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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution
Interferometric Measurement of Transient Absorption and Refraction Spectra with Dual Frequency Comb JunWoo Kim, Tai Hyun Yoon, and Minhaeng Cho J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b09262 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018
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The Journal of Physical Chemistry
Interferometric Measurement of Transient Absorption and Refraction Spectra with Dual Frequency Comb JunWoo Kim,† Tai Hyun Yoon,*,†, § Minhaeng Cho*,†, ‡
†
Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Seoul 02841, Republic of Korea §
‡
Department of Physics, Korea University, Seoul 02841, Republic of Korea
Department of Chemistry, Korea University, Seoul 02841, Republic of Korea *E-mail:
[email protected],
[email protected] ABSTRACT We demonstrate that a dual frequency comb-transient absorption (DFC-TA) technique can be combined with a time-domain interferometric detection to measure both the transient absorption and refraction spectra of molecules in solution. To do this, the pump-probe signal field of DFC-TA is allowed to interfere with a time-delayed local oscillator field in time domain. We show that this DFC interferometric pump-probe spectroscopy (DFC-IPS) technique has a unique ability to extract the phase and amplitude information of pump-probe signal using
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just a single-scan data, while conventional techniques require an independent signal measured without the pump field for the normalization of the pump-probe spectrum. As a proof-ofprinciple experiment, we here show that the DFC-IPS enables us to simultaneously measure the frequency-resolved (from 650 to 950 nm) transient absorption and refraction signals with an exceptionally broad dynamic range from femtosecond to nanosecond without using a mechanical translational stage for pump-probe time-scanning. We anticipate that our DFC-IPS technique with femtosecond time-resolution capability will be useful to investigate photo-induced chemical and biological reactions covering broad dynamic ranges.
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The Journal of Physical Chemistry
1. INTRODUCTION
Linear spectroscopy such as absorption and luminescence measurement methods provides invaluable information about time and ensemble-averaged structure of molecules in condensed phases. However, the lack of time resolvability limits its use in studying a wide range of chemical and biological reaction dynamics. Therefore, time-resolved spectroscopy has long been used to study a variety of photo-chemical and physical processes such as energy transfer processes in photosynthetic complexes1-3, photo-thermal energy conversion4-5, photovoltaics6, and fundamental solvation dynamics7-8.
Transient absorption (TA) measurement is a simple and useful time-resolved spectroscopic technique, where the pump-induced change of probe absorption is measured with respect to the time delay between the pump and probe pulses. For example, the probe beam intensity can decrease due to excited state absorptions (ESA) when the probe field frequency is in resonance to a high-lying energy state from the excited state by the pump, whereas it can rather increase due to the ground-state bleach (GSB) and stimulated emission (SE) processes. By adding dispersive optics to the TA apparatus, one can further obtain the pump-probe or TA spectrum at a given pump-probe delay time providing the time-domain information about detailed photo-excited state dynamics of molecules of interest. Due to the versatility and experimental simplicity of TA measurement, it has been widely used for studying photochemical reactions3, 9-10 and even for developing time-resolved optical microscopy11.
In the conventional TA or more generally pump-probe measurement studies, the pumpprobe time-delay, T, is controlled by varying the optical-path length between the pump and probe beams with a mechanical translational stage. However, it has been shown that two mode-locked
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lasers with slightly detuned repetition rates can be used to automatically scan T to investigate the population dynamics of a molecular ensemble.12-13 Due to the difference between the two repetition rates, the time delay between the pulses generated by two different oscillators increase linearly right after the time that the pump and probe pulses overlap. This automatic time-delay generation is called asynchronous optical sampling and it has been utilized for time-domain THz spectroscopy14-15 and dual frequency comb (DFC) based experiments16-20. Recently, we have shown that DFC-based femtosecond TA (DFC-TA) measurement for a dye solution, where the TA signals decaying on the time scales from femtosecond to nanosecond were obtained without utilizing any mechanical time-delay-scanning device.21 Our DFC-TA using two repetition frequency-tunable optical frequency-comb (OFC) lasers and interferometric optical triggering method have definite advantages with improved scan rate, high time-resolution, and multichannel measurement capability as compared to the conventional pump-probe method with a single mode-locked laser. In the present work, we further demonstrate that DFC-based pump-probe measurement can be combined with interferometric detection of the complex (both real and imaginary parts of) pump-probe signal. This DFC-based interferometric time and frequency-resolved pump-probe spectroscopy (DFC-IPS) does not need dispersive optics nor array detector to measure the pumpprobe spectrum at a given waiting time T. Moreover, it allows us to measure both real (transient refraction, TR) and imaginary (TA) parts of the nonlinear responses of molecules in condensed phase with just a single time-domain interference scan, since the same signal measured at the steady-state is used for the normalization of the pump-probe spectrum (see Eq. (4) later). The important aspects for the success of our DFC-IPS are (1) highly stable repetition frequencies of two independent OFC lasers, which are critical for both automatic and precise pump-probe time
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The Journal of Physical Chemistry
delay scans with uniform spatial beam quality and (2) exceptionally fast and accurate optical triggering system, which reduces the time jitter to be less than 1 fs for repetitive T-scans. Thereby, the present DFC-IPS method has advantages in exceptionally broad dynamic range from a few fs to 10 ns as well as in broad frequency range from 315 THz to 460 THz (650 nm to 950 nm in wavelength). Overall, this paper is organized as follows. In Sec. 2, we provide experimental details such as dual-frequency comb source, phase stability, optical triggering method, interferometric detection scheme, fast scan approach, and data analysis method. In Sec. 3, a theoretical description of the present DFC-IPS is presented. Experimental results and discussion are presented in Sec. 4. The main results are summarized in Sec. 5.
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