The action of femtosecond pulses from mode locked lasers on photoactive systems, e.g., as solar cells and light emitting diodes, can induce nonlinearity in the incoherent responses such as fluorescence and photocurrent. The group develops and uses different techniques based on phase modulation of laser beams to isolate the different nonlinear signals from rather large linear background. The nonlinear signals can be used to investigate the ultrafast dynamics in the photoactive systems that contribute to the functionality of the devices.
The group also uses multiphoton fluorescence and photocurrent microscopy in order to investigate the sources of heterogeneity and their effects on the functionality of photoactive devices.
Photocurrent and fluorescence detected multiphoton microscopy:
Q. Shi, S. Ghosh, P. Kumar, L. C. Folkers, S. K. Pal, T. Pullerits, K. J. Karki, "Variations in the composition of the phases lead to the differences in the optoelectronic properties of MAPbBr3 thin films and crystals", J. Phys. Chem. C 122, 21817-21823 (2018).
S. Ghosh, Q. Shi, B. Pradhan, P. Kumar, Z. Wang, S. Acharya, S. Kalyan Pal, T. Pullerits, K. J. Karki, "Phonon coupling with excitons and free carriers in Formamidinium lead bromide perovskite nanocrystals", J. Phys. Chem. Lett., 9, 4245-4250 (2018).
Q. Shi, S. Ghosh, A. S. Sarkar, P. Kumar, Z. Wang, S. Kalyan Pal, T. Pullerits, K. J. Karki, "Variation in the photocurrent response due to different emissive states in methylammonium lead bromide perovskites", J. Phys. Chem. C 122, 3818–3823 (2018).
S. Ghosh, S. K. Pal, K. J. Karki, T. Pullerits, "Ion migration heals trapping centers in CH3NH3PbBr3 perovskite", ACS Energy Lett. 2, 2133-2139 (2017).
K. J. Karki, M. Abdellah, W. Zhang, T. Pullerits, "Different emissive states in the bulk and at the surface of methylammonium lead bromide perovskite revealed by two-photon micro-spectroscopy and lifetime measurements", Appl. Phys. Lett. Photonics 1, 046103 (2016).
Nonlinear spectroscopy based on phase modulation:
J. Chen, A. Krasavin, P. Ginzburg, A. V. Zayats, T. Pullerits, K. J. Karki, "Evidence of high-order nonlinearities in supercontinuum white-light generation from a gold nanofilm", ACS Photonics 5, 1927--1932 (2018).
F. A. Damtie, A. Wacker, T. Pullerits, K. J. Karki, "Two-dimensional action spectroscopy of excitonic systems: Explicit simulation using a phase-modulation technique", Phys. Rev. A 96, 053830 (2017).
K. J. Karki, "Increasing the density of modes in an optical frequency comb by cascaded four-wave mixing in a nonlinear optical fiber", Phys. Rev. A 96, 043802 (2017).
V. Al. Osipov, X. Shang, T. Hansen, T. Pullerits, K. J. Karki, "Nature of relaxation processes revealed by the action signals of intensity-modulated light fields", Phys. Rev. A 94, 053845 (2016).
K. J. Karki, L. Kringle, A. H. Marcus, T. Pullerits, "Phase-synchronous detection of coherent and incoherent nonlinear signals", J. Opt. 18, 015504 (2015).
K. J. Karki, J. R. Widom, J. Seibt, I. Moody, M. C. Lonergan, T. Pullerits, A. H. Marcus, "Coherent two-dimensional photocurrent spectroscopy in a PbS quantum dot photocell", Nat. Commun. 5, 5869 (2014).