- Email: [email protected]
Standardized specifications of 2D optical spectrometers
Results in Chemistry 1 (2019) 100001
Contents lists available at ScienceDirect
Results in Chemistry journal homepage: www.elsevier.com/locate/rechem
Standardized specifications of 2D optical spectrometers Daniel B. Turner Department of Chemistry, New York University, New York, NY 10003, USA
a r t i c l e
i n f o
Article history: Received 23 March 2019 Accepted 26 April 2019 Available online xxxx
a b s t r a c t In recent years two-dimensional (2D) optical spectrometers have proliferated, and a large number of designs and refinements are available. While researchers typically provide a few instrumentation specifications, many important quantifiable technical parameters are unreported. This lack of disclosure is a barrier to new researchers. Many of the unreported parameters can produce spectral artifacts and noise and are therefore likely responsible for reproducibility problems. This microarticle contains a list of parameters that can be used for all wavelength ranges and spectrometer designs, with a focus on the most common implementation of kHz laser systems using the BOX beam geometry. Adoption of this standardized reporting protocol will lead to further spectrometer refinements and improve reproducibility. © 2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).
1. Introduction
2. Discussion
Many groups have developed and refined 2D optical spectrometers for studying systems that absorb across a vast range of the electromagnetic spectrum, from THz frequencies through the IR and visible and even into the UV [1–5]. Analogous to the widespread use of 2D nuclear magnetic resonance (NMR) [6], the number of biological, chemical, and physical mechanisms studied using 2D optical spectroscopy is astonishingly diverse. Researchers typically report a few parameters of each spectrometer design or implementation, but these are insufficient to enable others to reproduce results and suggests that some researchers may be unaware of all the key characteristics. While some texts adequately describe the concepts of 2D optical spectroscopy [1,7], resources for the key aspects of technical implementation [8] are lacking. A standardized reporting protocol for 2D optical spectrometers will enable researchers to distinguish how instrumentation differences affect measured spectra and eventually lead to a convergence on optimal spectrometer designs. 2D electronic spectroscopy has demonstrated large differences in measured spectra of identical samples. For example, the 2D ES of a photosynthetic complex measured by some of the same researchers under the same conditions are quite distinct [9–11]. In a second example, large differences appear among the 2D spectra of the laser dye cresyl violet perchlorate published by ≥7 groups [12–19]. Reproducibility is an important aspect of science, and vast disparities among measurements motivate the adoption of standardized protocols.
In 2D optical spectroscopy, the key parameters can be grouped into three broad categories: (I) light source, (II) spectrometer design, and (III) data acquisition and processing. Currently, only the properties marked with an asterisk (*) have been broadly adopted.
E-mail address: [email protected].
(I) The parameters of the light source include: 1. A normalized laser pulse spectrum.* 2. The repetition rate of the source laser.* 3. The laser fluence at the sample position, either per beam or the total of all excitation beams.* 4. The measured pulse duration.* This can be supplemented by using a phase-retrieval method [20] to extract and report the spectral phase profile, ϕ(ω), of the pulse. 5. Shot-to-shot intensity (I) stability—characterized as the standard deviation over the mean, σ I =I—based on measurements of individual pulse intensities using a linear measuring device [21]. Our group, for example, uses an unbiased photodiode, a current preamplifier, and a DAQ card to measure the intensity of ∼100 k laser shots from the noncollinear optical parameter amplifier (NOPA) [22]. Typical stability values for our NOPA are 0.5%. Some groups report the standard deviation of the mean. However, the standard deviation over the mean is the preferred way to report the amount of variability in a set of data from its mean value [23]. 6. Shot-to-shot spectral stability. Some light sources exhibit significant spectral fluctuations between laser shots in addition to intensity fluctuations. Therefore researchers should quantify the degree to which the laser spectrum ‘wobbles’. A consistent method would be to acquire a large number of spectra from
https://doi.org/10.1016/j.rechem.2019.100001 2211-7156/© 2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
D.B. Turner / Results in Chemistry 1 (2019) 100001
