Ultrafast dynamics of hemin aggregates.

The effects of solvents on the conformation of hemin and their implications on the dynamics of the complex have been studied using the time-resolved optical Kerr effect (OKE) with 35 fs laser pulses (at a central wavelength of 800 nm). The OKE enabled estimation to be made of the third-order nonlinear electronic susceptibility (χ(3)) of hemin solutions: it was found to be significantly smaller than that in hemin thin films. The real and imaginary components of χ(3) were negative in both the solvents, suggesting that one-photon as well as two-photon absorption processes contribute to the nonlinear electronic susceptibility of hemin. Our study of the ultrafast heme dynamics not only unveils the instantaneous electronic response related to electronic susceptibility but also brings to the fore a novel libration process that has hitherto remained undetected. The hindered rotation in the femtosecond domain that may be responsible for this libration process possibly stems from π-π hemin oligomers formed in aqueous solution. The present results provide new insights into the conformational dynamics in the self-assembly of heme oligomers that may also be significant in certain pathogenic conditions where free heme is formed in biological systems.

Inscription of type I and depressed cladding waveguides in lithium niobate using a femtosecond laser.

We describe two types of waveguides (type I and depressed cladding) inscribed in lithium niobate using a variable repetition rate (200 kHz-25 MHz), 270 fs duration fiber laser. The type I modification-based waveguides have propagation losses in the range from 1.2 to 10 dB/cm at 1550 nm, depending on experimental parameters. These waveguides are not permanent; they deteriorate over time. Such deterioration of waveguides can be slowed down from 30 days to 100 days by pre-annealing the samples and by writing at a 720 kHz laser repetition rate. The propagation losses measured at 1550 nm show significant improvement for pre-annealed samples. The depressed cladding-inscribed waveguides are permanent, but the propagation loss depends on the number of damage tracks. A track separation of ∼1 µm between adjacent damage tracks yields the lowest propagation loss of 0.5 dB/cm at 1550 nm for a 40 µm diameter waveguide. We observe multimode guidance for sizes in the range of 20-80 µm in these waveguide structures at 1550 nm. Their crystalline nature is found to remain intact, as inferred from second-harmonic generation within the waveguide region.

Femtosecond supercontinuum generation in water in the vicinity of absorption bands.

We show that it is possible to overcome the perceived limitations caused by absorption bands in water so as to generate supercontinuum (SC) spectra in the anomalous dispersion regime that extend well beyond 2000 nm wavelength. By choosing a pump wavelength within a few hundred nanometers above the zero-dispersion wavelength of 1048 nm, initial spectral broadening extends into the normal dispersion regime and, in turn, the SC process in the visible strongly benefits from phase-matching and matching group velocities between dispersive radiation and light in the anomalous dispersion regime. Some of the SC spectra are shown to encompass two and a half octaves.

Influencing supercontinuum generation by phase distorting an ultrashort laser pulse.

We show that the spectral distribution of the supercontinuum (SC) generated in barium fluoride is amenable to alteration simply by controlling the second- and third-order phase distortion of incident femtosecond-duration pulses. The second- and third-order phase distortions are controlled by an acoustic-optic programmable dispersive filter (AOPDF). The spectral extent on the blue side of the SC is influenced by independently varying the phase distortion of an ultrashort laser pulse.

Selective breaking of bonds in water with intense, 2-cycle, infrared laser pulses.

One of the holy grails of contemporary science has been to establish the possibility of preferentially breaking one of several bonds in a molecule. For instance, the two O-H bonds in water are equivalent: given sufficient energy, either one of them is equally likely to break. We report bond-selective molecular fragmentation upon application of intense, 2-cycle pulses of 800 nm laser light: we demonstrate up to three-fold enhancement for preferential bond breaking in isotopically substituted water (HOD). Our experimental observations are rationalized by means of ab initio computations of the potential energy surfaces of HOD, HOD(+), and HOD(2+) and explorations of the dissociation limits resulting from either O-H or O-D bond rupture. The observations we report present a formidable theoretical challenge that need to be taken up in order to gain insights into molecular dynamics, strong field physics, chemical physics, non-adiabatic processes, mass spectrometry, and time-dependent quantum chemistry.

