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LETTER

Applied Physics Express 14, 122006 (2021)
https://doi.org/10.35848/1882-0786/ac39b1

Low-frequency multiplex CARS microscopy with a high-repetition near-infrared
supercontinuum laser
Yusuke Arashida1, Atsushi Taninaka1,2, Takahiro Ochiai1,2, Hiroyuki Mogi1, Shoji Yoshida1, Masamichi Yoshimura3,
Osamu Takeuchi1, and Hidemi Shigekawa1*
1Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan
TAKANO Co., Ltd., Miyada-mura, Kamiina-gMiyada-muraun, Nagano, 399-4301, Japan
3
Graduate School of Engineering, Toyota Technological Institute, Hisakata, Tempaku, Nagoya 468-8511, Japan2*

E-mail: hidemi @ims.tsukuba.ac.jp

Received October 28, 2021; revised November 12, 2021; accepted November 15, 2021; published online November 25, 2021
We have developed a multiplex coherent anti-Stokes Raman scattering microscope effective for low-wavenumber measurement by combining ahigh-repetition supercontinuum light source of 1064 nm and a near-infrared high-sensitivity InGaAs diode array. This system could observe thelow-wavenumber region down to 55 cm−1 with high sensitivity. In addition, using spectrum shaping and spectrum modulation techniques, we
simultaneously realized a wide bandwidth (<1800 cm−1), high wavenumber resolution (9 cm−1), high efficiency, and increasing signal-to-noise
ratio by reducing the effect of the background shape in the low-wavenumber region. Spatial variation of a sulfur crystal phase transition withmetastable states was visualized. © 2021 The Author(s). Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd
Supplementary material for this article is available onlineThe chemical information obtained by Raman scattering
is actively used in various ﬁelds such as bioscience,physical chemistry, material science, and structural
mechanics.1–6) In particular, Raman scattering has been
widely applied to bioimaging microscopes by taking advantage of its noncontact, minimally invasive, and label-freecharacteristics.7) Measurement efﬁciency has been markedly
improved by stimulated Raman spectroscopy and coherent
anti-Stokes Raman spectroscopy (CARS), which utilize
pulsed laser technology.8–12) A wide bandwidth, high-frequency resolution, and high throughput were realized simultaneously by a multiplex CARS method that utilized spectrum shaping technology.13–16)
In Raman spectroscopy, the main cause of damage is thethermal process caused by light absorption, which is a factor
that makes analysis by Raman spectroscopy difﬁcult. In
addition, although the efﬁciency is signiﬁcantly improved bymeasurement methods such as CARS, since Raman spectroscopy measures weak signals, light other than Raman
scattered light such as ﬂuorescence affects the measurement.
Since many materials ﬂuoresce in the ultraviolet–visible
region, it is necessary to use light with a longer wavelengththan the ultraviolet–visible region to reduce the ﬂuorescence.
However, a high-sensitivity Si image sensor, which is
widely used for observing visible light, has a signiﬁcantly
reduced sensitivity to light with a wavelength of 1 μm or
more, corresponding to a wavenumber of about 600 cm−1 orless. In such a low-frequency region, important informationsuch as structure of compounds containing relatively heavy
atoms such as P, S, and Fe as well as hydrates and polymers,
appears.17–21) Therefore, the development of a new technology is strongly required.
In this study, we have developed a multiplex CARS
microscope that is effective for low-wavenumber measurementby combining a high-repetition infrared supercontinuum (SC)laser and an infrared high-sensitivity diode array. A highefﬁciency, wide bandwidth, high wavenumber resolution, andlow background were simultaneously realized.

