Sumit Singhal

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Education / Work History

• Postdoctoral Fellow, University of Colorado (2019-present)
• Ph.D. Scholar, IIT Kanpur (2010-2018)

Thesis Abstract

Femtosecond Laser Pulse Train Induced Photothermal Lensing in Nonlinear Optical Medium

The central idea of this thesis is to manipulate and assess the lensing effect of the nonlinear optical medium to increase the potential of routine techniques and devices relying on it. The underlying physical mechanism and analysis of the lensing phenomena differ drastically with sample under investigation. Still in general, mechanism leading to Kerr lensing is much faster than the photothermal lensing. So, distinct mechanisms can be responsible for lensing of an NLO material at different time scale. Response time of the material to the incident light is of below sub picosecond for Kerr lensing effect, whereas, it is around millisecond for photothermal lensing effect. Ultrafast response enabled Kerr lensing to play a crucial role in the emerging field of laser and photonics technology used in broad range of applications such as mode locked lasers, ultrafast switches, optical signal processing, quantum computing, optical sensors, and many more. Photothermal lensing, however, are used generally to investigate thermodynamic and optical properties of the materials. For experimental studies, I employ femtosecond laser pulse train to induce photothermal lens and Kerr lens in the nonlinear optical (NLO) medium. This thesis has been arranged into six chapters which are listed below Chapter 1 contains a brief introduction of the various nonlinear optical properties of the materials. However, focus is on the third order nonlinear properties, namely, nonlinear refraction and two-photon absorption as only nonlinear optical properties relevant to this thesis. It starts with the definition and importance of nonlinear optical medium, and proceeds with description of the formalism required to understand these phenomena. Two-photon absorption process and lensing phenomena occurrences due to Kerr effect as well as photothermal effect is discussed in detail. In Chapter 2, principle of measurement techniques used in this thesis are described in detail. A well-known single beam Z-scan technique is used for the characterization of lensing effect in the steady state. A time resolved technique is employed for the study of photothermal lensing phenomena especially. The study of nonlinear optical phenomena requires high intensity laser source, and for the use of the mentioned techniques, laser must be well characterized. A brief overview of all the laser systems employed in the distinct experimental setups constructed in this thesis is given here, however other fine details of the particular experiment are given in concerned chapters. Standard autocorrelation techniques used to measure pulse duration and knife edge method used for spatial characteristics of laser beam like beam radius are also presented. In chapter 3, Nonlinear absorption and Kerr type nonlinear properties of pure CS2 are investigated by standard Z-scan method using high repetition rate (HRR) femtosecond laser coming directly from oscillator without amplification. High repetition of laser pulses causes substantial cumulative heating of the sample. Consequently, sample also exhibits strong photothermal lensing effect along with Kerr lensing effect. Single beam Zscan method is sensitive to both effects and cannot distinguish through steady state measurement. Here, I demonstrate that the photothermal lensing effect can be removed if a liquid sample is flowed at a flow rate above or equal to a sufficient value. It allows us to measure only Kerr lensing accurately with HRR lasers. In Chapter 4, Thermal lens measurement of influence of laser power and molecular sizes on the convective heat transfer in alcohols is performed. Thermal lens spectrometry/spectroscopy is a very sensitive analytical technique. This technique is anticipated as an alternative analytical tool to fluorescence or transmittance based methods. It has been practiced avoiding the chance of convective heat transfer in the process of thermal lens formation in almost every case. But, here I illustrate that convection can influence the thermal lens signal significantly, and it can be studied by thermal lens technique. In Chapter 5, I focus on isolating molecular effect of convection by novel time resolved photothermal lens spectroscopy, whereas, both laser power and molecular size collectively affected the convection in chapter 4. Beside that, A new analytical model is included rather than using existing semiempirical treatment of thermal lens signal as used in chapter 4. The model follows all basic assumptions that were included in a widely accepted Shen’s model except neglecting the convection. Only important mathematical steps are shown in the main section; however, intermediate steps between crucial mathematical expressions are given in the appendix. I apply the new model to study the convection effect in alkane samples which fits well in approximation regime since effect of convection are more prone to alkane than methanol due to lack of hydrogen bonding, unlike methanol. Chapter 6 contains conclusions and future prospects.

Publications

These include only those published in our lab.

