Debjit Roy

• Researchgate

Education / Work History

• Postdoctoral Research Associate, The University of Wisconsin–Madison (present)
• Ph.D. Scholar, IIT Kanpur (2016)

Thesis Abstract

Control and application of two photon fluorescence in microscopy and optical tweezers

The broad two photon absorption spectra of fluorophores along with the large spectral width of excitation laser pulses lead to the simultaneous excitation of different fluorophores. Control using selective excitation of chromophores is required to overcome such a problem. I performed such a control of TPF in microscopy using a single laser beam. Precise control of the delay between pairs of ultrafast laser pulses can effectively serve such a purpose by exploiting both the coherent vibrational wavepacket interferometry as well as the selective suppression of fluorescence by stimulated emission depletion. I also explored the effect of 20–50 GHz pulse-train on selective TPF suppression and a compared with the pulse-pair case. Optical trapping of tiny Rayleigh particles using high repetition rate femtosecond laser pulses occurs at a much lower average power as compared to that of a continuous wave laser. In this regard, I have shown the optical trapping of fluorophore coated latex bead spheres (diameter 100 nm) and fluorescent quantum dots at average powers of only a few tens of milliwatts. The gigantic peak power of the femtosecond laser pulses even at a very low average power enables such optical tweezing events. The TPF generated from the optically trapped Rayleigh particles by the trapping femtosecond laser pulses serves as a detection scheme for these Rayleigh particles. This is fortunate as they cannot be otherwise visualized easily under the conventional bright field imaging techniques because their sizes are below the diffraction limited spot size. I have also explored the effectiveness of TPF detection scheme in optical traps. Rather than just being a good background free detection scheme, I showed that TPF generated from the trapped objects can be used as their position marker. Extending on this, the optical tweezers system has been calibrated by measuring the trap stiffness using power spectrum method. When the sizes of the optically trapped objects are more than the laser beam focal spot size, partial illumination of the trapped objects occur. If the trapped objects are fluorophore coated, then the fluorophore molecules present only on the illuminated surface of the optically trapped objects undergo laser induced two photon absorption processes. Fluorescence energy transfer occurs from the fluorophore molecules present on the illuminated zone to those present on the non-illuminated zone of the trapped object surface. This leads to a counterintuitive decay of the two photon fluorescence signal with time. I have shown this phenomenon by optically trapping fluorophore coated polystyrene microspheres of diameters 1 μm, 2 μm and 4μm, respectively. A simple model that matches these experimental results has been proposed to explain this fact. The high gradient force of the optical tweezers arising from the tight focusing of the transverse Gaussian laser beam drags the particle towards the center of the optical trap. For the trapping of smaller particles, this would result in multiple particle trapping and as a consequence, an optically bound cluster would form. I have studied the structure and dynamics of such optically bound clusters formed by the optical trapping of multiple 0.5 μm diameter polystyrene microspheres that are fluorophore coated. This has been determined from the fluorescence resonance energy transfer process occurring between the fluorophores present on the illuminated and non-illuminated surface of the trapped microspheres using linearly versus circularly polarized trapping laser beams.

Research Topic / Interest

Contact me to know my current interests.

Publications

These include only those published in our lab.

1. Fluorophore Discrimination by Tracing Quantum Interference in Fluorescence Microscopy. A. K. De, D. Roy, and D. Goswami, Physical Review A 83(1), 015402 (2011) [Abstract] [PDF] [BibTeX]

   Abstract: We show fluorescence-detected quantum interference in a microscope setup and demonstrate selective enhancement or suppression of fluorophores using femtosecond pulse-pair excitation with periodic modulation of the interpulse phase.

    BibTeX: @article{deFluorophoreDiscriminationTracing2011,
     title = {Fluorophore Discrimination by Tracing Quantum Interference in Fluorescence Microscopy},
     author = {De, Arijit Kumar and Roy, Debjit and Goswami, Debabrata},
     date = {2011-01-21},
     journaltitle = {Physical Review A},
     shortjournal = {Phys. Rev. A},
     volume = {83},
     number = {1},
     pages = {015402},
     doi = {10/c73nhs},
     url = {https://link.aps.org/doi/10.1103/PhysRevA.83.015402},
     urldate = {2019-08-14}
   }
   

2. Selective Suppression of Two-Photon Fluorescence in Laser Scanning Microscopy by Ultrafast Pulse-Train Excitation. A. K. De, D. Roy, and D. Goswami, Journal of Biomedical Optics 15(6), 060502 (2010) [Abstract] [PDF] [BibTeX]

   Abstract: Selective excitation of a particular fluorophore in the presence of others demands clever design of the optical field interacting with the molecules. We describe the use of 20- to 50-GHz pulse-train excitation leading to two-photon absorption, followed by successive one-photon stimulated emission as a potential technique in the context of controlling two-photon molecular fluorescence, with applications in microscopy.

    BibTeX: @article{deSelectiveSuppressionTwophoton2010,
     title = {Selective Suppression of Two-Photon Fluorescence in Laser Scanning Microscopy by Ultrafast Pulse-Train Excitation},
     author = {De, Arijit Kumar and Roy, Debjit and Goswami, Debabrata},
     date = {2010-11},
     journaltitle = {Journal of Biomedical Optics},
     shortjournal = {JBO},
     volume = {15},
     number = {6},
     pages = {060502},
     issn = {1083-3668, 1560-2281},
     doi = {10/cp9rx2},
     url = {https://www.spiedigitallibrary.org/journals/Journal-of-Biomedical-Optics/volume-15/issue-6/060502/Selective-suppression-of-two-photon-fluorescence-in-laser-scanning-microscopy/10.1117/1.3509383.short},
     urldate = {2019-08-14}
   }
   

3. Selective Two-Photon Fluorescence Suppression by Ultrafast Pulse-Pair Excitation: Control by Selective One-Color Stimulated Emission. A. K. De, D. Roy, and D. Goswami, Journal of Biomedical Optics 16(10), 100505 (2011) [Abstract] [PDF] [BibTeX]

   Abstract: Controlling two-photon molecular fluorescence leading to selective fluorophore excitation has been a long sought after goal in fluorescence microscopy. In this letter, we thoroughly explore selective fluorescence suppression through simultaneous two-photon absorption by two different fluorophores followed by selective one-photon stimulated emission for one particular fluorophore. We achieve this by precisely controlling the time delay between two identical ultrafast near infrared laser pulses.

