arXiv subjects

Valery Milner

Publications and source records attributed to Valery Milner.

31 records · Page 2Linked to original sources

arXiv2014

Observation of nondispersing classical-like molecular rotation

Using the technique of an optical centrifuge, we produce rotational wave packets which evolve in time along either classical-like or non-classical trajectories. After releasing O2 and D2 molecules from the centrifuge, we track their field-free rotation by monitoring the molecular angular distribution with velocity map imaging. Due to the dispersion of the created rotational wave packets in oxygen, we observe a gradual transition between "dumbbell"-shaped and "cross"-shaped distributions, both rotating with a classical rotation frequency. In deuterium, a much narrower rotational wave packet is produced and shown to evolve in a truly classical non-dispersing fashion.

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↗

arXiv2012

Coherent anti-Stokes Raman spectroscopy in the presence of strong resonant signal from background molecules

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 from the dominating molecular species (O$_{2}$). Coherent ro-vibrational dynamics lead to long "silence windows" of zero signal from oxygen molecules. In these silence windows, the detected signal stems solely from the minority species (CO$_{2}$) enabling background-free detection and characterization of the O$_2$/CO$_2$ mixing ratio. In comparison to a Fourier analysis, our technique does not require femtosecond time resolution or time-delay scanning.

physics.optics↗

arXiv2012

Enhancing strong-field induced molecular vibration with femtosecond pulse shaping

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 excitation are explored, and the differences of their potential in increasing the excitation efficiency are discussed.

physics.atom-ph↗

arXiv2012

Coherent Ro-vibrational Revivals in a Thermal Molecular Ensemble

We report an experimental and theoretical study of the evolution of vibrational coherence in a thermal ensemble of nitrogen molecules. Rotational dephasing and rephasing of the vibrational coherence is detected by coherent anti-Stokes Raman scattering. The existence of ro-vibrational coupling and the discrete energy spectrum of the rotational bath lead to a whole new class of full and fractional ro-vibrational revivals. Following the rich ro-vibrational dynamics on a nanosecond time scale with sub-picosecond time resolution enables us to determine the second-order ro-vibrational constant $gamma_e$ and assess new possibilities of controlling decoherence.

quant-ph↗

arXiv2011

Prospects of coherent control in turbid media: Bounds on focusing broadband laser pulses

We study the prospects of controlling transmission of broadband and bi-chromatic laser pulses through turbid samples. The ability to focus transmitted broadband light is limited via both the scattering properties of the medium, and the technical characteristics of the experimental set-up. There are two time scales, given by pulse stretching in the near- and far-field regions, which define the maximum bandwidth of a pulse amenable to focusing. In the geometric optics regime of wave propagation in the medium, a single set-up can be optimal for focusing light at frequencies w and nw simultaneously, providing the basis for the 1 + n coherent quantum control. Beyond the regime of geometric optics, we discuss a simple solution for the shaping, which provides the figure of merit for one's ability to focus simultaneously several transmission modes.

physics.optics↗

arXiv2011

Interference spectroscopy with coherent anti-Stokes Raman scattering of noisy broadband pulses

We propose a new technique for comparing two Raman active samples. The method employs optical interference of the signals generated via coherent anti-Stokes Raman scattering (CARS) of broadband laser pulses with noisy spectra. It does not require spectrally resolved detection, and no prior knowledge about either the Raman spectrum of the samples, or the spectrum of the incident light is needed. We study the proposed method theoretically, and demonstrate it in a proof-of-principle experiment on Toluene and ortho-Xylene samples.

quant-ph↗

arXiv2009

Photon delocalization transition in dimensional crossover in layered media

We report a crossover in optical propagation in random layered media from localization towards diffusion as the interaction of the wave with the sample is transformed from one to three-dimensional due to nonuniformity in the layer thickness. The crossover occurs at the point that the lateral spread of the wave equals the transverse coherence length in the transmitted speckle pattern.

cond-mat.mes-hall↗

arXiv2008

Complete transfer of populations from a single state to a pre-selected superposition of states using Piecewise Adiabatic Passage

We develop a method for executing robust and selective transfer of populations between a single level and pre-selected superpositions of energy eigenstates. Viewed in the frequency domain, our method amounts to executing a series of simultaneous adiabatic passages into each component of the target superposition state. Viewed in {the} time domain, the method works by accumulating the wavefunction of the target wave packet as it revisits the Franck Condon region, in what amounts to an extension of the Piecewise Adiabatic Passage technique [ Shapiro et.al., Phys. Rev. Lett. 99, 033002 (2007)] to the multi-state regime. The viability of the method is verified by performing numerical tests for the Na_2 molecule.

quant-ph↗

arXiv2008

Narrowband spectroscopy by all-optical correlation of broadband pulses

High peak power ultrafast lasers are widely used in nonlinear spectroscopy but often limit its spectral resolution because of the broad frequency bandwidth of ultrashort laser pulses. Improving the resolution by achieving spectrally narrow excitation of, or emission from, the resonant medium by means of multi-photon interferences has been the focus of many recent developments in ultrafast spectroscopy. We demonstrate an alternative approach, in which high resolution is exercised by detecting narrow spectral correlations between broadband excitation and emission optical fields. All-optical correlation analysis, easily incorporated into the traditional spectroscopic setup, enables direct, robust and simultaneous detection of multiple narrow resonances with a single femtosecond pulse.

physics.optics↗