arXiv subjects

Alexander A. Milner

Publications and source records attributed to Alexander A. Milner.

16 recordsLinked to original sources

arXiv2026

Coherent spectroscopy of collective excitations in superfluid helium far from equilibrium

Ultrafast dynamics of collective excitations in superfluids remains largely unexplored beyond the roton region of the Landau excitation spectrum, despite the importance of such dynamics for understanding nonequilibrium processes in these systems. Here, we employ ultrafast coherent control with sequences of femtosecond pulses to perform spectroscopy of multiple quasiparticles in superfluid helium far from equilibrium. By measuring the time-resolved optical birefringence, we track the nonequilibrium dynamics of quasiparticle pairs associated with rotons, maxons and the Pitaevskii plateau region. The spectral lineshape of the roton peak is explained by an ab initio theoretical analysis of the roton-roton interaction. We also reveal strong energy shifts and short lifetimes of both maxon and Pitaevskii-plateau pairs, as well as an influence of the quasiparticle effective mass on the phase of their coherent response to laser pulses. These results demonstrate the ability to extract previously inaccessible information about collective excitations in a strongly interacting quantum fluid by probing its nonequilibrium dynamics on picosecond and sub-picosecond timescales.

cond-mat.other↗

arXiv2025

Control of molecular rotation in helium nanodroplets with an optical centrifuge

We experimentally demonstrate that the rotation of molecules embedded in helium nanodroplets can be controlled with an optical centrifuge, allowing for the study of molecular dynamics inside the strongly interacting many-body environment of superfluid helium at variable levels of rotational excitation. By doping the droplets with dimers of nitric oxide, (NO)$_2$, and measuring the degree of their centrifuge-induced alignment as a function of time, we show both the forced in-field rotation of molecules in a continuous range of frequencies, as well as the field-free resonant rotation with a long nanosecond-scale decay. The ability to control and monitor the rotational dynamics of molecular rotors inside the superfluid medium may shed new light on superfluidity and the interaction of superfluids with defects at the atomic level.

quant-ph↗

arXiv2023

Coherent control of molecular rotation in superfluid helium

We experimentally demonstrate control of molecular rotation in bulk superfluid $^4\mathrm{He}$. Metastable helium dimers, $\mathrm{He}_2^*$, are rotationally excited by a periodic train of linearly polarized femtosecond laser pulses. We show that the degree of rotational excitation of $\mathrm{He}_2^*$ can be enhanced or suppressed by varying the period of the pulse train, whereas the directionality of molecular rotation can be controlled by the relative angle between the polarization vectors of pulses in the train. The experimental results are in agreement with numerical calculations, based on a simple model, in which $\mathrm{He}_2^*$ molecules do not interact with the superfluid.

quant-ph↗

arXiv2023

Controlled excitation of rotons in superfluid helium with an optical centrifuge

We experimentally demonstrate a controlled transfer of angular momentum to roton pairs in superfluid helium. The control is executed with an optical centrifuge and detected with coherent time- and frequency-resolved Raman scattering. We show that the sign of the Raman shift, and hence the orientation of the angular momentum transferred from the laser field to the rotons, is dictated by the centrifuge. The magnitude of the shift reflects the two-roton energy and indicates that the centrifuge-induced hot roton pairs are far from the equilibrium with the colder quantum bath. The observed decay of the coherent Raman signal suggests that the decoherence is governed by the scattering on thermal rotons and phonons. The demonstrated method offers ways of examining microscopic origins of superfluidity by controlling collective excitations in superfluids.

cond-mat.quant-gas↗

arXiv2023

Dynamics of molecular rotors in bulk superfluid helium

Molecules immersed in liquid helium are excellent probes of superfluidity. Their electronic, vibrational and rotational dynamics provide valuable clues about the superfluid at the nanoscale. Here we report on the experimental study of the laser-induced rotation of helium dimers inside the superfluid $^4\mathrm{He}$ bath at variable temperature. The coherent rotational dynamics of $\mathrm{He}_2^*$ is initiated in a controlled way by ultrashort laser pulses, and tracked by means of time-resolved laser-induced fluorescence. We detect the decay of rotational coherence on the nanosecond timescale and investigate the effects of temperature on the decoherence rate. The observed temperature dependence suggests a non-equilibrium evolution of the quantum bath, accompanied by the emission of the wave of second sound. The method offers new ways of studying superfluidity with molecular nano-probes under variable thermodynamic conditions.

quant-ph↗

arXiv2021

Rotation of polarization of light propagating through a gas of molecular super-rotors