Figure 1 shows the outline of the developed multiplex
CARS microscopy system. The excitation was carried out by
a SC light source (FIU15, NKT Photonics) obtained bypassing the output of a mode-locked laser (center wavelength: 1064 nm, pulse duration: 5 ps) through a highly
nonlinear ﬁber (HNF, FIU-15). The wavelength range is
400–2400 nm, the repetition frequency frep is variable( frep = 78/n MHz, n = 1 to 40), and the maximum pulse
energy is 78 nJ. Visible light components were removed inadvance with a long-pass ﬁlter (LPF: FELH1050, Thorlabs).
The SC light was divided into transmitted light (angularfrequency of ωs) and reﬂected light (ωp) by an ultrasteep LPF(ULPF1: LP02-1064RU-25, Semlock). At this time, thelinewidth was narrowed by passing the ωp-light through abandpass ﬁlter (BPF: 1064.2-0.6, Alluxa) with a full width at
half maximum (FWHM) of 0.7 nm. The two beams werecombined by the ULPF1 after adjusting the optical path
length. By transmitting the combined light through ULPF2(LP01-1064-RE, Semlock), the light with wavelengths
shorter than 1058 nm was blocked. The obtained light was
focused on a sample using an objective lens
(LCPLN100XIR, Olympus) with NA = 0.85 installed in an
inverted optical microscope (IX83, Olympus).
The light reﬂected from the sample was collected by thesame objective lens (epi-detection) and separated from thelaser light by reﬂecting it on ULPF2. The obtained signal wasmeasured with a spectroscope (SpetraPro 2300i, Princeton
Instruments, grating of 600 grooves mm−1) and a nitrogencooled InGaAs photodiode array (PyLoN-IR-1024-1.7). As
will be described later, a resolution of 9.6 cm−1 was obtained.
The quantum efﬁciency is 0.2 or less in the visible lightregion, but a high quantum efﬁciency (>0.8) is possible atwavelengths of 950 nm or more.
The CARS process is a third-order nonlinear opticalprocess generated by irradiating light of ωp and ωs, as shownin Fig. 1(b). The vibration mode Ωi of the sample is excitedby the difference in frequency between ωp and ωs and theanti-Stokes line whose frequency is shifted by Ωi from ωp isContent from this work may be used under the terms of the Creative Commons Attribution 4.0 license. Any further distribution of thiswork must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
© 2021 The Author(s). Published on behalf of
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(a)

(b)

(c)

Fig. 1. (Color online) Developed system and analysis method. (a) Setup of the infrared low-frequency multiplex CARS system using a supercontinuum laser.
HNF: highly nonlinear ﬁber, (U)LPF: (ultrasteep) long-pass ﬁlter, and BPF: bandpass ﬁlter. The light spot position was controlled by the XY stage (NanoBioS200, MCL Inc.) on which the sample was placed. Spot sizes of ωp, ωs and the spatial resolution are shown in Figs. S4 and S5 (supporting information).
Since the laser beam could not be completely removed by one reﬂection, it was reﬂected twice more by ULPF3. This part can be simpliﬁed if a short-pass ﬁlter
with a narrow edge width near 1064 nm is used. (b) Energy diagram of CARS. Ωi: frequency of a vibration mode. (c) The spectrum of the laser incident on thesample. Inset: spectra of wp around 1064 nm with (red) and without (light red) the BPF.

measured. As shown in Fig. 1(c), the spectrum at the sample
position is a combination of the narrow-linewidth beam ωpand the wideband beam ωs. An enlarged view of the spectrumof ωp is shown in the inset. To improve the spectral
resolution, a part of the spectrum of ωp with a ﬁnite widthis narrowed using a BPF with a linewidth of 0.7 nm(FWHM).
CARS measurement generally produces broad light withthe same frequency as the signal. This is called non-resonantbackground (NRB), and it occurs strongly, especially in the
low-frequency region. Previous studies have attempted to
improve the signal-to-noise (SN) ratio by utilizing theinterference between the CARS signal and NRB;12) a notch
was made in the broad spectrum and ωCARS was extracted
from the difference between the two spectra, which wasmeasured by slightly changing the wavelength of the notch.
In our study, ωCARS was obtained by taking the difference
between the two spectra measured by changing the wavelength of ωp by δω using a BPF and a rotation stage(ELL18K/M, Thorlabs). The advantage of modulating thepeak-type ωp frequency is that the SN can be improved
because the intensity of ωp can be increased compared withthe case of using a notch described above.
However, in this method, attention must be paid to thespectral shape caused by the interference between CARS and
NRB. Assuming that the complex electric ﬁelds generated by
CARS and NRB are ECARS and ENR, respectively, thefrequency dependence of the detected light intensity can be
expressed* E CARS].
I (w ) = ∣ E CARS + ENR ∣2 = ∣ E CARS ∣2 + ∣ENR ∣2 + 2 Re [E NR(1 )The third term on the right side is produced by interference.
When ECARS has a resonance peak shape and ENR is a

