2D IR spectroscopy with phase-locked
pulse pairs from a birefringent delay line
Julien R
´
ehault,
1
Margherita Maiuri,
1
Cristian Manzoni,
1
Daniele
Brida,
2
Jan Helbing,
3
and Giulio Cerullo
1,∗
1
IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32,
20133 Milano, Italy
2
Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457
Konstanz, Germany
3
Department of Chemistry, University of Z
¨
urich, Winterthurerstrasse 190, 8057, Z
¨
urich,
Switzerland
*giulio.cerullo@polimi.it
Abstract: We introduce a new scheme for two-dimensional IR spec-
troscopy in the partially collinear pump-probe geometry. Translating bire-
fringent wedges allow generating phase-locked pump pulses with excep-
tional phase stability, in a simple and compact setup. A He-Ne tracking
scheme permits to scan continuously the acquisition time. For a proof-of-
principle demonstration we use lithium niobate, which allows operation up
to 5
μ
m. Exploiting the inherent perpendicular polarizations of the two
pump pulses, we also demonstrate signal enhancement and scattering sup-
pression.
OCIS codes: (320.7160) Ultrafast technology; (300.6530) Spectroscopy, ultrafast; (300.6300)
Spectroscopy, Fourier transforms; (300.6340) Spectroscopy, infrared.
References and links
1. P. Hamm, M. Lim, and R. M. Hochstrasser, “Structure of the amide I band of peptides measured by femtosecond
nonlinear-infrared spectroscopy,” J. Phys. Chem. B 102, 6123–6138 (1998).
2. P. Hamm and M. T. Zanni, Concepts and Methods of 2D Infrared Spectroscopy (Cambridge University, 2011).
3. M. Cho, Two-Dimensional Optical Spectroscopy (CRC Press, 2009).
4. G. D. Goodno, G. Dadusc, and R. J. D. Miller, “Ultrafast heterodyne-detected transient-grating spectroscopy
using diffractive optics,” J. Opt. Soc. Am. B 15, 1791–1794 (1998).
5. M. C. Asplund, M. T. Zanni, and R. M. Hochstrasser, “Two-dimensional infrared spectroscopy of peptides by
phase-controlled femtosecond vibrational photon echoes,” Proc. Natl. Acad. Sci. U.S.A. 97, 8219–8224 (2000).
6. S.-H. Shim, D. B. Strasfeld, E. C. Fulmer, and M. T. Zanni, “Femtosecond pulse shaping directly in the mid-IR
using acousto-optic modulation,” Opt. Lett. 31, 838–840 (2006).
7. L. P. DeFlores, R. A. Nicodemus, and A. Tokmakoff, “Two-dimensional Fourier transform spectroscopy in the
pump-probe geometry,” Opt. Lett. 32, 2966 (2007).
8. U. Selig, F. Langhojer, F. Dimler, T. Lhrig, C. Schwarz, B. Gieseking, and T. Brixner, “Inherently phase-stable
coherent two-dimensional spectroscopy using only conventional optics,” Opt. Lett. 33, 2851–2853 (2008).
9. J. Helbing and P. Hamm, “Compact implementation of Fourier transform two-dimensional IR spectroscopy with-
out phase ambiguity,” J. Opt. Soc. Am. B 28, 171–178 (2011).
10. V. Cervetto, J. Helbing, J. Bredenbeck, and P. Hamm, “Double-resonance versus pulsed Fourier transform two-
dimensional infrared spectroscopy: An experimental and theoretical comparison,” J. Chem. Phys. 121, 5935–
5942 (2004).
11. E. H. G. Backus, S. Garrett-Roe, and P. Hamm, “Phasing problem of heterodyne-detected two-dimensional in-
frared spectroscopy,” Opt. Lett. 33, 2665–2667 (2008).
12. W. Xiong and M. T. Zanni, “Signal enhancement and background cancellation in collinear two-dimensional
spectroscopies,” Opt. Lett. 33, 1371–1373 (2008).
13. J. R
´
ehault and J. Helbing, “Angle determination and scattering suppression in polarization-enhanced two-
dimensional infrared spectroscopy in the pump-probe geometry,” Opt. Express 20, 21665–21677 (2012).