individual laser shots, normalize each spectrum, and then prepare an errorbar plot of the fluctuations at each pixel for the set of measured spectra [24]. 7. Shot-to-shot intensity correlations. Some researchers have reported large correlations in intensity between adjacent laser shots, while other groups report largely ‘white’ (uncorrelated) noise. White noise requires fewer pulses to reach convergence. Only a few groups have report this value [22,25,26]. 8. A property of a collimated broadband light source that is more difficult to quantify is its angular dispersion. Angular dispersion induces frequency-dependent phase-matching conditions that will produce artifacts in 2D spectra. Strict quantification of angular dispersion is complicated [27], and an estimated approach is likely adequate. Users can translate a pinhole placed at the sample position an equal amount in all four directions and measure the spectrum of the scattered light in the farfield. The transverse displacement distance in each direction should be the (calculated) beam waist. Reporting the five values of λmax for each beam is a reliable indicator of the focusing conditions. 9. The optical polarization configuration of the laser beams. Many researchers use polarization schemes—denoted in various ways [28–30] such as (xxxx), (HH,VV), (+75∘,−75∘, 0∘, 0∘), or (σ+, σ−, σ−, σ+)—to infer useful information about the sample based on selection rules or by invoking isotropic averaging theories. These results are influenced by the purity of the polarization of the incoming beams and the polarization dependence of the optical elements after the sample. Thus, the polarization configuration and an assessment of the purity of the optical polarization should be reported. (II) The parameters of the spectrometer design include: 1. The phase-matching conditions, such as pump–probe or BOX beam geometries.* 2. The short-term and long-term phase stability of the nonlinear signal.* The time windows chosen for the two types of measurements should be at least the time required for a single 2D scan and the time required for a 3D scan, respectively [31]. 3. Directional-filter and phase-mismatch parameters [32]. These quantities are calculated from the laser bandwidth, the beam geometry, and the sample thickness, and they estimate distortions that arise in 2D spectra. The distortions include narrowed or square-shaped peaks. Fig. (1) reveals that improving these parameters leads to spectra with larger surface areas. 4. A key indication of the robustness of a spectrometer is its ability to maintain a stable nonlinear signal. Researchers should quantify the stability by reporting fluctuations of the homodyne nonlinear signal at {τ1 = 0, τ2 ≠ 0}. Using τ2 ≠ 0 eliminates fluctuations caused by the nonresonant response, which often contains higher-order components that are inherently less stable [33]. This is best measured using shot-by-shot acquisitions and quantified as a standard deviation over the mean. 5. Most coherent 2D optical spectroscopy measurements to date have required an ‘indirect’ detection dimension, meaning a dimension that is stepped while the emitted spectrum is measured directly. Poor accuracy of the delay stage(s) can distort peaks in a 2D spectrum. Thus researchers should report the accuracy or bidirectional repeatability—not resolution—of the delay stage(s). 6. Scattered light can contaminate measured spectra significantly and produce peaks with incorrect shapes. To quantify scatter, researchers should translate one delay stage away from the time zero position and then quantify the fractional amount of spectral fringe that appears on the recorded heterodyne spectrum for each excitation beam. This procedure should occur before chopping or phase-cycling schemes are applied. 7. For spectrometers that use pulse shapers, the calibrations procedures and an assessment of artifacts including pulse replica,
phase-cycling errors, and space-time coupling [31,34,35] should be described. (III) The parameters of the data acquisition and processing include: 1. Artifacts arise when multiple pulses are binned before digitizing [22,36]. Therefore researchers should state the number of spectra binned. 2. Related to the convergence speed issue, researchers should report the number of shots averaged per acquisition. In our laboratory, for example, we typically acquire and analyze ∼500 quads of laser shots in a 2D measurement using dual-beam chopping [18]. 3. Researcher should state if they use a data-rejection algorithm or not [37]. 4. Averaging of ratiometric measurements can be done several ways, although only one or two methods are optimal. Thus the averaging formula should be chosen carefully and reported [18,22,38]. 5. Because light sources have intensity and spectral fluctuations, researchers should use and describe the balanced-detection scheme [18,24].
