IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

We present a combined experimental and theoretical study of the charging dynamics of helium nanodroplets doped with atoms of different species and irradiated by intense near-infrared laser pulses (≤1015 W cm−2). In particular, we elucidate the interplay of dopant ionization inducing the ignition of a helium nanoplasma, and the charging of the dopant atoms driven by the ionized helium host. Most efficient nanoplasma ignition and charging is found when doping helium droplets with xenon atoms, in which case high charge states of both helium (He2+) and of xenon (Xe21+) are detected. In contrast, only low charge states of helium and dopants are measured when doping with potassium and calcium atoms. Classical molecular dynamics simulations which include focal averaging generally reproduce the experimental results and provide detailed insights into the correlated charging dynamics of guest and host clusters. © 2017 Informa UK Limited, trading as Taylor & Francis Group.

Department Of Physics

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Journal of Modern Optics

Charging dynamics of dopants in helium nanoplasmas

In this Letter, we report on the design, fabrication, and implementation of a novel plasmon-mode-driven low-threshold near-infrared (NIR) random laser (RL) in the 850–900 nm range based on plasmonic ZnS@Au core-shell scatterers. Plasmon modes in the NIR region are used for nanoscale scatterer engineering of ZnS@Au core-shell particles to enhance scattering, as against pristine ZnS. This plasmonic scattering enhancement coupled with femtosec-ond (fs) laser pumping is shown to cause a three-fold lasing threshold reduction from 325 μJ∕cm2 to 100 μJ∕cm2 and a mode Q-factor enhancement from 200 to 540 for ZnS@Au-based RL, as compared to pristine ZnS-based RL. Local field enhancement due to plasmonic ZnS@Au scatterers, as evidenced in the finite-difference time-domain simulation, further adds to this enhancement. This work demonstrates a novel scheme of plasmonic mode coupling in the NIR region and fs excitation in a random laser photonic system, overcoming the inherent deficiencies of weak absorption of gain media and poor scattering cross sections of dielectric scatterers for random lasing in the NIR spectrum. © 2017 Optical Society of America.

GENERAL

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Optics Letters

Femtosecond laser-pumped plasmonically enhanced near-infrared random laser based on engineered scatterers

Intense ultrashort laser produced plasmas are a source for high brightness, short burst of X-rays, electrons, and high energy ions. Laser energy absorption and its disbursement strongly depend on the laser parameters and also on the initial size and shape of the target. The ability to change the shape, size, and material composition of the matter that absorbs light is of paramount importance not only from a fundamental physics point of view but also for potentially developing laser plasma sources tailored for specific applications. The idea of preparing mesoscopic particles of desired size/shape and suspending them in vacuum for laser plasma acceleration is a sparsely explored domain. In the following report we outline the development of a delivery mechanism of microparticles into an effusive jet in vacuum for laser plasma studies. We characterise the device in terms of particle density, particle size distribution, and duration of operation under conditions suitable for laser plasma studies. We also present the first results of x-ray emission from micro crystals of boric acid that extends to 100 keV even under relatively mild intensities of 1016 W/cm2. © 2017 Author(s).

Review of Scientific Instruments

A source to deliver mesoscopic particles for laser plasma studies

We report the observation of electron-transfer-mediated decay (ETMD) involving magnesium (Mg) clusters embedded in helium (He) nanodroplets. ETMD is initiated by the ionization of He followed by removal of two electrons from the Mg clusters of which one is transferred to the He ion while the other electron is emitted into the continuum. The process is shown to be the dominant ionization mechanism for embedded clusters for photon energies above the ionization potential of He. For Mg clusters larger than five atoms we observe stable doubly ionized clusters. Thus, ETMD provides an efficient pathway to the formation of doubly ionized cold species in doped nanodroplets. © 2016 American Physical Society.

Physical Review Letters

Enhanced Ionization of Embedded Clusters by Electron-Transfer-Mediated Decay in Helium Nanodroplets

Helium nanodroplets irradiated by intense near-infrared laser pulses ignite and form highly ionized nanoplasmas even at laser intensities where helium is not directly ionized by the optical field, provided the droplets contain a few dopant atoms. We present a combined theoretical and experimental study of the He nanoplasma ignition dynamics for various dopant species. We find that the efficiency of dopants to ignite a nanoplasma in helium droplets strongly varies and mostly depends on (i) the number of free electrons each dopant donates upon ionization, (ii) the pick-up process, and (iii) the hitherto unexplored effect of the dopant location in or on the droplet. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

New Journal of Physics

Efficiency of dopant-induced ignition of helium nanoplasmas

Doubly excited Rydberg states of helium (He) have been studied in nanodroplets using synchrotron radiation. Although qualitatively similar to their atomic counterparts, the Fano resonances in droplets are broader and exhibit blueshifts which increase for the higher excited states. However, varying the droplet size hardly affects the shapes of the resonances. Furthermore, additional dipole-forbidden resonances appear which are not seen in the He atom. We discuss these features in terms of localized atomic states perturbed by the surrounding He atoms. © 2016 American Physical Society.

