Broadband stimulated Raman scattering with
Fourier-transform detection
Julien Réhault,
1,2
Francesco Crisafi,
1
Vikas Kumar,
1
Gustavo Ciardi,
1
Marco
Marangoni,
1
Giulio Cerullo
1
and Dario Polli
1,
*
1
IFN-CNR, Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133, Milano, Italy.
2
Current address: Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
*
dario.polli@polimi.it
Abstract: We propose a new approach to broadband Stimulated Raman
Scattering (SRS) spectroscopy and microscopy based on time-domain
Fourier transform (FT) detection of the stimulated Raman gain (SRG)
spectrum. We generate two phase-locked replicas of the Stokes pulse after
the sample using a passive birefringent interferometer and measure by the FT
technique both the Stokes and the SRG spectra. Our approach blends the very
high sensitivity of single-channel lock-in balanced detection with the spectral
coverage and resolution afforded by FT spectroscopy. We demonstrate our
method by measuring the SRG spectra of different compounds and
performing broadband SRS imaging on inorganic blends.
2015 Optical Society of America. One print or electronic copy may be made for
personal use only. Systematic reproduction and distribution, duplication of any
material in this paper for a fee or for commercial purposes, or modifications of the
content of this paper are prohibited.
Online abstract in the OSA Journal:
http://dx.doi.org/10.1364/OE.23.025235
OCIS codes: (320.7090) Ultrafast lasers; (290.5910) Scattering, stimulated Raman; (300.6300)
Spectroscopy, Fourier transforms.
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1. Introduction
Raman microscopy is gaining increasing recognition in biomedical optics [1] due to its
capability of non-invasive, label-free imaging of tissues and cells, based on their intrinsic
vibrational response [2,3]. In spontaneous Raman (SR) [4] a quasi-monochromatic laser at
frequency
p
(“pump”) excites the molecule to a virtual state, which then relaxes to the ground
state emitting photons at lower frequencies
S
(“Stokes”). The inelastic frequency shifts Ω=
p
-
S
match the molecular vibrations and the resulting vibrational spectrum reflects the molecular
structure, providing a detailed picture of the biochemical composition of the specimen. The
main drawback of SR is its very weak scattering cross section, 10-12 orders of magnitude lower
than that of absorption. This makes it difficult to separate the weak Raman-shifted light from
the intense elastic scattering and from sample and substrate fluorescence, preventing probing
dilute species and in vivo imaging, due to the long integration times needed.
Coherent Raman Scattering (CRS) [5] can overcome these limitations, exploiting the third-
order nonlinear optical response of the sample to pump and Stokes pulses in order to set up and
detect a vibrational coherence within the ensemble of molecules inside the laser focus. When
the difference between pump and Stokes frequencies matches a characteristic vibrational
frequency , i.e.
p
-
S
= , then all the molecules in the focal volume are resonantly excited
and vibrate in phase; this vibrational coherence enhances the Raman response by many orders
of magnitude with respect to the incoherent SR process. The two most widely employed CRS
techniques are Coherent Anti-Stokes Raman Scattering (CARS) [6] and Stimulated Raman
Scattering (SRS) [7,8]. CARS detects the coherent radiation at the anti-Stokes (blue-shifted)
frequency
aS
=
p
+ . SRS monitors either the stimulated emission of Stokes photons from
a virtual state of the sample to the investigated vibrational state (the so-called Stimulated Raman
Gain, SRG) or the absorption of pump photons from the ground state (Stimulated Raman Loss,
SRL). CARS and SRS have both advantages and drawbacks and are actively developed for
high-speed vibrational imaging.
