Self-focusing Process Research Articles

Evolution of a Gaussian laser beam in warm collisional magnetoplasma

In this paper, the spatial evolution of an intense circularly polarized Gaussian laser beam propagated through a warm plasma is investigated, taking into account the ponderomotive force, Ohmic heating, external magnetic field, and collisional effects. Using the momentum transfer and energy equations, both modified electron temperature and electron density in plasma are obtained. By introducing the complex dielectric permittivity of warm magnetized plasma and using the complex eikonal function, coupled differential equations for beam width parameter are established and solved numerically. The effects of polarization state of laser and magnetic field on the laser spot size evolution are studied. It is observed that in case of the right-handed polarization, an increase in the value of external magnetic field causes an increase in the strength of the self-focusing, especially in the higher values, and consequently, the self-focusing occurs in shorter distance of propagation. Moreover, the results demonstrate the existence of laser intensity and electron temperature ranges where self-focusing can occur, while the beam diverges outside of these regions; meanwhile, in these intervals, there exists a turning point for each of intensity and temperature in which the self-focusing process has its strongest strength. Finally, it is found that the self-focusing effect can be enhanced by increasing the plasma frequency (plasma density).

A Systematic Review of Efficacy of the Attention Training Technique in Clinical and Nonclinical Samples.

The Attention Training Technique (ATT; Wells, 1990) is a brief metacognitive treatment strategy aimed at remediating self-focused processing and increasing attention flexibility in psychological disorder. We systematically reviewed and examined the efficacy of ATT in clinical and nonclinical samples. Scientific databases were searched from 1990 to 2014 and 10 studies (total N = 295) met inclusion criteria. Single-case data were meta-analyzed using the improvement rate difference, and standardized between and within-group effect sizes (ESs) were examined across 4 analogue randomized controlled trials (RCTs). Single-case outcomes indicated that ATT yields large ES estimates (pooled ES range: 0.74-1.00) for anxiety and depressive disorders. Standardized ESs across the RCTs indicated that ATT yields greater treatment gains than reference groups across majority outcomes (adjusted Cohen's d range: 0.40-1.23). These preliminary results suggest ATT may be effective in treating anxiety and depressive disorders and help remediate some symptoms of schizophrenia. Although a limited number of studies with small sample sizes warrants caution of interpretation, ATT appears promising and future studies will benefit from adequately powered RCTs.

Temporal Airy pulses for controlled high aspect ratio nanomachining of dielectrics

Understanding the interplay between optical pulse parameters and ultrafast material response is critical in achieving efficient and controlled laser nanomachining. In general, the key to initiate material processing is the deposition of a sufficient energy density within the electronic system. In dielectrics this critical energy density corresponds typically to a plasma frequency in the near-IR spectral region. Creating this density instantaneously with ultrashort laser pulses of a few tens of femtoseconds pulse duration in the same spectral region, the penetration depth into the material will strongly decrease with increasing electron density. Consequently, staying below this critical density will allow deep penetration depths. This calls for delayed ionization processes to deposit the energy for processing, thus introducing the temporal structure of the laser pulses as a control parameter. In this contribution we demonstrate this concept experimentally and substantiate the physical picture with numerical calculations. Bandwidth-limited pulses of 30 fs pulse duration are stretched up to 1.5 ps either temporally symmetrically or temporally asymmetrically. The interplay between pulse structure and material response is optimally exploited by the asymmetrically structured temporal Airy pulses leading to the inherently efficient creation of high aspect ratio nanochannels. Depths in the range of several micrometers and diameters around 250 nm are created within a single laser shot and without making use of self-focusing and filamentation processes. In addition to the machining of nanophotonic devices in dielectrics, the technique has the potential to enhance laser-based nanocell surgery and cell poration techniques.

Mitigating the relativistic laser beam filamentation via an elliptical beam profile.

It is shown that the filamentation instability of relativistically intense laser pulses in plasmas can be mitigated in the case where the laser beam has an elliptically distributed beam profile. A high-power elliptical Gaussian laser beam would break up into a regular filamentation pattern-in contrast to the randomly distributed filaments of a circularly distributed laser beam-and much more laser power would be concentrated in the central region. A highly elliptically distributed laser beam experiences anisotropic self-focusing and diffraction processes in the plasma channel ensuring that the unstable diffractive rings of the circular case cannot be produced. The azimuthal modulational instability is thereby suppressed. These findings are verified by three-dimensional particle-in-cell simulations.

