Efficient ultrafast all-optical modulation in a nonlinear crystalline gallium phosphide nanodisk at the anapole excitation

Abstract

High–refractive index nanostructured dielectrics have the ability to locally enhance electromagnetic fields with low losses while presenting high third-order nonlinearities. In this work, we exploit these characteristics to achieve efficient ultrafast all-optical modulation in a crystalline gallium phosphide (GaP) nanoantenna through the optical Kerr effect (OKE) and two-photon absorption (TPA) in the visible/near-infrared range. We show that an individual GaP nanodisk can yield differential reflectivity modulations of up to ~40%, with characteristic modulation times between 14 and 66 fs, when probed at the anapole excitation (AE). Numerical simulations reveal that the AE represents a unique condition where both the OKE and TPA contribute with the same modulation sign, maximizing the response. These findings highly outperform previous reports on sub–100-fs all-optical switching from resonant nanoscale dielectrics, which have demonstrated modulation depths no larger than 0.5%, placing GaP nanoantennas as a promising choice for ultrafast all-optical modulation at the nanometer scale.

Metasurface Photoelectrodes for Enhanced Solar Fuel Generation

“…[8][9][10] Of particular interest are nonradiative excitations, such as the anapole, which lead to light confinement into the resonator and therefore high electromagnetic field strengths within the material. [11][12][13] In this type of excitation, far-field scattering is minimized and near-field energy inside the material reaches its maximum, making it particularly attractive, e.g., for photo catalytic applications. In a previous study, we demonstrated on the single particle level that resonant coupling of the anapole excitation wavelength to electronic transitions in substoichiometric TiO 2−x leads to enhanced electron-hole pair generation rates and catalytic yields under sub-bandgap excitation.…”

Deep Optical Switching on Subpicosecond Timescales in an Amorphous Ge Metamaterial

“…These results presented here show great merit for all‐optical switching when weighing the three main considerations of power consumption, switching speed and signal modulation. A comparison with the results in this work to state‐of‐the‐art all‐optical switching platforms [ 11,15,18,22,27–29,45 ] is given in Table 2 . Both the reported differential and absolute modulation values (if readily available) are given from the referenced works to highlight their overall modulation responses and better illustrate the comparison to and significance of this presented work.…”

“…[ 16,23,24 ] The latter is enabled by the presence of a direct electronic gap, which facilitates ultrafast relaxation of FCs and showing recombination times as short as several picoseconds. Although typically, indirect band gap materials do not employ efficient FC relaxation pathways to reduce the relaxation time to the sub‐picosecond time scale, [ 1,19,20,25–30 ] amorphous materials [ 29,31 ] have shown promise for nanophotonic devices and ultrafast FC relaxation. The lack of long‐range order in amorphous materials leads to the disappearance of extended states (e.g., Bloch states for electrons) resulting in localized states which leads to a stronger electron–phonon coupling and faster free carrier relaxation.…”

All‐Optical Modulation with Dielectric Nanoantennas: Multiresonant Control and Ultrafast Spatial Inhomogeneities

“…All‐dielectric nanoantennas have been proposed as alternatives to metallic ones [ 17 ] because of the possibility to excite Mie resonances inside nanostructures made of high refractive index materials experiencing much lower Ohmic losses. [ 18 ] Promising candidates for such configurations, also in view of their high third‐order optical nonlinearity, are conventional semiconductors, including silicon, [ 19,20 ] that can be easily operated in transmission when the nanoantennas are supported on a silica substrate, and gallium arsenide [ 21 ] and gallium phosphide, [ 22 ] mostly suitable for high reflectivity configurations. In particular, all‐optical reflectivity modulation on a few picoseconds time scale has been demonstrated from GaAs nanoantennas [ 21 ] by exploiting the magnetic dipole resonance in nanopillar structures pumped above bandgap with a femtosecond laser at relatively low fluences.…”

“…Here, the pump‐induced refractive index modulation is ascribable to the photogeneration of free carriers. An even faster modulation speed (on the sub‐ps time scale) has been recently reported from GaP nanodisks [ 22 ] by exploiting instantaneous optical Kerr effect and two‐photon absorption when pumping below the bandgap, even though at the expense of higher fluences of the control pulse. A comparison between the two nonlinear regimes of modulation (i.e., with the energy of the pump photon below or above the bandgap of the semiconductor) was conducted by some of the present authors for Si nanobrick metasurfaces.…”