DNA damage by OH radicals produced using intense, ultrashort, long wavelength laser pulses.

We probe femtosecond laser induced damage to aqueous DNA, relying on strong-field interaction with water wherein electrons and free radicals are generated in situ; these, in turn, interact with DNA plasmids under physiological conditions, producing nicks. Exposure to intense femtosecond pulses of 1350 and 2200 nm light induces single strand breaks and double strand breaks (DSBs) in DNA. At the longer wavelength (and at higher intensities), rotationally hot OH radicals induce DSBs, producing linear DNA. Strand breaks occur due to single or multiple OH hits on DNA. With 2200 nm light, DSBs are formed mostly by the action of two OH radicals; use of OH scavengers establishes that the probability of a two-hit event reduces much faster than a one-hit event as scavenger concentration is increased. Thermal effects do not induce DSBs with 2200 nm light.

Carrier-envelope-phase effects in ultrafast strong-field ionization dynamics of multielectron systems: Xe and CS2.

Carrier-envelope-phase- (CEP) stabilized 5 and 22 fs pulses of intense 800 nm light are used to probe the strong-field ionization dynamics of 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(+)) by 30%-50%, depending on phase, reflecting the phase dependence of nonsequential ionization and its contribution to the formation of higher charge states. Ion yields for Xe(q+) (q = 2-4) with CEP-stabilized pulses are enhanced (by up to 50%) compared to those with CEP-unstabilized pulses. Such enhancement is particularly pronounced with 2-cycle (5 fs) pulses and is distinctly phase dependent. Orbital shape and symmetry affect how CS(2) responds to variations in optical field that are effected as CEP is altered, keeping intensity constant. Molecular fragmentation is found to depend on field strength (not intensity); the relative enhancement of fragmentation when CEP-stabilized 2-cycle pulses are used is found to be at the expense of molecular ionization.

Emotional intelligence, perceived stress and burnout among resident doctors: an assessment of the relationship.

BACKGROUND: Burnout is an increasing problem among the medical fraternity, especially resident doctors. We assessed the relationship between burnout, emotional intelligence and perceived stress in an attempt to explore a probable solution. METHODS: Residents in the departments of medicine (n=29) and surgery (n=27) were assessed with a specially designed proforma consisting of participant's profile sheet, Shirom-Melamed Burnout Measure (SMBM), Trait Emotional Intelligence Questionnaire and Perceived Stress Scale (PSS). Correlation and mediational analysis were used to analyse the data. RESULTS: The findings showed a positive correlation of burnout with perceived stress and a negative correlation with trait emotional intelligence (EI). Perceived stress also showed a negative correlation with trait EI. Mediation analysis showed that perceived stress mediates the effect of trait EI on burnout. CONCLUSION: Enhancing EI through training may be an important intervention for targeting burnout.

Adipokines and body fat composition in South Asians: results of the Metabolic Syndrome and Atherosclerosis in South Asians Living in America (MASALA) study.

