Dipankar Mondal

• Researchgate

Education / Work History

• Postdoctoral Research Associate, State University of New York (2017-present)
• Ph.D. Scholar, IIT Kanpur (2017)
• M.Sc., IIT Kanpur (2008-2010)

Thesis Abstract

Application of Contact less Micromanipulation using One- and Two- color Fermtosecond Optical Tweezers

Femtosecond optical tweezers (FOT) can trap micron to nano sized particles and measure nano to piconewton forces with high peak power under optically benign condition. I have designed and built two FOT set up with inverted microscope geometry one of them is one color FOT and other is two-color FOT. This newly developed FOT uses a femtosecond dual beam pulsed tweezers with an IR laser at 1560 nm as a heating source as well as a single color FOT with NIR laser centered at 780 nm to control the trapped volume temperature in bio medium. I have explored the applications of FOT to control and measure temperature and the corresponding microrheological properties by probing Brownian motion. Due to temperature rise there is continuous thermal energy transfer from solvent to the trapped bead that changes its kinetic energy. I also have elucidated the nonlinear effect of laser polarization on trapping stiffness in polar binary mixture. The non-ideal mixing of water-alcohol also exhibit nonlinear concentration dependent micro-viscosity changes. I have performed absolute calibration both in frequency domain as well as time domain. Absolute calibration with power spectrum method, which is a frequency domain measurement, resolution is restricted by measurement time. On the other hand, our newly developed on the fly absolute calibration combined with normalized position autocorrelation method (NPAF) and equipartition theorem has higher resolution and can measure absolute viscosity of Newtonian as well as non-Newtonian fluids. The advantage of pulsed laser based tweezers has also been explored in determining structure and dynamics of nanoparticle clusters through two-photon fluorescence (TPF).

Research Topic / Interest

Contact me to know my current interests.

Publications

These include only those published in our lab.

1. Calibration of Femtosecond Optical Tweezers as a Sensitive Thermometer. D. Mondal and D. Goswami, in Optical Trapping and Optical Micromanipulation XII (International Society for Optics and Photonics, 2015), 9548, p. 95481N [Abstract] [PDF] [BibTeX]

   Abstract: We present cumulative perturbation effects of femtosecond laser pulses on an optical tweezer. Our experiments involve a dual wavelength high repetition rate femtosecond laser, one at the non-heating wavelength of 780 nm while the other at 1560 nm to cause heating in the trapped volume under low power (100-800 μW) conditions. The 1560 nm high repetition rate laser acts as a resonant excitation source for the vibrational combination band of the hydroxyl group (OH) of water, which helps create the local heating effortlessly within the trapping volume. With such an experimental system, we are the first to observe direct effect of temperature on the corner frequency deduced from power spectrum. We can, thus, control and measure temperature precisely at the optical trap. This observation has lead us to calculate viscosity as well as temperature in the vicinity of the trapping zone. These experimental results also support the well-known fact that the nature of Brownian motion is the response of the optically trapped bead from the temperature change of surroundings. Temperature rise near the trapping zone can significantly change the viscosity of the medium. However, we notice that though the temperature and viscosity are changing as per our corner frequency calculations, the trap stiffness remains the same throughout our experiments within the temperature range of about 20 K.

    BibTeX: @inproceedings{mondalCalibrationFemtosecondOptical2015,
     title = {Calibration of Femtosecond Optical Tweezers as a Sensitive Thermometer},
     booktitle = {Optical {{Trapping}} and {{Optical Micromanipulation XII}}},
     author = {Mondal, Dipankar and Goswami, Debabrata},
     date = {2015-08-25},
     volume = {9548},
     pages = {95481N},
     publisher = {{International Society for Optics and Photonics}},
     doi = {10/gf5msk},
     url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/9548/95481N/Calibration-of-femtosecond-optical-tweezers-as-a-sensitive-thermometer/10.1117/12.2187097.short},
     urldate = {2019-08-01},
     eventtitle = {Optical {{Trapping}} and {{Optical Micromanipulation XII}}}
   }
   

2. Controlling the Effect on Solvent by Resonant Excitation in Femtosecond Optical Tweezer. D. Mondal and D. Goswami, in Optics in the Life Sciences (OSA, 2015), p. OtT4E.3 [Abstract] [PDF] [BibTeX]

   Abstract: Observation of direct effect of temperature rise in water around optical trap center is presented. Irradiation of IR laser with low power during pulsed 780 nm trapping significantly changes solvent viscosity keeping trap stiffness unaffected.

    BibTeX: @inproceedings{mondalControllingEffectSolvent2015,
     title = {Controlling the Effect on Solvent by Resonant Excitation in Femtosecond Optical Tweezer},
     booktitle = {Optics in the {{Life Sciences}}},
     author = {Mondal, Dipankar and Goswami, Debabrata},
     date = {2015},
     pages = {OtT4E.3},
     publisher = {{OSA}},
     location = {{Vancouver}},
     doi = {10/gf5msw},
     url = {https://www.osapublishing.org/abstract.cfm?URI=OTA-2015-OtT4E.3},
     urldate = {2019-08-01},
     eventtitle = {Optical {{Trapping Applications}}},
     isbn = {978-1-55752-954-1}
   }
   

3. Controlling Local Temperature in Water Using Femtosecond Optical Tweezer. D. Mondal and D. Goswami, Biomedical Optics Express 6(9), 3190–3196 (2015) [Abstract] [PDF] [BibTeX]

   Abstract: A novel method of directly observing the effect of temperature rise in water at the vicinity of optical trap center is presented. Our approach relies on changed values of corner frequency of the optical trap that, in turn, is realized from its power spectra. Our two color experiment is a unique combination of a non-heating femtosecond trapping laser at 780 nm, coupled to a femtosecond infrared heating laser at 1560 nm, which precisely controls temperature at focal volume of the trap center using low powers (100-800 µW) at high repetition rate. The geometric ray optics model quantitatively supports our experimental data.