    BibTeX: @article{deSelectiveTwophotonFluorescence2011,
     title = {Selective Two-Photon Fluorescence Suppression by Ultrafast Pulse-Pair Excitation: Control by Selective One-Color Stimulated Emission},
     shorttitle = {Selective Two-Photon Fluorescence Suppression by Ultrafast Pulse-Pair Excitation},
     author = {De, Arijit K. and Roy, Debjit and Goswami, Debabrata},
     date = {2011-10},
     journaltitle = {Journal of Biomedical Optics},
     shortjournal = {JBO},
     volume = {16},
     number = {10},
     pages = {100505},
     issn = {1083-3668, 1560-2281},
     doi = {10/fqkzpp},
     url = {https://my.pcloud.com/publink/show?code=XZoNxr7Zgv99AAhjLt4lAwzfutsBwYsdJNvy},
     urldate = {2019-08-02}
   }
   

4. Spatio-Temporal Control in Multiphoton Fluorescence Laser-Scanning Microscopy. A. K. De, D. Roy, and D. Goswami, in Multiphoton Microscopy in the Biomedical Sciences X (International Society for Optics and Photonics, 2010), 7569, p. 756929 [Abstract] [PDF] [BibTeX]

   Abstract: The broad spectral window of an ultra-short laser pulse and the broad overlapping multiphoton absorption spectra of common fluorophores restrict selective excitation of one fluorophore in presence of others during multiphoton fluorescence microscopy. Also spatial resolution, limited by the fundamental diffraction limit, is governed by the beam profile. Here we show our recent work on selective fluorescence suppression using a femtosecond pulse-pair excitation which is equivalent to amplitude shaping using a pulse shaper. In addition, prospects of laser beam shaping in imaging are also briefly discussed.

    BibTeX: @inproceedings{deSpatiotemporalControlMultiphoton2010,
     title = {Spatio-Temporal Control in Multiphoton Fluorescence Laser-Scanning Microscopy},
     booktitle = {Multiphoton {{Microscopy}} in the {{Biomedical Sciences X}}},
     author = {De, Arijit Kumar and Roy, Debjit and Goswami, Debabrata},
     date = {2010-02-26},
     volume = {7569},
     pages = {756929},
     publisher = {{International Society for Optics and Photonics}},
     doi = {10/c529hw},
     url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/7569/756929/Spatio-temporal-control-in-multiphoton-fluorescence-laser-scanning-microscopy/10.1117/12.838287.short},
     urldate = {2019-08-14},
     eventtitle = {Multiphoton {{Microscopy}} in the {{Biomedical Sciences X}}}
   }
   

5. Stable Optical Trapping of Latex Nanoparticles with Ultrashort Pulsed Illumination. A. K. De, D. Roy, A. Dutta, and D. Goswami, Applied Optics 48(31), G33–G37 (2009) [Abstract] [PDF] [BibTeX]

   Abstract: Here we report how ultrafast pulsed illumination at low average power results in a stable three-dimensional (3D) optical trap holding latex nanoparticles which is otherwise not possible with continuous wave lasers at the same power level. The gigantic peak power of a femtosecond pulse exerts a huge instantaneous gradient force that has been predicted theoretically earlier and implemented for microsecond pulses in a different context by others. In addition, the resulting two-photon fluorescence allows direct observation of trapping events by providing intrinsic 3D resolution.

    BibTeX: @article{deStableOpticalTrapping2009,
     title = {Stable Optical Trapping of Latex Nanoparticles with Ultrashort Pulsed Illumination},
     author = {De, Arijit Kumar and Roy, Debjit and Dutta, Aveek and Goswami, Debabrata},
     date = {2009-11-01},
     journaltitle = {Applied Optics},
     shortjournal = {Appl. Opt., AO},
     volume = {48},
     number = {31},
     pages = {G33-G37},
     issn = {2155-3165},
     doi = {10/b9sxj4},
     url = {https://www.osapublishing.org/ao/abstract.cfm?uri=ao-48-31-G33},
     urldate = {2019-08-14}
   }
   

6. Towards Stable Trapping of Single Macromolecules in Solution. A. K. De, D. Roy, and D. Goswami, in Optical Trapping and Optical Micromanipulation VII (International Society for Optics and Photonics, 2010), 7762, p. 776203 [Abstract] [PDF] [BibTeX]

   Abstract: The implementation of high instantaneous peak power of a femtosecond laser pulse at moderate time-averaged power (~10 mW) to trap latex nanoparticles, which is otherwise impossible with continuous wave illumination at similar power level, has recently been shown [De, A. K., Roy, D., Dutta, A. and Goswami, D. "Stable optical trapping of latex nanoparticles with ultrashort pulsed illumination", Appd. Opt., 48, G33 (2009)]. However, direct measurement of the instantaneous trapping force/stiffness due to a single pulse has been unsuccessful due to the fleeting existence (~100 fs) of the laser pulse compared with the much slower time scale associated with the available trapping force/stiffness calibration techniques, as discussed in this proceeding article. We also demonstrate trapping of quantum dots having dimension similar to macromolecules.

    BibTeX: @inproceedings{deStableTrappingSingle2010,
     title = {Towards Stable Trapping of Single Macromolecules in Solution},
     booktitle = {Optical {{Trapping}} and {{Optical Micromanipulation VII}}},
     author = {De, Arijit Kumar and Roy, Debjit and Goswami, Debabrata},
     date = {2010-08-27},
     volume = {7762},
     pages = {776203},
     publisher = {{International Society for Optics and Photonics}},
     doi = {10/ckqvtz},
     url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/7762/776203/Towards-stable-trapping-of-single-macromolecules-in-solution/10.1117/12.862364.short},
     urldate = {2019-08-14},
     eventtitle = {Optical {{Trapping}} and {{Optical Micromanipulation VII}}}
   }
   

7. Two-Photon Fluorescence Diagnostics of Femtosecond Laser Tweezers. A. K. De, D. Roy, and D. Goswami, CURRENT SCIENCE 101(7), 4 (2011) [Abstract] [PDF] [BibTeX]

   Abstract: We show how two-photon fluorescence signal can be used as an effective detection scheme for trapping particles of any size in comparison to methods using back-scattered light. Development of such a diagnostic scheme allows us a direct observation of trapping a single nanoparticle, which shows new directions to spectroscopy at the single-molecule level in solution.

    BibTeX: @article{deTwophotonFluorescenceDiagnostics2011,
     title = {Two-Photon Fluorescence Diagnostics of Femtosecond Laser Tweezers},
     author = {De, Arijit Kumar and Roy, Debjit and Goswami, Debabrata},
     date = {2011},
     journaltitle = {CURRENT SCIENCE},
     volume = {101},
     number = {7},
     pages = {4},
     url = {https://my.pcloud.com/publink/show?code=XZyaxr7ZQuCnGQH0PFBeRMoMq6mRURASwEJk}
   }
   

8. Fluorescence Advantages with Microscopic Spatiotemporal Control. D. Goswami, D. Roy, and A. K. De, in Design and Quality for Biomedical Technologies VI (International Society for Optics and Photonics, 2013), 8573, p. 857302 [Abstract] [PDF] [BibTeX]

