Welcome to the website of Seungchul Kim's research group at Pusan National University, exploring the frontiers of ultrafast laser technologies, utilizing material analysis, plasmonics, and innovative precision measurement/sensor infrastructures. Ultrafast/Ultra-precision technology is a new paradigm and not just an emerging research trend, as it can be applied to many existing fields of science and technology. Time, frequency and length are the most important basic physical quantities. The ‘ultrafast photonics’ enabled the ‘ultra-precision’ measurement of these quantities in the last decade with the ‘frequency comb’: 0.000000000000001 second in time, 0.000000000000001 Hz in frequency and 0.000000000001 m in length. These extreme precision will benefit all the measurements in cutting-edge technologies and fundamental sciences.
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High harmonic generation High Harmonic Generation (HHG) is a nonlinear optical process in which intense laser light interacts with a medium—typically a gas or solid—to generate new photons with much higher energies, corresponding to harmonics of the original laser frequency. This process can produce coherent light in the extreme ultraviolet (EUV) or even soft X-ray regions. HHG is widely used in attosecond science, ultrafast spectroscopy, and the development of compact, high-frequency light sources. The person interested: San Kim |
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Plasmon-enhanced high harmonic generation
Plasmon-enhanced high harmonic generation (HHG) is a technique that leverages the strong local field enhancement provided by plasmonic nanostructures to boost the efficiency of HHG processes. In conventional HHG, extremely high laser intensities are required to drive nonlinear electron dynamics in gases or solids, which limits integration and scalability. By using plasmonic nanostructures, such as nanoantennas or metasurfaces, the optical field can be locally intensified at the nanoscale, enabling efficient harmonic generation with significantly lower input power. This approach holds promise for compact, on-chip coherent extreme ultraviolet (EUV) sources and ultrafast photonic applications. The person interested: San Kim |
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Stable plasmon-enhanced third harmonic generation
Plasmonic nanoantennas enhance nonlinear optical processes by concentrating light at the nanoscale. However, due to the Gaussian intensity profile of a focused laser beam, nanoantennas at the beam edges produce less stable third-harmonic generation (THG) signals compared to those at the center. This spatial inhomogeneity causes fluctuations in the THG yield when the beam position shifts. To address this, the authors propose a method to design nanoantenna array density that reduces the ratio of ambiguous nanoantennas at the beam boundary—termed the Ratio of Ambiguity (ROA). A lower ROA leads to more stable THG output, making this approach useful for designing reliable sensors and nonlinear optical devices. The person interested: Tae-In Jeong |
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Frequency-resolved Optical Grating (FROG) •FROG is a technique for measuring both the amplitude and phase of ultrashort laser pulses in the time-frequency domain. •It uses a nonlinear optical process to generate a signal, which is then spectrally resolved to obtain a two-dimensional FROG trace. •The pulse’s temporal electric field and phase are retrieved by applying an iterative reconstruction algorithm to the measured FROG trace. The person interested: Tae-In Jeong |
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Phase-Enabled Nonlinear Gating with Unbalanced Intensity (PENGUIN)
•PENGUIN is a technique for measuring the complete characterization of ultrafast pulse without spectrometer. •PENGUIN introducing an intensity asymmetry into a conventional nonlinear interferometric autocorrelation preserves some spectral phase information within the autocorrelation signal, which enables the full reconstruction of the original electric field. •The performance of the PENGUIN is compared with general method of FROG, which show same result but require short time. The person interested: Tae-In Jeong |
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Optical frequency comb An optical frequency comb is a laser source whose spectrum consists of a series of equally spaced, discrete frequency lines, resembling the teeth of a comb. It is typically generated using mode-locked femtosecond lasers, which emit a train of ultra-short pulses. These pulses produce a comb-like structure in the frequency domain, characterized by two key parameters: the repetition rate, which determines the spacing between the comb lines, and the carrier-envelope offset frequency, which accounts for the phase difference between the carrier wave and the pulse envelope. Optical frequency combs have become essential tools in precision metrology and spectroscopy. They enable ultra-accurate frequency measurements and are widely used in optical atomic clocks, laser stabilization, frequency calibration, and dual-comb spectroscopy. Applications also extend to LIDAR, remote sensing, and fundamental quantum optics. The development of optical frequency combs has had a profound impact on science and technology, earning the 2005 Nobel Prize in Physics. By linking optical and microwave frequency domains, they have enabled new levels of precision in timekeeping and spectral analysis, and continue to drive advancements in fields ranging from quantum science to astronomy and molecular diagnostics. The person interested: San Kim |
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Frequency stability and linewidth analysis of interleaved combs generated by a PDMS phase modulator
