(Invited) High Index Dielectric Nanoantennas for Light Management, Controlled Photoluminescence of Quantum Emitters, and Semiconductor-Based Plasmonics

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Recently, an alternative to plasmonics emerged with high-refractive index dielectric nanostructures (e.g. silicon, n≈4 in the visible range), which offer the same kind of applications as nanostructures of noble metals, but with interesting advantages and despite lower field enhancement and confinement volume. Thus, they can be used to enhance and control light scattering efficiency, nonlinear emission, Purcell effect, etc. Furthermore, silicon nanostructures offer several key advantages: absorption losses are far weaker than in metals for wavelengths longer than the direct band gap, presence of strong magnetic resonances much more difficult to obtain in metals, and access to semiconductor (CMOS) technology for reproducible large-scale nanostructure fabrication. The first mean for controlling optical properties is given by manipulating Mie resonances instead of local surface plasmon resonances (LSPR). The Mie resonances can be adjusted by modifying the size, shape, and material of those nanostructures. The other mean is obtained by doping the nanostructures generating free carriers, thus leading to tunable LSPRs as function of dopant concentration (contrary to metals which have a fixed free electron concentrations). We present two examples describing the interest of using silicon nanoantennas, which are: (i) emission rate modification of quantum emitters placed in the silicon nanoantenna near-field (we focus also on the important role of both electric and magnetic components of light), and (ii) tunable Si-based plasmonics in the MIR to NIR spectral range by tuning dopant concentration and structural parameters of Si nanostructures. We show the effect of a Si nanoantenna on the spontaneous emission of quantum emitters placed in its vicinity. The emission rate corresponds to the local density of photonic states (LDOS), which is modified by the nanostructure. Thus, it can be controlled (enhancement or quenching) by properly designing the nanoantenna. For application in field-enhanced spectroscopy and single molecule detection, the goal is to obtain the highest enhancement. We discuss results of photoluminescence enhancement of various emitters placed around simple Si nanoantennas, with a special focus on the photoluminescence of Eu3+-doped thin films deposited on the Si nanoantennas. These rare earth ions exhibit both electric dipole (ED) and magnetic dipole (MD) transitions of nearly equal strength. We show that these ED and MD transitions are very sensitive to the electric and magnetic LDOS, respectively. This can be evidenced using Si nanoantennas, which support both electric and magnetic resonances of comparable strength. Due to the very high index, they also give the unique opportunity to separate and redistribute in the nearfield the energy of the magnetic and electric parts of the electromagnetic field, otherwise inextricably connected in the far field. In a second part, we present theoretical results about doped Si nanostructures based on the Green Dyadic Method initially used for studying optical properties in metal nanostructures. We show that cuboid Si nanostructures of a few tenths of nm in each dimension, with carrier concentration around 2.6 1021 cm-3, give LSPR close to the telecom wavelength at 1.54 μm. Experimentally, an original strategy has been developed to incorporate and active dopant at concentrations above 1020 cm-3. Such strategy involves out-of-equilibrium annealing by using nanosecond laser pulses and liquid phase epitaxy. We can thus expect tuning the LSPR frequency in the MIR to NIR range depending on the dopant concentration and the nanostructure shape. Preliminary experimental results are shown and compared to theory.

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