    BibTeX: @article{mondalControllingLocalTemperature2015,
     title = {Controlling Local Temperature in Water Using Femtosecond Optical Tweezer},
     author = {Mondal, Dipankar and Goswami, Debabrata},
     date = {2015-09-01},
     journaltitle = {Biomedical Optics Express},
     shortjournal = {Biomed. Opt. Express, BOE},
     volume = {6},
     number = {9},
     pages = {3190--3196},
     issn = {2156-7085},
     doi = {10/gf5msh},
     url = {https://www.osapublishing.org/boe/abstract.cfm?uri=boe-6-9-3190},
     urldate = {2019-08-01}
   }
   

4. Controlling and Tracking of Colloidal Nanostructures through Two-Photon Fluorescence. D. Mondal and D. Goswami, Methods and Applications in Fluorescence 4(4), 044004 (2016) [Abstract] [PDF] [BibTeX]

   Abstract: Multiphoton absorbing dye-coated trapped spherical bead at the focal plane of femtosecond optical tweezers shows nonlinear optical (NLO) phenomena. One such NLO process of two-photon fluorescence (TPF) has been used for the background-free imaging of a femtosecond laser-trapping event. Due to the high peak powers of femtosecond laser pulses with low average powers, it is possible to not only trap single nanospheres, but encourage optically directed self-assembly. The TPF signatures of trapped particles show evidence of such a directed self-assembly process which, in turn, can provide information about the structural dynamics during the process of cluster formation. We are able to trap and characterize structure and dynamics in 3D until pentamer formation from the decay characteristics of trapping at the focal plane.

    BibTeX: @article{mondalControllingTrackingColloidal2016,
     title = {Controlling and Tracking of Colloidal Nanostructures through Two-Photon Fluorescence},
     author = {Mondal, Dipankar and Goswami, Debabrata},
     date = {2016-10-07},
     journaltitle = {Methods and Applications in Fluorescence},
     shortjournal = {Methods Appl. Fluoresc.},
     volume = {4},
     number = {4},
     pages = {044004},
     issn = {2050-6120},
     doi = {10/gf5mrw},
     url = {http://stacks.iop.org/2050-6120/4/i=4/a=044004?key=crossref.b299c380a1fd88f37b79e579221d3e7a},
     urldate = {2019-08-01}
   }
   

5. 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},
     number = {10},
     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}
   }
   

6. 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}
   }
   

7. Femtosecond Laser-Induced Photothermal Effect for Nanoscale Viscometer and Thermometer. D. Mondal, S. Singhal, and D. Goswami, in Selected Topics in Photonics, A. Pradhan and P. K. Krishnamurthy, eds., IITK Directions (Springer Singapore, 2018), pp. 13–17 [Abstract] [PDF] [BibTeX]

   Abstract: A new method of utilizing photothermal effect at nano-volume dimensions to measure viscosity is presented here that can, in turn, provide the surrounding temperature. Our measurements use high repetition rate, low average power, femtosecond laser pulses that induce photothermal effect that is highly influence by the convective mode of heat transfer. This is especially important for absorbing liquids, which is unlike the typical photothermal effects that are due to such ultrashort pulses. Typical thermal processes involve only conductive mode of heat transfer and are phenomenological in nature. Inclusion of convective mode results in additional molecular characteristics of the thermal process. We measure traditional thermal lens with femtosecond pulse train through geometric beam divergence of a collimated laser beam co-propagating with the focused heating laser beam. The refractive index gradient in the sample arising from a focused heating laser creates a thermal lens, which is measured. On the other hand, the same heat gradient from the focusing heating laser beam generates a change in local viscosity in the medium, which changes the trapped stiffness of an optically trapped microsphere in its vicinity. We use co-propagating femtosecond train of laser pulses at 1560 and 780 nm wavelengths for these experiments. We also show from the bulk thermal studies that use of water as sample has the advantage of using conductive mode of heat transfer for femtosecond pulse train excitation.

    BibTeX: @incollection{mondalFemtosecondLaserInducedPhotothermal2018,
     title = {Femtosecond {{Laser-Induced Photothermal Effect}} for {{Nanoscale Viscometer}} and {{Thermometer}}},
     booktitle = {Selected {{Topics}} in {{Photonics}}},
     author = {Mondal, Dipankar and Singhal, Sumit and Goswami, Debabrata},
     editor = {Pradhan, Asima and Krishnamurthy, Pradeep Kumar},
     date = {2018},
     series = {{{IITK Directions}}},
     pages = {13--17},
     publisher = {{Springer Singapore}},
     location = {{Singapore}},
     doi = {10.1007/978-981-10-5010-7_2},
     url = {https://doi.org/10.1007/978-981-10-5010-7_2},
     urldate = {2019-08-01},
     isbn = {978-981-10-5010-7}
   }
   

8. Femtosecond Optical Tweezers as Sensitive Nano-Thermometer. D. Mondal and D. Goswami, in Frontiers in Optics 2016 (OSA, 2016), p. JTh2A.117 [Abstract] [PDF] [BibTeX]

   Abstract: Nano-volume temperature rise around optical trap is measured by exploiting non radiative relaxation in solvents. Co-propagating 1560 nm femtosecond laser pulses change solvent temperature and viscosity while trap-stiffness is unaffected during 780 nm trapping.

    BibTeX: @inproceedings{mondalFemtosecondOpticalTweezers2016,
     title = {Femtosecond Optical Tweezers as Sensitive Nano-Thermometer},
     booktitle = {Frontiers in {{Optics}} 2016},
     author = {Mondal, Dipankar and Goswami, Debabrata},
     date = {2016},
     pages = {JTh2A.117},
     publisher = {{OSA}},
     location = {{Rochester, New York}},
     doi = {10/gf5mrv},
     url = {https://www.osapublishing.org/abstract.cfm?URI=FiO-2016-JTh2A.117},
     urldate = {2019-08-01},
     eventtitle = {Frontiers in {{Optics}}},
     isbn = {978-1-943580-19-4}
   }
   

9. Microrheology Study of Aqueous Suspensions of Laponite Using Femtosecond Optical Tweezers. D. Mondal, A. Jha, Y. M. Joshi, and D. Goswami, in Optics in the Life Sciences Congress (2017), Paper OtW2E.1 (Optical Society of America, 2017), p. OtW2E.1 [Abstract] [PDF] [BibTeX]

   Abstract: We have observed microrheological aging dynamics of Laponite® suspensions using femtosecond optical tweezers (FOTs). Our on the fly calibration in time domain has been used to probe microscopic structural changes in the complex fluid.

