IIT Kanpur

Education / Work History

  • Ph.D. Scholar, IIT Kanpur (2011-present)
  • M.Sc, IIT Kanpur (2009-2011)
  • B.Sc, Presidency College, Kolkata (2009)

Research Topic / Interest

My main interests are:

  • Femtosecond Shaped Pulse Bioimaging
  • Molecular Dynamics

Publications

These include only those published in our lab.

  1. Comparative Study of the Real-Time Optical Trapping in the Rayleigh Regime for Continuous and Pulsed Lasers. S. N. Bandyopadhyay, T. Gaur, and D. Goswami, (2020) [Abstract] [PDF] [BibTeX]

    Abstract: Simulating real-time scenarios to predict optical trapping behavior for continuous wave (CW) as well as femtosecond pulsed lasers is a challenging problem. This is especially so, because, for a tightly focused laser, one must also include optical Kerr effect as well as thermal nonlinearity. We show here the distinct differences between CW and femtosecond pulsed laser mediated optical trapping via their effect in scattering and gradient forces in the Rayleigh regime. In our newly developed model, it will also be easier to predict the stability of the trap in real optical trapping scenario as well as provide information regarding choice of solvents, probes, and optical parameters like laser type, power range, wavelength, etc. as may the cases be.

     BibTeX: @article{bandyopadhyayComparativeStudyRealtime2020,
      title = {Comparative Study of the Real-Time Optical Trapping in the {{Rayleigh}} Regime for Continuous and Pulsed Lasers},
      author = {Bandyopadhyay, Soumendra Nath and Gaur, Tushar and Goswami, Debabrata},
      date = {2020-02-01},
      url = {http://arxiv.org/abs/2002.00227},
      urldate = {2020-02-06},
      archiveprefix = {arXiv},
      eprint = {2002.00227},
      eprinttype = {arxiv},
      file = {/run/media/haozeke/Storage/Zotero/storage/REXLETZ6/Bandyopadhyay et al. - 2020 - Comparative study of the real-time optical trappin.pdf;/run/media/haozeke/Storage/Zotero/storage/ZUYJ29VK/2002.html},
      keywords = {Physics - Applied Physics,Physics - Optics},
      primaryclass = {physics}
    }
    
  2. Direct Observation of Coherent Oscillations in Solution Due to Microheterogeneous Environment. D. K. Das, K. Makhal, S. N. Bandyopadhyay, and D. Goswami, Scientific Reports 4, 6097 (2014) [Abstract] [PDF] [BibTeX]

    Abstract: We report, for the first time, direct observation of coherent oscillations in the ground-state of IR775 dye due to microheterogeneous environment. Using ultrafast near-infrared degenerate pump-probe technique centered at 800 nm, we present the dynamics of IR775 in a binary mixture of methanol and chloroform at ultra-short time resolution of 30 fs. The dynamics of the dye in binary mixtures, in a time-scale of a few fs to ~740 ps, strongly varies as a function of solvent composition (volume fraction). Multi-oscillation behavior of the coherent vibration was observed, which increased with decreasing percentage of methanol in the dye mixture. Maximum number of damped oscillations were observed in 20% methanol. The observed vibrational wavepacket motion in the ground-state is periodic in nature. We needed two cosine functions to fit the coherent oscillation data as two different solvents were used. Dynamics of the dye molecule in binary mixtures can be explained by wavepacket motion in the ground potential energy surface. More is the confinement of the dye molecule in binary mixtures, more is the number of damped oscillations. The vibrational cooling time, τ2, increases with increase in the confinement of the system. The observed wavepacket oscillations in ground-state dynamics continued until 1.6 ps.

     BibTeX: @article{dasDirectObservationCoherent2014,
      title = {Direct {{Observation}} of {{Coherent Oscillations}} in {{Solution}} Due to {{Microheterogeneous Environment}}},
      author = {Das, Dipak Kumar and Makhal, Krishnandu and Bandyopadhyay, Soumendra Nath and Goswami, Debabrata},
      date = {2014-08-18},
      journaltitle = {Scientific Reports},
      volume = {4},
      pages = {6097},
      issn = {2045-2322},
      doi = {10/gf5mth},
      url = {https://www.nature.com/articles/srep06097},
      urldate = {2019-08-01},
      file = {/run/media/haozeke/Storage/Zotero/storage/96KMQTBM/dgoswamiFemtoLab-Das_et_al_2014_Direct_Observation_of_Coherent_Oscillations_in_Solution_due_to.pdf;/run/media/haozeke/Storage/Zotero/storage/SE6JFJ4V/srep06097.html},
      keywords = {_tablet},
      langid = {english},
      note = {00000}
    }
    
  3. On the Interferometric Coherent Structures in Femtosecond Supercontinuum Generation. S. Dinda, S. N. Bandyopadhyay, and D. Goswami, Applied Physics B 122(5), 148 (2016) [Abstract] [PDF] [BibTeX]

    Abstract: We report structured interferometric features in femtosecond supercontinuum generation (FSG) with incident laser powers that are near threshold for FSG. We argue that near threshold, these structures arise from the coherent superposition of pulses that are split initially into two daughter pulses during FSG process. Increase in the input pulse energy generates multiple daughter fragments in the temporal domain to an extent that correlated interference structures are not measurable.

