We report an experiment to measure the femtosecond electric field of the signal emitted from an optical third-order nonlinear interaction in carbon dioxide molecules. Using degenerate four-wave mixing with femtosecond near infrared laser pulses in combination with the ultra-weak femtosecond pulse measurement technique of TADPOLE, we measure the nonlinear signal electric field in the time domain at different time delays between the interacting pulses. The chirp extracted from the temporal phase of the emitted nonlinear signal is found to sensitively depend on the electronic and rotational contributions to the nonlinear response. While the rotational contribution results in a nonlinear signal chirp close to the chirp of the input pulses, the electronic contribution results in a significantly higher chirp which changes with time delay. Our work demonstrates that electric field-resolved nonlinear spectroscopy offers detailed information on nonlinear interactions at ultrafast time scales.
We present a comprehensive study of enantioselective orientation of chiral molecules excited by a pair of delayed cross-polarized femtosecond laser pulses. We show that by optimizing the pulses' parameters, a significant (~ 10%) degree of enantioselective orientation can be achieved at zero and at five kelvin rotational temperatures. This study suggests a set of reasonable experimental conditions for inducing and measuring strong enantioselective orientation. The strong enantioselective orientation and the wide availability of the femtosecond laser systems required for the proposed experiments…
A pair of coherent femtosecond pulse excitations applied to a molecule with strong electron-phonon coupling creates a coherent superposition of a low momentum and a high momentum wavepacket in the vibrational states of both the excited state and the ground state of the coherent transition. As the excited state is accelerated further, interference between the high momentum ground state contribution and the low momentum excited state contribution causes a photon echo. This photon echo is a direct consequence of quantum interference between separate vibrational trajectories and can therefore prov…
Impulsive orientation of symmetric-top molecules excited by two-color femtosecond pulses is considered. In addition to the well-known transient orientation appearing immediately after the pulse and then reemerging periodically due to quantum revivals, we report the phenomenon of field-free long-lasting orientation. Long-lasting means that the time averaged orientation remains non-zero until destroyed by other physical effects, e.g. intermolecular collisions. The effect is caused by the combined action of the field-polarizability and field-hyperpolarizability interactions. The dependence of deg…
We study the excitation of asymmetric-top (including chiral) molecules by two-color femtosecond laser pulses. In the cases of non-chiral asymmetric-top molecules excited by an orthogonally polarized two-color pulse, we demonstrate, classically and quantum mechanically, three-dimensional orientation. For chiral molecules, we show that the orientation induced by a cross-polarized two-color pulse is enantioselective along the laser propagation direction, namely, the two enantiomers are oriented in opposite directions. On the short time scale, the classical and quantum simulations give results tha…
Quantum mechanical phenomena, such as electronic coherence and entanglement, play a key role in achieving the unrivalled efficiencies of light-energy conversion in natural photosynthetic light-harvesting complexes, and triggered the growing interest in the possibility of organic quantum computing. Since biological systems are intrinsically heterogeneous, clear relations between structural and quantum-mechanical properties can only be obtained by investigating individual assemblies. However, single-molecule techniques to access ultrafast coherences at physiological conditions were not available…
We investigate the prospects of creating broad rotational wave packets by means of molecular interaction with long sequences of intense femtosecond pulses. Using state-resolved rotational Raman spectroscopy of oxygen, subject to a sequence of more than 20 laser pulses with peak intensities exceeding $10^{13}$ W/cm$^{2}$ per pulse, we show that the centrifugal distortion is the main obstacle on the way to reaching high rotational states. We demonstrate that the timing of the pulses can be optimized to partially mitigate the centrifugal limit. The cumulative effect of a long pulse sequence resul…
We present a combined experimental and theoretical study on the photodissociation dynamics of ion-pair formation in O$_2$ following resonant two-photon absorption of a 9.3 eV femtosecond pulse, where the resulting O$^+$ ions are detected using 3-D momentum imaging. Ion-pair formation states of $^3Σ^-_g$ and $^3Π_g$ symmetry are accessed through predissociation of optically dark continuum Rydberg states converging to the B $^2Σ^-_g$ ionic state, which are resonantly populated via a mixture of both parallel-parallel and parallel-perpendicular two-photon transitions. This mixture is evident in th…
We investigate the dynamics of diatomic molecules subjected to both a femtosecond mid-infrared laser pulse and Gaussian white noise. The stochastic Schrödinger equation with a Morse potential is used to describe the molecular vibrations under noise and the laser pulse. For weak laser intensity, well below the dissociation threshold, it is shown that one can find an optimum amount of noise that leads to a dramatic enhancement of the dissociation probability. The enhancement landscape which is shown as a function of both the noise and the laser strength, exhibits a global maximum. A frequency-re…
The capability of generating two intense, femtosecond x-ray pulses with controlled time delay opens the possibility of performing time-resolved experiments for x-ray induced phenomena. We have applied this capability to study the photoinduced dynamics in diatomic molecules. In molecules composed of low-Z elements, \textit{K}-shell ionization creates a core-hole state in which the main decay mode is an Auger process involving two electrons in the valence shell. After Auger decay, the nuclear wavepackets of the transient two-valence-hole states continue evolving on the femtosecond timescale, lea…
