Stokes lasers

When a pump and a Stokes laser beam coincide on the sample and their difference frequency matches a particular molecular vibrational frequency, then SRS appears in the form of a gain of the Stokes pulse intensity and a loss of the pump pulse intensity, as first observed by Woodbury and Ng in 1962 [170] and by Jones and Stoicheff in 1964 [171], respectively (see Fig. 6.1). SRS has long been recognized as a highly sensitive spectroscopic tool for chemical analyses in the condensed and gas phases [172, 173, 29, 174]. For example, a shot-noise limited SRS spectrum of a single molecular monolayer was demonstrated by Heritage and Allara in 1980 [175]. In this section, we discuss the fundamental properties and applications of SRS microscopy, as was first successfully demonstrated by Nandakumar et al. [20] and subsequently reported by several research teams [21, 12, 13, 22]. 

Coherent anti-Stokes Raman scattering (CARS) microscopy is an emerging technology. By tuning a pump laser and a Stokes laser to a Raman-active molecular vibration, molecular selectivity and faster measurement speed can be obtained. This approach has been used to track the phase segregation, crystallisation and dissolution of paclitaxel from biocompatible excipients and films providing kinetic data not achievable through standard Raman microscopy methods [56]. 

STIRAP Transfer of population by means of Stimulated Raman Adiabatic Passage, using a pump and Stokes laser. Population in a three-level system is completely transferred without populating the intermediate state if the Stokes laser precedes the pump laser in a counterintuitive order. 

Population transfer in a three-level system can be achieved by using one laser (known as the pump laser, which may be either continuous wave or pulsed) to connect the ground and intermediate levels, and a second laser (the Stokes laser ) to connect the intermediate and final levels. This method, known as stimulated Raman adiabatic passage or STIRAP, is illustrated in Fig. 22. In this example, the three levels have a A-type configuration, where... 

FIGURE 23 Illustration of the STIRAP technique used for coherent population transfer. Shown are the time evolution of (a) the Rabi frequencies of the pump and Stokes lasers, (b) the mixing angle, (c) the dressed-state eigenvalues, and (d) the populations of the initial and final levels. [Reproduced with permission from Bergmann, K., Theuer, H., and Shore, B. W. (1998) Rev. Mod. Phys. 70, 1003.]... 

FIGURE 25 Efficiency of the transfer of population in S02 by STIRAP as a function of time delay between the pump and Stokes laser pulses. [Reproduced with permission from Halfmann, T., and Bergmann, K. (1996). J. Chem. Phys. 104, 7068. Copyright American Institute of Physics.]... 

Figure 7 Schematic of the laser system used in the Raman FID and echo experiments. PC = Pulse compressor AOM = acousto-optic modulator PD = photodiode FB = feedback electronics PBS = polarizing beamsplitter 3PBF = 3-plate birefringent filter SDL/LDL = Stokes/Laser dye laser P = pellicle AC = autocorrelator OC = output coupler LBO/KDP = doubling crystals. Final pulses have widths of 0.5-1 ps and energies of 0.3-1 mJ (From Ref. 6.)... 

The CARS signal is detected by a Model RCA C31024A fast photomultiplier (PM). The intensities of pump and Stokes laser are measured by photodiodes D1 and D2, respectively. The output signals of PM, D1 and D2 serve as input signals for the computer system. By means of the D1 output the pump laser triggers the complete electronic system for each laser shot. The computer controls the stepping motor of the double monochromator, the wavelength of the Stokes laser, positions of mirrors 1-5 and the shutter. 

As mentioned previously, the incident pump and Stokes laser beams must be aligned in a precise manner so that the CARS generation process is properly phased. Since gases are virtually dispersionless, i. e. the refractive index is nearly a constant over a wide frequency range, the photon energy conservation condition ujas = in-... 

In particular we consider A-systems, depicted in Fig. 18, where the lasers 1 and 2 are, respectively, called pump and Stokes lasers. We use here resonant... 

The usual RWA consists in neglecting the 0-dependent operator V. The first term of V) (319) contains the counterrotating terms of the pump laser on the 1-2 transition and of the Stokes laser on the 2-3 transition. The next two terms correspond to the interactions of the pump laser on the 2-3 transition and of the Stokes laser on the 1-2 transition. Following the hypothesis (316), we neglect the first two terms and keep the last term, which becomes large (see Ref. 38 for details) when maxJ Oi(t), fi2(t) ] approaches or overcomes 8. The (approximate) effective one-mode Floquet Hamiltonian is thus... 

