Coherent Raman Technique Research Articles

Coherent phonon relaxation in strontium titanate

Ultrafast decays of the intrinsic phonon modes within 260–790 cm−1 range of frequencies, which are symmetry forbidden in the first-order Raman scattering in bulk SrTiO3 crystal, have been traced using time-resolved coherent Raman technique. The effective phonon decay times are within 0.86–1.38 ps and yield corresponding linewidths of 7.7–12.3 cm−1 for optical lattice vibrations. Symmetry breakage mechanisms related to free charged carriers resulting from vacancies do not appear to play a major role.

Ultrafast Phonon Decay in Complex Oxides

The decay of multiple Raman active vibrations has been directly traced, in time, in technologically important wide bandgap semiconduction oxides such as BaSnO3 (BSO), STiO3 (STO), and KTiOPO4 (KTP) crystal, which have important applications in laser frequency conversion. A time-domain coherent Raman technique, with excellent time (~120 fs) and spectral resolutions, has been applied to measure the ultrafast decay rates of optical phonons with 350–1500 cm−1 frequencies. Phonon decay mechanisms via phonon energy loss due to second- and third-order parametric processes have been discussed. The correspondingly high equivalent spectral resolution allowed for the determination of the phonon line bandwidths to be within 7.2–8.3 cm−1 (BSO), 8.5–9.7 cm−1 (STO), and 6.2–18.6 cm−1 (KTP).

Time-Domain Observation of Surface-Enhanced Coherent Raman Scattering with 105-106 Enhancement.

Combining surface-enhanced Raman scattering (SERS) with the coherent nonlinear Raman technique is a promising route for achieving higher sensitivity and time-resolved SERS measurements, yet such attempts have just been started. Here, we report time-domain Raman measurements of trans-1,2-bis(4-pyridyl)ethylene (BPE) adsorbed on gold nanoparticle assemblies (GNAs), which were carried out with impulsive stimulated Raman spectroscopy using sub-8 fs pulses. We observe coherent nuclear wavepacket motion of BPE on GNAs with drastic enhancement through the surface plasmon resonance, which provides information on the Raman-active vibrations in the time domain. Through Fourier transform of the measured time-domain Raman data, we obtained SERS spectra of BPE on GNAs with enhancement factors as high as 105-106. The present study not only demonstrates applicability of time-domain nonlinear Raman techniques in SERS, i.e., surface-enhanced impulsive stimulated Raman spectroscopy (SE-ISRS), but also provides a technical basis for femtosecond time-resolved SE-ISRS experiments to track ultrafast dynamics of the adsorbates.

Real-time monitoring of blood using coherent anti-Stokes Raman spectroscopy

We demonstrate a real-time method of measuring the vibrational Raman spectrum of whole blood. Using a novel coherent Raman technique, we record the vibrational spectrum of the red blood cells from picoliters in blood in milliseconds. This method will allow real-time in vivo blood monitoring.

Excitation of laser-induced thermal gratings in the gas phase via Raman-active transitions

A systematic account of the properties of a recently developed coherent Raman technique is given. This is a time-resolved technique, based on the generation of thermal gratings due to a population change induced by stimulated Raman pumping of molecular levels. Excitation of gratings is achieved by spatially and temporally overlapping intensity interference patterns independently generated by two lasers with appropriate wavelengths. By employing this technique, it is possible to investigate gas transport properties and molecular energy relaxation processes, as has been shown for carbon dioxide. The transitions of the Q-branches of the ν 1/2 ν 2 Fermi dyad were used for Raman pumping in neat gas and in mixtures with argon. A possibility to perform spectroscopy of Raman-active transitions has also been demonstrated.

Laser-induced gratings in the gas phase excited by Raman-active transitions

We report on a new time-resolved coherent Raman technique that is based on the generation of thermal gratings following a population change among molecular levels that is induced by stimulated Raman pumping. This is achieved by use of spatially and temporally overlapping intensity interference patterns generated independently by two lasers. When this technique is used in carbon dioxide, making use of transitions that belong to the Q branches of the v(1)/2v Fermi dyad, it is possible to investigate molecular energy transfer processes. A further unique feature of this method is spectral resolution that is better than that achieved by coherent anti-Stokes Raman scattering.

