Download High spectral resolution multiplex CARS spectroscopy using chirped

Invited Paper
High spectral resolution multiplex CARS spectroscopy using chirped
pulses
K.P. Knutsen, J.C. Johnson, A.E. Miller, P.B. Petersen, R.J. Saykally*
Department of Chemistry, University of California/Berkeley, Berkeley, CA, USA 94720
ABSTRACT
A novel technique for achieving high spectral resolution with a femtosecond laser system is presented. Transformlimited 800 nm, 90 femtosecond (fs) pulses pass off two gratings, stretching the pulse in time to a pulse width of several
picoseconds due to an induced linear temporal chirp directly proportional to the grating separation. This chirped pulse
is the degenerate pump (ωp) and probe (ωp') pulse for the CARS experiment. When overlapped in time with the 1050
nm, 90 fs transform-limited Stokes (ωS) pulse, only a fraction of the chirped ωp' pulse generates the CARS signal,
thereby creating a temporal slit that defines the spectral resolution of the technique. Spectra for liquid methanol and
liquid isooctane are presented, with ~6 cm-1 spectral resolution achieved for isooctane. Resonance enhancement and the
mechanism of achieving high spectral resolution are shown by adjusting the ωS wavelength and ωp delay relative to the
ωS pulse.
Keywords: Coherent Anti-Stokes Raman Scattering (CARS), chirp, multiplex
1. INTRODUCTION
Coherent Anti-stokes Raman Scattering (CARS) is a nonlinear process wherein the interaction of three beams, ωp, the
pump (which is usually degenerate) and ωs (the Stokes beam) generates the anti-Stokes signal at the frequency ωas =
2ωp - ωs. By tuning the energy difference ωp - ωs into a molecular resonance, usually a vibrational transition, one can
achieve large enhancements in the signal. The CARS signal is dependent upon the types of chemical bonds and the
environment that surrounds those bonds and is therefore very useful in determining the chemical composition of a
system. For example, the protein secondary structures α-helices and β-sheets have different Raman frequencies,
allowing for identification and assessment of relative quantities of the two species within a sample.
Standard experimental design with narrow bandwidth (picosecond) lasers have high spectral resolution, but they have
the disadvantage of requiring step-scanning through the frequencies to generate a spectrum, making direct comparison
difficult and experimentally more challenging. Recent efforts have been devoted towards developing multiplex CARS
setups that can measure an entire spectrum at once, allowing for direct comparison of the relative intensities and shifts
of vibrations within the bandwidth of the a broadband (femtosecond) laser.
There are various designs for multiplex CARS, all based on the fundamental principle of employing a broadband light
source to simultaneously excite several vibrational modes in the sample, followed by a probe pulse that is of narrow
bandwidth. One option is to use a synchronized femtosecond/picosecond laser combination, wherein the picosecond
laser defines the spectral resolution.1,2 Another approach demonstrated in sum frequency generation spectroscopy is to
start with a femtosecond light source, physically disperse the bandwidth through the use of dispersive optics (e.g. prisms
or gratings), and then introduce a slit in the dispersed beam to decrease its bandwidth, effectively converting the
femtosecond laser into a picosecond laser.3 We introduce a novel variant on the latter design, wherein the spectral
resolution of our instrument is defined by a temporal rather than a physical slit. Through the use of chirped laser pulses,
the spectral resolution is defined by a temporal overlap of a transform-limited 90 fs pulse with a temporally chirped
*
230
Corresponding author. Email address: [email protected]
Multiphoton Microscopy in the Biomedical Sciences IV, edited by
Ammasi Periasamy, Peter T. C. So, Proceedings of SPIE Vol. 5323
(SPIE, Bellingham, WA, 2004) · 1605-7422/04/$15 · doi: 10.1117/12.527830
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pulse stretched to several picoseconds. The advantages of this technique are that the spectral resolution can be easily
adjusted with gratings separation to match the natural line widths of the transitions, thereby increasing the resonant
CARS signal obtained from the sample, or that one can obtain arbitrarily low spectral resolution. With chirped pulses
rather than transform-limited pulses, the temporally longer beam decreases the peak power on the sample and thus
reduces photo-damage.
2. EXPERIMENTAL
2.1 Optical design for chirped CARS
Femtosecond pulses for the pump and Stokes beams are generated by a home-built Ti:Sapphire oscillator centered at
793 nm (40 fs, 300 mW, 3.4 nJ, 88 MHz), pumped by a cw Nd:VO4 532 nm laser (3 W, Spectra Physics, Millenia Vs).
