Aleksandr Vaskin

Ultrafast all-optical tuning of direct-gap semiconductor metasurfaces

Optical metasurfaces are regular quasi-planar nanopatterns that can apply diverse spatial and spectral transformations to light waves. However, metasurfaces are no longer adjustable after fabrication, and a critical challenge is to realise a technique of tuning their optical properties that is both fast and efficient. We experimentally realise an ultrafast tunable metasurface consisting of subwavelength gallium arsenide nanoparticles supporting Mie-type resonances in the near infrared. Using transient reflectance spectroscopy, we demonstrate a picosecond-scale absolute reflectance modulation of up to 0.35 at the magnetic dipole resonance of the metasurfaces and a spectral shift of the resonance by 30 nm, both achieved at unprecedentedly low pump fluences of less than 400 μJ cm–2. Our findings thereby enable a versatile tool for ultrafast and efficient control of light using light.Metasurfaces are not adjustable after fabrication, and a critical challenge is to realise a technique of tuning their optical properties that is both fast and efficient. Here, Shcherbakov et al. realise an ultrafast tunable metasurface with picosecond-scale large absolute reflectance modulation at low pump fluences.

Huygens’ Metasurfaces Enabled by Magnetic Dipole Resonance Tuning in Split Dielectric Nanoresonators

Dielectric metasurfaces that exploit the different Mie resonances of nanoscale dielectric resonators are a powerful platform for manipulating electromagnetic fields and can provide novel optical behavior. In this work, we experimentally demonstrate independent tuning of the magnetic dipole resonances relative to the electric dipole resonances of split dielectric resonators (SDRs). By increasing the split dimension, we observe a blue shift of the magnetic dipole resonance toward the electric dipole resonance. Therefore, SDRs provide the ability to directly control the interaction between the two dipole resonances within the same resonator. For example, we achieve the first Kerker condition by spectrally overlapping the electric and magnetic dipole resonances and observe significantly suppressed backward scattering. Moreover, we show that a single SDR can be used as an optical nanoantenna that provides strong unidirectional emission from an electric dipole source.

An all-dielectric metasurface as a broadband optical frequency mixer.

A frequency mixer is a nonlinear device that combines electromagnetic waves to create waves at new frequencies. Mixers are ubiquitous components in modern radio-frequency technology and microwave signal processing. The development of versatile frequency mixers for optical frequencies remains challenging: such devices generally rely on weak nonlinear optical processes and, thus, must satisfy phase-matching conditions. Here we utilize a GaAs-based dielectric metasurface to demonstrate an optical frequency mixer that concurrently generates eleven new frequencies spanning the ultraviolet to near-infrared. The even and odd order nonlinearities of GaAs enable our observation of second-harmonic, third-harmonic, and fourth-harmonic generation, sum-frequency generation, two-photon absorption-induced photoluminescence, four-wave mixing and six-wave mixing. The simultaneous occurrence of these seven nonlinear processes is assisted by the combined effects of strong intrinsic material nonlinearities, enhanced electromagnetic fields, and relaxed phase-matching requirements. Such ultracompact optical mixers may enable a plethora of applications in biology, chemistry, sensing, communications, and quantum optics.Frequency mixers are hard to achieve at optical frequencies because it is difficult to meet different phase-matching conditions. Here, the authors show that GaAs metasurfaces can mix laser beams to generate eleven new wavelengths through different nonlinear optical processes occurring simultaneously.

Frequency-mixing in GaAs dielectric metasurfaces

We experimentally demonstrate resonantly enhanced nonlinear optical processes such as 2nd-, 3rd-, and 4th-harmonic generations, sum-frequency generation, four-wave mixing processes, etc., in the visible and near-IR using GaAs dielectric metasurfaces.

Emission enhancement from MoS 2 monolayers with silicon nanoantennas

Transition-metal-dichalcogenides (TMDs), which exhibit an indirect electronic band gap as bulk crystals, can become direct semiconductors in the monolayer phase [1]. Such monolayer TMDs show unique optical properties arising from the strong two-dimensional confinement of excitons as well as from the reduction in crystal symmetry. However, the strong mismatch in length scale between the sub-nanometer thickness of an atomically thin crystal sheet and the wavelength of propagating infrared or visible light leads to a rather weak light-matter interaction. By tailoring the near-field environment of monolayer TMDs, resonant optical antennas can strongly modify the excitation response [2]. While research efforts targeted at tailoring and enhancing light-matter interactions in monolayer TMDs have so far been limited to plasmonic nanoantennas, here we concentrate on high-index dielectric nanoantennas, which can show negligible intrinsic losses and thus a high radiation efficiency in the visible and near-infrared spectral range.

Spatial and spectral tailoring of visible light emission with mie resonances in silicon nanoantenna arrays

Similar to their plasmonic counterparts, dielectric nanoantennas have the ability to manipulate the emission properties of nanoscale emitters placed in their vicinity. Most importantly, they can increase the radiative decay rate of emitters by coupling to Mie-type resonances and/or shape the emission into directional patterns [1]. While considerable theoretical work has been dedicated to the study of coupled system consisting of Mie-resonant dielectric nanoantennas and nanoemitters [2, 3], experimental realizations of such systems are sparse due to the difficulty of integrating suitable nanoemitters with designed dielectric nanoantennas in a defined way [4]. Here we experimentally investigate visible light emission from square arrays (lattice constant is 560 nm) of silicon nanoantennas exhibiting dipolar magnetic Mie-type resonances. The nanoantennas are fabricated by electron beam-lithography on thin-films of hydrogenated amorphous silicon (a-Si:H) on a glass substrate. The unstructured wafers show an intrinsic photoluminescence (PL) centered at 740 nm. A scanning-electron micrograph (SEM) of a typical sample is shown in Fig. 1 (a).

Probing and mapping optical fields in Si disk arrays with Eu 3

We experimentally study enhancement of spontaneous emission of Eu3+ at the magnetic dipole transition by Mie-resonances in silicon nanodisks and estimate the spectral branching ratio as the function of the nanodisk radius.

Ultrafast modulation of femtosecond laser pulses in direct-gap semiconductor metasurfaces with magnetic resonances

We experimentally demonstrate ultrafast all-optical modulation in periodic arrays of subwavelength gallium arsenide nanodisks with localized Mie-type resonances. The efficient laser pulse modulation of up to 90% on the picosecond scale in the vicinity of the magnetic dipole resonance at a low pump fluence below 400 µJ/cm2 is shown by transient reflectance spectroscopy.