Krishnandu Makhal

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Education / Work History

• Postdoctoral Research Associate, University of Michigan (2017-present)
• Ph.D. Scholar, IIT Kanpur (2010-2017)
• M.Sc., Banaras Hindu University (2008-2010)

Thesis Abstract

Third Order Optical Nonlinearities and Ultra-fast Dynamics at Visible and Near-IT Wavelengths

The central theme of this thesis is twofold: One is to investigate third-order optical nonlinearities of conjugated dye molecules (organic chromophores) in the condensed phase i.e. in organic and inorganic solvents by means of Z-scan studies. These molecules absorb in visible and/or near-infra-red (NIR) wavelengths. In the present study laser pulses with widths ranging from a few hundreds of nanoseconds to tens of femtoseconds are employed with tunable wavelengths spanning the visible to NIR regions. The second theme of this thesis is to study the ultrafast dynamics of the aforesaid chromophores with femtosecond pulses employing pump-probe spectroscopy. Several subclasses, such as, degenerate and non-degenerate pump-probe technique, UV-vis pump and supercontinuum probe technique in addition to transient absorption, time correlated single photon counting (TCSPC) techniques are employed to monitor and explore the relaxation process and pathways. To understand the effect of molecular geometry and the possible effects of molecular structure due to electron delocalization on third-order nonlinearity, full geometry optimization has also been carried out using time dependent density functional theory (TDDFT) with the help of Gaussian09 software package. This combined experimental and theoretical investigations resulted in various new material properties and their ultrafast relaxation pathways, which were never explored earlier for the molecules studied. These studies also open up new applications of the NIR-absorbing dyes in the visible region at non-resonant excitations.

Research Topic / Interest

Nonlinear Spectroscopy and Ultrafast Optics. My research is based on Intense Light matter interaction and the consequences which follow after it. Trying to explain the nonlinear phenomena through Experiments like nonlinear Raman scattering, Three and four wave mixing, SHG, Optical Kerr Effect and magneto optical Kerr effect Spectroscopy. Exploring the Magnetization at Optical frequencies in dielectrics through Magneto-electric interactions experimentally.

Activities and Societies: My dissertation was on Third order nonlinearities and dynamics in visible and near infrared wavelengths. Carried my experiments using home built Pump Probe setup, Transient Absorption Spectroscopy, Two photon absorption and fluorescence spectroscopy and with Z scan experiments. Invested efforts on newly synthesized chromophores and on dielectrics.Activities and Societies: My dissertation was on Third order nonlinearities and dynamics in visible and near infrared wavelengths. Carried my experiments using home built Pump Probe setup, Transient Absorption Spectroscopy, Two photon absorption and fluorescence spectroscopy and with Z scan experiments. Invested efforts on newly synthesized chromophores and on dielectrics.

Publications

These include only those published in our lab.

1. Excited State Absorption and Relaxation Dynamics in a Series of Heptamethine Dyes under Femtosecond and Nanosecond Excitations. K. Makhal, S. Maurya, and D. Goswami, Physica Scripta 94(9), 095501 (2019) [Abstract] [PDF] [BibTeX]

   Abstract: We show that the near-infrared-absorbing polymethine cyanine dyes exhibit nonlinear absorption (NLA) in the visible region, where they have negligible or very low linear absorption. In chloroform solvent, the studied cyanines were found to be photo-unstable and exhibited photo-degradation under prolonged exposures as compared to methanol and dimethyl sulphoxide solvents, where they were stable. Excitation with femtosecond (fs) pulses in the visible region (400–750 nm) exhibits excited state absorption (ESA) as characterized by positive differential absorption (ΔOD) in transient absorption signal. Non-resonant excitations in the visible region are governed by reverse saturable absorption (RSA) mechanism leading to ESA. Excitation with nanosecond pulses also shows NLA owing to ESA from triplet states as compared to that from singlet states under fs excitations. Single beam Z-scan studies were performed with nanosecond (ns) pulses to evaluate ground (σ gr) and excited state (σ ex) absorption cross-sections, which confirms the mechanism of RSA as σ ex were found to be greater than σ gr. The population relaxation from higher excited singlet states shows ultrafast behavior with multi-exponential decay components. The fast time component decay varies from a few hundreds of fs (τ 1) to some hundreds of picoseconds (τ 2), and the long-time decay is in the ns domain (τ 3).

