ANTI-STOKES STIMULATED RAMAN SCATTERING IN PHOTONIC CRYSTALS Nikolay S. Makarov, SPbSU ITMO/MSU, MT Victor G. Bespalov, S.I. Vavilov State Optical Institute Introduction Stimulated Raman scattering (SRS) is widely used for discrete frequency conversion of pulsed and continuous-wave lasers [1-5]. It is especially interesting to use anti-Stokes SRS-radiation, however, its efficiency in usual conditions is small enough, that is why some methods of its increasing were proposed [610]. For example, in papers [8-10] we have proposed to use quasi-phase matching conditions for efficient anti-Stokes SRS generation in media with alternating of the third-order nonlinearity ((3)) along propagation direction. By numerical simulations we showed that for each Raman-active medium there is an optimal value of the input Stokes seed intensity at which the maximal efficiency of generation (up to 35%) is reached. This value is much more than at simple focusing in Raman-active media and reaches to the efficiency of phase matched conversion (50%). In papers [11-12] authors have proposed to use photonic crystals for realization of quasi-phase matching conditions at various harmonic generation. It was shown that using photonic crystals allows greatly decreasing of active medium length and increasing of harmonic intensities. Today there are only a few studies of SRS in photonic crystals, for example, [13], where the authors have studied the generation of Stokes SRS component in Bragg gratings. By considering of propagation of pump and first-order forward and backward Stokes waves, the authors have studied shaping of slow and practically stationary solitons due to interacting of SRS, Kerr nonlinearity, grating dispersion and light reflection at its layers boundaries. Since the fully correct modeling of SRS in photonic crystals is complicated enough, we try to evaluate the possibility of efficient anti-Stokes generation in one-dimension photonic crystals. Also, we present some comparison of results for quasi-phase matching SRS in hydrogen and barium nitrate with results for photonic crystals and discuss the enhancement of anti-Stokes generation in photonic crystals. Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] Principles of quasi-phase matching Nonlinearity (3) Nonlinearity (2) (3)0 L к I2w Lк H2 c-axis E (3)=0 H2 H2 H2 Raman-active medium d31 z Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] Model We consider one-dimension photonic crystals wits the lengths of layers l1 and l2 with permittivities 1 and 2 correspondingly. Crystal layers are considered with the same Raman transition frequency, while the pump and Stokes seed initial waves are perpendicular to layers of photonic crystal. By varying of layers lengths and permittivities it is possible to chouse the conditions at which the effective wave mismatching of interacting waves is near to zero. pump 1 l2 l2 2 2 1 pump Stokes Stokes anti-Stokes l1 l1 Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] Dispersion of photonic crystal At first approximation it is possible to consider photonic crystal as a continuous medium with effective dispersion, given by [14]: 2 1l1 2 2 l2 1 2 2 1l1 2 2 l2 cos cos keff l1 l2 cos sin 2 sin 1 2 where keff is an effective wave vector of radiation with wavelength , propagating through photonic crystal. k1 ();n1() At first approximation as continuous medium k2();n2() keffective();neffective() Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] Steady-state SRS coupled differential equations system 2 2 dE0 g 0 dz 2 E1 E1 E0 1 dE1 g * iz * E E e E 1 0 0 E1 E0 2 dz dE1 g 1 * * iz dz 2 E1E0 E0 E1e E0 1 – wave mismatching, g – steady-state Raman gain coefficient, j – frequencies of interacting waves, Ej – complex amplitudes of waves Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] The interacting of waves in photonic crystal in conditions of quasi-phase matching 1=1; 2=13; l1=175.0000 nm; l2=287.8774; =0.263 rad/cm; the efficiency of Stokes and anti-Stokes generation are about ~33.3% and ~30.1% correspondingly Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] The required layers length accuracy (the difference between maximal and minimal effective wave mismatching in photonic crystal at optimal conditions while changing the length of one layers by 0.01 nm as a function on contrast in layers refractive indices) Modeling shows that the appropriate wave