1. Exploring the Critical Role of Detection Aperture in Thermal Lens Measurements. S. Singhal, I. Bhattacharyya, and D. Goswami, in 2015 Workshop on Recent Advances in Photonics (WRAP) (2015), pp. 1–4 [Abstract] [BibTeX]

   Abstract: The critical role of the detection aperture size on the measurement of thermal lens signals has been explored by using mode-mismatched pump-probe thermal lens spectroscopy in pure methanol. Femtosecond laser pulses at 1560 nm are used to create thermal lens signal, which is simultaneously probed by a collinear 780 nm pulse train through a partially open aperture of different sizes placed after the sample. The trend in the time resolved thermal lens signals reveals that aperture size allowing less than half of the incident probe light is critical in providing accuracy of the thermal lens spectroscopy.

    BibTeX: @inproceedings{singhalExploringCriticalRole2015,
     title = {Exploring the Critical Role of Detection Aperture in Thermal Lens Measurements},
     booktitle = {2015 {{Workshop}} on {{Recent Advances}} in {{Photonics}} ({{WRAP}})},
     author = {Singhal, S. and Bhattacharyya, I. and Goswami, D.},
     date = {2015-12},
     pages = {1--4},
     doi = {10/gf5msd},
     eventtitle = {2015 {{Workshop}} on {{Recent Advances}} in {{Photonics}} ({{WRAP}})}
   }
   

2. Importance of Hydrogen Bonding in Thermal Lens Study of Highly Absorbing Liquids. S. Singhal, P. P. Roy, and D. Goswami, in Frontiers in Optics 2015 (2015), Paper FTu5E.5 (Optical Society of America, 2015), p. FTu5E.5 [Abstract] [PDF] [BibTeX]

   Abstract: Effect of thermal load on highly absorbing hydrogen bonded liquids are explored. We find heat transfer is better facilitated in lesser hydrogen bonded liquids by convection through molecular motion in addition to conduction.

    BibTeX: @inproceedings{singhalImportanceHydrogenBonding2015,
     title = {Importance of {{Hydrogen Bonding}} in {{Thermal Lens Study}} of {{Highly Absorbing Liquids}}},
     booktitle = {Frontiers in {{Optics}} 2015 (2015), Paper {{FTu5E}}.5},
     author = {Singhal, Sumit and Roy, Partha Pratim and Goswami, Debabrata},
     date = {2015-10-18},
     pages = {FTu5E.5},
     publisher = {{Optical Society of America}},
     doi = {10/gf5msf},
     url = {https://www.osapublishing.org/abstract.cfm?uri=FiO-2015-FTu5E.5},
     urldate = {2019-08-01},
     eventtitle = {Frontiers in {{Optics}}}
   }
   

3. Measurement of Pure Optical Nonlinearity in Carbon Disulfide with a High-Repetition-Rate Femtosecond Laser. S. Singhal, S. Dinda, and D. Goswami, Applied Optics 56(3), 644 (2017) [PDF] [BibTeX]

    BibTeX: @article{singhalMeasurementPureOptical2017,
     title = {Measurement of Pure Optical Nonlinearity in Carbon Disulfide with a High-Repetition-Rate Femtosecond Laser},
     author = {Singhal, Sumit and Dinda, Sirshendu and Goswami, Debabrata},
     date = {2017-01-20},
     journaltitle = {Applied Optics},
     shortjournal = {Appl. Opt.},
     volume = {56},
     number = {3},
     pages = {644},
     issn = {0003-6935, 1539-4522},
     doi = {10/gf5mrm},
     url = {https://www.osapublishing.org/abstract.cfm?URI=ao-56-3-644},
     urldate = {2019-08-01}
   }
   

4. Molecular Size and Mass Sensitive Femtosecond Thermal Spectrometer. S. Singhal, S. Goswami, A. Banerjee, and D. Goswami, in 2019 URSI Asia-Pacific Radio Science Conference (AP-RASC) (2019), pp. 1–3 [Abstract] [BibTeX]

   Abstract: Though a single ultrashort pulse has inconceivable thermal effect, highly repetitive femtosecond lasers often result in some heating effects. Instead of fretting over the thermal effect, we have used this to develop the time-resolved photothermal lens spectroscopy for molecular sensitivity.

    BibTeX: @inproceedings{singhalMolecularSizeMass2019,
     title = {Molecular {{Size}} and {{Mass Sensitive Femtosecond Thermal Spectrometer}}},
     booktitle = {2019 {{URSI Asia-Pacific Radio Science Conference}} ({{AP-RASC}})},
     author = {Singhal, Sumit and Goswami, Sonaly and Banerjee, A. and Goswami, Debabrata},
     date = {2019-03},
     pages = {1--3},
     doi = {10/gf5mqh},
     eventtitle = {2019 {{URSI Asia-Pacific Radio Science Conference}} ({{AP-RASC}})}
   }
   

5. Sensitive Dual Beam Thermal Lens Detection of Convection in Methanol. S. Singhal and D. Goswami, in 13th International Conference on Fiber Optics and Photonics (OSA, 2016), p. P1A.16 [Abstract] [PDF] [BibTeX]

   Abstract: We show that dual beam mode mismatched thermal lens (TL) technique is more sensitive than the single beam one. Presence of unusual peak inside the dual beam TL signal directly indicates strong convection.

