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Tham khảo tài liệu 'coherence and ultrashort pulse laser emission part 18', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Ultrashort Laser Pulses Applications 671 Cumpston et al. 1999 Maruo et al. 1997 . Both the two-photon microscopy and two-photon polymerization take advantage of the transparency of the medium for the fundamental wavelength which allows the laser propagation inside the medium with minimal losses and the nonthermal character of the ultrashort pulses interaction that does not heat the samples avoiding thermal effects that can modify the samples properties. Moving to the third order susceptibility some phenomena are extensions of the second order ones such as the four wave mixing governed by x 3 ro ro1 ro2 ro3 Trager 2007 in which three photons at frequencies ro1 ro2 and ro3 interact to generate a fourth one. Depending on the particular frequencies involved several phenomena as Coherent AntiStokes Raman Scattering CARS Zumbusch et al. 1999 Degenerate Four Wave Mixing DFWM Joo Albrecht 1993 Optical Phase Conjugation Yau et al. 1996 Stimulated Brillouin Scattering among others Yariv Yeh 2007 can be generated. In the degenerate case for a single beam the third harmonic described by x 3 3ro ro ro ro is generated and some applications are extensions of the second order ones Trager 2007 . Another class of phenomena emerge when the effects arising from the third order susceptibility x 3 ro ro ro -ro come into play for a single beam. Under these conditions the third-order polarization is P 3 xj M-a E0j t e-iat E0j E e-iat E0j f e-iat j -a c X t l0 t 6 where I0 t E0j ty E0j t is the beam intensity. Expression 6 shows that the polarization oscillates at the same frequency of the exciting beam and it is proportional to its intensity and this implies Butcher Cotter 1990 Shen 1984 that the total refractive index of the material n must have a dependence on the pulse intensity according to n r t n0 n2I r t 7 where n0 is the linear usual refractive index of the material I is the pulse intensity and n2 is the nonlinear refractive index given by n2 a Re x 3 ro ro ro -ro where the .
