We experimentally demonstrate enhanced third-harmonic generation from indium tin oxide nanolayers at telecommunication wavelengths with an efficiency that is approximately 600 times larger than crystalline silicon (Si). The increased optical nonlinearity of the fabricated nanolayers is driven by their epsilon-near-zero response, which can be tailored on-demand in the near-infrared region. The present material platform is obtained without any specialized nanofabrication process and is fully compatible with the standard Si-planar technology. The proposed approach can lead to largely scalable and highly integrated optical nonlinearities in Si-integrated devices for information processing and optical sensing applications.

We perform a comparative study of second-harmonic generation (SHG) from indium tin oxide (ITO) and from titanium nitride (TiN) nanolayers excited in the near-infrared spectrum. Both materials are compatible with Si technology and are candidate platforms for integrated nonlinear optics. In this work, we fabricate ITO samples with an ε-near-zero (ENZ) condition, which can be continuously tailored in the 1150−1670 nm spectral range, and TiN samples with a metallic behavior in the same spectral range. For the ITO nanolayers, we observe tunability and enhancement of the SHG intensity when the samples are excited at their respective ENZ condition, in agreement with the electromagnetic modeling and analogous to its third-harmonic generation studied earlier. On the other hand, we show that the SHG efficiency of TiN nanolayers is lower by a factor of 50. We determine experimentally that the dominant component of the second-order susceptibility for our best ITO nanolayer is χ zzz (2ω) = 0.18 pm V −1 , and we theoretically predict that the SHG process is enhanced up to 4 orders of magnitude when resonantly pumping the nanolayer at the ENZ wavelength with respect to a wavelength at 2000 nm. Remarkably, the resulting SHG efficiency is comparable with a crystalline quartz plate with thickness 0.5 mm used as a reference in our experiments in reflection configuration. Our study clearly indicates that ITO nanolayers with engineered ENZ conditions are a promising material platform for surface nonlinearities, with possible applications to nonlinear metasurfaces, Si-based flat optics, and sensing.

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The all-inorganic perovskite nanocrystals are currently in the research spotlight owing to their physical stability and superior optical properties—these features make them interesting for optoelectronic and photovoltaic applications. Here, we report on the observation of highly efficient carrier multiplication in colloidal CsPbI3 nanocrystals prepared by a hot-injection method. The carrier multiplication process counteracts thermalization of hot carriers and as such provides the potential to increase the conversion efficiency of solar cells. We demonstrate that carrier multiplication commences at the threshold excitation energy near the energy conservation limit of twice the band gap, and has step-like characteristics with an extremely high quantum yield of up to 98%. Using ultrahigh temporal resolution, we show that carrier multiplication induces a longer build-up of the free carrier concentration, thus providing important insights into the physical mechanism responsible for this phenomenon. The evidence is obtained using three independent experimental approaches, and is conclusive.

In the present Letter, we demonstrate how the design of metallic nanoparticle arrays with large electric field enhancement can be performed using the basic paradigm of engineering, namely the optimization of a well-defined objective function. Such optimization is carried out by coupling a genetic algorithm with the analytical multiparticle Mie theory. General design criteria for best enhancement of electric fields are obtained, unveiling the fundamental interplay between the near-field plasmonic and radiative photonic coupling. Our optimization approach is experimentally validated by surface-enhanced Raman scattering measurements, which demonstrate how genetically optimized arrays, fabricated using electron beam lithography, lead to order of ten improvement of Raman enhancement over nanoparticle dimer antennas, and order of one hundred improvement over optimal nanoparticle gratings. A rigorous design of nanoparticle arrays with optimal field enhancement is essential to the engineering of numerous nanoscale optical devices such as plasmon-enhanced biosensors, photodetectors, light sources and more efficient nonlinear optical elements for on chip integration.