Time-resolved three-color coherent Raman scattering applied to polycrystalline and opaque solids

A new time-resolved coherent Raman technique (three-color picosecond coherent anti-Stokes Raman scattering), which uses a backscattering geometry and three colors obtained from three different lasers, is demonstrated. The advantage of this technique over conventional two-color picosecond coherent anti-Stokes Raman scattering is that it is insensitive to sample morphology. It is used to study time-dependent vibrational dephasing from opaque, polycrystalline solids at low temperature. A quantitative treatment of the limiting apparatus time response, where each pulse has a different duration and where there is arrival-time jitter between the pulses, is derived. New measurements of vibrational dynamics in low-temperature carbon disulfide indicate that polycrystalline samples are more homogeneous, on a microscopic level, than the large single crystals used in previous work. Raman line broadening in a sample of naphthalene in which strain is induced intentionally by grinding and compression is also investigated.

Ultrasensitive coherent Raman technique with picosecond lasers

The use of picosecond, Q-switched lasers and advanced polarization schemes has led to the development of a coherent Raman technique with the sensitivity of coherent anti-Stokes Raman spectroscopy experiments but without the troublesome phase-matching requirements. Experiments in dilute solutions of benzene indicate a limit of sensitivity for the current apparatus of 2.5×10−4 M in two minutes of signal averaging over 150 cm−1. Possible applications to the in situ study of passive films and thin films on transparent media are discussed.

Prolonged-excitation coherent Raman spectroscopy with spectral resolution beyond the transition linewidth using two tunable picosecond dye lasers

A time-resolved coherent anti-Stokes Raman technique is demonstrated that yields a spectral resolution beyond the linewidth obtained in spontaneous Raman spectroscopy. Two picosecond dye lasers, independently tunable with low timing jitter, are used. The coherent material excitation is generated by long pump pulses and monitored by short delayed probing pulses. Transiently narrowed Raman spectra are recorded by scanning the lasers over the resonances of interest and simultaneously monitoring the coherent scattering signal as a function of the excitation frequency. The technique is applied to molecular vibrations in liquids. The differences between the technique presented and standard coherent anti-Stokes Raman spectroscopy are demonstrated.

Protein-ligand interactions in lumazine protein and in Desulfovibrio flavodoxins from resonance coherent anti-Stokes Raman spectra.

The resonance coherent anti-Stokes Raman technique was used to obtain vibrational spectra of flavin in flavodoxins from Desulfovibrio gigas and Desulfovibrio vulgaris and of the simpler 6,7-dimethyl-8-ribityllumazine chromophore in the blue fluorescence lumazine protein from the bioluminescent bacterium Photobacterium phosphoreum. In the region examined, 1100-1700 cm-1, the Raman spectrum of the lumazine is less crowded than that of the flavin and this facilitates assignment of observed frequencies to particular vibrational modes. Certain modes are not affected by chromophore binding to the protein, but others are changed in frequency or intensity in a way that can be rationalized by expected interactions of the chromophore with the amino acid residues of the binding site. For example, a tentative assignment of change in hydrogen bonding at N(5) is suggested as the cause of the frequency shift for the chromophore in both flavodoxins (free-bound, 1582-1572 cm-1) and for C(4)=O in glucose oxidase (1359-1364 cm-1) and lumazine protein (1359-1362 cm-1). Shifts of the C(2)-N(3) mode in D2O may arise from hydrogen-bonding changes at C(2)=O in lumazine protein (1284-1291 cm-1), flavodoxin (1300-1280 cm-1), and glucose oxidase (1297-1287 cm-1). Bonding at N(3)-H may be the origin of changes in the frequency or intensity of the amide III mode in riboflavin binding protein and glucose oxidase. A stacking interaction is suggested for the change in a pyrimidine ring mode in FAD (1508 cm-1) since this mode is found at 1504 cm-1 in 30% Me2SO/H2O, where the adenine and pyrimidine are unstacked. The results clearly indicate different interactions in the binding sites of those proteins studied to date.

Sensitivity limitations for CW stimulated Raman spectroscopy

It is demonstrated that cw stimulated Raman spectroscopy using low-power pump and probe sources is a viable coherent Raman technique for obtaining Raman spectra. A multipass Raman cell configuration is shown to be effective in providing enhancement of the output gain signal and the study of representative Raman modes in mixtures of several common solvents gives an experimental basis for assessing the ultimate sensitivity of the technique both for studies in dense media and high resolution applications in gaseous media.