We present a detailed theoretical and experimental study of the rotation of the plane of polarization of light traveling through a gas of fast-spinning molecules. This effect is similar to the polarization drag phenomenon predicted by Fermi a century ago and it is a mechanical analog of the Faraday effect. In our experiments, molecules were spun up by an optical centrifuge and brought to the super-rotor state that retains its rotation for a relatively long time. Polarizability properties of fast-rotating molecules were analyzed considering the rotational Doppler effect and Coriolis forces. We used molecular dynamics simulations to account for intermolecular collisions. We found, both experimentally and theoretically, a nontrivial nonmonotonic time dependence of the polarization rotation angle. This time dependence reflects transfer of the angular momentum from rotating molecules to the macroscopic gas flow, which may lead to the birth of gas vortices. Moreover, we show that the long-term behavior of the polarization rotation is sensitive to the details of the intermolecular potential. Thus, the polarization drag effect appears as a novel diagnostic tool for the characterization of intermolecular interaction potentials and studies of collisional processes in gases.

physics.optics↗

arXiv2021

Observation of mechanical Faraday effect in gas media

We report the experimental observation of the rotation of the polarization plane of light propagating in a gas of fast-spinning molecules (molecular super-rotors). In the observed effect, related to Fermi's prediction of "polarization drag" by a rotating medium, the vector of linear polarization tilts in the direction of molecular rotation due to the rotation-induced difference in the refractive indices for the left and right circularly polarized components. We use an optical centrifuge to bring the molecules in a gas sample to ultrafast unidirectional rotation and measure the polarization drag angles of the order of 0.2 milliradians in a number of gases under ambient conditions. We demonstrate an all-optical control of the drag magnitude and direction, and investigate the robustness of the mechanical Faraday effect with respect to molecular collisions.

physics.atom-ph↗

arXiv2021

Controlling the degree of rotational directionality in laser-induced molecular dynamics

We demonstrate experimentally a method of varying the degree of directionality in laser-induced molecular rotation. To control the ratio between the number of clockwise and counter-clockwise rotating molecules (with respect to a fixed laboratory axis), we change the polarization ellipticity of the laser field of an optical centrifuge. The experimental data, supported by the numerical simulations, show that the degree of rotational directionality can be varied in a continuous fashion between unidirectional and bidirectional rotation. The control can be executed with no significant loss in the total number of rotating molecules. The technique could be used for studying the effects of orientation of the molecular angular momentum on molecular collisions and chemical reactions. It could also be utilized for controlling magnetic and optical properties of gases, as well as for the enantioselective detection of chiral molecules.

physics.chem-ph↗

arXiv2020

Laser control of molecular rotation: Expanding the utility of an optical centrifuge

Since its invention in 1999, optical centrifuge has become a powerful tool for controlling molecular rotation and studying molecular dynamics and molecular properties at extreme levels of rotational excitation. The technique has been applied to a variety of molecular species, from simple linear molecules to symmetric and asymmetric tops, to molecular ions and chiral enantiomers. Properties of isolated ultrafast rotating molecules, so-called molecular superrotors, have been investigated, as well as their collisions with one another and interaction with external fields. The ability of an optical centrifuge to spin a particular molecule of interest depends on both the molecular structure and the parameters of the centrifuge laser pulse. An interplay between these two factors dictates the utility of an optical centrifuge in any specific application. Here, we discuss the strategy of assessing and adjusting the properties of the centrifuge to those of the molecular rotors, and describe two practical examples of optical centrifuges with very different characteristics, implemented experimentally in our laboratory.

physics.ins-det↗

arXiv2019

Photosensitivity of La2CuO4 in the vicinity of the phase boundary antiferromagnet-spin glass

The spectral, temperature and magnetic field dependencies of the linear dichroism in the reflection of light from the surface of an undoped single crystal of La2CuO4 are studied. The changes of the dichroism absolute value and its response to the external magnetic field induced by a weak optical illumination were detected. The set of experimental results is analyzed from the point of view of the high sensitivity of optical and magnetic properties of cuprates to the density of charge carriers localized in CuO2 planes. It is assumed that the poor reproducibility of some of the observed effects is related to the proximity (on the x-T diagram of magnetic states) to the antiferromagnet - spin glass phase boundary.

cond-mat.supr-con↗

arXiv2019

Observation of persistent orientation of chiral molecules by laser field with twisted polarization

Molecular chirality is an omnipresent phenomenon of fundamental significance in physics, chemistry and biology. For this reason, search for novel techniques for enantioselective control, detection and separation of chiral molecules is of particular importance. It has been recently predicted that laser fields with twisted polarization may induce persistent enantioselective field-free orientation of chiral molecules. Here we report the first experimental observation of this phenomenon using propylene oxide molecules ($\mathrm{CH_{3}CHCH_{2}O}$, or PPO) spun by an optical centrifuge - a laser pulse, whose linear polarization undergoes an accelerated rotation around its propagation direction. We show that PPO molecules remain oriented on a time scale exceeding the duration of the centrifuge pulse by several orders of magnitude. The demonstrated long-time field-free enantioselective orientation opens new avenues for optical manipulation, discrimination, and, potentially, separation of molecular enantiomers.