State-of-the-art CRS microscopy has reached extremely high imaging speed (up to the
video rate [9,10]), but working only at one vibrational frequency (single-color CRS). This
provides limited chemical selectivity in complex heterogeneous systems, such as cells and
tissues, with spectrally overlapping chemical species. Broadband CRS microscopy has been so
far mostly implemented in the CARS configuration [11-17]. In this method, an ultra-broadband
laser pulse, or a combination of two properly detuned pump and Stokes pulses, first create the
vibrational coherence in the sample. A narrowband pulse then probes the coherence by
transferring the vibrational spectrum to the blue-shifted anti-Stokes region, thus in the absence
of any linear background light, enabling direct signal detection by photon counting. Despite this
advantage, CARS suffers from the so-called non-resonant background (NRB) caused by the
purely electronic third-order nonlinear response of the molecules or the medium in which they
are embedded, i.e. not mediated by any characteristic vibration of the sample. NRB may
strongly distort or even overwhelm the molecular vibrational spectrum of interest. The
extraction of quantitative information from these altered spectra requires the application of
complex phase-retrieval algorithms [15,18]. Furthermore, since the CARS signal is homodyne
detected, it scales quadratically with the number of oscillators in the focal volume, making it
hard to perform quantitative measurements and to probe dilute species. This problem can be
alleviated by exploiting the NRB as a phase-locked local oscillator, allowing to perform
heterodyne detection and to recover the linear concentration dependence [15, 19].
SRS does not suffer from NRB and, since the signal is heterodyne detected, it inherently
provides a linear dependence of the signal on the sample concentration; however, its extension
to the broadband modality is technically challenging. SRS in fact requires the detection of a
very small differential signal (SRG/SRL) on top of the intense Stokes/pump pulse, which is
only possible through sensitive modulation transfer schemes. To achieve high-speed imaging
by SRS, the detection chain needs to be carefully designed to provide sensitivity levels close to
the shot noise limit. In the single-color regime this is typically accomplished by high-frequency
amplitude modulation of one of the laser beams (pump or Stokes) followed by lock-in detection
of its counterpart (Stokes or pump). Extension of this approach to a broadband configuration
would require a multichannel high-frequency lock-in amplifier, which is not readily available.
Only a few experimental demonstrations of broadband SRS microscopy have been
presented so far. A first approach is based on narrowband pump and Stokes pulses and single-
color detection. Either the pump or the Stokes spectrum is kept fixed, while the other is rapidly
tuned via a picosecond optical parametric oscillator and an electro-optical tunable Lyot filter
[20] or using two synchronized laser sources and a wavelength scanner in a 4-f scheme [21].
For every corresponding Raman frequency, the sample is raster scanned either line-by-line or
frame-by-frame and multispectral images are finally reconstructed by stacking the data. The
achieved tuning range is however narrow (~300 cm
-1
) due to the limited achievable tuning
range/bandwidth of the laser. Another somewhat similar approach has also been recently
demonstrated using a time-encoded method [22]. It has the advantages of high spectral
resolution (better than 3 cm
-1
) and large spectral coverage (600-cm
-1
for a single laser
configuration and gap-free from 250 to 3150 cm
-1
combining four different lasers), but at the
expense of using continuous-wave lasers with low power densities which limit the SRS signal
amplitude.
A second approach to broadband SRS involves the use of a broadband Stokes (pump) pulse
combined with multichannel detection. This approach is conceptually very powerful as it avoids
frequency tuning and enables parallel detection of all frequencies, but poses the technical
challenge of developing a multichannel detection with the speed and sensitivity required for
SRS. Among the proposed experimental schemes, a first group employs an array of
photodetectors, each one connected to a specific input port of a multi-channel lock-in amplifier
[23,24] This solution is extremely powerful, but it requires a complex and costly dedicated
electronics, making it difficult to translate it to mainstream biomedical applications. A recently
proposed approach, which is much simpler yet very effective, involves the use of an array of
tuned LC filters [25]. An alternative scheme makes use of a spectrometer equipped with a
CCD/CMOS/NMOS linear image sensor [26-28]. This solution is mainly limited by the full-
well capacity of the pixels, of the order of 10
6
electrons, which reduces the achievable signal-
to-noise ratio to ΔI
S
/I
S
10
-3
for a single spectrum (depending on the number of photons
illuminating each pixel). As the read-out speed typically cannot exceed a few kilohertz, it would
require several seconds or even minutes to achieve a sensitivity down to ΔI
S
/I
S
=10
-5
, needed for
most applications. A third method is based on the modulation of different spectral portions of
the broadband pulse at different frequencies using an acousto-optical tunable filter device, so
that the different Raman lines can be measured simultaneously and distinguished by
demodulating the signal at different frequencies [29]. Finally, spectral focusing [30] of
broadband pump/Stokes pulses in combination with rapid variations of their delay can be used
[31,32].
