Stokes beam

FIGURE 9.7 Schematic of an in-line interferometer. The anti-Stokes local oscillator field is collinearly overlapped with the pnmp and Stokes beams on a dichroic mirror (DM). All fields are focused by a microscope objective (MO) into the sample (S), and the total signal at the anti-Stokes frequency is detected throngh a spectral bandpass filter (F) at the photodetector. 

Such problems can be circumvented if the local oscillator co-travels with the pump and Stokes excitation beams into the microscope, as depicted in Figure 9.7. In this configuration, the local oscillator is generated either in-line (Andresen et al. 2006 Lee et al. 2007 Yacoby et al. 1980) or in a compact interferometer (Potma et al. 2006), which minimizes phase fluctuations. An active stabilization scheme is relatively easily incorporated into the interferometer (Krishnamachari et al. 2006). Because the local oscillator propagates through the same optics as the pump and Stokes beams, no temporal phase differences are accumulated beyond the interferometer. 

The interference process in this collinear approach is, however, different from the interference realized by mixing the local oscillator and the CARS field on a beam splitter. Interference takes place in the sample, which, in the presence of multiple frequencies, mediates the transfer of energy between the beams that participate in the nonlinear process. The local oscillator mixes with the anti-Stokes polarization in the focal volume, and is thus coherently coupled with the pump and Stokes beams in the sample through the third-order polarization of the material. In other words, the material s polarization, and its ability to radiate, is directly controlled in this collinear interferometric scheme. Under these conditions, energy from the local oscillator may flow to the pump and Stokes fields, and vice versa. For instance, when the local oscillator field is rout of phase with the pump/Stokes-induced anti-Stokes polarization in the focal interaction volume, complete depletion of the local oscillator may occur. The energy of the local oscillator field is not redistributed in terms... 

FIGURE 9.11 Calculated angle-resolved CARS radiation intensity in the case of a Stokes beam with a HGOO phase pattern (a) and in the case of a Stokes beam with a HGOl phase profile (b). Note that no CARS intensity along the optical axis is seen. 

Figure 23 VMPof HOD (v"qjj = 1) (left panel) and (v"oi3 = 1) (right panel) at 193 nm performed by scanning the Stokes beam via the rovibrational levels (a) and (d) respective CARS signals (b) and (e)LIF of the/f2(4)lineof A(v = 0)<-3f(v" = 0) transition of the OD photofragments, and (c) and (f) of the OH photofragments. The intensity scales in (b) and (c) are similar. The numbers above the peaks mark the g-branch rotational transitions monitored by CARS. Reproduced with permission from Ref. [80]. Copyright (1991) AIP Publishing LLC.

The CARS system used to measure temperature and species concentrations in the combustor zone is composed of a single-mode ruby-laser oscillator-amplifier with a repetition rate of 1 Hz and a ruby-pumped, near-infrared broad-band dye laser. The two laser beams are combined collinearly and focused first into a cell containing a nonresonant reference gas and then into the sample volume (approximately 30-u diam. x 2 cm) in the combustion region. The anti-Stokes beams produced in the sample and reference volumes are directed to spatially separated foci on the entrance slit of a spectrometer and detected by separate photomultiplier tubes. An optional means of detection is provided for the sample signal in the form of an optical multichannel analyzer (OMA), which makes it possible to obtain single-pulse CARS spectra. 

The principle of photoacoustic Raman spectroscopy (107) is similar to that of CARS. When two laser beams, vp (pump beam) and vs (Stokes beam), impinge on a gaseous sample contained in a cell (Fig. 3-43), these two beams interact when the resonance condition, vp — vs = vM, is met, where vM is a frequency of a Raman-active mode. This results in the amplification of the Stokes beam and the attenuation of the pump beam. Each Stokes photon thus... 

Figure 3-43 Schematic representation of the photoacoustic Raman scattering (PARS) process, (a) A simple energy level diagram illustrating the Raman interaction that occurs in the PARS process, (b) Basic elements of the PARS experimental arrangement. The pump beam is attenuated and the Stokes beam is amplified by the stimulated Raman process that takes place where the beams overlap in the gas sample cell. For each Stokes photon created by the Raman process, one molecule is transferred from the lower state to the upper state of the transition. Collisional relaxation of these excited molecules produces a pressure change that is detected by a microphone. (Reproduced with permission from Ref. 107.)...

Figure 3-44 Photoacoustic rotational Raman spectrum of CO2 at a pressure of 80 kPa (600 torr). The rotational line spacing is about 3.1 cm 1. Laser powers of the pump and Stokes beams were 3.3 MW and 120 kW, respectively. (Reproduced with permission from Ref. 107.)...

Other methods include tip-enhanced Raman using 20- to 30-nm diameter Au or Ag tips, polarized Raman, stimulated Raman, micro-Raman spectroscopy, and coherent anti-Stokes Raman spectroscopy (CARS), where two laser beams are combined to generate an anti-Stokes beam, and so on. 

If the intensity of the Raman Stokes beam at I si is high enough, this beam itself can generate a new beam at the second Stokes frequency v 2 = si = in a man-... 

In order to observe backward scattered CARS signals from the surface of an opaque sample a special CARS set-up has been made, which is essentially the standard configuration as displayed in Fig. 3.6-8 with specific modifications. The latter is shown in detail in Fig. 3.6-9. Here, the sample area from Fig. 3.6-8 is reproduced together with the modification. In this case, the Stokes beam runs parallel above the pump beam. Both... 

Time-resolved coherent anti-Stokes Raman scattering has produced much important information on the dynamics of molecular vibrations and rotations (Laubereau and Kaiser, 1978 Duppen et al., 1983 Akhmanov et al., 1985 Angeloni et al., 1988). Particularly vibrational dephasing time constants in liquids and solids could be determined with this technique (Laubereau et al., 1978 Kohles and Laubereau, 1987 Weber and Rice, 1988 Bron et al., 1989). In these experiments pump and tunable Stokes beams serve to produce... 

Another practical CARS technique places the Stokes beam inside the pump beam when using a Nd YAG laser with Donut profile as pump laser source (Marko and Rimai, 1979). This method which has been termed USED (unstable resonator spatially enhanced detection) CARS (Eckbreth et al,. 1984 Eckbreth and Anderson, 1985) also increases the spatial resolution. 

Figure 1. A - 87Rb levels used in the experiments (Di line Zeeman sublevels are not shown in this simplified scheme the ground-state hyperfine splitting is 6.835 GHz). The write (retrieve) laser and Stokes (anti-Stokes) beam are illustrated in red (blue). B - After the optical pumping pulse (provided by the retrieve laser), the 1.6/us-long write pulse is followed by the retrieve pulse after a controllable delay Td. C - Schematic of the experimental setup. SI, S2 (AS1, AS2) denote the avalanche photodiodes for the Stokes (anti-Stokes) light.

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