   Abstract: We present a clever design concept of using femtosecond laser pulses in microscopy by selective excitation or de-excitation of one fluorophore over the other overlapping one. Using either a simple pair of femtosecond pulses with variable delay or using a train of laser pulses at 20-50 Giga-Hertz excitation, we show controlled fluorescence excitation or suppression of one of the fluorophores with respect to the other through wave-packet interference, an effect that prevails even after the fluorophore coherence timescale. Such an approach can be used both under the single-photon excitation as well as in the multi-photon excitation conditions resulting in effective higher spatial resolution. Such high spatial resolution advantage with broadband-pulsed excitation is of immense benefit to multi-photon microscopy and can also be an effective detection scheme for trapped nanoparticles with near-infrared light. Such sub-diffraction limit trapping of nanoparticles is challenging and a two-photon fluorescence diagnostics allows a direct observation of a single nanoparticle in a femtosecond high-repetition rate laser trap, which promises new directions to spectroscopy at the single molecule level in solution. The gigantic peak power of femtosecond laser pulses at high repetition rate, even at low average powers, provide huge instantaneous gradient force that most effectively result in a stable optical trap for spatial control at sub-diffraction limit. Such studies have also enabled us to explore simultaneous control of internal and external degrees of freedom that require coupling of various control parameters to result in spatiotemporal control, which promises to be a versatile tool for the microscopic world.

    BibTeX: @inproceedings{goswamiFluorescenceAdvantagesMicroscopic2013,
     title = {Fluorescence Advantages with Microscopic Spatiotemporal Control},
     booktitle = {Design and {{Quality}} for {{Biomedical Technologies VI}}},
     author = {Goswami, Debabrata and Roy, Debjit and De, Arijit K.},
     date = {2013-03-13},
     volume = {8573},
     pages = {857302},
     publisher = {{International Society for Optics and Photonics}},
     doi = {10/gf5npw},
     url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/8573/857302/Fluorescence-advantages-with-microscopic-spatiotemporal-control/10.1117/12.2009500.short},
     urldate = {2019-08-02},
     eventtitle = {Design and {{Quality}} for {{Biomedical Technologies VI}}}
   }
   

9. Elucidating Two Photon FRET and Its Application through Femtosecond Optical Tweezers. D. Mondal, D. Roy, S. Dinda, A. Singh, and D. Goswami, in Advanced Photonics 2016 (IPR, NOMA, Sensors, Networks, SPPCom, SOF) (OSA, 2016), p. NoTu2D.4 [Abstract] [PDF] [BibTeX]

   Abstract: We have observed two photon fluorescence resonance energy transfer (FRET) from optically trapped bead coated with multiple dyes. The fluorescence obtained from trapped particles is useful to measure biomechanical property of that confined system.

    BibTeX: @inproceedings{mondalElucidatingTwoPhoton2016,
     title = {Elucidating {{Two Photon FRET}} and Its Application through Femtosecond Optical Tweezers},
     booktitle = {Advanced {{Photonics}} 2016 ({{IPR}}, {{NOMA}}, {{Sensors}}, {{Networks}}, {{SPPCom}}, {{SOF}})},
     author = {Mondal, Dipankar and Roy, Debjit and Dinda, Sirshendu and Singh, Ajitesh and Goswami, Debabrata},
     date = {2016},
     pages = {NoTu2D.4},
     publisher = {{OSA}},
     location = {{Vancouver}},
     doi = {10/gf5mr4},
     url = {https://www.osapublishing.org/abstract.cfm?URI=NOMA-2016-NoTu2D.4},
     urldate = {2019-08-01},
     eventtitle = {Novel {{Optical Materials}} and {{Applications}}},
     isbn = {978-1-943580-14-9}
   }
   

10. ‪Control of Selective Two-Photon Fluorescence Suppression by One-Color Pulse-Pair Excitation‬. D. Roy, A. K. De, and D. Goswami, Asian Journal of Physics 31(3-5), 475–478 (2022) [Abstract] [PDF] [BibTeX]

    Abstract: For studying the selective excitation of a particular fluorophore in fluorescence microscopy, the effect of the laser pulse chosen is immense. To study the ramifications of this selection, we present a thorough study of one-color stimulated emission depletion of the two-photon fluorescence of selective fluorophores by changing the wavelength and delay between two identical laser pulses.

     BibTeX: @article{royControlSelectiveTwophoton2022,
      title = {‪{{Control}} of Selective Two-Photon Fluorescence Suppression by One-Color Pulse-Pair Excitation‬},
      author = {Roy, Debjit and De, Arijit K. and Goswami, Debabrata},
      date = {2022},
      journaltitle = {Asian Journal of Physics},
      volume = {31},
      number = {3-5},
      pages = {475--478},
      url = {https://scholar.google.com/citations?view_op=view_citation&hl=en&user=OZ651FIAAAAJ&sortby=pubdate&citation_for_view=OZ651FIAAAAJ:jtI9f0ekYq0C},
      urldate = {2022-12-31}
    }
    

11. Elucidating Microscopic Structure and Dynamics in Optically Tweezed Environments. D. Roy, D. Mondal, and D. Goswami, Chemical Physics Letters 621, 203–208 (2015) [Abstract] [PDF] [BibTeX]

    Abstract: To probe the structure and dynamics of molecules under optical trapping conditions, we exploit the effect of femtosecond Fluorescence Resonance Energy Transfer (FRET) between dye molecules coated on the surface of polystyrene microspheres of various sizes suspended in water. The use of femtosecond laser pulses enables sensitive detection through two-photon fluorescence (TPF). Unlike conventional backscatter signal, the TPF signal shows a slow counterintuitive decay for the trapped microspheres when they are not fully within the laser illuminated volume. This decay is a characteristic sign of the occurrence of the FRET process. For microspheres with sizes less than the trapping focal volume, trapping of multiple particles can occur leading to the formation of optically bound clusters. Using different laser polarizations, we also extract information about the structure and dynamics of such optically bound clusters as a consequence of FRET.

     BibTeX: @article{royElucidatingMicroscopicStructure2015,
      title = {Elucidating Microscopic Structure and Dynamics in Optically Tweezed Environments},
      author = {Roy, Debjit and Mondal, Dipankar and Goswami, Debabrata},
      date = {2015-02-04},
      journaltitle = {Chemical Physics Letters},
      shortjournal = {Chemical Physics Letters},
      volume = {621},
      pages = {203--208},
      issn = {0009-2614},
      doi = {10/f6zd8x},
      url = {http://www.sciencedirect.com/science/article/pii/S0009261414009658},
      urldate = {2019-08-01}
    }
    

12. Exploring the Physics of Efficient Optical Trapping of Dielectric Nanoparticles with Ultrafast Pulsed Excitation. D. Roy, D. Goswami, and A. K. De, Applied Optics 54(23), 7002 (2015) [Abstract] [PDF] [BibTeX]

    Abstract: Stable optical trapping of dielectric nanoparticles with low power high-repetition-rate ultrafast pulsed excitation has received considerable attention in recent years. However, the exact role of such excitation has been quite illusive so far since, for dielectric micron-sized particles, the trapping efficiency turns out to be similar to that of continuous-wave excitation and independent of pulse chirping. In order to provide a coherent explanation of this apparently puzzling phenomenon, we justify the superior role of high-repetition-rate pulsed excitation in dielectric nanoparticle trapping which is otherwise not possible with continuous-wave excitation at a similar average power level. We quantitatively estimate the optimal combination of pulse peak power and pulse repetition rate leading to a stable trap and discuss the role of inertial response on the dependence of trapping efficiency on pulse width. In addition, we report gradual trapping of individual quantum dots detected by a stepwise rise in a two-photon fluorescence signal from the trapped quantum dots which conclusively proves individual particle trapping.

