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Mountain echoes are a well-known phenomenon, where an impulse excitation is mirrored by the rocks to generate a replica of the original stimulus, often with reverberating recurrences. For spin echoes in magnetic resonance and photon echoes in atomic and molecular systems the role of the mirror is played by a second, time delayed pulse which is able to reverse the flow of time and recreate the original event. Recently, laser-induced rotational alignment and orientation echoes were introduced for molecular gases, and discussed in terms of rotational-phase-space filamentation. Here we present, for the first time, a direct spatiotemporal analysis of various molecular alignment echoes by means of coincidence Coulomb explosion imaging. We observe hitherto unreported spatially rotated echoes, that depend on the polarization direction of the pump pulses, and find surprising imaginary echoes at negative times. PACS numbers:In 1950, Erwin Hahn reported [1] that if a spin system is irradiated by two properly timed and shaped pulses, a third pulse appears at twice the delay between the first two. The intuitive explanation was given in terms of time reversal, namely the second pulse reverses the direction of propagation of the original excitation, leading to reappearance of the original impulse [2]. In the absence of interaction with the environment, the full original excitation is recovered, but with environmental influences, various dephasing and energy loss processes may be probed. Following the original discovery in the realms of spins, echoes were observed in many other nonlinear physical situations such as photon echo [3], cyclotron echo [4], plasma wave echo [5], echoes in cold atoms [6,7], cavity QED [8], and even in particle accelerators [9,10]. Echoes form the basis for many modern methodologies ranging from Magnetic Resonance Imaging (MRI) [11] to short-wavelength radiation generation in free-electron * lasers [12][13][14][15]. Echoes are a classical phenomenon that is different from another well-known effect: quantum revivals [16][17][18] which are caused by the energy quantization of physical systems. Recently, a new type of echoes was introduced: molecular alignment echoes [19,20]. When a gas of molecules undergoes excitation by an ultrashort laser pulse, the molecules experience a torque causing transient alignment of the ensemble along the laser polarization axis (for a review of laser molecular alignment, see [21][22][23][24]). A pair of time-delayed laser pulses results in three alignment events: two of them immediately following each excitation, and a third one, an echo, that appears with a delay equal to that between the exciting pulses. This delay can be shorter than the rotational revival time, so that the echo provides access to rapidly dephasing molecular dynamics. The formation of these echoes is caused by the kick-induced filamentation of the rotational phase space [19], a phenomenon well known in the physics of particle accelerators [32]. Moreover, fractional echoes were predicted and observed in mo...

Future nanoscale soft matter design will be driven by the biological paradigms of hierarchical self-assembly and long-lived nonequilibrium states. To reproducibly control the low-energy self-assembly of nanomaterials for the future, we must first learn the lessons of biology. Many cellular organelles exhibit highly ordered cubic membrane structures. Determining the mechanistic origins of such lipid organelle complexity has been elusive. We report the first observation of the complete sequence of major transformations in the conversion from a 1D lamellar membrane to 3D inverse bicontinuous cubic nanostructure. Characterization was enabled by adding a steric stabilizer to dispersions of lipid nanoparticles which increased the lifetime of very short-lived nonequilibrium intermediate structures. By using synchrotron small-angle X-ray scattering and cryo-transmission electron microscopy we observed and characterized initial lipid bilayer contacts and stalk formation, followed by membrane pore development, pore evolution into 2D hexagonally packed lattices, and finally creation of 3D bicontinuous cubic structures.

We demonstrate that oral delivery of self-assembled nanostructured nanoparticles consisting of 5-fluorouracil (5-FU) lipid prodrugs results in a highly effective, target-activated, chemotherapeutic agent, and offers significantly enhanced efficacy over a commercially available alternative that does not self-assemble. The lipid prodrug nanoparticles have been found to significantly slow the growth of a highly aggressive mouse 4T1 breast tumour, and essentially halt the growth of a human MDA-MB-231 breast tumour in mouse xenografts. Systemic toxicity is avoided as prodrug activation requires a three-step, enzymatic conversion to 5-FU, with the third step occurring preferentially at the tumour site. Additionally, differences in the lipid prodrug chemical structure and internal nanostructure of the nanoparticle dictate the enzymatic conversion rate and can be used to control sustained release profiles. Thus, we have developed novel oral nanomedicines that combine sustained release properties with target-selective activation.

An intense phase-controlled orthogonally polarized two-color ultrashort laser pulse is used to singly ionize and dissociate H_{2} into a neutral hydrogen atom and a proton. Emission-direction and kinetic-energy dependent asymmetric dissociation of H_{2} is observed as a function of the relative phase of the orthogonally polarized two-color pulse. Significant asymmetric proton emission is measured in the direction between two polarization axes. Our numerical simulations of the time-dependent Schrödinger equation reproduce many of the observed features. The asymmetry is attributed to the coherent superposition of two-dimensional nuclear wave packets with opposite parities, which have the same energies and overlap in the same emission directions.

A molecule can be optically accelerated to rotate unidirectionally at a frequency of a few terahertzes which is many orders higher than the classical mechanical rotor. Such a photon-induced ultrafast molecular unidirectional rotation has been well explored as a controllable spin of the molecular nuclear wave packet. Although it has been observed for more than 10 years, a complete imaging of the unidirectional rotating nuclear wave packet is still missing, which is essentially the cornerstone of all the exploring applications. Here, for the first time, we experimentally visualize the time-dependent evolution of the double-pulse excited molecular unidirectional rotation by Coulomb explosion imaging the rotational nuclear wave packet. Our results reveal comprehensive details undiscovered in pioneering measurements, which exhibits as a joint of the quantum revival of the impulsively aligned rotational wave packet and its unidirectional rotation following the angular momentum conservation. The numerical simulations well reproduce the experimental observations and intuitively revivify the thoroughgoing evolution of the rotational wave packet.Comment: 15 pages, 5 figure

No abstract

A phase-controlled orthogonal two-color (OTC) femtosecond laser pulse is employed to probe the time delay of photoelectron emission in the strong-field ionization of atoms. The OTC field spatiotemporally steers the emission dynamics of the photoelectrons and meanwhile allows us to unambiguously distinguish the main and sideband peaks of the above-threshold ionization spectrum. The relative phase shift between the main and sideband peaks, retrieved from the phase-of-phase of the photoelectron spectrum as a function of the laser phase, gradually decreases with increasing electron energy, and becomes zero for the fast electron which is mainly produced by the rescattering process. Furthermore, a Freeman resonance delay of 140±40 attoseconds between photoelectrons emitted via the 4f and 5p Rydberg states of argon is observed.