     BibTeX: @article{dindaInterferometricCoherentStructures2016,
      title = {On the Interferometric Coherent Structures in Femtosecond Supercontinuum Generation},
      author = {Dinda, Sirshendu and Bandyopadhyay, Soumendra Nath and Goswami, Debabrata},
      date = {2016-05-11},
      journaltitle = {Applied Physics B},
      shortjournal = {Appl. Phys. B},
      volume = {122},
      pages = {148},
      issn = {1432-0649},
      doi = {10/gf5mr5},
      url = {https://my.pcloud.com/publink/show?code=XZfaxr7ZhXTWojTzmcfBOA16JDESUyR5fRWV},
      urldate = {2019-08-01},
      file = {/run/media/haozeke/Storage/Zotero/storage/2I8YT8K7/dgoswamiFemtoLab-Dinda_et_al_2016_On_the_interferometric_coherent_structures_in_femtosecond_supercontinuum.pdf},
      keywords = {_tablet,CCl4,Multiphoton Ionization,Nonlinear Refractive Index,Optical Frequency Comb,Optical Kerr Effect},
      langid = {english},
      note = {00000},
      number = {5}
    }
    
  4. Rapid Programmable Pulse Shaping of Femtosecond Pulses at the MHz Repetition Rate. S. Dinda, S. N. Bandyopadhyay, and D. Goswami, OSA Continuum 2(4), 1386 (2019) [Abstract] [PDF] [BibTeX]

    Abstract: We have shown experimentally the successful engineering of femtosecond pulse shaping at a 76 MHz repetition rate input pulse with an acousto-optic modulator (AOM). High repetition rate (HRR) femtosecond laser pulse shaping using an AOM in the Fourier plane was incomprehensible because of its intrinsic 100 kHz acoustic update limit. We demonstrate an effective way of pulse selection and a calibration routine (Fourier shift theorem), which enables generation of ∼10 MHz shaped output pulses from the HRR input pulse train. We have generated a temporally shifted rectangular shaped pulse profile by applying modulation on both the phase and amplitude of the ‘sinc’ RF modulation function.

     BibTeX: @article{dindaRapidProgrammablePulse2019,
      title = {Rapid Programmable Pulse Shaping of Femtosecond Pulses at the {{MHz}} Repetition Rate},
      author = {Dinda, Sirshendu and Bandyopadhyay, Soumendra Nath and Goswami, Debabrata},
      date = {2019-04-15},
      journaltitle = {OSA Continuum},
      shortjournal = {OSA Continuum},
      volume = {2},
      pages = {1386},
      issn = {2578-7519},
      doi = {10.1364/OSAC.2.001386},
      url = {https://my.pcloud.com/publink/show?code=XZDaxr7Z7j2h0Ej5kjfj9pVqlqwvHViq6oeX},
      urldate = {2019-08-01},
      file = {/run/media/haozeke/Storage/Zotero/storage/NPHNENMX/dgoswamiFemtoLab-Dinda_et_al_2019_Rapid_programmable_pulse_shaping_of_femtosecond_pulses_at_the_MHz_repetition.pdf},
      keywords = {_tablet},
      langid = {english},
      note = {00000},
      number = {4}
    }
    
  5. Manifesting the Effects of Thermal Nonlinearity in Optical Trapping for Rayleigh Regime. T. Gaur, S. N. Bandyopadhyay, and D. Goswami, in 2019 URSI Asia-Pacific Radio Science Conference (AP-RASC) (IEEE, 2019), pp. 1–5 [Abstract] [PDF] [BibTeX]

    Abstract: Since long the thermal effects have not been much explored in the optical trapping theory, in this paper, we are establishing the effects of optically induced thermal nonlinearity in the medium of optical trapping in the Rayleigh regime for both continuous wave and a pulsed laser. For a single beam, optical tweezers with high numerical aperture (N.A.) objectives are used as a routine. In such a tight focusing scenario, both optical nonlinearity and thermal effects may prevail in the cases of continuous wave (C.W.) and pulsed laser-mediated optical trapping events. In this paper we will introduce the effects sequentially, starting from optical nonlinearity and methods to implement this effect and subsequently introduce the thermal nonlinearity in the medium. The effects are significantly different when compared between CW and pulsed optical tweezers and will be discussed in detail in this paper.

