Femtosecond Pump-probe Technique Research Articles

Coherent terahertz acoustic vibrations in polar and semipolar gallium nitride-based superlattices

The femtosecond optical pump-probe technique is used to generate and detect the coherent acoustic vibrations with a frequency 0.83×1012Hz in polar (0001) and semipolar (112¯2) InGaN∕GaN superlattices. The measured amplitude of the vibrations in the semipolar sample is about five times smaller that in the polar one. Analysis of the experimental data and theoretical estimates suggest that, in addition to piezoelectric, deformation potential provides essential contribution to both generation and detection of vibrations, especially in the semipolar superlattice.

铬膜的时间分辨反射率的实验研究

The transient time-resolved reflectivity of chromium film is studied by femtosecond pump-probe technique with a 70-fs laser. Experimental results show that the reflectivity change increases with the power of the pump laser. The fast decrease of the reflectivity occurs between 0-200 fs which is mainly due to the electron-electron interaction. Subsequencely, the slower recovery of the reflectivity between 200-900 fs is mainly due to the electron-phonon coupling process. The reflectivity after 900 fs rises little to a near-constant value for the thermal equilibrium of the system. The experimental results can be explained properly with numerical simulation of the two-temperature model. It is helpful for understanding of the electron ultrafast dynamics in chromium film.

Femtosecond pump-probe technique assisted by surface plasmon resonance

Femtosecond pump-probe technique assisted by surface plasmon resonance （SPR） is investigated numerically in this paper. Our calculations show that the signal ratio ΔR/R is increased by 2—3 orders of magnitude higher than the standard pump-probe technique， where ΔR and R are the intensity change and total intensity of the probe beam， respectively. It should also be pointed out that ΔR/R is sensitive to the incident angle of the pump beam and the thickness of the metal film. In particular for the metal-absorbing material system， ΔR induced by the hot electrons in the metal is only 2 percent of that induced by excited absorbing material， so ΔR/R is mainly related to the dynamics of the absorbing material. This technique will be interesting in investigating the ultrafast surface dynamics of thin films.

Observation of coherent acoustic and optical phonons in bismuth nanowires by a femtosecond pump-probe technique

Coherent acoustic and optical phonon oscillations in Bi nanowire samples were studied with a femtosecond pump-probe technique. Laser pulses of 50 fs excited simultaneously acoustic oscillations at a frequency of about 9.5 GHz and optical phonons in the terahertz range. The transmission signal of nanowires on a glass substrate and the signal of light scattered from freestanding nanowires were measured. The acoustic velocity in nanowires was found to be close to that of bulk polycrystalline material. The changes in the optical phonon frequency at different laser fluences were simulated taking into account excitation inhomogeneity, lattice anharmonicity, diffusion, and recombination of the carriers and gave good agreement with experimental results.

Excited-state dynamics in light-harvesting complex of Rhodobacter sphaeroides

Photodynamics of peripheral antenna complexes, light-harvesting complex (LH2) of Rhodobacter (Rb) Sphaeroides 601, was studied using femtosecond pump-probe technique at different laser wavelengths. The obtained results reveal dramatic dynamical evolutions within B800 and B850 absorption bands of antenna complexes LH2. At excitation wavelength around 835 nm, a sharp photobleaching signal was observed which was assigned to the contribution of the two-exciton state, which was further confirmed by the power dependence measurement. Rate equations with eight-level scheme were used to calculate the population evolution in LH2 and the transient dynamics under femtosecond pulse excitation. The research results prove that not only the transition from ground state to one-exciton state but also that from one-exciton state to two-exciton state contribute to the photodynamics of B850.

Femtosecond Optical Detection of Quasiparticle Dynamics in Single-Crystal Bi2Sr2CaCu2O8+δ

Quasiparticle dynamics of an optimally doped Bi2Sr2CaCu2O8+δ single crystal is investigated by the femtosecond pump-probe technique. Temperature dependences of amplitude of the photoinduced differential reflectivity and the relaxation time show the evidence of strong phonon bottleneck. The experimental results are analysed by the Rothwarf–Taylor model.

Lumped oscillations of a nanofilm at adhesion bond

Femtosecond pump-probe laser technique has been applied for the diagnostics of a film adhesion to a substrate. A low frequency motion additional to acoustic vibrational modes of the film has been detected and related to film oscillation as a lumped mass element at a weak adhesion bond. The measurements of this low-frequency provide information on the strength of the interface. Long life time of the observed oscillation has remained unexplained.

Sub-THz coherent phonon coupling between solid and liquid water

Sub-THz coherent acoustic phonon coupling between solid and liquid water was studied by the femtosecond pump-probe technique. We used InGaN/GaN heterostructures to generate narrow-band coherent acoustic phonons at frequencies up to 1.4 THz. The measured ratio of reflectivities at GaN-water interfaces to that at GaN-air interfaces was found to strongly decrease and exhibit an abnormal oscillation with the increase in phonon frequency. The possible mechanisms resulting in the characteristics of experimental observations are briefly discussed in this report.

