Ultrafast dynamics of atoms, molecules and clusters in strong fields


Our laboratory of ultrafast laser science at Tata Institute of Fundamental Research now routinely produces intense infrared laser pulses of 800 nm wavelength with pulse duration as short as 5 fs, long enough for barely two optical cycles, with carrier envelope phase (CEP) stabilization and control. Our initial results on ultrafast dynamics have already resulted in 3 publications in Physical Review Letters. This is the first such facility in South Asia and amongst the few in the world.

The CEP dependence of strong-field ionization processes in multielectron systems has been explored for the first time and experiments have been conducted with 5 fs pulses that demonstrate a measure of control on how atomic and molecular systems respond to instantaneous variations in optical field.Intense few-cycle pulses in which the optical field can be fixed via CEP-control opens new vistas for probing and controlling the moment at which an electron wavepacket is “born”, and its subsequent motion on attosecond timescales.

Amongst the first experiments conducted on cluster dynamics in the few-cycle, strong-field regime have revealed an unexpected angular dependence of Coulomb explosion products, with implications for the practical realization of table-top acceleration schemes.

Ultrafast molecular rearrangements have been studied in small molecules; these constitute among the first such studies and reveal, for the first time, that proton migration can occur of timescales of one vibrational period.

[1] Intense Two-Cycle Laser Pulses Induce Time-Dependent Bond Hardening in a Polyatomic Molecule
K Dota, M Garg, AK Tiwari, JA Dharmadhikari, AK Dharmadhikari, D Mathur
Physical Review Letters 108 (7), 073602
[2] Carrier-Envelope-Phase Effects in Ultrafast Strong-Field Ionization Dynamics of Multielectron Systems: Xe and CS_ {2}
D Mathur, K Dota, AK Dharmadhikari, JA Dharmadhikari
Physical Review Letters 110 (8), 083602, 2013


Ultrafast non-linear optics


Axicon-generated Bessel beams have been used to carry out a systematic study of the physics that governs propagation of intense, femtosecond-long pulses in a transparent crystal. Our interest is based on the realization that a Bessel beam is, mathematically, a solution of Bessel’s differential equation, a consequence of separable solutions to Laplace’s equation and the Helmholtz equation in cylindrical coordinates and ideal Bessel beams possess the property of maintaining a tight focus over long propagation lengths by virtue of the diffraction-free propagation property of their central peak over long distance. Their non-diffractive property implies that, in principle, Bessel beams are capable of carrying infinite energy!

The propagation of intense ultrashort pulses in the transparent glasses modifies the spectral extent of the pulse. We have measured the spectral broadening in the incident pulse upto three octaves during pulse propagation in the normal, zero and anomalous dispersion regimes of the transparent materials. An optical parametric amplifier was used to tailor the wavelength for various dispersion regimes.

[1] Femtosecond laser filamentation in condensed media with Bessel beams
K Dota, A Pathak, JA Dharmadhikari, D Mathur, AK Dharmadhikari
Physical Review A 86 (2), 023808, 2012


Biological physics


Optical trapping of healthy and malaria-infected human red blood cells have revealed significant changes in cell membrane elasticity. The membrane elasticity of normal cells in the proximity of infected cells is also affected. This information is new and has only been possible to obtain in my laboratory because of the successful integration, in one home-made apparatus, of optical trapping, fluorescence measurements, and a liquid flow-cell.

A tightly-focused, low-powered, infrared laser beam has been successfully utilized to generate nano-bubbles encrusted with carbon nanotubes (CNT); such CNT-encrusted bubbles have been shown to generate broadband radiation spanning the wavelength region 400-1064 nm and open new possibilities of highly-localized white light therapy in biomedical environments.

Toxicity of CNTs has recently been discovered in a number of laboratories; this discovery apparently thwarts CNT-based schemes for directed drug delivery. Tightly-focused, low-powered laser beams have been utilized in my laboratory to demonstrate a new scheme for physical disruption of localized concentrations of CNTs in flowing biomedical environments. The scheme relies on nano-bubbles to act as efficient scavengers of CNT bundles.

We have successfully integrated high-resolution Raman spectroscopy with optical trapping to make a Raman Tweezers set-up that has been utilized to spectroscopically probe single red blood cells that have been chemically stressed. The Raman Tweezers set-up opens new vistas for spectroscopic investigations of a wide variety of cellular processes with an in-built ability to study live cells kept under physiological conditions.
We have also experimentally demonstrated that DNA damage can be induced by very low-energy electrons and OH-radicals. These experiments were conducted using an ultrashort, high-intensity laser which produced a hot plasma filament in a liquid (water) containing DNA plasmids; electrons and OH-radicals produced with the hot plasma caused in-situ strand breakages in DNA, which were monitored using gel electrophoresis. OH-radicals are four times more efficient in causing strand breakages.