We present an analysis of two experimental approaches to controlling the directionality of molecular rotation with ultrashort laser pulses. The two methods are based on the molecular interaction with either a pair of pulses (a "double kick" scheme) or a longer pulse sequence (a "chiral pulse train" scheme). In both cases, rotational control is achieved by varying the polarization of and the time delay between the consecutive laser pulses. Using the technique of polarization sensitive resonance-enhanced multi-photon ionization, we show that both methods produce significant rotational directiona…
We present experimental results for the ionization of aniline and benzene molecules subjected to intense ultrashort laser pulses. Measured parent molecular ions yields were obtained using a recently developed technique capable of three-dimensional imaging of ion distributions within the focus of a laser beam. By selecting ions originating from the central region of the focus, where the spatial intensity distribution is nearly uniform, volumetric-free intensity-dependent ionization yields were obtained. The measured data revealed a previously unseen resonant-like multiphoton ionization process.…
We report a source of free electron pulses based on a field emission tip irradiated by a low-power femtosecond laser. The electron pulses are shorter than 70 fs and originate from a tip with an emission area diameter down to 2 nm. Depending on the operating regime we observe either photofield emission or optical field emission with up to 200 electrons per pulse at a repetition rate of 1 GHz. This pulsed electron emitter, triggered by a femtosecond oscillator, could serve as an efficient source for time-resolved electron interferometry, for time-resolved nanometric imaging and for synchrotrons.
Visualizing and controlling electron dynamics over femtosecond timescale play a key role in the design of next-generation electronic devices. Using simulations, we demonstrate the electronic oscillation inside the naphthalene molecule can be tracked by means of the tuning of delay time between two identical femtosecond laser pulses. Both the frequency and decay time of the oscillation are detected by the tunneling charge through the junction of scanning tunneling microscopy. And the tunneling charge is sensitive to the carrier-envelope phase (CEP) for few-cycle long optical pulses. While this …
Optical spectroscopy with broadband femtosecond laser pulses often involves simultaneous excitation of multiple molecular species with close resonance frequencies. Interpreting the collective optical response from molecular mixtures typically requires Fourier analysis of the detected time-resolved signal. We propose an alternative method of separating coherent optical responses from two molecular species with neighboring excitation resonances (here, vibrational modes of oxygen and carbon dioxide). We utilize ro-vibrational coupling as a mechanism of suppressing the strong vibrational response …
Delayed cavity-free forward lasing at the wavelengths of 391 and 428 nm was observed in recent experiments in air or pure nitrogen pumped with an intense femtosecond laser pulse at wavelength of 800~nm. The mechanism responsible for the lasing is highly controversial. In this article we explain the delayed emission by the presence of long-lived polarizations coupling simultaneously ground state X$^2Σ_g^+$ to states A$^2Π_u$ and B$^2Σ_u^+$ of singly ionized nitrogen molecules N$_2^+$. Ionization of neutral nitrogen molecules in a strong laser field and subsequent ion excitation are described by…
This work investigates the utility of femtosecond pulse shaping in increasing the efficiency of Raman excitation of molecules in the strong-field interaction regime. We study experimentally and theoretically the effect of pulse shaping on the strength of non-resonant coherent anti-Stokes Raman scattering in iodine vapor at laser intensities exceeding $10^{13}$ W/cm$^2$. We show that unlike the perturbative case, shaping strong non-resonant laser pulses can increase the signal strength beyond that observed with the transform-limited excitation. Both adiabatic and non-adiabatic schemes of excita…
Interaction of light with media often occurs with a femtosecond response time. Its measurement by conventional techniques requires the use of femtosecond lasers and sophisticated time-gated optical detection1-3. Here we demonstrate that by exploiting quantum interference of entangled photons it is possible to measure the phase relaxation time of a media on the femtosecond time scale (down to 100 fs) using accessible continuous wave laser and single-photon counting. We insert the sample in the Hong-Ou-Mandel interferometer4 and infer the phase relaxation time from the modification of the two-ph…
We propose a method to reconstruct the vibrational quantum state of molecules excited by a general excitation laser pulse. Unlike existing methods, we do not require the molecules before excitation to be in a pure state, allowing us to treat the important case of initially thermally excited molecules. Even if only a single initial level is appreciably populated, initial levels with small populations can still give major contributions to the unknown vibrational state, making it essential to take them into account. In addition to the excitation pulse, the method uses two incident, short laser pu…
Electron and vibrational dynamics of molecules are commonly studied by subjecting them to two interactions with a fast actinic pulse that prepares them in a nonstationary state and after a variable delay period $T$, probing them with a Raman process induced by a combination of a broadband and a narrowband pulse. This technique known as femtosecond stimulated Raman spectroscopy (FSRS) can effectively probe time resolved vibrational resonances. We show how FSRS signals can be modeled and interpreted using the stochastic Liouville equations (SLE) originally developed for NMR lineshapes. The SLE p…
