Ghazaleh Kafaie Shirmanesh

Dual-Gated Active Metasurface at 1550 nm with Wide (>300°) Phase Tunability

Active metasurfaces composed of electrically reconfigurable nanoscale subwavelength antenna arrays can enable real-time control of scattered light amplitude and phase. Achievement of widely tunable phase and amplitude in chip-based active metasurfaces operating at or near 1550 nm wavelength has considerable potential for active beam steering, dynamic hologram rendition, and realization of flat optics with reconfigurable focal lengths. Previously, electrically tunable conducting oxide-based reflectarray metasurfaces have demonstrated dynamic phase control of reflected light with a maximum phase shift of 184° ( Nano Lett. 2016 , 16 , 5319 ). Here, we introduce a dual-gated reflectarray metasurface architecture that enables much wider (>300°) phase tunability. We explore light-matter interactions with dual-gated metasurface elements that incorporate two independent voltage-controlled MOS field effect channels connected in series to form a single metasurface element that enables wider phase tunability. Using indium tin oxide (ITO) as the active metasurface material and a composite hafnia/alumina gate dielectric, we demonstrate a prototype dual-gated metasurface with a continuous phase shift from 0 to 303° and a relative reflectance modulation of 89% under applied voltage bias of 6.5 V.

Dynamically controlled Purcell enhancement of visible spontaneous emission in a gated plasmonic heterostructure

Emission control of colloidal quantum dots (QDs) is a cornerstone of modern high-quality lighting and display technologies. Dynamic emission control of colloidal QDs in an optoelectronic device is usually achieved by changing the optical pump intensity or injection current density. Here we propose and demonstrate a distinctly different mechanism for the temporal modulation of QD emission intensity at constant optical pumping rate. Our mechanism is based on the electrically controlled modulation of the local density of optical states (LDOS) at the position of the QDs, resulting in the modulation of the QD spontaneous emission rate, far-field emission intensity, and quantum yield. We manipulate the LDOS via field effect-induced optical permittivity modulation of an ultrathin titanium nitride (TiN) film, which is incorporated in a gated TiN/SiO2/Ag plasmonic heterostructure. The demonstrated electrical control of the colloidal QD emission provides a new approach for modulating intensity of light in displays and other optoelectronics.The dynamic control of light emission from quantum dots is generally controlled via optical or electrical pumping. Here, Lu et al. electrically control the local density of states around a quantum dot to modulate its visible light emission properties.

Towards dynamic polarization control with optical metasurfaces (Conference Presentation)

Optical polarization is an important characteristic of electromagnetic waves that has a significant impact on number of applications, such as information delivery, 3D imaging, and quantum computation. Metasurfaces, sort of artificially designed planar structure, have attracted immense attention due to their ability to control the amplitude and phase of electromagnetic waves at a subwavelength scale. Metasurfaces hold promise for the fields of nonlinear dynamics, light beam shaping, quantum computation, etc. Beside these promising applications, metasurfaces can also be used for versatile polarization generation in a compact device dimension. Therefore, metasurfaces can be used for creation of flat optical devices with novel functionalities. In this talk, I will discuss how we can generate versatile polarization states by using metasurfaces. Firstly, I will describe geometric phase metasurfaces, which can be used for passive polarization control. We demonstrate six metasurface chips integrated on a single sample, in which each chip is responsible for generating one specific polarization state thus generating versatile polarization sates. Subsequently, I will discuss a scheme of active polarization modulation by using indium tin oxide (ITO)-based tunable metasurfaces. By suitably biasing the metasurface, the linearly-polarized incident light can be actively converted to a cross-polarized, circularly-polarized or elliptically-polarized light.

>270-degree phase shift in aluminum gate tunable conducting oxide metasurfaces for continuous optical beam steering at 1550 nm (Conference Presentation)

In the last several years, metasurfaces have demonstrated promise as both flat optical elements to replace conventional three-dimensional components (prisms or lenses) as well as to access functions that are unachievable in conventional optics. To date, the functional performance of metasurfaces have typically been encoded at the time of fabrication, which fixes the achievable phase and amplitude for each elements in an array. However if actively controlled metasurface elements can be designed to dynamically control the phase shift and amplitude change imposed by each metasurface element, we could realize phased arrays to enable complex spatio-temporal wavefront engineering. nWe report here design and experimental demonstration of a tunable conducting oxide metasurface that achieves such active control by incorporating materials with voltage-tunable optical permivitties, such as indium tin oxide (ITO), into a metasurface [1]. We design a metasurface that consists of an aluminum back plane, HfO2 gate dielectric followed by a 14 nm thick ITO active layer, and a periodic array of aluminum patch antennas. We choose the dimensions of the Al antennas so that the antenna magnetic dipole resonance occurs at 1550 nm. By applying a gate bias between the Al antenna and ITO active layer, charge accumulation or depletion occurs at the ITO/HfO2 interface. This results in modulation of the ITO complex permittivity, thus altering the metasurface reflection phase and amplitude. The designed metasurface is capable of >270° phase shift. Our design enables independent control of each metasurface element enabling electrical control of the metasurface phase profile, which is an essential requirement for demonstration of continious beam steering.n n[1] Y.-W. Huang et al., “Gate-Tunable Conducting Oxide Metasurfaces”, Nano Letters 16, 5319-5325 (2016).

Plasmonic nanophotonic modulators

Developing a compact, low power and high speed electro-optic modulator is crucial for overcoming the performance bottleneck of electronics. We review progress in chip based silicon compatible plasmonic modulator design, and discuss recent designs which have switching energy close to 1 fJ/bit.

Tunable optical response and purcell enhancement of gated plasmonic structures

We experimentally demonstrate plasmonic nanostructures that enable dynamic electrical control of the phase and/or amplitude of the plane wave reflected from the nanostructures. We also demonstrate dynamically controlled Purcell enhancement of spontaneous emission of InP quantum dots (QDs) coupled to plasmonic heterostructures.

Field-effect modulation of the local density of optical states in a reflectarray metasurface(Conference Presentation)

During recent years, advances in the design of arrays of subwavelength optical elements with special electromagnetic properties have enabled quasi two-dimensional structures that control and manipulate electromagnetic phase, amplitude and polarization. Active control of the response of metasurfaces is possible using transparent conducting oxides such as Indium Tin Oxide (ITO) as a tunable active material [1]. Changing the complex permittivity of ITO by applying a voltage yields modulation of reflected wave phase and amplitude. To achieve this, we designed subwavelength antenna arrays consisting of a gold back reflector and gold fishbone antennas. Planar dielectric layers containing a gate tunable layer of ITO are sandwiched between the back reflector and the antenna. The obtained structure shows resonance around 1.5 µm. As a result, based on the 1.54 µm photoluminescence emission of Er doped Al2O3 films, we embedded trivalent erbium ions as quantum emitters inside an alumina host within the metasurface in order to enhance the local density of optical states (LDOS). Simulations indicate the designed structure shows a significant LDOS enhancement (of order of hundreds). By applying a bias between the antenna and the ITO layer, across an HfO2 gate dielectric, we can control the permittivity of ITO and hence dynamically modulate the decay rate of quantum emitters embedded within the structure. In this way, we can achieve LDOS enhancement modulation of about 325%. 1. Y. W Huang, H. W. H. Lee, R. Sokhoyan, R. Pala, K. Thyagarajan, S. Han, D. P. Tsai, H. A. Atwater, “Gate-tunable conducting oxide metasurfaces”. (arXiv:1511.09380).