    BibTeX: @inproceedings{mondalMicrorheologyStudyAqueous2017,
     title = {Microrheology {{Study}} of {{Aqueous Suspensions}} of {{Laponite}} Using {{Femtosecond Optical Tweezers}}},
     booktitle = {Optics in the {{Life Sciences Congress}} (2017), Paper {{OtW2E}}.1},
     author = {Mondal, Dipankar and Jha, Anushka and Joshi, Yogesh M. and Goswami, Debabrata},
     date = {2017-04-02},
     pages = {OtW2E.1},
     publisher = {{Optical Society of America}},
     doi = {10/gf5mrb},
     url = {https://www.osapublishing.org/abstract.cfm?uri=OTA-2017-OtW2E.1},
     urldate = {2019-08-01},
     eventtitle = {Optical {{Trapping Applications}}}
   }
   

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{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},
      number = {9},
      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}
    }
    

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},
      number = {1},
      pages = {700},
      issn = {2045-2322},
      doi = {10/gf5mqb},
      url = {http://www.nature.com/articles/s41598-018-36856-5},
      urldate = {2019-08-01}
    }
    

12. Precise Control and Measurement of Temperature with Femtosecond Optical Tweezers. D. Mondal and D. Goswami, Biophysical Journal 110(3), 500a (2016) [Abstract] [PDF] [BibTeX]

    Abstract: Optical traps have often been used for physical manipulation and transport within liquids for studying bio-systems. In this connection, though a lot of work has focused on the property of the trapped particle, there is little effort on utilizing the effect of the trapping environment. Here we demonstrate a novel method for exploiting the effect of trapping environment in observing the temperature rise in liquids directly at the vicinity of an optical trap center. Our approach utilizes the photo-thermal effect at micro-volume dimension to measure temperature, which could eventually be extended to in-vivo conditions. Our two-color experiment is a unique combination of a non-heating femtosecond trapping laser at 780 nm, which is coupled to a femtosecond infrared heating laser at 1560 nm. Femtosecond pulsed laser heating is much more effective than the traditional continuous-wave laser, which increases the sensitivity of our measurements. The heating laser precisely controls temperature at the focal volume of the trap center using low powers at high repetition rate. The changed values of corner frequency of the optical trap due to this local heating is recognized from its power spectra. The solvent having hydroxyl group is very sensitive to 1560 nm laser due to non-radiative relaxation from their higher excited state. Since most bio-systems contain hydroxyl group, they would be highly responsive to our heating laser, while our trapping laser is highly transparent for such systems. Our method could be used to specifically heat a particular cell to evaluate its faster cell division and also can be utilized for the possible phototherapy of cancer cells. The advantage of this technique over the conventional phototherapy is that this is highly localized due to focusing through high numerical aperture resulting in a high gradient of temperature.

     BibTeX: @article{mondalPreciseControlMeasurement2016,
      title = {Precise {{Control}} and {{Measurement}} of {{Temperature}} with {{Femtosecond Optical Tweezers}}},
      author = {Mondal, Dipankar and Goswami, Debabrata},
      date = {2016-02},
      journaltitle = {Biophysical Journal},
      shortjournal = {Biophysical Journal},
      volume = {110},
      number = {3},
      pages = {500a},
      issn = {00063495},
      doi = {10/gf5mr6},
      url = {https://linkinghub.elsevier.com/retrieve/pii/S0006349515038564},
      urldate = {2019-08-01}
    }
    

13. Precise Control and Measurement of Solid–Liquid Interfacial Temperature and Viscosity Using Dual-Beam Femtosecond Optical Tweezers in the Condensed Phase. D. Mondal, P. Mathur, and D. Goswami, Physical Chemistry Chemical Physics 18(37), 25823–25830 (2016) [Abstract] [PDF] [BibTeX]

    Abstract: We present a novel method of microrheology based on femtosecond optical tweezers, which in turn enables us to directly measure and control in situ temperature at microscale volumes at the solid–liquid interface. A noninvasive pulsed 780 nm trapped bead spontaneously responds to changes in its environment induced by a co-propagating 1560 nm pulsed laser due to mutual energy transfer between the solvent molecules and the trapped bead. Strong absorption of the hydroxyl group by the 1560 nm laser creates local heating in individual and binary mixtures of water and alcohols. “Hot Brownian motion” of the trapped polystyrene bead is reflected in the corner frequency deduced from the power spectrum. Changes in corner frequency values enable us to calculate the viscosity as well as temperature at the solid–liquid interface. We show that these experimental results can also be theoretically ratified.

     BibTeX: @article{mondalPreciseControlMeasurement2016a,
      title = {Precise Control and Measurement of Solid–Liquid Interfacial Temperature and Viscosity Using Dual-Beam Femtosecond Optical Tweezers in the Condensed Phase},
      author = {Mondal, Dipankar and Mathur, Paresh and Goswami, Debabrata},
      date = {2016},
      journaltitle = {Physical Chemistry Chemical Physics},
      volume = {18},
      number = {37},
      pages = {25823--25830},
      doi = {10/gf5mr8},
      url = {https://pubs.rsc.org/en/content/articlelanding/2016/cp/c6cp03093a},
      urldate = {2019-08-01}
    }
    

14. Sensitive in Situ Nanothermometer Using Femtosecond Optical Tweezers. D. Mondal and D. Goswami, Journal of Nanophotonics 10(2), 026013 (2016) [Abstract] [PDF] [BibTeX]

    Abstract: We report the rise in temperature in various liquid media adjacent to a trapped bead. A nonheating laser at 780 nm has been used to optically trap a 500-nm radius polystyrene bead, while a simultaneous irradiation with a copropagating 1560-nm high-repetition-rate femtosecond laser led to temperature rise in various trapping media. Vibrational combination band of the hydroxyl group in the trapping media resulted in high absorption of 1560-nm laser. This, in turn, gave us control over the trapping media temperature at the focus of the optical trap.

