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.

Transition metal dichalcogenide materials have recently been shown to exhibit a variety of intriguing optical and electronic phenomena. Focusing on the optical properties of semiconducting WS2 nanotubes, we show here that these nanostructures exhibit strong light-matter interaction and form exciton-polaritons. Namely, these nanotubes act as quasi 1-D polaritonic nano-systems and sustain both excitonic features and cavity modes in the visible-near infrared range. This ability to confine light to subwavelength dimensions under ambient conditions is induced by the high refractive index of tungsten disulfide. Using "finite-difference time-domain" (FDTD) simulations we investigate the interactions between the excitons and the cavity mode and their effect on the extinction spectrum of these nanostructures. The results of FDTD simulations agree well with the experimental findings as well as with a phenomenological coupled oscillator model which suggests a high Rabi splitting of ∼280 meV. These findings open up possibilities for developing new concepts in nanotube-based photonic devices.

Second-harmonic generation (SHG) spectra of single and coupled porous silicon-based photonic crystal microcavities are studied in both frequency and wave vector domains. For the fundamental field resonant to the microcavity mode the second-harmonic intensity is enhanced by 102 times in comparison with that outside the photonic band gap. SHG spectroscopy in identical microcavities coupled through the intermediate Bragg reflector reveals two SHG peaks if the fundamental field is in resonance with the splitted mode of coupled microcavities. The spatial confinement of the resonant fundamental radiation is directly probed at the microcavity cleavage by scanning near-field optical microscopy.

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A form-birefringent plasmonic metamaterial of the subwavelength thickness is used to convert the light's polarization state in a way to cover the whole Poincaré sphere's surface by adjusting the experimental configuration. This optical anisotropy is induced by grating surface plasmon polaritons of a nanoslit array made in a thin golden film with the narrow spectral Fano resonance. Phase delay between linearly polarized states introduced by the sample reaches the value of 0.85 in the visible corresponding to the effective ordinaryextraordinary refractive index difference of ⌬n Ӎ 10.4. Thin metallic films shaped at the nanoscale have become of exceptional interest due to their electromagnetic response being highly dependent on the pattern configuration which allows one to pre-engineer their optical properties. Extraordinary optical transmission 1 media, negative refractive index metamaterials, 2 and artificially anisotropic 3,4 and chiral 5,6 metamaterials are the few examples of plasmonic objects acquiring their unusual optical properties through nanostructuring. Optical response of such materials is often defined by the excitation of propagating surface plasmon polaritons ͑SPPs͒ which are coupled onto the metallic surface through the periodicity of the structure itself. 7,8 In some cases, these excitations give rise to sharp Fano-type spectral lines 9-11 which have been recently intensively used for manipulation of transmittance of the nanostructured metal films from almost perfect 12,13 to suppressed transmission. 14 Controlling the polarization state of transmitted or reflected light with anisotropic plasmonic nanostructures has been studied for a long time 15 and contemporary results in this area include giant specific linear birefringence and dichroism [16][17][18][19][20] in the spectral vicinity of plasmonic resonances. However, high phase delays between ordinary and extraordinary waves could not be achieved with broad spectral lines of about 100 nm full width at half maximum used in preceding works for the reasons given in the discussion section of this paper.In this Brief Report we show how a narrow Fano resonance in a plasmonic metamaterial could be utilized to induce huge phase delay between transmitted linearly polarized eigenstates. It is demonstrated that the resulting birefringence can be controlled by varying the angle of incidence in the range of only 8°from ⌬n = 4.4 to ⌬n = 10.4, where ⌬n is the difference between effective refractive indices of ordinary and extraordinary waves. It is shown that the polarization state of the light output from a system with a Fano resonance could cover the whole Poincaré sphere which was unattainable with previously concerned structured films of subwavelength thicknesses.A system which optical response consists of a coherent superposition of Lorentz-resonance response and background signal would possess a spectral line shape called Fano resonance. 21 The general expression for the complex transfer function of a Fano-resonance system can be written as follows:where...