    BibTeX: @inproceedings{singhalSensitiveDualBeam2016,
     title = {Sensitive Dual Beam Thermal Lens Detection of Convection in Methanol},
     booktitle = {13th {{International Conference}} on {{Fiber Optics}} and {{Photonics}}},
     author = {Singhal, Sumit and Goswami, Debabrata},
     date = {2016},
     pages = {P1A.16},
     publisher = {{OSA}},
     location = {{Kanpur}},
     doi = {10/gf5mrn},
     url = {https://www.osapublishing.org/abstract.cfm?URI=Photonics-2016-P1A.16},
     urldate = {2019-08-01},
     eventtitle = {International {{Conference}} on {{Fibre Optics}} and {{Photonics}}},
     isbn = {978-1-943580-22-4}
   }
   

6. Thermal Lens Study of NIR Femtosecond Laser-Induced Convection in Alcohols. S. Singhal and D. Goswami, ACS Omega 4(1), 1889–1896 (2019) [Abstract] [PDF] [BibTeX]

   Abstract: We use time-resolved thermal lens (TL) experiments to examine the convective heat transfer at microscale in the first eight members of the homologous series of primary alcohols. TL measurements enable a direct study of these primary alcohols without adding any chromophore as a function of varying heat loads created via femtosecond laser pulses at 1560 nm. Convective heat transfer leads to the asymmetrical and reduced thermal gradient, which substantially weakens the TL signal. The inflection in the time profile of the TL signal of methanol at higher powers is attributed to the greater molecular convection in methanol compared to other samples. This inflection dies out with a decrease in laser power. Our results demonstrate that the convection is more prominent at higher laser powers in all samples, and it modifies the trend in the steady-state TL signal of different alcohols with pump laser power. Methanol also has the highest steady-state TL among the primary alcohol series at low laser powers. The maxima in the TL signal are shifted systematically from methanol to ethanol and then to propanol as the laser power increases. Semiempirical analysis of time-resolved TL signal by using the latest theoretical TL model enabled us to extract the coefficient of convective heat transfer in methanol at different laser powers. In addition to that, analysis of other members of alcohol series at the highest (7.3 mW) laser power shows that convection is more facile in short-chain alcohols compared to the long-chain alcohols.

    BibTeX: @article{singhalThermalLensStudy2019,
     title = {Thermal {{Lens Study}} of {{NIR Femtosecond Laser-Induced Convection}} in {{Alcohols}}},
     author = {Singhal, Sumit and Goswami, Debabrata},
     date = {2019-01-31},
     journaltitle = {ACS Omega},
     shortjournal = {ACS Omega},
     volume = {4},
     number = {1},
     pages = {1889--1896},
     issn = {2470-1343, 2470-1343},
     doi = {10/gf5mp4},
     url = {http://pubs.acs.org/doi/10.1021/acsomega.8b02956},
     urldate = {2019-08-01}
   }
   

7. Unraveling the Molecular Dependence of Femtosecond Laser-Induced Thermal Lens Spectroscopy in Fluids. S. Singhal and D. Goswami, The Analyst 10.1039.C9AN01082C (2020) [Abstract] [PDF] [BibTeX]

   Abstract: Fluid systems exhibit thermal lens effects due to laser irradiation accompanied by convection and in contrast primarily conductive heat dissipation is observed in solids. , Fluid systems exhibit thermal lens effects due to laser irradiation accompanied by convection and in contrast primarily conductive heat dissipation is observed in solids. The presence of a significant convective mode modifies the temperature gradient in fluids resulting in a deviation of the experimental results from theories that are based on pure conduction. Herein, we present a carefully designed femtosecond laser experiment that keeps the heat generation process constant in order to account for the effect of molecular properties on thermal dissipation. We derive a theoretical model that introduces and characterizes the additional convective heat transfer in thermal lens (TL) spectroscopy which explains our observed experimental results. We measured the TL signal for a series of liquid aliphatic alkanes, ranging from hexane to decane, and their comparison has proven the validity of our model. The influence of convective heat transfer on the TL signal is predicted in terms of the dimensionless Peclet number ( P E ). The lower values of P E for alkanes with longer carbon chains indicate that the convective flow of heat slows down substantially for larger molecules.

