Enhanced Nonlinear Refractive Index inε-Near-Zero Materials

Abstract

New propagation regimes for light arise from the ability to tune the dielectric permittivity to extremely low values. Here we demonstrate a universal approach based on the low linear permittivity values attained in the epsilon-near-zero (ENZ) regime for enhancing the nonlinear refractive index, which enables remarkable light-induced changes of the material properties. Experiments performed on Al-doped ZnO (AZO) thin films show a six-fold increase of the Kerr nonlinear refractive index (n2) at the ENZ wavelength, located in the 1300 nm region. This in turn leads to ultrafast light-induced refractive index changes of the order of unity, thus representing a new paradigm for nonlinear optics.The nonlinear optical response of matter to light is, by its very nature, a perturbative and hence typically weak effect. Applications, e.g. for nonlinear optical switches or quantum optics, are therefore largely underpinned by the continuous endeavour to attain stronger and more efficient light-matter interactions. Nonlinear mechanisms can typically be classified as resonant or non-resonant, depending on the frequency of light with respect to the characteristic electronic resonances of the material. Non resonant nonlinearities, like those present in transparent crystals or amorphous materials (e.g. fused silica glass), are generally weak and require high light intensities and/or very long samples to take advantage of an extended light matter interaction. Conversely, resonant nonlinearities can be several orders of magnitude stronger, but this comes at the price of introducing detrimental losses. A typical example is that of metals, which both reflect and absorb light strongly [1][2][3]. An alternative approach to enhance the nonlinear response of a material consists of creating artificial electromagnetic resonances, for example by stacking materials of different refractive index or using other types of composite materials [4][5][6][7][8][9][10][11]. Creating resonant metaldielectric stacks and composites yields a very strong nonlinear enhancement [12][13][14], but inevitably exacerbates the detrimental role of linear and nonlinear losses. Here we propose a different approach to enhance the effective nonlinearity without resorting to optical resonances. Our approach relies on enhancing the nonlinear effect, measured in terms of the nonlinear Kerr index n 2 , rather than on a direct enhancement of the intrinsic χ (3) nonlinear susceptibility. As we show below, this enhancement arises due to the fact that the nonlinear refractive index is a function of both the nonlinear susceptibility and the linear refractive index. Recent progress in material design and fabrication has provided access to the full range of linear optical properties bounded by dielectric and metallic regimes. Of particular relevance for this work are materials which exhibit a real part of the dielectric permittivity that is zero, or close to zero, such as transparent conducting oxides where their permittivity cross over is typically located in the near infrared s...

Large optical nonlinearity of nanoantennas coupled to an epsilon-near-zero material

“…Furthermore, the zero-permittivity wavelength, strength of the nonlinear response, and the losses depend on the optical properties of the ENZ material. In comparison to previous works [17,18] where strong nonlinear responses were reported in ENZ materials, here we show that many of these constraints can be overcome by incorporating engineered nanostructures on an ENZ host material. Specifically, we report a new approach to engineer an optical medium with an unprecedentedly large intensity-dependent refractive index using nanoantennas coupled to a thin ENZ material.…”

“…These features make the system flexible enough to engineer both the magnitude and sign of the nonlinear response simply by engineering the linear dispersion, by appropriately choosing the antenna parameters,and by choosing an ENZ host that exhibits the required nonlinear response. [17,18].We demonstrate the principle of operation using a metasurface [22,23, 24,25,26,27,28] consisting of a two-dimensional array of gold optical antennas 2 on a thin ENZ layer, which in turn is deposited on a glass substrate (Fig. 1b).…”

“…Thus, ENZ material with lower intrinsic damping could increase the saturation threshold. The nonlinear response could be further enhanced by maximizing the overlap between the near-field of the antennas and the ITO layer, by choosing antennas (dielectric or metallic) with higher quality factors, and by choosing an ENZ material with lower intrinsic damping [18,36] .We note that when the ENZ medium is sufficiently thick, the antenna resonance gets locked to the zero permittivity wavelength of the ENZ material [37]. Nevertheless a metasurface that incorporates a thick ENZ material will also exhibit enhanced nonlinear response because a dynamic change in the substrate's refractive index necessarily modifies the resonance wavelength of the antenna.…”

“…These features make the system flexible enough to engineer both the magnitude and sign of the nonlinear response simply by engineering the linear dispersion, by appropriately choosing the antenna parameters,and by choosing an ENZ host that exhibits the required nonlinear response. [17,18].…”

Improving the transmittance of an epsilon-near-zero-based wavefront shaper

“…Among the huge diversity of metamaterials, Epsilon-Near-Zero (ENZ) materials consider the particular case where the dielectric constant reaches zero. These metamaterials have already demonstrated to behave unconventionally, with their ability to bend the light [5,6], to tunnel light through subwavelength channels [7] or to exalt optical nonlinearities [8], promising a new paradigm for nonlinear photonics and nonlinear plasmonics. …”