Debabrata Goswami
@iitk.ac.in
Professor of Chemistry
IIT Kanpur
RESEARCH INTERESTS
Femtoscience, Coherent Control, Ultrafast Pulse Shaping, Optical Tweezers, Multiphoton Microscopy, Quantum Computing
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Habib Ali, Afreen Anjum, and Debabrata Goswami
Elsevier BV
Syed Jehanger Shah, Ajitesh Singh, Debabrata Goswami, Masatoshi Ishida, and Sankar Prasad Rath
Royal Society of Chemistry (RSC)
Substantial molecular motion of ‘nano-size’ molecules controlled by light or heat has been demonstrated in which two structural isomers reversibly ‘open’ and ‘close’ their cavities.
Habib Ali and Debabrata Goswami
Springer Science and Business Media LLC
Ashwini Kumar Rawat, Subhajit Chakraborty, Amit Kumar Mishra, and Debabrata Goswami
Elsevier BV
Debabrata Goswami
Frontiers Media SA
For studying any event, measurement can never be enough; “control” is required. This means mere passive tracking of the event is insufficient and being able to manipulate it is necessary. To maximize this capability to exert control and manipulate, both spatial and temporal domains need to be jointly accounted for, which has remained an intractable problem at microscopic scales. Simultaneous control of dynamics and position of an observable event requires a holistic combination of spatial and temporal control principles, which gives rise to the field of spatiotemporal control. For this, we present a novel femtosecond pulse-shaping approach. We explain how to achieve spatiotemporal control by spatially manipulating the system through trapping and subsequently or simultaneously exerting temporal control using shaped femtosecond pulses. By leveraging ultrafast femtosecond lasers, the prospect of having temporal control of molecular dynamics increases, and it becomes possible to circumvent the relaxation processes at microscopic timescales. Optical trapping is an exemplary demonstration of spatial control that results in the immobilization of microscopic objects with radiation pressure from a tightly focused laser beam. Conventional single-beam optical tweezers use continuous-wave (CW) lasers for achieving spatial control through photon fluxes, but these lack temporal control knobs. We use a femtosecond high repetition rate (HRR) pulsed laser to bypass this lack of dynamical control in the time domain for optical trapping studies. From a technological viewpoint, the high photon flux requirement of stable optical tweezers necessitates femtosecond pulse shaping at HRR, which has been a barrier until the recent Megahertz pulse shaping developments. Finally, recognizing the theoretical distinction between tweezers with femtosecond pulses and CW lasers is of paramount interest. Non-linear optical (NLO) interactions must be included prima facie to understand pulsed laser tweezers in areas where they excel, like the two-photon-fluorescence-based detection. We show that our theoretical model can holistically address the common drawback of all tweezers. We are able to mitigate the effects of laser-induced heating by balancing this with femtosecond laser-induced NLO effects. An interesting side-product of HRR femtosecond-laser-induced thermal lens is the development of femtosecond thermal lens spectroscopy (FTLS) and its ability to provide sensitive molecular detection.
Debabrata Goswami
SPIE
Optical tweezers have become an important tool for various applications over the past couple of decades. However, as their range of applications increases, the calibration and sensitivity of detection schemes need to keep pace. Additionally, practical optical tweezer setups become very complex if there is more than one laser-beam trap. When an optical tweezer setup uses a high-repetition-rate femtosecond laser beam to immobilize the trapping object, the technique is known as a Femtosecond optical tweezer (FOT). The instantaneous trapping potential is due to the high peak power of each laser pulse. In contrast, the sustained stable trapping regime results from the high repetition rate of successive pulses. FOTs provide critical advantages in sensitivity through in situ two-photon detection capabilities due to the sensitive detection of background-free two-photon fluorescence. For a tightly focused beam as used in an optical tweezer, cumulative heating can occur despite the minimal absorption cross-section of the trapping medium or the trapped particle, which reaches its maximum value near the focus. A temperature gradient from the laser focal spot is thus generated outwards from the laser focus in the medium, creating a refractive index gradient across the focusing region. The refractive index attains its minimum value at the focus, gradually increasing as a function of increasing distance from it. Since the trapping force and potential depend on the refractive index of the medium, the thermal effect impacts the force and potential of the trapped particle significantly. FOTs offer an interesting balance of thermal aspects with inherent nonlinearities. In addition to providing sensitive measurements with super-resolution capabilities, FOTs also allow for sensitive monitoring of the colloidal aggregation processes, which is presented in some detail here.