     BibTeX: @article{mondalSensitiveSituNanothermometer2016,
      title = {Sensitive in Situ Nanothermometer Using Femtosecond Optical Tweezers},
      author = {Mondal, Dipankar and Goswami, Debabrata},
      date = {2016-05},
      journaltitle = {Journal of Nanophotonics},
      shortjournal = {JNP},
      volume = {10},
      number = {2},
      pages = {026013},
      issn = {1934-2608, 1934-2608},
      doi = {10/f832c9},
      url = {https://www.spiedigitallibrary.org/journals/Journal-of-Nanophotonics/volume-10/issue-2/026013/----Custom-HTML----Sensitive/10.1117/1.JNP.10.026013.short},
      urldate = {2019-08-01}
    }
    

15. In Situ Temperature Control and Measurement with Femtosecond Optical Tweezers: Offering Biomedical Application. D. Mondal and D. Goswami, in Imaging, Manipulation, and Analysis of Biomolecules, Cells, and Tissues XV (International Society for Optics and Photonics, 2017), 10068, p. 100681T [Abstract] [PDF] [BibTeX]

    Abstract: We present here the control and measurement of temperature rise using femtosecond optical tweezers at near infrared (NIR) region. Based on our theoretical development, we have designed our experimental techniques. The high temporal sensitivity of position autocorrelation and equipartition theorem is simultaneously applied to elucidate temperature control and high precision measurement around focal volume. Experimentally we have made the benign NIR wavelength to induce local heating by adding very low fluorescent dye molecule with low average power. Local temperature control in aqueous solution exciting within optically absorbing window of the low quantum yield molecules can be possible due to non-radiative relaxation via thermal emission. The stochastic nature of Brownian particle has enough information of its surroundings. We have mapped the nano-dimension beam waist environment by probing the fluctuation of trapped particle. We have observed up to 30K temperature rise from room temperature at sub micro molar concentration. The gradient of temperature is as sharp as the fluence of pulsed laser focused by high numerical aperture objective. Thus, pulsed laser radiation always allows finer surgical techniques involving minimal thermal injuries. Our new techniques with multiphoton absorbing non-fluorescent dye can further be used to selective phototherapeutic diagnosis of cancer cells due to peak power dependent nonlinear phenomenon (NLO).

     BibTeX: @inproceedings{mondalSituTemperatureControl2017,
      title = {In Situ Temperature Control and Measurement with Femtosecond Optical Tweezers: Offering Biomedical Application},
      shorttitle = {In Situ Temperature Control and Measurement with Femtosecond Optical Tweezers},
      booktitle = {Imaging, {{Manipulation}}, and {{Analysis}} of {{Biomolecules}}, {{Cells}}, and {{Tissues XV}}},
      author = {Mondal, Dipankar and Goswami, Debabrata},
      date = {2017-02-16},
      volume = {10068},
      pages = {100681T},
      publisher = {{International Society for Optics and Photonics}},
      doi = {10/gf5mrg},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10068/100681T/In-situ-temperature-control-and-measurement-with-femtosecond-optical-tweezers/10.1117/12.2251892.short},
      urldate = {2019-08-01},
      eventtitle = {Imaging, {{Manipulation}}, and {{Analysis}} of {{Biomolecules}}, {{Cells}}, and {{Tissues XV}}}
    }
    

16. Spatiotemporal Control of Energy Transfer in Optically Trapped Systems. D. Mondal, D. Roy, and D. Goswami, in 2015 International Young Scientists Forum on Applied Physics (YSF) (2015), pp. 1–4 [Abstract] [BibTeX]

    Abstract: We demonstrate control over two photon fluorescence (TPF) resonance energy transfer under femtosecond optically tweezed condition. We also extract information about the structure and dynamics of the trapped particles and their clusters as observed from the TPF decay times of the trapped multiple microspheres (0.50 μm size) by varying the polarization of the trapping laser. This micron to nano regime of the energy transfer process has provided us the necessary control over the molecular level energy transfer rate due to the different overlap integral of excitation and emission spectra of the dye that is coated on the surface of 1.0 μm polystyrene particles. Our background free detection method thus provides additional structural information about multiple trapping events.

     BibTeX: @inproceedings{mondalSpatiotemporalControlEnergy2015,
      title = {Spatiotemporal Control of Energy Transfer in Optically Trapped Systems},
      booktitle = {2015 {{International Young Scientists Forum}} on {{Applied Physics}} ({{YSF}})},
      author = {Mondal, D. and Roy, D. and Goswami, D.},
      date = {2015-09},
      pages = {1--4},
      doi = {10/gf5msz},
      eventtitle = {2015 {{International Young Scientists Forum}} on {{Applied Physics}} ({{YSF}})}
    }
    

17. Temperature Control and Measurement with Tunable Femtosecond Optical Tweezers. D. Mondal and D. Goswami, in Optical Trapping and Optical Micromanipulation XIII (International Society for Optics and Photonics, 2016), 9922, p. 992210 [Abstract] [PDF] [BibTeX]

    Abstract: We present the effects of wavelength dependent temperature rise in a femtosecond optical tweezers. Our experiments involve the femtosecond trapping laser tunable from 740-820 nm at low power 25 mW to cause heating in the trapped volume within a homogeneous solution of sub micro-molar concentration of IR dye. The 780 nm high repetition rate laser acts as a resonant excitation source which helps to create the local heating effortlessly within the trapping volume. We have used both position autocorrelation and equipartion theorem to evaluate temperature at different wavelength having different absorption coefficient. Fixing the pulse width in the temporal domain gives constant bandwidth at spatial domain, which makes our system behave as a tunable temperature rise device with high precision. This observation leads us to calculate temperature as well as viscosity within the vicinity of the trapping zone. A mutual energy transfer occurs between the trapped bead and solvents that leads to transfer the thermal energy of solvents into the kinetic energy of the trap bead and vice-versa. Thus hot solvated molecules resulting from resonant and near resonant excitation of trapping wavelength can continuously dissipate heat to the trapped bead which will be reflected on frequency spectrum of Brownian noise exhibited by the bead. Temperature rise near the trapping zone can significantly change the viscosity of the medium. We observe temperature rise profile according to its Gaussian shaped absorption spectrum with different wavelength.

