Jaehun Park

Vacuum Electronic High Power Terahertz Sources

Recent research and development has been incredibly successful at advancing the capabilities for vacuum electronic device (VED) sources of powerful terahertz (THz) and near-THz coherent radiation, both CW or average and pulsed. Currently, the VED source portfolio covers over 12 orders of magnitude in power (mW-to-GW) and two orders of magnitude in frequency (from <; 0.1 to >; 10 THz). Further advances are still possible and anticipated. They will be enabled by improved understanding of fundamental beam-wave interactions, electromagnetic mode competition and mode control, along with research and development of new materials, fabrication methods, cathodes, electron beam alignment and focusing, magnet technologies, THz metrology and advanced, broadband output radiation coupling techniques.

Dielectric relaxation change of water upon phase transition of a lipid bilayer probed by terahertz time domain spectroscopy

We investigate the influence of the 1, 2-ditetradecanoyl-sn-glycero-3-phosphocholine lipid bilayer phases on the water reorientation dynamics with terahertz time domain spectroscopy. The phase of the lipids was controlled by the temperature in the range of 14-35 °C. During the gel-to-fluid phase transition, the hydration water ratio drastically changed from 0.3 to 0.6. The absorption coefficient of the hydration water increased with the temperature in the gel phase and then decreased in the fluid phase. The dielectric relaxation time of the lipid solution decreased initially but then increased after the phase transition. This indicates that the hydration water reorientation dynamics are restricted by lipids and that this phenomenon is pronounced in a biologically relevant fluid phase.

Generation, transport, and detection of linear accelerator based femtosecond-terahertz pulses

The generation and detection of intense terahertz (THz) radiation has drawn a great attention recently. The dramatically enhanced energy and peak electric field of the coherent THz radiation can be generated by coherent superposition of radiated fields emitted by ultrafast electron bunches. The femtosecond (fs)-THz beamline construction at the Pohang Accelerator Laboratory (PAL) was completed in the end of 2009. The fs-THz beamline at PAL can supply ultrafast and intense fs-THz radiation from a 75 MeV linear accelerator. The radiation is expected to have frequency up to 3 THz (∼100 cm(-1)) and the pulse width of <200 fs with pulse energy up to 10 μJ. This intense THz source has great potential for applications in nonlinear optical phenomena and fields such as material science, biomedical science, chemistry, and physics, etc.

High-power femtosecond-terahertz pulse induces a wound response in mouse skin

Terahertz (THz) technology has emerged for biomedical applications such as scanning, molecular spectroscopy, and medical imaging. Although a thorough assessment to predict potential concerns has to precede before practical utilization of THz source, the biological effect of THz radiation is not yet fully understood with scant related investigations. Here, we applied a femtosecond-terahertz (fs-THz) pulse to mouse skin to evaluate non-thermal effects of THz radiation. Analysis of the genome-wide expression profile in fs-THz-irradiated skin indicated that wound responses were predominantly mediated by transforming growth factor-beta (TGF-β) signaling pathways. We validated NFκB1- and Smad3/4-mediated transcriptional activation in fs-THz-irradiated skin by chromatin immunoprecipitation assay. Repeated fs-THz radiation delayed the closure of mouse skin punch wounds due to up-regulation of TGF-β. These findings suggest that fs-THz radiation initiate a wound-like signal in skin with increased expression of TGF-β and activation of its downstream target genes, which perturbs the wound healing process in vivo.

Coherently controlled spin precession in canted antiferromagnetic YFeO3 using terahertz magnetic field

We investigate the details of the precessional motion of the magnetic moment in canted antiferromagnetic YFeO3, which is excited by a linearly polarized terahertz (THz) pulse at room temperature. By tuning the spectral component of the input THz pulse around the quasi-ferromagnetic mode located near 0.3 THz, we have experimentally clarified the resonance effect in the THz control of the spin state. We were able to confirm this result from the simulation based on the Landau–Lifshitz–Gilbert equation with the two sub-lattice model for the canted antiferromagnet. Finally, we discuss a crossover from a linear to a nonlinear magnetic response to the input THz pulse during the THz-induced precessional switching of the magnetization.