OBJECTIVE: To investigate whether leptin and adiponectin are associated with body fat composition in a South Asian population independent of metabolic variables. DESIGN: Cross-sectional study. SUBJECTS: 150 South Asian men and women, between the ages of 45-79 years, in the San Francisco Bay Area without pre-existing clinical cardiovascular disease. MEASUREMENTS: Blood samples were obtained to measure glucose metabolism variables, lipid profiles and adipokines. Total body fat was determined using dual-energy X-ray absorptiometry. Abdominal computed tomography was used to measure subcutaneous, visceral and hepatic fat. RESULTS: Average body mass index (BMI) was overweight at 26.1±4.6 kg m(-2) and did not differ by sex. However, women had significantly more total body fat (P<0.001) and subcutaneous fat (P<0.001) than men, whereas men had significantly more visceral fat (P<0.001) and hepatic fat (P=0.04) than women. Women had significantly higher levels of adiponectin (P<0.01) and leptin (P<0.01). In sex-stratified analyses, leptin was strongly associated with all-body composition measures in women (P<0.05) as well as in men (P<0.05 except for hepatic fat), whereas there was an insignificant trend towards an inverse association between adiponectin and body composition in both women and men, which was significant in combined bivariate analyses. In multivariate analyses, leptin was strongly associated with all measures of adiposity, including BMI (P<0.001), total body fat (P<0.001), visceral fat (P<0.001) and hepatic fat (P=0.01). However, adiponectin's inverse association with adiposity was significantly attenuated by high-density lipoprotein (HDL), triglycerides and insulin resistance. The association between adipokines and diabetes was markedly attenuated after adjusting for body composition. CONCLUSION: Despite only modestly elevated BMI, South Asians have elevated levels of total and regional adiposity. Leptin is strongly associated with adiposity, whereas adiponectin's association with adiposity is attenuated by metabolic variables in South Asians. Adipokines in association with adiposity have an important role in the development of diabetes.

Optical trapping in an absorbing medium: from optical tweezing to thermal tweezing.

We report on optical trapping in a weakly absorbing medium, hemin, an iron-containing porphyrin that is an important component of hemoglobin. By altering the hemin concentration we are able to control the amount of optical energy that is absorbed; changing the hemin concentration from <12 mg/ml to >45 mg/ml enables the onset of thermal trapping to be observed. By estimating the trap strength using two different methods we are readily able to differentiate between the optical trapping and thermal trapping regimes. We also deduce the rise in temperature that occurs within the laser focal volume: temperature changes of 5-24 K are observed for laser power values of 10-90 mW for hemin concentrations of 0-50 mg/ml.

Intense two-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 such as TMS do not exhibit a prominent molecular ion (TMS(+)) as unimolecular dissociation into [Si(CH(3))(3)](+) proceeds very fast. Under a 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 wave packet. The time dependence of this bond-hardening process is verified using longer-duration (≥100 fs) pulses; the relatively slower falloff of optical field in such pulses allows the initially trapped wave packet to leak out, thereby rendering TMS(+) unstable once again.

Effect of intense, ultrashort laser pulses on DNA plasmids in their native state: strand breakages induced by 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, 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.

Optical-tweezer-induced microbubbles as scavengers of carbon nanotubes.

A modified optical tweezers set-up has been used to generate microbubbles in flowing, biologically relevant fluids and human whole blood that contains carbon nanotubes (CNTs) using low power (< or =5 mW), infrared (1064 nm wavelength), continuous wave laser light. Temperature driven effects at the tweezers' focal point help to optically trap these microbubbles. It is observed that proximate CNTs are driven towards the focal spot where, on encountering the microbubble, they adhere to it. Such CNT-loaded microbubbles can be transported both along and against the flow of surrounding fluid, and can also be exploded to cause fragmentation of the bundles. Thus, microbubbles may be used for scavenging, transporting and dispersal of potentially toxic CNTs in biologically relevant environments.

Communication: Ionization and Coulomb explosion of xenon clusters by intense, few-cycle laser pulses.

Intense, ultrashort pulses of 800 nm laser light (12 fs, approximately 4 optical cycles) of peak intensity 5x10(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 expansion due to Coulombic and hydrodynamic pressures. 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.

Bright visible emission from carbon nanotubes spatially constrained on a micro-bubble.

We report emission of broadband light in the spectral range 500 nm - 900 nm from single walled carbon nanotubes (SWNTs) in a liquid environment upon irradiance by a very low power (typically <5 mW), continuous-wave laser source in a tweezers setup. We show (i) formation of micro-bubbles upon irradiation of fluids containing bundles of SWNTs, (ii) optical trapping of such micro-bubbles, (iii) adhesion of SWNTs on the surface of such micro-bubbles, and (iv) bright emission of white light due to tweezer-induced localized heating of spatially-constrained SWNTs.

Strong fields induce ultrafast rearrangement of H atoms in H2O.

