Extremely short pulses via resonantly induced transparency

We study a novel method to produce extremely short pulses of radiation in a resonant medium via induced transparency by means of adiabatic periodic modulation of atomic transition frequencies by far-off-resonant laser field, which causes linear Stark splitting of atomic energy levels resulting in partial transparency of an optically deep medium and drastic spectral modification of an incident resonant radiation. We find the regimes where the output spectrum corresponds to extremely short pulses and discuss several possible experimental realizations of generation of attosecond pulses in Li²⁺ ions and femtosecond pulses in atomic hydrogen with commercially available facilities.     

Standard-based method for proton-electron double resonance imaging of oxygen

Proton-electron double resonance imaging (PEDRI) has been utilized for indirect determination of oxygen concentrations in aqueous samples and living systems. Due to the complexity of the problem, there are seven oxygen related parameters that need to be measured to determine the distribution of oxygen. We present an improved approach in which image intensities from only two PEDRI acquisitions with different EPR irradiation powers are required to determine the distribution of a paramagnetic probe and oxygen in an analyzed sample. This is achieved using three reference samples with known concentrations of a paramagnetic probe and oxygen placed inside the resonator together with the measurement sample. An EPR-off image, which has low signal intensity at low magnetic field (0.02 T) is not required for the calculations, significantly reducing the total time of the experiments and the noise while enhancing the accuracy of these oxygen measurements. The Finland trityl radical was used as the paramagnetic probe and oxygen concentrations could be accurately measured and imaged over the physiological range from 0 to 240 μM.     

Optical modeling of vesicular volcanic ash particles

We present a realistic shape model for nonspherical, vesicular particles and use the model to derive single-scattering properties of volcanic fine-ash particles. Light-scattering computations with discrete-dipole approximation reveal that, qualitatively, scattering by the model particles resembles that of the measured, real volcanic ash particles. Comparison of compact and vesicular ash shows that porosity promotes positive degree of linear polarization and decreases the depolarization ratio for both large and small vesicles. Yet, the single-scattering properties of ash particles with large vesicles are found to be surprisingly similar to those of compact ash particles. A comparison with Mie computations of equal-volume spheres indicates that for small size parameters, the spherical shape underestimates the asymmetry parameter of volcanic ash particles; whereas, for larger size parameters, it is overestimated.     

High sensitivity ICLAS of H2O in the 12,580-13,550 cm transparency window

High-sensitivity Intracavity Laser Absorption Spectroscopy (ICLAS) is used to measure the high resolution absorption spectrum of H₂¹⁸O between 12,580 and 13,550 cm⁻¹. This spectral region covers the 3v+δ polyad of very weak absorption. Four isotopologues of water (H₂¹⁸O, H₂¹⁶O, H₂¹⁷O, HD¹⁸O) are found to contribute to the observed spectrum. Spectrum analysis is performed with the aid of variational calculations and allowed for assigning 1126 lines belonging to H₂¹⁸O, while only 160 H₂¹⁸O lines are included in the HITRAN-2008 database. Altogether, 823 accurate energy levels of H₂¹⁸O are determined from transitions attributed to 26 upper vibrational states, 438 of them being reported for the first time. New information includes energy levels of four newly observed vibrational states of H₂¹⁸O: (2 4 0), (1 4 1), (0 4 2) and (2 3 1) at 13,167.718, 13,212.678, 13,403.71 and 15,073.975 cm⁻¹, respectively. H₂¹⁸O transitions involving highly excited bending states like (1 6 0), (0 6 1), (0 7 1), (1 7 0), (0 9 0) and even (0 10 0) have been identified as a result of an intensity borrowing from stronger bands via high-order resonance interactions. Thirty-six new energy levels of H₂¹⁷O, present with a 2% relative concentration in our sample, could be determined. The rotational structure of the (0 2 3) state of HD¹⁸O at 13,245.497 cm⁻¹ is also reported for the first time.     

Controlled passage through resonance in mechanical systems

Methods of passing through resonance zones in mechanical systems are discussed and a new method based on the speed-gradient energy control of two subsystems (rotor and support) is presented. Two typical problems of passing through resonance for one- and two-dimensional motion of the support are posed and analyzed by computer simulation. The control algorithms based on the speed-gradient method and averaging allow one to significantly reduce the required level of the controlling torque. The proposed algorithms have a small number of design parameters. Compared with the known algorithms the proposed ones are more simple for design and exhibit stronger robustness properties.     

Pair-structure matrix of random collection of particles: Implications for light scattering

Scattering of light fields from random collections of particles of different types is considered. A new matrix called pair-structure matrix characterizing the correlations between particles of the same and of different types is introduced. This matrix reduces to the pair-structure factor of scattering collection [Opt. Lett. 34, 1762 (2009)] in the case when the particles in the collection are all of the same type. For illustration of the importance of the new matrix the formula for the degree of coherence of a polychromatic plane wave scattered from a collection of particles of different types is derived.     

How geometry controls the tearing of adhesive thin films on curved surfaces

Flaps can be detached from a thin film glued on a solid substrate by tearing and peeling. For flat substrates, it has been shown that these flaps spontaneously narrow and collapse in pointy triangular shapes. Here we show that various shapes, triangular, elliptic, acuminate, or spatulate, can be observed for the tears by adjusting the curvature of the substrate. From combined experiments and theoretical models, we show that the flap morphology is governed by simple geometric rules.     

Optically controllable photonic structures with zero absorption

We show the possibility to periodically modulate the refractive index in a homogeneous resonant atomic medium in space or/and time while simultaneously keeping vanishing absorption or gain. Such modulation is based on periodic resonant enhancement of the refractive index, controlled by an external optical field, and opens the way to produce coherently controllable photonic structures. We suggest the possible implementation of the proposed scheme in rare-earth doped crystals with excited state absorption.     

Anomalous effects of hydrostatic pressure on ice surface self-diffusion

This paper presents an experimental study of the formation time of the groove that occurs at the intersection of a grain boundary and the free surface of a pure ice sample immersed in silicone oil at a pressure of approximately 10 MPa and temperature higher than ? 25 °C. Using the theoretical approach presented by Mullins (1957) [14], the surface diffusion coefficient is obtained. It is shown that, at 10 MPa, the ice surface diffusion coefficient is more than twice that observed at atmospheric pressure.     

Arrest of self-focusing collapse in femtosecond air filaments: higher order Kerr or plasma defocusing?

Experimentally measured conical emission rings on the blue side of the filament supercontinuum of a 800 nm 50 fs pulse in air are reproduced in simulations with plasma and the third-order Kerr as the nonlinear terms. This agreement indicates plasma as the dominant mechanism arresting the self-focusing collapse. The higher order Kerr terms with the recently measured coefficients stop the collapse at a lower intensity than the plasma does and lead to the spherical angle-wavelength spectrum without blueshifted rings.