     BibTeX: @inproceedings{mondalTemperatureControlMeasurement2016,
      title = {Temperature Control and Measurement with Tunable Femtosecond Optical Tweezers},
      booktitle = {Optical {{Trapping}} and {{Optical Micromanipulation XIII}}},
      author = {Mondal, Dipankar and Goswami, Debabrata},
      date = {2016-09-16},
      volume = {9922},
      pages = {992210},
      publisher = {{International Society for Optics and Photonics}},
      doi = {10/gf5mr2},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/9922/992210/Temperature-control-and-measurement-with-tunable-femtosecond-optical-tweezers/10.1117/12.2237708.short},
      urldate = {2019-08-01},
      eventtitle = {Optical {{Trapping}} and {{Optical Micromanipulation XIII}}}
    }
    

18. 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}
    }
    

19. 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}
    }
    

20. Femtosecond Laser-Induced Photothermal Effect for Nanoscale Viscometer and Thermometer. D. Mondal, S. Singhal, and D. Goswami, in Selected Topics in Photonics, A. Pradhan and P. K. Krishnamurthy, eds., IITK Directions (Springer Singapore, 2018), pp. 13–17 [Abstract] [PDF] [BibTeX]

    Abstract: A new method of utilizing photothermal effect at nano-volume dimensions to measure viscosity is presented here that can, in turn, provide the surrounding temperature. Our measurements use high repetition rate, low average power, femtosecond laser pulses that induce photothermal effect that is highly influence by the convective mode of heat transfer. This is especially important for absorbing liquids, which is unlike the typical photothermal effects that are due to such ultrashort pulses. Typical thermal processes involve only conductive mode of heat transfer and are phenomenological in nature. Inclusion of convective mode results in additional molecular characteristics of the thermal process. We measure traditional thermal lens with femtosecond pulse train through geometric beam divergence of a collimated laser beam co-propagating with the focused heating laser beam. The refractive index gradient in the sample arising from a focused heating laser creates a thermal lens, which is measured. On the other hand, the same heat gradient from the focusing heating laser beam generates a change in local viscosity in the medium, which changes the trapped stiffness of an optically trapped microsphere in its vicinity. We use co-propagating femtosecond train of laser pulses at 1560 and 780 nm wavelengths for these experiments. We also show from the bulk thermal studies that use of water as sample has the advantage of using conductive mode of heat transfer for femtosecond pulse train excitation.

     BibTeX: @incollection{mondalFemtosecondLaserInducedPhotothermal2019,
      langid = {english},
      location = {{Singapore}},
      title = {Femtosecond {{Laser}}-{{Induced Photothermal Effect}} for {{Nanoscale Viscometer}} and {{Thermometer}}},
      isbn = {978-981-10-5010-7},
      url = {https://doi.org/10.1007/978-981-10-5010-7_2},
      booktitle = {Selected {{Topics}} in {{Photonics}}},
      series = {{{IITK Directions}}},
      publisher = {{Springer Singapore}},
      urldate = {2019-08-01},
      date = {2018},
      pages = {13-17},
      author = {Mondal, Dipankar and Singhal, Sumit and Goswami, Debabrata},
      editor = {Pradhan, Asima and Krishnamurthy, Pradeep Kumar},
      doi = {10.1007/978-981-10-5010-7_2}
    }
    

21. Microrheology Study of Aqueous Suspensions of Laponite Using Femtosecond Optical Tweezers. D. Mondal, A. Jha, Y. M. Joshi, and D. Goswami, in Optics in the Life Sciences Congress (2017), Paper OtW2E.1 (Optical Society of America, 2017), p. OtW2E.1 [Abstract] [PDF] [BibTeX]

    Abstract: We have observed microrheological aging dynamics of Laponite® suspensions using femtosecond optical tweezers (FOTs). Our on the fly calibration in time domain has been used to probe microscopic structural changes in the complex fluid.

     BibTeX: @inproceedings{mondalMicrorheologyStudyAqueous2018,
      langid = {english},
      title = {Microrheology {{Study}} of {{Aqueous Suspensions}} of {{Laponite}} Using {{Femtosecond Optical Tweezers}}},
      url = {https://www.osapublishing.org/abstract.cfm?uri=OTA-2017-OtW2E.1},
      doi = {10/gf5mrb},
      eventtitle = {Optical {{Trapping Applications}}},
      booktitle = {Optics in the {{Life Sciences Congress}} (2017), Paper {{OtW2E}}.1},
      publisher = {{Optical Society of America}},
      urldate = {2019-08-01},
      date = {2017-04-02},
      pages = {OtW2E.1},
      author = {Mondal, Dipankar and Jha, Anushka and Joshi, Yogesh M. and Goswami, Debabrata}
    }
    

22. In Situ Temperature Control and Measurement with Femtosecond Optical Tweezers: Offering Biomedical Application. D. Mondal and D. Goswami, in Imaging, Manipulation, and Analysis of Biomolecules, Cells, and Tissues XV (International Society for Optics and Photonics, 2017), 10068, p. 100681T [Abstract] [PDF] [BibTeX]

    Abstract: We present here the control and measurement of temperature rise using femtosecond optical tweezers at near infrared (NIR) region. Based on our theoretical development, we have designed our experimental techniques. The high temporal sensitivity of position autocorrelation and equipartition theorem is simultaneously applied to elucidate temperature control and high precision measurement around focal volume. Experimentally we have made the benign NIR wavelength to induce local heating by adding very low fluorescent dye molecule with low average power. Local temperature control in aqueous solution exciting within optically absorbing window of the low quantum yield molecules can be possible due to non-radiative relaxation via thermal emission. The stochastic nature of Brownian particle has enough information of its surroundings. We have mapped the nano-dimension beam waist environment by probing the fluctuation of trapped particle. We have observed up to 30K temperature rise from room temperature at sub micro molar concentration. The gradient of temperature is as sharp as the fluence of pulsed laser focused by high numerical aperture objective. Thus, pulsed laser radiation always allows finer surgical techniques involving minimal thermal injuries. Our new techniques with multiphoton absorbing non-fluorescent dye can further be used to selective phototherapeutic diagnosis of cancer cells due to peak power dependent nonlinear phenomenon (NLO).