The birefringence of a red blood cell (RBC) has been quantitatively monitored as it becomes infected by a malarial parasite. Large changes have been discovered in the cell's refractive index at different stages of malarial infection, shedding new light on the competition between shape- and form-birefringence in RBCs. These experiments demonstrate the possibility of using birefringence to establish early stages of infected parasites and of assessing various factors that contribute to birefringence in normal and infected cells; our results have implications for the development and use of non-invasive techniques of quantifying changes in cell properties induced by malaria disease pathology.

Optical trapping has been used in conjunction with fluid flow technology to dissect the mechanics and spatiotemporaldynamics of how neural progenitor/stem cells (NSCs) adhere and aggregate. Hitherto unavailable information hasbeen obtained on the most probable minimum time (~5 s) and most probable minimum distance of approach (4–6 mm)required for irreversible adhesion of proximate cells to occur. A lower limit of the adhesive force by which NSCs aggregate (~18 pN) has been discovered. These findings, which we also validate bycomputational modeling, have important implications for the neurosphere assay: once aggregated, neurospheres cannotdisassemble merely by being subjected to shaking or by thermal effects. Our findings provide quantitative affirmation to thenotion that the neurosphere assay may not be a valid measure of clonality and ‘‘stemness’’.

Assembling Neurospheres: Dynamics of neural progenitor/stem cell aggregation probed using an optical trap
In work done in collaboration with Uma Ladiwala of the Centre for Excellence in Basic Science, Mumbai, we have used optical trapping (tweezing) in conjunction with fluid flow technology to dissect the mechanics and spatio-temporal dynamics of how neural progenitor/stem cells (NSCs) adhere and aggregate. Hitherto unavailable information has been obtained on the most probable minimum time (~5 s) and most probable minimum distance of approach (4-6 µm) required for irreversible adhesion of proximate cells to occur. Our experiments also allow us to study and quantify the spatial characteristics of filopodial- and membrane-mediated adhesion, and to probe the functional dynamics of NSCs to quantify a lower limit of the adhesive force by which NSCs aggregate (~18 pN). Our findings, which we also validate by computational modeling, have important implications for the neurosphere assay: once aggregated, neurospheres cannot disassemble merely by being subjected to shaking or by thermal effects. Our findings provide quantitative affirmation to the notion that the neurosphere assay may not be a valid measure of clonality and “stemness”. Post-adhesion dynamics were also studied and oscillatory motion in filopodia-mediated adhesion was observed. Furthermore, we have also explored the effect of the removal of calcium ions: both filopodia-mediated as well as membrane-membrane adhesion were inhibited. On the other hand, F-actin disrupted the dynamics of such adhesion events such that filopodia-mediated adhesion was inhibited but not membrane-membrane adhesion.

 

[1] Tank Treading of Optically Trapped Red Blood Cells in Shear Flow
H Basu, AK Dharmadhikari, JA Dharmadhikari, S Sharma, D Mathur
Biophysical journal 101 (7), 1604-1612, 2011
[2] Optical-tweezer-induced microbubbles as scavengers of carbon nanotubes
H Ramachandran, AK Dharmadhikari, K Bambardekar, H Basu, JA Dharmadhikari, S Sharma and D Mathur
Nanotechnology 21 (24), 245102


Ultrafast photonics


A new method has been developed for controlling the wettability of polymer and glass surfaces by single-step patterning on surfaces with a femtosecond laser oscillator. Appropriate choice of patterning parameters enables the creation of isotropic as well as anisotropic wetting.  Contact angle measurements on water droplets reveal that the patterned surfaces on PMMA are more hydrophobic than pristine ones while patterning on glass coverslips induces hydrophilicity which, as is demonstrated for the first time, can be used for spatial confinement of biological cells. Spectroscopic studies confirm that it is laser-induced surface modification rather than chemical change that causes the observed alteration of wetting properties.

[1] Femtosecond Laser-Induced Dot-pattern Formation in BK7 Glasses
John Thomas, Santhosh Chidangil, Anuj Bhatnagar, Rodney Bernard, Jayashree A Dharmadhikari, Aditya Dharmadhikari, Deepak Mathur
International Conference on Fibre Optics and Photonics
[2] Axicon-based writing of waveguides in BK7 glass
JA Dharmadhikari, R Bernard, AK Bhatnagar, D Mathur, AK Dharmadhikari
Optics Letters 38 (2), 172-174