Femtosecond Time-resolved Four-wave Mixing Research Articles

Two-photon resonances in femtosecond time-resolved four-wave mixing spectroscopy: β-carotene

Femtosecond time-resolved pump-degenerate four-wave mixing (pump-DFWM) spectroscopy has been used to study the ultrafast dynamics of beta-carotene involving several electronic and vibrational states. An initial pump pulse, resonant with the S(0)-to-S(2) transition, excites the molecular system and a DFWM process, resonant with the S(1)-to-S(n) transition, is used to probe the relaxation pathways. The transient shows a peculiar decay behavior, which is due to the contributions of resonant DFWM signal of the excited S(1) state, nonresonant DFWM signal of the ground S(0) state and vibrational hot S(0)* state, and the two-photon resonant DFWM signal of the ground S(0) state. We have used a kinetic model including all the signal contributions to successfully fit the transient. The time constants extracted are in very good agreement with the known values for beta-carotene. For comparison, a two-pulse pump-probe experiment was performed measuring the transient absorption at the wavelength of the DFWM experiment.

The investigation of excited-state processes in biologically relevant molecules by means of femtosecond time-resolved four-wave mixing spectroscopy

The investigation of excited-state processes in biologically relevant molecules by means of femtosecond time-resolved four-wave mixing spectroscopy

Femtosecond ground state dynamics of gas phase N 2O 4 and NO 2

Non-resonant femtosecond time-resolved four-wave mixing (FWM) data on gas phase NO 2 and N 2O 4 are presented. The initial rotational dephasing is observed during the first picosecond after excitation. Fast vibrational dynamics (average value 133±1 fs beats) are observed for dinitrogen tetroxide and are assigned to the ν 3 N–N stretching mode in the ground state ( 1 A g ) . Rotational revivals on the picosecond time scale are recorded for both samples. No evidence of a long-lived excited state participation was found using photon echo (PE) or virtual echo (VE) pulse sequences with 800 nm laser pulses, however, NO 2 photoproducts were observed following N 2O 4 excitation.

Femtosecond laser-controlled selective excitation of vibrational modes on a multidimensional ground state potential energy surface

In this Letter, we report the controlled selective excitation of vibrational modes in the electronic ground state of polymers of diacetylene. The technique applied for exciting and probing is femtosecond time-resolved four-wave mixing spectroscopy with broad-band detection. We use a coherent anti-Stokes Raman scheme, where the difference of pump and Stokes laser wavelengths is chosen to achieve resonance with several chain modes of the polydiacetylenes. By varying the timing and the phases of the femtosecond laser pulses, the timing and intensity of these modes can be strongly influenced.

Separation of vibrational and rotational coherences with polarized femtosecond time-resolved four-wave mixing spectroscopy

Separation of vibrational and rotational coherences with polarized femtosecond time-resolved four-wave mixing spectroscopy

CCD broadband detection technique for the spectral characterization of the inhomogeneous signal in femtosecond time-resolved four-wave mixing spectroscopy

CCD broadband detection technique for the spectral characterization of the inhomogeneous signal in femtosecond time-resolved four-wave mixing spectroscopy

Determination of wave packet dynamics by femtosecond time-resolved pump-dump-probe and four-wave mixing techniques

Femtosecond time-resolved four-wave mixing (FWM) spectroscopy is performed in order to investigate molecular dynamics in iodine molecules in the gas phase. Depending on both the timing of the laser pulses and on the applied FWM technique different dynamics are observed in the FWM transient signals. By using the time-evolution diagrams the varying contribution of ground and excited state dynamics can be explained conclusively. Further, a femtosecond three-color pump-dump-probe scheme in combination with a time-of-flight mass selective detection unit has been applied to study vibrational wave packet dynamics in the electronic ground state of cold K 2 molecules. A spectral analysis of the time domain signal reveals two different wave packet contributions: hot wave packets from a stimulated Raman process and cold ones from stimulated emission pumping.

Simulation of femtosecond time-resolved four-wave mixing experiments on I2

Quantum calculations on time-resolved coherent spectroscopy of gas-phase I2 samples are presented. The total polarization induced in the molecules depends on the delay-time between the applied laser pulses. Different contributions reflect the nuclear dynamics in the ground or an excited electronic state. Dependent on the pulse delay-time on one hand and the wavelengths employed on the other, the signal reflects dynamics in a single or in both electronic states. In the case that several initially populated vibrational states contribute to the signal, special care has to be taken of the experimental detection process.

Coupled absorber-cavity system: Observation of a characteristic nonlinear response

We investigate the nonlinear dynamics of the composite absorber-cavity system by performing femtosecond time-resolved four-wave mixing experiments on a semiconductor microcavity. The nonlinear response exhibits an unexpected temporal behavior that distinctively differs from that of other multilevel systems. Accompanying model calculations show that the observed behavior is a direct consequence of the mixed electron-photon character of the absorber-cavity system.

Femtosecond time-resolved four-wave mixing spectroscopy in iodine vapour

Femtosecond time-resolved four-wave mixing (FWM) spectroscopy is performed using degenerate laser frequencies in order to investigate molecular dynamics in iodine molecules in the gas phase. It is demonstrated that by varying the timing as well as the wavenumber position for the signal detection different dynamics can be accessed. This is due to a selection of contributions (e.g. described by Feynman diagrams) to the nonlinear polarization of the process. The wave packet dynamics of the electronic excited B as well as the X ground state of iodine can be monitored by degenerate FWM, coherent anti-Stokes Raman scattering or coherent Stokes Raman scattering (CSRS) processes all included in the same experiment.

Quantum Coherence of Continuum States in the Valence Band of GaAs Quantum Wells.

Quantum beats of heavy and light holes in GaAs quantum wells are investigated in femtosecond time-resolved four-wave mixing and transmission experiments as a function of optical excitation energy. Under nonexcitonic excitation conditions, the four-wave mixing signal disappears due to the immediate loss of the interband coherence of continuum states. In the transmission experiment, the quantum beats are observed up to excess energies of 75 meV above the exciton resonances. The experimental data clearly demonstrate the coherence of continuum states in the valence band. Changes of the beat frequency with the excitation energy are due to the dispersion of the valence bands.

Femtosecond time-resolved four-wave mixing from biexcitons in GaAs quantum wells: Dominance of the interaction-induced signal.

Time-resolved (TR) four-wave mixing (FWM) experiments are performed on a high-quality GaAs quantum-well sample where biexcitons make the dominant contribution under certain experimental conditions. TR-FWM peaks well after both beams passed through, regardless of relative contributions of biexcitons to FWM. Our experiments show that the diffraction of the interaction-induced field dominates FWM from biexcitons, as well as from excitons.

Vibrational frequencies and dephasing times in excited electronic states by femtosecond time-resolved four-wave mixing

Time-resolved degenerate four-wave mixing (TRDFWM) for an electronically resonant system in a phase-matching configuration that measures population decay is reported. Because the spectral width of input light exceeds the vibrational Bohr frequency of a strong Raman active mode, the vibrational coherence produces strong oscillations in the TRDFWM signal together with the usual population decay from the excited electronic state. The data are analyzed in terms of a four-level system: ground and excited electronic states each split by a vibrational quantum of a Raman active mode. Absolute frequencies and their dephasing times of the vibrational modes at ≈590 cm −1 are obtained for the excited as well as the ground electronic state. The vibrational dephasing rate in the excited electronic state is about an order of magnitude faster than that in the ground state, the origin of which is speculated upon.