This seed pulse is amplified by a regenerative amplifier (90 fs, 2.2 W, 1kHz, Spectra Physics, Spitfire, pumped by 2
diode-pumped 8W, 527nm Nd:YLF Spectra Physics, Evolution X) before entering an optical parametric amplifier
(OPA) (1.15-2.6 µm, 100 fs, 60 µJ at 2100 nm, Light Conversion, TOPAS) which generates the near-infrared idler
pulse for the Stokes beam. A small amount (5%) of the unchirped, transform-limited 800 nm beam from the amplifier
is picked off by a dichroic mirror before the OPA. This pulse passes 4 times off of two gratings (800nm blaze, 830
grooves/mm, Edmund Scientific), stretching the pulse in time due to an induced linear temporal chirp directly
proportional to the grating separation. The transform-limited near-infrared (2100 nm) idler beam from the OPA is
doubled by a BBO crystal to 1050 nm (ωS), temporally overlapped with a fraction of the chirped 800 nm pulse (ωp, ωp')
using a delay stage. The ωS beam reflects from the edge of a silver mirror, while the ωp beam passes closely by the
edge. The use of a silver mirror rather than a dichroic mirror allows for greater tunability of the system, with no
dependence on the reflectivity of the dichroic coating. The two beams converge and are collinear entering the excitation
objective (Olympus, 10x, 0.3 N.A.) of an inverted microscope (Olympus, IX-71), and thus collinearly excite the sample
with ~3 µJ ωp and ~1 µJ ωS. An identical collection objective collimates the CARS signal, which is then directed to a
fiber optic that directs the signal over to a spectrograph/CCD (Roper Scientific). This simplified setup for detecting a
broadband signal from the sample with high spectral sensitivity allows for the detection of a CARS spectrum over ~300
cm-1 full-width half-maximum (FWHM) region of interest in a single pulse, facilitating the detection of relative changes
in peaks within the experimental bandwidth.
2x idler, ωs
Chirped 800 nm, ωp
objective
Spectrometer/
CCD
sample
Grating 1
objective
CARS
Fiber optic
From OPA
Unchirped 800 nm from amplifier
Grating 2
Figure 1. Optical design for chirped CARS. Unchirped, transform-limited 800 nm (ωp) pulse from amplifier passes 4 times between
two gratings, stretching the pulse in time due to an induced linear negative temporal chirp directly proportional to the grating
separation. Grayscale represents the chirp, with bluer (light gray) wavelengths leading the redder (dark gray) wavelengths in time.
The transform-limited near-infrared (2100 nm) idler pulse from the OPA is doubled by a BBO crystal to 1050 nm (ωS) and is
temporally overlapped with the chirped pulse using a delay stage.
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231
2.2 Autocorrelation of pulses
In order to determine the extent of the chirp effected by the separation distance of the diffraction gratings, the pulse
width of the ωp pulse was measured for each grating separation. A homebuilt Michelson autocorrelator was employed,
wherein the beams were split 50-50 by a beam-splitter, focused, and recombined in a 0.3mm BBO doubling crystal.
The resultant sum frequency signal was spatially filtered and collected on a PMT (Hamamatsu, R212). To measure the
autocorrelation, one arm of the interferometer was translated in known steps, manually by a micrometer in 0.1mm steps
for the longer (>1ps) pulses, and automatically by a picomotor (New Focus, 9064) in ~25 nm steps for the short,
unchirped pulses. The largest source of error for the short pulse experiments is that due to the picomotor step-size. The
actual distance traveled by the picomotor screw was measured with a caliper for each measurement, and the step size
was then calibrated for each measurement.
60
pulsewidth (ps)
50
Signal (V)
1.4
40
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
20
30
40
60
80
time (ps)
20
2
R = 0.996
10
0
0
5
10
15
20
25
30
35
gratings separation (cm)
Figure 2. Linear relation between the separation of the gratings and the measured pulse width, directly related to the linear chirp
induced by the configuration of the gratings, R2 = 0.996. Insert: Representative autocorrelation measurement of the 800 nm (ωp)
pulse width. Squares are the raw data and the line is the Gaussian fit to the data, R2 = 0.997. The separation of the gratings was 13.5
cm, yielding a measured pulsewidth of 22.2 ps.
