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OPTICS LETTERS / Vol. 40, No. 6 / March 15, 2015
Stable and high-power few cycle supercontinuum for 2D ultrabroadband electronic spectroscopy Boris Spokoyny, Christine J. Koh, and Elad Harel* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA *Corresponding author: [email protected] Received December 12, 2014; revised January 27, 2015; accepted February 2, 2015; posted February 3, 2015 (Doc. ID 230485); published March 10, 2015 Broadband supercontinuum (SC) pulses in the few cycle regime are a promising source for spectroscopic and imaging applications. However, SC sources are plagued by poor stability, greatly limiting their utility in phase-resolved nonlinear experiments such as 2D photon echo spectroscopy (2D PES). Here, we generated SC by two-stage filamentation in argon and air starting from 100 fs input pulses, which are sufficiently high-power and stable to record timeresolved 2D PE spectra in a single laser shot. We obtain a total power of 400 μJ/pulse in the visible spectral range of 500–850 nm and, after compression, yield pulses with duration of 6 fs according to transient-grating frequency-resolved optical gating (TG-FROG) measurements. We demonstrate the method on the laser dye, Cresyl Violet, and observe coherent oscillations indicative of nuclear wavepacket dynamics. © 2015 Optical Society of America OCIS codes: (320.6629) Supercontinuum generation; (320.5520) Pulse compression; (190.4380) Nonlinear optics, four-wave mixing. http://dx.doi.org/10.1364/OL.40.001014
Two-dimensional photon echo spectroscopy (2D PES) is a powerful method to measure the vibrational and electronic structure of complex molecules in the condensed phase [1,2]. Unlike conventional pump-probe spectroscopy, 2D spectroscopy separates the temporal and spectral resolution of the experiment by recoding the signal as a function of time delays between pulses in the fourwave mixing sequence. It is highly desirable, therefore, to incorporate spectrally broad and temporally short excitation sources into 2D spectroscopic methods. Supercontinuum (SC) generation offers a means by which to convert readily available ultrafast sources into octave spanning and single or few cycle pulses [3]. Intense femtosecond pulses focused in transparent media results in significant spectral broadening by self-phase modulation [4], resulting in SC emission. SC generation in hollow core, photonic crystal fibers, and filamentation in gaseous or liquid media has been extensively explored [5–12]. These new SC sources offer opportunities to investigate new spectral and temporal regimes and have potential in applications, including optical imaging, microscopy [13–15], spectroscopy [16–18], optical metrology [19,20], and telecommunications [21,22]. However, SC remains problematic for multidimensional coherent spectroscopies because of the inherent instability of the nonlinear SC generation process, greatly limiting its practical implementation. Recently, we circumvented this limitation by using single-shot detection to measure the 2D PE spectrum of a cyanine dye, IR144, using an SC excitation source [23]. This method, which we call white gradient-assisted photon echo spectroscopy, (WHITE GRAPES), creates a spatiotemporal gradient that multiplexes the acquisition of multiple time delays, greatly improving the speed of 2D PES measurements [24]. As shown in Fig. 1, GRAPES uses the pulse fronts of each beam to create a time delay across the sample that is proportional to the crossing angle and the spatial extent of beams 1 and 2. Therefore, by using line focusing, all of the time delays that are parametrically sampled in a point-by-point (PBP) 2D 0146-9592/15/061014-04$15.00/0
sequence, are captured in one shot by using a 2D area detector. A major advantage of GRAPES is that it is highly immune to pulse instability which would otherwise yield noise in the Fourier domain of the experiment. Yet, SC remains challenging for experiments that require stability during the second period or waiting time, T, where dynamics occur. Here, we show that the stability of the SC is sufficient to record WHITE GRAPE spectra as a function of T in the dye, Cresyl Violet. Specifically, we demonstrate sufficient stability to observe quantum coherence beating signals corresponding to nuclear wave packet dynamics. Although SC generation in the few cycle regime has been successfully demonstrated [25–27], most approaches require either a source that is ultrashort
