We demonstrate two different coherent anti-Stokes Raman scattering (CARS) microscopy and microspectroscopy methods based on the spectral focusing mechanism. The first method uses strongly chirped broadband pulses from a single Ti:sapphire laser and generates CARS signals at the fingerprint region. Fast modulation of the time delay between the pump and Stokes laser pulses coupled with lock-in signal detection significantly reduces the nonresonant background and produces Raman-like CARS signals with a spectral resolution of 20 cm−1. The second method generates CARS signals in the CH (carbon-hydrogen) stretching region with IR supercontinuum pulses from a photonic crystal fiber. The spectral resolution of 30 cm−1 is achieved. Maximum entropy method is used to retrieve a Raman-equivalent CARS spectrum from lipid membranes. Chemical imaging and microspectroscopy are demonstrated with various samples.
We demonstrate a new CARS microscopy method based on fast switching of effective vibrational excitation frequency
from chirped femtosecond laser pulses. Broadband pump and Stokes pulses excite a single vibrational mode with a high
spectral resolution when the two pulses are identically chirped and their pulse durations are approaching the dephasing
time of the excited vibrational state. This "spectral focusing" mechanism is applied to CARS microscopy with a single
broadband Ti:Sapphire laser. The vibrational excitation frequency is controlled simply by the time delay between the
pump and Stokes pulses and fast switching of the excitation frequency (~100 kHz) is achieved with a Pockels cell and
polarization optics. Lock-in detection of the difference between the two CARS signals at nearby vibrational frequencies
not only eliminates the non-resonant background but also generates a spectral line shape similar to the spontaneous
Raman scattering. We demonstrate both micro-spectroscopy and vibrational imaging with various samples.
A new multiplex CARS (coherent anti-Stokes Raman scattering) microscopy technique with a single ultrafast laser pulse
is demonstrated. All the pump, Stokes, probe pulses are selected inside a single broadband cavity dumping Ti:Sapphire
oscillator laser pulse. The measured CARS signal is a coherent sum of the resonant and non-resonant signals, leading to
a complicated vibrational line shape due to the spectral interference. The resonant and non-resonant CARS signals,
however, have different symmetries in the time domain due to the causality principle of the vibrationally resonant
excitation. A new Fourier Transform Spectral interferometry (FTSI) is developed to extract the full complex quantity of
the vibrationally resonant signal against the non-resonant one utilizing the different time symmetry. This method can
generate Raman-like vibrational spectrum in a single experimental measurement, which can be readily applied to a
vibrational hyperspectral imaging. Current sensitivity, available CARS window and application to hyperspectral
vibrational microscopy are discussed.
The non-resonant background signal has been the major obstacle in coherent anti-Stokes Raman scattering (CARS)
spectroscopy and microscopy. This unwanted background is generated by the electronic response of the sample. It not
only obscures the desired signal but also results in spectral interference with the desired vibrationally resonant CARS
signal, making it difficult to assign vibrational peaks using characteristic spontaneous Raman spectra. We show that the
non-resonant background can be used as a local oscillator for spectral interferometric CARS spectroscopy. Two different
techniques are discussed to extract the vibrationally resonant multiplex CARS spectrum and discriminate it against the
much larger non-resonant background. The pump, Stokes and probe pulses are all selected inside a single broadband
ultrafast pulse (bandwidth ~1800 cm-1) by a phase- and/or polarization-controlled pulse-shaping technique. The first
technique generates two spectral interference CARS signals simultaneously, and the normalized difference of these two
signals provides an amplified background-free broadband resonant CARS spectrum over 400-1500 cm-1. The second
method generates a single spectral interference CARS signal by a phase-only pulse shaping. A Fourier transform spectral
interferometry (FTSI) method is used to retrieve the Raman-equivalent CARS spectrum from the measured spectral
signal. Both methods enhance the resonant CARS signal by utilizing the non-resonant background as a local oscillator
for homodyne mixing.
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