Grating separation
0
7.5 cm
13.5 cm
34.1 cm
Pulse width
0.09 ± 0.01 ps
9.4 ± 0.2 ps
22.4 ± 0.4 ps
53 ± 1 ps
Spectral resolution
300 cm-1
3 cm-1
1 cm-1
0.5 cm-1
Table 1. Autocorrelation measurements of chirped and unchirped 800 nm (ωp) pulse widths, and the resultant spectral resolution
effected by the chirped-pulse CARS.
The pulse widths of the chirped and unchirped ωp pulse, plus that of the unchirped ωS pulse were measured and are
given in Table 1. A plot of the chirped pulse widths to diffraction grating separation is given in Fig. 2. As can be seen,
the pulse width caused by the temporal chirp is proportional to the linear distance between the gratings.4 The negative
chirp induced by the gratings corresponds to blue wavelengths preceding the red wavelengths, stretching the pulse in
time. The linear chirp, and therefore the effective pulsewidth of the pulses on the sample determine the achievable
spectral resolution of this chirped pulse CARS technique.
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A CARS signal is generated only when the pulses are overlapped in time at the sample, as defined by the shorter,
transform-limited 90 fs ωS pulse. The rest of the time that the longer, chirped ωp pulse is on the sample there is no
CARS signal generated. Therefore, an effective temporal slit is introduced into the optical paths since the 90 fs pulse
overlaps with only a small portion of the ωp pulse in time, and due to the temporal chirp, this implies overlap with only
a small portion of the spectrum as well, as shown in Fig. 3. Equation 1 describes the effective spectral resolution of the
beam in the limiting case that the chirped pulse is much longer than the transform-limited pulse:
(∆tS/∆tp) x ∆ωp = ∆ωspec
(1)
where ∆tS is the pulse width of the transform-limited ωS pulse, ∆tp is the pulse width of the chirped ωp pulse, ∆ωp is the
bandwidth of the chirped ωp pulse and ∆ωspec is the resulting effective spectral resolution. For example, a chirped pulse
of 10 ps duration overlaps in time for roughly 100 fs with a 100 fs pulse of 300 cm-1 spectral bandwidth, achieving
.1ps/10ps * 300 cm-1 = 3cm-1 spectral resolution.
10 ps
100 fs
t
Figure 3. A temporal slit yields high spectral resolution, determined by the product of the ratio of the pulse widths and the bandwidth
of the transform-limited pulse. Total multiplex bandwidth is mainly determined by the bandwidth of the fs transform-limited pulse
(300cm-1 for 100 fs pulse).
3. RESULTS
As shown in Fig. 4, the resonant enhancement achieved with high spectral resolution chirped CARS dominates the
CARS spectrum of liquid methanol, with only minimal spectral distortion resulting from interference with the nonresonant electronic CARS background. The line widths of the spectra are equal to the natural (homogeneouslybroadened) width. For the symmetric C-H stretch at 2825cm-1, the observed line width is 25 cm-1. The asymmetric
stretch appears more affected by the non-resonant background, exhibiting a small shoulder at 2910 cm-1, and increasing
the apparent line width to 38 cm-1 FWHM. The spontaneous Raman spectrum of methanol exhibits a broader
asymmetric stretch as well whose line width is roughly 30-40 cm-1.5 These line widths remain the same for all grating
separations measured, with effective spectral resolutions ranging from 3 to 0.5 cm-1, implying natural line widthlimited spectral resolution.
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233
CARS signal (a.u.)
2000
1800
1600
2940 cm
1400
(38 cm FWHM)
2825 cm
-1
-1
-1
(25 cm FWHM)
-1
1200
1000
800
600
400
200
0
3200
3000
2800
2600
-1
Raman shift (cm )
Figure 4. Methanol CARS spectrum of the methyl symmetric and asymmetric C-H stretches at 2825 cm-1 and 2940 cm-1,
respectively. Slight spectral distortion due to interference with non-resonant CARS signal is evident for the latter peak. Spectral
resolution 1 cm-1; total spectral bandwidth 300 cm-1.
As shown in the energy level diagram Fig. 5, the observed multiplex chirped CARS spectrum for liquid methanol is
generated by first exciting the sample to a virtual state by the pump (ωp) pulse that is spectrally narrow, as determined
by rate of the chirp. Next, the spectrally broad Stokes pulse (ωS) populates both the asymmetric and symmetric C-H
stretches within liquid methanol, whose relative populations depend on the intensity of the ωS pulse at the energy
difference between ωp and ωS. The resonance enhanced transitions for both vibrations are highlighted. Once several
vibrational states are excited by the ωp and ωS pulses, the spectrally narrow chirped probe pulse (ωp'), as determined by
the temporal slit, resolves the two excited state transitions, generating the observed chirped CARS spectrum with
natural, homogeneously-broadened line widths.