600
500 +1
excitation frequency (THz)
2
0
400
–1
600
500 +1
0
400 400
–1
500
600
emission frequency (THz) Fig. 1. We measured these 2D ES of cresyl violet perchlorate in methanol in the same laboratory with the same laser bandwidth. In the bottom spectrum, we improved the geometry to minimize phase-mismatch and directional-filter distortions, installed delay stages with improved accuracy, and improved the ‘phasing’ algorithm. These refinements decreased noise and increased the effective spectral surface area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
D.B. Turner / Results in Chemistry 1 (2019) 100001
6. ‘Phasing’ the 2D spectrum to a pump–probe spectrum is always imperfect. Researchers should therefore report the total normalized error. We have found that spectra with integrated errors N0.3% have unreliable features [39]. 7. During analysis, researchers should not interpolate the directly detected dimension because the Fourier transformations used in spectral interferometry can distort peaks [40]. Therefore the spectral interferometry procedure should be reported.
3. Conclusion & outlook One leading researcher recently proposed that 2D spectroscopy will evolve into a “dominant spectroscopic method throughout the sciences” [41]. This will require users to adopt a more analytic mindset [42] to make the measured 2D spectra more rigorous and therefore more broadly useful. Of the 23 parameters detailed above, research groups currently report only ∼6. Adopting the full set of standardized specifications by the 2D spectroscopy community will decrease barriers to entry, improve reproducibility, and enhance the general perception of the field. The list presented here applies most directly to spectrometers that operate at kHz repetition rates using the BOX beam geometry. Spectrometers that use less-conventional configurations—for example those that operate at MHz frequencies, use the pump–probe geometry, or do not use optical detection [4]—will need to modify certain items on the list. Researchers using such instruments are encouraged to propose a similar list that is appropriate to their implementation. Acknowledgments DBT thanks former coworkers and students for their contributions to these technical improvements and acknowledges financial support from the Alfred P. Sloan Foundation and the National Science Foundation under CAREER Grant No. CHE–1552235. References [1] P. Hamm, M.T. Zanni, Concepts and Methods of 2D Infrared Spectroscopy, Cambridge University Press, 2011. [2] M. Woerner, W. Kuehn, P. Bowlan, K. Reimann, T. Elsaesser, Ultrafast twodimensional terahertz spectroscopy of elementray excitations in solids, New J. Phys. 15 (2013), 025039. [3] B.A. West, P.G. Giokas, B.P. Molesky, A.D. Ross, A.M. Moran, Toward twodimensional photon echo spectroscopy with 200 nm laser pulses, Opt. Express 21 (2013) 2118–2125. [4] F.D. Fuller, J.P. Ogilvie, Experimental implementations of two-dimensional Fourier transform electronic spectroscopy, Annu. Rev. Phys. Chem. 66 (2015) 667–690. [5] V.I. Prokhorenko, A. Picchiotti, M. Pola, A.G. Dijkstra, R.J.D. Miller, New insights into the photophysics of DNA nucleobases, J. Phys. Chem. Lett. 7 (2016) 4445–4450. [6] D.M. Jonas, Optical analogs of 2D NMR, Science 300 (2003) 1515–1517. [7] S. Mukamel, Principles of Nonlinear Optical Spectroscopy, Oxford University Press, New York, 1995. [8] N. Tkachenko, Optical Spectroscopy: Methods and Implementation, 1st ed. Elsevier Science, 2006. [9] T. Brixner, J. Stenger, H.M. Vaswani, M. Cho, R.E. Blankenship, G.R. Fleming, Twodimensional spectroscopy of electronic couplings in photosynthesis, Nature 434 (2005) 625{628. [10] G.S. Engel, T.R. Calhoun, E.L. Read, T.-K. Ahn, T. Mancal, Y.-C. Cheng, R.E. Blankenship, G.R. Fleming, Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems, Nature 446 (2007) 782{786. [11] G. Panitchayangkoon, D. Hayes, K.A. Fransted, J.R. Caram, E. Harel, J. Wen, R.E. Blankenship, G.S. Engel, Longlived quantum coherence in photosynthetic complexes at physiological temperature, Proc. Natl. Acad. Sci. U. S. A. 107 (2010), 12766{12770. [12] J.A. Myers, K.L.M. Lewis, P.F. Tekavec, J.P. Ogilvie, Two-dimensional Fourier transform electronic spectroscopy with a pulse-shaper, in: P. Corkum, S. Silvestri, K.A. Nelson, E. Riedle, R. Schoenlein (Eds.), Ultrafast Phenomena XVI: Proceedings of the 16th International Conference Springer Series in Chemical Physics, 92, Heidelberg, Germany 2009, pp. 956–958.
