The action of femtosecond pulses from the mode locked lasers on photoactive systems such as solar cells and light emitting diodes can induce nonlinearity in the incoherent response, 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.
Nonlinear spectroscopy based on phase modulation:
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).
S. Fu, A. Sakurai, L. Liu, F. Edman, T. Pullerits, V. Öwall, K. J. Karki, "Generalized lock-in amplifier for precision measurement of high frequency signals", Rev. Sci. Instrum 84, 115101 (2013).
Photocurrent and fluorescence detected multiphoton microscopy:
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).