Physical Review A

Fano resonances observed in helium nanodroplets

In this article we present a perspective on the current state of the art in the photoionization of atomic clusters in few-cycle near-infrared laser pulses. Recently, several studies have reported intriguing phenomena associated with the photoionization of clusters by pulses as short as ∼10 fs which approach the natural timescales of collective electronic motion in such nanoscale aggregates. In contrast to the dynamics occurring on few- and sub-picosecond timescales where ionic motion sets in and plays a key role marked by resonant plasmon oscillations, the few-cycle limit precludes cluster expansion due to the nuclear motion of ionic constituents. Thus, pulses lasting just a few optical cycles explore a new “impulsive” regime for the first time in cluster nanoplasmas wherein ions essentially remain “frozen”. Along with the perspective on this new regime, we present first measurements of photoelectron distributions and temperatures.

Physical Chemistry Chemical Physics

Photoionization of clusters in intense few-cycle near infrared femtosecond pulses

Helium nanodroplets are widely used as a cold, weakly interacting matrix for spectroscopy of embedded species. In this work, we excite or ionize doped He droplets using synchrotron radiation and study the effect onto the dopant atoms depending on their location inside the droplets (rare gases) or outside at the droplet surface (alkali metals). Using photoelectron-photoion coincidence imaging spectroscopy at variable photon energies (20-25 eV), we compare the rates of charge-transfer to Penning ionization of the dopants in the two cases. The surprising finding is that alkali metals, in contrast to the rare gases, are efficiently Penning ionized upon excitation of the (n = 2)-bands of the host droplets. This indicates rapid migration of the excitation to the droplet surface, followed by relaxation, and eventually energy transfer to the alkali dopants. © 2013 American Chemical Society.

Journal of Physical Chemistry A

Charge transfer and penning ionization of dopants in or on helium nanodroplets exposed to EUV radiation

The ionization dynamics of pure He nanodroplets irradiated by Extreme ultraviolet radiation is studied using Velocity-Map Imaging PhotoElectron- PhotoIon COincidence spectroscopy. We present photoelectron energy spectra and angular distributions measured in coincidence with the most abundant ions He+, He 2+, and He 3+. Surprisingly, below the autoionization threshold of He droplets, we find indications for multiple excitation and subsequent ionization of the droplets by a Penning-like process. At high photon energies we observe inelastic collisions of photoelectrons with the surrounding He atoms in the droplets. © 2013 AIP Publishing LLC.

Journal of Chemical Physics

Extreme ultraviolet ionization of pure He nanodroplets: Mass-correlated photoelectron imaging, Penning ionization, and electron energy-loss spectra

Two-component nanoplasmas generated by strong-field ionization of doped helium nanodroplets are studied in a pump–probe experiment using few-cycle laser pulses in combination with molecular dynamics simulations. High yields of helium ions and a pronounced resonance structure in the pump–probe transients which is droplet size dependent reveal the evolution of the dopant-induced helium nanoplasma with an active role for He shells in the ensuing dynamics. The pump–probe dynamics is interpreted in terms of strong inner ionization by the pump pulse and resonant heating by the probe pulse which controls the final charge states detected via the frustration of electron–ion recombination.

Evolution of dopant-induced helium nanoplasmas

The ultra-fast dynamics of He nanodroplets (103–105 atoms) in intense (1…7 ×1014 W/cm2), few-cycle (∼10 fs), infrared (∼790 nm) laser pulses has been investigated as a function of the number of dopant rare-gas atoms, the laser intensity and the rare-gas species. We find the “ignition” behaviour predicted by theory for 20 fs resulting in the complete ionisation and disintegration of the droplet, otherwise entirely transparent, initiated by just a few, less than 5 dopant atoms.

Multiphoton Processes and Attosecond Physics

Ignition of Doped Helium Nanodroplets in Intense Few-Cycle Laser Pulses

We demonstrate ultrafast resonant energy absorption of rare-gas doped He nanodroplets from intense few-cycle (∼10fs) laser pulses. We find that less than 10 dopant atoms "ignite" the droplet to generate a nonspherical electronic nanoplasma resulting ultimately in complete ionization and disintegration of all atoms, although the pristine He droplet is transparent for the laser intensities applied. Our calculations at those intensities reveal that the minimal pulse length required for ignition is about 9 fs. © 2011 American Physical Society.

Dopant-induced ignition of helium nanodroplets in intense few-cycle laser pulses

Aerodynamic investigations on the models of two types of bus coaches in operation with a transport corporation show that their drag coefficients are considerably large. An attempt has been made to evolve experimentally a streamlined shape with smallest drag coefficient within the operating constraints in India. The streamlined configuration was obtained by rounding the frontal corners systematically and tapering the roof at the rear end. It was possible to obtain a drag coefficient of 0.42 compared to the values of 0.93 and 0.63 for the unstreamlined configurations. © Copyright 1989 Society of Automotive Engineers, Inc.