김대식 교수

Four-wave-mixing signals from excitons in high-quality GaAs quantum wells show an unusually slow temporal evolution, in stark contrast to the behavior expected for a noninteracting two-level system. We show that Coulomb interaction effects, generally neglected in the analyses of four-wave-mixing experiments, dominate the signals by 2 orders of magnitude. Numerical calculations based on extended optical Bloch equations for semiconductors provide good qualitative agreement with the data.

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We report a strong (300:1) excitonic resonance in time-resolved (100 fs) four-wave mixing in GaAs quantum wells as a function of the incident photon energy and show that it results from the large difference in the dephasing rates of excitons and free carriers. We conclude that excitons dominate even when they make up only a small fraction of the excited states. These results, in conjuction with new results in exciton-phonon scattering, provide new insights into previous studies performed with sp

We report the first direct measurement of carrier-carrier scattering rates in a degenerate electron system. Our results on modulation-doped quantum wells, using femtosecond four-wave-mixing (FWM) techniques, demonstrate a strong inhibition of scattering near the Fermi edge, and exhibit other behavior predicted by Landau Fermi liquid theory. Time-resolved (100 fs) FWM measurements clearly show photon echoes, demonstrating the inhomogeneous character of this intrinsic system, as well as prompt sig

Abstract Phase‐change phenomena have been an attractive research theme for decades due to the dynamic transition of material properties providing extraordinary capabilities for versatile optical device applications. Even at the terahertz (THz) frequency regime, phase‐change materials (PCMs) promote the development of dynamic devices, especially when combined with a plasmonic approach delivering strong field enhancement and localization. According to the design of plasmonic metamaterials or hybri

We have utilized subpicosecond laser pulses to excite and probe hot electrons and nonequilibrium longitudinal-optical (LO) phonons in bulk GaAs by Raman scattering. We find that the photoexcited hot electrons cool at a rate much faster than predicted by intravalley scattering of LO phonons via the Fr\"ohlich interaction. On the other hand, this fast cooling rate can be accounted for satisfactorily by intervalley scattering. As a result of this very rapid cooling, the temperature of the hot LO ph

Hot electrons and phonons excited in GaAs by subpicosecond laser pulses have been studied by inelastic light scattering for photoexcited electron densities varying between ${10}^{17}$ and ${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. Transient overshoot of longitudinal-optical (LO) phonon temperature above the electron temperature has been observed. This is explained by the fast production of zone-center LO phonons by hot electrons combined with slower reabsorption of the emitted phono

We show that a low-intensity femtosecond pulse is severely distorted while propagating through a relatively thin (7000 \AA{}) GaAs multiple-quantum-well sample and that this pulse distortion depends critically on the dephasing time ${\mathit{T}}_{2}$ and the total thickness l. An interferometric measurement reveals the existence of well-defined nodes at which the envelope function changes its sign. This pulse distortion significantly affects femtosecond experiments such as pump-probe or four-wav

We report the first observation of a confined-to-propagating transition of LO phonons in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As superlattices (SL's). Picosecond Raman scattering was used to investigate the generation rate of nonequilibrium GaAs LO phonons in two series of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As SL's with constant GaAs layer widths (${\mathit{L}}_{\mathit{z}}$) and x, while the ${\mat

Abstract The ideals of reconfigurable metasurfaces would be operation in a broad frequency range with a high extinction ratio and fatigue resistivity. In this paper, all the above is achieved in the microwave regime by transforming a bare metallic film into well‐controlled nanometer sized gaps in a fully reversible manner. It is shown that adjacent metallic patterns deposited at different times can form “zero‐nanometer gaps,” or “zerogaps,” while maintaining the optical and electrical connectivi

Abstract Through the manipulation of metallic structures, light–matter interaction can enter into the realm of quantum mechanics. For example, intense terahertz pulses illuminating a metallic nanotip can promote terahertz field–driven electron tunneling to generate enormous electron emission currents in a subpicosecond time scale. By decreasing the dimension of the metallic structures down to the nanoscale and angstrom scale, one can obtain a strong field enhancement of the incoming terahertz fi

A technique using a single picosecond laser beam to excite and probe photoexcited hot electron and hole plasma by inelastic light scattering is proposed. The cooling rate of the hot electrons is determined by varying the pulse width of the laser beam. The technique is illustrated by measuring the subpicosecond cooling of hot carriers in GaAs and InGaAs. The advantages and limitations of the technique are discussed.

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With recent advances in nanofabrication technology, various metallic gap structures with gap widths reaching a few to sub-nanometer, and even 'zero-nanometer', have been realized. At such regime, metallic gaps not only exhibit strong electromagnetic field confinement and enhancement, but also incorporate various quantum phenomena in a macroscopic scale, finding applications in ultrasensitive detection using nanosystems, enhancement of light-matter interactions in low-dimensional materials, and u

We demonstrate photoexcited carrier response of metallic nanogap-patterned semiconductor using optical pump-terahertz probe spectroscopy. Metallic nanogap facilitates observing surface carrier dynamics of bulk semiconductors by means of strong field confinement and enhancement of electromagnetic waves at nanoscale. Here, we observe that the enhanced terahertz transmission change of photoexcited InP has a nonmonotonic behavior (increases and then decreases) as reducing the gap size, which origina