Mohsen Rahmani

Realization of Variable Three‐Dimensional Terahertz Metamaterial Tubes for Passive Resonance Tunability

A novel three-dimensional metamaterials tube is investigated to achieve passive resonance tunability. Varying its diameter, the resonance frequency shows a blue-shift from 0.75 to 1.13 THz. FDTD simulation reveals that this resonance tunability is attributed to destructive magnetic coupling among neighboring SRRs on the curved surface of the metamaterials tube. This blue-shift provides a new approach to achieve fl exible resonance tunability into higher terahertz frequency. Meanwhile, the 3D solidcore metamaterials tube, wrapping the planar meta materials against transparent sample in terahertz regime, can be applied to identify the materials by resonance shift. This ultra-sensitive sensing means can measure a refractive index change down to 0.0075. Terahertz metamaterials have attracted much research interest in the past decade due to their unique electromagnetic properties. [ 1–4 ] A split ring resonator (SRR) is one of the typical metamaterials structures. In order to extend its resonance bandwidth, terahertz metamaterials with resonance tunability were developed recently. [ 5 , 6 ] Previous experimental studies on tunable metamaterials are focused on active tunability by tuning the optical properties of the substrates using external sources. [ 7–11 ] Passive tunability, without any external constituents, is of great signifi cance to realize resonance tunability with simple setup and convenient operation. Structural tunability in the multi-layer metamaterials was proposed to tune the resonance frequency by the relative position of SRRs in the neighboring layers, which demonstrates a potential way to realize passive tunability. [ 12–14 ]

Beyond the hybridization effects in plasmonic nanoclusters: diffraction-induced enhanced absorption and scattering.

It is demonstrated herein both theoretically and experimentally that Youngs interference can be observed in plasmonic structures when two or three nanoparticles with separation on the order of the wavelength are illuminated simultaneously by a plane wave. This effect leads to the formation of intermediate-field hybridized modes with a character distinct of those mediated by near-field and/or far-field radiative effects. The physical mechanism for the enhancement of absorption and scattering of light due to plasmonic Youngs interference is revealed, which we explain through a redistribution of the Poynting vector field and the formation of near-field subwavelength optical vortices.

Use of a gold reflecting-layer in optical antenna substrates for increase of photoluminescence enhancement.

We report on a straightforward way to increase the photoluminescence enhancement of nanoemitters induced by optical nanotantennas. The nanoantennas are placed above a gold film-silica bilayer, which produces a drastic increase of the scattered radiation power and near field enhancement. We demonstrate this increase via photoluminescence enhancement using an organic emitter of low quantum efficiency, Tetraphenylporphyrin (TPP). An increase of the photoluminescence enhancement by a factor larger than three is observed compared to antennas without the reflecting-layer. In addition, we study the possibility of influencing the polarization of the light emitted by utilizing asymmetry of dimer antennas.

Temporal broadening of attosecond photoelectron wavepackets from solid surfaces

The response of solids to electromagnetic fields is of crucial importance in many areas of science and technology. Many fundamental questions remain to be answered about the dynamics of the photoexcited electrons that underpin this response, which can evolve on timescales of tens to hundreds of attoseconds. How, for example, is the photoexcited electron affected by the periodic potential as it travels in the solid, and how do the other electrons respond in these strongly correlated systems? Furthermore, control of electronic motion in solids with attosecond precision would pave the way for the development of ultrafast optoelectronics. Attosecond electron dynamics can be traced using streaking, a technique in which a strong near-infrared laser field accelerates an attosecond electron wavepacket photoemitted by an extreme ultraviolet light pulse, imprinting timing information onto it. We present attosecond streaking measurements on the wide-bandgap semiconductor tungsten trioxide, and on gold, a metal used in many nanoplasmonic devices. Information about electronic motion in the solid is encoded on the temporal properties of the photoemitted electron wavepackets, which are consistent with a spread of electron transport times to the surface following photoexcitation.