Guest molecule dynamics and guest-specific degassing phenomenon of binary gas hydrate investigated by terahertz time-domain spectroscopy

The behaviour of clathrate hydrates in the THz region could provide useful information about inclusion compounds. In this study, terahertz time-domain spectroscopy (THz-TDS) revealed the dynamics of gaseous guest molecules inside the water framework of a clathrate hydrate. Distinct footprints of each gaseous molecule (CH4, H2 and O2) inside the gas hydrate lattice were analysed. Thermally stimulated gas hydrates give out gas molecules (secondary guests) to remain secure, and each gas has a specific degassing temperature. In this way, they form the stable sII THF hydrate with empty small cages (512) which can survive at temperatures up to 278 K.

Structural evolution and carrier scattering of Si nanowires as a function of oxidation time

We investigated the morphological characteristics of the cross-sectional shape of Si-core nanowires (NWs) as a function of oxidation time. In the case of as-grown Si NWs, the Si cores were hexagons with a 3-fold symmetry, not a 6-fold symmetry, and this shape was transformed into a triangular shape with rounded edges after thermal oxidation. Structural changes in the cross-section of the Si-core NWs were related to the surface free energy. Moreover, the morphological change in the oxide shell during the oxidation treatment suggested that the stress relaxation process is closely related to the structural evolution of the Si core. The change in defect states generated in Si/SiO2 core/shell NWs during structural evolution was investigated by low-temperature photoluminescence and optical pump–THz probe spectroscopy measurements. In particular, surface defect states formed in the interfacial region between the Si core and the oxide shell were reduced during the oxidation process. Appropriate control of the surface state affected carrier scattering: the relaxation time of photogenerated carriers was significantly increased due to the reduction in surface defects.

Biological effects of femtosecond-terahertz pulses on C57BL/6 mouse skin.

Dear Editor: Terahertz (THz; 1 THz=1012 Hz≈33 cm-1) radiation, comprising electromagnetic waves propagating at frequencies in the THz range, lies between infrared and microwave radiation in the electromagnetic spectrum, and it shares some properties of each. Specifically, it travels in a straight line and is nonionizing. The THz spectral range typically covers frequencies between 0.1 and 10 THz, and THz radiation can interact with biological molecules, cells, and tissues1. However, until recently, this region of the electromagnetic spectrum has remained as a so-called THz-gap because of the lack of radiation sources. The recent advents of practical sources that cover the THz-gap have prompted its diverse applications including those in medical imaging2, telecommunication3, and security systems4. In the near future, people will be exposed to more and more THz radiation, and the exposure effects on skin will be a major concern because THz radiation is mostly absorbed by the skin due to its low penetrance of liquid water. A few studies have reported the effects of the millimeter waves on keratinocytes in vitro5-7; however, we do not know of any studies that have used in vivo models to examine the biological changes in the skin caused by THz radiation. In this current study, the femtosecond (fs)-THz beamline at the Pohang Accelerator Laboratory was used as the THz light source8. The optical pulses were produced in a 3.2 W Ti:sapphire regenerative laser amplifier with a center wavelength of 800 nm and a pulse width of approximately 180 fs, which was delivered to a 10×10 mm ZnTe crystal to generate fs-THz pulses via optical rectification. The fs-THz pulse has frequency up to 3 THz (wavelength, approximation 0.01 cm) and the pulse width of <200 fs with a repetition rate of 1 KHz. Before exposure, the THz pulse energy, measured with a Golay cell (Microtech Instruments Inc., Eugene, OR, USA) and a lock-in amplifier (SR830; Stanford Research Systems Inc., Sunnyvale, CA, USA), was approximately 0.26 nJ/pulse. To minimize the absorption by water vapor, a region of the THz beam path was enclosed and purged with dry air. A THz low-pass filter was used to remove the extra visible source during the process. We exposed the back skin of 8-week-old male C57BL/6 mice to the fs-THz beam and examined the histological and molecular biological changes. Briefly, the hair on the back of the mice was clipped and they were positioned in the customized acryl restrainer manufactured for this experiment. After taking a precise position with the visible beam in targeting the spot, the back, the skin (size, 1×1 cm2) was exposed to the fs-THz beam for 1 hour. Accumulated energy in the targeted area for 1 hour was calculated as approximately 1.15 J/cm2. Skin biopsy samples were taken at 1 and 24 hours after exposure. Mice in the sham group were treated in the same manner except for the fs-THz beam exposure. Skin samples were stained with hematoxylin and eosin for histological assessment. We found that fs-THz irradiation did not induce any obvious histological changes in either 1 or 24 hours (Fig. 1). In addition, neither inflammatory cells nor damaged skin cells, such as sunburn cells, were observed. The collagen fibers in the dermis also seemed to be normal. Fig. 1 Histological observations on the back skin of C57BL/6 mice exposed to the femtosecond-Terahertz (fs-THz) beam for 1 hour (HE ×200 and ×400). Skin samples were biopsied at 1 and 24 hours after exposure. However, the quantitative real-time polymerase chain reaction analysis for the genes screened through microarray analysis reveals that some genes in the fs-THz-irradiated mice were either biologically activated or suppressed. For instance, the gene transcription of substance P (SP), a neuropeptide that is released from the C and Aδ fibers and is involved in local inflammation, including dermatoses, decreased significantly at 24 hours after exposure (Fig. 2A). On the contrary, the transcription of calcitonin gene-related peptide, another neuropeptide that is also associated with neurogenic inflammation in eczema, increased significantly in fs-THz-irradiated mice (Fig. 2B). We cannot provide a suitable explanation for this discrepancy because the biological effects of THz radiation are not yet fully understood. Moreover, the transcription of the transient receptor potential vanilloid (TRPV) 1 and 4, which are kinds of ion channels, seemed to decrease at 24 hours after exposure, but the changes were not significant due to the small sample size (Fig. 2C, D). Further, we performed the same experiment on BALB/c nude mice, in which they were exposed to the fs-THz beam for 1 hour, and biopsy samples were taken at 12 and 36 hours after exposure. We observed that the transcription of the TRPV1 gene decreased significantly at 36 hours after exposure (Fig. 2E). Interestingly, TRPV1 has been reported to be able to release SP9; therefore, it could be hypothesized that fs-THz pulses exert their biological effects primarily through the modulation of ion channels. However, further investigation is necessary to confirm this. Fig. 2 Gene expression in the femtosecond-Terahertz (fs-THz)-irradiated mouse skin. The back skin of C57BL/6 mice (sham, n=4; fs-THz, n=5; A~D) and BALB/c nude mice (sham, n=3; fs-THz, n=4; E~F) were exposed to the fs-THz beam for 1 hour. Skin samples were biopsied ... In conclusion, our data show that fs-THz pulses have biological effects at the molecular level, even though the mean energy of the radiation was quite low to induce any noticeable histological changes. This study has some limitations; THz radiation at different frequencies, power, and pulse durations were not studied, and it is difficult to presume the clinical meaning of this study. Moreover, we did not test higher energy of THz radiation because increasing the exposure time was difficult due to the limit of anesthesia. In spite of these limitations, this is the first study to demonstrate the molecular biological effects of THz radiation on the skin using an animal model to the best of our knowledge, and we hope that our results promote further investigations in the future.

