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...

We fabricated slotted photonic crystal waveguides and cavities supporting resonant modes in air. Their peculiar geometry enables the detection of refractive index changes in a given analyte with high sensitivity because of the large overlap between the optical mode and the analyte. This yields a high figure of merit for the sensitivity of the device and we are able to report values of S=Δλ/Δn over 1500. By applying a photonic crystal heterostructure to the slotted geometry, we are able to create high quality-factor cavities essential for realizing low detection limits up to Q=50 000.

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Molecular dyes, plasmonic nanoparticles and colloidal quantum dots are widely used in biomedical optics. Their operation is usually governed by spontaneous processes, which results in broad spectral features and limited signal-to-noise ratio, thus restricting opportunities for spectral multiplexing and sensing. Lasers provide the ultimate spectral definition and background suppression, and their integration with cells has recently been demonstrated. However, laser size and threshold remain problematic. Here, we report on the design, high-throughput fabrication and intracellular integration of semiconductor nanodisk lasers. By exploiting the large optical gain and high refractive index of GaInP/AlGaInP quantum wells, we obtain lasers with volumes 1000-fold smaller than the eukaryotic nucleus (Vlaser < 0.1 µm3), lasing thresholds 500-fold below the pulse energies typically used in two-photon microscopy (Eth ≈ 0.13 pJ), and excellent spectral stability (<50 pm wavelength shift). Multiplexed labeling with these lasers allows cell-tracking through micro-pores, thus providing a powerful tool to study cell migration and cancer invasion.

We present a generic technique allowing size-based all-optical sorting of gold nanoparticles. Optical forces acting on metallic nanoparticles are substantially enhanced when they are illuminated at a wavelength near the plasmon resonance, as determined by the particle's geometry. Exploiting these resonances, we realize sorting in a system of two counter-propagating evanescent waves, each at different wavelengths that selectively guide nanoparticles of different sizes in opposite directions. We validate this concept by demonstrating bidirectional sorting of gold nanoparticles of either 150 or 130 nm in diameter from those of 100 nm in diameter within a mixture.