Strong Nonlinear Optical Response in the Visible Spectral Range with Epsilon‐Near‐Zero Organic Thin Films

Abstract

The optical properties of a medium are described by the dielectric permittivity ε and the refractive index N, where ε is a measure how much polarization is induced upon application of an optical electric field, while refractive index N determines how the optical phase develops as optical wave propagates in the optical medium by associating momentum k and energy ω. Optical epsilon-near-zero (ENZ) material possesses the permittivity |ε| → 0 and the phase velocity of optical wave becomes very large while the group velocity is slowing down significantly, owing to the relation between refractive index and permittivity, ε = N .[1] Related to nonlinear optical processes, this simple equation also implies that the optical Kerr nonlinearity is strongly enhanced in the ENZ spectral range. [2] Enhanced Kerr nonlinearities are observed in metamaterials such as conducting oxides and doped inorganic semiconductor thin films showing epsilon-nearzero (ENZ) response in the infrared region. However, to achieve ENZ in the visible, artificial metamaterials with more complex nanostructures have to be specifically designed. [2,4-bis[8-hydroxy-1,1,7,7-tetramethyljulolidin-9-yl] squaraine] organic thin films, ENZ responses between 450 and 620 nm are demonstrated. Both nonlinear refractive index and nonlinear absorption coefficient are enhanced by more than two orders of magnitude in the ENZ spectral region. These optical effects in the visible spectral range come from the strongly dispersive permittivity of molecular aggregates resulting from the coupling of excitonic transition dipoles. These findings open the path toward a next generation of high-performance solution-processable organic nonlinear optical materials with ENZ properties that can be tuned by molecular engineering. Here, using sodium [5,6-dichloro-2-[[5,6-dichloro-1-ethyl-3-(4-sulphobutyl)-benzimidazol-2-ylidene]-propenyl]-1-ethyl-3-(4-sulphobutyl)-benzimidazolium hydroxide] and

Quasi-confined ENZ mode in an anisotropic uniaxial thin slab

“…We refer the reader to a recent review for more informations on this topic [15]. ENZ materials can be found naturally in polar materials close to optical phonon resonances, in transparent conductive oxide close to their plasma frequency, but have also been engineered to reach visible wavelength using metamaterials [16] or organic molecules [17]. Strategies to enhance the ENZ properties by loss-compensation have also been proposed [18].…”

Large optical nonlinearity enhancement under electronic strong coupling

“…T hird-order optical nonlinear effects are intrinsic characteristics of a material. Because many significant optical nonlinear phenomena, such as four wave mixing, optical modulation, self-focusing and stimulated Raman scattering, are caused by the third-order optical nonlinear susceptibility 1 , they have been explored extensively in various materials ranging from metals 2 , semiconductors [3][4][5] , 2D materials 6,7 , topological insulators 8 to organic materials [9][10][11][12][13] . An ideal nonlinear optical material should possess large refractive index change at low optical power.…”

Spectrally Tailored Light‐Matter Interaction in Lithography‐Free Functional Nanomaterials

“…Negative ε 1 values were reported in the visible at the short wavelength side of strong absorption bands in TiO x N y compounds; however, this was attributed to effective medium effects resulting from the granular structure of the material (TiN nanoparticles in a TiO x N y matrix) . Negative ε 1 values were also reported in the visible for organic materials consisting of J aggregates and shown to be induced by excitonic resonances. This enables plasmonic phenomena in organic materials, which can be tuned by laser irradiation.…”