non-phase-sensitive background, the spectrum with the Fano
shape appears when the NRB amplitude is sufﬁciently larger
than the CARS signal [see Fig. S1(a) available online atstacks.iop.org/APEX/14/122006/mmedia]. On the other
hand, when the effect of interference by NRB is small, aspectrum with a peak appears {see Fig. S1(b)}. Then, when
the difference between the two spectra obtained by changing
ωp is considered, the spectrum has a structure with the largest
slope at the frequency Ωi. To obtain an accurate Raman shift,
it is necessary to consider ﬁtting with the degree ofinterference.
First, the bandwidth of the IR excitation multiplex CARS
microscope was evaluated. A polyimide ﬁlm with a thicknessof 12.5 μm was used as a sample. Polyimide is a polymer inwhich aromatic compounds are linked via an imide bond andhas high thermal, mechanical, and chemical stabilities.
However, polyimide exhibits strong ﬂuorescence over theentire visible light range.22) Therefore, when Raman measurement is performed with a visible light source, processing
is necessary to reduce the background.22) Numerous vibrational modes derived from the polyimide structure wereobserved in a wide band from 586 to 1788 cm−1 (see Fig.
S2). The attribution of each peak according to previous
studies23–25) is as described in the caption of Fig. S2. The
upper limit of 1800 cm−1 is due to the negative dispersion of
the broadband Stokes light (see Fig. S3).
Next, low-wavenumber CARS spectra and removal of
NRB is discussed. A sulfur crystal was used as a sample,
whose structures at room temperature and pressure are known
to be α-S8 (pale yellow orthorhombic), β-S8 (pale yellow
monoclinic), and γ-S8 (pale yellow needle-shaped monoclinic). All have a cyclic octamer ring S8 as a unit structure,
the α-type is the most stable at 95.6 °C or lower, and theβ-type is the most stable above 95.6 °C. In this experiment,

122006-2© 2021 The Author(s). Published on behalf of
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Fig. 2. (Color online) CARS spectra obtained for the sulfur crystal at room
temperature. Red and blue curves are raw spectra (top lines) obtained at room
temperature with slightly different wavelengths of λp. Exposure time was
100 ms with the laser power of 24.0 mW at the sample. The green curve isthe difference between the red and blue spectra (middle line). The orange
curve is the numerical derivative of the blue spectrum (bottom line) withrespect to the Raman shift k. dA/dk showed the same shape in the region
larger than 100 cm−1 (not shown here).

