Yin-Chieh Huang

Observation of huge nonlinear absorption enhancement near exciton resonance in GaN

Huge nonlinear-absorption enhancement was observed in GaN system at the excitonic transition wavelength. This excitonic enhancement is attributed to four-wave-mixing type nonlinear processes. The peak nonlinear coefficient is at least 3000 cm/GW corresponding to an enhancement factor of >200. The dephasing time of the exciton was also observed to be 60-160 fs in GaN thin film at room temperature.

Femtosecond dynamics of exciton bleaching in bulk GaN at room temperature

Femtosecond transient transmission pump–probe technique was used to investigate exciton dynamics in a nominally undoped GaN thin film at room temperature. An exciton ionization time of 100–250 femtoseconds was observed by the time-resolved pump–probe measurement. A comparison experiment with pre-excited free carriers also confirmed the observation of the exciton ionization process in bulk GaN.

Triple-optical autocorrelation for direct optical pulse-shape measurement

Triple optical autocorrelation of femtosecond optical pulses was realized simply with third-harmonic-generation technique. This optical technique provides complete knowledge of transient pulse intensity variation directly in time domain. Only analytic calculation is needed to obtain the pulse-shape from data without direction-of-time ambiguity. Combined with a spectral measurement and the Gerchberg–Saxton algorithm, except for pulses with complete temporal and spectral symmetry that will cause a twofold ambiguity, exact phase variation in time can also be retrieved through an iterative calculation with an O(n) complexity.

Piezoelectric-field-enhanced lateral ambipolar diffusion coefficient in InGaN/GaN multiple quantum wells

Lateral diffusion behavior of two-dimensional carrier gas in InGaN/GaN multiple quantum wells was investigated using optical transient transmission measurements. A large ambipolar diffusion coefficient was observed, which was attributed to the enhancement by a strong piezoelectric field. This large ambipolar diffusion coefficient was found to increase with increased well width with a value on the order of 3000 cm2/s for a 62 A well-width sample.

Characterization of ultrashort optical pulses with third-harmonic-generation based triple autocorrelation

We present a method to obtain complete information of femtosecond pulses. By measuring triple-optical autocorrelation directly with third-harmonic generation, without spectral information, a temporal pulse shape can be obtained by analytical calculation without direction-of-time ambiguity. Combining the resulting optical pulse shape with its corresponding optical spectrum, the exact phase and color variations in time can all be recovered with a Gerchberg-Saxton algorithm through an iterative calculation with an O(n) complexity.

Direct temporal intensity measurement of ultrashort optical pulses using third-harmonic-generation based triple correlation

Summary form only given. Because electronic devices (streak camera etc.) are too slow to measure temporal evolution of ultrashort optical pulses, many techniques were developed to retrieve temporal pulse shape. Most of them extract the temporal intensity either by assuming an analytic pulse shape (autocorrelation) or with the help of spectral measurements (including FROG and SPIDER.) All, of these techniques rely on either interference or various nonlinear effect including second harmonic generation and optical Kerr effect. However, in the later 1960s it was shown,that a triple correlation is sufficient to determine the temporal intensity of, a laser pulse with a direct mathematical calculation. In this report, we demonstrate direct temporal intensity measurement of ultrashort optical pulses using a novel third-harmonic-generation (THG) based triple correlation method. Using THG process in a single GaN thin film, the optical pulse intensity profile, from a mode-locked Cr:forsterite laser was directly obtained with the background free triple correlation trace without any spectral information and pulseshape assumption. This is different from the previous demonstration where triple correlation was obtained through a complex combination of second-harmonic generation and sum-frequency-generation. In order to retrieve the corresponding phase information, a simple genetic algorithm was also developed for the first time based on, the direct optical spectrum measurement with improved O(n) complexity than O(n/sup 2/) in.

Characterization of ultrashort optical pulses: a comparison between TOAD and FROG

Comparisons between second-harmonic-generation based frequency resolved optical gating (SHG FROG) and triple-optical autocorrelation for direct pulse-shape measurement (TOAD), have been made by computer simulation and experiments. Results show that TOAD has better accuracy than FROG.

Femtosecond carrier dynamics in GaN

Ultrafast carrier dynamics in an unintentionally doped GaN sample was investigated using femtosecond transient transmission measurements. Special attention was focused on bandtail states. The transient responses suggest that the shallow bandtail states are extended states and deep bandtail states are localized states. The carriers in shallow bandtail states are found to externally thermalize within 500 fs, at the same rate as the above bandgap carriers. The carriers in deep bandtail states are, on the other hand, dominated by carrier transfer into the lower energy states through phonon assisted tunneling.