Subhajit Chakraborty, Yang Xu, Ann Roberts, Debabrata Goswami, and Trevor A Smith
IOP Publishing
Abstract Evanescent wave-induced fluorescence spectroscopy (EWIFS) is a widely used technique for probing the interfacial behavior of different complex media in investigations of samples in the physical, chemical, and biological sciences. This technique takes advantage of the sharply decaying evanescent field, established following total internal reflection (TIR) at the interface of two media, for spatially identifying the photoluminescence characteristics of the sample. The generation of the evanescent field requires the refractive index of the second medium to be lower than that of the first, so a major disadvantage of this increasingly widely used spectroscopic technique is the inability to exploit the advantages of EWIFS to image a sample with a higher refractive index than the incident substrate medium. A proposed configuration in which a thin, low refractive index intermediate layer is established between the TIR substrate and a high refractive index sample is investigated. We illustrate that this arrangement does not afford the desired advantages of evanescent field-induced fluorescence measurements for investigating high refractive index media.
Ashwini Kumar Rawat, Subhajit Chakraborty, Amit Kumar Mishra, and Debabrata Goswami
Elsevier BV
D K Das, K Makhal, and Debabrata Goswami
IOP Publishing
Abstract Probing transient states in molecules having vibronic transitions with femtosecond (fs) laser pulses often results in coherent oscillations either in the ground state, the excited states, or both. We find such coherent oscillations are highly solvent-dependent and provide a holistic overview of the pump-probe experiments for ultrafast dye dynamics at interfaces. For molecules dissolved in single solvents, modulations in oscillations occur due to transitions in the sub-vibrational levels of the electronic state. For binary solvents, in particular, these modulations are strongly sensitive to solvent compositions. The changes induced by various solvent compositions are drastic enough to act as a control parameter for dynamical control processes. We demonstrate an end-to-end understanding of ground-state coherent oscillations, vibrational cooling, ground-state recovery processes, and excited-state dynamics through a series of experiments. We further present a methodology for establishing such control using near-infrared dyes to measure the oscillations with fs pump-probe techniques. In the case of immiscible binary solvents, the same method allows us to investigate the liquid–liquid interface. Our control methodology is validated by an experiment using a cyanine dye dissolved in dimethyl sulfoxide, interfaced with neat diethyl-ether. The dye dynamics are retarded on moving from the bulk dye solution towards the interface with the neat diethyl-ether. When sampled along the direction of the vector pointing from the bulk towards the near interface, monotonically decreasing time constants are obtained. This result strongly suggests the importance of microheterogeneity in interfacial dynamics.
S. N. Bandyopadhyay, Ajitesh Singh, Krishnavir Singh and D. Goswami
We present a microscopic study of water–dimethyl sulfoxide (DMSO) binary mixtures using optical tweezers and thermal lens techniques. Binary mixtures of DMSO with water show anomalous behavior due to the specific hydrogen bonding ability of DMSO. We use a tightly focused femtosecond laser at a low average power to optically trap microspheres with diameters of 1 micron for use as probes. The binary mixture exhibits various viscosities, depending on its composition ratio, and hence different trapped particle characteristic frequencies (corner frequencies) due to Brownian motion. The power spectrum density method is used to obtain the corner frequency from forward-scattered data. Thus, using low-power optical tweezer experiments, we find that the maximum viscosity occurs at a DMSO mole fraction of 0.276. At higher powers, the propensity for trapping is highly diminished. It may be surprising to note that these viscosity values obtained from the corner frequencies do not exactly match those published in the literature. However, this deviation can be attributed to the thermal behavior of the binary mixture, which affects the Brownian motion and hence the obtained viscosity values. Studies at the microscopic level can thus provide a newer perspective on these already important binary mixtures. Intensity-dependent measurements further confirm the contribution of thermal effects in this study.
Sayan Kundu, Niranjan Chatterjee, Subhajit Chakraborty, Arjit Gupta, Debabrata Goswami, and Santosh K. Misra
Elsevier BV
Sonaly Goswami and Debabrata Goswami
Elsevier
Rohit Goswami, Ruhila S., Amrita Goswami, Sonaly Goswami, and Debabrata Goswami
IEEE
High performance computing (HPC) clusters are typically managed in a restrictive manner; the large user base makes cluster administrators unwilling to allow privilege escalation. Here we discuss existing methods of package management, including those which have been developed with scalability in mind, and enumerate the drawbacks and advantages of each management methodology. We contrast the paradigms of containerization via docker, virtualization via KVM, pod-infrastructures via Kubernetes, and specialized HPC packaging systems via Spack and identify key areas of neglect. We demonstrate how functional programming due to reliance on immutable states has been leveraged for deterministic package management via the nix-language expressions. We show its associated ecosystem is a prime candidate for HPC package management. We further develop guidelines and identify bottlenecks in the existing structure and present the methodology by which the nix ecosystem should be developed further as an optimal tool for HPC package management. We assert that the caveats of the nix ecosystem can easily mitigated by considerations relevant only to HPC systems, without compromising on functional methodology and features of the nix-language. We show that benefits of adoption in terms of generating reproducible derivations in a secure manner allow for workflows to be scaled across heterogeneous clusters. In particular, from the implementation hurdles faced during the compilation and running of the d-SEAMS scientific software engine, distributed as a nix-derivation on an HPC cluster, we identify communication protocols for working with SLURM and TORQUE user resource allocation queues. These protocols are heuristically defined and described in terms of the reference implementation required for queue-efficient nix builds.