     BibTeX: @inproceedings{gaurManifestingEffectsThermal2019,
      title = {Manifesting the {{Effects}} of {{Thermal Nonlinearity}} in {{Optical Trapping}} for {{Rayleigh Regime}}},
      booktitle = {2019 {{URSI Asia}}-{{Pacific Radio Science Conference}} ({{AP}}-{{RASC}})},
      author = {Gaur, Tushar and Bandyopadhyay, Soumendra Nath and Goswami, Debabrata},
      date = {2019-03},
      pages = {1--5},
      publisher = {{IEEE}},
      location = {{New Delhi, India}},
      doi = {10/gf5mqf},
      url = {https://my.pcloud.com/publink/show?code=XZqaxr7ZmnVNxAn4Y4mRbGS3d4A7FjmCWEv7},
      urldate = {2019-08-01},
      eventtitle = {2019 {{URSI Asia}}-{{Pacific Radio Science Conference}} ({{AP}}-{{RASC}})},
      file = {/run/media/haozeke/Storage/Zotero/storage/J9D85AX6/dgoswamiFemtoLab-Gaur_et_al_2019_Manifesting_the_Effects_of_Thermal_Nonlinearity_in_Optical_Trapping_for.pdf},
      isbn = {978-90-825987-5-9},
      keywords = {_tablet},
      langid = {english},
      note = {00000}
    }
    
  6. Resolution Enhancement through Microscopic Spatiotemporal Control. D. Goswami, D. Das, and S. N. Bandyopadhyay, Faraday Discussions 177(0), 203–212 (2015) [Abstract] [PDF] [BibTeX]

    Abstract: Operating at biologically benign conditions, multi-photon fluorescence imaging microscopy has benefitted immensely from recent developments in microscopic resolution enhancement. Fluorescence microscopy continues to be the best choice for experiments on live specimens, however, multi-photon fluorescence imaging often suffers from overlapping fluorescence of typical dyes used in microscopy, limiting its scope. This limitation has been the focus of our research where we show that by making simple modifications to the laser pulse structure, it is possible to resolve these overlapping fluorescence complications. Specifically, by using pairs of femtosecond pulses with variable delay in place of single pulse excitation, we show controlled fluorescence excitation or suppression of one of the fluorophores over the other through wave-packet interferometry. Such an effect prevails even after the fluorophore coherence timescale, which effectively results in a higher spatial resolution. Here we extend the effect of our pulse-pair technique to microscopic axial resolution experiments and show that such pairs of pulses can also ‘enhance’ axial resolution.

     BibTeX: @article{goswamiResolutionEnhancementMicroscopic2015,
      title = {Resolution Enhancement through Microscopic Spatiotemporal Control},
      author = {Goswami, Debabrata and Das, Dhiman and Bandyopadhyay, Soumendra Nath},
      date = {2015-04-14},
      journaltitle = {Faraday Discussions},
      shortjournal = {Faraday Discuss.},
      volume = {177},
      pages = {203--212},
      issn = {1364-5498},
      doi = {10/gf5msv},
      url = {https://my.pcloud.com/publink/show?code=XZzvxr7Z0vbvki6gLSQhR76QzRco87Moa8Fy},
      urldate = {2019-08-01},
      file = {/run/media/haozeke/Storage/Zotero/storage/CTZQVZMG/dgoswamiFemtoLab-Goswami_et_al_2015_Resolution_enhancement_through_microscopic_spatiotemporal_control.pdf;/run/media/haozeke/Storage/Zotero/storage/CGDJXRI2/c4fd00177j.html},
      keywords = {_tablet},
      langid = {english},
      note = {00000},
      number = {0}
    }
    
  7. Structured Interferometry Features in Femtosecond Supercontinuum: Towards Better Understanding of Supercontinuum for Bio Applications. D. Goswami, S. Dinda, and S. N. Bandyopadhyay, in Optical Biopsy XV: Toward Real-Time Spectroscopic Imaging and Diagnosis (International Society for Optics and Photonics, 2017), 10060, p. 1006019 [Abstract] [PDF] [BibTeX]

    Abstract: We report structured interferometry features in femtosecond supercontinuum generated with power near to supercontinuum generation threshold. We argue that these structures arise from the coherent superposition of supercontinuum generated from different sources of supercontinuum as pulse split into two daughter pulses. Increase in input pulse energy creates several more temporal pulse fragments and disrupts interference resulting in the typical feature of continuous broad supercontinuum. Such an understanding of supercontinuum generation process is critical to the use of supercontinuum as the light source for use in understanding the time dynamics and imaging of bio systems.

