Influence Of Laser Repetition Rate Research Articles

Damage visualization using laser-generated residual guided waves with optimization of laser scanning path

Laser ultrasonic scanning is intensively used in guided wave-based non-destructive testing due to its non-contact scanning, automated scanning path and full wavefield measurement. However, in practical applications, the selected repetition rate of laser generation is limited due to the existence of residual guided waves generated by consecutive pulsed laser impingements, leading to a low scanning speed. To overcome such limitations, the formation of residual wavefield is investigated and an optimization method of laser scanning path is developed to improve the damage visualization using the residual guided waves. First, the energy partition of residual wavefield is investigated based on diffuse wavefield approximation and transient statistical energy analysis. Then, a virtual superposition method is proposed to obtain the synthetic wavefields with different laser repetition rates. The influence of laser repetition rate on the wavefield directivity and energy distribution is analyzed. Moreover, according to the ultrasonic attenuation property of the test specimen, an optimized sub-block raster scanning path is proposed to minimize the spatial misalignment of the energy distribution in residual wavefield. Experiments are conducted on an aluminum alloy plate with multiple artificial notches using a 1-kHz-repetition-rate scanning laser ultrasonic system. Based on the residual wavefield signal energy, damage visualization results before and after laser scanning path optimization are compared. Results show that the proposed method is not only more efficient during the laser scanning process, but also more capable of distinguishing damaged areas from the intact structure.

Influence of laser repetition rate on the Cu2ZnSn(SSe)4 thin films synthesized via pulsed laser deposition technique

Cu2ZnSn(SSe)4 (CZTSSe) thin films are considered as a viable source for the future electricity generation using solar energy. In this report, CZTSSe thin films were synthesized by pulsed laser deposition (PLD) technique as a function of laser pulse repetition rate onto the Mo coated glass substrates. Structural, surface morphological, optical and electrical properties of CZTSSe thin films have been investigated by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and UV–vis spectroscopy. The XRD and Raman analysis confirmed the phase pure polycrystalline structure of thin films of CZTSSe synthesized at high pulse repetition rate. Moreover, the presence of the XRD peak at 27.72° corresponding to (112) plane reveals the tetragonal structure of CZTSSe. The band gap energy of synthesized CZTSSe thin films is found around 1.20eV which is optimum for the solar energy harvesting. To the best of our knowledge, this is the first report on the deposition of CZTSSe via PLD technique by using different pulse repetition rate.

Influence of laser repetition rate on the structural and optical properties of GaN layers grown on sapphire (0001) by laser molecular beam epitaxy

High-quality GaN layers were grown on sapphire (0001) substrates using laser molecular beam epitaxy (LMBE) by laser ablating a solid GaN target at different laser repetition rates (10–40 Hz) under a constant supply of r.f. nitrogen plasma. The effect of laser repetition rate on the structural and optical properties of GaN layers was systematically studied using high-resolution X-ray diffraction (HRXRD), field emission scanning electron microscopy, atomic force microscopy, Raman spectroscopy and photoluminescence (PL) spectroscopy. High-resolution X-ray rocking curve measurements revealed highly c-axis oriented GaN layers on sapphire grown at 30 Hz with a calculated screw dislocation density of ~1.42 × 107 cm−2, whereas the GaN layers grown at 10 or 40 Hz consisted the screw dislocation density in the range of 108–109 cm−2. Surface morphological analysis revealed a change in grain size as well as surface roughness as a function of laser repetition rate and is explained on the basis of growth kinetics. Vibrational Raman spectroscopy revealed that the GaN layer grown at 10 Hz shows an in-plane compressive stress of ~1 GPa, while the film grown at 30 Hz exhibits a minimum stress of ~0.3 GPa. The PL measurements show a highly luminescent band-to-band emission of GaN at 3.44 eV for the 10 Hz grown highly strained GaN layer and at 3.41 eV for the less strained film grown at 30 Hz along with a broad defect band emission centered around 2.28 eV. It is found that the GaN layers grown at 30 Hz have excellent structural and optical properties. We expect that the less strained thin and highly oriented GaN film grown by LMBE can further be utilized for developing prodigious low-temperature-grown nitride-based multilayer structures and devices.

Influence of laser repetition rate on ferroelectric properties of pulsed laser deposited BaTiO3 films on platinized silicon substrate

BaTiO3 thin films were deposited by pulsed laser deposition on Pt–Si at different laser pulse repetition frequencies. X-ray diffraction spectra show that preferred oriented films can be grown by adjusting the pulse repetition frequency. Enhanced dielectric and ferroelectric properties obtained in films deposited at 1 Hz is attributed to preferred orientation, low strain and homogeneous grain distribution. The films deposited at 1 Hz show an impressive remanent polarization of 21.4 μC/cm2 with a coercive field of 70.0 kV/cm. The shift in Curie temperature, which stems from changing the laser pulse repetition frequency, is associated with the strain state in the film.

TiO 2 thin films prepared by PLD for photocatalytic applications

Photocatalytic titanium oxide (TiO 2) thin films are synthesized by the Nd:YAG pulsed laser deposition (PLD) technique. The influence of laser repetition rate and substrate temperature on the crystalline structure is investigated by X-ray diffraction (XRD) measurement. Films deposited at a substrate temperature of 150 °C exhibit the anatase form, regardless of the repetition frequency. In films deposited at 300 °C, however, when the repetition rate is increased the crystal form changes from rutile to anatase. The change in crystal form is probably caused by a drop in the surface temperature of the growing film. The effect is very sensitive to repetition rate, which changes the deposition rate. The results suggest that control of the laser-pulse repetition rate in PLD is a very important factor for obtaining highly efficient photocatalytic TiO 2 thin films.