     BibTeX: @inproceedings{mondalSituTemperatureControl2018,
      title = {In Situ Temperature Control and Measurement with Femtosecond Optical Tweezers: Offering Biomedical Application},
      volume = {10068},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10068/100681T/In-situ-temperature-control-and-measurement-with-femtosecond-optical-tweezers/10.1117/12.2251892.short},
      doi = {10/gf5mrg},
      shorttitle = {In Situ Temperature Control and Measurement with Femtosecond Optical Tweezers},
      eventtitle = {Imaging, {{Manipulation}}, and {{Analysis}} of {{Biomolecules}}, {{Cells}}, and {{Tissues XV}}},
      booktitle = {Imaging, {{Manipulation}}, and {{Analysis}} of {{Biomolecules}}, {{Cells}}, and {{Tissues XV}}},
      publisher = {{International Society for Optics and Photonics}},
      urldate = {2019-08-01},
      date = {2017-02-16},
      pages = {100681T},
      author = {Mondal, Dipankar and Goswami, Debabrata}
    }
    

23. Femtosecond Optical Tweezers as Sensitive Nano-Thermometer. D. Mondal and D. Goswami, in Frontiers in Optics 2016 (OSA, 2016), p. JTh2A.117 [Abstract] [PDF] [BibTeX]

    Abstract: Nano-volume temperature rise around optical trap is measured by exploiting non radiative relaxation in solvents. Co-propagating 1560 nm femtosecond laser pulses change solvent temperature and viscosity while trap-stiffness is unaffected during 780 nm trapping.

     BibTeX: @inproceedings{mondalFemtosecondOpticalTweezers2017,
      langid = {english},
      location = {{Rochester, New York}},
      title = {Femtosecond Optical Tweezers as Sensitive Nano-Thermometer},
      isbn = {978-1-943580-19-4},
      url = {https://www.osapublishing.org/abstract.cfm?URI=FiO-2016-JTh2A.117},
      doi = {10/gf5mrv},
      eventtitle = {Frontiers in {{Optics}}},
      booktitle = {Frontiers in {{Optics}} 2016},
      publisher = {{OSA}},
      urldate = {2019-08-01},
      date = {2016},
      pages = {JTh2A.117},
      author = {Mondal, Dipankar and Goswami, Debabrata}
    }
    

24. Controlling and Tracking of Colloidal Nanostructures through Two-Photon Fluorescence. D. Mondal and D. Goswami, Methods Appl. Fluoresc. 4(4), 044004 (2016) [Abstract] [PDF] [BibTeX]

    Abstract: Multiphoton absorbing dye-coated trapped spherical bead at the focal plane of femtosecond optical tweezers shows nonlinear optical (NLO) phenomena. One such NLO process of two-photon fluorescence (TPF) has been used for the background-free imaging of a femtosecond laser-trapping event. Due to the high peak powers of femtosecond laser pulses with low average powers, it is possible to not only trap single nanospheres, but encourage optically directed self-assembly. The TPF signatures of trapped particles show evidence of such a directed self-assembly process which, in turn, can provide information about the structural dynamics during the process of cluster formation. We are able to trap and characterize structure and dynamics in 3D until pentamer formation from the decay characteristics of trapping at the focal plane.

     BibTeX: @article{mondalControllingTrackingColloidal2017,
      langid = {english},
      title = {Controlling and Tracking of Colloidal Nanostructures through Two-Photon Fluorescence},
      volume = {4},
      issn = {2050-6120},
      url = {http://stacks.iop.org/2050-6120/4/i=4/a=044004?key=crossref.b299c380a1fd88f37b79e579221d3e7a},
      doi = {10/gf5mrw},
      number = {4},
      journaltitle = {Methods Appl. Fluoresc.},
      urldate = {2019-08-01},
      date = {2016-10-07},
      pages = {044004},
      author = {Mondal, Dipankar and Goswami, Debabrata}
    }
    

25. Temperature Control and Measurement with Tunable Femtosecond Optical Tweezers. D. Mondal and D. Goswami, in Optical Trapping and Optical Micromanipulation XIII (International Society for Optics and Photonics, 2016), 9922, p. 992210 [Abstract] [PDF] [BibTeX]

    Abstract: We present the effects of wavelength dependent temperature rise in a femtosecond optical tweezers. Our experiments involve the femtosecond trapping laser tunable from 740-820 nm at low power 25 mW to cause heating in the trapped volume within a homogeneous solution of sub micro-molar concentration of IR dye. The 780 nm high repetition rate laser acts as a resonant excitation source which helps to create the local heating effortlessly within the trapping volume. We have used both position autocorrelation and equipartion theorem to evaluate temperature at different wavelength having different absorption coefficient. Fixing the pulse width in the temporal domain gives constant bandwidth at spatial domain, which makes our system behave as a tunable temperature rise device with high precision. This observation leads us to calculate temperature as well as viscosity within the vicinity of the trapping zone. A mutual energy transfer occurs between the trapped bead and solvents that leads to transfer the thermal energy of solvents into the kinetic energy of the trap bead and vice-versa. Thus hot solvated molecules resulting from resonant and near resonant excitation of trapping wavelength can continuously dissipate heat to the trapped bead which will be reflected on frequency spectrum of Brownian noise exhibited by the bead. Temperature rise near the trapping zone can significantly change the viscosity of the medium. We observe temperature rise profile according to its Gaussian shaped absorption spectrum with different wavelength.