H atoms in H(2)O are rearranged by strong optical fields generated by intense 9.3 fs laser pulses to form H(2)(+). This atomic rearrangement is ultrafast: It occurs within a single laser pulse. 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 degrees. Bond formation on the ultrafast time scale of molecular vibrations (10 fs for H(2)(+)) and in strong fields has hitherto not been reported.

Polarization and energy stability of filamentation-generated few-cycle pulses.

Polarization properties and energy stability are measured for few-cycle pulses that are generated by filamentation in dual Ar-filled tubes in tandem. The dual-tube geometry enhances the contribution of self-phase modulation to spectral broadening. The polarization extinction ratio (I(perpendicular)/I(parallel) is improved for the beam transmitted through the second tube compared to the first tube and of the incident laser beam. Polarization control of few-cycle pulses is realized in simple fashion by a half-wave plate placed prior to the dual-tube assembly. We show that intensity clamping in the filament affords a major advantage in accomplishing a significant reduction in energy fluctuations compared to those inherent in the incident laser beam.

Umbilical artery histomorphometry: a link between the intrauterine environment and kidney development.

Prematurity is a risk factor for hypertension, vascular stiffness, nephron deficit and adult onset cardiorenal disease. The vascular tree and kidneys share morphogenic drivers that promote maturation in utero before 36 weeks of gestation. Vascular elastin accrual terminates after birth leaving collagen to promote vascular stiffness. Our objective was to determine if the histomorphometry of the umbilical artery, an extension of the aorta, parallels nephron mass across gestational age groups. From a cohort of 54 newborns, 32 umbilical cord specimens were adequate for evaluation. The umbilical cord was sectioned, stained with trichrome, and digitalized. Muscular and collagenous areas of the umbilical artery were measured in pixels using the Image J 1.48q software. Total kidney volume was measured by ultrasound and factored by body surface area (TKV/BSA). The umbilical artery total area was significantly greater in term v. preterm infants (9.3±1.3 v. 7.0±2.0 mm2; P<0.05) and increased with gestational age; while the percent muscular and collagen areas were independent of gestational age (R 2=0.04; P=ns). Percent muscular area correlated positively with TKV/BSA (r=0.53; P=0.002); while an increase in collagen correlated inversely with kidney mass (r=-0.53; P=0.002). In conclusion, an enhanced % muscular area and presumed vascular elasticity was associated with increased renal mass in all infants. Umbilical artery histomorphometry provides a link between the intrauterine environment, vascular and kidney development.

Euler buckling-induced folding and rotation of red blood cells in an optical trap.

We investigate the physics of an optically driven micromotor of biological origin. When a single, live red blood cell (RBC) is placed in an optical trap, the normal biconcave disc shape of the cell is observed to fold into a rod-like shape. If the trapping laser beam is circularly polarized, the folded RBC rotates. A model based on geometric considerations, using the concept of buckling instabilities, captures the folding phenomenon; the rotation of the cell is rationalized using the Poincaré sphere. Our model predicts that (i) at a critical power of the trapping laser beam the RBC shape undergoes large fluctuations, and (ii) the torque that is generated is proportional to the power of the laser beam. These predictions are verified experimentally. We suggest a possible mechanism for the emergence of birefringent properties in the RBC in the folded state.

Femtosecond laser written channel waveguides in tellurite glass.

We have made and characterized a new, erbium-doped tellurite glass that has high glass transition temperature. Addition of phosphate is found to increase the phonon energy. The peak emission cross section is 6 x 10(-21) cm(2) at 1537 nm and the fluorescence lifetime of the (4)I(13/2)-(4)I(15/2) transition is 4.1 ms. We have written 2-D channel waveguides in this glass using focused, 45-fs pulses from an amplified Ti:sapphire laser at different laser energies and writing speeds. Migration of atoms towards the periphery of the waveguides occurs, leading to refractive index changes. Channels show waveguiding at 1310 nm which is promising for the fabrication of integrated lasers and broadband amplifiers.