#206811 - $15.00 USD
Received 19 Feb 2014; revised 21 Mar 2014; accepted 21 Mar 2014; published 7 Apr 2014
(C) 2014 OSA
21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009063 | OPTICS EXPRESS 9063
14. J. R
´
ehault and J. Helbing, “Exploring the polarization degrees of freedom in collinear two-dimensional infrared
spectroscopy,” EPJ Web of Conferences 41, 05003 (2013).
15. S.-H. Shim and M. T. Zanni, “How to turn your pump-probe instrument into a multidimensional spectrometer:
2D IR and vis spectroscopies via pulse shaping,” Phys. Chem. Chem. Phys. 11, 748–761 (2009).
16. D. R. Skoff, J. E. Laaser, S. S. Mukherjee, C. T. Middleton, and M. T. Zanni, “Simplified and economical 2D IR
spectrometer design using a dual acousto-optic modulator,” Chem. Phys. 422, 8–15 (2013).
17. D. Brida, C. Manzoni, and G. Cerullo, “Phase-locked pulses for two-dimensional spectroscopy by a birefringent
delay line,” Opt. Lett. 37, 3027–3029 (2012).
18. M. T. Zanni, N.-H. Ge, Y. S. Kim, and R. M. Hochstrasser, “Two-dimensional IR spectroscopy can be designed
to eliminate the diagonal peaks and expose only the crosspeaks needed for structure determination,” Proc. Natl.
Acad. Sci. USA 98, 11265–11270 (2001).
19. J. R
´
ehault, V. Zanirato, M. Olivucci, and J. Helbing, “Linear dichroism amplification: Adapting a long-known
technique for ultrasensitive femtosecond IR spectroscopy,” J. Chem. Phys. 134, 124516–124516–10 (2011).
20. D. N. Nikogosyan, Nonlinear Optical Crystals: A Complete Survey (Springer, 2005).
21. D. E. Zelmon, D. L. Small, and D. Jundt, “Infrared corrected Sellmeier coefficients for congruently grown lithium
niobate and 5 mol.% magnesium oxide-doped lithium niobate,” J. Opt. Soc. Am. B 14, 3319–3322 (1997).
22. N. Demird
¨
oven, M. Khalil, O. Golonzka and A. Tokmakoff, “Dispersion compensation with optical materials for
compression of intense sub-100-fs mid-infrared pulses” Opt. Lett. 27, 433 (2002).
23. C. T. Middleton, A. M. Woys, S. S. Mukherjee, and M. T. Zanni, “Residue-specific structural kinetics of proteins
through the union of isotope labeling, mid-IR pulse shaping, and coherent 2D IR spectroscopy,” Methods 52,
12–22 (2010).
24. S. T. Roberts, J. J. Loparo, K. Ramasesha, and A. Tokmakoff, “A fast-scanning Fourier transform 2D IR interfer-
ometer,” Opt. Commun. 284, 1062–1066 (2011).
25. K. F. Lee, A. Bonvalet, P. Nuernberger, and M. Joffre, “Unobtrusive interferometer tracking by pathlength oscil-
lation for multidimensionalspectroscopy,” Opt. Express 17, 12379–12384 (2009).
26. M. Downs and K. Raine, “An unmodulated bi-directional fringe-counting interferometer system for measuring
displacement,” Precision Engineering 1, 85–88 (1979).
27. L. Lepetit, G. Ch
´
eriaux, and M. Joffre, “Linear techniques of phase measurement by femtosecond spectral inter-
ferometry for applications in spectroscopy,” J. Opt. Soc. Am. B 12, 2467–2474 (1995).
28. U. Schlarb and K. Betzler, “Refractive indices of lithium niobate as a function of wavelength and composition,”
J. Appl. Phys. 73, 3472–3476 (1993).
29. R. Bloem, S. Garrett-Roe, H. Strzalka, P. Hamm, and P. Donaldson, “Enhancing signal detection and completely
eliminating scattering using quasi-phase-cycling in 2D IR experiments,” Opt. Express 18, 27067–27078 (2010).
30. F. Perakis and P. Hamm, “Two-dimensional infrared spectroscopy of supercooled water,” J. Phys. Chem. B 115,
5289–5293 (2011).
31. T. Steinel, J. B. Asbury, J. Zheng, and M. D. Fayer, “Watching hydrogen bonds break: a transient absorption
study of water,” J. Phys. Chem. A 108, 10957–10964 (2004).