    BibTeX: @article{singhalUnravelingMolecularDependence2020,
     title = {Unraveling the Molecular Dependence of Femtosecond Laser-Induced Thermal Lens Spectroscopy in Fluids},
     author = {Singhal, Sumit and Goswami, Debabrata},
     date = {2020},
     journaltitle = {The Analyst},
     shortjournal = {Analyst},
     pages = {10.1039.C9AN01082C},
     issn = {0003-2654, 1364-5528},
     doi = {10/ggfxvz},
     url = {http://xlink.rsc.org/?DOI=C9AN01082C},
     urldate = {2019-12-28}
   }
   

8. Thermal Lens Study of NIR Femtosecond Laser-Induced Convection in Alcohols. S. Singhal and D. Goswami, ACS Omega 4(1), 1889–1896 (2019) [Abstract] [PDF] [BibTeX]

   Abstract: We use time-resolved thermal lens (TL) experiments to examine the convective heat transfer at microscale in the first eight members of the homologous series of primary alcohols. TL measurements enable a direct study of these primary alcohols without adding any chromophore as a function of varying heat loads created via femtosecond laser pulses at 1560 nm. Convective heat transfer leads to the asymmetrical and reduced thermal gradient, which substantially weakens the TL signal. The inflection in the time profile of the TL signal of methanol at higher powers is attributed to the greater molecular convection in methanol compared to other samples. This inflection dies out with a decrease in laser power. Our results demonstrate that the convection is more prominent at higher laser powers in all samples, and it modifies the trend in the steady-state TL signal of different alcohols with pump laser power. Methanol also has the highest steady-state TL among the primary alcohol series at low laser powers. The maxima in the TL signal are shifted systematically from methanol to ethanol and then to propanol as the laser power increases. Semiempirical analysis of time-resolved TL signal by using the latest theoretical TL model enabled us to extract the coefficient of convective heat transfer in methanol at different laser powers. In addition to that, analysis of other members of alcohol series at the highest (7.3 mW) laser power shows that convection is more facile in short-chain alcohols compared to the long-chain alcohols.

    BibTeX: @article{singhalThermalLensStudy2020,
     langid = {english},
     title = {Thermal {{Lens Study}} of {{NIR Femtosecond Laser}}-{{Induced Convection}} in {{Alcohols}}},
     volume = {4},
     issn = {2470-1343, 2470-1343},
     url = {http://pubs.acs.org/doi/10.1021/acsomega.8b02956},
     doi = {10/gf5mp4},
     number = {1},
     journaltitle = {ACS Omega},
     urldate = {2019-08-01},
     date = {2019-01-31},
     pages = {1889-1896},
     author = {Singhal, Sumit and Goswami, Debabrata}
   }
   

9. Molecular Size and Mass Sensitive Femtosecond Thermal Spectrometer. S. Singhal, S. Goswami, A. Banerjee, and D. Goswami, in 2019 URSI Asia-Pacific Radio Science Conference (AP-RASC) (2019), pp. 1–3 [Abstract] [BibTeX]

   Abstract: Though a single ultrashort pulse has inconceivable thermal effect, highly repetitive femtosecond lasers often result in some heating effects. Instead of fretting over the thermal effect, we have used this to develop the time-resolved photothermal lens spectroscopy for molecular sensitivity.

    BibTeX: @inproceedings{singhalMolecularSizeMass2020,
     title = {Molecular {{Size}} and {{Mass Sensitive Femtosecond Thermal Spectrometer}}},
     doi = {10/gf5mqh},
     eventtitle = {2019 {{URSI Asia}}-{{Pacific Radio Science Conference}} ({{AP}}-{{RASC}})},
     booktitle = {2019 {{URSI Asia}}-{{Pacific Radio Science Conference}} ({{AP}}-{{RASC}})},
     date = {2019-03},
     pages = {1-3},
     author = {Singhal, S. and Goswami, S. and Banerjee, A. and Goswami, D.}
   }
   

10. Measurement of Pure Optical Nonlinearity in Carbon Disulfide with a High-Repetition-Rate Femtosecond Laser. S. Singhal, S. Dinda, and D. Goswami, Appl. Opt. 56(3), 644 (2017) [PDF] [BibTeX]

