Phase-locked Femtosecond Pulses Research Articles

Polarization-Directed Surface Plasmon Polariton Launching.

The relative intensities of propagating surface plasmons (PSPs) simultaneously launched from opposing edges of a symmetric trench structure etched into a silver thin film may be controllably varied by tuning the linear polarization of the driving field. This is demonstrated through transient multiphoton photoemission electron microscopy measurements performed using a pair of spatially separated phase-locked femtosecond pulses. Our measurements are rationalized using finite-difference time domain simulations, which reveal that the coupling efficiency into the PSP modes is inversely proportional to the magnitude of the localized surface plasmon fields excited at the trench edges. Our combined experimental and computational results allude to the interplay between localized and propagating surface plasmon modes in the trench; strong coupling to the localized modes at the edges correlates to weak coupling to the PSP modes. Polarization-directed PSP launching measurements reveal an optimal PSP contrast ratio of 4.2 using a 500 nm wide trench.

Alternative view of two-dimensional spectroscopy.

Femtosecond two-dimensional (2D) spectroscopy has become a widely employed method for the investigation of the dynamics of complex chemical and biological systems. In 2D spectroscopy, the sample is excited with three phase-locked femtosecond pulses, and the signal is heterodyned with the local oscillator field. The 2D spectrum is obtained by double Fourier transform with respect to the time delay between the first two pulses and the time delay between the third pulse and the local oscillator field. We show that 2D optical signals can alternatively be measured and computationally simulated as four-wave-mixing signals generated by two femtosecond pulses and two one-sided continuous-wave (CW) pulses. The first femtosecond pulse and one-sided CW pulse create the doorway state, while the second femtosecond pulse and one-sided CW pulse create the window state. This picture relates 2D spectroscopy to other mixed time-frequency-domain techniques, which is useful for the interpretation of the corresponding signals. Moreover, it allows a computationally efficient evaluation of 2D spectra.

Coherent control of interfering wave packets in dissociating HD+ molecules: the role of phase and delay time

The coherent control of interference between dissociating wave packets of the HD+ molecules generated by a pair of time-delayed and phase-locked femtosecond laser pulses is theoretically studied by using the time-dependent quantum wave packet method. The density function in both coordinate and momentum representation are presented and discussed. It is demonstrated that the interference pattern is observed in both coordinate and momentum density functions. The interference undergoes a π-phase shift when the delay time between the two phase-locked femtosecond laser pulses is changed by half an optical period. In particular, the number of interference fringes, the fringe spacing in the R-dependent density distribution |ψ(R)|2, and the modulation period of the energy-dependent distribution of the fragments P(E) can be tuned by two phase-locked femtosecond pulses.

Coherent enhancement in two-photon fluorescence in molecular system induced by phase-jump modulated pulse

Coherent control of two-photon fluorescence (TPF) of 2',7'-dichlorofluorescein in methanol solution was experimentally investigated by shaping the femtosecond pulse with the phase jump. The experimental results indicated that the TPF intensity induced by the shaped femtosecond pulses with certain phase jump could be coherently enhanced. The physical mechanisms for TPF enhancement in the molecular system were explicitly discussed and analyzed, which could be attributed to the wave-packet constructive interference in the excited states. Finally, two phase-locked femtosecond pulses were used to explore the wave-packet constructive interference in the excited states of 2',7'-dichlorofluorescein, which validate experimentally the proposed mechanism.

On the paradigm of coherent control: the phase-dependent light–matter interaction in the shaping window

Coherent control is unquestioningly a powerful method to investigate the interaction of femtosecond light pulses with ultrafast molecular processes. In order to exploit to a great extent the capabilities of coherent control, a judicious analysis of experimental control results is required, which is especially true for the manipulation of matter waves with phase-locked femtosecond pulses. Due to its importance for the optical control of chemical reactions, we discuss the tailoring of wavepackets and the interpretation of such coherent control results in the region where the pump and probe pulses temporally overlap. In this region, called the shaping window, the observed dynamics are phase dependent and may misleadingly resemble oscillatory molecular dynamics. In a transient absorption experiment, part of these oscillations is successfully identified as intra-multipulse interferences and the phenomenon is qualitatively and quantitatively explained by the spectral appearance of the unshaped laser pulse. Furthermore a strategy is presented based on phase-cycling for easy differentiation between interference effects and true wavepacket dynamics. These new findings have important consequences for the interpretation of adaptive coherent control results.

Designing of wave packets by using delayed phase-locked femtosecond pulses of different durations

Ab-initio study on the quantum interference of two vibrational wavepackets on A1Σ+ state of NaH molecule excited by two delayed phase-locked 4 femtosecond pulses (pump and control) has revealed that coherent control over populating selective vibrational levels or selective product state can be achieved. We have investigated here how the quantum interference of two wavepackets can be controlled to design a wavepacket which resembles one or two vibrational levels on the excited bound state leading to large overlap with these vibrational levels and hence enhancing the cross section of excitation to selected vibrational levels wiping out the population flow to other nearby levels covered by the bandwidth of the pulses. It will be shown here that the designing of interfering wavepacket can be controlled by controlling the pulse duration and carrier frequency of two delayed phase-locked ultrashort pulses.

Femtosecond phase control of spatial localization of the optical near-field in a metal nanoslit array

We demonstrate spatial control of optical near-fields by femtosecond phase shaping in one-dimensional plasmonic structures. The near-field images display striking temporal-phase dependence, switching between double- and single-peak images within one lattice constant. The change of the near-field distribution is studied in the time and spectral domain. The spectral composition change observed by varying the time delay between two phase-locked femtosecond pulses explains the spatial control of the near-field images. Modal expansion calculations of linear light transmission using the surface impedance boundary condition are in excellent agreement with experiments.

Coherent Exciton Dynamics in GaAs-Based Semiconductor Structures

We show that a very powerful tool in the investigation of the coherent exciton dynamics in semiconductors is provided by the study of the emitted light after resonant excitation from pairs of phase-locked femtosecond pulses. Under these conditions, not only the full dynamics of the coherent transients (dephasing times, quantum beat periods, etc.) can be obtained from linear experiments, but it can also be obtained a straightforward discrimination between the coherent or incoherent character of the emission by means of spectral filtering.