3
[13] D.B. Turner, K.E. Wilk, P.M.G. Curmi, G.D. Scholes, Comparison of electronic and vibrational coherence measured by two-dimensional electronic spectroscopy, J. Phys. Chem. Lett. 2 (2011) 1904{1911. [14] I.A. Heisler, R. Moca, F.V.A. Camargo, S.R. Meech, Two-dimensional electronic spectroscopy based on conventional optics and fast dual chopper data acquisition, Rev. Sci. Instrum. 85 (2014), 063103. [15] L.A. Bizimana, J. Brazard, W.P. Carbery, T. Gellen, D.B. Turner, Resolving molecular vibronic structure using highsensitivity two-dimensional electronic spectroscopy, J. Chem. Phys. 143 (2015), 164203. [16] B. Spokoyny, C.J. Koh, E. Harel, Stable and high-power few cycle supercontinuum for 2D ultrabroadband electronic spectroscopy, Opt. Lett. 40 (2015) 1014–1017. [17] X. Ma, J. Dostál, T. Brixner, Broadband 7-fs differactive-optic-based 2d electronic spectroscopy using hollow-core fiber compression, Opt. Express 24 (2016) 20781–20791. [18] T. Gellen, L.A. Bizimana, W.P. Carbery, I. Breen, D.B. Turner, Ultrabroadband twoquantum two-dimensional electronic spectroscopy, J. Chem. Phys. 145 (2016), 064201. [19] S. Draeger, S. Roeding, T. Brixner, Rapid-scan coherent 2D fluorescence spectroscopy, Opt. Express 25 (2017), 281609. [20] R. Trebino, K.W. DeLong, D.A. Fittinghoff, J.N. Sweetser, M.A. Kumbugel, B.A. Richman, D.J. Kane, Measuring ultrashort laser pulses in the time-frequency domain using frequency-resolved optical gating, Rev. Sci. Instrum. 68 (1997) 3277–3296. [21] C.A. Werley, S.M. Teo, K.A. Nelson, Pulsed laser noise analysis and pump-probe signal detection with a data acquisition card, Rev. Sci. Instrum. 82 (2011), 123108. [22] J. Brazard, L.A. Bizimana, D.B. Turner, Accurate convergence of transient-absorption spectra using pulsed lasers, Rev. Sci. Instrum. 86 (2015), 053106. [23] J.R. Taylor, An Introduction to Error Analysis: The Study of Uncertainties in Physical Measurements, University Science Books, 1997. [24] A.L. Dobryakov, S.A. Kovalenko, A. Weigel, J.L. Perez-Lustres, J. Lange, A. Müller, N.P. Ernsting, Femtosecond pump/supercontinuum-probe spectroscopy: optimized setup and signal analysis for single-shot spectral referencing, Rev. Sci. Instrum. 81 (2010), 113106. [25] D. Polli, L. Lüer, G. Cerullo, High-time-resolution pumpprobe system with broadband detection for the study of timedomain vibrational dynamics, Rev. Sci. Instrum. 78 (2007), 103108. [26] C. Schriever, S. Lochbrunner, E. Riedle, D.J. Nesbitt, Ultrasensitive ultraviolet-visible 20 f. absorption spectroscopy of low vapor pressure molecules in the gas phase, Rev. Sci. Instrum. 79 (2008), 013107. [27] E. Hecht, Optics, 5th ed. Pearson, 2016. [28] G.D. Scholes, Selection rules for probing biexcitons and electron spin transitions in isotropic quantum dot ensembles, J. Chem. Phys. 121 (2004), 10104{10110. [29] E.L. Read, G. Schlau-Cohen, G.S. Engel, J. Wen, R.E. Blankenship, G.R. Fleming, Visualization of excitonic structure in the Fenna-Matthews-Olson photosynthetic complex by polarization-dependent two-dimensional electronic spectroscopy, Biophys. J. 