32. R. Maksimenka, P. Nuernberger, K. F. Lee, A. Bonvalet, J. Milkiewicz, C. Barta, M. Klima, T. Oksenhendler,
P. Tournois, D. Kaplan, and M. Joffre, “Direct mid-infrared femtosecond pulse shaping with a calomel acousto-
optic programmable dispersive filter,” Opt. Lett. 35, 3565–3567 (2010).
33. S. P. Davis, M. C. Abrams, and J. W. Brault, Fourier Transform Spectrometry (Academic Press, 2001).
34. K. Ataka, T. Kottke, and J. Heberle, “Thinner, smaller, faster: IR techniques to probe the functionality of biolog-
ical and biomimetic systems,” Angew. Chem. 49, 5416-5424 (2010).
35. H. Rhee, Y.-G. June, J.-S. Lee, K.-K. Lee, J.-H. Ha, Z. H. Kim, S.-J. Jeon, and M. Cho, “Femtosecond charac-
terization of vibrational optical activity of chiral molecules,” Nature 458, 310–313 (2009).
1. Introduction
Since its first demonstration by Hamm et al. [1], two-dimensional infrared spectroscopy (2D
IR) has developed as an important technique to describe molecular structure, measure cou-
plings between vibrational modes of a molecule and track structural dynamics on the picosec-
ond timescale [2,3]. While 2D IR was first realized in the frequency domain [1], time domain
techniques are usually preferred [4–9], as they have better spectral resolution in the pump fre-
quency axis, and because they maintain the time resolution afforded by the spectral bandwidth
of the laser pulses [10]. The two schemes most widely used for time-domain 2D IR are the
heterodyne detected three-pulse photon-echo and the partially collinear pump-probe geometry.
The heterodyne photon-echo configuration is the most versatile and sensitive but also the
most complex way of performing 2D spectroscopy. It requires the overlap of four independent
beams at the sample, two pumps, one probe and one heterodyning pulse, and interferometric
#206811 - $15.00 USD
Received 19 Feb 2014; revised 21 Mar 2014; accepted 21 Mar 2014; published 7 Apr 2014
(C) 2014 OSA
21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009063 | OPTICS EXPRESS 9064
stabilization of the pulses in pairs, the two pumps and the probe with the heterodyning pulse.
The phase between the two pump beams is scanned and must be determined accurately at
the sample [11] for proper recovery of the line-shape. This configuration allows to record,
by inverting the ordering of the two pump pulses, the so-called rephasing and non-rephasing
spectra. They can provide some additional insight into the origin of cross and diagonal peaks,
but high spectral resolution is best afforded by their sum, the absorptive spectrum. An additional
advantage of the photon echo configuration is the independent control of the intensity and
polarization of the four pulses.
In comparison, the partially collinear configuration leads naturally to absorptive lineshapes,
requires that only two beams (one for the two pump pulses and one for the probe) be over-
lapped on the sample and allows a straightforward determination of the relative delay of the
collinear pump pulses [7, 9]. The probe beam has the dual purpose of generating the nonlin-
ear polarization and heterodyning the signal (self-heterodyning), as the signal is emitted in the
probe direction. For these reasons, this configuration is in principle less flexible and sensitive
than the heterodyne photon-echo, but control of the polarization and intensity of the four fields
interactions is still possible by the use of polarizers in both the pump and probe beams [12–14].
If we compare it to standard pump-probe spectroscopy, the additional requirement of 2D
spectroscopy is to resolve the frequency of the pump pulses. This can be done in the time
domain by scanning the delay t
1
between two pump pulses and Fourier transforming the probe
data along t
1
to finally obtain the additional frequency axis of the 2D spectra. The accurate
control or determination of delay t
1
, or coherence time, within a fraction of the optical cycle
is the key to obtain properly phased 2D spectra, and this is the main technical hurdle of 2D
spectroscopy in the time domain. Current implementations of collinear 2D IR spectroscopy
make use of a pulse shaper [15, 16] or an interferometer [7, 9]. When using a pulse shaper,
the phase and delay t
1
between the two pulses are fully controlled, and can be changed on a
pulse to pulse basis, allowing fast and efficient scanning of t
1
. However, pulse shapers in the
mid-IR are expensive and significantly increase the complexity of the experimental setup. With
an interferometer, t
1
can be scanned continuously and determined precisely by monitoring the
fringes of an auxiliary visible beam collinear with the IR one.
