ABSTRACT
Materials show nonlinear responses under an intense light irradiation. These responses are used widely in electro-optical devices as especially higher harmonic, sum, and difference frequencies generations. In these cases, the electric displacement D can be written as follows: https://www.w3.org/1998/Math/MathML"> D = ε E = ε 0 E + P = ε 0 E + P ( 1 ) ( ω s ) + P ( 2 ) ( ω s ) + P 3 ( ω s ) + ⋯ = ε 0 [ χ ( 1 ) E + χ ( 2 ) E 2 + χ ( 3 ) E 3 + ⋯ ] https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781315221403/d4ef19b2-c195-459c-8305-986f5591a121/content/equ3_1.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>