Self-regulated propagation of intense infrared pulses in elongated soft-x-ray plasma amplifiers

Increasing the electron density of collisionally pumped plasma-based soft-x-ray lasers offers promising opportunities to deliver ultrashort pulses. However, strong nonlinear effects, such as overionization-induced refraction and self-focusing, hinder the propagation of the laser beam and thus the generation of elongated volume of lasing ions to be pumped. Using a particle-in-cell code and a ray-tracing model we demonstrate that optically preformed waveguides allow for addressing those issues through a self-regulation regime between self-focusing and overionization processes. As a result, guiding intense pulses over several millimeters leads to the implementation of saturated plasma amplifiers.

Dedifferentiation of emotion regulation strategies in the aging brain.

Different emotion regulation strategies are distinctly represented in the brains of younger adults. Decreasing a reaction to a negative situation by reinterpreting it (reappraisal) relies on cognitive control regions in the prefrontal cortex, while distracting away from a stressor involves more posterior medial structures. In this study, we used Multi-Voxel pattern analyses (MVPA) to examine whether reappraisal and distraction strategies have distinct representations in the older adult brain, or whether emotion regulation strategies become more dedifferentiated in later life. MVPA better differentiated the two emotion regulation strategies for younger adults than for older adults, and revealed the greatest age-related differences in differentiation in the posterior medial cortex (PMC). Univariate analyses revealed equal PMC recruitment across strategies for older adults, but greater activity during distraction than reappraisal for younger adults. The PMC is central to self-focused processing, and thus our findings are consistent with the possibility that focusing on the self may be a default mechanism across emotion regulation strategies for older people.

Optimization of the regime of shortening of relativistically strong laser pulses in the process of excitation of a plasma wake wave

A detailed theoretical investigation is performed for the self-compression of relativistically strong laser pulses under plasma wake wave excitation conditions. An analytical expression is obtained for the characteristic length of laser pulse self-compression in the one-dimensional case. A good agreement between the theoretical and numerical calculation of the dependence of the wave-packet compression length on both amplitude and plasma concentration is demonstrated. The influence of the wave beam self-focusing radiation process on the rate of shortening of the laser pulse duration is considered.

Antecedents of maternal sensitivity during distressing tasks: integrating attachment, social information processing, and psychobiological perspectives.

Predictors of maternal sensitivity to infant distress were examined among 259 primiparous mothers. The Adult Attachment Interview, self-reports of personality and emotional functioning, and measures of physiological, emotional, and cognitive responses to videotapes of crying infants were administered prenatally. Maternal sensitivity was observed during three distress-eliciting tasks when infants were 6 months old. Coherence of mind was directly associated with higher maternal sensitivity to distress. Mothers' heightened emotional risk was indirectly associated with lower sensitivity via mothers' self-focused and negative processing of infant cry cues. Likewise, high physiological arousal accompanied by poor physiological regulation in response to infant crying was indirectly associated with lower maternal sensitivity to distress through mothers' self-focused and negative processing of infant cry cues.

Laser beam induced nanoscale spot through nonlinear “thick” samples: A multi-layer thin lens self-focusing model

Self-focusing is a well-researched phenomenon. Nanoscale spots can be achieved through self-focusing, which is an alternative method for achieving high-density data storage, high-resolution light imaging, and maskless nanolithography. Several research groups have observed that self-focusing spots can be reduced to nanoscale levels via incident laser power manipulation. Self-focusing spots can be analyzed by solving the nonlinear Schrödinger equation and the finite difference time domain method. However, both procedures are complex and time-consuming. In the present work, a multi-layer thin-lens self-focusing model that considers diffraction effects and changes of refractive index along the radial and film thickness directions is proposed to analyze the self-focusing behavior and traveling process of light beams intuitively. The self-focusing behaviors of As2S3 are simulated, and results show that a nanoscale self-focusing spot with a radius of about 0.12 μm can be formed at the bottom of nonlinear sample when the incident laser power exceeds 4.25 mW. Our findings are basically consistent with experimental reports and provide a good method for analyzing and understanding the self-focusing process. An appropriate application schematic design is also provided.

Self-compression of terawatt level picosecond 10 μm laser pulses in NaCl

We propose and numerically validate a self-compression regime of powerful 10 μm laser radiation (power 500 GW, energy 1.5 J, pulse duration 2.5 ps) in a NaCl crystal, taking into account its nonlinear and dispersive properties. We demonstrate numerically that the proposed scheme—a solid state compressor containing four NaCl plates (total thickness 19 cm) and divided by spatial filters—avoids the small-scale self-focusing process and reduces by an order of magnitude the initial pulse duration, from 2.5 ps up to 250 fs (seven periods of oscillation) and increases output power from 0.5 TW up to 2.2 TW.

