arXiv subjects

D. Mathur

Publications and source records attributed to D. Mathur.

At least 19 recordsLinked to original sources

arXiv2014

The Interface Region Imaging Spectrograph (IRIS)

The Interface Region Imaging Spectrograph (IRIS) small explorer spacecraft provides simultaneous spectra and images of the photosphere, chromosphere, transition region, and corona with 0.33-0.4 arcsec spatial resolution, 2 s temporal resolution and 1 km/s velocity resolution over a field-of-view of up to 175 arcsec x 175 arcsec. IRIS was launched into a Sun-synchronous orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a 19-cm UV telescope that feeds a slit-based dual-bandpass imaging spectrograph. IRIS obtains spectra in passbands from 1332-1358, 1389-1407 and 2783-2834 Angstrom including bright spectral lines formed in the chromosphere (Mg II h 2803 Angstrom and Mg II k 2796 Angstrom) and transition region (C II 1334/1335 Angstrom and Si IV 1394/1403 Angstrom). Slit-jaw images in four different passbands (C II 1330, Si IV 1400, Mg II k 2796 and Mg II wing 2830 Angstrom) can be taken simultaneously with spectral rasters that sample regions up to 130 arcsec x 175 arcsec at a variety of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to emission from plasma at temperatures between 5000 K and 10 MK and will advance our understanding of the flow of mass and energy through an interface region, formed by the chromosphere and transition region, between the photosphere and corona. This highly structured and dynamic region not only acts as the conduit of all mass and energy feeding into the corona and solar wind, it also requires an order of magnitude more energy to heat than the corona and solar wind combined. The IRIS investigation includes a strong numerical modeling component based on advanced radiative-MHD codes to facilitate interpretation of observations of this complex region. Approximately eight Gbytes of data (after compression) are acquired by IRIS each day and made available for unrestricted use within a few days of the observation.

astro-ph.SR↗

arXiv2013

Bond-selective fragmentation of water molecules with intense, ultrafast, carrier envelope phase stabilized laser pulses

Carrier envelope phase (CEP) stabilized pulses of intense 800 nm light of 5 fs duration are used to probe the dissociation dynamics of dications of isotopically-substituted water, HOD. HOD$^{2+}$ dissociates into either H$^+$ + OD$^+$ or D$^+$ + OH$^+$. The branching ratio for these two channels is CEP-dependent; the OD$^+$/OH$^+$ ratio (relative to that measured with CEP-unstabilized pulses) varies from 150% to over 300% at different CEP values, opening prospects of isotope-dependent control over molecular bond breakage. The kinetic energy released as HOD$^{2+}$ Coulomb explodes is also CEP-dependent. Formidable theoretical challenges are identified for proper insights into the overall dynamics which involve non-adiabatic field ionization from HOD to HOD$^+$ and, thence, to HOD$^{2+}$ via electron rescattering.

physics.atom-ph↗

arXiv2013

Carrier envelope phase effects in ultrafast, strong-field ionization dynamics of multielectron systems: Xe and CS$_2$

Carrier envelope phase (CEP) stabilized 5 fs and 22 fs pulses of intense 800 nm light are used to probe the strong-field ionization dynamics of multielectron entities, xenon and carbon disulfide. We compare ion yields obtained with and without CEP-stabilization: with 8-cycle (22 fs) pulses, Xe$^{6+}$ yields are suppressed (relative to Xe$^+$ yields) by between 30% and 50%, depending on phase, reflecting the phase dependence of non-sequential ionization and its contribution to the formation of higher charge states. On the other hand, ion yields for Xe$^{q+}$ ($q$=2-4) with CEP-stablized pulses are enhanced (by up to 50%) compared to those with CEP-unstabilized pulses. Such enhancment is particulary pronounced with 2-cycle (5 fs) pulses and is distinctly phase-dependent. Orbital shape and symmetry are found to have a bearing on the response of CS$_2$ to variations in optical field that are effected as CE phase is controllably altered, keeping the overall intensity constant. Molecular fragmentation is found to depend on field strength (not intensity); the observed relative enhancement of fragmentation when CEP-stabilized 2-cycle pulses are used is found to be at the expense of molecular ionization.

physics.atom-ph↗

arXiv2012

Intense 2-cycle laser pulses induce time-dependent bond-hardening in a polyatomic molecule

A time-dependent bond-hardening process is discovered in a polyatomic molecule (tetramethyl silane, TMS) using few-cycle pulses of intense 800 nm light. In conventional mass spectrometry, symmetrical molecules like TMS do not exhibit a prominent molecular ion (TMS$^+$) as unimolecular dissociation into [Si(CH$_3$)$_3]^+$ proceeds very fast. Under strong field and few-cycle conditions, this dissociation channel is defeated by time-dependent bond-hardening: a field-induced potential well is created in the TMS$^+$ potential energy curve that effectively traps a wavepacket. The time-dependence of this bond hardening process is verified using longer-duration ($\geq$ 100 fs) pulses; the relatively "slower" fall-off of optical field in such pulses allows the initially trapped wavepacket to leak out, thereby rendering TMS$^+$ unstable once again. Our results are significant as they demonstrate (i) optical generation of polyatomic ions that are normally inaccessible and (ii) optical control of dynamics in strong fields, with distinct advantages over weak-field control scenarios that demand a narrow bandwidth appropriate for a specified transition.

