Biomedical Engineering Reference
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A: excitation with
transform-limited
fs-pulses
B: excitation with
transform-limited
ps-pulses
C: 'spectral focusing' with
chirped fs-pulses
p
r
r
S
time,
r
r
Fig. 6.2.
Illustration of the frequency-time dependences of pump and Stokes pulses
in three different CRS excitation pulse schemes and their corresponding spectral
resolution of Raman shifts.
A
Using a pair of transform-limited femtosecond pulses
of broad spectral and narrow temporal widths results in a broad bandwidth of
Raman shifts that exceeds the line width of a single Raman resonance.
B
Using
transform-limited picosecond pulses of broad temporal and narrow spectral width
readily provides high spectral resolution matching the Raman resonance line width
to be probed. Selection of a Raman resonance shifted by ΔΩ
r
is achieved by tuning
thefrequencyofoneofthelaserbeamsbythesameamount.
C
'Spectral focusing'
of a pair of identically linear chirped pump and Stokes femtosecond pulses results in
a narrow
instantaneous
frequency difference in the CRS process, thus also providing
narrow-bandwidth CRS excitation. Selection of a Raman resonance shifted by ΔΩ
r
is achieved by adjusting the time delay Δ
τ
between the pulses. Shifted pulses in (
B)
and (
C)
are depicted hatched
femtosecond pulse and the consequential poor spectral CRS selectivity are
illustrated. To achieve not only higher spectral resolution that allows the
selective excitation of a particular vibrational resonance but also higher sen-
sitivity in CRS detection, it is therefore desirable to use narrow-bandwidth
excitation schemes.
As shown in Fig. 6.2B, this can be directly achieved by using transform-
limited pump and Stokes pulses, of which the spectral bandwidth matches
the Raman resonance line width to be probed [37]. The temporal width of
the pulses is typically
2
.
9cm
−
1
.
A frequency-resolved CRS spectrum is obtained by tuning the wavelength of
∼
∼
5 ps, corresponding to a spectral width of
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