     BibTeX: @article{royExploringPhysicsEfficient2015,
      title = {Exploring the Physics of Efficient Optical Trapping of Dielectric Nanoparticles with Ultrafast Pulsed Excitation},
      author = {Roy, Debjit and Goswami, Debabrata and De, Arijit K.},
      date = {2015-08-10},
      journaltitle = {Applied Optics},
      shortjournal = {Appl. Opt.},
      volume = {54},
      number = {23},
      pages = {7002},
      issn = {0003-6935, 1539-4522},
      doi = {10/gf5msm},
      url = {https://www.osapublishing.org/abstract.cfm?URI=ao-54-23-7002},
      urldate = {2019-08-01}
    }
    

13. Polarization Modulated Ultrafast Pulse-Pair Control in Two-Photon Fluorescence Microscopy. D. Roy, A. K. De, and D. Goswami, in Frontiers in Optics 2011/Laser Science XXVII (OSA, 2011), p. JWA3 [Abstract] [PDF] [BibTeX]

    Abstract: Selective fluorescence suppression for two spectrally overlapping fluorophores that undergo simultaneous two-photon absorption from identical femtosecond pulses through precise control in relative timing and polarization of the exciting pulse-pair.

     BibTeX: @inproceedings{royPolarizationModulatedUltrafast2011,
      title = {Polarization Modulated {{Ultrafast Pulse-Pair Control}} in {{Two-Photon Fluorescence Microscopy}}},
      booktitle = {Frontiers in {{Optics}} 2011/{{Laser Science XXVII}}},
      author = {Roy, Debjit and De, Arijit Kumar and Goswami, Debabrata},
      date = {2011},
      pages = {JWA3},
      publisher = {{OSA}},
      location = {{San Jose, California}},
      doi = {10/gf5nqh},
      url = {https://www.osapublishing.org/abstract.cfm?URI=FiO-2011-JWA3},
      urldate = {2019-08-02},
      eventtitle = {Frontiers in {{Optics}}},
      isbn = {978-1-55752-917-6}
    }
    

14. Towards Spatio-Temporal Control in Optical Trapping. D. Roy, A. K. De, and D. Goswami, in Optical Trapping and Optical Micromanipulation VI (International Society for Optics and Photonics, 2009), 7400, p. 74000G [Abstract] [PDF] [BibTeX]

    Abstract: Using both continuous-wave (CW) and high repetition rate femtosecond lasers, we present stable 3-dimensional trapping of 1μm polystyrene microspheres. We also stably trapped 100nm latex nanoparticles using the femtosecond mode-locked laser at a very low average power where the CW lasers cannot trap, demonstrating the significance of the fleeting temporal existence of the femtosecond pulses. Trapping was visualized through dark-field microscopy as well as through a noise free detection using two-photon fluorescence as a diagnostics tool owing to its intrinsic 3- dimensional resolution. Comparison between a Gaussian versus a flat-top Gaussian beam profile demonstrates the importance of laser spatial mode in optical trapping.

     BibTeX: @inproceedings{roySpatiotemporalControlOptical2009,
      title = {Towards Spatio-Temporal Control in Optical Trapping},
      booktitle = {Optical {{Trapping}} and {{Optical Micromanipulation VI}}},
      author = {Roy, Debjit and De, Arijit Kumar and Goswami, Debabrata},
      date = {2009-08-20},
      volume = {7400},
      pages = {74000G},
      publisher = {{International Society for Optics and Photonics}},
      doi = {10/fpdhxs},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/7400/74000G/Towards-spatio-temporal-control-in-optical-trapping/10.1117/12.824372.short},
      urldate = {2019-08-14},
      eventtitle = {Optical {{Trapping}} and {{Optical Micromanipulation VI}}}
    }
    

15. Structure and Dynamics of Optically Directed Self-Assembly of Nanoparticles. D. Roy, D. Mondal, and D. Goswami, Scientific Reports 6, 23318 (2016) [Abstract] [PDF] [BibTeX]

    Abstract: Self-assembly of nanoparticles leading to the formation of colloidal clusters often serves as the representative analogue for understanding molecular assembly. Unravelling the in situ structure and dynamics of such clusters in liquid suspensions is highly challenging. Presently colloidal clusters are first isolated from their generating environment and then their structures are probed by light scattering methods. In order to measure the in situ structure and dynamics of colloidal clusters, we have generated them using the high-repetition-rate femtosecond laser pulse optical tweezer. Since the constituent of our dimer, trimer or tetramer clusters are 250 nm radius two-photon resonant fluorophore coated nanospheres under the optical trap, they inherently produce Two-Photon Fluorescence, which undergo intra-nanosphere Fluorescence Energy Transfer. This unique energy transfer signature, in turn, enables us to visualize structures and orientations of these colloidal clusters during the process of their formation and subsequent dynamics in a liquid suspension. We also show that due to shape-birefringence, orientation and structural control of these colloidal clusters are possible as the polarization of the trapping laser is changed from linear to circular. We thus report important progress in sampling the smallest possible aggregates of nanoparticles, dimers, trimers or tetramers, formed early in the self-assembly process.

     BibTeX: @article{royStructureDynamicsOptically2016,
      title = {Structure and Dynamics of Optically Directed Self-Assembly of Nanoparticles},
      author = {Roy, Debjit and Mondal, Dipankar and Goswami, Debabrata},
      date = {2016-03-23},
      journaltitle = {Scientific Reports},
      volume = {6},
      pages = {23318},
      issn = {2045-2322},
      doi = {10/f8fjqk},
      url = {https://www.nature.com/articles/srep23318},
      urldate = {2019-08-01}
    }
    

16. Two-Photon Fluorescence Tracking of Colloidal Clusters. D. Roy, D. Mondal, and D. Goswami, Journal of Fluorescence 26(4), 1271–1277 (2016) [Abstract] [PDF] [BibTeX]

    Abstract: In situ dynamics of colloidal cluster formation from nanoparticles is yet to be addressed. Using two-photon fluorescence (TPF) that has been amply used for single particle tracking, we demonstrate in situ measurement of effective three-dimensional optical trap stiffness of nanoparticles and their aggregates without using any position sensitive detector. Optical trap stiffness is an essential measure of the strength of an optical trap. TPF is a zero-background detection scheme and has excellent signal-to-noise-ratio, which can be easily extended to study the formation of colloidal cluster of nanospheres in the optical trapping regime. TPF tracking can successfully distinguish colloidal cluster from its monomer.

