Exciton-phonon Interaction in Carbon Nanotubes and Excited State Dynamics of Red Fluorescent Proteins

Abstract

Observation of reactions in real time enables us directly to access the information on the reaction dynamics such as structural change, pathway of energy/charge flow, and coupling between energy states. However, many reactions are barely understandable because they occur in extremely short time scale inaccessible with ordinary spectroscopy. Time-resolved ultrafast spectroscopy has been promoted by the progress in ultrafast light sources. Ultrafast light sources are characterized by their short pulse duration and high peak intensity, and the former allows the observation of reactions with high temporal resolution while the latter opens the way to investigate excited state dynamics with nonlinear spectroscopy. Wavelength tunability of light sources is also a requirement of the ultrafast light sources. Especially, tunable femtosecond pulses in near-infrared (NIR) region is of great interest since they have the prospects for the study on optical properties of semiconductor, deep tissue imaging by multiphoton microscopy. This study consists of three parts: development of cavity-dumped femtosecond optical parametric oscillator (OPO) operating in near-infrared region discussed in chapter III, excition-phonon interaction in single-walled carbon nanotubes (SWNT) in chapter IV, and excited sate dynamics of red fluorescent proteins are presented in chapter V. Chapter III reports a synchronously pumped OPO based on a periodically poled stoichiometric lithium tantalate (PPSLT) crystal which is tunable in near-infrared region. This OPO adopts optical parametric generation (OPG) which decompose pump pulse into signal pulse and idler pulses and quasi-phase matching to utilize the largest nonlinear coefficient of PPSLT to maximize efficiency of frequency conversion, OPG in this case. Signal pulse whose wavelength falls into near-infrared region is chosen to be oscillated and amplified in the cavity to deliver pulse energy over 100 nJ up to 600 kHz and 70 nJ even at 1 MHz of repetition rate solidly. The pulse duration is as short as 42 fs, which is shorter than the PPSLT-based OPO reported before by Rowley et al. We have also explored the dispersion property of the OPO output because it is the information of importance that tells us what compensation of dispersion should be like to bring better quality in pulse. We measured the cross-correlation between the direct output of OPO and the Gaussian-shape short pulse and finally found that the OPO operates with positive group velocity dispersion (GVD) and negative third order dispersion (TOD). Therefore a grating compressor can provide better quality in the pulse after the compensation than a prism compressor does, however, the effect may not be dramatic owing to the extremely large TOD value. Such large TOD is one of the reason that the wings appear pronouncedly in the time profile of the pulse. This is a new approach to investigate properties of a light source beyond the measurements of the pulse energy and the pulse width, which brings more plentiful information. In chapter IV, exciton-phonon interaction in (6,5) single-walled carbon nanotube was investigated with transient absorption (TA) based on third order nonlinear spectroscopy. Recent experiments and theories suggested that nanotubes start their coherent radial breathing mode (RBM) by initially expanding or shrinking their diameter depending on the nanotube type. Furthermore, they could be reversed to initial shrinking or expanding motion in diameter, depending on the resonance type, E11 or E22. Coherent phonon (CP) wave packets are created by the impulsive excitation, and their motions can be recorded directly by probe pulse in TA experiment. With TA, direct measurement of vibrational dynamics can be obtained including the phase that provides critical information about initial motion of RBM. Because TA is based on third order nonlinear phenomena, CP provides vibrational dynamics of the excited state, whereas resonance Raman gives only the frequencies of the ground state. In this work, we have measured the CP wave packet motions of the E11 state well as the ground state. From the detailed information on the phonon motion and the theoretical calculation of the CP signals using third order nonlinear response theory, we confirmed the vibrational mode in E11 state. It is suggested that the diameter of the (6,5) SWNT is smaller in E11 state, and the RBM starts as the shrinking motion in the E11 state. Chapter V reports on the excited state dynamics of red fluorescent proteins (RFP) investigated with time-resolved fluorescence spectroscopy. RFP emitting red-shifted fluorescence is a probe with prospects because they have lower cellular autofluorescence background, lower light scatter, and higher tissue transmission compared to fluorophores operating at shorter wavelengths. However, the origin of such large Stokes shift of RFP is still ambiguous. Therefore, we measured the spectral change of fluorescence of RFP and compared the results with the results of simulation on the structure of RFP. One of the interesting feature found in time-resolved spectroscopy, is unusual blue-shift of the fluorescence spectrum occurring on nanoseconds timescales for the TagRFP675 and mKate/M41Q variants. Classical molecular dynamics simulations suggest TagRFP675 has three conformers in ground state, whereas mKate/M41Q has two. In addition to the simulation results, the fact that mKate/M41Q experiences large change of emission anisotropy in time and shows isoemissive point in area-normalized TRFS, it is highly probable for mKate/M41Q to have two emissive state. Furthermore, mKate/M41Q shows shorter lifetime at longer wavelength of fluorescence, therefore it is natural the red emitting state decays faster than the blue one does to give the blue-shift in the spectra of mKate/M41Q. On the other hand, TagRFP675 shows little change of emission anisotropy in time and no isoemissive point in area-normalized TRFS, therefore only one emissive state is involved in the fluorescence of TagRFP675. The larger flexibility of TagRFP675 compared to the other RFP variants, can lead significantly shifted excited state compared ground state. Spectral blue-shift of TagRFP675 can be explained by a model whose fully relaxed excited state of TagRFP675 exists above the one of the minima in ground state with small shift toward the left side of that local minimum. With these simple model, the spectral blue-shifts of mKate/M41Q and TagRFP675 are explained.