Usually a Gaussian distribution may be assumed for the spectral profiles of the pump and the Stokes laser. In actual practice the convolution over the dye profile is eliminated by dividing the experimental CARS spectrum with a non-resonant reference spectrum collected with a suitable reference gas as the medium. 

A second laser is required to produce fhe Stokes photons for the CARS interaction. This can be achieved with a separate Stokes laser with its own pulse control system or, as is shown in Figure 13.8, some of the laser energy from the pump laser can be used to optically pump a laser suited to the CARS system. The symchro-nization of the timing between the pump and Stokes laser systems must be accurate to within 1 ms to ensure that the two laser beams arrive at the same place at the same time. The advantage of fhe latter laser system is that only one timing circuit is needed to keep the pump... 

The wavelength of fhe Stokes laser depends on the species to be probed with the CARS system. For nitrogen thermometry, a rhodamine dye laser is used that produces laser energy at about 607 nm and the CARS or anti-Stokes wavelength is 473 nm. Table 13.1 lists the Stokes and anti-Stokes wavelengths for species of infer-esf in air breafhing combustion assuming a 532 nm pump laser. 

Noise contribution from the many modes in the Stokes laser can be reduced by the use of a modeless dye laser invented by Ewart [67]. Fortunately nitrogen CARS spectra are less affected by this due to the large number of spectral lines that can be probed together [68]. 

Hz however, higher repetition rafe lasers radiaf-ing in the same wavelength region do exist. The problem with using these high repetition rate lasers is that the CARS data acquisition hardware is limited by the rate at which it can store the spectra. If fhe number of points per spectrum was reduced and/or the data capture rate limit is increased then lasers with higher repetition rate could be used. The next most important component in the CARS setup is the detection system. As indicated above, a sensitive detector with the least susceptibility to noise and non-linearities will result in measurements with the least machine dependant statistics. Commercially available dye lasers can be used for the Stokes laser however, for reasons of expediency, simplicity, and economy, a home built dye laser is sufficient. 

Figure 3.15 The STIRAP via a continuum experiment. Left panel The coupling scheme in He. The pump and Stokes lasers couple the initially populated state 2s So to the target state 4s So via the ionization continuum (solid lines). Population transferred to the target state can be ionized by the pump laser (dotted line), too. An off-resonant Raman-type transition between the initial and target states is also indicated (dash-dotted lines). Right panel The measured electron signals versus laser tuning of the pump laser in the two-photon resonance range for the counter-intuitive pulse ordering. The circles and the triangles indicate slow and fast electrons, respectively. The solid lines show numerical simulations. Taken from Ref. [237].

In a series of papers on various anti-Stokes laser media. White and Henderson [13] demonstrated such laser emission at 178 nm from I, at 149 nm from Br, and 410 nm from In. White [l4] also proposed that anti-Stokes lasers with emission from 100 to 206 nm could be produced based on metastable population inversions in the group VI elements 0, S, Se from selective photodissociation of N2O, OCS, and OCSe by VUV radiation. In recent experiments, Ludewigt et al. [15] have achieved anti-Stokes... 

The Stokes laser generates a coherent superposition of the wavefunctions of levels 2) and 3). The states 2) and 3) are, however, not occupied before the pump pulse arrives. The wavefunction oscillates between levels 2) and 3) with the Rabi frequency which depends on the intensity of the Stokes pulse and its detuning from resonance. Now the pump pulse comes with a time delay At with respect to the Stokes pulse, where At is smaller than the width of the Stokes pulse, which means that the two pulses still overlap (Fig. 7.15b). This places the molecule at a coherent superposition of levels 1) and 2) and 2) and 3). If the delay At, the detuning A v and the intensities of the two lasers are correctly chosen, the population in level 11) can be completely transferred into level 13) without creating a population in level 2) (Fig. 7.15c). The coherently excited levels 1) and 2) are described by the wavefunction... 

What is serendipitious in combustion is that if the Stokes sources are positioned to observe the major products of hydrocarbon-air reactions, namely CO2 and H2O, the dual broadband frequency differences cover all the important diatomic constituents, i.e. N2, CO and NO if sufficiently abundant. If the CO2 Stokes laser is centered near 1320 cm midway between the and 21/2 modes, then O2 can be observed as well in the low frequency tall of the dye laser. Most broadband dyes possess unnarrowed FWHH on the order of 150 to 200 cm and base widths at least double this value. We have found that the dye DCM dissolved in DMSO, well suited to the H2O Raman freqtiencles, possesses a FWHH of 350 cm . Using this dye, a very broad frequency difference range can be achieved. 

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