     BibTeX: @article{royTwoPhotonFluorescenceTracking2016,
      title = {Two-{{Photon Fluorescence Tracking}} of {{Colloidal Clusters}}},
      author = {Roy, Debjit and Mondal, Dipankar and Goswami, Debabrata},
      date = {2016-07-01},
      journaltitle = {Journal of Fluorescence},
      shortjournal = {J Fluoresc},
      volume = {26},
      number = {4},
      pages = {1271--1277},
      issn = {1573-4994},
      doi = {10/f82fs3},
      url = {https://doi.org/10.1007/s10895-016-1814-3},
      urldate = {2019-08-01}
    }
    

17. Stable Optical Trapping of Latex Nanoparticles with Ultrashort Pulsed Illumination. A. K. De, D. Roy, A. Dutta, and D. Goswami, Appl. Opt., AO 48(31), G33–G37 (2009) [Abstract] [PDF] [BibTeX]

    Abstract: Here we report how ultrafast pulsed illumination at low average power results in a stable three-dimensional (3D) optical trap holding latex nanoparticles which is otherwise not possible with continuous wave lasers at the same power level. The gigantic peak power of a femtosecond pulse exerts a huge instantaneous gradient force that has been predicted theoretically earlier and implemented for microsecond pulses in a different context by others. In addition, the resulting two-photon fluorescence allows direct observation of trapping events by providing intrinsic 3D resolution.

     BibTeX: @article{deStableOpticalTrapping2010,
      langid = {english},
      title = {Stable Optical Trapping of Latex Nanoparticles with Ultrashort Pulsed Illumination},
      volume = {48},
      issn = {2155-3165},
      url = {https://www.osapublishing.org/ao/abstract.cfm?uri=ao-48-31-G33},
      doi = {10/b9sxj4},
      number = {31},
      journaltitle = {Appl. Opt., AO},
      urldate = {2019-08-14},
      date = {2009-11-01},
      pages = {G33-G37},
      author = {De, Arijit Kumar and Roy, Debjit and Dutta, Aveek and Goswami, Debabrata}
    }
    

18. Towards Spatio-Temporal Control in Optical Trapping. D. Roy, A. K. De, and D. Goswami, in Optical Trapping and Optical Micromanipulation VI (International Society for Optics and Photonics, 2009), 7400, p. 74000G [Abstract] [PDF] [BibTeX]

    Abstract: Using both continuous-wave (CW) and high repetition rate femtosecond lasers, we present stable 3-dimensional trapping of 1μm polystyrene microspheres. We also stably trapped 100nm latex nanoparticles using the femtosecond mode-locked laser at a very low average power where the CW lasers cannot trap, demonstrating the significance of the fleeting temporal existence of the femtosecond pulses. Trapping was visualized through dark-field microscopy as well as through a noise free detection using two-photon fluorescence as a diagnostics tool owing to its intrinsic 3- dimensional resolution. Comparison between a Gaussian versus a flat-top Gaussian beam profile demonstrates the importance of laser spatial mode in optical trapping.

     BibTeX: @inproceedings{roySpatiotemporalControlOptical2010,
      title = {Towards Spatio-Temporal Control in Optical Trapping},
      volume = {7400},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/7400/74000G/Towards-spatio-temporal-control-in-optical-trapping/10.1117/12.824372.short},
      doi = {10/fpdhxs},
      eventtitle = {Optical {{Trapping}} and {{Optical Micromanipulation VI}}},
      booktitle = {Optical {{Trapping}} and {{Optical Micromanipulation VI}}},
      publisher = {{International Society for Optics and Photonics}},
      urldate = {2019-08-14},
      date = {2009-08-20},
      pages = {74000G},
      author = {Roy, Debjit and De, Arijit Kumar and Goswami, Debabrata}
    }
    

19. Two-Photon Fluorescence Tracking of Colloidal Clusters. D. Roy, D. Mondal, and D. Goswami, J Fluoresc 26(4), 1271–1277 (2016) [Abstract] [PDF] [BibTeX]

    Abstract: In situ dynamics of colloidal cluster formation from nanoparticles is yet to be addressed. Using two-photon fluorescence (TPF) that has been amply used for single particle tracking, we demonstrate in situ measurement of effective three-dimensional optical trap stiffness of nanoparticles and their aggregates without using any position sensitive detector. Optical trap stiffness is an essential measure of the strength of an optical trap. TPF is a zero-background detection scheme and has excellent signal-to-noise-ratio, which can be easily extended to study the formation of colloidal cluster of nanospheres in the optical trapping regime. TPF tracking can successfully distinguish colloidal cluster from its monomer.

     BibTeX: @article{royTwoPhotonFluorescenceTracking2017,
      langid = {english},
      title = {Two-{{Photon Fluorescence Tracking}} of {{Colloidal Clusters}}},
      volume = {26},
      issn = {1573-4994},
      url = {https://doi.org/10.1007/s10895-016-1814-3},
      doi = {10.1007/s10895-016-1814-3},
      number = {4},
      journaltitle = {J Fluoresc},
      urldate = {2019-08-01},
      date = {2016-07-01},
      pages = {1271-1277},
      author = {Roy, Debjit and Mondal, Dipankar and Goswami, Debabrata}
    }
    

20. Elucidating Two Photon FRET and Its Application through Femtosecond Optical Tweezers. D. Mondal, D. Roy, S. Dinda, A. Singh, and D. Goswami, in Advanced Photonics 2016 (IPR, NOMA, Sensors, Networks, SPPCom, SOF) (OSA, 2016), p. NoTu2D.4 [Abstract] [PDF] [BibTeX]

    Abstract: We have observed two photon fluorescence resonance energy transfer (FRET) from optically trapped bead coated with multiple dyes. The fluorescence obtained from trapped particles is useful to measure biomechanical property of that confined system.