We perform a comprehensive investigation of Förster resonant energy transfer in a room-temperature thermal mixture of ammonia molecules and rubidium Rydberg atoms. Fully state-resolved measurement of the Rydberg-atom populations is achieved by combining millimeter-wave state transfer with state-selective field ionization. This allows aspects of the energy transfer process such as state dependence, ammonia pressure dependence, and dependence on the energy resonance condition to be investigated in detail. Our results pave the way for future quantum experiments combining polar molecules and Rydbe…
The paper explores the prospects of observing the phenomenon of dynamical Anderson localisation via non-resonant Raman-type rotational excitation of molecules by periodic trains of short laser pulses. We define conditions for such an experiment, and show that current femtosecond technology used for non-adiabatic laser alignment of linear molecules is sufficient for this task. Several observables which can serve as indicator for Anderson localisation are suggested for measurement, and the influence of experimental limitations imposed by laser intensity noise, finite pulse duration, limited numb…
We describe a simple and efficient setup to generate and characterize femtosecond quadrature-entangled pulses. Quantum correlations equivalent to about 2.5 dB squeezing are efficiently and easily reached using the non-degenerate parametric amplification of femtosecond pulses through a single-pass in a thin (0.1 mm) potassium niobate crystal. The entangled pulses are then individually sampled to characterize the non-separability and the entropy of formation of the states. The complete experiment is analysed in the time-domain, from the pulsed source of quadrature entanglement to the time-resolv…
A new scheme is described for pulsed squeezed light generation using femtosecond pulses parametrically deamplified through a single pass in a thin (0.1mm) potassium niobate KNbO3 crystal, with a significant deamplification of about -3dB. The quantum noise of each individual pulse is registered in the time domain using a single-shot homodyne detection operated with femtosecond pulses and the best squeezed quadrature variance was measured to be 1.87 dB below the shot noise level. Such a scheme provides the basic ressource for time-resolved quantum communication protocols.
Two-photon photoassociation of hot magnesium atoms by femtosecond laser pulses, creating electronically excited magnesium dimer molecules, is studied from first principles, combining \textit{ab initio} quantum chemistry and molecular quantum dynamics. This theoretical framework allows for rationalizing the generation of molecular rovibrational coherence from thermally hot atoms [L. Rybak \textit{et al.}, Phys. Rev. Lett. {\bf 107}, 273001 (2011)]. Random phase thermal wave functions are employed to model the thermal ensemble of hot colliding atoms. Comparing two different choices of basis func…
Using tomographic reconstruction we determine the complete internuclear quantum state, represented by the Wigner function, of a dissociating I2 molecule based on femtosecond time resolved position and momentum distributions of the atomic fragments. The experimental data are recorded by timed ionization of the photofragments with an intense 20 fs laser pulse. Our reconstruction method, which relies on Jaynes' maximum entropy principle, will also be applicable to time resolved position or momentum data obtained with other experimental techniques.
Time-resolved femtosecond x-ray diffraction patterns from laser-excited molecular iodine are used to create a movie of intramolecular motion with time and space resolution of $30~$fs and $0.3$ Å. The high spatial fidelity is due to interference between the moving excitation and the static initial charge distribution. This x-ray interference has not been employed to image internal motion in molecules before. The initial state is used as the local oscillator for heterodyne amplification of the excited charge distribution to retrieve real-space movies of atomic motion on Ångstrom and femtosecond …
A new physical implementation for quantum computation is proposed. The vibrational modes of molecules are used to encode qubit systems. Global quantum logic gates are realized using shaped femtosecond laser pulses which are calculated applying optimal control theory. The scaling of the system is favourable, sources for decoherence can be eliminated. A complete set of one and two quantum gates is presented for a specific molecule. Detailed analysis regarding experimental realization shows that the structural resolution of today's pulse shapers is easiliy sufficient for pulse formation.
We explore the novel capabilities offered by attosecond extreme ultraviolet and x-ray pulses that can be now generated by free-electron lasers and high-harmonics generation sources for probing photon-induced electron dynamics in molecules. We theoretically analyze how spatial and temporal dependence of charge migration in a pentacene molecule can be followed by means of time-resolved photoelectron microscopy on the attosecond time scale. Performing the analysis, we accurately take into account that an attosecond probe pulse leads to considerable spectral broadening. We demonstrate that the exc…
We investigate the effect of incoherent perturbations on atomic photoionization due to a femtosecond mid-infrared laser pulse by solving the time-dependent stochastic Schrödinger equation. For a weak laser pulse which causes almost no ionization, an addition of a Gaussian white noise to the pulse leads to a significantly enhanced ionization probability. Tuning the noise level, a stochastic resonance-like curve is observed showing the existence of an optimum noise for a given laser pulse. Besides studying the sensitivity of the obtained enhancement curve on the pulse parameters, such as the pul…
