Molecular alignment and orientation by laser fields has attracted significant attention in recent years, mostly due to new capabilities to manipulate the molecular spatial arrangement. Molecules can now be efficiently prepared for ionization, structural imaging, orbital tomography, and more, enabling, for example, shooting of dynamic molecular movies. Furthermore, molecular alignment and orientation processes give rise to fundamental quantum and classical phenomena like quantum revivals, Anderson localization, and rotational echoes, just to mention a few. We review recent progress on the visualization, coherent control, and applications of the rich dynamics of molecular rotational wave packets driven by laser pulses of various intensities, durations, and polarizations. In particular, we focus on the molecular unidirectional rotation and its visualization, the orientation of chiral molecules, and the three-dimensional orientation of asymmetric-top molecules. Rotational echoes are discussed as an example of nontrivial dynamics and detection of prepared molecular states.
The ability to control the amplitude, phase and polarization states of light on subwavelength scales established metasurfaces as miniaturized alternatives to standard, the relatively large optical components. So far, most of these ultrathin elements operate in the linear regime, and do not change the frequency of the light transmitted or reflected from them. Using our better control over the response of the metasurfaces, we demonstrate a special class of metasurfaces that act as frequency-converting optical components [1-3]. Through nonlinear generation, plasmonic meta-atoms are used as the metasurfaces’ building blocks and a 2π phase shift can be imparted on the nonlinear wave. Similar to the linear metasurfaces case, the laws governing nonlinear optics can be generalized to include nonlinear phase gradients. In phase matched interactions for example, the anomalous signal generated in a collinear wave mixing scheme is emitted into another direction [2]. We demonstrate optical elements such as blazed gratings and lenses operating through four-wave mixing and third-harmonic generation. Additionally, we devise a novel type of computer-generated hologram that can reconstruct complex images at the third harmonic frequency of the reading beam [3]. Polarization-multiplexed, three-dimensional and dynamical nonlinear holograms are fabricated in ultrathin elements by multilayer nanolithography, paving the way to a class of devices that can manipulate optical beams in unprecedented ways.
[1] E. Almeida and Y. Prior. Scientific Reports 5, 10033 (2015)
[2] E. Almeida, G. Shalem and Y. Prior, Nature Communications 7, 10367 (2016)
[3] E. Almeida, O. Bitton and Y. Prior, Nature Communications 7, 12533 (2016)
Femtosecond laser ablation occurs on timescales faster than the thermalization of the excited electrons and the lattice in solid materials. The ultrafast deposition of energy competes with the slower electron-phonon energy redistribution, raising the question of what is the optimal pulse duration for efficient deposition of energy while minimizing peripheral damage, and whether the shortest pulse is always the most efficient. We studied femtosecond laser ablation of silicon and several metals, varied the pulse duration while keeping all other parameters equal, and looked for optimal conditions. The main findings in our study are that at low fluences, not too high above the ablation threshold, the shortest pulses are the most efficient, whereas under high fluence conditions, well above the ablation threshold, longer pulses ablate more efficiently. In order to facilitate eventual direct, real time optimization, we developed a diagnostics tool for the monitoring of the ablation efficiency over a wide range of pulse durations. The intensity of the emission at atomic lines (i.e. the 289 nm line in Silicon, calibrated by plasma emission at other wavelengths) provides such information, while optical and AFM microscopy provide reliable information about the quality of ablated structures.
The construction and the operation properties of an organically doped, sol-gel cladded optical fiber pH sensor, are described. The silica-entrapped indicator in the fluorescence-based device was fluorescein, pumped with a continuous wave (cw) argon 488 nm laser. The transmitted signal through the sensing fiber yielded a response in the pH range of 4 - 7, where signal level increased from acid to base. The device is durable and renewable. When tested over more than 8 weeks it retained its response, as demonstrated by dozens of cycles of measurements each lasting a few hours. The probe is easily prepared under regular room conditions by simple decladding of the fiber and sol-gel recoating. System design and setup are attractive due to modularity and discardable low cost probe tips.
An extension of the non-Markovian jump model of laser noise to the case of correlated frequency jumps is considered. The model parameters are the mean jump time the rms jump size and a correlation parameter which quantifies the degree of correlation between two successive jumps. A technique to obtain exact expressions for correlation functions is described. While the basic jump processes are stationary both the phase and frequency fluctuations are not. The field autocorrelation function for the pure phase jump process is stationary for all values of the correlation parameter. For the frequency jump process the average over all initial times of the two time field autocorrelation function is non zero only for fully anticorrelated jumps.
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