     BibTeX: @inproceedings{mondalElucidatingTwoPhoton2017,
      langid = {english},
      location = {{Vancouver}},
      title = {Elucidating {{Two Photon FRET}} and Its Application through Femtosecond Optical Tweezers},
      isbn = {978-1-943580-14-9},
      url = {https://www.osapublishing.org/abstract.cfm?URI=NOMA-2016-NoTu2D.4},
      doi = {10/gf5mr4},
      eventtitle = {Novel {{Optical Materials}} and {{Applications}}},
      booktitle = {Advanced {{Photonics}} 2016 ({{IPR}}, {{NOMA}}, {{Sensors}}, {{Networks}}, {{SPPCom}}, {{SOF}})},
      publisher = {{OSA}},
      urldate = {2019-08-01},
      date = {2016},
      pages = {NoTu2D.4},
      author = {Mondal, Dipankar and Roy, Debjit and Dinda, Sirshendu and Singh, Ajitesh and Goswami, Debabrata}
    }
    

21. Structure and Dynamics of Optically Directed Self-Assembly of Nanoparticles. D. Roy, D. Mondal, and D. Goswami, Scientific Reports 6, 23318 (2016) [Abstract] [PDF] [BibTeX]

    Abstract: Self-assembly of nanoparticles leading to the formation of colloidal clusters often serves as the representative analogue for understanding molecular assembly. Unravelling the in situ structure and dynamics of such clusters in liquid suspensions is highly challenging. Presently colloidal clusters are first isolated from their generating environment and then their structures are probed by light scattering methods. In order to measure the in situ structure and dynamics of colloidal clusters, we have generated them using the high-repetition-rate femtosecond laser pulse optical tweezer. Since the constituent of our dimer, trimer or tetramer clusters are 250 nm radius two-photon resonant fluorophore coated nanospheres under the optical trap, they inherently produce Two-Photon Fluorescence, which undergo intra-nanosphere Fluorescence Energy Transfer. This unique energy transfer signature, in turn, enables us to visualize structures and orientations of these colloidal clusters during the process of their formation and subsequent dynamics in a liquid suspension. We also show that due to shape-birefringence, orientation and structural control of these colloidal clusters are possible as the polarization of the trapping laser is changed from linear to circular. We thus report important progress in sampling the smallest possible aggregates of nanoparticles, dimers, trimers or tetramers, formed early in the self-assembly process.

     BibTeX: @article{royStructureDynamicsOptically2017,
      langid = {english},
      title = {Structure and Dynamics of Optically Directed Self-Assembly of Nanoparticles},
      volume = {6},
      issn = {2045-2322},
      url = {https://www.nature.com/articles/srep23318},
      doi = {10/f8fjqk},
      journaltitle = {Scientific Reports},
      urldate = {2019-08-01},
      date = {2016-03-23},
      pages = {23318},
      author = {Roy, Debjit and Mondal, Dipankar and Goswami, Debabrata}
    }
    

22. Exploring the Physics of Efficient Optical Trapping of Dielectric Nanoparticles with Ultrafast Pulsed Excitation. D. Roy, D. Goswami, and A. K. De, Appl. Opt. 54(23), 7002 (2015) [Abstract] [PDF] [BibTeX]

    Abstract: Stable optical trapping of dielectric nanoparticles with low power high-repetition-rate ultrafast pulsed excitation has received considerable attention in recent years. However, the exact role of such excitation has been quite illusive so far since, for dielectric micron-sized particles, the trapping efficiency turns out to be similar to that of continuous-wave excitation and independent of pulse chirping. In order to provide a coherent explanation of this apparently puzzling phenomenon, we justify the superior role of high-repetition-rate pulsed excitation in dielectric nanoparticle trapping which is otherwise not possible with continuous-wave excitation at a similar average power level. We quantitatively estimate the optimal combination of pulse peak power and pulse repetition rate leading to a stable trap and discuss the role of inertial response on the dependence of trapping efficiency on pulse width. In addition, we report gradual trapping of individual quantum dots detected by a stepwise rise in a two-photon fluorescence signal from the trapped quantum dots which conclusively proves individual particle trapping.

     BibTeX: @article{royExploringPhysicsEfficient2016,
      langid = {english},
      title = {Exploring the Physics of Efficient Optical Trapping of Dielectric Nanoparticles with Ultrafast Pulsed Excitation},
      volume = {54},
      issn = {0003-6935, 1539-4522},
      url = {https://www.osapublishing.org/abstract.cfm?URI=ao-54-23-7002},
      doi = {10/gf5msm},
      number = {23},
      journaltitle = {Appl. Opt.},
      urldate = {2019-08-01},
      date = {2015-08-10},
      pages = {7002},
      author = {Roy, Debjit and Goswami, Debabrata and De, Arijit K.}
    }
    

23. Elucidating Microscopic Structure and Dynamics in Optically Tweezed Environments. D. Roy, D. Mondal, and D. Goswami, Chemical Physics Letters 621, 203–208 (2015) [Abstract] [PDF] [BibTeX]

    Abstract: To probe the structure and dynamics of molecules under optical trapping conditions, we exploit the effect of femtosecond Fluorescence Resonance Energy Transfer (FRET) between dye molecules coated on the surface of polystyrene microspheres of various sizes suspended in water. The use of femtosecond laser pulses enables sensitive detection through two-photon fluorescence (TPF). Unlike conventional backscatter signal, the TPF signal shows a slow counterintuitive decay for the trapped microspheres when they are not fully within the laser illuminated volume. This decay is a characteristic sign of the occurrence of the FRET process. For microspheres with sizes less than the trapping focal volume, trapping of multiple particles can occur leading to the formation of optically bound clusters. Using different laser polarizations, we also extract information about the structure and dynamics of such optically bound clusters as a consequence of FRET.

     BibTeX: @article{royElucidatingMicroscopicStructure2016,
      title = {Elucidating Microscopic Structure and Dynamics in Optically Tweezed Environments},
      volume = {621},
      issn = {0009-2614},
      url = {http://www.sciencedirect.com/science/article/pii/S0009261414009658},
      doi = {10/f6zd8x},
      journaltitle = {Chemical Physics Letters},
      urldate = {2019-08-01},
      date = {2015-02-04},
      pages = {203-208},
      author = {Roy, Debjit and Mondal, Dipankar and Goswami, Debabrata}
    }
    

24. Fluorescence Advantages with Microscopic Spatiotemporal Control. D. Goswami, D. Roy, and A. K. De, in Design and Quality for Biomedical Technologies VI (International Society for Optics and Photonics, 2013), 8573, p. 857302 [Abstract] [PDF] [BibTeX]

    Abstract: We present a clever design concept of using femtosecond laser pulses in microscopy by selective excitation or de-excitation of one fluorophore over the other overlapping one. Using either a simple pair of femtosecond pulses with variable delay or using a train of laser pulses at 20-50 Giga-Hertz excitation, we show controlled fluorescence excitation or suppression of one of the fluorophores with respect to the other through wave-packet interference, an effect that prevails even after the fluorophore coherence timescale. Such an approach can be used both under the single-photon excitation as well as in the multi-photon excitation conditions resulting in effective higher spatial resolution. Such high spatial resolution advantage with broadband-pulsed excitation is of immense benefit to multi-photon microscopy and can also be an effective detection scheme for trapped nanoparticles with near-infrared light. Such sub-diffraction limit trapping of nanoparticles is challenging and a two-photon fluorescence diagnostics allows a direct observation of a single nanoparticle in a femtosecond high-repetition rate laser trap, which promises new directions to spectroscopy at the single molecule level in solution. The gigantic peak power of femtosecond laser pulses at high repetition rate, even at low average powers, provide huge instantaneous gradient force that most effectively result in a stable optical trap for spatial control at sub-diffraction limit. Such studies have also enabled us to explore simultaneous control of internal and external degrees of freedom that require coupling of various control parameters to result in spatiotemporal control, which promises to be a versatile tool for the microscopic world.

