Active metasurfaces

Nanophotonics Australasia 2017, 2018

Recently, metasurfaces have gained popularity due to their ability to offer a spatially varying phase response, low intrinsic losses and high transmittance. Here, we demonstrate numerically and experimentally a silicon metasurface at THz frequencies that converts a Gaussian beam into a Vortex beam independent of the polarization of the incident beam. The metasurface consists of an array of sub-wavelength silicon cross resonators made of a high refractive index material on substrates such as sapphire and CaF 2 that are transparent at IR-THz spectral range. With these substrates, it is possible to create phase elements for a specific spectral range including at the molecular finger printing around 10 µm as well as at longer THz wavelengths where secondary molecular structures can be revealed. This device offers high transmittance and a phase coverage of 0 to 2π. The transmittance phase is tuned by varying the dimensions of the meta-atoms. To demonstrate wavefront engineering, we used a discretized spiraling phase profile to convert the incident Gaussian beam to vortex beam. To realize this, we divided the metasurface surface into eight angular sectors and chose eight different dimensions for the crosses providing successive phase shifts spaced by π/4 radians for each of these sectors. Photolithography and reactive ion etching (RIE) were used to fabricate these silicon crosses as the dimensions of these cylinders range up to few hundreds of micrometers. Large 1-cm-diameter optical elements were successfully fabricated and characterised by optical profilometry.

Advances in terahertz technology rely on the combination of novel materials and designs. As new devices are demonstrated to address the terahertz gap, the ability to perform high-efficiency beam control will be integral to making terahertz radiation a practical technology. Here, we use a metasurface composed of nonuniform dielectric resonator antennas on a ground plane to achieve efficient beam focusing at 1 THz. The dielectric resonators are made of high-resistivity silicon, which is a low-loss, nondispersive material for terahertz waves. The resonators operate around the resonance of the displacement current in the silicon, which is crucial to attaining high efficiency. The reflectarray's capacity to focus terahertz radiation is experimentally verified, and hence by the principle of antenna reciprocity, it can also be employed as a terahertz collimator. The demonstrated device can therefore be deployed for high-gain terahertz antennas. Further measurements show that the loss of the reflectarray is negligible, which confirms the high efficiency of the dielectric resonators. This finding will enable the design of efficient flat-profile terahertz reflectarrays and metasurfaces to serve arbitrary beam control requirements in the near and far fields.

IEEE Long Island Systems, Applications and Technology (LISAT) Conference 2014, 2014

Beam steering has been traditionally achieved by mechanical means. Even though these mechanical techniques have evolved over the past few decades, non-mechanical approaches, due to benefits such as increased speed, lower costs and reduced complexity, have gained considerable interest. In this work, we propose a non-mechanical beam steering of terahertz radiation method using a transient, or dynamic lithography process. The experimental setup is based on a pumpprobe technique, with a Ti:Sa pulse laser at 800 nm wavelength as laser source. The high power pump beam is used to "write" a metasurface pattern composed of v-shaped antennas on a high purity float zone silicon substrate. The wavefront of the pump beam is modulated by a liquid crystal spatial light modulator (LCSLM), such that, by the time the beam reaches the silicon substrate, the illumination pattern has the appearance of an array of antennas. Upon incidence on the silicon substrate, the beam generates electron-hole pairs in the illuminated areas, therefore creating structures with metallic-like properties. The presence of carriers and implicitly the metallic quality of the structures are ensured as long the radiation is incident on the substrate. This process was named transient or dynamic lithography, due to its non-destructive property relative to the silicon wafer. The probe beam, less powerful, is used to generate the terahertz signal. This is achieved by a photo-conductive antenna. Subsequently, the terahertz beam probes the pattern projected on the silicon substrate by the pump beam. Due to the electron-hole pairs previously generated in the substrate, the antenna structures will respond to the terahertz radiation in a way similar to metallic antennas. The terahertz beam is therefore steered by the pseudo-metallic antenna array. The detection is achieved with an un-cooled bolometer terahertz video camera. This beam steering technique is very promising due to its flexibility in quickly changing the direction of the steered beam, by "rewriting" the antenna arrays on the silicon substrate, without any mechanical movement of optical elements.

Advanced Optical Materials, 2019

Metasurfaces offer a highly flexible platform for controlling the propagation and localization of electromagnetic waves. Due to the relatively large size of commonly used resonators, various undesirable effects including spatial dispersion and spurious diffraction occur, thus limiting the metasurface performance. To overcome these problems, one straightforward approach is to utilize deeply subwavlength meta-units. In contrast to conventional approaches that minimize the resonator size by reshaping the metallic patches, we reshape the capacitive gaps, an approach which is more robust to material loss, minimizing the problem of overdamping. As an example, we introduce a novel design based on interdigital capacitors (meander gap) with extremely subwavelength gaps for use in the terahertz frequency range. The size of our new resonator can be reduced to below λ/30 in a reflective-type terahertz metasurface, while maintaining the 2π phase shift required for full wavefront control. Using an advanced electron-beam lithography technique, we perform a proof-ofconcept experiment and fabricate a 5mm × 5mm beam deflector, with the capacitive gaps as small as 300 nm (∼ λ/1130). The device performance is characterized using angle-resolved time-domain spectroscopy. Our study provides useful insight for ultra-compact metadevices based on deeply subwavelength meta-units working at terahertz frequencies and beyond.

