Center for Relativistic Laser Science

UP 2018

Abstract: We present a dual frequency comb transient absorption (DFC-TA) technique that does not rely on mechanical-time-delay scan. DFC-TA measures photo-physical or -chemical processes occurring in several nanoseconds with femtosecond time resolution and millisecond data-acquisition time.

Optical frequency comb (OFC) has brought a remarkable advancement in gas-phase spectroscopy due to its high precision and accuracy. OFC is characterized by two parameters, i.e., repetition rate fr and carrier-envelop-offset frequency fceo, which need to be stabilized to maintain phase coherence. The frequency resolution of OFC spectroscopy is determined by the bandwidth of a single comb line. In dual frequency comb (DFC) system with two phase-locked OFC lasers, the slightly detuned repetition rates of the two OFCs linearly changes the time delay between the pulses from different OFCs. Here, the fixed carrier-envelop-phase (CEP) of OFC is critical for achieving interferometric spectroscopy with the DFC, where the interference provides phase and amplitude information of transmitted field [1,2]. Although DFC is a useful spectroscopic tool, DFC has not been actively employed for the studies of molecular reaction dynamics in condensed phases. The high frequency resolution of DFC cannot be brought out in condensed phases. The electronic coherence time of condensed phase, < 20 fs, is extremely shorter than that of gas phase, which is longer than tens of picosecond. In other words, most of pulses go to waste in condensed phase, because DFC has to scan as long as 1/fr, which is several nanoseconds. In addition, the broad electronic transition lines of molecules in condensed phases require a broadband light source. Previously, we experimentally demonstrate a condensed-phase linear spectroscopy with a broadband DFC system, which is based on Ti:sapphire laser [3]. The electronic transition line of a dye solution was well resolved even with a single interferogram measurement.

Optical frequency comb (OFC) has brought a remarkable advancement in gas-phase spectroscopy due to its high precision and accuracy. OFC is characterized by two parameters, i.e., repetition rate fr and carrier-envelop-offset frequency fceo, which need to be stabilized to maintain phase coherence. The frequency resolution of OFC spectroscopy is determined by the bandwidth of a single comb line. In dual frequency comb (DFC) system with two phase-locked OFC lasers, the slightly detuned repetition rates of the two OFCs linearly changes the time delay between the pulses from different OFCs. Here, the fixed carrier-envelop-phase (CEP) of OFC is critical for achieving interferometric spectroscopy with the DFC, where the interference provides phase and amplitude information of transmitted field [1,2]. Although DFC is a useful spectroscopic tool, DFC has not been actively employed for the studies of molecular reaction dynamics in condensed phases. The high frequency resolution of DFC cannot be brought out in condensed phases. The electronic coherence time of condensed phase, < 20 fs, is extremely shorter than that of gas phase, which is longer than tens of picosecond. In other words, most of pulses go to waste in condensed phase, because DFC has to scan as long as 1/fr, which is several nanoseconds. In addition, the broad electronic transition lines of molecules in condensed phases require a broadband light source. Previously, we experimentally demonstrate a condensed-phase linear spectroscopy with a broadband DFC system, which is based on Ti:sapphire laser [3]. The electronic transition line of a dye solution was well resolved even with a single interferogram measurement.

The single-scan DFC-TA data of IR125 ethanol solution are shown Fig. 1b. At first sight, both DFC-TA data with two fr’s, 1.0 kHz (Fig. 1b, red) and 12.5 Hz (Fig. 1b, black), well represent the population relaxation dynamics of IR125 ethanol solution. In addition, an ultrafast rise feature, which completes in 100 fs, in the single scan (Fig. 1b, inset, black) and averaged data (Fig. 1b, inset, green) with fr = 12.5 Hz (t = 1.95 fs) is also well resolved, while the TA data with fr = 1.0 kHz does not show the ultrafast dynamics due to its large time-step, t = 156 fs. Our experimental results with novel DFC-TA technique clearly show that one can study chemical reaction dynamics or physical processes occurring in the range from femtoseconds to nanoseconds. The data collection time is as short as about 0.08 s. In other words, ultra-broad dynamic range measurement of photo-chemical and -physical processes with fs time resolution becomes possible with our DFC-TA.

The interferometric optical trigger is only possible with frequency comb due to the fixed carrier envelope phase, otherwise nonlinear signal should be used to trigger data collection. Often, triggering with a nonlinear signal is inefficient because a large of amount of pulse energy is consumed just for light-induced triggering and its signal fluctuation is another source of time jitter error. Here, we checked that the time jitter from the fluctuation is negligible in our interferometric-trigger system. As can be seen in Fig. 1c, we could extract information on vibronically coupled normal modes and their relaxation times, which indicates that time jitter originating from the interferometric triggering is indeed negligible.

DFC-TA developed by us has interesting advantages over the conventional TA technique employing mechanical stage to scan the pump-probe time delay. One of the most notable features of DFC-TA is its fast scanning capability. By virtue of the fast data acquisition rate, DFC-TA could be applicable to microscopy. We anticipate that the present DFC-TA method will be of use to further develop coherent multidimensional spectroscopy in the near future.

References

[1] I. Coddington, W. C. Swann, and N. R. Newbury, “Coherent dual-comb spectroscopy at high signal-to-noise ratio,” Phys. Rev. A 82, 043817 (2010)

[2] B. Bernhardt, A. Ozawa, P. Jacquet, M. Jacquey, Y. Kobayashi, T. Udem, R. Holtzwarth, G. Guelachvili, T. W. Hänsch, N. Picqué, “Cavity-enhanced dual-comb spectroscopy,” Nat. Photon. 4, 55 (2009)

[3] B. Cho, T. H. Yoon, and M. Cho, “Dual comb spectroscopy of molecular electronic transitions,” (submitted)

[4] JW. Kim, B, Cho, T. H. Yoon, and M. Cho, “Dual frequency comb transient absorption: Measurement of ultrabroad dynamic range from femtosecond to nanosecond relaxations,” (submitted)