We recently introduced a novel optical configuration, called TWINS (Translating-Wedge-
Based Identical Pulses eNcoding System) which solves in a simple, compact and cost-effective
way the problem of building a collinear delay line with interferometric stability. This device
exploits the birefringence of an optical material to impose an arbitrary delay on two orthogonal
polarization components by continuously varying the material thickness. TWINS has already
been successfully implemented in the visible range using as birefringent material
β
-barium bo-
rate and demonstrated excellent phase stability (better than
λ
/360) and reproducibility for 2D
electronic spectroscopy. Here, we extend the TWINS concept to 2D IR spectroscopy, using as
birefringent material lithium niobate, whose transparency extends out to 5 micron. We imple-
mented a He-Ne tracking scheme very similar to the one used by Helbing et Hamm [9], that
allows to scan t
1
continuously and record a full 2D spectrum in less than 30 s.
Besides its compactness and simplicity, advantage of the TWINS configuration is that it
naturally produces a pair of perpendicularly polarized pump pulses. They can be projected
to the same polarization with an additional polarizer for conventional 2D spectroscopy [17].
However, it is also possible to use the two perpendicularly polarized pulses to remove scattering
and enhance signal with the use of polarizers in the probe beam [13,14,18,19].
2. Measurement principle
When a light beam propagates in a birefringent material, it experiences a refractive index that
depends on its polarization. The orientation of the optical axis with respect to the light propaga-
#206811 - $15.00 USD
Received 19 Feb 2014; revised 21 Mar 2014; accepted 21 Mar 2014; published 7 Apr 2014
(C) 2014 OSA
21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009063 | OPTICS EXPRESS 9065
tion direction determines two main polarization axes, the ordinary and the extraordinary axis.
As a consequence, the group velocity of a pulse propagating in such materials depends on its
polarization. This effect can be used to generate two orthogonally polarised pulse replicas, and
to finely tune their relative delay by adjusting the material thickness. The pulses generated this
way are also intrinsically phased-locked, as they propagate along the same path. This effect is
employed in the TWINS device [17], schematically shown in Fig. 1. Let us first consider block
B, consisting of two wedges with optical axis rotated by 90
◦
. Upon transverse displacement of
this block, light polarized in X direction is not affected, as it propagates in the two wedges with
the same ordinary index of refraction, and thus experiences a constant optical path. In contrast,
the light polarized in the Y direction propagates with ordinary index of refraction in the first
wedge, and with the extraordinary index in the second wedge. In a negative birefringence ma-
terial (n
o
>n
e
), the X polarized pulses is delayed relative to the Y polarized pulses, depending
on the position of the moving wedge pair.
Z
Y
X
α
t1
45°
YX X Y
A
B
C
L
X
Y
d
Δr
no>ne
Fig. 1. Principle of the TWINS. Block A introduces a constant negative delay. Block B
moves to scan t
1
. Block C corrects the parallelism and front tilt of the beams. Arrows
indicate the orientation of the optical axis in each element. Below each pulse is indicated
its polarization. Inset : Path for X and Y-polarized beams inside the TWINS. Angles are
exaggerated for clarity.
As shown in the inset of Fig. 1, the perpendicularly polarized beams created by a single
pair of wedges are not exactly parallel, they suffer from spatial chirp, and the phase front of
the extraordinary beam is slightly tilted. This is corrected by the second pair of static wedges,
after which the two beams are now parallel and almost collinear. The insertion of the second
wedge pair into the beam is kept as small as possible to minimize dispersion. When both prism
pairs are in contact, the X-polarized pulse is not displaced. In contrast, the Y polarization is
refracted at every interface. It is again parallel to the X-polarized pulse but it is displaced by
an amount Δr depending on the first wedge pair insertion L (proportional to t
1
), the distance
between the two wedge pairs d, the apex angle of the wedges
α
and the birefringence of the
material. When d=10 mm and L=0 mm (t
1
=0 ps), the calculated beam displacement is Δr =105
μ
m. Every additional delay of 1 ps, corresponding to a translation L of 3.2 cm, increases Δr by
50
μ
m. Here, we could neglect the influence of the lateral displacement Δr of the Y polarization
relative to the X polarization, as the beam size (8mm) is significantly bigger than the maximal
displacement (170
μ
m overall). In the case of smaller beam, for longer scans or for materials
with higher birefringence, this effect should be carefully taken into account as it could lower
#206811 - $15.00 USD
Received 19 Feb 2014; revised 21 Mar 2014; accepted 21 Mar 2014; published 7 Apr 2014
(C) 2014 OSA
21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009063 | OPTICS EXPRESS 9066
the signal size at long t
1
delays. This effect could distort the lineshapes, or affect the balance
between rephasing and non-rephasing contributions. In fact, the phase-matching direction of
the signal will be affected at long t
1
and not be any more collinear with the probe. A solution
to this problem could be to reflect back the beams into the TWINS, which would produce a
perfectly collinear pair of pulses while doubling the delay t
1
.