95 (2008) 847–856. [30] K.W. Stone, D.B. Turner, K. Gundogdu, S.T. Cundiff, K.A. Nelson, Exciton-exciton correlations revealed by two-quantum, two-dimensional Fourier transform optical spectroscopy, Acc. Chem. Res. 42 (2009) 1452–1461. [31] D.B. Turner, K.W. Stone, K. Gundogdu, K.A. Nelson, Invited article: the coherent optical laser beam recombination technique (COLBERT) spectrometer: coherent multidimensional spectroscopy made easier, Rev. Sci. Instrum. 82 (2011), 081301. [32] M.K. Yetzbacher, N. Belabus, K.A. Kitney, D.M. Jonas, Propagation, beam geometry, and detection distortions of peak shapes in two-dimensional Fourier transform spectra, J. Chem. Phys. 126 (2007), 044511. [33] R.W. Boyd, Nonlinear Optics, 2nd ed. Academic Press, San Diego, 2003. [34] J.C. Vaughan, T. Feurer, K.W. Stone, K.A. Nelson, Analysis of replica pulses in femtosecond pulse shaping with pixelated devices, Opt. Express 14 (2006) 1314–1328. [35] F. Frei, R. Bloch, T. Feurer, Influence of finite spatial resolution on single- and doublepass femtosecond pulse shapers, Opt. Lett. 35 (2010) 4072–4074. [36] B. Lang, Photometrics of ultrafast and fast broadband electronic transient absorption spectroscopy: State of the art, Rev. Sci. Instrum. 89 (2018), 093112. [37] K.E.H. Anderson, S.L. Sewall, R.R. Cooney, P. Kambhampati, Noise analysis and noise reduction methods in kilohertz pump-probe experiments, Rev. Sci. Instrum. 78 (2007), 073101. [38] S.D. McClure, D.B. Turner, P.C. Arpin, T. Mirkovic, G.D. Scholes, Coherent oscillations in the PC577 cryptophyte antenna occur in the excited electronic state, J. Phys. Chem. B 118 (2014) 1296–1308. [39] B.P. Petkov, T.A. Gellen, C.A. Farfan, W.P. Carbery, B.E. Hetzler, D. Trauner, X. Li, W.J. Glover, D.J. Ulness, D.B. 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Contents lists available at ScienceDirect
Results in Chemistry journal homepage: www.elsevier.com/locate/rechem
Standardized specifications of 2D optical spectrometers Daniel B. Turner Department of Chemistry, New York University, New York, NY 10003, USA
a r t i c l e
i n f o
Article history: Received 23 March 2019 Accepted 26 April 2019 Available online xxxx
a b s t r a c t In recent years two-dimensional (2D) optical spectrometers have proliferated, and a large number of designs and refinements are available. While researchers typically provide a few instrumentation specifications, many important quantifiable technical parameters are unreported. This lack of disclosure is a barrier to new researchers. Many of the unreported parameters can produce spectral artifacts and noise and are therefore likely responsible for reproducibility problems. This microarticle contains a list of parameters that can be used for all wavelength ranges and spectrometer designs, with a focus on the most common implementation of kHz laser systems using the BOX beam geometry. Adoption of this standardized reporting protocol will lead to further spectrometer refinements and improve reproducibility. © 2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).