Debabrata Goswami
SPIE
Single-beam optical tweezers that use continuous wave (CW) lasers for trapping microscopic particles can be understood in terms of force-balancing light pressure from a tightly focused laser beam. High-repetition-rate femtosecond lasers for single-beam optical trapping research have matured as a technique and have garnered increasing interest. There are important differences between the theoretical models for femtosecond laser tweezers and the CW tweezers, e.g., in the sensitive detection of background-free two-photon fluorescence. The instantaneous trapping potential is due to the high peak power of each laser pulse, while the sustained stable trapping regime is a consequence of the high repetition rate of successive pulses. Simulating real-time scenarios for predicting optical trapping behavior continues to be a challenging problem. However, the capability and usefulness of optical tweezers setups with both CW and pulsed lasers are well established. For a tightly focused beam as used in an optical tweezer, cumulative heating can occur despite the minimal absorption cross-section of the trapping medium or the trapped particle, which reaches its maximum value near the focus. A temperature gradient from the laser focal spot is thus generated outwards from the laser focus in the medium, creating a refractive index gradient across the focusing region. The refractive index attains its minimum value at the focus, gradually increasing as a function of increasing distance from it. Since the trapping force and potential depend on the refractive index of the medium, the thermal effect impacts the force and potential of the trapped particle significantly. With CW lasers, computational evidence of temperature rise at the focus of optical tweezers has been posited, which, unfortunately, is not a feasible approach for ultrafast lasers, given their inherent computational complexities. A better understanding of high photon-flux induced processes and a working model of the single-beam optical tweezers that could address both CW and pulsed lasers would be ideal for elucidating the effects of this inherent thermal gradient of the optical tweezers. We demonstrate a framework that includes all possible nonlinear effects arising from high photon flux interactions and validate this with experimental results. Our approach allows a coherent and consistent treatment for both CW and ultrafast cases. We have the purely thermal nonlinear effects for the CW laser case, while for the ultrafast laser case, we include both the thermal and the Kerr type nonlinearities. Such a source-sensitive model is amenable to high throughput computations when coupled with a suitable paradigm for modeling experimental conditions as well.
Abhishek Dey, Debabrata Goswami, Kenneth Karlin, Edward I. Solomon, and T. Daniel P. Stack
Royal Society of Chemistry (RSC)
Anjali Anilkumar, Philip Ash, Akhil R. Chakravarty, Peter Comba, Serena DeBeer, Abhishek Dey, Apparao Draksharapu, Debabrata Goswami, Shinobu Itoh, Kenneth Karlin,et al.
Royal Society of Chemistry (RSC)
Philip Ash, Akhil R. Chakravarty, Peter Comba, Abhishek Dey, Debabrata Goswami, Christof Martin Jäger, Kenneth Karlin, Subrata Kundu, Salvatore La Gatta, Rocío López Domene,et al.
Royal Society of Chemistry (RSC)
Ashwini Kumar Rawat, Subhajit Chakraborty, Amit Kumar Mishra, and Debabrata Goswami
Elsevier BV
Debabrata Goswami
IOP Publishing
Abstract Typical single-beam optical tweezers use continuous wave (CW) lasers, which can be explained through force balancing the light pressure from a tightly focused laser beam used for trapping microscopic particles. Recent years have also seen a surge in single-beam optical trapping research with high-repetition-rate femtosecond lasers that has shown certain differences from the CW tweezers, one of which is its sensitive detection capability of the ultrashort pulse induced background free two-photon fluorescence signals. The high peak power of each laser pulse is enough to provide instantaneous trapping potential, while the high repetition rate ensures sustained stable trapping from the successive pulses. Though the capability and usefulness of the optical-tweezers are well established, for both CW and pulsed lasers, simulating real-time scenarios to predict optical trapping behaviour remains a challenging problem. This is especially true for femtosecond laser tweezers since high peak powers are involved when the laser is tightly focused for achieving the tweezing action. The nonlinear optical effect and thermal nonlinearity become much more significant for femtosecond optical trapping. We demonstrate the importance of including these nonlinear interactions for femtosecond pulsed laser mediated optical trapping via their effect in scattering and gradient forces in the Rayleigh regime. Our optical-tweezers model includes thermal and optical nonlinear interactions, making it easier to predict the optical-trap stability in real optical trapping scenarios for both CW and pulsed lasers. Our model provides predictive metrics for choosing solvents, probes, and several optical parameters, which can be validated from our experiments.
Soumendra Nath Bandyopadhyay, Tushar Gaur, and Debabrata Goswami
Elsevier BV