    BibTeX: @article{makhalExcitedStateAbsorption2019,
     title = {Excited State Absorption and Relaxation Dynamics in a Series of Heptamethine Dyes under Femtosecond and Nanosecond Excitations},
     author = {Makhal, Krishnandu and Maurya, Sidharth and Goswami, Debabrata},
     date = {2019-06},
     journaltitle = {Physica Scripta},
     shortjournal = {Phys. Scr.},
     volume = {94},
     number = {9},
     pages = {095501},
     issn = {1402-4896},
     doi = {10/gf5mpz},
     url = {https://doi.org/10.1088%2F1402-4896%2Fab0064},
     urldate = {2019-08-01}
   }
   

2. Extracting Third Order Optical Nonlinearities of Mn(III)-Phthalocyanine Chloride Using High Repetition Rate Femtosecond Pulses. K. Makhal, P. Mathur, S. Maurya, and D. Goswami, Journal of Applied Physics 121(5), 053103 (2017) [PDF] [BibTeX]

    BibTeX: @article{makhalExtractingThirdOrder2017,
     title = {Extracting Third Order Optical Nonlinearities of {{Mn}}({{III}})-{{Phthalocyanine}} Chloride Using High Repetition Rate Femtosecond Pulses},
     author = {Makhal, Krishnandu and Mathur, Paresh and Maurya, Sidharth and Goswami, Debabrata},
     date = {2017-02-06},
     journaltitle = {Journal of Applied Physics},
     shortjournal = {Journal of Applied Physics},
     volume = {121},
     number = {5},
     pages = {053103},
     issn = {0021-8979},
     doi = {10/gf5mrh},
     url = {https://aip.scitation.org/doi/abs/10.1063/1.4974966},
     urldate = {2019-08-01}
   }
   

3. pH Effect on Two-Photon Cross Section of Highly Fluorescent Dyes Using Femtosecond Two-Photon Induced Fluorescence. K. Makhal and D. Goswami, Journal of Fluorescence 27(1), 339–356 (2017) [Abstract] [PDF] [BibTeX]

   Abstract: Effect of solution pH on two-photon absorption cross-section of highly fluorescent Coumarin and Rhodamine dyes with high repetition rate femtosecond laser pulses at 780 nm is presented using two-photon induced fluorescence technique. A correspondence in the measured two-photon and single-photon cross-section values is seen when the pH changes from acidic to basic conditions (pH = 2–10) for solutions in 1:1 water-ethanol binary mixture. By plotting changes in the single-photon and two-photon fluorescence in this pH range, the excited state pKa values are found. The ground state pKa values are also affected by the protonation deprotonation equilibrium as a result of variation in pH from acidic to basic, which are characterized by changes in absorbance spectra. Most of these single-photon and two-photon induced fluorescence spectra show characteristic blue shifts. Different fluorescence quantum yields calculated at each pH reflect a change in structure corresponding to their associated properties as a result of acid base equilibrium.