     BibTeX: @inproceedings{goswamiStructuredInterferometryFeatures2017,
      title = {Structured Interferometry Features in Femtosecond Supercontinuum: Towards Better Understanding of Supercontinuum for Bio Applications},
      shorttitle = {Structured Interferometry Features in Femtosecond Supercontinuum},
      booktitle = {Optical {{Biopsy XV}}: {{Toward Real}}-{{Time Spectroscopic Imaging}} and {{Diagnosis}}},
      author = {Goswami, Debabrata and Dinda, Sirshendu and Bandyopadhyay, Soumendra N.},
      date = {2017-02-17},
      volume = {10060},
      pages = {1006019},
      publisher = {{International Society for Optics and Photonics}},
      doi = {10/gf5mrd},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10060/1006019/Structured-interferometry-features-in-femtosecond-supercontinuum--towards-better-understanding/10.1117/12.2253561.short},
      urldate = {2019-08-01},
      eventtitle = {Optical {{Biopsy XV}}: {{Toward Real}}-{{Time Spectroscopic Imaging}} and {{Diagnosis}}},
      file = {/run/media/haozeke/Storage/Zotero/storage/74G8UKR8/12.2253561.html},
      note = {00000}
    }
    
  8. Elucidating Optical Field Directed Hierarchical Self-Assembly of Homogenous versus Heterogeneous Nanoclusters with Femtosecond Optical Tweezers. D. Mondal, S. N. Bandyopadhyay, and D. Goswami, PLOS ONE 14(10), e0223688 (2019) [Abstract] [PDF] [BibTeX]

    Abstract: Insights into the morphology of nanoclusters would facilitate the design of nano-devices with improved optical, electrical, and magnetic responses. We have utilized optical gradient forces for the directed self-assembly of colloidal clusters using high-repetition-rate femtosecond laser pulses to delineate their structure and dynamics. We have ratified our experiments with theoretical models derived from the Langevin equation and defined the valid ranges of applicability. Our femtosecond optical tweezer-based technique characterizes the in-situ formation of hierarchical self-assembled clusters of homomers as well as heteromers by analyzing the back focal plane displacement signal. This technique is able to efficiently distinguish between nano-particles in heterogeneous clusters and is in accordance with our theory. Herein, we report results from our technique, and also develop a model to describe the mechanism of such processes where corner frequency changes. We show how the corner frequency changes enables us to recognize the structure and dynamics of the coagulation of colloidal homogeneous and heterogeneous clusters in condensed media over a broad range of nanoparticle sizes. The methods described here are advantageous, as the backscatter position-sensitive detection probes the in-situ self-assembly process while other light scattering approaches are leveraged for the characterization of isolated clusters.

     BibTeX: @article{mondalElucidatingOpticalField2019,
      title = {Elucidating Optical Field Directed Hierarchical Self-Assembly of Homogenous versus Heterogeneous Nanoclusters with Femtosecond Optical Tweezers},
      author = {Mondal, Dipankar and Bandyopadhyay, Soumendra Nath and Goswami, Debabrata},
      date = {2019-10-31},
      journaltitle = {PLOS ONE},
      shortjournal = {PLOS ONE},
      volume = {14},
      pages = {e0223688},
      issn = {1932-6203},
      doi = {10/ggfxvx},
      url = {https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0223688},
      urldate = {2019-12-28},
      file = {/run/media/haozeke/Storage/Zotero/storage/P2AMQKQV/dgoswamiFemtoLab-Mondal_et_al_2019_Elucidating_optical_field_directed_hierarchical_self-assembly_of_homogenous.pdf},
      keywords = {Dimers,Lasers,Microspheres,Molecular self assembly,Monomers,Nanoparticles,Optical lenses,Polystyrene},
      langid = {english},
      note = {00000},
      number = {10}
    }
    
  9. On-the-Fly Calibrated Measure and Remote Control of Temperature and Viscosity at Nanoscale. D. Mondal, S. N. Bandyopadhyay, P. Mathur, and D. Goswami, ACS Omega 3(9), 12304–12311 (2018) [Abstract] [PDF] [BibTeX]

    Abstract: A novel on-the-fly calibration method of optical tweezers is presented, which enables in situ control and measure of absolute temperature and viscosity at nanoscale dimensions. Such noncontact measurement and control at the nanoscale are challenging as the present techniques only provide off-line measurements that do not provide absolute values. Additionally, some of the present methods have a low spatial resolution. We simultaneously apply the high temporal sensitivity of position autocorrelation and equipartition theorem to precisely measure and control in situ temperature and the corresponding microrheological property around the focal volume of the trap at high spatial resolution. The femtosecond optical tweezers (FOTs) use a single-beam high repetition rate laser for optical trapping to result in finer temperature gradients in comparison to the continuous-wave laser tweezers. Such finer temperature gradients are due to the additional nonlinear optical (NLO) phenomena occurring only at the nanoscale focal plane of the FOTs. Because NLO processes are laser peak power-dependent, they promote an effective study of physical properties occurring only at the focal plane. Using FOTs at optically benign near-infrared wavelengths, we demonstrate microrheological control and measurement in water by adding a highly absorbing yet low fluorescent dye (IR780).

