven Strain," ACS Nano 18, 30966 (2024).
T. G. Park†, M. Kang†, E.-T. Oh†, A. Ham, K. Kang*, F. Rotermund*, "Low surface recombination velocity and enhanced photocarrier dynamics in Bi2O2Se nanosheets," Adv. Opt. Mater. 13, 2403138 (2025).
E.-T. Oh†, T. G. Park†, S.-E. Lim, H. R. Na, S.-H. Chun, S. Lee, F. Rotermund*, "Mode-selective and anomalous decoherence mechanisms of interlayer phonons in a topological Insulator," ACS Nano 20, 624 (2026).
Ultrafast spintronics
Future electronic devices require extended functionality beyond electronic charge and operating speeds beyond gigahertz frequencies. Optical manipulation of spin angular momentum has attracted attention due to its potential to accelerate the speed of information engineering to femtosecond time scales, and thus, terahertz (THz) frequencies. Strong THz field pulse and optical pulse can manipulate magnetic excitations with THz frequencies (Fig. a-c), and conversely, spin transfer and relaxation can emit THz pulse, making it efficient spintronic THz emitter (Fig. d-e).
Related publications
J.-H. Park†, W. T. Kim†, W. Won, J.-H. Kang, S. Lee, B.-G. Park, B. S. Ham, Y. Jo, F. Rotermund*, K.-J. Kim*, “Observation of spin-glass-like characteristics in ferromagnetic TbCo through energy-level-selective approach,” Nat. Commun. 13, 5530 (2022).
I. C. Yu, B.-G. Park, S. B. Kim, N. Noh, J. M. Yuk, Y. Kim, F. Rotermund*, "Study on microscopic origin of spin excitation in TmFeO3 by time-resolved muticolor optical pump-probe spectroscopy," Curr. Appl. Phys. 62, 54 (2024).
I. C. Yu†, J. H. Oh†, J.-H. Shim, K.-J. Kim, B.-G. Park, D.-H. Kim, K.-W. Kim, H.-W. Lee, K.-J. Lee*, F. Rotermund*, "Laser-induced coherent spin change due to spin-orbit coupling," npj Spintronics 3, 4 (2025).
I. C. Yu†, S. Ko†, W. T. Kim, B.-G. Park, B. S. Ham, K.-J. Kim*, F. Rotermund*, "Photon energy-dependent ultrafast magnetization dynamics in magnetic heterostructures," Commun. Mater. 7, 15 (2026).
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