Time-resolved Pump-probe System Research Articles

Polaron lifetime enhancement of P3HT thin film under femtosecond laser irradiation and its ultrafast mechanism

The morphology and properties of poly (3-hexylthiophene) (P3HT) thin films after femtosecond laser irradiation with various fluences were studied. Two types of final ablation morphology were characterized through optical microscope (OM), atomic force microscope (AFM) and scanning electron microscope (SEM). At lower fluences, nano-bumps were formed in the central area. At higher fluences, nano-bumps were formed in the entire irradiated area and the material in the center disintegrated. Time-resolved pump–probe system was used to investigate laser − P3HT ultrafast interaction and the principle of morphological differences, which revealed that the Coulomb explosion was the reason for the morphology change with fluence increasing. Photoluminescence (PL) spectra indicated that the transition of the PL signal from enhancement to quenching with various laser fluences was related to the effect of laser on morphology and molecular exciton. A maximal 2 fold lifetime enhancement of bound polaron pair dissociation and 1.3 fold lifetime improvement of defect trapping were achieved at higher fluences, which contributed to obtaining more effective free carriers and subsequent photocurrent. These results promote the potential applications of femtosecond laser on P3HT and other organic semiconductors, especially organic photovoltaics (OPV) devices.

Dynamics of optical nonlinearities in GaN

The dynamics of optical nonlinearities in bulk GaN crystal are investigated by a modified time-resolved pump-probe system with phase object at 532 nm under picosecond pulse excitation. The sample's optical nonlinearities can be determined by measuring the normalized transmittance with/without an aperture in the far field. The different nonlinear mechanisms in GaN are separated. Experimental results show that the nonlinear response of GaN is a combination of bound-electronic and free-carrier nonlinearities mechanisms. With the theory of two-photon-induced free-carrier nonlinearity, the related optical nonlinear parameters, including free-carrier nonlinearities and free-carrier life time which are difficult to be determined, are obtained.

Temperature Dependence of All-Optical Ultrafast Magnetization Switching in TbFeCo

Using a time-resolved pump-probe system, we experimentally demonstrated that all-optical magnetization switching in TbFeCo can be observed at both room temperature and a low temperature of 110 K. This switching can only be triggered by a circularly polarized pump beam within a very narrow range of pulse energies. We believe these findings will lead to a better understanding of optically induced magnetization reversal on rare-earth transition metal alloys and provide an ultrafast and efficient way for future magnetic recording technique.

Excited state refraction of C70/toluene studied by using 4f coherent imaging system with phase object

Dynamic nonlinearities of C70/toluene solution are measured and analysed by an improved picosecond time-resolved pump-probe system based on a nonlinear imaging technique with phase object. The photophysical parameters are determined by the five-level model, which is adopted to interpret the experimental data. The change of refraction index per unit density of the excited state obtained by a numerically simulation is a critical factor to determine the nonlinear behaviour of C70 in picosecond time regime.

Investigation of nonlinear refraction of the C60/toluene solution by a time-resolved pump-probe system based on a 4f nonlinear imaging technique with the phase object

Excited state nonlinear refraction of the C60/toluene solution is studied using a picosecond time-resolved pump-probe system based on a 4f nonlinear imaging technique with the phase object. Negative nonlinear refraction of the C60/toluene solution is observed after zero-delay time besides normal nonlinear absorption. By using a five-energy-band model, both the change of refraction index per unit density of the first singlet excited state and the first triplet excited state of the C60 molecule having a negative sign are determined. The nonlinear refraction, as well as the dynamics of neat toluene and its excited state contribution to the solution, is also investigated.

Energy transfer induced by femtosecond two-beam coupling in two Kerr liquids

Femtosecond two-beam coupling is demonstrated in N,N′-dimethylformamide and dimethyl sulfoxide by using a time-resolved pump-probe system at a wavelength of 800 nm. As the temporal delay between pulses is varied within the coherence time, transient energy transfer from one pulse to another occurs. Because of the asymmetry of the normalized transmittance, two-photon absorption is considered in the two liquids. A theory based on two-beam coupling in perpendicular linear polarization is used to interpret the experimental results. Good agreement is obtained between the theory and experiment. Moreover, by introducing a valuable variable, the ratio of the relaxation times of the two Kerr liquids is directly characterized.

Time-resolved pump-probe technology with phase object for measurements of optical nonlinearities

A new time-resolved pump-probe system with phase object, which is based on the PO Z-scan technique and the standard pump-probe system, is presented. This new method can simultaneously investigate the dynamic of nonlinear absorption and refraction conveniently. On the other hand, both degenerate and nondegenerate pump and probe beams in any polarization states can be used in this system. The nonlinear optical properties of tetrasulfonated copper phthalocyanine dissolved in dimethyl sulfoxide solution are investigated by using this system.

Excited-state nonlinearity measurements of ZnPcBr_4/DMSO

Excited-state absorptive and refractive nonlinearities of a ZnPcBr4/DMSO solution are studied using a modified picosecond time-resolved pump-probe system based on a 4f nonlinear imaging technique with phase object. With the help of additional nanosecond laser Z-scan measurements, the parameters associated with the five-level model used to interpret the experimental results are uniquely determined. Unusually large absorption cross sections for the first singlet excited state and for the lowest triplet excited state as well as a rather short intersystem crossing time are found in this system, suggesting ZnPcBr4 (a phthalocyanine) to be an effective optical limiter. The intensity-dependent refractive index of neat dimethyl sulfoxide (DMSO) and the excited-state contribution to the nonlinear refractive index of the solution are also determined.

Investigation of dynamics of Kerr liquids by time-resolved pump-probe system based on 4f nonlinear imaging technique with phase object

A newly developed method for the characterization of the optical nonlinearities and dynamics is applied to Kerr liquids. The time-resolved pump-probe system based on the 4f nonlinear imaging technique with phase object is used to obtain the diffraction pattern of the nonlinear filter induced in the liquid CS2 placed in the Fourier plane by a charge-coupled device at various delay times. A theory based on two-beam coupling in perpendicular linear polarizations is used to interpret the measurement results. Good agreement is obtained between theory and experiment, suggesting a new method for simultaneous measurements of both magnitude and sign of the intensity-dependent refractive index as well as the dynamics of the Kerr liquids.

Time-resolved pump-probe system based on a nonlinear imaging technique with phase object

A nonlinear imaging technique with phase object, which can deduce nonlinear absorption and refraction coefficients by single laser-shot exposure, is expanded to a time-resolved pump-probe system by introducing a pump beam with a variable temporal delay. This new system, in which both degenerate and nondegenerate pump and probe beams in any polarization states can be used, can simultaneously measure dynamic nonlinear absorption and refraction conveniently. In addition, the sensitivity of this new pump-probe system is more than twice that of the Z-scan-based system. The semiconductor ZnSe is used to demonstrate this system.

Femtosecond near-field scanning optical microscopy study of molecular thin films.

A near-field scanning optical microscope has been combined with a two-colour time-resolved pump-probe measurement system. It has a noise-equivalent transmittance change of 5.0 x 10-5 for a probe pulse with an intensity of 30 nW. The system has been used for evaluating molecular thin films that have a domain structure, particularly for observing a gate action of the single domains. The results include key features to understand an origin of the domains and suggest that the film composition is uniform over a distance of several micrometres.