Jian-Chin Liang

Two-photon absorption study of GaN

Two-photon absorption coefficients of GaN for below band gap ultraviolet wavelength and midgap infrared wavelength were measured by using femtosecond pulsewidth autocorrelation and Z-scan techniques. Large two-photon absorption coefficients were obtained. Taking advantage of the large two-photon absorption, we have demonstrated two-photon confocal imaging of a GaN thin film. Direct correlation was found between the yellow luminescence and suppression of bandedge luminescence.

Large coherent acoustic-phonon oscillation observed in InGaN/GaN multiple-quantum wells

We have observed coherent acoustic-phonon oscillation in InGaN/GaN multiple-quantum wells. With femtosecond pulse excitation, photogenerated carriers screen out the strained-induced piezoelectric field and initiate the coherent acoustic-phonon oscillation. The resulted modulation of the piezoelectric field will then cause large optical property oscillation through the quantum-confined Stark effect.

Femtosecond Z-scan measurement of GaN

Nonlinear refractive index n2 of GaN was measured for the below-band-gap ultraviolet (UV) wavelength region using Z-scan techniques with femtosecond UV pulses. A large nonlinear refractive index of −2.9±1.2×10−12 cm2/W was obtained at a wavelength of 368 nm. The distribution of n2 versus wavelength was found to be consistent with a model described by the quadratic Stark effect, which is the dominant factor contributed to the nonlinear refractive index near the band gap. Our experiments show that GaN is an excellent nonlinear material with large negative n2 in the UV region.

Coherent optical control of acoustic phonon oscillations in InGaN/GaN multiple quantum wells

We demonstrate coherent optical control of phonon oscillations using a femtosecond control pulse. The experiments were performed in InGaN/GaN multiple quantum wells. Coherent acoustic phonon oscillations was initiated with an UV femtosecond pulse. The subsequent manipulation, including magnitude and phase, of the coherent acoustic phonon oscillations was achieved using another UV femtosecond pulse by controlling the pulse time delay and intensity.

Large near resonance third order nonlinearity in GaN

We have studied third order nonlinearities, including two-photon absorption coefficient β and nonlinear refractive index n2, of GaN in below bandgap ultraviolet (UV) wavelength regime by using UV femtosecond pulses. Two-photon absorption was investigated by demonstrating femtosecond UV pulsewidth autocorrelation in a GaN thin film while femtosecond Z-scan measurements revealed information for both n2 and β. The distribution of n2 versus wavelength was found to be consistent with a model described by the quadratic Stark effect, which is the dominant factor contributed to the nonlinear refractive index near the bandgap. Large β on the order of 10 cm/GW and large negative n2 with a magnitude on the order of several 10−12 cm2/W were obtained. The β at near mid-gap infrared (IR) wavelength was also found to be on the order of several cm/GW by using two-photon-type autocorrelations in a GaN thin film. Taking advantage of the large two-photon absorption at mid-gap wavelengths, we have demonstrated excellent image quality on two-photon confocal microscopy, including two-photon-scanning-photoluminescence imaging and two-photon optical-beam-induced current microscopy, on a GaN Hall measurement sample and an InGaN green light emitting diode.

Studies of carrier dynamics in unintentionally doped gallium nitride bandtail states

Ultrafast carrier dynamics of bandtail states in an unintentionally doped gallium nitride sample was investigated using femtosecond transient transmission measurements. The transient responses of shallow bandtail states resemble those of above band gap extended states. The transient responses of the deep bandtail states are, on the other hand dominated by carrier transfer into the lower energy states through phonon assisted tunneling suggesting that the deep bandtail states are localized states.

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.

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.

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.

Coherent control of acoustic phonon oscillations in InGaN/GaN multiple-quantum-wells

Summary form only given. Progress in femtosecond lasers and ultrafast spectroscopy technology has enabled us to generate and directly observe the coherent oscillations of phonon modes. We demonstrate the coherent control of the initiated coherent acoustic phonon oscillation using a separate control UV pulse. The experiments were performed at room temperature on 14 periods InGaN-GaN MQWs.