E
ωp
ωs
ωp′
ωas
vasym = 2920 cm-1
vsym = 2825 cm-1
v = ground state
Figure 5. Multiplex CARS energy level diagram centered on the asymmetric C-H stretch of liquid methanol. The pump (ωp) pulse is
spectrally narrow, as determined by rate of the chirp. The Stokes pulse (ωS) is broad spectrally, with resonance enhanced transitions
highlighted. The spectrally narrow chirped probe pulse (ωp') resolves the two excited state transitions in liquid methanol, generating
the observed chirped CARS spectrum with natural, homogeneously-broadened line widths.
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The best observed spectral resolution achieved was for liquid isooctane, with two peaks clearly resolved and
reproducibly observed, with a splitting of only 5 cm-1, as seen in the Figure 6 inset. Numerous aliphatic C-H vibrations
were observed: the CH3 asymmetric stretch at 2963 cm-1; the CH2 asymmetric stretch at 2930 cm-1; the CH3 symmetric
stretch consisting of two resolvable transitions at 2906 and 2901 cm-1; and the CH3 non-symmetric deformation at 2875
cm-1. The two CH3 symmetric stretch peaks remain resolvable for all gratings separations, and their resonances were
confirmed by adjusting the ωS wavelengths and relative ωp delay.
5
CARS signal (a.u.)
3.5x10
5
3.0x10
2963 cm
-1
2906 cm
-1
2901 cm
-1
5
2.5x10
5
2.0x10
5
2920
1.5x10
2930 cm
5
1.0x10
2910
2900
2890
-1
2875 cm
-1
4
5.0x10
0.0
3100
3000
2900
2800
-1
2700
Raman shift (cm )
Figure 6. CARS spectrum of liquid isooctane. Observed aliphatic C-H transitions include: the CH3 asymmetric stretch at 2963 cm-1;
the CH2 asymmetric stretch at 2930 cm-1; the CH3 symmetric stretch consisting of two resolvable transitions at 2906 and 2901 cm-1;
and the CH3 non-symmetric deformation at 2875 cm-1. Inset more clearly shows the two resolved CH3 symmetric stretch transitions.
Resonance enhancement is confirmed by observing the chirped CARS spectra for different ωS wavelengths. As shown
in Fig 7a, for ωS wavelengths ranging from 1075 to 1025 nm the maximum CARS signal for liquid methanol is centered
at the same wavelength. Since the maximum CARS intensity for the two dominant vibrational transitions for liquid
methanol are defined by ωp + ωasym and ωp + ωsym, where ωasym and ωsym are the vibrational frequencies of the
asymmetric and symmetric C-H stretches, respectively, the center CARS wavelength will not change as the ωS
wavelength is adjusted, as shown in the energy level diagrams in Fig. 7b. As the maximum spectral intensity of the ωS
pulse is shifted across the vibrational energy levels, the relative intensities of the two C-H peaks change due to different
excited populations of the two vibrational energy levels. There is also an increase in the non-resonant contribution to
the overall CARS spectrum as ωp - ωS moves further from the C-H resonances, which causes the well-known distortion
of the resonant CARS peak1 and a subtle shift in the peak position for each C-H stretch. As seen in Table 2, the
resonant CARS signal for the 2825 cm-1 peak shifts slightly to the red as the ωS wavelength shifts more to the blue due
to an increase in the non-resonant CARS signal that grows in to the red of the resonant peaks. This convolution of the
increasingly stronger non-resonant CARS signal at redder wavelengths with the constant wavelength resonant CARS
signal distorts the peak shape of the resonant signal towards the redder wavelengths, causing the observed shift to the
red for the maximum of the resonant CARS peak. The opposite is true for the 2940 cm-1 peak.
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235
CARS signal (a.u.)
40000
35000
1075 nm
1062.5
1050 nm
1037.5 nm
1025 nm
30000
25000
20000
15000
10000
5000
0
a
640
650
660
670
wavelength (nm)
E
b
Figure 7. Effect of changing ωS on observed CARS spectrum. a) Observed CARS spectra for ωS between 1025 and 1075 nm. b)
Energy level diagrams for decreasing wavelength of ωS (from left to right). CARS spectra are centered at the same wavelength;
however, the relative intensities of the two C-H peaks change due to different excited populations as the maximum intensity of the ωS
pulse shifts across the vibrational energy levels. Spectral distortion caused by mixing of the resonant and non-resonant CARS
signals.