Applied Physics Letters, 2017

Switchable metasurfaces fabricated on a doped epi-layer have become an important platform for developing techniques to control terahertz (THz) radiation, as a DC bias can modulate the transmission characteristics of the metasurface. To model and understand this performance in new device configurations accurately, a quantitative understanding of the bias-dependent surface characteristics is required. We perform THz variable angle spectroscopic ellipsometry (VASE) on a switchable metasurface as a function of DC bias. By comparing these data with numerical simulations, we extract a model for the response of the metasurface at any bias value. Using this model, we predict a giant bias-induced phase modulation in a guided wave configuration. These predictions are in qualitative agreement with our measurements, offering a route to efficient modulation of THz signals.

This paper covers our recent work on terahertz re-flectarray antennas, providing a broad, critical perspective, and contrasting different approaches. The reflectarray antenna is a well-established device that offers significant control and freedom over the directionality and characteristics of its radiation pattern. Such a capability is critical to the successful development of commercially viable terahertz technologies. In this paper, the design, fabrication, and experimental characterization of four terahertz reflectarray devices is presented, based on two different classes of terahertz resonator. The first class is the metallic resonator, and three such reflectarray devices are presented, with each offering its own particular birefringent behavior. The second class is the dielectric resonator, which promises higher efficiency than the metallic resonator, and one such reflectarray device is presented. Devices such as these provide significant design freedom for defining particular beam-shaping operations for diverse application requirements. It is hoped that, with future advances in terahertz resonator technology, reflectarray antennas will prove instrumental in facilitating numerous promising applications of the terahertz range, including high-volume communications, non-invasive medical imaging, and security screening.

Optics express, 2014

Metamaterials offer exciting opportunities that enable precise control of amplitude, polarization and phase of the light beam at a subwavelength scale. A gradient metasurface consists of a class of anisotropic subwavelength metamaterial resonators that offer abrupt amplitude and phase changes, thus enabling new applications in optical device design such as ultrathin flat lenses. We propose a highly efficient gradient metasurface lens based on a metal-dielectric-metal structure that operates in the terahertz regime. The proposed structure consists of slotted metallic resonator arrays on two sides of a thin dielectric spacer. By varying the geometrical parameters, the metasurface lens efficiently manipulates the spatial distribution of the terahertz field and focuses the beam to a spot size on the order of a wavelength. The proposed flat metasurface lens design is polarization insensitive and works efficiently even at wide angles of incidence.

We demonstrate a terahertz flat lens based on tri-layer meta-surfaces allowing for broadband linear polarization conversion , where the phase can be tuned through a full 2π range by tailoring the geometry of the subwavelength resonators. The lens functionality is realized by arranging these resona-tors to create a parabolic spatial phase profile. The fabricated 124-μm-thick device is characterized by scanning the beam profile and cross section, showing diffraction-limited focus-ing and ∼68% overall efficiency at the operating frequency of 400 GHz. This device has potential for applications in terahertz imaging and communications, as well as beam control in general.

2018

We report on THz MEMS sensors suitable for large focal plane arrays and readout schemes compatible with real-time imaging. Terahertz absorption near 100 %, optimized to particular monochromatic quantum cascade laser (QCL) illumination sources, was achieved using metal-dielectric metasurfaces. MEMS devices were designed using metasurface absorbers as structural components, allowing for streamlined fabrication of very efficient detectors in two different configurations. In the first scheme, bi-material sensors were used, where the heat from the absorbers is converted into mechanical deformation. The angular displacement, proportional to the absorbed THz radiation, was then optically probed. In the second configuration, THz to IR conversion was achieved whereas the front side of the metasurface absorbs THz and the backside served as an efficient infrared emitter, allowing its temperature to be probed directly by a commercial, thermal (infrared) camera. The devices are comprised of ultr...

Scientific Reports

constituted by subwavelength elements printed on a grounded dielectric slab. these antennas exploit the interaction between a cylindrical surface wave (sW) wavefront and an anisotropic impedance boundary condition (BC) to produce an almost arbitrary aperture field. They are extremely thin and excited by a simple in-plane monopole. By tailoring the BC through the shaping of the printed elements, these antennas can be largely customized in terms of beam shape, bandwidth and polarization. In this paper, we describe new designs and their implementation and measurements. It is experimentally shown for the first time that these antennas can have aperture efficiency up to 70%, a bandwidth up to 30%, they can produce two different direction beams of high-gain and similar beams at two different frequencies, showing performances never reached before.