     BibTeX: @inproceedings{goswamiFluorescenceAdvantagesMicroscopic2014,
      title = {Fluorescence Advantages with Microscopic Spatiotemporal Control},
      volume = {8573},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/8573/857302/Fluorescence-advantages-with-microscopic-spatiotemporal-control/10.1117/12.2009500.short},
      doi = {10/gf5npw},
      eventtitle = {Design and {{Quality}} for {{Biomedical Technologies VI}}},
      booktitle = {Design and {{Quality}} for {{Biomedical Technologies VI}}},
      publisher = {{International Society for Optics and Photonics}},
      urldate = {2019-08-02},
      date = {2013-03-13},
      pages = {857302},
      author = {Goswami, Debabrata and Roy, Debjit and De, Arijit K.}
    }
    

25. Polarization Modulated Ultrafast Pulse-Pair Control in Two-Photon Fluorescence Microscopy. D. Roy, A. K. De, and D. Goswami, in Frontiers in Optics 2011/Laser Science XXVII (OSA, 2011), p. JWA3 [Abstract] [PDF] [BibTeX]

    Abstract: Selective fluorescence suppression for two spectrally overlapping fluorophores that undergo simultaneous two-photon absorption from identical femtosecond pulses through precise control in relative timing and polarization of the exciting pulse-pair.

     BibTeX: @inproceedings{royPolarizationModulatedUltrafast2012,
      langid = {english},
      location = {{San Jose, California}},
      title = {Polarization Modulated {{Ultrafast Pulse}}-{{Pair Control}} in {{Two}}-{{Photon Fluorescence Microscopy}}},
      isbn = {978-1-55752-917-6},
      url = {https://www.osapublishing.org/abstract.cfm?URI=FiO-2011-JWA3},
      doi = {10/gf5nqh},
      eventtitle = {Frontiers in {{Optics}}},
      booktitle = {Frontiers in {{Optics}} 2011/{{Laser Science XXVII}}},
      publisher = {{OSA}},
      urldate = {2019-08-02},
      date = {2011},
      pages = {JWA3},
      author = {Roy, Debjit and De, Arijit Kumar and Goswami, Debabrata}
    }
    

26. Two-Photon Fluorescence Diagnostics of Femtosecond Laser Tweezers. A. K. De, D. Roy, and D. Goswami, CURRENT SCIENCE 101(7), 4 (2011) [Abstract] [PDF] [BibTeX]

    Abstract: We show how two-photon fluorescence signal can be used as an effective detection scheme for trapping particles of any size in comparison to methods using back-scattered light. Development of such a diagnostic scheme allows us a direct observation of trapping a single nanoparticle, which shows new directions to spectroscopy at the single-molecule level in solution.

     BibTeX: @article{deTwophotonFluorescenceDiagnostics2012,
      langid = {english},
      title = {Two-Photon Fluorescence Diagnostics of Femtosecond Laser Tweezers},
      volume = {101},
      url = {https://my.pcloud.com/publink/show?code=XZyaxr7ZQuCnGQH0PFBeRMoMq6mRURASwEJk},
      number = {7},
      journaltitle = {CURRENT SCIENCE},
      date = {2011},
      pages = {4},
      author = {De, Arijit Kumar and Roy, Debjit and Goswami, Debabrata}
    }
    

27. Selective Two-Photon Fluorescence Suppression by Ultrafast Pulse-Pair Excitation: Control by Selective One-Color Stimulated Emission. A. K. De, D. Roy, and D. Goswami, JBO 16(10), 100505 (2011) [Abstract] [PDF] [BibTeX]

    Abstract: Controlling two-photon molecular fluorescence leading to selective fluorophore excitation has been a long sought after goal in fluorescence microscopy. In this letter, we thoroughly explore selective fluorescence suppression through simultaneous two-photon absorption by two different fluorophores followed by selective one-photon stimulated emission for one particular fluorophore. We achieve this by precisely controlling the time delay between two identical ultrafast near infrared laser pulses.

     BibTeX: @article{deSelectiveTwophotonFluorescence2012,
      title = {Selective Two-Photon Fluorescence Suppression by Ultrafast Pulse-Pair Excitation: Control by Selective One-Color Stimulated Emission},
      volume = {16},
      issn = {1083-3668, 1560-2281},
      url = {https://my.pcloud.com/publink/show?code=XZoNxr7Zgv99AAhjLt4lAwzfutsBwYsdJNvy},
      doi = {10/fqkzpp},
      shorttitle = {Selective Two-Photon Fluorescence Suppression by Ultrafast Pulse-Pair Excitation},
      number = {10},
      journaltitle = {JBO},
      urldate = {2019-08-02},
      date = {2011-10},
      pages = {100505},
      author = {De, Arijit K. and Roy, Debjit and Goswami, Debabrata}
    }
    

28. Fluorophore Discrimination by Tracing Quantum Interference in Fluorescence Microscopy. A. K. De, D. Roy, and D. Goswami, Phys. Rev. A 83(1), 015402 (2011) [Abstract] [PDF] [BibTeX]

    Abstract: We show fluorescence-detected quantum interference in a microscope setup and demonstrate selective enhancement or suppression of fluorophores using femtosecond pulse-pair excitation with periodic modulation of the interpulse phase.

     BibTeX: @article{deFluorophoreDiscriminationTracing2012,
      title = {Fluorophore Discrimination by Tracing Quantum Interference in Fluorescence Microscopy},
      volume = {83},
      url = {https://link.aps.org/doi/10.1103/PhysRevA.83.015402},
      doi = {10/c73nhs},
      number = {1},
      journaltitle = {Phys. Rev. A},
      urldate = {2019-08-14},
      date = {2011-01-21},
      pages = {015402},
      author = {De, Arijit Kumar and Roy, Debjit and Goswami, Debabrata}
    }
    

29. Selective Suppression of Two-Photon Fluorescence in Laser Scanning Microscopy by Ultrafast Pulse-Train Excitation. A. K. De, D. Roy, and D. Goswami, JBO 15(6), 060502 (2010) [Abstract] [PDF] [BibTeX]

    Abstract: Selective excitation of a particular fluorophore in the presence of others demands clever design of the optical field interacting with the molecules. We describe the use of 20- to 50-GHz pulse-train excitation leading to two-photon absorption, followed by successive one-photon stimulated emission as a potential technique in the context of controlling two-photon molecular fluorescence, with applications in microscopy.