2940 cm-1 peak
CARS maximum (nm)
ωS (nm)
1075
655.0
1062.5
655.1
1050
655.2
1037.5
655.2
1025
655.2
2825 cm-1 peak
CARS maximum (nm)
ωS (nm)
1075
659.8
1062.5
659.9
1050
660.0
1037.5
660.0
1025
660.1
Table 2. Observed resonant CARS wavelength for decreasing ωS wavelengths. Subtle shift in CARS maximum due to increased
contribution of non-resonant CARS background.
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When the delay between the ωp and ωS pulses is adjusted, the overall CARS spectrum shifts in wavelength as the pump
wavelength used for generating the CARS signal changes (see Fig. 8a). This happens because the virtual states that the
pump and probe wavelengths excite are different, therefore increasing or decreasing the observed CARS wavelength, as
shown in the energy level diagrams in Fig. 8b. Fig 8a shows the effect that 0.85 ps steps in the delay stage has on the
observed chirped CARS spectrum. Zero delay time is defined as the delay stage position that yields the largest CARS
signal. Given the degeneracy of the ωp and ωp' pulses in the experimental setup, it is not possible to delay the final ωp'
pulse relative to the ωS pulse, as is done in several CARS setups to improve signal and decrease non-resonant electronic
background. The spectra are scaled to more readily observe the changes in the spectra as the delay in the ωp pulse is
adjusted. The chirped CARS signal is much larger when the delay matches the optimal spectral overlap between the ωp
and ωS pulses for resonance enhancement (see Fig. 8a). As the delay is increased, the energy difference between ωp and
ωS steps closer to the energy difference of the 2825 cm-1 resonance, resulting in relative increase of the 2825 cm-1 peak
compared to the 2940 cm-1 peak. A coincidence of the 2940 and 2825 cm-1 peaks occurs at delay times of 3.4 ps. Since
the chirped ωp pulse was 9.4 ps, the ratio of the delay to the pulse width is 0.36. The product of that ratio and the
bandwidth of ωS (300 cm-1) is 109 cm-1, roughly the spacing between the two peaks (115 cm-1). Therefore, the 2940cm1
peak will appear at roughly the 2825 cm-1 peak position for a 3.4 ps delay. This consistency further proves the
mechanism of the observed chirped CARS spectrum.
CARS signal (a.u.)
40000
2940 cm
-1.7 ps delay (x2)
t=0 (m ax intensity)
+1.7 ps delay (x2)
+3.4 ps delay (x10)
-1
35000
30000
2825 cm
-1
25000
20000
15000
10000
5000
0
a
640
645
650
655
660
wavelength (nm )
E
b
Figure 8. Effect of ωp delay on CARS spectrum. a. Scaled spectra of liquid methanol for adjusting the delay between ωp to ωS. For
increased delays, the 2825 cm-1 peak increases in relative intensity compared to the 2940 cm-1 peak. Delays of 3.4 ps results in an
overlap of the 2825 cm-1 peak and 2940 cm-1 peak, as determined by the ratio of delay to ωp bandwidth. b. Energy level diagrams
for adjusting delay.
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237
For the liquid methanol chirped CARS experiments, ωp and ωS were centered at 799 nm and 1040 nm, respectively.
The wavelength of the optimal delay (t=0, no delay) of the ωp pulse was calculated to be 797.4 nm for the 2940 cm-1
peak of methanol. The calculated value of the Stokes pulse was 1041.7 nm. Both wavelengths are near the maximum
intensity spectrally for each pulse, yielding a very large resonance enhancement and subsequently large chirped CARS
signal. The spectral overlap for resonance enhancement is worse for the 2825cm-1 peak. Since the temporal slit defines
the ωp wavelength, the calculated wavelength for ωp was 797.4 nm for the 2825 cm-1 peak at maximum CARS intensity
as well. However, the calculated ωS wavelength was 1029.2 nm, roughly half the maximum spectral intensity for ωS.
Given that the cross-sections for both C-H stretches of liquid methanol are comparable, the difference in ωS spectral
intensity causes the 2825 cm-1 peak to be roughly half the intensity of the 2940 cm-1 peak.