     BibTeX: @inproceedings{mondalTemperatureControlMeasurement2017,
      title = {Temperature Control and Measurement with Tunable Femtosecond Optical Tweezers},
      volume = {9922},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/9922/992210/Temperature-control-and-measurement-with-tunable-femtosecond-optical-tweezers/10.1117/12.2237708.short},
      doi = {10/gf5mr2},
      eventtitle = {Optical {{Trapping}} and {{Optical Micromanipulation XIII}}},
      booktitle = {Optical {{Trapping}} and {{Optical Micromanipulation XIII}}},
      publisher = {{International Society for Optics and Photonics}},
      urldate = {2019-08-01},
      date = {2016-09-16},
      pages = {992210},
      author = {Mondal, Dipankar and Goswami, Debabrata}
    }
    

26. 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}
    }
    

27. Sensitive in Situ Nanothermometer Using Femtosecond Optical Tweezers. D. Mondal and D. Goswami, JNP 10(2), 026013 (2016) [Abstract] [PDF] [BibTeX]

    Abstract: We report the rise in temperature in various liquid media adjacent to a trapped bead. A nonheating laser at 780 nm has been used to optically trap a 500-nm radius polystyrene bead, while a simultaneous irradiation with a copropagating 1560-nm high-repetition-rate femtosecond laser led to temperature rise in various trapping media. Vibrational combination band of the hydroxyl group in the trapping media resulted in high absorption of 1560-nm laser. This, in turn, gave us control over the trapping media temperature at the focus of the optical trap.

     BibTeX: @article{mondalSensitiveSituNanothermometer2017,
      title = {Sensitive in Situ Nanothermometer Using Femtosecond Optical Tweezers},
      volume = {10},
      issn = {1934-2608, 1934-2608},
      url = {https://www.spiedigitallibrary.org/journals/Journal-of-Nanophotonics/volume-10/issue-2/026013/----Custom-HTML----Sensitive/10.1117/1.JNP.10.026013.short},
      doi = {10/f832c9},
      number = {2},
      journaltitle = {JNP},
      urldate = {2019-08-01},
      date = {2016-05},
      pages = {026013},
      author = {Mondal, Dipankar and Goswami, Debabrata}
    }
    

28. Precise Control and Measurement of Temperature with Femtosecond Optical Tweezers. D. Mondal and D. Goswami, Biophysical Journal 110(3), 500a (2016) [Abstract] [PDF] [BibTeX]

    Abstract: Optical traps have often been used for physical manipulation and transport within liquids for studying bio-systems. In this connection, though a lot of work has focused on the property of the trapped particle, there is little effort on utilizing the effect of the trapping environment. Here we demonstrate a novel method for exploiting the effect of trapping environment in observing the temperature rise in liquids directly at the vicinity of an optical trap center. Our approach utilizes the photo-thermal effect at micro-volume dimension to measure temperature, which could eventually be extended to in-vivo conditions. Our two-color experiment is a unique combination of a non-heating femtosecond trapping laser at 780 nm, which is coupled to a femtosecond infrared heating laser at 1560 nm. Femtosecond pulsed laser heating is much more effective than the traditional continuous-wave laser, which increases the sensitivity of our measurements. The heating laser precisely controls temperature at the focal volume of the trap center using low powers at high repetition rate. The changed values of corner frequency of the optical trap due to this local heating is recognized from its power spectra. The solvent having hydroxyl group is very sensitive to 1560 nm laser due to non-radiative relaxation from their higher excited state. Since most bio-systems contain hydroxyl group, they would be highly responsive to our heating laser, while our trapping laser is highly transparent for such systems. Our method could be used to specifically heat a particular cell to evaluate its faster cell division and also can be utilized for the possible phototherapy of cancer cells. The advantage of this technique over the conventional phototherapy is that this is highly localized due to focusing through high numerical aperture resulting in a high gradient of temperature.

     BibTeX: @article{mondalPreciseControlMeasurement2017,
      langid = {english},
      title = {Precise {{Control}} and {{Measurement}} of {{Temperature}} with {{Femtosecond Optical Tweezers}}},
      volume = {110},
      issn = {00063495},
      url = {https://linkinghub.elsevier.com/retrieve/pii/S0006349515038564},
      doi = {10/gf5mr6},
      number = {3},
      journaltitle = {Biophysical Journal},
      urldate = {2019-08-01},
      date = {2016-02},
      pages = {500a},
      author = {Mondal, Dipankar and Goswami, Debabrata}
    }
    

29. Precise Control and Measurement of Solid–Liquid Interfacial Temperature and Viscosity Using Dual-Beam Femtosecond Optical Tweezers in the Condensed Phase. D. Mondal, P. Mathur, and D. Goswami, Physical Chemistry Chemical Physics 18(37), 25823–25830 (2016) [Abstract] [PDF] [BibTeX]

    Abstract: We present a novel method of microrheology based on femtosecond optical tweezers, which in turn enables us to directly measure and control in situ temperature at microscale volumes at the solid–liquid interface. A noninvasive pulsed 780 nm trapped bead spontaneously responds to changes in its environment induced by a co-propagating 1560 nm pulsed laser due to mutual energy transfer between the solvent molecules and the trapped bead. Strong absorption of the hydroxyl group by the 1560 nm laser creates local heating in individual and binary mixtures of water and alcohols. “Hot Brownian motion” of the trapped polystyrene bead is reflected in the corner frequency deduced from the power spectrum. Changes in corner frequency values enable us to calculate the viscosity as well as temperature at the solid–liquid interface. We show that these experimental results can also be theoretically ratified.

     BibTeX: @article{mondalPreciseControlMeasurement2016b,
      langid = {english},
      title = {Precise Control and Measurement of Solid–Liquid Interfacial Temperature and Viscosity Using Dual-Beam Femtosecond Optical Tweezers in the Condensed Phase},
      volume = {18},
      url = {https://pubs.rsc.org/en/content/articlelanding/2016/cp/c6cp03093a},
      doi = {10/gf5mr8},
      number = {37},
      journaltitle = {Physical Chemistry Chemical Physics},
      urldate = {2019-08-01},
      date = {2016},
      pages = {25823-25830},
      author = {Mondal, Dipankar and Mathur, Paresh and Goswami, Debabrata}
    }
    

30. Controlling Local Temperature in Water Using Femtosecond Optical Tweezer. D. Mondal and D. Goswami, Biomed. Opt. Express, BOE 6(9), 3190–3196 (2015) [Abstract] [PDF] [BibTeX]

    Abstract: A novel method of directly observing the effect of temperature rise in water at the vicinity of optical trap center is presented. Our approach relies on changed values of corner frequency of the optical trap that, in turn, is realized from its power spectra. Our two color experiment is a unique combination of a non-heating femtosecond trapping laser at 780 nm, coupled to a femtosecond infrared heating laser at 1560 nm, which precisely controls temperature at focal volume of the trap center using low powers (100-800 µW) at high repetition rate. The geometric ray optics model quantitatively supports our experimental data.