     BibTeX: @article{mondalOntheFlyCalibratedMeasure2018,
      title = {On-the-{{Fly Calibrated Measure}} and {{Remote Control}} of {{Temperature}} and {{Viscosity}} at {{Nanoscale}}},
      author = {Mondal, Dipankar and Bandyopadhyay, Soumendra Nath and Mathur, Paresh and Goswami, Debabrata},
      date = {2018-09-30},
      journaltitle = {ACS Omega},
      shortjournal = {ACS Omega},
      volume = {3},
      pages = {12304--12311},
      issn = {2470-1343},
      doi = {10/gff5s6},
      url = {https://doi.org/10.1021/acsomega.8b01572},
      urldate = {2019-10-01},
      file = {/run/media/haozeke/Storage/Zotero/storage/P35MVUQV/dgoswamiFemtoLab-Mondal_et_al_2018_On-the-Fly_Calibrated_Measure_and_Remote_Control_of_Temperature_and_Viscosity.pdf},
      note = {00001},
      number = {9}
    }
    
  10. On-the-Fly Calibrated Measure and Remote Control of Temperature and Viscosity at Nanoscale. D. Mondal, S. N. Bandyopadhyay, P. Mathur, and D. Goswami, ACS Omega 3(9), 12304–12311 (2018) [Abstract] [PDF] [BibTeX]

    Abstract: A novel on-the-f ly calibration method of optical tweezers is presented, which enables in situ control and measure of absolute temperature and viscosity at nanoscale dimensions. Such noncontact measurement and control at the nanoscale are challenging as the present techniques only provide off-line measurements that do not provide absolute values. Additionally, some of the present methods have a low spatial resolution. We simultaneously apply the high temporal sensitivity of position autocorrelation and equipartition theorem to precisely measure and control in situ temperature and the corresponding microrheological property around the focal volume of the trap at high spatial resolution. The femtosecond optical tweezers (FOTs) use a single-beam high repetition rate laser for optical trapping to result in finer temperature gradients in comparison to the continuous-wave laser tweezers. Such finer temperature gradients are due to the additional nonlinear optical (NLO) phenomena occurring only at the nanoscale focal plane of the FOTs. Because NLO processes are laser peak power-dependent, they promote an effective study of physical properties occurring only at the focal plane. Using FOTs at optically benign near-infrared wavelengths, we demonstrate microrheological control and measurement in water by adding a highly absorbing yet low fluorescent dye (IR780).

     BibTeX: @article{mondalOntheFlyCalibratedMeasure2018a,
      title = {On-the-{{Fly Calibrated Measure}} and {{Remote Control}} of {{Temperature}} and {{Viscosity}} at {{Nanoscale}}},
      author = {Mondal, Dipankar and Bandyopadhyay, Soumendra Nath and Mathur, Paresh and Goswami, Debabrata},
      date = {2018-09-30},
      journaltitle = {ACS Omega},
      shortjournal = {ACS Omega},
      volume = {3},
      pages = {12304--12311},
      issn = {2470-1343, 2470-1343},
      doi = {10/gff5s6},
      url = {http://pubs.acs.org/doi/10.1021/acsomega.8b01572},
      urldate = {2019-08-01},
      file = {/run/media/haozeke/Storage/Zotero/storage/9V2JNDW8/dgoswamiFemtoLab-Mondal_et_al_2018_On-the-Fly_Calibrated_Measure_and_Remote_Control_of_Temperature_and_Viscosity.pdf},
      keywords = {_tablet},
      langid = {english},
      note = {00001},
      number = {9}
    }
    
  11. Polarization Induced Control of Optical Trap Potentials in Binary Liquids. D. Mondal, S. Dinda, S. N. Bandyopadhyay, and D. Goswami, Scientific Reports 9(1), 700 (2019) [PDF] [BibTeX]
     BibTeX: @article{mondalPolarizationInducedControl2019,
      title = {Polarization Induced Control of Optical Trap Potentials in Binary Liquids},
      author = {Mondal, Dipankar and Dinda, Sirshendu and Bandyopadhyay, Soumendra Nath and Goswami, Debabrata},
      date = {2019-12},
      journaltitle = {Scientific Reports},
      shortjournal = {Sci Rep},
      volume = {9},
      pages = {700},
      issn = {2045-2322},
      doi = {10.1038/s41598-018-36856-5},
      url = {http://www.nature.com/articles/s41598-018-36856-5},
      urldate = {2019-08-01},
      file = {/run/media/haozeke/Storage/Zotero/storage/JCHJ485E/dgoswamiFemtoLab-Mondal_et_al_2019_Polarization_induced_control_of_optical_trap_potentials_in_binary_liquids.pdf},
      keywords = {_tablet},
      langid = {english},
      note = {00000},
      number = {1}
    }
    