Time-resolved infrared spectroscopy of the lowest triplet state of thymine and thymidine

Vibrational spectra of the lowest energy triplet states of thymine and its 2′-deoxyribonucleoside, thymidine, are reported for the first time. Time-resolved infrared (TRIR) difference spectra were recorded over seven decades of time from 300 fs to 3 μs using femtosecond and nanosecond pump-probe techniques. The carbonyl stretch bands in the triplet state are seen at 1603 and ∼1700 cm −1 in room-temperature acetonitrile- d 3 solution. These bands and additional ones observed between 1300 and 1450 cm −1 are quenched by dissolved oxygen on a nanosecond time scale. Density-functional calculations accurately predict the difference spectrum between triplet and singlet IR absorption cross sections, confirming the peak assignments and elucidating the nature of the vibrational modes. In the triplet state, the C4 O carbonyl exhibits substantial single-bond character, explaining the large (∼70 cm −1) red shift in this vibration, relative to the singlet ground state. Femtosecond TRIR measurements unambiguously demonstrate that the triplet state is fully formed within the first 10 ps after excitation, ruling out a relaxed 1 nπ ∗ state as the triplet precursor.

Laser-excited acoustic oscillations in silver and bismuth nanowires

Coherent acoustic oscillations in Bi and Ag nanowire samples were studied with a femtosecond pump-probe technique and detection of the scattered light. The observed optical and acoustic properties reflect the nanostructure of these materials. The electronic and lattice contributions to the excitation of coherent acoustic phonons are described using a two-temperature model. The excitation is performed at different laser fluences, and the high density of optically induced excitations modifies the state of the lattice. A transient state with softening of the lattice was observed in Ag nanowire samples.

Long-lived coherent traveling acoustic pulses induced by femtosecond laser pulses

In this paper, we systematically study the generation and propagation of coherent acoustic pulses in a metal–dielectric system using a two-color femtosecond pump–probe technique at different probe angles. A long-lived acoustic oscillation is observed in a borosilicate glass coated with gold and shows different attenuations and amplitude at different probe wavelengths. Our study demonstrates that the two-color optical pump–probe technique can be used as a noninvasive tool to study acoustic properties of dielectric materials.

Predissociation Dynamics of B State of Methyl Iodide with Femtosecond Pump-probe Technique

The predissociation dynamics of B Rydberg state of methyl iodide has been studied with femtosecond two-color pump-probe time-of-flight spectra at pump pulse of 400nm and probe pulse 800 nm. The dominant product channels are the CH3I+ and CH3+ formation. The time-dependent signals for CH3I+ and CH3+ ions are obtained. Both of the signal curves can be fitted by biexponential decays with time constants of O 1 and O 2, O 1 was assigned to the lifetimes of high Rydberg states, which can be accessed by absorbing three 400 nm pump pulses and O 2 reflects the dynamics of B Rydberg state, which is accessed with two pump pulses. The lifetime of B Rydberg state is determined to be about 1.57 ps, which is incredibly consistent with the previous studies. The results were interpreted as a multiphoton dissociative ionization processes.

Ultrafast dynamics in ZnO/ZnMgO multiple quantum wells

We have investigated carrier relaxation and exciton recombination dynamics inZnO/ZnMgO multiple quantum wells using femtosecond pump–probe techniques at roomtemperature. For a probe energy above the band gap, the hot carriers exhibit aneffective relaxation by longitudinal optical phonon scattering with a cooling time of700–850 fs. By detecting the emission near the band-gap, a longer decay time of a fewpicoseconds was observed which is attributed to acoustic phonon scattering. Asthe probe energy is decreased further, the decay time continues to increase dueto the transitions of exciton recombination or localized carrier recombination.

Ultrafast Photoinduced Electron Transfer in Directly Linked Porphyrin−Ferrocene Dyads

The ultrafast electron transfer occurring upon Soret excitation of three new porphyrin-ferrocene (XP-Fc) dyads has been studied by femtosecond up-conversion and pump-probe techniques. In the XP-Fc dyads (XP-Fcs) designed in this study, the ferrocene moiety is covalently bonded to the meso positions of 3,5-di-tert-butylphenyl zinc porphyrin (BPZnP-Fc), pentafluorophenyl zinc porphyrin (FPZnP-Fc), and 3,5-di-tert-butylphenyl free-base porphyrin (BPH2P-Fc). Charge separation and recombination in the XP-Fcs were confirmed by transient absorption spectra, and the lifetimes of the charge-separated states were estimated from the decay rate of the porphyrin radical anion band to be approximately 20 ps. The charge-separation rates of the XP-Fcs were found to be >10(13) s-1 from the S2 state and 6.3x10(12) s-1 from the S1 state. Charge separation from the S2 state was particularly efficient for BPZnP-Fc, whereas the main reaction pathway was from the S1 state for BPH2P-Fc. Charge separation from the S2 and S1 states occurred at virtually the same rate in benzene and tetrahydrofuran and was much faster than their solvation times. Analysis of these results using semiquantum Marcus theory indicates that the magnitude of the electronic-tunneling matrix element is rather large and far outside the range of nonadiabatic approximation. The pump-probe data show the presence of vibrational coherence during the reactions, suggesting that wavepacket dynamics on the adiabatic potential energy surface might regulate the ultrafast reactions.