1. Introduction
2. Discussion
Many groups have developed and refined 2D optical spectrometers for studying systems that absorb across a vast range of the electromagnetic spectrum, from THz frequencies through the IR and visible and even into the UV [1–5]. Analogous to the widespread use of 2D nuclear magnetic resonance (NMR) [6], the number of biological, chemical, and physical mechanisms studied using 2D optical spectroscopy is astonishingly diverse. Researchers typically report a few parameters of each spectrometer design or implementation, but these are insufficient to enable others to reproduce results and suggests that some researchers may be unaware of all the key characteristics. While some texts adequately describe the concepts of 2D optical spectroscopy [1,7], resources for the key aspects of technical implementation [8] are lacking. A standardized reporting protocol for 2D optical spectrometers will enable researchers to distinguish how instrumentation differences affect measured spectra and eventually lead to a convergence on optimal spectrometer designs. 2D electronic spectroscopy has demonstrated large differences in measured spectra of identical samples. For example, the 2D ES of a photosynthetic complex measured by some of the same researchers under the same conditions are quite distinct [9–11]. In a second example, large differences appear among the 2D spectra of the laser dye cresyl violet perchlorate published by ≥7 groups [12–19]. Reproducibility is an important aspect of science, and vast disparities among measurements motivate the adoption of standardized protocols.
In 2D optical spectroscopy, the key parameters can be grouped into three broad categories: (I) light source, (II) spectrometer design, and (III) data acquisition and processing. Currently, only the properties marked with an asterisk (*) have been broadly adopted.
E-mail address: [email protected].
(I) The parameters of the light source include: 1. A normalized laser pulse spectrum.* 2. The repetition rate of the source laser.* 3. The laser fluence at the sample position, either per beam or the total of all excitation beams.* 4. The measured pulse duration.* This can be supplemented by using a phase-retrieval method [20] to extract and report the spectral phase profile, ϕ(ω), of the pulse. 5. Shot-to-shot intensity (I) stability—characterized as the standard deviation over the mean, σ I =I—based on measurements of individual pulse intensities using a linear measuring device [21]. Our group, for example, uses an unbiased photodiode, a current preamplifier, and a DAQ card to measure the intensity of ∼100 k laser shots from the noncollinear optical parameter amplifier (NOPA) [22]. Typical stability values for our NOPA are 0.5%. Some groups report the standard deviation of the mean. However, the standard deviation over the mean is the preferred way to report the amount of variability in a set of data from its mean value [23]. 6. Shot-to-shot spectral stability. Some light sources exhibit significant spectral fluctuations between laser shots in addition to intensity fluctuations. Therefore researchers should quantify the degree to which the laser spectrum ‘wobbles’. A consistent method would be to acquire a large number of spectra from
https://doi.org/10.1016/j.rechem.2019.100001 2211-7156/© 2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
D.B. Turner / Results in Chemistry 1 (2019) 100001
individual laser shots, normalize each spectrum, and then prepare an errorbar plot of the fluctuations at each pixel for the set of measured spectra [24]. 7. Shot-to-shot intensity correlations. Some researchers have reported large correlations in intensity between adjacent laser shots, while other groups report largely ‘white’ (uncorrelated) noise. White noise requires fewer pulses to reach convergence. Only a few groups have report this value [22,25,26]. 8. A property of a collimated broadband light source that is more difficult to quantify is its angular dispersion. Angular dispersion induces frequency-dependent phase-matching conditions that will produce artifacts in 2D spectra. Strict quantification of angular dispersion is complicated [27], and an estimated approach is likely adequate. Users can translate a pinhole placed at the sample position an equal amount in all four directions and measure the spectrum of the scattered light in the farfield. The transverse displacement distance in each direction should be the (calculated) beam waist. Reporting the five values of λmax for each beam is a reliable indicator of the focusing conditions. 