Sungkyu Yu

Photoluminescence studies of excitonic transitions in GaN epitaxial layers

GaN epitaxial layers on sapphire substrates were grown by the rotating disk metal organic chemical vapor deposition technique. Excitonic transitions from conduction band to spin-orbit split valence bands were observed. At 12 K we observed donor bound exciton and a very weak acceptor bound exciton. The temperature dependence of luminescence peak positions of free-excitons A and B were fitted to the Varshni’s equation to study the variation of the band gap with temperature. The linewidth of the free exciton (A) was studied as a function of temperature and was explained by theoretical model considering the scattering of excitons with acoustic phonons and longitudinal optical phonons. In the 12 K spectrum we also observed phonon-assisted excitonic transitions. The activation energy of the free exciton (A) was found to be 26 meV, while that of the donor bound exciton was 7 meV. The binding energy of the donor was estimated as 35 meV and that of the acceptor as 250 meV. The band gap of GaN was found to be 3.505...

Magnesium acceptor levels in GaN studied by photoluminescence

Magnesium doped GaN epitaxial layers were grown by metal-organic chemical vapor deposition on sapphire substrate. Energy levels of these acceptors were investigated by systematic photoluminescence measurements in the temperature range of 12–300 K. Magnesium concentration was varied from <1×1019 to higher than 5×1019 cm−3. Photoluminescence measurements were made on the as-grown and annealed samples. We have observed various transitions related to donor to acceptor and their phonon replicas, conduction band to acceptors, and free excitons. Their dependence on temperature, concentration of the magnesium impurity and annealing conditions was discussed. In our study, two important observations were made. First, very deep level luminescence was not observed even in the highly magnesium doped as-grown samples. Second, free exciton transitions including valence band splittings were observed for the first time in the Mg-doped materials, demonstrating the high quality of the samples.

MANY-BODY EFFECTS ON MODULATION-DOPED INAS/GAAS QUANTUM DOTS

The excitation intensity dependent photoluminescence spectra of various modulation-doped InAs/GaAs quantum dots exhibit the band filling and band-gap renormalization. From the time-resolved photoluminescence spectra, in addition to the interaction of the carriers in quantum dots with the remote ionized impurities in a GaAs barrier, the screening due to the two-dimensional charges is found to mainly affect the carrier lifetime in the modulation-doped quantum dots.

Strong instantaneous contribution in femtosecond degenerate four‐wave mixing from 350 μm InP due to virtual excitation

We have observed strong four‐wave mixing (FWM) signals from the third up to the seventh order for a 350 μm undoped InP at 10 K with an excitation at far below the band gap. The third order spectrally resolved FWM signal shifts continuously to blue as time delay moves away from positive to negative. The peak intensities of the third and fifth order time‐integrated FWM signals decrease rapidly as the detuning increases, as (detuning)−2 and (detuning)−6, respectively. The FWM signal at far below the band gap is attributable to the strong instantaneous contribution of the excitation pulse.

Time-resolved four-wave mixing signal in thick bulk GaAs

We experimentally demonstrate that the temporal shape of the four-wave mixing (FWM) signal in thick semiconductors is significantly influenced by the absorption of the generated FWM signal during its propagation through the sample as well as the pulse broadening effect, by performing the time-integrated (TI) and time-resolved (TR) FWM experiments, and by measuring the pulse shape of the transmitted probe beam in the presence of a pump beam for a 500- and a 100-μm-thick undoped GaAs. We find that the temporal shape of the TR-FWM signal in the vicinity of the exciton resonance depends on the sample thickness and time delay between two incident pulses. As the excitation laser is tuned far below the exciton resonance, however, the propagation effect of the FWM signal itself through the sample becomes dramatically weaker and the temporal shape of the FWM signal resembles the distorted probe pulse shape. Furthermore, the peak intensities of the TI- FWM signals near the exciton resonance are smaller in the thick...

Time-resolved spectroscopy of InAs quantum dots using one-side modulation-doping technique: renormalization and screening

We present the optical properties of modulation-doped InAs/GaAs quantum dots (QDs). Bandgap renormalization is clearly observed from a red shift of the emission peaks on the QDs with various modulation-doping concentrations, demonstrating that exchange interactions of electrons within single states have no effect on renormalization. Screening due to the electrons at modulation-doped layer as well as the photo-generated carriers in GaAs matrix plays an important role in reducing the radiative recombination rate.

Femtosecond degenerate four-wave mixing in 500 μm undoped GaAs and 350 μm undoped InP far below band gap

Rather strong third- and fifth-order four-wave mixing (FWM) signals from a 500 μm thick undoped GaAs and a 350 μm thick undoped InP in detuning ranges of 80–170 meV below the band gap are observed. The spectrally resolved (SR) FWM signals for both samples far below the band gap are blue-shifted significantly with respect to the spectrum of the excitation laser pulse for negative time delays, whereas they are red-shifted for positive time delays. The temporal and spectral shapes of the FWM signals for both samples far below the band gap are nearly consistent with those of the excitation laser pulse but are remarkably different from the excitonic properties. The shifts of SR-FWM signals far below the band gap are interpreted as due to the instantaneously created virtual states by the temporal overlap of the excitation laser pulses.

Optical Third Harmonic Generation of Poly(2-Alkoxy-1,4-Phenylenevinylene)'S

Abstract A series of poly (2-alkoxy-1,4-phenylenevinylene)s was synthesized by water-soluble precursor method. In this paper, three poly(1,4-phenylenevinylene) (PPV) derivatives were included, poly (2-methoxy-1,4-phenylenevinylene), poly(2-butoxy-1,4-phenylenevinylene) and poly (2-dodecyloxy-1,4-phenylenevinylene). These polymers were characterized using UV-visible, FT-IR spectroscopy and DSC, TGA thermal analyzer. The third order nonlinear optical susceptibilities of the polymers were determined using third harmonic generation (THG) method at 1907 nm, fundamental wavelength. Measured x (3) values are 10−11–10−12 esu order and as the side chain length is increased, the x (3) value is decreased because of the side chain effect on conjugation and of volume fraction.

Temperature dependence of a femtosecond degenerate four-wave mixing signal from 350-µm-thick undoped InP far below the band gap

We experimentally investigated the temporal and spectral profiles of the four-wave mixing (FWM) signals from 350-µm-thick undoped InP at various temperatures when the laser was tuned from the exciton resonance to 100 meV below the band gap. The temporal and spectral shapes of the observed FWM signals well below the band gap were nearly consistent with those of the excitation laser pulse and exhibited significantly different behavior from the excitonic properties in thin samples. Furthermore, we observed temperature-independent large blue shifts of the spectrally resolved FWM signals far below the band gap with respect to the excitation laser spectrum when the time delay moved from positive to negative. Based on our experimental observations, we conclude that this behavior can be explained not simply by the excitonic effects but by the instantaneously created virtual states and the frequency-broadening effect.

Temperature and density dependence of exciton lifetimes in GaAs/AlGaAs multiple quantum wells

The photoluminescence linewidths and excition lifetimes of free excitons in GaAs/AlGaAs multiple quantum wells were systematically investigated as a function of temperature, quantum well width, and carrier density. The experimental results showed that the exciton decay processes were strongly related to the linewidth of the exciton and the exciton binding energy.