Observation of ultrafast carrier dynamics in amorphous Ge2Sb2Te5 films induced by femtosecond laser pulses

The femtosecond pump-probe technique was used to study the carrier dynamics of amorphous Ge2Sb2Te5 films. With carrier density at around 1020−1021 cm−3, carriers were excited within 1 ps and recovered to the initial state for less than 3 ns. On the picosecond time scale, the carrier relaxation consists of two components: a fast process within 5 ps and a slow process after 5 ps. The relaxation time of the fast component is a function of carrier density, which increases from 1.9 to 4.3 ps for the carrier density changing from 9.7×1020 cm−3 to 3.1×1021 cm−3. A possible interpretation of the relaxation processes is elucidated. In the first 5 ps the relaxation process is dominated by an intraband carrier relaxation and the carrier trapping. It is followed by a recombination process of trapped carriers at later delay time.

Photoinduced Fano resonance of coherent phonons in zinc

Utilizing femtosecond optical pump-probe technique, we have studied transient Fano resonance in zinc. At high excitation levels the Fourier spectrum of the coherent ${E}_{2g}$ phonon exhibits strongly asymmetric line shape, which is well modeled by the Fano function. The Fano parameter $(1∕Q)$ was found to be strongly excitation fluence dependent while depending weakly on the initial lattice temperature. We attribute the origin of the Fano resonance to the coupling of a coherent phonon to the electronic continuum, with their transition probabilities strongly renormalized in the vicinity of the photoinduced structural transition.

Lifetime and anisotropy decay of excited Coumarin 30 measured by a femtosecond pump–probe technique

We measured the lifetime of Coumarin 30 dye molecules excited with 400 nm in ethanol solution with the pump–probe stimulated emission fluorescence technique using femtosecond laser pulses that provided high temporal resolution of about 50 fs. Both fluorescence and stimulated emission signals were observed. The stimulated emission was induced at 550 nm. The lifetime of the excited state was determined to be (1.9± 0.3) ns and the rotational depolarization time was measured to be (120± 20) ps. Our results are in agreement with data obtained previously by a different technique.

Ultrafast processes of photoinduced charge and energy transfer in nanostructural fullerene-metal films

The primary stages of photoinduced processes in tin-doped C60 fullerene films have been studied using a femtosecond pump-probe technique with 150-fs laser excitation pulses (λ = 400 nm) and differential transmission and reflection probing in the 1100–1700 nm range. The relaxation dynamics strongly depends both on the metal-to-fullerene ratio in the film and on the mutual distribution (packing) of components in the nanocomposite material. The observed response signal dynamics is related to features in the charge carrier generation, energy transfer between fullerene molecules, and charge transport between metal and fullerene.

Ultrafast mapping of H2+ (D2+) nuclear wave packets using time-resolved Coulomb explosion imaging

The time evolution of H2+ (D2+) nuclear wave packets is studied exploiting a combination of coincident Coulomb explosion imaging and femtosecond pump–probe techniques. Using two 25 fs laser pulses, we map the motion of the dissociating molecular ion, observe an enhanced ionization rate at an internuclear separation of ∼11 au and resolve trajectories due to the one- and two-photon Floquet channels. With two 7 fs pulses, we are able to visualize the vibrational motion of the bound part of the wave packet, which exhibits counterintuitive quantum behaviour and dephases within about 100 fs, in agreement with recent numerical simulations.

Femtosecond optical investigation of electron–lattice interactions in an ensemble and a single metal nanoparticle

Electron–lattice energy exchange is investigated in an ensemble of silver nanoparticles of mean diameter 9 nm and in a single 30-nm particle using a femtosecond pump–probe technique. The dependences of the measured transient transmission change and of the electron energy loss kinetics on the excitation amplitude are compared to the results of numerical simulations of nonequilibrium electron relaxation and of the two-temperature model. The good agreement between the theoretical and experimental data indicates that, for the studied low particle density samples, hot-electron cooling is dominated by electron–lattice coupling in a nanoparticle both for weak and large electron heating with a minor influence of their surrounding environment (glass or polymer matrix).