Influence of group-velocity-dispersion effects on the propagation of femtosecond laser pulses in air at different pressures

The influence of group-velocity-dispersion (GVD) effects on the propagation of femtosecond laser pulses in air at different pressures is investigated by numerically solving the extended nonlinear Schr\"odinger equation. It is observed that GVD has a great influence on the collapse distance ${L}_{\mathrm{c}}$ of the self-focusing short laser pulses when the pressure is relatively high (e.g., 10 atm), in which case the semiempirical formula describing the collapse distance of self-focusing laser pulse [Dawes and Marburger, Phys. Rev. 179, 862 (1969)] is no longer applicable, while GVD has little influence on the self-focusing process of longer laser pulses. Through the results of numerical simulations as well as analytical analysis, we find that the initial duration of the laser pulse is the main factor that determines the GVD effect on the propagation and the increase of the pressure and pulse compression during the propagation enhance this effect. The spectral data show that GVD also influences the spectrum broadening in high-pressure cases.

Gaussian beam evolution in nonlinear inhomogeneous plasma

AbstractThe method of nonlinear complex geometrical optics (NCGO) is proposed in this paper for description of the evolution of a spatially narrow Gaussian beam (GB) in an inhomogeneous nonlinear plasma. NCGO method deals with first-order ordinary differential equations for the complex curvature of the wave front and for GB amplitude and for second-order ordinary differential equation for GB width. Thus, NCGO simplifies the description of GB diffraction and self-focusing effects as compared to the known methods of plasma physics and this way it can be assumed to be attractive and comprehensive approach in problems of plasma heating by electromagnetic waves. Moreover, we demonstrate in this paper some regularity for nonlinear inhomogeneous plasma in the framework of which central ray of a GB is not subjected to nonlinear refraction within NCGO method boundary applicability. On the contrary, the beam width, wave front curvature, and GB amplitude are modified by diffraction and self-focusing processes. General properties of the beam propagation are illustrated with results of numerical modeling for two particular cases: GB diffraction and self-focusing along curvilinear trajectory with torsion in axially symmetric plasma column and GB reflection from nonlinear inhomogeneous plasma layer. We prove in this paper that NCGO is new effective method of plasma physics, which can be applied for improvement of ray tracing techniques and plasma diagnostics.

Self-focus and transmission of relativistic electron beam in a dynamically loaded plasma

In order to further study the radiation of the relativistic electron beam-ion channel experimentally and theoretically, the propagation of a relativistic electron beam in neutral gas and its self-focusing process are investigated. Particle in cell (PIC) simulation shows that the electron beam can self-focus and transmit the dynamically loaded plasma through impact ionization. The transverse and the longitude inhomogeneities of the ion background have significant effects on the transport properties of the electron beam. Base on these researches, a model of transmission of electron beam in a transverse non-uniform ion background is supposed. And the condition of self-focus is given. The numerical results show that the transverse inhomogeneity will lead to the mixed phase transmission of the electron beam, and the inner electrons can defocus near the focus point, which is consistent with the PIC simulation. The PIC simulation also shows that due to the self-focusing of the electron beam, there are much more ions to be ionized at the focus point, which will capture the lower-energy electrons after collision, the capture electron effect will significantly reduce the efficiency of the transmission of the electron beam. But the distribution of the captured electrons in the longitude direction is quasi-periodic, which acts as the electrostatic Wiggler field. These may achieve the dynamical loading of the electrostatic Wiggler field. These results give new clues to the further study of electron beam-plasma system in experiment and the establishment of theoretical models.

Shape control of Pd-based nanocrystals via quasi-solid-state reactions

We demonstrated here a quasi-solid-state method for the shape-controlled growth of Pd-based nanocrystals. Tiny amount of solvent is needed to form clusters which can aggregate and then evolve into nanocrystals with different morphologies. It is proposed that the shape-controlled growth processes proceed primarily by the aggregation of the clusters and the subsequent interparticle growth via solid-state self-focusing process.

Broadband colored-crescent generation in a singleβ-barium-borate crystal by intense femtosecond pulses

A visible colored crescent with a bandwidth broader than 220 nm is observed experimentally by loosely focused femtosecond pulses in a bulk quadratic nonlinear crystal (\ensuremath{\beta}-BBO crystal) at certain incident angles. Through the analysis based on a simple collinear phase-matching model, we suggest that the colored crescent might be the coexistence of spontaneous parametric down-conversions (SPDCs) in the infrared range and the corresponding efficient second-order harmonic generations (SHGs) that occur in a wide spectrum. We further provide a possible mechanism for the SHG process in which the phase-mismatching angles of the frequency doubling of SPDCs in \ensuremath{\beta}-BBO crystal are assumed to be compensated by the strong diffraction effect during the self-focusing process of the generated intense SPDC signals.