We generated a new interleaved comb light source with extremely narrow spacing on the order of kilohertz (kHz) by applying a PDMS-based phase modulator between the frequency lines of a conventional frequency comb arranged at regular intervals. To evaluate the frequency stability and linewidth of this light source, we interfered it with a reference frequency comb that has high frequency and phase stability and a linewidth narrower than 1 Hz, thereby generating a radio-frequency (RF) beat signal. By analyzing the resulting RF beat signal, we were able to quantitatively determine the linewidth of the interleaved comb and assess its frequency stability through long-term monitoring over more than 4000 seconds. As a result, we show that a multi-frequency acousto-optic phase modulation at a chip-scale of soft polydimethylsiloxane can readily support a 200-times higher 0.5-MHz spectral resolution for the frequency-comb-based spectroscopy, while co-located plasmonic nanostructures mediate the strong light-matter interaction. These results suggest the potential of polydimethylsiloxane acousto-optic phase modulation for cost-effective, compact, multifunctional chip-scale tools in diverse applications such as quantum spectroscopy, high-finesse cavity analysis, and surface plasmonic spectroscopy. The person interested: San Kim |
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Nonlinear harmonic comb generation in Fiber-embedded nanoparticle clusters
We used meniscus-guided 3D printing to fabricate nanoparticle clusters on an optical fiber core and studied the characteristics of the beam induced by the clusters. This printing method is a bottom-up fabrication process in which a nanoparticle solution is extruded through the meniscus formed between a substrate and a glass micropipette. In a dry condition, the water in the meniscus evaporates, resulting in a structure composed solely of nanoparticles. It has been reported that this method enables the fabrication of various three-dimensional structures, including helical and multilayer structures. By forming clusters with silicon nanoparticles with a high nonlinear coefficient and irradiating a femtosecond laser with frequency comb characteristics at a frequency ω onto the optical fiber, third harmonic generation (THG) is induced. The resulting 3ω signal maintains the frequency comb characteristics and can function as a comb generator producing a frequency at 3ω. The person interested: San Kim, Eunju Yang |
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2D single photon emitter
Two-dimensional single photon emitters (2D SPEs) are quantum light sources that emit one photon at a time from atomically thin materials. They are considered essential components for next-generation quantum technologies such as quantum communication, sensing, and computing. Single photon emission in 2D materials typically arises from localized defects or strain, which create discrete energy states within the bandgap. we investigate strain-induced single photon emission in 2D materials by using laser ablation to locally modify the material and induce strain. This method enables spatially controlled generation of quantum emitters without the need for complex fabrication processes. The goal of this research is to develop controllable and scalable platforms for integrated quantum photonic devices based on 2D materials. The person interested: Sehyeon Kim |
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Multichannel hierarchical analysis of time-resolved hyperspectral data (TRH)
The TRH system can capture all spectral features, whereas the RGB detector struggles due to the cancel-out effect from its broad response band. To address this spectral ambiguity, multichannel hierarchical analysis using a TRH system is introduced in colorimetric e-noses. The colorimetric sensor changes the spectrum by reacting with the gas, and the time-dependent spectral variations are measured through the TRH system. These variations are captured as a hyperspectral 3D data cube, which is converted into a 2D multi-channel spectrogram using a novel data processing method to address the high-dimensional complexity of the 3D data cube. A convolution filter was then used for hierarchical analysis of the multi-channel spectrogram, effectively capturing the complex gas-induced spectral patterns and temporal dynamics. (ACS Sensors 2024, ACS Sensors 2025) The person interested: Eunji Choi, Tae-In Jeong |
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Photodetection via Plasmonic Thermoelectric effects
Plasmonic thermoelectric effects offer a powerful mechanism for converting light into electrical signals by utilizing heat generated from surface plasmon resonances. In our research, we explore how plasmon-induced temperature gradients in metallic nanostructures can drive thermoelectric voltages, enabling bias-free and filter-free photodetection with intrinsic spectral selectivity. This approach allows us to engineer narrowband photodetectors that are compact, energy-efficient, and highly integrable. By designing plasmonic structures with tailored optical responses, we can control both the spectral position and polarization sensitivity of the device. The person interested: Sehyeon Kim, San Kim |
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Time Division Multiplexing based Multi-spectral Semantic Camera for LiDAR
We design a time‑division‑multiplexing (TDM) based multi‑spectral camera system for semantic object inference by the simultaneous acquisition of spatial and spectral information. By utilizing the TDM method with nanosecond pulses of five different wavelength, it is possible to acquire the spatial and sufficient spectral information simultaneously as well as a TOF based distance map using only a single photodetector. Our work presents a compact novel spectroscopic camera system for LiDAR application, which provides increased recognition performance and thus a great potential to improve safety and reliability in autonomous driving. The person interested: Jae-Young Kim, Tae-In Jeong, Sehyeon Kim |