Papers by Keyword: Pump-Probe

Abstract: Formation of triplet states in aqueous herbicide solutions excited by nanosecond radiation of 4^th harmonic of Nd:YAG laser (wavelength 266 nm) is revealed by the pump-probe method. During photosensitized 8-MOP oxidation of herbicides, formation of triplet states of the photosensitizer and of the hydrated electron is observed in the process of two-step photoionization of 8-MOP.

Abstract: Investigation of excess carrier dynamics in a 15R-SiC bulk layer grown by physical vapour transport (PVT) on 15R-SiC substrate has been carried out using pump-probe techniques: an interband carrier injection by a picosecond laser pulse and measuring the induced absorption and diffraction of a probe beam. For this task, differential transmittivity (DT) and light induced transient grating (LITG) techniques were used. Room temperature carrier lifetime varied in the 3 ns 8 ns range at excess carrier densities above ΔN₀ = 7×10¹⁷ cm^-3 and was ascribed to the recovery time of optically recharged carrier traps, and their activation energy of E_a = 75 meV was determined. The presence of recharged traps caused the injection-dependence of the diffusion coefficient D, whereby its value dropped below 0.1 cm²/s at ΔN₀ < 1×10¹⁸ cm^-3 and gradually increased up to 0.7 cm²/s at higher injections. At elevated temperatures (300 K < T < 700 K), when the traps are thermally activated, the diffusivity increased up to ~ 1.5 cm²/s and was independent on ΔN₀. The overgrown layer parameters were comparable to those of the used 15R PVT seed.

Abstract: Free carrier lifetimes and diffusion coefficients were determined in 6H-SiC bulk crystals grown by PVT on 6H-and 4H-SiC seeds varying the temperature from 300 K to 650 K and at excess carrier densities ΔN₀ from 10¹⁷ cm^-3 to 10¹⁹ cm^-3. Carrier generation was achieved by using a single or two-photon absorption of picosecond pulses at 351 and 532 nm, respectively. Fast and slow recombination transients revealed the decay time of free carriers and the presence of deep acceptor traps. The thermal trap activation energy E_a = 0.33 eV was determined in the 6H/4H sample and ascribed to the boron, while the presence of deeper traps is suggested in the 6H/6H sample. At room temperature and reaching conditions of trap saturation regime (ΔN₀ 10¹⁹ cm³), both crystals revealed the bipolar diffusion coefficient D_a 4 cm²/s. For comparison, we also determined the photoelectrical parameters in commercial 6H-SiC crystals grown by the Lely and PVT techniques.

Abstract: The effect process between femtosecond laser and matter had been introduced first. Then introduce the theory of strain pulse and the experiment set up.We detect transient reflectivity change on surface of GaAs at different delayed time using femtosecond transient reflection spectroscopy.We found it was made up of three parts: original scattering process of 100fs, carriers-lattice thermal equilibrium of 1.5ps and recombination process of 500ps.

Abstract: A novel faint signal processing method based on phase-lock amplification principle is developed to enhance the sensitivity of pump/probe-based laser generated and laser detected surface acoustic waves (LG/LD SAW) spectroscopy system. This novel method involves a “dead zone” filter located before a common phase-lock amplification unit, which wipes off most of the noise stacked on SAW signal. This paper explored the working principle of this novel signal processing method based on the knowledge of the characteristics of the system noise and the signal. The experiment result shows that the detection system’s sensitivity has been dramatically improved by this new signal processing method.

Abstract: Cu films were deposited on Si and K9 glass substrates by magnetron sputtering technique. The influences of varying thicknesses and substrates on the transient reflectivity of Cu films were studied by using femtosecond laser pump-probe technology. The results show that the transient reflectivity curve of Cu films in different thicknesses have the same trend except that when they reach the peak value and recover to the balance. When the laser power is 40 mW, the influence of Si and K9 substrates on the transient reflectivity curve of 20 nm Cu films is relatively small. But when the laser power is160 mW, the influence of Si and K9 substrates on the transient reflectivity curve of 20 nm Cu films have obvious difference, the former needs much less time to reach the thermal equilibrium than of the later. At the same time, the influence of different substrates on the transient reflectivity curve of 200 nm Cu is also small.

Abstract: In order to break the limitation of the speed of magnetic installation due to magnetic layer transfer characteristics, the ultrafast thermalization dynamic of Ni films and its composite films were studied by femtosecond laser pump-probe technique. The paper focuses on the research of the effect of cooling layer and annealing on the transient reflectivity curves. And the transient reflectivity signals of Ni films prepared under different parameters were measured. The results show that the cooling layer with the larger electron heat capacity constant and the stronger electron-phonon coupling constant can enhance the scattering efficiency of transient heat conduction. Compared to the unannealed sample, the composite film sample after annealing has faster recovery process.

Abstract: Experiment setup of femtosecond laser pump probe was established, the time resolution of time-delay setting reached 67fs. By use of femtosecond laser with width of 30fs and wavelength is 796nm the dependence of transient change of reflectivity on delayed time in GaAs was measured by pump-probe method. By calculating the change of complex index of refraction (%n), free-carrier effect, lattice-temperature and carrier recombination contributions to relaxation curve was analyzed. When the carrier density N is 1.44×1018/cm3, free-carrier contribution to refraction index %nFC is -7.33×10-4, lattice-temperature %nLT is 0.85×10-4. Based on recombination rate equation, recombination lifetime of 980ps was deduced.