Third Order Optical Response

Both the commutation relations [Equation (15)] and the Hamiltonian [Equation (17)] contain higher order products of Bj and Bn. These higher terms do not contribute to the third-order optical response and were neglected. 

J.L. Bredas, C. Adant, P. Tackx, A. Persoons, Third-Order Optical Response in Organic Materials Theoretical and Experimental Aspects, Chem. Rev. 94 (1994)... 

At this point, we restrict the multi-state quantum chromophore to the ground ( 0 R)), first-excited ( 1 R)), and second-exdted ( 2 R)) adiabatic states, which is the minimum number of states required to describe third-order optical response in ultrafast IR experiments. It is assumed that only transitions between 0 R) and 11 . R) and between 11 i ), and 2 R) are allowed. Also, the rotating wave approximation (RWA) is invoked, such that the field-matter interaction term may be written as ... 

The increasing use of optical fibre in the telecommunications network will, ultimately, require all-optical signal processing to exploit the full bandwidth available. This has led to a search for materials with fast, large third order optical nonlinearities. Most of the current materials either respond in the nanosecond regime or the nonlinearity is too small (1-3). Organic materials are attractive because of their ultra-fast, broadband responses and low absorption. However the main problem in the materials studied to date, e.g. polydiacetylenes (4) and aromatic main chain polymers (5), has been the small nonlinear coefficients. 

As the local electric field in the particles is enhanced at the SPR, the metal nonlinear optical response can be amplified as compared to the bulk solid one. Moreover, the intrinsic nonlinear properties of metals may themselves be modified by effects linked with electronic confinement. These interesting features have led an increasing number of people to devote their research to the study of nonlinear optical properties of nanocomposite media for about two decades. Tire third-order nonlinear response known as optical Kerr effect have been particularly investigated, both theoretically and experimentally. It results in the linear variation of both the refraction index and the absorption coefficient as a function of light intensity. These effects are usually measured by techniques employing pulsed lasers. 

Thus, subphthalocyanine 17 exhibits a large resonant third-order optical nonlinearity and fast response. The measured resonant third-order NLO response is two orders of magnitude larger than the off-resonant values of chloroboron subphthalocyanine, and of the same order of magnitude as the off-resonant values of polydiacetylene due to resonant enhancement. 

Barzoukas and Blanchard-Desce proposed an approach of molecular engineering using multivalence-bond state models [55]. Push-pull polyenes were shown also to present an enhancement of the TPA response and a loss of transparency of molecules, as a function of the increase of the polyenic chain length [56,57]. Trends observed in these polyenic systems are supported by the large third-order optical nonlinearities measured in asymmetric carotenoids, in which the role of the large value of dipole moment difference A/z was shown [58]. 

Polycondensation and imidization of w,w -diaminobenzophenone and pyromellitic dianhydride under microwave radiation was also carried out. The product polyimide was obtained in a two-step process. It is claimed that this product of microwave radiation polymerization compares favorably with a product of conventional thermal polymerization, because it exhibits third-order nonlinear optical coefficient of 1.642 x 10 esu and response time of 24 ps. The third-order optical nonlinearity of this polymer is dependent on the chain length and the molecular structure. 

Xie and Rao have given a brief review of the third order optical non-linearities (related to y) in the higher flillerenes (Cyo-Cge) and compared their response to carbon nanotube and polyenic systems. They find that, in tubular fiillerenes, doping can greatly increase y. 

Nonlinear optical properties of PTs which exhibit ultrafast responses and large nonlinearities attributed to one-dimensionality and delocalization of n-electrons along the polymer chains are also described [403,404]. Poly(4,4 -dipentoxy-2,2 -bithiophene) and poly(4,4 -dipentoxy-2,2 5, 2"-terthiophene) show a fast and high third-order nonlinearity [405]. Third-order nonlinearities depend on the nature of the polymer backbone and only slightly on the substituents [406], The optical transparency and the third-order optical nonlinearities can be tailored in random copolymers of 3-methylthiophene and methyl methacrylate [407]. A solution-processable thiophene copolymer with a side... 

Third-Order Optical Polarization and Non-linear Response Functions... 

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