    BibTeX: @article{makhalPHEffectTwoPhoton2017,
     title = {{{pH Effect}} on {{Two-Photon Cross Section}} of {{Highly Fluorescent Dyes Using Femtosecond Two-Photon Induced Fluorescence}}},
     author = {Makhal, Krishnandu and Goswami, Debabrata},
     date = {2017-01-01},
     journaltitle = {Journal of Fluorescence},
     shortjournal = {J Fluoresc},
     volume = {27},
     number = {1},
     pages = {339--356},
     issn = {1573-4994},
     doi = {10/f9rpw6},
     url = {https://doi.org/10.1007/s10895-016-1963-4},
     urldate = {2019-08-01}
   }
   

4. Reverse Saturable Absorption Followed by Anomalous Saturable Absorption in Rhodamine-700. K. Makhal, S. Maurya, and D. Goswami, in 13th International Conference on Fiber Optics and Photonics (OSA, 2016), p. Tu5D.2 [Abstract] [PDF] [BibTeX]

   Abstract: Anomalous SA (saturable absorption) in presence of reverse saturable absorption (RSA) in Rhodamine-700 is reported employing Z-scan with 100 ns, 250 Hz pulses in methanol. SA is observed only at a particular intensity range otherwise RSA survives.

    BibTeX: @inproceedings{makhalReverseSaturableAbsorption2016,
     title = {Reverse {{Saturable Absorption}} Followed by {{Anomalous Saturable Absorption}} in {{Rhodamine-700}}},
     booktitle = {13th {{International Conference}} on {{Fiber Optics}} and {{Photonics}}},
     author = {Makhal, Krishnandu and Maurya, Sidharth and Goswami, Debabrata},
     date = {2016},
     pages = {Tu5D.2},
     publisher = {{OSA}},
     location = {{Kharagpur}},
     doi = {10/gf5mrp},
     url = {https://www.osapublishing.org/abstract.cfm?URI=Photonics-2016-Tu5D.2},
     urldate = {2019-08-01},
     eventtitle = {International {{Conference}} on {{Fibre Optics}} and {{Photonics}}},
     isbn = {978-1-943580-22-4}
   }
   

5. Third-Order Nonlinear Optical Response and Ultrafast Dynamics of Tetraoxa[22]Porphyrin(2.1.2.1)s. K. Makhal, S. Arora, P. Kaur, D. Goswami, and K. Singh, Journal of Materials Chemistry C 4(40), 9445–9453 (2016) [Abstract] [PDF] [BibTeX]

   Abstract: Ultrafast dynamics and third-order nonlinear optical studies of tetraoxa[22]porphyrin(2.1.2.1)s are reported using a single beam Z-scan technique with femtosecond (fs) and nanosecond (ns) pulses at respective near-IR and visible wavelengths. Excited state absorption mediated reverse saturable absorption was observed at 527 nm while two-photon absorption was observed at 800 nm. High nonlinear refractive index coefficient (γ) and nonlinear absorption coefficient (β) values and the fast response times were attributed to the availability of polarizable π-electrons of the cyclic conjugated aromatic porphyrin framework. Our proposed mechanism of third-order nonlinearity was verified using ultrafast transient absorption spectroscopy. With the observed fast electronic nonlinearities the studied molecules are potential candidates for use as optical limiters.

    BibTeX: @article{makhalThirdorderNonlinearOptical2016,
     title = {Third-Order Nonlinear Optical Response and Ultrafast Dynamics of Tetraoxa[22]Porphyrin(2.1.2.1)s},
     author = {Makhal, Krishnandu and Arora, Shafali and Kaur, Paramjit and Goswami, Debabrata and Singh, Kamaljit},
     date = {2016},
     journaltitle = {Journal of Materials Chemistry C},
     volume = {4},
     number = {40},
     pages = {9445--9453},
     doi = {10/gf5mr7},
     url = {https://pubs.rsc.org/en/content/articlelanding/2016/tc/c6tc03239g},
     urldate = {2019-08-01}
   }
   

6. Excited State Absorption and Relaxation Dynamics in a Series of Heptamethine Dyes under Femtosecond and Nanosecond Excitations. K. Makhal, S. Maurya, and D. Goswami, Phys. Scr. 94(9), 095501 (2019) [Abstract] [PDF] [BibTeX]

