Time-resolved Pump-probe Technique Research Articles

Photoionization of gas-phase versus ion-beam-desorbed dopamine with femtosecond laser pulses.

We have investigated the photoionization of gas-phase and ion-beam desorbed dopamine using femtosecond laser pulses at wavelengths of 800, 400, 267, and 200 nm. Photoionization of gas-phase dopamine is found to produce the molecular ion, and three fragment ions at all four wavelengths, with the branching ratios strongly wavelength dependent. Photoionization at 400 and 267 nm yields the highest molecular ion signal, while that at 800 and 200 nm produces very little molecular ion signal. An excited-state lifetime of approximately 10 ps following 267-nm excitation has been measured for dopamine using time-resolved pump-probe techniques. The short-lived excited state suggests that internal conversion, intersystem crossing, and/or dissociation is a concern when ionizing at this wavelength using longer laser pulses. Photoionization of ion-beam-desorbed dopamine exhibits a large degree of fragmentation at all four wavelengths, though 267-nm photoionization produces the highest yield of dopamine fragment ions. Power dependence studies show a high degree of internal excitation. A direct comparison of ion yields obtained for photoionization of ion-beam-desorbed dopamine at 267 nm to that for SIMS shows a 20-fold increase in signal.

Coherent Phonons in Ag-BaO Thin Films

Coherent acoustic phonons have been observed in Ag nanoscale particles (NSPs) embedded in BaO thin film by means of the femtosecond time-resolved pump-probe technique. The oscillatory component in the signal corresponds to a very low-frequency acoustic phonon mode of Ag NSPs. The generation of coherent acoustic phonons is attributed to the resonant excitation of localized surface plasmon.

Nonresonant optical nonlinearity of ZnO composite nanoparticles with different interfacial chemical environments

ZnO nanoparticles coated by surfactant molecules were synthesized by microemulsion method. Under proper thermal treatment, a new ZnO/surfactant composite nanoparticles were formed and exhibited an unique optical property. The third-order optical nonlinearity χ(3) of ZnO composite nanoparticles with different interfacial chemical environment were measured by single-beam Z-scan technique, the sign and magnitude of both the real and imaginary parts of χ(3) at 790 nm were measured to be: -5.2×10−16 m2/W and 11.6 cm/Gw for ZnO/DBS composite nanoparticles (DBS, dodecyl benzene sulfonate, anionic surfactant); and -2.2×10−17 m2/W, 0.45 cm/Gw for ZnO/CTAB composite nanoparticles (CTAB, cetyltrimethyl ammonium bromide, cationic surfactant). The ultrafast nonlinear response time (∼250 fs) measured by time-resolved pump-probe technique at excitation wavelength of 647 nm suggests that the optical nonlinearity below band-gap originate mainly from a rapid electronic polarization process or virtual process such as the optical Stark effect.

Coherent dynamics of zone-folded acoustic phonons in GaAs/AlAs superlattices

The zone-folded longitudinal acoustic phonons in GaAs/AlAs superlattices are studied by the time-resolved pump-probe technique. Up to seven phonon modes are clearly observed and they are classified into standing and propagating modes according to their wave vectors. The standing modes are generated by the selective excitation of the well layers and the propagation modes are generated as a phonon wave packet at the substrate interface,

Nonresonant Optical Nonlinearities of Surface Chemically Modified ZnO Composite Nanoparticles

With surface chemical modification, we have obtained core-shell structure ZnO nanoparticles. The nonresonant third-order nonlinear optical refraction of this new composite material was found to be -3.8 × 10-10 esu by Z-scan measurements with a femtosecond pulsed laser. The ultrafast nonlinear response time similar 250 fs measured by time-resolved pump-probe techniques suggests that the nonlinearity of this new material originates mainly from a rapid electronic polarization process or virtual process such as the optical Stark effect.

Coherent THz Plasmons in GaAs: Transition from “Pure” Plasmons to Coupled Plasmon–Phonon Modes

We report on the observation of coherent THz plasmons in GaAs using the femtosecond time-resolved pump–probe technique. The samples are GaAs/AlGaAs modulation doped single heterostructures with a depletion field below the 2D-channel in the intrinsic GaAs substrate. For the first time, several coherent oscillations (up to five full periods) are clearly visible in the frequency range of 1.5 to 5 THz. The observed oscillation frequencies increase with increasing carrier densities. Our experimental data are in good agreement with theoretical Ensemble-Monte-Carlo simulations.

