The last two decades have seen a renaissance of interest in coherent Raman scattering techniques, partly due to the emergence of attractive applications in optical microscopy. In general, coherent Raman scattering describes nonlinear light-matter interactions which can exhibit significant resonance enhancements under vibrationally resonant excitation. The main processes employed are coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS). In both cases, vibrational selectivity arises from resonances in the third-order nonlinear optical response of molecules. Coherent Raman signals are often several orders of magnitude stronger than spontaneous Raman scattering signals. Very soon after the invention of the laser and in numerous experiments ever since, this advantage has been exploited in spectroscopic applications. The recent surge in publications on coherent Raman scattering applications, however, is owed to the successful combination of these spectroscopic approaches with optical microscopy. The main motivation for this is the ability of coherent Raman scattering to generate chemically selective contrasts in microscopy with high sensitivity and no need for labeling. This avoids the shortcomings of spontaneous Raman or infrared microscopy, such as low signal intensity, susceptibility to sample autofluorescence, and low spatial resolution, and permits the recording of highly resolved microscopic images with frame rates up to video rates.

Because efficient coherent Raman scattering requires the use of ultrashort pulsed laser sources, the development of new laser sources and sensitive excitation and detection schemes has always been of great importance to its further advancement. In addition, the chemical synthesis of Raman tags has become a pivotal factor in exploring novel applications of CARS and SRS microscopy. Currently, a broad variety of modalities of spectroscopy and microscopy based on CARS or SRS are exploited and applied in numerous scientific, industrial, and biomedical applications, including combustion diagnosis, food inspection, cosmetics, neural imaging, drug discovery, intraoperative diagnosis, flow cytometry, and endoscopy. Yet, challenges remain in further extending the capabilities of coherent Raman scattering and making it a mainstream technology for spectroscopy and imaging.

The primary goal of this Special Topic is to highlight basic principles, advanced techniques, laser sources, and applications of spectroscopy and imaging based on the effect of coherent Raman scattering, namely coherent Raman spectroscopy and imaging. Specifically, the Special Topic contains one Tutorial, one Perspective, and ten original research articles as described below.

H. Rigneault and P. Berto1 provide a comprehensive Tutorial on coherent Raman light-matter interaction, which forms a basis for understanding coherent Raman scattering processes such as CARS and SRS. Specifically, the paper provides a digest introduction to the fundamental physics of coherent Raman scattering with simple physical pictures and derivations. The tutorial is an excellent educational tool suitable for students and newcomers with various backgrounds to shape the future of coherent Raman spectroscopy and imaging.

C. Zhang and J.-X. Cheng2 provide a timely Perspective on the future of coherent Raman microscopy. Specifically, they introduce the history, early development, recent technical innovations, and enabling applications of coherent Raman microscopy. The perspective also discusses the advantages and disadvantages of different modalities and the remaining challenges and possible future directions of coherent Raman microscopy. Consequently, the paper provides an excellent overview of recent advances in the field for students and researchers who are interested in developing or using coherent Raman microscopy.

This Special Topic comprises articles on CARS spectroscopy, microscopy, and endoscopy together with their biomedical applications in neuron imaging. Yoneyama et al.3 report on the development of a method for ultra-broadband multiplex CARS microspectroscopy using a sub-100-ps microchip laser source and fiber amplifier, which enables intracellular molecular fingerprinting of live HeLa cells. Müller et al.4 show how applying shaped laser pulses in a single beam for integrated CARS and mid-infrared spectroscopy brings the advantages of high spatial resolution and easy access to vibrational fingerprints. Langbein et al.5 report on the realization of dual-polarization epi-heterodyne CARS microscopy with balanced detection, which provides background-free chemically specific image contrast, shot-noise limited detection, and phase sensitivity. Zirak et al.6 present a rigid and compact CARS endoscope design integrated with a gradient index lens and successfully demonstrate CARS endoscopic imaging on murine spinal cord tissue with high resolution in a large field of view. Hirose et al.7 illustrate a different CARS rigid endoscope design with two optical fibers for delivering pump and Stokes beams separately and demonstrate label-free imaging of rat sciatic nerves, representing an important step toward applications in neurosurgery.

