Kang-Jeon Han

Monoclinic and correlated metal phase in VO(2) as evidence of the Mott transition: coherent phonon analysis.

In femtosecond pump-probe measurements, the appearance of coherent phonon oscillations at 4.5 and 6.0 THz indicating the rutile metal phase of VO2 does not occur simultaneously with the first-order metal-insulator transition (MIT) near 68 degrees C. The monoclinic and correlated metal (MCM) phase between the MIT and the structural phase transition (SPT) is generated by a photoassisted hole excitation, which is evidence of the Mott transition. The SPT between the MCM phase and the rutile metal phase occurs due to subsequent Joule heating. The MCM phase can be regarded as an intermediate nonequilibrium state.

Chirality-Selective Excitation of Coherent Phonons in Carbon Nanotubes by Femtosecond Optical Pulses

Using predesigned trains of femtosecond optical pulses, we have selectively excited coherent phonons of the radial breathing mode of specific-chirality single-walled carbon nanotubes within an ensemble sample. By analyzing the initial phase of the phonon oscillations, we prove that the tube diameter initially increases in response to ultrafast photoexcitation. Furthermore, from excitation profiles, we demonstrate that an excitonic absorption peak of carbon nanotubes periodically oscillates as a function of time when the tube diameter undergoes coherent radial breathing mode oscillations.

Synchronously pumped optical parametric oscillator based on periodically poled MgO-doped lithium niobate

We demonstrate a room-temperature operation of the near-infrared femtosecond optical parametric oscillator based on MgO-doped stoichiometric periodically-poled lithium niobate, which is synchronously pumped by a Kerr-lens mode-locked Ti:sapphire laser. Wide tunability in the range from 0.98 microm to 1.50 microm was enabled for a single set of cavity mirrors by incorporating a BK7 window as an output coupler. For the output coupling ratio of 3.7%, the threshold pumping power of 460 mW and the slope power conversion efficiency of 37% were achieved. By controlling dispersion values with intra-cavity prisms, femtosecond pulses as short as 66 fs could be obtained.

Properties of (Zn,Cr)Te semiconductor deposited at room temperature by magnetron sputtering

We report the fabrication of (Zn,Cr)Te films at room temperature by magnetron sputtering. Various structural and elemental characterizations revealed there was only a zinc blende phase from the ZnTe host and Cr atoms were distributed uniformly in these films. The magnetization measurement by superconducting quantum interference device magnetometer clearly showed that the samples were ferromagnetic at low temperatures with Curie temperature around 150K. The magnetic circular dichroism measurements confirmed that the observed ferromagnetism was originated from the interaction of substitutional Cr ions and ZnTe host. Transport measurement revealed typical semiconductor behaviors with the large negative magnetoresistance observed.

Magnetization-induced optical nonlinearity in ferromagnetic GaMnAs

We report the observation of a coherent nonlinear signal in pump-probe experiments on a ferromagnetic GaMnAs. The coherent signal, which is originating due to coherent interaction between pump and probe beams, depends on the polarization configuration of each beam and follows the sample magnetization as it changes with the applied magnetic field and∕or the sample temperature.

Temperature Dependence of the Coherent Radial Breathing Mode Oscillations in Single Walled Carbon Nanotubes

We report on temperature dependence of the radial breathing mode(RBM) frequency and its decay time in single walled carbon nanotubes (SWCNTs), which were studied by the femtosecond pump and probe technique. With increasing temperature, we found out that the RBM frequency and phonon decay time shows on abnormal dramatic change around 70degC for a certain type of carbon nanotubes, which may be originated from the surfactant effect.

Generation of high-repetition rate pulse trains up to 20 THz using the femtosecond pulse shaping

Using the pulse shaping technique where the added amplitude or phase information in spectral domain modifies the pulse evolution in time domain, we could generate terahertz-rate trains with high repetition rate up to 20 THz. The advanced phase mask design was applied such that fixed energy interval constitutes one period rather than fixed wavelength interval does in spectral domain.

Carrier dynamics in ZnO nanorods revealed by pump-probe and the time-resolved photoluminescence

Using a mode-locked Ti:sapphire laser, we have performed time-resolved pump-probe and PL measurements for ZnO nanorods of which the size were determined to be 30-40 nm in diameter and about 1 mum in length by filed-emission scanning electron microscopy (FESEM). The excited carrier dynamics was analyzed from the complementary time-resolved processes in pump-probe and PL.

Carrier Dynamics and Magnetization‐induced Nonlinearity in Ferromagnetic GaMnAs

Using a femtosecond Ti:sapphire laser, we have performed time‐resolved transmission/reflection measurements. Carrier dynamics is studied for a series of GaMnAs layers and the observation of magnetization induced nonlinearity signal is reported.