     BibTeX: @article{deSelectiveSuppressionTwophoton2011,
      title = {Selective Suppression of Two-Photon Fluorescence in Laser Scanning Microscopy by Ultrafast Pulse-Train Excitation},
      volume = {15},
      issn = {1083-3668, 1560-2281},
      url = {https://www.spiedigitallibrary.org/journals/Journal-of-Biomedical-Optics/volume-15/issue-6/060502/Selective-suppression-of-two-photon-fluorescence-in-laser-scanning-microscopy/10.1117/1.3509383.short},
      doi = {10/cp9rx2},
      number = {6},
      journaltitle = {JBO},
      urldate = {2019-08-14},
      date = {2010-11},
      pages = {060502},
      author = {De, Arijit Kumar and Roy, Debjit and Goswami, Debabrata}
    }
    

30. Towards Stable Trapping of Single Macromolecules in Solution. A. K. De, D. Roy, and D. Goswami, in Optical Trapping and Optical Micromanipulation VII (International Society for Optics and Photonics, 2010), 7762, p. 776203 [Abstract] [PDF] [BibTeX]

    Abstract: The implementation of high instantaneous peak power of a femtosecond laser pulse at moderate time-averaged power (~10 mW) to trap latex nanoparticles, which is otherwise impossible with continuous wave illumination at similar power level, has recently been shown [De, A. K., Roy, D., Dutta, A. and Goswami, D. "Stable optical trapping of latex nanoparticles with ultrashort pulsed illumination", Appd. Opt., 48, G33 (2009)]. However, direct measurement of the instantaneous trapping force/stiffness due to a single pulse has been unsuccessful due to the fleeting existence (~100 fs) of the laser pulse compared with the much slower time scale associated with the available trapping force/stiffness calibration techniques, as discussed in this proceeding article. We also demonstrate trapping of quantum dots having dimension similar to macromolecules.

     BibTeX: @inproceedings{deStableTrappingSingle2011,
      title = {Towards Stable Trapping of Single Macromolecules in Solution},
      volume = {7762},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/7762/776203/Towards-stable-trapping-of-single-macromolecules-in-solution/10.1117/12.862364.short},
      doi = {10/ckqvtz},
      eventtitle = {Optical {{Trapping}} and {{Optical Micromanipulation VII}}},
      booktitle = {Optical {{Trapping}} and {{Optical Micromanipulation VII}}},
      publisher = {{International Society for Optics and Photonics}},
      urldate = {2019-08-14},
      date = {2010-08-27},
      pages = {776203},
      author = {De, Arijit Kumar and Roy, Debjit and Goswami, Debabrata}
    }
    

31. Spatio-Temporal Control in Multiphoton Fluorescence Laser-Scanning Microscopy. A. K. De, D. Roy, and D. Goswami, in Multiphoton Microscopy in the Biomedical Sciences X (International Society for Optics and Photonics, 2010), 7569, p. 756929 [Abstract] [PDF] [BibTeX]

    Abstract: The broad spectral window of an ultra-short laser pulse and the broad overlapping multiphoton absorption spectra of common fluorophores restrict selective excitation of one fluorophore in presence of others during multiphoton fluorescence microscopy. Also spatial resolution, limited by the fundamental diffraction limit, is governed by the beam profile. Here we show our recent work on selective fluorescence suppression using a femtosecond pulse-pair excitation which is equivalent to amplitude shaping using a pulse shaper. In addition, prospects of laser beam shaping in imaging are also briefly discussed.

     BibTeX: @inproceedings{deSpatiotemporalControlMultiphoton2011,
      title = {Spatio-Temporal Control in Multiphoton Fluorescence Laser-Scanning Microscopy},
      volume = {7569},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/7569/756929/Spatio-temporal-control-in-multiphoton-fluorescence-laser-scanning-microscopy/10.1117/12.838287.short},
      doi = {10/c529hw},
      eventtitle = {Multiphoton {{Microscopy}} in the {{Biomedical Sciences X}}},
      booktitle = {Multiphoton {{Microscopy}} in the {{Biomedical Sciences X}}},
      publisher = {{International Society for Optics and Photonics}},
      urldate = {2019-08-14},
      date = {2010-02-26},
      pages = {756929},
      author = {De, Arijit Kumar and Roy, Debjit and Goswami, Debabrata}
    }
    

32. Stable Optical Trapping of Latex Nanoparticles with Ultrashort Pulsed Illumination. A. K. De, D. Roy, A. Dutta, and D. Goswami, Appl. Opt., AO 48(31), G33–G37 (2009) [Abstract] [PDF] [BibTeX]

    Abstract: Here we report how ultrafast pulsed illumination at low average power results in a stable three-dimensional (3D) optical trap holding latex nanoparticles which is otherwise not possible with continuous wave lasers at the same power level. The gigantic peak power of a femtosecond pulse exerts a huge instantaneous gradient force that has been predicted theoretically earlier and implemented for microsecond pulses in a different context by others. In addition, the resulting two-photon fluorescence allows direct observation of trapping events by providing intrinsic 3D resolution.

     BibTeX: @article{deStableOpticalTrapping2011,
      langid = {english},
      title = {Stable Optical Trapping of Latex Nanoparticles with Ultrashort Pulsed Illumination},
      volume = {48},
      issn = {2155-3165},
      url = {https://www.osapublishing.org/ao/abstract.cfm?uri=ao-48-31-G33},
      doi = {10/b9sxj4},
      number = {31},
      journaltitle = {Appl. Opt., AO},
      urldate = {2019-08-14},
      date = {2009-11-01},
      pages = {G33-G37},
      author = {De, Arijit Kumar and Roy, Debjit and Dutta, Aveek and Goswami, Debabrata}
    }
    

33. Towards Spatio-Temporal Control in Optical Trapping. D. Roy, A. K. De, and D. Goswami, in Optical Trapping and Optical Micromanipulation VI (International Society for Optics and Photonics, 2009), 7400, p. 74000G [Abstract] [PDF] [BibTeX]

    Abstract: Using both continuous-wave (CW) and high repetition rate femtosecond lasers, we present stable 3-dimensional trapping of 1μm polystyrene microspheres. We also stably trapped 100nm latex nanoparticles using the femtosecond mode-locked laser at a very low average power where the CW lasers cannot trap, demonstrating the significance of the fleeting temporal existence of the femtosecond pulses. Trapping was visualized through dark-field microscopy as well as through a noise free detection using two-photon fluorescence as a diagnostics tool owing to its intrinsic 3- dimensional resolution. Comparison between a Gaussian versus a flat-top Gaussian beam profile demonstrates the importance of laser spatial mode in optical trapping.

     BibTeX: @inproceedings{roySpatiotemporalControlOptical2011,
      title = {Towards Spatio-Temporal Control in Optical Trapping},
      volume = {7400},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/7400/74000G/Towards-spatio-temporal-control-in-optical-trapping/10.1117/12.824372.short},
      doi = {10/fpdhxs},
      eventtitle = {Optical {{Trapping}} and {{Optical Micromanipulation VI}}},
      booktitle = {Optical {{Trapping}} and {{Optical Micromanipulation VI}}},
      publisher = {{International Society for Optics and Photonics}},
      urldate = {2019-08-14},
      date = {2009-08-20},
      pages = {74000G},
      author = {Roy, Debjit and De, Arijit Kumar and Goswami, Debabrata}
    }