Stimulated emission Raman scattering

Figure 10.14 (a) PL intensity and FWHM of the emission peak at 565 nm taken from the AC7 crystals as a function of Xex- (b) Changes in kem of the emission peaks that appeared at various X x- Reproduced from H. Yanagi, A. Yoshiki, S. Hotta and S. Kobayashi, Mirrorless lasing from thiophene/phenylene co-oligomer crystals based on stimulated resonance Raman scattering, J. Appl. Phys., 96, 4240-4244 (2004) with permission from the American Institute of Physics... 

In Table I we summarize the tuning ranges of stimulated emission obtained so far by stimulated polariton and spin-flip Raman scattering. 

If J" —> J excitation is accompanied or followed by deexcitation J —> J" in a stimulated emission process (SEP), then the population efficiency of the level can be increased considerably. It is now known [248, 347] that the process might be made more effective by applying the A-configuration scheme in which the first-step (J" — J ) excitation pulse is applied after the second-step (J — J") pulse which, at first glance, seems surprising. This process is called stimulated Raman scattering by delayed pulses (STIRAP). The population transfer here takes place coherently and includes coordination of the Rabi nutation phase in both transitions. 

Up to now/ the dimer laser system has been described alone in terms of population inversion between suitable energy levels/ and for this description the condition S2 > A 2 is indeed the only necessary condition for cw laser oscillation/ as long as the thermal population density in the lower laser level remains negligibly low. However/ as this optically pumped laser system is a coherently excited three level system/ the coherent emission can also be described as stimulated Raman scattering/ which is resonantly enhanced by the common level 3 of the pump and laser transitions. This coupled two photon or Raman process does not require a population inversion between levels 3 and 2 and introduces qualitatively new aspects which appreciably influence and change the normal laser behaviour. For a detailed and deeper description of the coherently excited three level dimer... 

Stimulated Raman gain Stimulated Raman gain (SRG) and inverse Raman scattering (IRS) are closely related. While one involves stimulated gain at a Stokes-shifted frequency, the other involves stimulated loss at an anti-Stokes-shifted frequency. SRG can be viewed as an induced emission process at the Stokes frequency. Both SRG and IRS are coherent processes. 

Recently, a XeCl laser (308 nm) with high spectral brightness has been used by Maeda and Takahashi [lO] to excite up to the 11th AS component at 128.3 nm in H2 gas at 10 atm. This source and therefore the AS Raman emission is tunable over the bandwidth of the XeCl laser ( V IOA). The potential for emission at much shorter wavelengths now exists with excitation of stimulated Raman scattering with an E-beam pumped Ar2 ex-cimer laser operating at 124 to 127.5 nm with 2 MW output power. In preliminary experiments, Sasaki et al. [ll] have generated X IOO kW power in the first and second order Stokes emission (at 134 and 141 nm) of H2 at 8 atm pressure. 

G. Zhong He, A. Kuhn, S. Schiemann, K. Bergmann, Population transfer by stimulated Raman scattering with delayed pulses and by the stimulated-emission pumping method a comparative study. J. Opt. Soc. Am. B 7, 1960 (1990)... 

It is well-known that gain can be obtained by stimulated Raman scattering without population inversion between the lower, initial state a> and the higher, final state c> of the two-photon transition. Near resonance, that is when the laser photon frequency almost equals the transition frequency u, the overall transition takes place mainly ac-cording to the following scheme (1) absorption of a laser photon of frequency (2) stimulated emission of a Stokes laser photon of frequency as depicted in Fig. la. This process prevails over the reverse process shown in Fig. lb, provided. ... 

On the basis of the experimental data presented above, one cjin exclude on the onset several models for the "blue peak" emission. Firstly, spin-flip collision induced population inversion on the Di transition is not involved - due to both the off-resonant character of the emission and its independence on the buffer gas pressure. Similarly, pressure induced extra resonances are rejected. Stimulated electronic Raman and three photon scattering effects, both by a two or three level system, are dependent on the laser detuning and neither their frequencies are to the blue in the vicinity of the Di line (figure 2) thus, these processes are also excluded. 

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