physics.atom-ph↗

arXiv2011

Laser-driven accelerated growth of dendritic patterns

We report a scheme for very significantly accelerating growth of dendritic patterns in diverse liquids, making use of only a few hundred microwatts of laser power in the presence of an efficient absorber like carbon nanotubes (CNTs). The CNTs act as a heat source that drives dendritic growth; their anisotropy ensures a rich diversity of branched patterns. We rationalize the unprecedented speed of dendritic growth using a diffusion equation for the temperature field with an additional source term. Close to the heat source, the well-established microscopic solvability theory is seen to break down. Our method opens new vistas for experimental and theoretical studies of pattern formation in liquids.

cond-mat.mtrl-sci↗

arXiv2011

Effect of intense, ultrashort laser pulses on DNA plasmids in their native state: strand breakages induced by {\it in-situ} electrons and radicals

Single strand breaks are induced in DNA plasmids, pBR322 and pUC19, in aqueous media exposed to strong fields generated using ultrashort laser pulses (820 nm wavelength, 45 fs pulse duration, 1 kHz repetition rate) at intensities of 1-12 TW cm$^{-2}$. The strong fields generate, {\it in situ}, electrons and radicals that induce transformation of supercoiled DNA into relaxed DNA, the extent of which is quantified. Introduction of electron and radical scavengers inhibits DNA damage; results indicate that OH radicals are the primary (but not sole) cause of DNA damage.

physics.bio-ph↗

arXiv2010

Ionization and Coulomb explosion of Xenon clusters by intense, few-cycle laser pulses

Intense, ultrashort pulses of 800 nm laser light (12 fs, $\sim$4 optical cycles) of peak intensity 5$\times$10$^{14}$ W cm$^{-2}$ have been used to irradiate gas-phase Xe$_n$ clusters ($n$=500-25,000) so as to induce multiple ionization and subsequent Coulomb explosion. Energy distributions of exploding ions are measured in the few-cycle domain that does not allow sufficient time for the cluster to undergo Coulomb-driven expansion. This results in overall dynamics that appear to be significantly different to those in the many-cycle regime. One manifestation is that the maximum ion energies are measured to be much lower than those obtained when longer pulses of the same intensity are used. Ion yields are cluster-size independent but polarization dependent in that they are significantly larger when the polarization is perpendicular to the detection axis than along it. This unexpected behavior is qualitatively rationalized in terms of a spatially anisotropic shielding effect induced by the electronic charge cloud within the cluster.

physics.atm-clus↗

arXiv2010

Effect of intense, ultrashort laser pulses on DNA plasmids in their native state: strand breakages induced by {\it in-situ} electrons

Single strand breaks are induced in DNA plasmids, pBR322 and pUC19, in aqueous media by intense ultrashort laser pulses (820 nm wavelength, 45 fs pulse duration, 1 kHz repetition rate) at intensities of 1-12 TW cm$^{-2}$. The intense laser radiation generates, {\it in situ}, electrons that induce transformation of supercoiled DNA into relaxed DNA. The extent of electron-mediated relaxation of DNA structure is quantified. Introduction of electron and radical scavengers inhibits DNA damage.

physics.bio-ph↗

arXiv2010

Strong-field ionization and fragmentation of large, gas-phase clusters in the few-cycle domain

Intense 3-cycle pulses (10 fs) of 800 nm laser light are utilized to measure energy distributions of ions emitted following Coulomb explosion of Ar$_n$ clusters ($n$=400-900) upon their irradiation by peak intensitis of 5$\times$10$^{14}$ W cm$^{-2}$. The 3-cycle pulses do not afford the cluster sufficient time to undergo Coulomb-driven expansion, resulting in overall dynamics that appear to be very different to those in the many-pulse regime. The peak ion energies are much lower than those obtained when 100 fs pulses of the same intensity are used; they are almost independent of the size of the cluster (over the range 400-900 atoms). Ion yields are a factor of 20 larger in the direction that is perpendicular to the laser polarization vector than along it. This unexpected anisotropy is qualitatively rationalized using molecular dynamics calculations in terms of shielding by an electronic charge cloud within the cluster that is spatially asymmetric.

physics.atm-clus↗

arXiv2008

Strong fields induce ultrafast rearrangement of H-atoms in H$_2$O

H-atoms in H$_2$O are rearranged by strong optical fields generated by intense, 10 fs laser pulses to form H$_2^+$, against prevailing wisdom that strong fields inevitably lead to multiple molecular ionization and the subsequent Coulomb explosion into fragments. This atomic rearrangement is shown to occur within a single 10 fs pulse. Comparison with results obtained with $\sim$300-attosecond long strong fields generated using fast Si$^{8+}$ ions helps establish thresholds for field strength and time required for such rearrangements. Quantum-chemical calculations reveal that H$_2^+$ originates in the $^1$A state of H$_2$O$^{2+}$ when the O-H bond elongates to 1.15 a.u. and the H-O-H angle becomes 120$^o$. Bond formation on the ultrafast timescale of molecular vibrations (10 fs for H$_2^+$) has hitherto not been reported.

