Optics and Photonics Journal, 2013, 3, 38-42
doi:10.4236/opj.2013.32B009 Published Online June 2013 (http://www.scirp.org/journal/opj)
Selective Excitation of Two-pulse Femtosecond Coherent
Anti-Stokes Raman Scattering in a Mixture
Hui Zhang1, Xia nghua Feng1, Zhongqiu Yu1, Xia Li1, Shian Zhang2, Zhenrong Sun2
1Institute of Science, PLA Information Engineering University, Zhengzhou, China
2State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, China
Received 2013
ABSTRACT
In this paper, we experimentally study the selective excitation of two-pulse femtosecond coherent anti-Stokes Raman
scattering (CARS) in a mixture of dibromomethane (CH2Br2) and chloroform (CHCl3) by adaptive pulse shaping based
on genetic algorithm. Second harmonic generation frequency-resolved optical gating (SHG-FROG) traces indicate that
the spectral amplitude and phase of the optimal pulse are both modulated. Finally, we discuss the physical mechanism
for the selective excitation of femtosecond CARS based on the retrieved information from SHG-FROG traces.
Keywords: Coherent Anti-Stokes Raman Scattering; Selective Excitation; Pulse Shaping
1. Introduction
Coherent anti-Stokes Raman scattering (CARS), a nonlin-
ear four-wave-mixing (FWM) process, is one of the most
important nonlinear spectroscopic techniques, [1-5] and
has been widely employed to investigate the molecular
dynamical process. The selective excitation of special
Raman mode can enhance the CARS spectroscopic con-
trast, and therefore it can be applied on microscopic in-
vestigation of complex molecular structure. However, for
femtosecond CARS process, several Raman modes fal-
ling within the broad spectrum of the femtosecond pulses
can be simultaneously excited, and it results in the poor
selectivity between the different Raman modes. A prom-
ising method for the mode-selective excitation is coher-
ent control by the femtosecond pulse shaping technique.
Here, the femtosecond pulse is shaped by modulating the
spectral amplitude and/or phase to control the light-matter
interaction and achieve the desired outcomes. So far, co-
herent control by shaping the femtosecond pulse has been
widely utilized to realize the selective excitation of fem-
tosecond CARS. [6-19] Such as, the shaped femtosecond
pulse with the simple spectral phase pattern of -step,
sinusoidal, chirped or binary function has been employed
to realize the selective excitation of the femtosecond CARS.
[8-14] Moreover, the use of adaptive feed- back control
based on genetic algorithm or evolution strategy to selec-
tively excite the femtosecond CARS has also been re-
ported.[15-19]
In our previous study on the selective excitation of one
or more Raman modes in the benzene (C6H6) solution by
the optimal control of two-pulse femtosecond CARS,[19]
it was demonstrated that both the spectral amplitude and
phase of the optimal pump and probe pulses are strongly
modulated. In this paper, we further explore the physical
control processes. We experimentally study the selective
excitation of two-pulse femtosecond coherent anti-Stokes
Raman scattering (CARS) in a mixture of dibromomethane
(CH2Br2) and chloroform (CHCl3) by shaping femtosec-
ond laser pulse based on genetic algorithm, the CARS
signal from chloroform can be enhanced and simultane-
ously the CARS signal from dibromomethane is effec-
tively suppressed. The original and optimal laser pulses
are characterized by second harmonic generation fre-
quency-resolved optical gating (SHG-FROG) technique,
and finally the physical control mechanisms are explic-
itly discussed and analyzed based on the retrieved infor-
mation from SHG-FROG traces.
2. Experiment Setup
The layout of the experimental arrangement is shown in
Figure 1. A Ti:sapphire mode-locked laser (Spectra-Physics
Spitfire amplifier) is used as the excitation source with
the pulse duration of about 50 fs and the center wave-
length of 800 nm. The output laser pulse is split into two
components. One is used to pump an optical parametric
amplifier (OPA) to generate the Stokes pulse. The other,
as the pump and probe pulses, is sent into a programma-
ble 4f pulse shaper. The pulse shaper is composed of a
pair of diffraction gratings with 1200 lines/mm and a pair
of concave mirrors with 200 mm focal length. A one-
dimensional programmable liquid-crystal spatial light
modulator array (SLM-256, CRI) is placed at the Fourier
Copyright © 2013 SciRes. OPJ
H. ZHANG ET AL. 39
plane of the shaper and used as updatable filter for the
spectral amplitude and phase modulation. The frequency
difference of the pump and Stokes pulses is set between
the Raman shifts of dibromomethane and chloroform,
and the dibromomethane and chloroform are mixed by
the volume ratio 1:1. Their CARS signal is detected by
the spectrometer equipped with a charge coupled device
(CCD). A computer is served for reading the CARS sig-
nal, evaluating the cost function and updating the SLM.
The original and optimal pump and probe pulses are
characterized by second harmonic generation frequency-
resolved optical gating (SHG–FROG) technique.[20]
3. Results and Discussion
Figure 2(a) shows CARS spectrum for the mixture of
dibromomethane and chloroform excited by the original
pulse. Two Raman signals are observed and labeled peak
1 (intensive) and peak 2 (weak). The peak 1 can be con-
tributed to the Raman mode of dibromomethane at 1388
cm-1 and the peak 2 can be attributed to the Raman mode
of chloroform at 1889 cm-1.[21,22]
Our experimental goal is to enhance the weak CARS
signal 2 from chloroform and maximally suppress the in-
tense CARS signal 1 from dibromomethane. In our ex-
periment, an optimal feedback control method based on
genetic algorithms is used to selectively excite special
Raman mode from the mixture of dibromomethane and
chloroform. Genetic algorithms allow a change in all of the
pixels of the spectral phase mask and a decision whether
the change is either accepted or rejected according to the
calculated fitness. Firstly, the initial voltage values are
generated randomly and loaded on the pixels of SLM as
the first generation. Secondly, the fitness for the contrast
ratio of the CARS signal intensity from chloroform to that
from dibromomethane is calculated, and then the voltage
values of the new spectral mask for the second genera-
tion are generated by genetic algorithm operation (select,
crossover and mutate). Finally, the abovementioned opti-
mization procedure is repetitively performed till the
Spec.