     BibTeX: @article{singhalMeasurementPureOptical2018,
      langid = {english},
      title = {Measurement of Pure Optical Nonlinearity in Carbon Disulfide with a High-Repetition-Rate Femtosecond Laser},
      volume = {56},
      issn = {0003-6935, 1539-4522},
      url = {https://www.osapublishing.org/abstract.cfm?URI=ao-56-3-644},
      doi = {10/gf5mrm},
      number = {3},
      journaltitle = {Appl. Opt.},
      urldate = {2019-08-01},
      date = {2017-01-20},
      pages = {644},
      author = {Singhal, Sumit and Dinda, Sirshendu and Goswami, Debabrata}
    }
    

11. Sensitive Dual Beam Thermal Lens Detection of Convection in Methanol. S. Singhal and D. Goswami, in 13th International Conference on Fiber Optics and Photonics (OSA, 2016), p. P1A.16 [Abstract] [PDF] [BibTeX]

    Abstract: We show that dual beam mode mismatched thermal lens (TL) technique is more sensitive than the single beam one. Presence of unusual peak inside the dual beam TL signal directly indicates strong convection.

     BibTeX: @inproceedings{singhalSensitiveDualBeam2017,
      langid = {english},
      location = {{Kanpur}},
      title = {Sensitive Dual Beam Thermal Lens Detection of Convection in Methanol},
      isbn = {978-1-943580-22-4},
      url = {https://www.osapublishing.org/abstract.cfm?URI=Photonics-2016-P1A.16},
      doi = {10.1364/PHOTONICS.2016.P1A.16},
      eventtitle = {International {{Conference}} on {{Fibre Optics}} and {{Photonics}}},
      booktitle = {13th {{International Conference}} on {{Fiber Optics}} and {{Photonics}}},
      publisher = {{OSA}},
      urldate = {2019-08-01},
      date = {2016},
      pages = {P1A.16},
      author = {Singhal, Sumit and Goswami, Debabrata}
    }
    

12. Exploring the Critical Role of Detection Aperture in Thermal Lens Measurements. S. Singhal, I. Bhattacharyya, and D. Goswami, in 2015 Workshop on Recent Advances in Photonics (WRAP) (2015), pp. 1–4 [Abstract] [BibTeX]

    Abstract: The critical role of the detection aperture size on the measurement of thermal lens signals has been explored by using mode-mismatched pump-probe thermal lens spectroscopy in pure methanol. Femtosecond laser pulses at 1560 nm are used to create thermal lens signal, which is simultaneously probed by a collinear 780 nm pulse train through a partially open aperture of different sizes placed after the sample. The trend in the time resolved thermal lens signals reveals that aperture size allowing less than half of the incident probe light is critical in providing accuracy of the thermal lens spectroscopy.

     BibTeX: @inproceedings{singhalExploringCriticalRole2016,
      title = {Exploring the Critical Role of Detection Aperture in Thermal Lens Measurements},
      doi = {10/gf5msd},
      eventtitle = {2015 {{Workshop}} on {{Recent Advances}} in {{Photonics}} ({{WRAP}})},
      booktitle = {2015 {{Workshop}} on {{Recent Advances}} in {{Photonics}} ({{WRAP}})},
      date = {2015-12},
      pages = {1-4},
      author = {Singhal, S. and Bhattacharyya, I. and Goswami, D.}
    }
    

13. Importance of Hydrogen Bonding in Thermal Lens Study of Highly Absorbing Liquids. S. Singhal, P. P. Roy, and D. Goswami, in Frontiers in Optics 2015 (2015), Paper FTu5E.5 (Optical Society of America, 2015), p. FTu5E.5 [Abstract] [PDF] [BibTeX]

    Abstract: Effect of thermal load on highly absorbing hydrogen bonded liquids are explored. We find heat transfer is better facilitated in lesser hydrogen bonded liquids by convection through molecular motion in addition to conduction.

     BibTeX: @inproceedings{singhalImportanceHydrogenBonding2016,
      langid = {english},
      title = {Importance of {{Hydrogen Bonding}} in {{Thermal Lens Study}} of {{Highly Absorbing Liquids}}},
      url = {https://www.osapublishing.org/abstract.cfm?uri=FiO-2015-FTu5E.5},
      doi = {10/gf5msf},
      eventtitle = {Frontiers in {{Optics}}},
      booktitle = {Frontiers in {{Optics}} 2015 (2015), Paper {{FTu5E}}.5},
      publisher = {{Optical Society of America}},
      urldate = {2019-08-01},
      date = {2015-10-18},
      pages = {FTu5E.5},
      author = {Singhal, Sumit and Roy, Partha Pratim and Goswami, Debabrata}
    }