Hyeyoung Ahn

Terahertz emission from vertically aligned InN nanorod arrays

Terahertz emission from indium nitride (InN) nanorods and InN film grown by molecular-beam epitaxy on Si(111) substrates has been investigated. Terahertz emission from InN nanorods is at least three times more intense than that from InN film and depends strongly on the size distribution of the nanorods. Surface electron accumulation at the InN nanorods effectively screens out the photo-Dember field in the accumulation layer formed under the surface. The nanorods with considerably large diameter than the thickness of accumulation layer are found to be dominant in the emission of terahertz radiation from InN nanorod arrays.

Intense terahertz emission from a-plane InN surface

We report a significant enhancement in terahertz emission from the indium nitride (InN) films grown along the a axis (a-plane InN), relative to the InN films grown along the c axis. The primary radiation mechanism of the a-plane InN film is found to be due to the acceleration of photoexcited carriers under the polarization-induced in-plane electric field perpendicular to the a axis, which effectively enhances the geometrical coupling of the radiation out of semiconductor. In addition, azimuthal angle dependence measurement shows that the p-polarized terahertz output consists of a large angularly independent component and a weak component with a distinctive fourfold rotation symmetry.

Spectroscopic ellipsometry study of wurtzite InN epitaxial films on Si(111) with varied carrier concentrations

Spectroscopic ellipsometry (SE) has been performed to determine the optical properties of the InN epitaxial films grown by nitrogen-plasma-assisted molecular-beam epitaxy on Si(111) substrates using a double-buffer technique. In addition to SE, cross-sectional transmission electron microscopy and x-ray diffraction reveal that epitaxially grown InN epilayer is homogeneous with high crystalline quality and does not include any metallic In. SE results analyzed by the Adachi’s model for the dielectric function of InN show that the optical absorption edge of InN varies in the range of 0.76–0.83 eV depending on the carrier concentration, which is determined by the thickness of the AlN buffer layer.

Terahertz spectroscopic study of vertically aligned InN nanorods

Terahertz time-domain spectroscopy has been used to investigate terahertz conductivity and dielectric response of indium nitride (InN) nanorod array and epitaxial film. The complex terahertz conductivity of InN film is well fitted by the Drude model, while the negative imaginary conductivity of the InN nanorods can be described by using the Drude-Smith model. The electron mobility of the InN film is 1217±58cm2∕Vs, while that of the InN nanorods is 80±5cm2∕Vs. The reduced mobility of carriers for the latter can be attributed to the restricted carrier transport within the nanorods.Terahertz time-domain spectroscopy has been used to investigate terahertz conductivity and dielectric response of indium nitride (InN) nanorod array and epitaxial film. The complex terahertz conductivity of InN film is well fitted by the Drude model, while the negative imaginary conductivity of the InN nanorods can be described by using the Drude-Smith model. The electron mobility of the InN film is 1217±58cm2∕Vs, while that of the InN nanorods is 80±5cm2∕Vs. The reduced mobility of carriers for the latter can be attributed to the restricted carrier transport within the nanorods.

X-ray generation enhancement from a laser-produced plasma with a porous silicon target

X-ray generation enhancement from a laser-produced plasma with a porous Si target is reported. For a porous surface formed on a Si wafer, the self-reflectivity of a femtosecond pulse becomes considerably small. The observed energy penetration depth is 25–30 μm, which is much larger than the skin depth of solid density matter. Using a porous Si target, the threshold of the pre-pulse intensity required for soft x-ray emission enhancement can be reduced. It also contributes to enhance the pre-pulse effect, and soft x-ray generation enhancement ranging from 1.6 to 6.5 times is observed depending on the pre-pulse intensity.

Temporal evolution of soft x-ray pulse emitted from aluminum plasma produced by a pair of Ti : sapphire laser pulses

The temporal and spectral evolution of soft x‐ray pulses (40–100 A) emitted from Al plasma produced by a pair of femtosecond Ti:sapphire laser pulses at normal incidence was studied. Both the soft x‐ray emission and the pulse duration increased with increase in the scale length of the preformed plasma. Prepulse enhanced soft x‐ray emission about 100 times with a pulse duration of 100–130 ps. A spectrally resolved time history revealed that the emission at shorter wavelengths started and decayed relatively more quickly, and emissions at longer wavelengths built up and decayed more slowly.

Strong green photoluminescence from InxGa 1- xN/GaN nanorod arrays

We report intense green photoluminescence (PL) from vertically aligned indium gallium nitride (InxGa(1-x)N) nanorod arrays. The formation of InxGa(1-x)N/GaN-heterostructure nanorods increases the localization depth of the radially confined carriers (> 100 meV). Temperature dependent PL peak energy of InGaN nanorods shows the characteristic S-shaped behavior, indicating the prominent carrier trapping in band-tail states associated with the nonuniformity of In content. Time-resolved PL (TRPL) response decays biexponentially and the dominant slow decay component of TRPL for InxGa(1-x)N nanorods is due to the transfer of excitons to the localized states before the radiative decay.

Absolute frequency measurement of rubidium 5S–7S two-photon transitions with a femtosecond laser comb

The absolute frequencies of rubidium 5S–7S two-photon transitions at 760 nm are measured to an accuracy of 20 kHz with an optical frequency comb based on a mode-locked femtosecond Ti:sapphire laser. The rubidium 5S–7S two-photon transitions are potential candidates for frequency standards and serve as important optical frequency standards for telecommunication applications. The accuracy of the hyperfine constant of the 7S1?2 state is improved by a factor of 5 in comparison with previous results.

Carrier dynamics in InN nanorod arrays

A fast initial decay was observed from InN nanorods with ~30 nm diameter, the lifetime of which is much shorter than the carrier cooling time, demonstrating the substantial surface-associated influence on carrier relaxation in nanorods.

Terahertz emission mechanism of magnesium doped indium nitride

We report carrier concentration-dependence of terahertz emission from magnesium doped indium nitride (InN:Mg) films. Near the critical concentration (nc∼1×1018 cm−3), the competition between two emission mechanisms determines the polarity of terahertz emission. InN:Mg with n>nc exhibits enhanced positive polarity terahertz emission compared to the undoped InN, which is due to the reduced screening of the photo-Dember field. For InN:Mg with n<nc, the polarity of terahertz signal changes to negative, indicating the dominant contribution of the surface electric field due to the large downward surface band bending within the surface layer extending over the optical absorption depth.