In this paper, we propose and demonstrate a new approach to broadband SRS based on
time-domain Fourier transform (FT) detection of the SRG spectrum. We use a passive
birefringent delay line that guarantees exceptional (</300) path-length stability and
reproducibility for time-domain detection of the broadband Stokes pulse. We further exploit the
linearity of the FT operator for time-domain measurement of the SRG spectrum by high-
frequency modulation of the pump and single-channel, lock-in detection. Our approach
conjugates the very high sensitivity of single-channel lock-in detection with the spectral
resolution afforded by FT spectroscopy. We demonstrate our method by measuring broadband
SRS spectra of different compounds and performing broadband SRS imaging of polymer
blends.
2. Principle of Fourier transform SRS
In order to understand the principle of our broadband FT-SRS, let us compare it with standard
multichannel SRS detection, the conceptual scheme of which is shown in Fig. 1(a). For the case
of SRG detection, a narrowband pump pulse and a broadband Stokes are synchronized,
collinearly combined by a dichroic beam splitter and focused on the sample. The intensity of
the pump beam is modulated at high frequency, ideally at half the repetition rate of the laser.
Modulation transfer to the broadband Stokes pulse occurs at the frequencies where SRG takes
place, due to the presence of specific molecular vibrations. After the sample (in the case of a
measurement in transmission), a filter rejects the pump photons and the Stokes spectrum is
detected using the aforementioned multi-channel scheme based on, e.g., a photodiode array and
a multi-channel lock-in amplifier (Fig. 1(a), gray box named (a1)) [23,24] or a tuned amplifier
[25]. It is also possible to combine a broadband pump with a narrowband Stokes and measure
the SRL of the pump, in a process known as Inverse Raman scattering [33].
Our approach (see Fig. 1(a), gray box named (a2)) is radically different, as it requires a
single photodetector and a single-channel lock-in amplifier. It makes use of the time-domain
measurement of spectra enabled by FT spectroscopy, in analogy with the FTIR technique used
in the mid-infrared region [34]. The Stokes pulse after the sample is sent to a linear
interferometer (e.g. a Michelson) that creates two collinear identical replicas of the pulse, with
relative delay τ. The two replicas are made to interfere as a function of τ onto a single detector,
giving rise to an interferogram as that sketched in red in Fig. 1(b). In the following, we will call
it “Stokes interferogram”. The FT of the interferogram with respect to τ gives the spectrum of
the Stokes pulse. The presence of SRG on the Stokes, induced by the pump pulse in a sample
with Raman gain, modifies the interferogram (see Fig. 1(b)) generating tails at long delays. Its
FT results in a different Stokes spectrum, as it now displays gain in the form of sharp peaks at
specific Stokes shifts. In Fig. 1(b), the gain is exaggerated, to highlight the concept, but in
practical conditions the differences between the two interferograms and between the two Stokes
spectra are as low as a few parts in 10
4
or 10
5
. By computing the difference of the two Stokes
spectra and normalizing over the Stokes spectrum one obtains the SRG spectrum.
For long delays the difference between the two red interferograms in Fig. 1(b) is
immediately visible because, in the presence of the pump, the interferogram shows persistent
oscillations that are not present without it. This is due to the fact that molecular vibrations have
coherence times of the order of a few picoseconds, much longer than the Stokes pulse duration
(≈20fs in our case), so that they manifest in the time domain as oscillations that persist long
after the Stokes interferogram has decayed. In the spectral domain, due to the well-known time-
frequency reciprocity, this results in narrow linewidths (few tens of wavenumbers, to be
compared with the Stokes linewidth of ≈700 cm
-1
FWHM in our case).
To measure the tiny pump-induced modification of the Stokes signal, we implement high-
speed modulation transfer and lock-in detection. Thanks to the linearity of the FT operator, the
difference between the FTs of two signals (see Fig. 1(b), upper part) is equal to the FT of the