김대식 교수

We report on terahertz electromagnetic field transmission through metallic slot antennas supported on various dielectric substrates. The substrate effect strongly modifies the transmission resonance compared to the free-standing case, while the resonance can be determined systematically by introducing an effective refractive index. The relative contribution of the substrate index to the effective index has been measured for various substrates, and shows a good agreement with theoretical calculat

We report on a 100 % transmission in one- and two-dimensional plasmonic metamaterials in the terahertz frequency range. At a specific frequency at which the plasmonic meta material is perfectly impedance matched to vacuum, the metal material becomes essentially invisible. In the case of a two-dimensional array of square holes, perfect impedance matching occurs when the effective dielectric constant becomes .2 for samples with hole coverages larger than 0.19. In a one-dimensional array of slits,

We show that a periodic array of one-dimensional slits in a perfect conductor admits 100 percent perfect transmission when proper resonance conditions are met. We present both analytic and numerical studies on the transmission by using a finite-difference time-domain (FDTD) computer simulation. We find that both the Fabry-Perot and the coupled evanescent surface modes contribute to the enhanced transmission peak in the terahertz spectral range. This theoretical prediction agrees well with experi

We have generated sub-10-fs laser pulses from a Kerr-lens mode-locked Ti:Sapphire laser using a single set of commercially available simple double-chirped mirrors. Double-chirped mirrors which are deliberately designed for negative group delay dispersion allow pulses with a high-quality spectral shape and with a spectral bandwidth as broad as 143 nm to be achieved. Broad spectral tunability over a 140 nm spectral range was achieved. Numerical simulations show that quadratic-chirped double-chirp

We clarify the positive and the negative roles of a surface plasmon polariton in light transmission through metallic nanoslit/hole structures. While conventional surface plasmon polaritons suppress the transmission due to the equipartition of diffraction orders, we show that “radiation dressed” surface plasmon polaritons (DSPP) mediate the extraordinary transmission. We determine the coupling strength of DSPP to radiation as a product of the geometric opening ratio, the aperture momentum, and th

We observed the high- and low-frequency E$_2$ optical phonon modes of wurtzite ZnO. Their dephasing times of 1.75 ps, 29.2 ps were obtained at room temperature. We tested polarization dependence of the coherent phonon excitation and the behavior is fully consistent with the selection of the Impulsive Stimulated Raman Scattering(ISRS), identifying the generation mechanism. Also, we studied the relations between the optical phonon intensity and the pulse width of the transform-limited pump pulse.

We have studied photoluminescence spectra of CdSe self-assembled quantum dots (QD's) embedded in Zn$_{1-x}$Mn$_x$Se ($x$ = 0.05 and 0.15) with and without an external magnetic field. Under a magnetic field, the photoluminescence from CdSe QD's was circularly polarized under light excitation above the bandgap energy of ZnMnSe, indicating carrier spin polarization in CdSe QD's. The time-resolved photoluminescence study showed that spin-flip occured very rapidly as photocarriers transfered from the

We study emission patterns in periodic nano-hole arrays perforated in a metal film. In the nearfield region, higher order multiple components of diffraction interfere with each other to generate complicated spatial patterns. These patterns simplify to a sinusoidal one in the intermediate region and become either homogeneous or sinusoidal in the far-field region, depending on whether the excitation wavelength is larger or smaller than the lattice constant. For an incident wavelength much smaller

A spatio-spectral measurement of transmited near field distribution from a nano optic structure is demonstrated by near field scanning optical miscroscopy (NSOM) with the point spectroscopic approach. This method provides spectral information of nano-optic system with sub-diffraction limit spatial resolution. The gold nano slit aray is a good model system where analytical calculation is posible by the modal expansion method for all diffraction orders with surface impedance boundary conditions. T

We measured the time-resolved differential reflectivity, R, of Ga1.xMnxAs for x 0.05 for various excitation wavelengths and compared with the signals from semi-insulating GaAs substrates. The sign of R from Ga1.xMnxAs (x = 0.015 and x = 0.03) was negative at 295 K for photon energies larger than bandgap, which was ascribed to defect-induced absorption or a reduction of exciton bleaching. We also discuss the screening of Mn alloy potential fluctuations by photocarriers in the time-resolved diffe

A low-temperature near-field scanning optical microscope (LT-NSOM) is designed and realized in order to study the optical properties of nanostructures at cryogenic temperatures in the UV-visible wavelength range. Due to the simple and extensive optical design, any type of NSOM operation mode is accessible. In particular, the localized photoluminescence spectra of nanostructures can be measured at temperatures from 13 K up to 300 K. We successfully demonstrate the performance of our LT-NSOM by in

본 고는 조선후기 공적 사적 기록에 등장하는 塾師라는 이들의 존재 양태와 역할을 규명하는 것을 목적으로 한다. 조선후기 문집과 실록 등에서 사용된 숙사의 의미를 검토하여 그들이 교육기관에 소속된 이들만이 아니라, 개인적으로 가르치던 이들에 대한 호칭이기도 하였음을 확인할 수 있었다. 이 시기 숙사는 교육기관에 소속되어 있는가와는 상관없이 초보적 수준의 유학교육을 담당하는 이들에게 사용되는 호칭이었다. 숙사들은 동몽들의 가장 가까이에서 가르치는 친근한 교사들이었다고 할 수 있다. 교육기관과 무관한 숙사들이 존재했다는 것은 조선후기의 유학교육을 이해할 때 교육기관과 그 역할에만 주목할 것이 아니라 교육 행위자에도 초점을 두고 접근할 필요가 있음을 시사한다.

We report on terahertz focusing through rectangular slit apertures on thin metal film by using terahertz time domain spectroscopy both in the far- and in the near-field. Our experiments show that far-field perfect transmission is maintained in the rectangular slit array even as the aperture coverage decreases. Near-field imaging supports the field enhancement in the single rectangular aperture indeed occurring at a frequency equal to the fundamental shape resonance frequency.

We present terahertz optical properties of GdBa2Cu3O7-x (GdBCO) superconducting thin films. GdBCO films with a thickness of about 105 nm were grown on a LaAlO3 (LAO) single crystal substrate using a conventional pulsed laser deposition (PLD) technique. Using an Ar ion milling system, the thickness of the GdBCO film was reduced to 58 nm, and its surface was also smoothened. Terahertz (THz) transmission spectra through two different GdBCO films are measured over the range between 0.2 and 1.5 THz u

When light passes through a subwavelength-scale square hole in a perfect conductor, the flow of elecrtromagnetic energy exhibits a strikingly rich behavior that depends on the geometry of the rectangular hole. We present a detailed calculation and demonstrate that funneling of electromagnetic energy occurs when certain resonance conditions are met.