Trauma History and Linguistic Self-Focus Moderate the Course of Psychological Adjustment to Divorce

Divorce is considered to be a significant life stressor, and yet little is known about how early life experiences predict adjustment to marital dissolution in adulthood. Using a sample of 99 recently-separated adults, we examine whether adults' self-reported history of traumatic experiences is associated with increased psychological distress and self-blame following divorce as well as linguistic indices of self-focus while discussing the divorce experience (more first person pronouns). We also evaluate whether among individuals with greater trauma histories, self-focused processing of the divorce experience prospectively predicts increase in psychological distress over 7.5 months. Participants reported on their trauma history and provided four minute speech samples regarding the divorce. At intake and 7.5 months later they reported on the psychological impact of the divorce. Results indicated that adults with greater trauma histories report more psychological distress at Time 1, more self-blame regarding the divorce, and more distress in response to discussing the divorce experience. Further, self-blame mediates the association between trauma history and the psychological impact of the divorce. Among adults with greater trauma histories, greater use of first person singular pronouns when discussing the divorce prospectively predicted increases in psychological distress. Findings are discussed in terms of their contribution to the literature on divorce and trauma.

Experimental control of steady state photorefractive self-focusing in InP:Fe at infrared wavelengths

This paper reports an experimental study of the self-focusing process in iron doped indium phosphide at an 1.06 micron wavelength, identifying the influence of temperature, beam intensity and background illumination for two different iron dopings. We point out that the iron ionization ratio is at the origin of different qualitative behavior previously reported and we show that it is possible to reproduce the said behaviors in the same crystal by applying a uniform illumination, allowing their eventual control for dynamic wave-guiding.

The Relationship of Post-Event Processing to Self-Evaluation of Performance in Social Anxiety

Socially anxious and control participants engaged in a social interaction with a confederate and then wrote about themselves or the other person (i.e., self-focused post-event processing [SF-PEP] vs. other-focused post-event processing [OF-PEP]) and completed several questionnaires. One week later, participants completed measures concerning their evaluation of their performance in the social interaction and the degree to which they engaged in post-event processing (PEP) during the week. Socially anxious individuals evaluated their performance in the social interaction more poorly than control participants, both immediately after and 1 week later. Socially anxious individuals assigned to the SF-PEP condition displayed fewer positive feelings about their performance compared to the socially anxious individuals in the OF-PEP condition as well as controls in either condition. Also, the trait tendency to engage in PEP moderated the effect of social anxiety on participants' evaluation of their performance in the interaction, such that high socially anxious individuals with high trait PEP scores evaluated themselves in the interaction more negatively at the later assessment. These results suggest that PEP and other self-evaluative processes may perpetuate the cycle of social anxiety.

Ruminative and mindful self-focused processing modes and their impact on problem solving in dysphoric individuals

A mindful experiential rather than an analytical ruminative mode of information processing is beneficial in depression. However studies have not taken into account people’s trait mindfulness or trait rumination. This study aimed to examine the effects of state and trait analytical rumination and experiential mindfulness on affect and social problem solving (SPS). Participants filled in the trait mindfulness and trait rumination questionnaires. After mood induction, dysphoric participants with and without a history of depression were assessed for SPS and affect before and after induction of mindful (experiential) or ruminative (analytic) processing modes. Results showed trait mindfulness was negatively correlated with the BDI and trait rumination was positively correlated with the BDI. There was no significant difference between groups after the induced experiential mindful or analytical ruminative processing modes. However, participants who scored high on trait rumination showed significant improvement in SPS after induced mindfulness processing. No such effect was found on trait mindfulness. These findings suggest a useful way to prevent high trait ruminators from developing depressive symptoms or depression. Experiential mindful processing may help reduce the risk of developing depressive symptoms by increasing social problem solving ability in those with low mood and high levels of trait rumination.

Sex differences in brain activation to anticipated and experienced pain in the medial prefrontal cortex

Previous studies on sex differences in neural responses to noxious stimuli yielded mixed results. Both increased and decreased brain activation in several brain areas in women as compared to men has been reported. The current event-related functional magnetic resonance imaging study used a parametric design with different levels of the intensity of electrical stimulation in order to investigate sex differences in brain activation during pain processing. Four intensity levels, which were determined individually according to subjective ratings, ranging from stimulation below the stimulus detection threshold to moderately painful stimuli, were applied. Females experienced mild and moderate pain at lower stimulus intensity than males. Pronounced sex differences in brain activation were found in response to stimulation below the detection threshold and for the most intense pain stimuli in the medial prefrontal cortex (MPFC). Under both the conditions, women showed stronger activation in a region of the pregenual MPFC, which has been implicated in introspective, self-focused information processing. The results suggest that women, as compared to men, show increased self-related attention during anticipation of pain and in response to intense pain.