This dissertation explores two main topics: Transient nonlinear refraction of air in Mid-IR spectral range and nonlinear optical properties of organometallic complexes. For seeing a vibrational and rotational Raman response the molecule should be Raman active. The first requirement for being a Raman active molecule is that the polarizability of molecule must be anisotropic. Linear symmetric molecules do have rotational Raman spectra. Not all the vibrational mode can be excited by a femtosecond pulse. The pulsewidth of our excitation beam should be less than the half of the vibration period. In this dissertation my excitation pulsewidth is not short enough to excite these vibrational modes and we ignored this contribution. The nonlinearity in both liquids and gases in general originates from both bound-electronic and nuclear responses. The bound-electronic response is an almost instantaneous response, however the nuclear response, simply because of the weight of nuclei, is a non-instantaneous response. For the nuclear response in the gas medium, we can ignore the libration and collision responses because of the low collision rates. We will also ignore the vibrational response since the bandwidth of the excitation pulse does not overlap with the first vibrational transitions of Oxygen and Nitrogen. For this reason the reorientational response is the only nuclear response contribution that we consider to the nonlinear refraction. Because of the low collision rates in the gases, the reorientational response does not damp quickly as it does in liquids and the molecules continue to rotate. Using short laser pulses with a broad bandwidth will excite coherently all the approximately rotational Raman lines. As a result, all the molecules that are occupying different rotational levels and therefore rotating at different rates will periodically rephase and results in pulsations of index of refraction. Since the nonlinearity arises from an instantaneous and non-instantaneous response we need to measure the nonlinearity with a time resolved technique. In addition, the relative polarization of pump and probe can greatly affect the observed nonlinear response. For these reasons we chose the time-resolved, polarization-sensitive, Beam-Deflection technique to perform our experiments. We performed both extremely nondegenerate and nearly degenerate experiments using the Near-IR and Mid-IR excitation beams and Mid-IR Probe to investigate the bound-electronic nonlinear refraction of air. To our knowledge, this is the first measurement of nonlinear refraction of air using both pump and probe in the Mid-IR. In addition to that, we explored the effect of pulsewidth of the excitation pulse on the nonlinear refraction by defining an effective nonlinear refractive index consisting of the effects of both the bound electronic response and reorientational response. We further analyzed the effect of changing the pressure and temperature on the pulsewidth dependence of effective nonlinear refraction. Finally, calculate the changes of effective nonlinear refraction for different layers of atmosphere using an atmospheric model From NASA. In the second major portion of this dissertation, we studied the nonlinear optical properties of organometallic complexes. These complexes were designed to have very high triplet quantum yields and fast intersystem crossing rates. Having very high singlet to triplet yields, makes these molecules good candidates for many applications. We studied seven different organometallic molecules. Three of these complexes were synthesized by our collaborators and the rest are commercially available. The majority of these molecules used iridium as the central metal while one used Ruthenium. Using the double pump-probe technique, which is a variation of pump-probe technique in order to decouple the singlet-triplet quantum yield from triplet cross-section. We performed our measurements using different sets of input fluences, using both pico-second and femto-second laser systems. We also developed a six-level electronic model that explains the complex nature of the interaction of optical pulses with some of these molecules. Our results show less than unity triplet quantum yields for these complexes. Since these molecules are believed to have very high triplet quantum yields, our results were contrary to our expectation.
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Doctor of Philosophy (Ph.D.)
College of Optics and Photonics
Optics and Photonics
Optics and Photonics
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Tofighi, Salimeh, "Transient Mid-IR Nonlinear Refraction in Air and Nonlinear Optical properties of Organometallic Complexes" (2020). Electronic Theses and Dissertations, 2020-. 143.