9. The optical polarization configuration of the laser beams. Many researchers use polarization schemes—denoted in various ways [28–30] such as (xxxx), (HH,VV), (+75∘,−75∘, 0∘, 0∘), or (σ+, σ−, σ−, σ+)—to infer useful information about the sample based on selection rules or by invoking isotropic averaging theories. These results are influenced by the purity of the polarization of the incoming beams and the polarization dependence of the optical elements after the sample. Thus, the polarization configuration and an assessment of the purity of the optical polarization should be reported. (II) The parameters of the spectrometer design include: 1. The phase-matching conditions, such as pump–probe or BOX beam geometries.* 2. The short-term and long-term phase stability of the nonlinear signal.* The time windows chosen for the two types of measurements should be at least the time required for a single 2D scan and the time required for a 3D scan, respectively [31]. 3. Directional-filter and phase-mismatch parameters [32]. These quantities are calculated from the laser bandwidth, the beam geometry, and the sample thickness, and they estimate distortions that arise in 2D spectra. The distortions include narrowed or square-shaped peaks. Fig. (1) reveals that improving these parameters leads to spectra with larger surface areas. 4. A key indication of the robustness of a spectrometer is its ability to maintain a stable nonlinear signal. Researchers should quantify the stability by reporting fluctuations of the homodyne nonlinear signal at {τ1 = 0, τ2 ≠ 0}. Using τ2 ≠ 0 eliminates fluctuations caused by the nonresonant response, which often contains higher-order components that are inherently less stable [33]. This is best measured using shot-by-shot acquisitions and quantified as a standard deviation over the mean. 5. Most coherent 2D optical spectroscopy measurements to date have required an ‘indirect’ detection dimension, meaning a dimension that is stepped while the emitted spectrum is measured directly. Poor accuracy of the delay stage(s) can distort peaks in a 2D spectrum. Thus researchers should report the accuracy or bidirectional repeatability—not resolution—of the delay stage(s). 6. Scattered light can contaminate measured spectra significantly and produce peaks with incorrect shapes. To quantify scatter, researchers should translate one delay stage away from the time zero position and then quantify the fractional amount of spectral fringe that appears on the recorded heterodyne spectrum for each excitation beam. This procedure should occur before chopping or phase-cycling schemes are applied. 7. For spectrometers that use pulse shapers, the calibrations procedures and an assessment of artifacts including pulse replica,
phase-cycling errors, and space-time coupling [31,34,35] should be described. (III) The parameters of the data acquisition and processing include: 1. Artifacts arise when multiple pulses are binned before digitizing [22,36]. Therefore researchers should state the number of spectra binned. 2. Related to the convergence speed issue, researchers should report the number of shots averaged per acquisition. In our laboratory, for example, we typically acquire and analyze ∼500 quads of laser shots in a 2D measurement using dual-beam chopping [18]. 3. Researcher should state if they use a data-rejection algorithm or not [37]. 4. Averaging of ratiometric measurements can be done several ways, although only one or two methods are optimal. Thus the averaging formula should be chosen carefully and reported [18,22,38]. 5. Because light sources have intensity and spectral fluctuations, researchers should use and describe the balanced-detection scheme [18,24].
600
500 +1
excitation frequency (THz)
2
0
400
–1
600
500 +1
0
400 400
–1
500
600
emission frequency (THz) Fig. 1. We measured these 2D ES of cresyl violet perchlorate in methanol in the same laboratory with the same laser bandwidth. In the bottom spectrum, we improved the geometry to minimize phase-mismatch and directional-filter distortions, installed delay stages with improved accuracy, and improved the ‘phasing’ algorithm. These refinements decreased noise and increased the effective spectral surface area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
D.B. Turner / Results in Chemistry 1 (2019) 100001
6. ‘Phasing’ the 2D spectrum to a pump–probe spectrum is always imperfect. Researchers should therefore report the total normalized error. We have found that spectra with integrated errors N0.3% have unreliable features [39]. 7. During analysis, researchers should not interpolate the directly detected dimension because the Fourier transformations used in spectral interferometry can distort peaks [40]. Therefore the spectral interferometry procedure should be reported.