PAL-XFEL soft X-ray scientific instruments and X-ray optics: First commissioning results

We report an overview of soft X-ray scientific instruments and X-ray optics at the free electron laser (FEL) of the Pohang Accelerator Laboratory, with selected first-commissioning results. The FEL exhibited a pulse energy of 200 μJ/pulse, a pulse width of <50 fs full width at half maximum, and an energy bandwidth of 0.44% at a photon energy of 850 eV. Monochromator resolving power of 10 500 was achieved. The estimated total time resolution between optical laser and X-ray pulses was <270 fs. A resonant inelastic X-ray scattering spectrometer was set up; its commissioning results are also reported.

Tuning the Fermi level with topological phase transition by internal strain in a topological insulator Bi2Se3 thin film

In a three-dimensional topological insulator Bi2Se3, a stress control for band gap manipulation was predicted but no systematic investigation has been performed yet due to the requirement of large external stress. We report herein on the strain-dependent results for Bi2Se3 films of various thicknesses that are grown via a self-organized ordering process. Using small angle X-ray scattering and Raman spectroscopy, the changes of d-spacings in the crystal structure and phonon vibration shifts resulted from stress are clearly observed when the film thickness is below ten quintuple layers. From the UV photoemission/inverse photoemission spectroscopy (UPS/IPES) results and ab initio calculations, significant changes of the Fermi level and band gap were observed. The deformed band structure also exhibits a Van Hove singularity at specific energies in the UV absorption experiment and ab initio calculations. Our results, including the synthesis of a strained ultrathin topological insulator, suggest a new direction for electronic and spintronic applications for the future.