  12. Polarization Induced Control of Optical Trap Potentials in Binary Liquids. D. Mondal, S. Dinda, S. N. Bandyopadhyay, and D. Goswami, Sci Rep 9(1), 700 (2019) [PDF] [BibTeX]
     BibTeX: @article{mondalPolarizationInducedControl2020,
      langid = {english},
      title = {Polarization Induced Control of Optical Trap Potentials in Binary Liquids},
      volume = {9},
      issn = {2045-2322},
      url = {http://www.nature.com/articles/s41598-018-36856-5},
      doi = {10.1038/s41598-018-36856-5},
      number = {1},
      journaltitle = {Sci Rep},
      urldate = {2019-08-01},
      date = {2019-12},
      pages = {700},
      author = {Mondal, Dipankar and Dinda, Sirshendu and Bandyopadhyay, Soumendra Nath and Goswami, Debabrata}
    }
    
  13. Rapid Programmable Pulse Shaping of Femtosecond Pulses at the MHz Repetition Rate. S. Dinda, S. N. Bandyopadhyay, and D. Goswami, OSA Continuum 2(4), 1386 (2019) [Abstract] [PDF] [BibTeX]

    Abstract: We have shown experimentally the successful engineering of femtosecond pulse shaping at a 76 MHz repetition rate input pulse with an acousto-optic modulator (AOM). High repetition rate (HRR) femtosecond laser pulse shaping using an AOM in the Fourier plane was incomprehensible because of its intrinsic 100 kHz acoustic update limit. We demonstrate an effective way of pulse selection and a calibration routine (Fourier shift theorem), which enables generation of ∼10 MHz shaped output pulses from the HRR input pulse train. We have generated a temporally shifted rectangular shaped pulse profile by applying modulation on both the phase and amplitude of the ‘sinc’ RF modulation function.

     BibTeX: @article{dindaRapidProgrammablePulse2019b,
      langid = {english},
      title = {Rapid Programmable Pulse Shaping of Femtosecond Pulses at the {{MHz}} Repetition Rate},
      volume = {2},
      issn = {2578-7519},
      url = {https://my.pcloud.com/publink/show?code=XZDaxr7Z7j2h0Ej5kjfj9pVqlqwvHViq6oeX},
      doi = {10.1364/OSAC.2.001386},
      number = {4},
      journaltitle = {OSA Continuum},
      urldate = {2019-08-01},
      date = {2019-04-15},
      pages = {1386},
      author = {Dinda, Sirshendu and Bandyopadhyay, Soumendra Nath and Goswami, Debabrata}
    }
    
  14. Manifesting the Effects of Thermal Nonlinearity in Optical Trapping for Rayleigh Regime. T. Gaur, S. N. Bandyopadhyay, and D. Goswami, in 2019 URSI Asia-Pacific Radio Science Conference (AP-RASC) (IEEE, 2019), pp. 1–5 [Abstract] [PDF] [BibTeX]

    Abstract: Since long the thermal effects have not been much explored in the optical trapping theory, in this paper, we are establishing the effects of optically induced thermal nonlinearity in the medium of optical trapping in the Rayleigh regime for both continuous wave and a pulsed laser. For a single beam, optical tweezers with high numerical aperture (N.A.) objectives are used as a routine. In such a tight focusing scenario, both optical nonlinearity and thermal effects may prevail in the cases of continuous wave (C.W.) and pulsed laser-mediated optical trapping events. In this paper we will introduce the effects sequentially, starting from optical nonlinearity and methods to implement this effect and subsequently introduce the thermal nonlinearity in the medium. The effects are significantly different when compared between CW and pulsed optical tweezers and will be discussed in detail in this paper.

     BibTeX: @inproceedings{gaurManifestingEffectsThermal2020,
      langid = {english},
      location = {{New Delhi, India}},
      title = {Manifesting the {{Effects}} of {{Thermal Nonlinearity}} in {{Optical Trapping}} for {{Rayleigh Regime}}},
      isbn = {978-90-825987-5-9},
      url = {https://my.pcloud.com/publink/show?code=XZqaxr7ZmnVNxAn4Y4mRbGS3d4A7FjmCWEv7},
      doi = {10/gf5mqf},
      eventtitle = {2019 {{URSI Asia}}-{{Pacific Radio Science Conference}} ({{AP}}-{{RASC}})},
      booktitle = {2019 {{URSI Asia}}-{{Pacific Radio Science Conference}} ({{AP}}-{{RASC}})},
      publisher = {{IEEE}},
      urldate = {2019-08-01},
      date = {2019-03},
      pages = {1-5},
      author = {Gaur, Tushar and Bandyopadhyay, Soumendra Nath and Goswami, Debabrata}
    }
    