3. Conclusion & outlook One leading researcher recently proposed that 2D spectroscopy will evolve into a “dominant spectroscopic method throughout the sciences” [41]. This will require users to adopt a more analytic mindset [42] to make the measured 2D spectra more rigorous and therefore more broadly useful. Of the 23 parameters detailed above, research groups currently report only ∼6. Adopting the full set of standardized specifications by the 2D spectroscopy community will decrease barriers to entry, improve reproducibility, and enhance the general perception of the field. The list presented here applies most directly to spectrometers that operate at kHz repetition rates using the BOX beam geometry. Spectrometers that use less-conventional configurations—for example those that operate at MHz frequencies, use the pump–probe geometry, or do not use optical detection [4]—will need to modify certain items on the list. Researchers using such instruments are encouraged to propose a similar list that is appropriate to their implementation. Acknowledgments DBT thanks former coworkers and students for their contributions to these technical improvements and acknowledges financial support from the Alfred P. Sloan Foundation and the National Science Foundation under CAREER Grant No. CHE–1552235. References [1] P. Hamm, M.T. Zanni, Concepts and Methods of 2D Infrared Spectroscopy, Cambridge University Press, 2011. [2] M. Woerner, W. Kuehn, P. Bowlan, K. Reimann, T. Elsaesser, Ultrafast twodimensional terahertz spectroscopy of elementray excitations in solids, New J. Phys. 15 (2013), 025039. [3] B.A. West, P.G. Giokas, B.P. Molesky, A.D. Ross, A.M. Moran, Toward twodimensional photon echo spectroscopy with 200 nm laser pulses, Opt. Express 21 (2013) 2118–2125. [4] F.D. Fuller, J.P. Ogilvie, Experimental implementations of two-dimensional Fourier transform electronic spectroscopy, Annu. Rev. Phys. Chem. 66 (2015) 667–690. [5] V.I. Prokhorenko, A. Picchiotti, M. Pola, A.G. Dijkstra, R.J.D. Miller, New insights into the photophysics of DNA nucleobases, J. Phys. Chem. Lett. 7 (2016) 4445–4450. [6] D.M. Jonas, Optical analogs of 2D NMR, Science 300 (2003) 1515–1517. [7] S. Mukamel, Principles of Nonlinear Optical Spectroscopy, Oxford University Press, New York, 1995. [8] N. Tkachenko, Optical Spectroscopy: Methods and Implementation, 1st ed. Elsevier Science, 2006. [9] T. Brixner, J. Stenger, H.M. Vaswani, M. Cho, R.E. Blankenship, G.R. Fleming, Twodimensional spectroscopy of electronic couplings in photosynthesis, Nature 434 (2005) 625{628. [10] G.S. Engel, T.R. Calhoun, E.L. Read, T.-K. Ahn, T. Mancal, Y.-C. Cheng, R.E. Blankenship, G.R. Fleming, Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems, Nature 446 (2007) 782{786. [11] G. Panitchayangkoon, D. Hayes, K.A. Fransted, J.R. Caram, E. Harel, J. Wen, R.E. Blankenship, G.S. Engel, Longlived quantum coherence in photosynthetic complexes at physiological temperature, Proc. Natl. Acad. Sci. U. S. A. 107 (2010), 12766{12770. [12] J.A. Myers, K.L.M. Lewis, P.F. Tekavec, J.P. Ogilvie, Two-dimensional Fourier transform electronic spectroscopy with a pulse-shaper, in: P. Corkum, S. Silvestri, K.A. Nelson, E. Riedle, R. Schoenlein (Eds.), Ultrafast Phenomena XVI: Proceedings of the 16th International Conference Springer Series in Chemical Physics, 92, Heidelberg, Germany 2009, pp. 956–958.
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