physics.chem-ph↗

arXiv2007

Control of the Onset of Filamentation in Condensed Media

Propagation of intense, ultrashort laser pulses through condensed media like crystals of BaF$_2$ and sapphire results in the formation of filaments. We demonstrate that the onset of filamentation may be controlled by rotating the plane of polarization of incident light. We directly visualize filamentation in BaF_2 via six-photon absorption-induced fluorescence and, concomitantly, by probing the spectral and spatial properties of white light that is generated.

physics.optics↗

arXiv2007

Possible EIT-like effects in strong-field photodissociation of carbon disulphide

CS$_2$ molecules are spatially aligned upon irradiation by intense (1-100 TW cm$^{-2}$), 35 ps pulses of 512 nm or 355 nm light. When both colours are simultaneously present, spatial alignment disappears. We draw analogy with weak-field illumination of an atom by two colours wherein EIT (electromagnetically induced transparency) inhibits absorption by virtue of dipoles not being induced in two-colour fields. In the strong field scenario, molecular alignment is a consequence of a dipole being induced in the direction of the applied electric field. It follows, therefore, that when dipoles are not induced, no spatial alignment is to be expected.

physics.atom-ph↗

arXiv2006

Characterization of doping levels in heteronuclear, gas-phase, van der Waals clusters and their energy absorption from an intense optical field

A simple mass spectrometric method has been developed to quantify dopant levels in heteronuclear clusters in the gas phase. The method is demonstrated with reference to quantification of the water content in supersonic beams of water-doped argon clusters. Such doped clusters have assumed much importance in the context of recently-reported doping-induced enhancement in the emission of energetic charged particles and photons upon their interaction with intense laser pulses. We have also measured the energy that a doped cluster absorbs from the optical field; we find that energy absorption increases with increasing level of doping. The oft-used linear model of energy absorption is found to be quantitatively inadequate.

physics.atm-clus↗

arXiv2006

All-optical Control of the Propagation of Intense Laser Light in Condensed Media

We experimentally demonstrate optical control of filamentation that occurs during propagation of intense, ultrashort laser pulses through crystals like barium fluoride and sapphire. Control is exercised by rotating the plane of polarization of the incident laser radiation and is demonstrated by directly visualizing filamentation in the bulk via six-photon absorption-induced fluorescence and, concomitantly, by probing the spectral and spatial properties of white light that is generated.

physics.plasm-ph↗

arXiv2006

Multiple optical traps from a single laser beam using a mechanical element

The use of a wire mesh facilitates creation of multiple optical traps for manipulation of small micron or sub-micron particles. Such an array of optical traps can be easily controlled. The trap that is formed in this manner is a continuous trap; it obviates the need to time share a laser beam among a set of positions, as is presently done in conventional multiple traps.

physics.optics↗

arXiv2005

Euler buckling in red blood cells: An optically driven biological micromotor

We investigate the physics of an optically-driven micromotor of biological origin. A single, live red blood cell, when placed in an optical trap folds into a rod-like shape. If the trapping laser beam is circularly polarized, the folded RBC rotates. A model based on the concept of buckling instabilities captures the folding phenomenon; the rotation of the cell is simply understood using the Poincarè sphere. Our model predicts that (i) at a critical intensity of the trapping beam the RBC shape undergoes large fluctuations and (ii) the torque is proportional to the intensity of the laser beam. These predictions have been tested experimentally. We suggest a possible mechanism for emergence of birefringent properties in the RBC in the folded state.

physics.bio-ph↗

arXiv2004

Strong light fields coax intramolecular reactions on femtosecond time scales

Energetic H$_2^+$ ions are formed as a result of intra-molecular rearrangement during fragmentation of linear alcohols (methanol, ethanol, propanol, hexanol, and dodecanol) induced by intense optical fields produced by 100 fs long, infrared, laser pulses of peak intensity 8$\times10^{15}$ W cm$^{-2}$. Polarization dependent measurements show, counterintuitively, that rearrangement is induced by the strong optical field within a single laser pulse, and that it occurs before Coulomb explosion of the field-ionized multiply charged alcohols.

physics.atom-ph↗

arXiv2003

All-optical switching with bacteriorhodopsin

All-optical, mirrorless switching and bistability is demonstrated with bacteriorhodopsin (bR). A low-power, 532 nm laser beam modulates the transmission of a cw laser beam at 635 nm that corresponds to peak absorption of the O-excited state in the bR photocycle. The switch has features like a NOT-gate; its switching contrast depends on the pulse width and average power of the modulating laser. The phase of the switching characteristics can be controlled by varying the pulse width and frequency of the modulating laser. Simulations based on a rate equation approach considering a six-state model of the bR photocycle successfully reproduce the experimental results.

physics.optics↗