     BibTeX: @article{mondalControllingLocalTemperature2016,
      langid = {english},
      title = {Controlling Local Temperature in Water Using Femtosecond Optical Tweezer},
      volume = {6},
      issn = {2156-7085},
      url = {https://www.osapublishing.org/boe/abstract.cfm?uri=boe-6-9-3190},
      doi = {10/gf5msh},
      number = {9},
      journaltitle = {Biomed. Opt. Express, BOE},
      urldate = {2019-08-01},
      date = {2015-09-01},
      pages = {3190-3196},
      author = {Mondal, Dipankar and Goswami, Debabrata}
    }
    

31. Calibration of Femtosecond Optical Tweezers as a Sensitive Thermometer. D. Mondal and D. Goswami, in Optical Trapping and Optical Micromanipulation XII (International Society for Optics and Photonics, 2015), 9548, p. 95481N [Abstract] [PDF] [BibTeX]

    Abstract: We present cumulative perturbation effects of femtosecond laser pulses on an optical tweezer. Our experiments involve a dual wavelength high repetition rate femtosecond laser, one at the non-heating wavelength of 780 nm while the other at 1560 nm to cause heating in the trapped volume under low power (100-800 μW) conditions. The 1560 nm high repetition rate laser acts as a resonant excitation source for the vibrational combination band of the hydroxyl group (OH) of water, which helps create the local heating effortlessly within the trapping volume. With such an experimental system, we are the first to observe direct effect of temperature on the corner frequency deduced from power spectrum. We can, thus, control and measure temperature precisely at the optical trap. This observation has lead us to calculate viscosity as well as temperature in the vicinity of the trapping zone. These experimental results also support the well-known fact that the nature of Brownian motion is the response of the optically trapped bead from the temperature change of surroundings. Temperature rise near the trapping zone can significantly change the viscosity of the medium. However, we notice that though the temperature and viscosity are changing as per our corner frequency calculations, the trap stiffness remains the same throughout our experiments within the temperature range of about 20 K.

     BibTeX: @inproceedings{mondalCalibrationFemtosecondOptical2016,
      title = {Calibration of Femtosecond Optical Tweezers as a Sensitive Thermometer},
      volume = {9548},
      url = {https://www.spiedigitallibrary.org/conference-proceedings-of-spie/9548/95481N/Calibration-of-femtosecond-optical-tweezers-as-a-sensitive-thermometer/10.1117/12.2187097.short},
      doi = {10.1117/12.2187097},
      eventtitle = {Optical {{Trapping}} and {{Optical Micromanipulation XII}}},
      booktitle = {Optical {{Trapping}} and {{Optical Micromanipulation XII}}},
      publisher = {{International Society for Optics and Photonics}},
      urldate = {2019-08-01},
      date = {2015-08-25},
      pages = {95481N},
      author = {Mondal, Dipankar and Goswami, Debabrata}
    }
    

32. Controlling the Effect on Solvent by Resonant Excitation in Femtosecond Optical Tweezer. D. Mondal and D. Goswami, in Optics in the Life Sciences (OSA, 2015), p. OtT4E.3 [Abstract] [PDF] [BibTeX]

    Abstract: Observation of direct effect of temperature rise in water around optical trap center is presented. Irradiation of IR laser with low power during pulsed 780 nm trapping significantly changes solvent viscosity keeping trap stiffness unaffected.

     BibTeX: @inproceedings{mondalControllingEffectSolvent2016,
      langid = {english},
      location = {{Vancouver}},
      title = {Controlling the Effect on Solvent by Resonant Excitation in Femtosecond Optical Tweezer},
      isbn = {978-1-55752-954-1},
      url = {https://www.osapublishing.org/abstract.cfm?URI=OTA-2015-OtT4E.3},
      doi = {10/gf5msw},
      eventtitle = {Optical {{Trapping Applications}}},
      booktitle = {Optics in the {{Life Sciences}}},
      publisher = {{OSA}},
      urldate = {2019-08-01},
      date = {2015},
      pages = {OtT4E.3},
      author = {Mondal, Dipankar and Goswami, Debabrata}
    }
    

33. Spatiotemporal Control of Energy Transfer in Optically Trapped Systems. D. Mondal, D. Roy, and D. Goswami, in 2015 International Young Scientists Forum on Applied Physics (YSF) (2015), pp. 1–4 [Abstract] [BibTeX]

    Abstract: We demonstrate control over two photon fluorescence (TPF) resonance energy transfer under femtosecond optically tweezed condition. We also extract information about the structure and dynamics of the trapped particles and their clusters as observed from the TPF decay times of the trapped multiple microspheres (0.50 μm size) by varying the polarization of the trapping laser. This micron to nano regime of the energy transfer process has provided us the necessary control over the molecular level energy transfer rate due to the different overlap integral of excitation and emission spectra of the dye that is coated on the surface of 1.0 μm polystyrene particles. Our background free detection method thus provides additional structural information about multiple trapping events.

     BibTeX: @inproceedings{mondalSpatiotemporalControlEnergy2016,
      title = {Spatiotemporal Control of Energy Transfer in Optically Trapped Systems},
      doi = {10/gf5msz},
      eventtitle = {2015 {{International Young Scientists Forum}} on {{Applied Physics}} ({{YSF}})},
      booktitle = {2015 {{International Young Scientists Forum}} on {{Applied Physics}} ({{YSF}})},
      date = {2015-09},
      pages = {1-4},
      author = {Mondal, D. and Roy, D. and Goswami, D.}
    }