OPTICS LETTERS / Vol. 40, No. 6 / March 15, 2015
Stable and high-power few cycle supercontinuum for 2D ultrabroadband electronic spectroscopy Boris Spokoyny, Christine J. Koh, and Elad Harel* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA *Corresponding author: [email protected] Received December 12, 2014; revised January 27, 2015; accepted February 2, 2015; posted February 3, 2015 (Doc. ID 230485); published March 10, 2015 Broadband supercontinuum (SC) pulses in the few cycle regime are a promising source for spectroscopic and imaging applications. However, SC sources are plagued by poor stability, greatly limiting their utility in phase-resolved nonlinear experiments such as 2D photon echo spectroscopy (2D PES). Here, we generated SC by two-stage filamentation in argon and air starting from 100 fs input pulses, which are sufficiently high-power and stable to record timeresolved 2D PE spectra in a single laser shot. We obtain a total power of 400 μJ/pulse in the visible spectral range of 500–850 nm and, after compression, yield pulses with duration of 6 fs according to transient-grating frequency-resolved optical gating (TG-FROG) measurements. We demonstrate the method on the laser dye, Cresyl Violet, and observe coherent oscillations indicative of nuclear wavepacket dynamics. © 2015 Optical Society of America OCIS codes: (320.6629) Supercontinuum generation; (320.5520) Pulse compression; (190.4380) Nonlinear optics, four-wave mixing. http://dx.doi.org/10.1364/OL.40.001014
Two-dimensional photon echo spectroscopy (2D PES) is a powerful method to measure the vibrational and electronic structure of complex molecules in the condensed phase [1,2]. Unlike conventional pump-probe spectroscopy, 2D spectroscopy separates the temporal and spectral resolution of the experiment by recoding the signal as a function of time delays between pulses in the fourwave mixing sequence. It is highly desirable, therefore, to incorporate spectrally broad and temporally short excitation sources into 2D spectroscopic methods. Supercontinuum (SC) generation offers a means by which to convert readily available ultrafast sources into octave spanning and single or few cycle pulses [3]. Intense femtosecond pulses focused in transparent media results in significant spectral broadening by self-phase modulation [4], resulting in SC emission. SC generation in hollow core, photonic crystal fibers, and filamentation in gaseous or liquid media has been extensively explored [5–12]. These new SC sources offer opportunities to investigate new spectral and temporal regimes and have potential in applications, including optical imaging, microscopy [13–15], spectroscopy [16–18], optical metrology [19,20], and telecommunications [21,22]. However, SC remains problematic for multidimensional coherent spectroscopies because of the inherent instability of the nonlinear SC generation process, greatly limiting its practical implementation. Recently, we circumvented this limitation by using single-shot detection to measure the 2D PE spectrum of a cyanine dye, IR144, using an SC excitation source [23]. This method, which we call white gradient-assisted photon echo spectroscopy, (WHITE GRAPES), creates a spatiotemporal gradient that multiplexes the acquisition of multiple time delays, greatly improving the speed of 2D PES measurements [24]. As shown in Fig. 1, GRAPES uses the pulse fronts of each beam to create a time delay across the sample that is proportional to the crossing angle and the spatial extent of beams 1 and 2. Therefore, by using line focusing, all of the time delays that are parametrically sampled in a point-by-point (PBP) 2D 0146-9592/15/061014-04$15.00/0
sequence, are captured in one shot by using a 2D area detector. A major advantage of GRAPES is that it is highly immune to pulse instability which would otherwise yield noise in the Fourier domain of the experiment. Yet, SC remains challenging for experiments that require stability during the second period or waiting time, T, where dynamics occur. Here, we show that the stability of the SC is sufficient to record WHITE GRAPE spectra as a function of T in the dye, Cresyl Violet. Specifically, we demonstrate sufficient stability to observe quantum coherence beating signals corresponding to nuclear wave packet dynamics. Although SC generation in the few cycle regime has been successfully demonstrated [25–27], most approaches require either a source that is ultrashort