  15. On-the-Fly Calibrated Measure and Remote Control of Temperature and Viscosity at Nanoscale. D. Mondal, S. N. Bandyopadhyay, P. Mathur, and D. Goswami, ACS Omega 3(9), 12304–12311 (2018) [Abstract] [PDF] [BibTeX]

    Abstract: A novel on-the-f ly calibration method of optical tweezers is presented, which enables in situ control and measure of absolute temperature and viscosity at nanoscale dimensions. Such noncontact measurement and control at the nanoscale are challenging as the present techniques only provide off-line measurements that do not provide absolute values. Additionally, some of the present methods have a low spatial resolution. We simultaneously apply the high temporal sensitivity of position autocorrelation and equipartition theorem to precisely measure and control in situ temperature and the corresponding microrheological property around the focal volume of the trap at high spatial resolution. The femtosecond optical tweezers (FOTs) use a single-beam high repetition rate laser for optical trapping to result in finer temperature gradients in comparison to the continuous-wave laser tweezers. Such finer temperature gradients are due to the additional nonlinear optical (NLO) phenomena occurring only at the nanoscale focal plane of the FOTs. Because NLO processes are laser peak power-dependent, they promote an effective study of physical properties occurring only at the focal plane. Using FOTs at optically benign near-infrared wavelengths, we demonstrate microrheological control and measurement in water by adding a highly absorbing yet low fluorescent dye (IR780).

     BibTeX: @article{mondalOntheFlyCalibratedMeasure2019,
      langid = {english},
      title = {On-the-{{Fly Calibrated Measure}} and {{Remote Control}} of {{Temperature}} and {{Viscosity}} at {{Nanoscale}}},
      volume = {3},
      issn = {2470-1343, 2470-1343},
      url = {http://pubs.acs.org/doi/10.1021/acsomega.8b01572},
      doi = {10/gff5s6},
      number = {9},
      journaltitle = {ACS Omega},
      urldate = {2019-08-01},
      date = {2018-09-30},
      pages = {12304-12311},
      author = {Mondal, Dipankar and Bandyopadhyay, Soumendra Nath and Mathur, Paresh and Goswami, Debabrata}
    }
    
  16. Structured Interferometry Features in Femtosecond Supercontinuum: Towards Better Understanding of Supercontinuum for Bio Applications. D. Goswami, S. Dinda, and S. N. Bandyopadhyay, in Optical Biopsy XV: Toward Real-Time Spectroscopic Imaging and Diagnosis (International Society for Optics and Photonics, 2017), 10060, p. 1006019 [Abstract] [PDF] [BibTeX]

    Abstract: We report structured interferometry features in femtosecond supercontinuum generated with power near to supercontinuum generation threshold. We argue that these structures arise from the coherent superposition of supercontinuum generated from different sources of supercontinuum as pulse split into two daughter pulses. Increase in input pulse energy creates several more temporal pulse fragments and disrupts interference resulting in the typical feature of continuous broad supercontinuum. Such an understanding of supercontinuum generation process is critical to the use of supercontinuum as the light source for use in understanding the time dynamics and imaging of bio systems.

     BibTeX: @inproceedings{goswamiStructuredInterferometryFeatures2018,
      title = {Structured Interferometry Features in Femtosecond Supercontinuum: Towards Better Understanding of Supercontinuum for Bio Applications},
      volume = {10060},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10060/1006019/Structured-interferometry-features-in-femtosecond-supercontinuum--towards-better-understanding/10.1117/12.2253561.short},
      doi = {10/gf5mrd},
      shorttitle = {Structured Interferometry Features in Femtosecond Supercontinuum},
      eventtitle = {Optical {{Biopsy XV}}: {{Toward Real}}-{{Time Spectroscopic Imaging}} and {{Diagnosis}}},
      booktitle = {Optical {{Biopsy XV}}: {{Toward Real}}-{{Time Spectroscopic Imaging}} and {{Diagnosis}}},
      publisher = {{International Society for Optics and Photonics}},
      urldate = {2019-08-01},
      date = {2017-02-17},
      pages = {1006019},
      author = {Goswami, Debabrata and Dinda, Sirshendu and Bandyopadhyay, Soumendra N.}
    }
    
  17. On the Interferometric Coherent Structures in Femtosecond Supercontinuum Generation. S. Dinda, S. N. Bandyopadhyay, and D. Goswami, Appl. Phys. B 122(5), 148 (2016) [Abstract] [PDF] [BibTeX]

    Abstract: We report structured interferometric features in femtosecond supercontinuum generation (FSG) with incident laser powers that are near threshold for FSG. We argue that near threshold, these structures arise from the coherent superposition of pulses that are split initially into two daughter pulses during FSG process. Increase in the input pulse energy generates multiple daughter fragments in the temporal domain to an extent that correlated interference structures are not measurable.

