Low-Frequency Coherent Raman Spectroscopy Using Spectral-Focusing of Chirped Laser Pulses

Utilization of chirped laser pulses to measure stimulated Raman scattering of organic liquids in the terahertz regime

“…A Ti:Sapphire regenerative amplifier provided 200 fs pulses with a repetition rate of 1 kHz (λ ∼ 800 nm, bandwidth ∼ 34 nm). Using anÖffner stretcher [21][22][23] , a linear chirp was imprinted to the fspulses expanding the pulse width to about 44 ps. Then, the pulse train passed an interferometer, where the beams were split by a 50/50 beam splitter (fused silica, thickness ∼ 2.3 mm) into a pump and a probe beam.…”

“…The spectrum shows an anti-symmetric line shape which can be explained by considering stimulated Raman scattering at the sample position: According to the linearity of the chirp, a temporal constant energy difference between the probe and the pump beam can be adjusted by changing the time delay between them with the delay stage of the interferometer (an illustration of the time-frequency correlation for both laser beams is shown in Figure 1). This energy difference causes an excitation of the sample by a stimulated Raman scattering process with respect to the corresponding third-order nonlinear susceptibility 23 . Thereby, during the overlap of the pulses, the intensity of the beam with higher energy photons is decreased (IRS) while the intensity of the beam with the lower energy photons is increased (SRGS) 24 .…”

“…In order to assign a frequency axis to the measured SRS signal, we can relate the difference frequency f d between the pump and the probe beam to the time delay τ using f d = b × τ 14,16,23 , where b denotes the chirp parameter. In doing so, the knowledge of the chirp parameter b is crucial for the evaluation of the measured spectra.…”

“…However, such an approach can be challenging when the laser beams exhibit a diffraction pattern and detection of small signals is required. Another approach using a time gating method by implementing an additional compressor was introduced before 23 . While these examples show that some effort is necessary to separate the pump and probe signals, it is possible to avoid the implementation of two laser sources or additional laser systems like an optical parametric oscillator.…”

“…For example, by using the frequency resolved optical gating (FROG) technique 28 it might be possible to directly monitor the time-frequency correlation of the ps-pulses at the sample position and eliminate the influence of prior optical systems (such as lenses or gratings) on the estimation of the chirp parameter. Due to the sharper spectral bands of crystals, one can expect to further increase the accuracy of the chirp parameter estimation by evaluating their spectral signatures 23 .…”