Cell
Len
CCD
OPA
Dela y
C
1
SL
C
2
G
21
G
11
Len
PC
Figure 1. The experimental arrangement for selective excita-
tion in stimulated Raman scattering. The programmable
liquid-crystal spatial light modulator (SLM-256) is used as
an updatable filter for spe ctral manipulation of the incident
pulses. The computer is used for calculating the cost funtion
and updating the spectr al filter of the SLM.
fitness approaches convergence and the contrast ratio
approaches to the optimal value. Figure 2(b) shows CARS
spectrum for the mixture of dibromomethane and chloro-
form excited by the optimal pulse. It is found that, Peak 1
from dibromomethane can be suppressed by one order of
magnitude, but the Peak 2 from chloroform can be effec-
tively enhanced by a factor of 8.
To experimentally indicate the physical control proc-
ess, second harmonic generation frequency resolved op-
tical gating (SHG-FROG) technique is employed to
characterize the original and optimal pump and probe
pulses, and the experimental results are shown in Figure
3. The SHG-FROG traces show that the optimal pulse is
strongly modulated. To further achieve the spectral in-
formation, the amplitude and phase of the original and
optimal pulses in frequency and time domain are calcu-
lated based on the SHG-FROG traces and presented in
Figure 4. As can be seen, the low wavelength compo-
nents in optimal pulse are attenuated and simultaneously
their phases are modulated (see Figure 4(c)). Moreover,
the optimal pulse becomes pulse train (see Figure 4(d)).
So we reasonably believe that the shaped pump and probe
pulses with the combined amplitude and phase modula-
tion should be the optimal field for the selective excita-
tion of the two-pulse femtosecond CARS.
640 660 680 700 720 740
0
2
4
6
CARS intensity (arb. units)
Wavelength (nm)
1
2
(a)
640 660 680 700 720 740
0.0
0.2
0.4
0.6
0.8
1.0
CARS intensity (arb. units)
Wav elen gth (nm )
2
1
(b)
Figure 2. Stimulated Raman spectrum for the mixture of
dibromomethane and chloroform is excited by the original
pulse (a) and the optimal pulse (b).
Copyright © 2013 SciRes. OPJ
H. ZHANG ET AL.
40
-300 -200 -1000100200300
390
395
400
405
410
Time delay (fs)
Wavelength (nm)
(a)
-300 -200 -1000100200300
390
395
400
405
410
Time delay (fs)
Wavelength (nm)
(b)
Figure 3. Second harmonic generation frequency resolved
gating (SHG-FROG) traces for the original (a) and optimal
pump pulse (b).
Next we discuss the physical mechanism for the selec-
tive excitation of femtosecond CARS based on the opti-
mal pulse characteristic. According to the theory of Ma-
linovskaya and Bucksbaum,[23] the stimulated Rman
transition can be suppressed by tailoring its correspond-
ing excitation frequency components. In our experiment,
it is obvious that the peak 1 is induced by the blue frequency
components of the pulse and the peak 2 is induced by the
red frequency components. The blue frequency compo-
nents of the optimal pulse are tailored (see Figure 4(c)),
which results in the peak 1 being suppressed. Further-
more, the optimal pulse is modulated into pulse train (see
Figure 4(d)), so the Stokes pulse will be prior to some
sub-pulses in the pulse train, thus a stimulated Raman
adiabatic passage (STIRAP) process can generate.[24,25]
In this case, more population can transfer, which results
in the peak 2 being enhanced.
0
10
20
30
0
2
4
6
0
10
20
30
-200 -1000100200
0.0
0.2
0.4
0.6
0.8
1.0
intensity
phase
intensity
phase
Time delay (fs)
Intensity (a.u.)
(d)
0
2
4
6
Phase (rad)
Phase (rad)
Phase (rad)Phase (rad)
775 800 825
0.0
0.2
0.4
0.6
0.8
1.0 intensity
phase
Intensity (a.u.)
Wavelength (nm)
Time delay (fs)
(c)
-200 -1000100200
0.0
0.2
0.4
0.6
0.8
1.0
(b)
775 800 825
0.0
0.2
0.4
0.6
0.8
1.0 intensity
phase
Intensity (a.u.)Intensity (a.u.)
Wavelength (nm)
(a)
Figure 4. The amplitude and phase for the original (a,b)
and optimal pulse (c,d) in frequency and time domains.
4. Conclusions
In summary, we have experimentally demonstrated the
selective excitation of CARS in the mixture of di-
bromomethane (CH2Br2) and chloroform (CHCl3) by
Copyright © 2013 SciRes. OPJ
H. ZHANG ET AL. 41
adaptive pulse shaping based on genetic algorithm. Sec-
ond harmonic generation frequency-resolved optical gat-
ing (SHG-FROG) traces indicate that the spectral ampli-
tude and phase of the optimal pulse are strongly modu-
lated. Based on the retrieved information from SHG-FROG
traces, it was proposed that the selective excitation of
femtosecond CARS was due to laser spectrum tailored
and pulse-train forming. We believe that these experiment-
tal results should have potential applications on stimu-
lated Raman spectrum.
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