     BibTeX: @article{dindaInterferometricCoherentStructures2017,
      langid = {english},
      title = {On the Interferometric Coherent Structures in Femtosecond Supercontinuum Generation},
      volume = {122},
      issn = {1432-0649},
      url = {https://my.pcloud.com/publink/show?code=XZfaxr7ZhXTWojTzmcfBOA16JDESUyR5fRWV},
      doi = {10/gf5mr5},
      number = {5},
      journaltitle = {Appl. Phys. B},
      urldate = {2019-08-01},
      date = {2016-05-11},
      pages = {148},
      author = {Dinda, Sirshendu and Bandyopadhyay, Soumendra Nath and Goswami, Debabrata}
    }
    
  18. Resolution Enhancement through Microscopic Spatiotemporal Control. D. Goswami, D. Das, and S. N. Bandyopadhyay, Faraday Discuss. 177(0), 203–212 (2015) [Abstract] [PDF] [BibTeX]

    Abstract: Operating at biologically benign conditions, multi-photon fluorescence imaging microscopy has benefitted immensely from recent developments in microscopic resolution enhancement. Fluorescence microscopy continues to be the best choice for experiments on live specimens, however, multi-photon fluorescence imaging often suffers from overlapping fluorescence of typical dyes used in microscopy, limiting its scope. This limitation has been the focus of our research where we show that by making simple modifications to the laser pulse structure, it is possible to resolve these overlapping fluorescence complications. Specifically, by using pairs of femtosecond pulses with variable delay in place of single pulse excitation, we show controlled fluorescence excitation or suppression of one of the fluorophores over the other through wave-packet interferometry. Such an effect prevails even after the fluorophore coherence timescale, which effectively results in a higher spatial resolution. Here we extend the effect of our pulse-pair technique to microscopic axial resolution experiments and show that such pairs of pulses can also ‘enhance’ axial resolution.

     BibTeX: @article{goswamiResolutionEnhancementMicroscopic2016,
      langid = {english},
      title = {Resolution Enhancement through Microscopic Spatiotemporal Control},
      volume = {177},
      issn = {1364-5498},
      url = {https://my.pcloud.com/publink/show?code=XZzvxr7Z0vbvki6gLSQhR76QzRco87Moa8Fy},
      doi = {10/gf5msv},
      number = {0},
      journaltitle = {Faraday Discuss.},
      urldate = {2019-08-01},
      date = {2015-04-14},
      pages = {203-212},
      author = {Goswami, Debabrata and Das, Dhiman and Bandyopadhyay, Soumendra Nath}
    }
    
  19. Direct Observation of Coherent Oscillations in Solution Due to Microheterogeneous Environment. D. K. Das, K. Makhal, S. N. Bandyopadhyay, and D. Goswami, Scientific Reports 4, 6097 (2014) [Abstract] [PDF] [BibTeX]

    Abstract: We report, for the first time, direct observation of coherent oscillations in the ground-state of IR775 dye due to microheterogeneous environment. Using ultrafast near-infrared degenerate pump-probe technique centered at 800 nm, we present the dynamics of IR775 in a binary mixture of methanol and chloroform at ultra-short time resolution of 30 fs. The dynamics of the dye in binary mixtures, in a time-scale of a few fs to ~740 ps, strongly varies as a function of solvent composition (volume fraction). Multi-oscillation behavior of the coherent vibration was observed, which increased with decreasing percentage of methanol in the dye mixture. Maximum number of damped oscillations were observed in 20% methanol. The observed vibrational wavepacket motion in the ground-state is periodic in nature. We needed two cosine functions to fit the coherent oscillation data as two different solvents were used. Dynamics of the dye molecule in binary mixtures can be explained by wavepacket motion in the ground potential energy surface. More is the confinement of the dye molecule in binary mixtures, more is the number of damped oscillations. The vibrational cooling time, τ2, increases with increase in the confinement of the system. The observed wavepacket oscillations in ground-state dynamics continued until 1.6 ps.

     BibTeX: @article{dasDirectObservationCoherent2015,
      langid = {english},
      title = {Direct {{Observation}} of {{Coherent Oscillations}} in {{Solution}} Due to {{Microheterogeneous Environment}}},
      volume = {4},
      issn = {2045-2322},
      url = {https://www.nature.com/articles/srep06097},
      doi = {10/gf5mth},
      journaltitle = {Scientific Reports},
      urldate = {2019-08-01},
      date = {2014-08-18},
      pages = {6097},
      author = {Das, Dipak Kumar and Makhal, Krishnandu and Bandyopadhyay, Soumendra Nath and Goswami, Debabrata}
    }