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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 ow 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.
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Abstract We report the experimental observation of molecular unidirectional rotation (UDR) echoes, and analyze their origin and behavior both classically and quantum mechanically. The molecules are excited by two time-delayed polarization-twisted ultrashort laser pulses and the echoes are measured by exploding the molecules and reconstructing their spatial orientation from the detected recoil ions momenta. Unlike alignment echoes which are induced by linearly polarized pulses, here the axial symmetry is broken by the twisted polarization, giving rise to molecular unidirectional rotation. We find that the rotation sense of the echo is governed by the twisting sense of the second pulse even when its intensity is much weaker than the intensity of the first pulse. In our theoretical study, we rely on classical phase space analysis and on three-dimensional quantum simulations of the laser-driven molecular dynamics. Both approaches nicely reproduce the experimental results. Echoes in general, and the unique UDR echoes in particular, provide new tools for studies of relaxation processes in dense molecular gases.
Introducing correlations between the spatial and temporal degrees of freedom of a pulsed optical beam (or wave packet) can profoundly alter its propagation in free space. Indeed, appropriate spatio-temporal spectral correlations can render the wave packet propagation-invariant: the spatial and temporal profiles remain unchanged along the propagation axis. The spatio-temporal spectral locus of any such wave packet lies at the intersection of the light-cone with tilted spectral hyperplanes. We investigate (2+1)D space-time propagation-invariant light sheets, and identify 10 classes categorized according to the magnitude and sign of their group velocity and the nature of their spatial spectrum - whether the low spatial frequencies are physically allowed or forbidden according to their compatibility with causal excitation and propagation. We experimentally synthesize and characterize all 10 classes using an experimental strategy capable of synthesizing space-time wave packets that incorporate arbitrary spatio-temporal spectral correlations.
Although diffractive spreading is an unavoidable feature of all wave phenomena, certain waveforms can attain propagation-invariance. A lesser-explored strategy for achieving optical selfsimilar propagation exploits the modification of the spatio-temporal field structure when observed in reference frames moving at relativistic speeds. For such an observer, it is predicted that the associated Lorentz boost can bring to a halt the axial dynamics of a wave packet of arbitrary profile. This phenomenon is particularly striking in the case of a self-accelerating beam -- such as an Airy beam -- whose peak normally undergoes a transverse displacement upon free-propagation. Here we synthesize an acceleration-free Airy wave packet that travels in a straight line by deforming its spatio-temporal spectrum to reproduce the impact of a Lorentz boost. The roles of the axial spatial coordinate and time are swapped, leading to `time-diffraction manifested in self-acceleration observed in the propagating Airy wave-packet frame.
Controlling the group velocity of an optical pulse typically requires traversing a material or structure whose dispersion is judiciously crafted. Alternatively, the group velocity can be modified in free space by spatially structuring the beam profile, but the realizable deviation from the speed of light in vacuum is small. Here we demonstrate precise and versatile control over the group velocity of a propagation-invariant optical wave packet in free space through sculpting its spatio-temporal spectrum. By jointly modulating the spatial and temporal degrees of freedom, arbitrary group velocities are unambiguously observed in free space above or below the speed of light in vacuum, whether in the forward direction propagating away from the source or even traveling backwards towards it.
Metamaterials represent one of the most vibrant fields of modern science and technology. They are generally dispersive structures in the direct and reciprocal space and time domains. Upon this consideration, I overview here a number of metamaterial innovations developed by colleagues and myself in the holistic framework of space and time dispersion engineering. Moreover, I provide some thoughts regarding the future perspectives of the area.
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