   Abstract: We show that the near-infrared-absorbing polymethine cyanine dyes exhibit nonlinear absorption (NLA) in the visible region, where they have negligible or very low linear absorption. In chloroform solvent, the studied cyanines were found to be photo-unstable and exhibited photo-degradation under prolonged exposures as compared to methanol and dimethyl sulphoxide solvents, where they were stable. Excitation with femtosecond (fs) pulses in the visible region (400–750 nm) exhibits excited state absorption (ESA) as characterized by positive differential absorption (ΔOD) in transient absorption signal. Non-resonant excitations in the visible region are governed by reverse saturable absorption (RSA) mechanism leading to ESA. Excitation with nanosecond pulses also shows NLA owing to ESA from triplet states as compared to that from singlet states under fs excitations. Single beam Z-scan studies were performed with nanosecond (ns) pulses to evaluate ground (σ gr) and excited state (σ ex) absorption cross-sections, which confirms the mechanism of RSA as σ ex were found to be greater than σ gr. The population relaxation from higher excited singlet states shows ultrafast behavior with multi-exponential decay components. The fast time component decay varies from a few hundreds of fs (τ 1) to some hundreds of picoseconds (τ 2), and the long-time decay is in the ns domain (τ 3).

    BibTeX: @article{makhalExcitedStateAbsorption2020,
     langid = {english},
     title = {Excited State Absorption and Relaxation Dynamics in a Series of Heptamethine Dyes under Femtosecond and Nanosecond Excitations},
     volume = {94},
     issn = {1402-4896},
     url = {https://doi.org/10.1088%2F1402-4896%2Fab0064},
     doi = {10/gf5mpz},
     number = {9},
     journaltitle = {Phys. Scr.},
     urldate = {2019-08-01},
     date = {2019-06},
     pages = {095501},
     author = {Makhal, Krishnandu and Maurya, Sidharth and Goswami, Debabrata}
   }
   

7. Extracting Third Order Optical Nonlinearities of Mn(III)-Phthalocyanine Chloride Using High Repetition Rate Femtosecond Pulses. K. Makhal, P. Mathur, S. Maurya, and D. Goswami, Journal of Applied Physics 121(5), 053103 (2017) [PDF] [BibTeX]

    BibTeX: @article{makhalExtractingThirdOrder2018,
     title = {Extracting Third Order Optical Nonlinearities of {{Mn}}({{III}})-{{Phthalocyanine}} Chloride Using High Repetition Rate Femtosecond Pulses},
     volume = {121},
     issn = {0021-8979},
     url = {https://aip.scitation.org/doi/abs/10.1063/1.4974966},
     doi = {10/gf5mrh},
     number = {5},
     journaltitle = {Journal of Applied Physics},
     urldate = {2019-08-01},
     date = {2017-02-06},
     pages = {053103},
     author = {Makhal, Krishnandu and Mathur, Paresh and Maurya, Sidharth and Goswami, Debabrata}
   }
   

8. pH Effect on Two-Photon Cross Section of Highly Fluorescent Dyes Using Femtosecond Two-Photon Induced Fluorescence. K. Makhal and D. Goswami, J Fluoresc 27(1), 339–356 (2017) [Abstract] [PDF] [BibTeX]

   Abstract: Effect of solution pH on two-photon absorption cross-section of highly fluorescent Coumarin and Rhodamine dyes with high repetition rate femtosecond laser pulses at 780 nm is presented using two-photon induced fluorescence technique. A correspondence in the measured two-photon and single-photon cross-section values is seen when the pH changes from acidic to basic conditions (pH = 2–10) for solutions in 1:1 water-ethanol binary mixture. By plotting changes in the single-photon and two-photon fluorescence in this pH range, the excited state pKa values are found. The ground state pKa values are also affected by the protonation deprotonation equilibrium as a result of variation in pH from acidic to basic, which are characterized by changes in absorbance spectra. Most of these single-photon and two-photon induced fluorescence spectra show characteristic blue shifts. Different fluorescence quantum yields calculated at each pH reflect a change in structure corresponding to their associated properties as a result of acid base equilibrium.