Directional second harmonic generation from AlGaAs nanoantennas (Conference Presentation)

Optical nanoantennas possess great potential for controlling the spatial distribution of light in the linear regime as well as for frequency conversion of the incoming light in the nonlinear regime. However, the usually used plasmonic nanostructures are highly restricted by Ohmic losses and heat resistance. Dielectric nanoparticles like silicon and germanium can overcome these constrains [1,2], however second harmonic signal cannot be generated in these materials due to their centrosymmetric nature. GaAs-based III-V semiconductors, with non-centrosymmetric crystallinity, can produce second harmonic generation (SHG) [3]. Unfortunately, generating and studying SHG by AlGaAs nanocrystals in both backward and forward directions is very challenging due to difficulties to fabricate III-V semiconductors on low-refractive index substrate, like glass. Here, for the first time to our knowledge, we designed and fabricated AlGaAs nanoantennas on a glass substrate. This novel design allows the excitation, control and detection of backwards and forwards SHG nonlinear signals. Different complex spatial distribution in the SHG signal, including radial and azimuthal polarization originated from the excitation of electric and magnetic multipoles were observed. We have demonstrated an unprecedented SHG conversion efficiency of 10-4; a breakthrough that can open new opportunities for enhancing the performance of light emission and sensing [4].nnReferencesn[1] A. S. Shorokhov et al. Nano Letters 16, 4857 (2016).n[2] G. Grinblat et al. Nano Letters 16, 4635 (2016).n[3] S. Liu et al. Nano Letters 16, 7191 (2016).n[4] R. Camacho et al. Nano Lett. 16, 7191 (2016).

Linearly polarized dipolar second harmonic generation from gold nano-antennas by controlling their radiation phase

A recurring theme in optics and photonics is the ability of metal nanostructures to imbue artificial materials with new functions. Metallic nano-antennas [1], so-called meta-atoms, are the building blocks of such metamaterials that boast unusual linear [2, 3] and nonlinear [4- 6] characteristics not observed in natural materials. Recently, nonlinear metamaterials have generated considerable excitement; while nonlinear effects in natural materials must gradually accumulate weak nonlinearity across macroscopic crystal dimensions, a small volume of metamaterial [7, 8], and even isolated antennas [9-11], can create a surprisingly strong effect. This capability stems from additional nanoscopic degrees of freedom that include couplings between the constituent nanoparticles within antennas or between antennas and material resonances [7, 9, 10]. Second harmonic generation (SHG) enables diversely coloured light sources through an energy exchange process between light at initial, ω, and final, 2ω, frequencies typically in optically thick non-linear crystals. Recently, metamaterials imbued with nonlinearity from their constituent nano-antennas have generated excitement by opening the possibility of wavelength-scale nonlinear optics. The nonlinear selection rules typically prevent dipole SHG from nano-antennas leading to much discussion concerning the best geometries. Following recent literature, we examine individual antennas that are designed to efficiently radiate SHG (ηSHG > 10-7W-1, corresponding to |χeff(2)| > 40pmV-1) by incorporating simple resonant elements tuned to light at both ω and 2ω. We show that antennas exhibiting both non-centro-symmetry and a mirror symmetry plane exhibit the strongest normal incidence SHG emission with a high degree of linear polarization. We confirm this by direct measurement of SHG in the back focal plane of isolated nano-antennas in a variety of configurations. We also show that antennas incorporating multiple 2ω-elements provide greater flexibility to ensure that both symmetries are met. By interpreting the SHG emission patterns using a multi-dipole model we also identify the phase and the orientation of each antenna element as key design parameters to control SHG emission pattern and polarisation. Metamaterials incorporating such antennas could enable compact nonlinear photonic nanotechnologies.

Hybrid gap plasmon waveguides on the silicon-on-insulator platform for adiabatic nanofocusing

We present a new class of silicon hybrid gap plasmon waveguides designed for adiabatic nanofocusing. Using a 3-photon absorption process in quantum dots, we show a 167±26 intensity enhancement for a 24nm wide waveguide.

Exploiting plasmonics for THz and infrared sensing

Bridging the gap in scale between the THz wavelength and the biomolecule sample sizes to be sensed is a challenging task. We tackle this mismatch by developing sensing platforms based on the concepts of designer surface plasmon polaritons and localized plasmons. We show that corrugated metallic surfaces, complementary split ring resonators and arrays of micro-dipoles provides enhanced THz-matter interaction times and strong interrogating evanescent fields. We will also demonstrate how transformation optics can be used to design broadband plasmonic semiconductor and metallic gap micro-antennas for terahertz-to-visible applications.

Attosecond streaking on gold films

We demonstrate attosecond streaking on thin Au films. This is the first streaking measurement on Au, and also the first demonstration of streaking on a non-crystalline, non-UHV-prepared sample.