mismatching is possible only at very accurate determining of lengths of crystal layers (~0.001 nm) Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] Effective pump wave vector (1=1, 2=2, l2=723.83 nm) Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] Effective Stokes wave vector (1=1, 2=2, l2=723.83 nm) Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] Effective anti-Stokes wave vector (1=1, 2=2, l2=723.83 nm) Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] Effective wave mismatching (1=1, 2=2, l2=723.83 nm) Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] The optimization of photonic crystal parameters Our simulations have shown that the optimal wave mismatching is equal to 0.008 rad/cm is possible at l1=174.9986 nm. Moreover, the appropriate wave mismatching is possible between l1=440 nm and l1=441 nm, while in the range of l1=970 nm – l1=1035 nm photonic crystal at given wavelengths is equivalent to a simple media with constant refractive index l1=174.9986 nm; l2=723.83 nm; 1=1; 2=2 =0.008 rad/cm Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] Comparison with layered hydrogen and barium nitrate: periodicity of layers Active layer length, cm Forward and backward SRS 0,8 0,7 0,6 0,5 0,4 0,3 0 10 20 layer number 30 40 Forward SRS Passive layer length, cm Forward SRS 0,9 Forward and backward SRS 1,3 1,25 1,2 1,15 1,1 1,05 1 0,95 0,9 0 10 20 layer number 30 40 The main difference between these two cases: using photonic crystals allows us to make Ramanactive medium significantly shorter and get some gain of its periodicity, however to achieve this effect we need to use a lot of thin layers. The simulations show that the maximum efficiency of anti-Stokes SRS generation is practically the same for both cases (about 30%) and is restricted by energy transformation from pump into Stokes and anti-Stokes waves. Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] 25 25 20 20 15 15 Eff_a, % Eff_a, % Comparison with layered hydrogen and barium nitrate: pump input parameters 10 10 5 5 Ip, GW/cm^2 0 0 0 0,2 0,4 0,6 0,8 1 0 2 4 Tp, ns 6 8 10 For both photonic crystals and conventional layered structures the effective generation occurs within a wide enough set of input pump parameters, that allows realization of the tuned source of anti-Stokes SRS generation. 0.15 GW / cm I 0 0.6 GW / cm T0 1 ns 2 2 Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] 5 ns T1 20 ns 25 25 20 20 Eff_a, % Eff_a, % Comparison with layered hydrogen and barium nitrate: Stokes input parameters 15 10 15 10 5 5 0 0 0 0 5 10 Ts, ns 15 20 25 30 0,005 0,01 0,015 0,02 0,025 Is, GW/cm^2 For both photonic crystals and conventional layered structures the effective generation occurs within a wide enough set of Stokes seed pulse parameters, at which the efficiency exceeds 20%. 0.001 GW / cm I S 0.025 GW / cm 5 ns TS 20 ns 2 2 Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] Comparison with layered hydrogen and barium nitrate: wavelength sensitivity 25 Eff_a, % 20 15 10 5 0 510 515 520 525 530 535 540 545 550 pump wavelength, nm All considered layered structures are very sensitive to the wavelength variations. This leads to sufficient decreasing of the anti-Stokes generation efficiency even at small wavelength variations about 0.75%. Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] Conclusion • We have proposed the method of increasing of efficiency of anti-Stokes SRSgeneration in one-dimension photonic crystals in conditions of quasi-phase matching • We have simulated photonic crystals with different contrast in layers refraction indices • Modeling has shown that using of photonic crystals allows to increase the efficiency on anti-Stokes SRS generation up to 30% and to design white-light coherent sources • However, it is required to provide high accuracy in layers lengths at manufacturing of photonic crystal, and requirements to this accuracy increase wits increasing of contrast in layers refraction indices • It is shown that the realization of quasi-phase matching conditions is possible in wide enough range of contrast in layers refraction indices (from 2/1=1.5 up to 2/1=13) Anti-Stokes stimulated Raman scattering in photonic crystals; San-Jose, 22 – 27 January 2005 Makarov N.S., [email protected] Bespalov V.G., [email protected] References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 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