Since the wedge system can introduce only positive delay between X and Y, we used an
additional birefringent plate with the extraordinary axis oriented along the X axis (Block A
in Fig. 1); this plate introduces a negative delay between X and Y, allowing to access zero
time delay during the scan of the moving wedges. A half-wave plate placed before the wedge
sequence tunes the balance between the X- and Y-polarized pulse intensity.
Our goal is to use the TWINS device to generate phase-locked pulse pairs in the mid-IR
spectral range. Table 1 reports a list of candidate birefringent materials which are transpar-
ent in the mid-IR [20]. Magnesium fluoride could be considered, as it is standard material for
waveplates in the mid-IR, but it has a too small birefringence for our application. Silver gal-
lium sulfide (AgGaS
2
) and selenide (AgGaSe
2
) are commonly used as non-linear material, they
have a good mid-IR transparency, but their relatively small birefringence require to build big
wedges (on the order of 10 cm). Thallium based crystal (Tl
4
HgI
6
and Tl
3
AsSe
3
) have interest-
ing properties but are still exotic and difficult to grow in large sizes, they also do not transmit
visible light, preventing alignment of the wedges with a visible laser beam. The most favourable
material, in terms of both transparency and birefringence, is Hg
2
Cl
2
(calomel). However this
material is not readily available and, for a proof of concept, we have selected LiNbO
3
, which
has a transparency window extending up to 5.2
μ
m, a reasonably high birefringence and is
commercially available in the size requested for our experiment.
Table 1. Characteristics of a selection of birefringent materials at 4
μ
m [20]. GVM: group
velocity mismatch between ordinary and extraordinary polarizations.
Material Ordinary index Birefringence (n
e
-n
o
) GVM (fs/mm) Range (
μ
m)
LiNbO
3
2.1142 -0,0583 310 0.4-5.2
Hg
2
Cl
2
1.8976 0.549 1847 0.4-20
CdGeAs
2
3.5431 0.096 423 2.4-11.5
AgGaS
2
2.4027 -0.05368 177 0.58-10.6
AgGaSe
2
2.6787 -0.0326 105 0.58-10.6
MgF
2
1.34883 0.0099 45 0.2-7
LiO
3
1.8163 -0.109 578 0.5-5
Tl
4
HgI
6
2.3914 0.0688 230 1.0-60
Tl
3
AsSe
3
3.3651 -0.187 639 2-12
LiNbO
3
has a negative birefringence (n
e
−n
o
) of the order of 5.8 10
−2
around 4
μ
m, meaning
that a wedge thickness of 6 mm results in a phase delay between the two pump pulses of 1.16
ps. Based on the Sellmeier coefficients of LiNbO
3
[21], the group delay is of 1.86 ps. The
relatively small birefringence of LiNbO
3
requires the use of rather large wedges to produce
the delay necessary for sufficient spectral resolution along the pump axis of a 2D IR spectrum.
With an angle of 10
◦
, the wedge needs to be longer than 3.2 cm for a group delay of 1.86 ps,
which becomes 4 cm if we account for our beam diameter of 8 mm. Block A has a thickness of
1.5 mm, and the overall thickness of the TWINS is ≈9 mm. The total group delay dispersion
(GDD) introduced by the TWINS is approx -15000 fs
2
, and over compensate the estimated 4000
fs
2
of the pulses coming out the infrared optical parametric amplifier. Still, the pump pulses
#206811 - $15.00 USD
Received 19 Feb 2014; revised 21 Mar 2014; accepted 21 Mar 2014; published 7 Apr 2014
(C) 2014 OSA
21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009063 | OPTICS EXPRESS 9067
![Table 1. Characteristics of a selection of birefringent materials at 4 μm [20]. GVM: group velocity mismatch between ordinary and extraordinary polarizations.](/figures/table-1-characteristics-of-a-selection-of-birefringent-v4tj8ikv.webp)