We introduce a hybrid technique that combines the robustness of frequency-resolved coherent anti-Stokes Raman scattering (CARS) with the advantages of time-resolved CARS spectroscopy. Instantaneous coherent broadband excitation of several characteristic molecular vibrations and the subsequent probing of these vibrations by an optimally shaped time-delayed narrowband laser pulse help to suppress the nonresonant background and to retrieve the species-specific signal. We used this technique for coherent Raman spectroscopy of sodium dipicolinate powder, which is similar to calcium dipicolinate (a marker molecule for bacterial endospores, such as Bacillus subtilis and Bacillus anthracis), and we demonstrated a rapid and highly specific detection scheme that works even in the presence of multiple scattering.

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We present a comparative analysis of spontaneous and coherent Raman scattering on pyridine. The instantaneous excitation of the molecular coherence is done by a pair of ultrashort preparation pulses. Then, a long narrowband probe pulse is scattered off the molecular vibrations. The described hybrid technique allows for the single-shot acquisition of a background-free coherent Raman spectrum within the excitation band and its straightforward comparison with the spontaneous Raman measurements, performed in the same setup. We report a 10(5)-fold increase in the efficiency of the Raman scattering process due to the broadband pump-Stokes preparation. The coherence magnitude (approximately 0.5x10(-3)) is inferred experimentally, without a priori knowledge about the molecular structure.

A double-clad Yb-doped all-normal-dispersion fiber laser with a narrow intra-cavity spectral filter is demonstrated to produce 22 nJ pulses at 42.5 MHz repetition rate. These pulses are characterized and compressed via mulitphoton intrapulse interference phase scan to as short as 42 fs and 10 nJ/pulse. Adaptive compression underlies the achievement of 250-kW peak power, which enables efficient second and third harmonic generation with spectra spanning 30 nm and 20 nm, respectively.

Coherent anti-Stokes Raman scattering (CARS) spectroscopy of gas-phase CO 2 is demonstrated using a single femtosecond (fs) laser beam. A shaped ultrashort laser pulse with a transform-limited temporal width of ∼7 fs and spectral bandwidth of ∼225 nm (∼3500 cm −1 ) is employed for simultaneous excitation of the CO 2 Fermi dyads at ∼1285 and ∼1388 cm −1 . CARS signal intensities for the two Raman transitions and their ratio as a function of pressure are presented. The signal-to-noise ratio of the single beam-generated CO 2 CARS signal is sufficient to perform concentration measurements at a rate of 1 kHz. The implications of these experiments for measuring CO 2 concentrations and rapid pressure fluctuations in hypersonic and detonation-based chemically reacting flows are also discussed.

A simple technique for the synthesis of optical pulse sequences is described, where the input laser spectrum is viewed as a superposition of independent but interlaced combs assigned to different sub-pulses. The devised concept enables intuitive programming of complex multi-pulse waveforms via one-dimensional phase-only shaping. Using this approach, we perform self-referenced cross-correlation measurements of various optical waveforms and demonstrate the generation and coding of shaped pulse sequences.

Single-beam coherent anti-Stokes Raman scattering spectroscopy of using a shaped 7 fs laser pulse

An adaptive pulse shaper controlled by multiphoton intrapulse interference phase scanning (MIIPS) was used, together with a prism-pair, to measure and cancel high-order phase distortions introduced by a high-numerical-aperture objective and other dispersive elements of a two-photon laser-scanning microscope. The delivery of broad-bandwidth (approximately 100 nm), sub-12-fs pulses was confirmed by interferometric autocorrelation measurements at the focal plane. A comparison of two-photon imaging with transform-limited and second-order-dispersion compensated laser pulses of the same energy showed a 6-to-11-fold improvement in the two-photon excitation fluorescence signal when applied to cells and tissue, and up to a 19-fold improvement in the second harmonic generation signal from a rat tendon specimen.