Ultrafast properties of femtosecond-laser-ablated GaAs and its application to terahertz optoelectronics.

We report on the first terahertz (THz) emitter based on femtosecond-laser-ablated gallium arsenide (GaAs), demonstrating a 65% enhancement in THz emission at high optical power compared to the nonablated device. Counter-intuitively, the ablated device shows significantly lower photocurrent and carrier mobility. We understand this behavior in terms of n-doping, shorter carrier lifetime, and enhanced photoabsorption arising from the ablation process. Our results show that laser ablation allows for efficient and cost-effective optoelectronic THz devices via the manipulation of fundamental properties of materials.

Optical absorption and photocurrent enhancement in semi-insulating gallium arsenide by femtosecond laser pulse surface microstructuring.

We observe an enhancement of optical absorption and photocurrent from semi-insulating gallium arsenide (SI-GaAs) irradiated by femtosecond laser pulses. The SI-GaAs wafer is treated by a regeneratively amplified Ti: Sapphire laser of 120 fs laser pulse at 800 nm wavelength. The laser ablation induced 0.74 µm periodic ripples, and its optical absorption-edge is shifted to a longer wavelength. Meanwhile, the steady photocurrent of irradiated SI-GaAs is found to enhance 50%. The electrical properties of samples are calibrated by van der Pauw method. It is found that femtosecond laser ablation causes a microscale anti-reflection coating surface which enhances the absorption and photoconductivity.

[Effect of post-annealing on the structure and fluorescence properties of YMnO3 thin films].

YMnO3 thin films deposited on Si (100) substrate by pulsed laser deposition at room temperature were annealed at different temperatures. The microstructure and fluorescent emission properties of these films were studied using XRD and UV-Vis spectroscopy. The results show that polycrystalline YMnO3 thin films can be obtained through post-annealing, in which hexagonal phase and orthorhombic phase coexist. And with increasing the temperature, the ratio of orthorhombic phase to hexagonal phase varies considerably. The results also show that fluorescence peaks in the wavelength range of 430-620 nm maybe originate from the transition from 5T2 to 5E of Mn3+. The intensity of fluorescence peaks is enhanced with increasing the temperature. However, the positions of fluorescence peaks remain invariable. These results indicate that the change in the film structure affects the transition probability greatly but almost has no effect on the position of energy level. Meanwhile, the relative intensity ratio of cyan fluorescence peak to green fluorescence peak is almost unchanged.

[Fluorescence emission properties of Ni-doped ZnO films].

Ni-doped ZnO films were deposited on Si(100) by pulsed laser deposition(PLD) at room temperature. Fluorescence emission properties of the films were measured using VARAIN Cary-Eclips 500 fluorescence spectrum analyzer. Two peaks centered respectively at about 360 and 380 nm were observed. The origin of the ultraviolet peak at 360 nm was investigated through doping Ni into the ZnO films. It was found that the intensity of this ultraviolet peak changed with Ni content while its position remained stable. The fluorescence emission of the samples was optimal when Ni:ZnO was 5 mol%, indicating that the peak centered at 360 nm might originate from the composite transition between the splitting valence band and conduction band, not from the entrance of the impurity energy level into the conduction band after doping.

[Photoluminescence spectrum of oxygen and nitrogen-doped Si-based film].

O and N-doped amorphous Si-based films were prepared by magnetron sputtering. The photoluminescence (PL) spectra from the films were measured. A series of intense PL bands located in red, green, blue, violet and ultraviolet regions were observed, whose intensities were affected by the contents of O and N and the temperature of the substrate (Ts) during deposition. Experimental results indicate that the red PL consists of wide band, which originates from quantum confinement effect, and the distinguishable peaks are related to O impurities. The green PL, which depends on N impurities, results from N defect level, and the type and position of its peaks are affected by Ts during deposition. The blue PL containing distinguishable peaks is related to complex O defect levels. The violet PL is composed of wide bands and a double peaks, and its PL intensity is influenced by the kind and content of impurity and Ts. In summary, the intense green and violet PL can be obtained when Ts during deposition is 750 degrees C and the contents of O and N impurities are moderate.