the sample was sandwiched between a gold mirror (PF10-03M03, Thorlabs) and a sapphire substrate (thickness of
0.1 mm).
The blue line in Fig. 2 shows the spectrum obtained fromthe sample at room temperature. Sharp peaks of 220 cm−1
and 470 cm−1 were observed in the broad spectrum. The redline overlapping the blue line shows the spectrum obtainedby changing λp by 0.4 nm by changing the angle of the BPF.
To distinguish the CARS signal from the NRB, we took the
difference between the red and blue spectra and obtained
the green spectrum. Three CARS peaks can be seen in the
spectrum. From the high-frequency side, 470 cm−1 is thesignal originating from the S–S bond stretching of the S8bond structure, 220 cm−1 is the signal from the symmetric
S–S bending (called the 220 cm−1 peak hereafter), and150 cm−1 is the signal from the asymmetric S–Sbending.17) All three peaks were hardly affected by interference and had a shape similar to that shown in Fig. S1(b).
However, since the NRB spectrum changes sharply at thepeak position of 150 cm−1, it is essential to conﬁrm theexistence of this peak in the difference spectrum.
For comparison, the orange line shows the result of
numerically differentiating the raw data. Multiple peaks
appeared in addition to the CARS signal, which is due tothe complex shape of the background at <650 cm−1, asshown in the raw data (top lines) in Fig. 2. Moreover, sincesuch NRB shape strongly depends on materials, the evaluation of signals becomes complicated. In contrast, obtaining asignal by taking the difference of two data can effectively
reduce such a problem. Namely, it was conﬁrmed that themethod of obtaining the spectrum using the BPF is especiallyeffective in distinguishing the CARS signal in the lowwavenumber region where the effect of NRB is relatively
large and characteristic. The wavenumber resolution of thedeveloped system was 9.6 cm−1 (FWHM), and the lower
limit of the measurement on the low-wavenumber side was
55 cm−1, which is the lowest value obtained thus far bymultiplex CARS using IR light with a wavelength of1064 nm or longer.
Next, we conﬁrmed the high efﬁciency of the developed
system. By IR excitation multiplex CARS, we achieved thesame high measurement efﬁciency as that in previous
research using visible light excitation, even on the lowwavenumber side of the ﬁngerprint region.17) Besides, IR
excitation could induce CARS while suppressing the sample
damages caused by light irradiation, as expected (see Fig.
S5).
Then, the change in the intensity and shift of the Raman
signal accompanying the phase transition of a sulfur crystal
was observed. The sample was heated to 120 °C to liquefy it,then it was naturally cooled to solidify it, and the changes
during the process were probed. Figure 3(a) shows the CARS
difference spectra when the temperature was increased fromroom temperature to 120 °C, obtained as shown in Fig. 2,
with each spectrum normalized by the peak height of
470 cm−1. Almost the same spectral shape was observed
from room temperature to 90 °C, but the intensity of the
220 cm−1 peak decreased at 95 °C. It is considered that the
sample underwent a phase transition to a monoclinic structure
at 95 °C. The sample was liqueﬁed at 105 °C, and the spectral
shape did not change further at higher temperatures. Since theeight-membered ring is stable up to 160 °C, three peaks
derived from S8 were observed even for the liquid.
Subsequently, Fig. 3(b) shows the results of observation
while cooling the sample from 120 °C. The spectrum of theliquid phase did not change until 60 °C below the melting
point of 105 °C. Since the wavelength of the 220 cm−1 peakshifted to the higher-frequency side by 3.5 cm−1, it isconsidered that the sample solidiﬁed at 60 °C, a temperature
lower than the conventional melting point, because of thesupercooling effect.
Figure 3(c) shows an enlarged view of the Raman spectra
obtained before (blue), at 95 °C; during heating (orange), forthe liquid (red), and at 60 °C; during cooling (green). The
peak position did not change signiﬁcantly between the initial
temperature and 95 °C; however, its shift to the lowwavenumber side was observed in the liquid phase. During
re-solidiﬁcation, it shifted to the higher-wavenumber sidethan that in the initial state.
Figure 3(d) shows the temperature dependence of themagniﬁcation of the shift shown in Fig. 3(c). The values on
the vertical axis were obtained by ﬁtting the spectra in
Figs. 3(a) and 3(b). For the ﬁtting function, the sum of two
Lorentz functions with slightly different center positions andopposite signs was used. The peak position, which was
initially at 222 cm−1, gradually decreased with heating andrapidly changed to 220 cm−1 at the liquefaction temperatureof 105 °C. When the sample was cooled after reaching
120 °C, there was no signiﬁcant change at 105 °C owing to
the effect of the supercooling. After the gradual shift to thehigh-frequency side, a sudden change in peak position fromabout 220.2–223.4 cm−1 occurred at 62 °C. The reason why
the wavenumber was different from that in the initial state isconsidered to be the generation of a γ-type phase instead of
the initial α-type phase.17) The slight decrease in frequency
when the β-type is generated is considered to be due to theincrease in the relative intensity of the lower-frequency sidein the double peak structure of the mode reported in the© 2021 The Author(s). Published on behalf of

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Appl. Phys. Express 14, 122006 (2021)

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Fig. 3. (Color online) Spectra obtained for the phase transition of a Sulfur crystal. Dependence of the substrate temperature of the difference spectra obtained
as shown in Fig. 2 with (a) increasing and (b) decreasing the substrate temperature. All spectra were normalized by the height of the 470 cm−1 peak. (c) Raman
spectra obtained before heating (blue), 95 °C during heating (orange), for the liquid (red), and at 60 °C during cooling (green). (d) Temperature dependence of
the Raman shift of the peak near 220 cm−1 (called the 220 cm−1 peak hereafter) obtained by ﬁtting the data in (a) and (b). When the ﬁtting was performed withLorentzian and Gaussian functions, the degree of shape matching depended on the temperature, but the peak position was the same for both. At 25 °C in (a),ﬁtting by the Lorentzian function is overlapped. The rate of heating and cooling is ∼1.5 °C min−1. The accuracy of the temperature is <0.2 °C for each
spectrum.

high-resolution measurement.17) The change in the peakintensity of 150 cm−1 appeared slightly different from that of
220 cm−1 when the temperature was increased. However, thetemperature dependence of the peak shift and the temperature
at which the intensity change occurred were similar for bothpeaks. In addition, the association with each phase wasconsistent with the previous study.17) The analysis of theRaman spectrum thus clariﬁed the details of the spectralchange due to the phase transition of a sulfur crystal during
heating and cooling.
Next, we visualized the phase transition. CARS spectra
were measured over a 50 × 50 μm2 area (100 × 100 points)
at different temperatures, and the results are shown in Fig. 4.
The temperature was increased from room temperature to