    BibTeX: @article{makhalPHEffectTwoPhoton2018,
     langid = {english},
     title = {{{pH Effect}} on {{Two}}-{{Photon Cross Section}} of {{Highly Fluorescent Dyes Using Femtosecond Two}}-{{Photon Induced Fluorescence}}},
     volume = {27},
     issn = {1573-4994},
     url = {https://doi.org/10.1007/s10895-016-1963-4},
     doi = {10/f9rpw6},
     number = {1},
     journaltitle = {J Fluoresc},
     urldate = {2019-08-01},
     date = {2017-01-01},
     pages = {339-356},
     author = {Makhal, Krishnandu and Goswami, Debabrata}
   }
   

9. Reverse Saturable Absorption Followed by Anomalous Saturable Absorption in Rhodamine-700. K. Makhal, S. Maurya, and D. Goswami, in 13th International Conference on Fiber Optics and Photonics (OSA, 2016), p. Tu5D.2 [Abstract] [PDF] [BibTeX]

   Abstract: Anomalous SA (saturable absorption) in presence of reverse saturable absorption (RSA) in Rhodamine-700 is reported employing Z-scan with 100 ns, 250 Hz pulses in methanol. SA is observed only at a particular intensity range otherwise RSA survives.

    BibTeX: @inproceedings{makhalReverseSaturableAbsorption2017,
     langid = {english},
     location = {{Kharagpur}},
     title = {Reverse {{Saturable Absorption}} Followed by {{Anomalous Saturable Absorption}} in {{Rhodamine}}-700},
     isbn = {978-1-943580-22-4},
     url = {https://www.osapublishing.org/abstract.cfm?URI=Photonics-2016-Tu5D.2},
     doi = {10.1364/PHOTONICS.2016.Tu5D.2},
     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 = {Tu5D.2},
     author = {Makhal, Krishnandu and Maurya, Sidharth and Goswami, Debabrata}
   }
   

10. Third-Order Nonlinear Optical Response and Ultrafast Dynamics of Tetraoxa[22]Porphyrin(2.1.2.1)s. K. Makhal, S. Arora, P. Kaur, D. Goswami, and K. Singh, Journal of Materials Chemistry C 4(40), 9445–9453 (2016) [Abstract] [PDF] [BibTeX]

    Abstract: Ultrafast dynamics and third-order nonlinear optical studies of tetraoxa[22]porphyrin(2.1.2.1)s are reported using a single beam Z-scan technique with femtosecond (fs) and nanosecond (ns) pulses at respective near-IR and visible wavelengths. Excited state absorption mediated reverse saturable absorption was observed at 527 nm while two-photon absorption was observed at 800 nm. High nonlinear refractive index coefficient (γ) and nonlinear absorption coefficient (β) values and the fast response times were attributed to the availability of polarizable π-electrons of the cyclic conjugated aromatic porphyrin framework. Our proposed mechanism of third-order nonlinearity was verified using ultrafast transient absorption spectroscopy. With the observed fast electronic nonlinearities the studied molecules are potential candidates for use as optical limiters.

     BibTeX: @article{makhalThirdorderNonlinearOptical2017,
      langid = {english},
      title = {Third-Order Nonlinear Optical Response and Ultrafast Dynamics of Tetraoxa[22]Porphyrin(2.1.2.1)s},
      volume = {4},
      url = {https://pubs.rsc.org/en/content/articlelanding/2016/tc/c6tc03239g},
      doi = {10.1039/C6TC03239G},
      number = {40},
      journaltitle = {Journal of Materials Chemistry C},
      urldate = {2019-08-01},
      date = {2016},
      pages = {9445-9453},
      author = {Makhal, Krishnandu and Arora, Shafali and Kaur, Paramjit and Goswami, Debabrata and Singh, Kamaljit}
    }