Pump-probe studies of carrier capture processes in semiconductor multiple-quantum-well waveguides

Carrier capture processes in GaAs-AlGaAs multiple-quantum-well (MQW) waveguides are monitored using the time-resolved pump-probe technique. With appropriate selections of wavelength and pulse recompression, subpicosecond pulses at various wavelengths can be obtained for both nondegenerate and degenerate pump-probe experiments with various polarization combinations. Two samples of different quantum-well structures are compared. It is found that the carrier capture times (defined as the time period to reach probe transmission maximum after the pump) are in the range from 15 to 50 ps in the samples used. In the degenerate pump-probe measurements, a two-component decay is observed for one of the samples in the case of TM pump and TE probe.

Subband gap carrier dynamics in low-temperature-grown GaAs

Measurements of the carrier relaxation dynamics in low-temperature-grown GaAs have been made with a femtosecond-resolution, time-resolved pump-probe technique using a subband-gap probe-beam wavelength. The transient absorption and index of refraction changes have been analyzed using a relaxation model with up to four different excited state populations. The carrier recombination time within the midgap trap states is found to be longer than the subpicosecond free-carrier trapping time. Two time scales are observed for the recombination rate, one of a few picoseconds and one of hundreds of picoseconds, indicating the presence of at least two different trap states for the free-carriers in this material.

Exciton dynamics in α-sexithienyl films

We present a characterization of the optical dynamics following photoexcitation in T 6 films. The relaxation of the excited state, generated by optical transitions at 3.2 eV, is measured using a time-resolved pump-probe technique. The relaxation kinetics consist of two main processes: a fast decay which depends on the excitation density and a slow decay which does not. We show that the fast dynamic is consistent with a time-dependent exciton annihilation model, which provides a value for the bimolecular rate constant of /2~ 3 × 10 −13 cm 3 s −1 2 . The slow decay is assigned to a monomolecular recombination process via exciton diffusion towards recombination centers. The obtained results are discussed within the wider context of photoexcitation in conducting polymers.

Picosecond strain waves in PTV

We studied the propagation of thermo-optically excited transient strain waves in thin films of PTV by means of the ps time-resolved pump-probe technique. We find that ultrasound at high frequencies (above 10GHz) does not properly propagate through the polymer film. Since ultrasound damping is frequency-Independent in the time interval of up to 3ns, we speculate that disorder-induced localization may be responsible for the high frequency cut-off.

Nonexponential relaxation in solid C 60 via time-dependent singlet exciton annihilation

We present measurements and detailed modeling of the dynamics of solid C 60 following photoexcitation. Optical pulses 60 fs in duration centered at 620 nm are used to excite the lowest optical band, and the relaxation of the photoexcitations is measured using a time-resolved pump-probe technique. The response is shown quantitatively to follow a time-dependent singlet exciton annihilation process consistent with long-range Förster annihilation. This result is supported by measurements of the dependence of the relaxation on excitation density and probe wavelength, by comparison of the response in thin film, crystalline, and solution samples, and by numerical modeling.

Coherent oscillation of zone-folded phonon modes in GaAs-AlAs superlattices.

We have observed a coherent oscillation of zone-folded acoustic phonons in GaAs-AlAs superlattices for the first time by means of a femtosecond time-resolved pump-probe technique. The oscillatory component in the time-resolved reflection signal corresponds to the upper branch (${B}_{2}$ symmetry) of the first-order doublet mode. Carriers were selectively excited in well layers and the ${B}_{2}$-symmetry phonon mode was selectively generated. The dephasing time of the ${B}_{2}$ mode was measured to be 70 ps from our experiments.

Exciton-polaritons in nonlinear optical phenomena in semiconductors: An overview of major developments

In the past decade, the availability of techniques for generating very intense tunable ultrashort-pulse laser radiation in the picoseconds and femtoseconds range have made it possible to use time-resolved pump probe techniques for studying the dynamics of the nonlinear response of bulk semiconductors and semiconductor quantum well structures to intense EM beams. Moreover, as a result of interest in nonlinear optical phenomena that might be used in optical memory, logic and switching applications, there has been an intensification of research on the effects of intense EM beams on optical properties in the vicinity of the exciton resonances and optical absorption edges of semiconductors. In this article we present an overview of key experimental and theoretical developments that underlie our understanding of the role played by exciton-polaritons in nonlinear optical phenomena in semiconductors and, in particular, the role played by exciton-polaritons in the nonlinear optical effects, including the optical Stark effect, which result from the virtual excitation of excitons, continuum electron-hole pairs and biexcitons by an ultrashort pulse of intense EM radiation at frequencies below the exciton resonance, and which exist only during the time of the pulse.