4. DISCUSSION
The use of chirped pulses in nonlinear optical spectroscopy is not in itself new. Cheng et al. described the effect that a
chirped pulse has on the observed CARS spectra in their multiplex CARS setup, noting that the overall spectral
bandwidth of their CARS signal decreased for increased chirp rate of the Stokes pulse, correctly attributing this to the
decreased overlap of the spectral components of the chirped pulse.2 However, more interesting effects occur when the
pump pulses are chirped. Theoretical papers by Naumov and Zheltikov propose the use of two transform-limited
picosecond pulses with one chirped picosecond pulse to achieve very high spectral resolution (~terahertz) CARS
signals, suggesting that the same idea could be used with chirped femtosecond lasers as well.6,7 Ambiguity in the
assignment of which pulses are chirped makes it difficult to compare directly with our experimental setup, but the
essential theory of the interactions for determining the spectral resolution of using such a pumping scheme are
presented.
An alternative to using chirp to obtain high spectral resolution is to delay the chirped probe pulse after the initial pump
and Stokes pulses to spatially disperse the temporal evolution of the CARS signal onto a CCD. Lang et al. have
recently used chirped pulses to map transient CARS spectrum of H2 in a single shot to measure the temperature of
flames.8 However, these proposed and observed experiments require a third, separate probe pulse to map out the
evolution in time of the H2 spectrum. As mentioned above, in our experiments the pump and probe pulse are degenerate
which does not allow for adjusting the delay between ωp and ωp'. Therefore, instead of using the chirp to map out the
time evolution of the spectrum, we use the chirp to create a temporal slit in order to obtain a high spectral resolution
spectrum of the sample.
Similarly, chirping the pulses could be considered as a simple linear pulse-shaping, similar to the work of Silberberg et
al.9 One of the advantages of these techniques is that of extracting information from the lower frequency Raman
transitions, difficult to experimentally measure. A simplification of that pulse-shaping technique was recently reported
by Gershgoren et al., using chirped pulses to achieve high spectral resolution.10 Through the use of induced beats in the
sample, taking the Fourier transform of the transient response at different delay times yields the spectrum, in this case, a
spectrum of the Kerr effect in CCl4. While similar in concept, the stepping of the delay between the ωp' and ωS pulses
and the complicated analysis of the resultant signal makes this design more complicated than that which we describe in
this paper.
What is novel about our technique is that it is unnecessary to reduce the bandwidth of the laser, instead using only the
chirp of the probe to achieve high spectral resolution, noting that even though the chirped pulse is several picoseconds
in duration, it still has the full spectral bandwidth of a femtosecond pulse, namely ~300 cm-1. Typical methods for
decreasing the total bandwidth of a pulse introduce a slit in the collimated beam after the second grating so that when
the pulse is recompressed, there are fewer total frequencies to recombine; therefore, the pulsewidth is correspondingly
increased and the bandwidth is decreased. However, we observed high spectral resolution without the introduction of a
slit. In our setup, by dispersing the CARS signal onto a CCD, we can observe the entire CARS spectrum in a single
shot, without the need for adjusting the delay of the pulses. Through the use of diffraction gratings rather than
dispersive media like a fiber or glass rod to induce the linear chirp, we can readily adjust the spectral resolution of our
technique to match the linewidth of the transition of interest, increasing the resonant CARS signal relative to the nonresonant signal through more efficient use of the laser power.
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One liability of this technique is that the chirped light not temporally overlapped with the femtosecond pulse does not
contribute to CARS signal, yet it still can lead to sample heating and possible photo-damage. However, the chirped
pulse has a lower peak power on the sample than does the femtosecond pulses, so this technique should cause less
photo-damage than if all of the pulses were femtosecond in duration.
5. CONCLUSION
We have shown that chirped-CARS allows for very simple control of the spectral resolution of the instrument,
achieving high spectral resolution in a novel way. Like other multiplex CARS techniques, chirped-CARS can be used
to record an entire spectrum at once, allowing for direct comparisons between peak intensities within the broadband
region of interest. This is a very important spectroscopic tool because the ability to monitor relative peak intensities and
shifts are critical to the analysis of the system. In addition, by matching the spectral resolution to the line widths of the
vibrations in the sample, CARS signal is more efficiently generated, increasing the ratio of resonant to non-resonant
background signals. The relative simplicity of this experimental setup allows for high spectral resolution CARS spectra
to be obtained with femtosecond laser systems.
ACKNOWLEDGEMENTS
The authors would like to thank the National Science Foundation and Lawrence Livermore National Lab for funding.
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