120 °C and then returned to room temperature. Figure 4(a)shows the mapping of the 1064 nm scattered light intensity
I0 at different temperatures. The optical contrast affected by
the shape, refractive index, and thickness of the sample wasvisualized. Some differences were observed in the images
between 25 °C and 80 °C during heating and between 60 °C
and 25 °C during cooling, despite the temperatures being
lower than the phase transition temperature. Namely,
gradual changes appear to occur in the crystal structure.
At 95 °C during cooling, the sample was in a liquid phaseowing to the effect of the supercooling, so a non-uniform
distribution appeared in the scattered light intensity. This isconsidered to represent the effect of the substrate on
multiple reﬂections.
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Appl. Phys. Express 14, 122006 (2021)
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(b)

Y. Arashida et al.

distributed around an average value of 221.5 cm−1. At 95 °C,
where the β-type is the most stable phase, the wavenumber was
distributed around 221 cm−1. Thus, the wavenumber shifted to alower value with increasing temperature.
On the other hand, when the sample was cooled from
120 °C, the sample was still in the liquid phase at 95 °C, andthe wavenumber was distributed below 220 cm−1. Upon further
cooling, the wavenumber shifted to the higher-wavenumber
side at 60 °C with the phase transition. An image with adifferent appearance from the scattered light and Raman
intensity distributions appeared, indicating the importance of
this high-resolution spectral analysis. Closer examinations
revealed a mixture of a region with a wavenumber of about
222 cm−1, which is often observed in the α-type phase, and aregion with a high wavenumber of up to 224 cm−1, as observedin Fig. 3(d). These results suggest that the sample was not asingle uniform crystal. The region with the high wavenumber isconsidered to be the γ-type phase from the discussion of Fig. 4.
It is considered that the γ-type phase did not immediately
disappear even when the temperature was returned to room
temperature, and the transition to the α-type phase took
required an even longer time (see Fig. S7 for more detail).
This method is highly expected to play an important role in the
precise evaluation of materials, for example, bio-cells andlithium-ion batteries, in which S plays an important role.
In conclusion, we have developed a multi-plex CARS
microscope that uses IR SC light as a light source. The
separation of the signal and background is extremely
important for low-wavenumber CARS. We have establisheda method of accurately distinguishing CARS signals. The
developed system has potential for further reﬁnement and ishighly expected to advance the micro-characteristic evaluation of materials, for example, containing heavy atoms andpolymers with low-wavenumber oscillations.

(c)

Acknowledgments We acknowledge the ﬁnancial supports of a Grant-inAid for Scientiﬁc Research (17H06088, JP19H02591) from Japan Society for the
Promotion of Science and Japan Core Research Evolutional Science andTechnology (CREST) (JPMJCR1875).

Fig. 4. (Color online) Visualization of the phase transition. The spatial
distributions of (a) the intensity of the scattered light of 1064 nm, Io, (b) the
intensity of the CARS signal of the 220 cm−1 peak, I220, and (c) themagnitude of the Raman shift of the 220 cm−1 peak are shown. k220 is thecentral wavenumber of the 220 cm−1 peak. The full scale of all images is
50 μm × 50 μm, and measurements were performed at 100 × 100 points. It
took 0.7 s to obtain two spectra at each point by changing the wavelength of
ωp by δω using the BPF and the rotation stage, as shown in Fig. 1, which canbe shortened about one order with a Galvano system.

The strength of the signal in the 220 cm−1 peak, I220, is
mapped in Fig. 4(b). The intensity distribution was different
from that of the scattered light shown in Fig. 4(b). To assist thediscussion of the distribution of each phase, Fig. 4(c) shows theresult of extracting and mapping the central wavenumber k220 of
the 220 cm−1 peak obtained by the same ﬁtting method as thatshown in Fig. S1(a). The red and blue colors represent the highand low-wavenumber sides, respectively. The wavenumber
distribution before heating was almost uniform, indicating thatthe α-type, which is stable at room temperature, occupied mostof the sample. However, there was also a part showing a highwavenumber of 224 cm−1. When the sample was heated to 80 °C, the wavelength shifted slightly to the lower side and was

Notes

The authors declare no competing interests.

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