physics.chem-ph↗

arXiv2019

Controlled enantioselective orientation of chiral molecules with an optical centrifuge

We report on the first experimental demonstration of enantioselective rotational control of chiral molecules with a laser field. In our experiments, two enantiomers of propylene oxide are brought to accelerated unidirectional rotation by means of an optical centrifuge. Using Coulomb explosion imaging, we show that the centrifuged molecules acquire preferential orientation perpendicular to the plane of rotation, and that the direction of this orientation depends on the relative handedness of the enantiomer and the rotating centrifuge field. The observed effect is in agreement with theoretical predictions and is reproduced in numerical simulations of the centrifuge excitation followed by Coulomb explosion of the centrifuged molecules. The demonstrated technique opens new avenues in optical enantioselective control of chiral molecules with a plethora of potential applications in differentiation, separation and purification of chiral mixtures.

physics.chem-ph↗

arXiv2014

Coherent spin-rotational dynamics of oxygen super rotors

We use state- and time-resolved coherent Raman spectroscopy to study the rotational dynamics of oxygen molecules in ultra-high rotational states. While it is possible to reach rotational quantum numbers up to $N \approx 50$ by increasing the gas temperature to 1500 K, low population levels and gas densities result in correspondingly weak optical response. By spinning O$_2$ molecules with an optical centrifuge, we efficiently excite extreme rotational states with $N\leqslant 109$ in high-density room temperature ensembles. Fast molecular rotation results in the enhanced robustness of the created rotational wave packets against collisions, enabling us to observe the effects of weak spin-rotation coupling in the coherent rotational dynamics of oxygen. The decay rate of spin-rotation coherence due to collisions is measured as a function of the molecular angular momentum and explained in terms of the general scaling law. We find that at high values of $N$, the rotational decoherence of oxygen is much faster than that of the previously studied non-magnetic nitrogen molecules. This may suggest a different mechanism of rotational relaxation in paramagnetic gases.

physics.chem-ph↗

arXiv2014

Effects of ultrafast molecular rotation on collisional decoherence

Using an optical centrifuge to control molecular rotation in an extremely broad range of angular momenta, we study coherent rotational dynamics of nitrogen molecules in the presence of collisions. We cover the range of rotational quantum numbers between J=8 and J=66 at room temperature and study a cross-over between the adiabatic and non-adiabatic regimes of rotational relaxation, which cannot be easily accessed by thermal means. We demonstrate that the rate of rotational decoherence changes by more than an order of magnitude in this range of J values, and show that its dependence on J can be described by a simplified scaling law.

physics.chem-ph↗

arXiv2013

Rotational spectroscopy with an optical centrifuge

We demonstrate a new spectroscopic method for studying electronic transitions in molecules with extremely broad range of angular momentum. We employ an optical centrifuge to create narrow rotational wave packets in the ground electronic state of $^{16}$O$_2$. Using the technique of resonance-enhanced multi-photon ionization, we record the spectrum of multiple ro-vibrational transitions between $X^3Σ_{g}^{-}$ and $C^3Π_{g}$ electronic manifolds of oxygen. Direct control of rotational excitation, extending to rotational quantum numbers as high as $N\gtrsim 120$, enables us to interpret the complex structure of rotational spectra of $C^3Π_{g}$ beyond thermally accessible levels.

physics.chem-ph↗

arXiv2013

Complete control, direct observation and study of molecular super rotors

Extremely fast rotating molecules carrying significantly more energy in their rotation than in any other degree of freedom are known as "super rotors". It has been speculated that super rotors may exhibit a number of unique properties. Theoretical studies showed that ultrafast molecular rotation may change the character of molecular scattering from solid surfaces, alter molecular trajectories in external fields, make super rotors stable against collisions, and lead to the formation of gas vortices. New ways of molecular cooling and selective chemical bond breaking by ultrafast spinning have been suggested. Bringing a large number of molecules to fast, directional and synchronous rotation is rather challenging. An efficient method of accelerating molecular rotation with an "optical centrifuge" has been proposed and successfully implemented, yet only indirect evidence of super rotors has been reported to date. Here we demonstrate the first direct observation of molecular super rotors and study their spectroscopic, dynamical and magneto-optical properties. Using the centrifuge technique, we control the degree of rotational excitation and detect molecular rotation with high spectral and temporal resolution. Frequency-resolved detection enables us to map out the energy of extreme rotation levels, two orders of magnitude above the room temperature limit, and quantify the onset of the centrifugal distortion. Femtosecond time resolution reveals highly coherent rotational dynamics with lower de-coherence rates at higher values of the molecular angular momentum, and the increase of the molecular moment of inertia due to the rotation-induced chemical bond stretching. In the presence of an external magnetic field, ultrafast molecular rotation is found to result in an optical birefringence of the molecular ensemble.

physics.optics↗