This Special Topic also contains articles on SRS microscopy and spectroscopy, including new applications, technical advances, and comparisons with CARS. Shi et al.8 report SRS imaging of in vivo protein synthesis in mouse brain, pancreas, liver, and colon tumors using deuterated amino acids introduced via arterial injection, identifying metabolic heterogeneity in protein synthesis. Kumar et al.9 report multiplex SRS spectroscopy with an interferometric Fourier transform method, which provides the complex Raman susceptibility of different liquids and their mixtures. Raanan et al.10 present a method of low-frequency vibrational spectroscopy based on stimulated Raman-induced Kerr lensing with a successful demonstration of Raman spectroscopy of neat liquids. Ito et al.11 demonstrate a sophisticated SRS detection method for suppressing an unwanted background signal based on the combination of impulsive excitation, spectral focusing, and time-resolved heterodyne detection. Bocklitz et al.12 report histopathological tissue imaging with CARS and SRS followed by cluster analysis of their hyperspectral dataset and discuss their advantages and disadvantages.

In conclusion, this Special Topic is intended to highlight the past, present, and future of coherent Raman spectroscopy and imaging. It is our hope that the Special Topic will serve as a forum for students and researchers to further expand the potential of coherent Raman spectroscopy and imaging. Our special thanks to Benjamin Eggleton, Editor-in-Chief, Benedetta Camarota, and Erinn Brigham, Journal Managers, for their technical assistance with publishing.

1.
H.
Rigneault
and
P.
Berto
,
APL Photonics
3
,
091101
(
2018
).
2.
C.
Zhang
and
J.-X.
Cheng
,
APL Photonics
3
,
090901
(
2018
).
3.
H.
Yoneyama
,
K.
Sudo
,
P.
Leproux
,
V.
Couderc
,
A.
Inoko
, and
H.
Kano
,
APL Photonics
3
,
092408
(
2018
).
4.
N.
Müller
,
L.
Brückner
, and
M.
Motzkus
,
APL Photonics
3
,
092406
(
2018
).
5.
W.
Langbein
,
D.
Regan
,
I.
Pope
, and
P.
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,
APL Photonics
3
,
092402
(
2018
).
6.
P.
Zirak
,
G.
Matz
,
B.
Messerschmidt
,
T.
Meyer
,
M.
Schmitt
,
J.
Popp
,
O.
Uckermann
,
R.
Galli
,
M.
Kirsch
,
M. J.
Winterhalder
, and
A.
Zumbusch
,
APL Photonics
3
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092409
(
2018
).
7.
K.
Hirose
,
S.
Fukushima
,
T.
Furukawa
,
H.
Niioka
, and
M.
Hashimoto
,
APL Photonics
3
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092407
(
2018
).
8.
L.
Shi
,
Y.
Shen
, and
W.
Min
,
APL Photonics
3
,
092401
(
2018
).
9.
V.
Kumar
,
A.
De la Cadena
,
A.
Perri
,
F.
Preda
,
N.
Coluccelli
,
G.
Cerullo
, and
D.
Polli
,
APL Photonics
3
,
092403
(
2018
).
10.
D.
Raanan
,
J.
Lüttig
,
Y.
Silberberg
, and
D.
Oron
,
APL Photonics
3
,
092501
(
2018
).
11.
T.
Ito
,
Y.
Obara
, and
K.
Misawa
,
APL Photonics
3
,
092405
(
2018
).
12.
T.
Bocklitz
,
T.
Meyer
,
M.
Schmitt
,
I.
Rimke
,
F.
Hoffmann
,
F.
von Eggeling
,
G.
Ernst
,
O.
Guntinas-Lichius
, and
J.
Popp
,
APL Photonics
3
,
092404
(
2018
).
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