Work packages

For the designing and modelling of the chiral metamaterials/metasurfaces we will employ complementary tools and approaches. Some of them have already been developed and used extensively by the consortium members, while others will be developed in the course of the project implementation.

For the theoretical analysis we will exploit different models to describe the helicity-sensitive transmission and reflection of layered chiral metamaterials ranging from simplified analytical approaches to calculations based on the electromagnetic response of the ensemble of chiral and bianisotropic scatterers. These models with help to reveal the physics behind the response observed (e.g. in the numerical or actual experiments) and also to predict main features of the metamaterial response, which depend on e.g. specific symmetry of individual meta-atoms and their mutual arrangements.

Graphene electromagnetics for metasurface tunability will be developed in line with the work done at the UEF for graphene multilayers when tunability have been achieved by using different substrates.

Electronic and optical properties of graphene and CNTs will be simulated by non-empiric quantum chemical density functional theory (DFT) method in the cluster approximation.

Plan A. The multilayer dielectric/semiconductor/metal substrates will be fabricated at the UEF and NAPATECH using the physical, atomic layer and chemical vapor deposition (CVD) techniques. The CVD will be employed to produce single- and multilayer graphene sheets, which will be used for enhancing and tuning the rotatory power of the chiral metamaterials by external stimuli.

The fabricated graphene sheets will be characterized in terms of homogeneity, crystallinity, roughness, and local conductivity. Scanning tunnelling microscopy studies will reveal morphology, local electronic properties, and a local interaction between the 2D sheet and the substrate. Angle-resolved photoemission spectroscopy (ARPES) technique at the IPST will be employed to study the band structure of the 2D materials and multilayers. Surface conductivity of the samples will be measured by using methods of the transmission THz spectroscopy.

Plan B. The chiral metamaterials based on the vertically alighted arrays of CNTs. UEF has all necessary expertise, equipment and manpower to work with CNT including patterning CNT arrays using laser pulses. In order to prevent the growth of the CNT forest on the prescribed areas of the silicon substrate, one can cover these areas by a thin copper film. Thus combining CNT synthesis with lithography techniques open an opportunity to control the geometry of the CNT forest pattern. Alternatively, the CNT forest on the silicon substrate can be sculptured by the laser beam, which can remove the nanotubes from the prescribed areas of the substrate. It has been demonstrated that CO2 laser beam can be employed for formation engraving a periodic array of the pyramidal-shaped CNT forest islands, which can be employed as 2D photonic crystal for the THz spectral range.

The chiral metamaterials based on multilayer dielectric/semiconductor/metal substrates will be fabricated at the ORC by using femtosecond micromachining. The substrates will be fabricated at the UEF and NAPA Technologies by the physical and atomic layer deposition (see Fig.5). The layer-specific micromachining [70, 71] will be performed by changing the position of the focal point of the laser beam along the propagation direction with micron resolution. The using of the femtosecond optical parametric amplifier for micromachining will enable laser writing in the spatially separated inner layers of the composite substrate.

The chiral metasurfaces based on the vertically aligned array of multiwall nanotubes will be fabricated at the the UEF by using a laser engraving technique followed by polymer covering. The metasurfaces will be designed for frequency-selective reflection in the THz range that enable polarization control by irradiation of intense laser beams and other external stimuli.

The chiral metamaterials that enable electric field polarization control will be fabricated by using metal (e.g. aluminium or gold) film deposited on a dielectric substrate. By using the UEF lithography facilities we will create a chiral (e.g. gammadion) hole pattern in the metal film preserving its continuity. The patterned film will be covered by dielectric (e.g. alumina) spacer followed by graphene sheet. By placing a thin layer of a solid electrolyte (e.g. polyethylene oxide and lithium perchlorate) on the top of graphene we will control the graphene Fermi energy and hence the surface conductivity of the 2D sheet.

3D metallic chiral structure, which is expected to show strong optical activity, will be fabricated at the IPST by using a unique high-resolution 3D printing technology (RECILS).  The morphology of the metasurface will be studied by using STM/SEM/HRTEM and optical microscopy techniques. The continuity of the metasurface will be examined by using AFM measurements, while its electrical conductivity metasurface will be compared with that of the virgin metal film. The crystallinity of the deposited graphene sheet will be studied by using micro-Raman measurements.

To probe ultrafast relaxation dynamics and to distinguish between different channels of relaxation, including charge transfer between layers forming the chiral meta-atom, we will employ multi-color pump-probe spectroscopy with ~100 fs temporal resolution. The proposed method is an invaluable tool to get a complete picture (temporal, spectral, spatial) of the excited carrier distribution. To realize selective excitation of different materials forming the heterostructure, we will use tunable pump (OPA). To probe the pump induced changes, femtosecond continuum with broad spectrum, spanning from 1700 to 400 nm will be employed. By measuring the pump induced transient changes in absorption spectrum, we will also be able to probe the temporal evolution of the heterostructure response in a wide (near IR-VIS) spectral range.

The measurement of 2D materials transmission/reflection in the THz range will be performed at the UEF and IPST. The THz-TDS setup based on GaAs emitter/detector and femtosecond Ti:Sapphire regenerative amplifier will be employed.

The measurements of the polarization state of the THz radiation will be performed at IPST by using a ultra-high sensitive THz polarization measurement system. This THz-TDS system is based on a GaP(111) nonlinear crystal for detection and Yb-based regenerative amplifier.

The results of the EM measurements will allow us to reveal effective permittivity and ac conductivity of the fabricated materials. The obtained material parameters will be fed back to synthesis stage to optimize (in terms of the performance) EM properties of the material.

In order to tune circular birefringence and dichroism we will irradiate the array of chiral meta-atoms with intense laser pulses capable to alter sheet conductivity of the 2D material and hence the constitutive parameters of the metamaterial. The measurements of the light-induced change of the optical rotation power and circular dichroism in the THz range will be performed at the IPST using an optical-pump terahertz-probe spectroscopy system. The system provides 50-fs pulses tunable in the visual and near-IR spectrum range that incident on the metamaterial prior the arrival of the terahertz radiation. In order to measure the THz polarization change varying in a very wide time range spanning from femtoseconds to nanoseconds, the system based on combination of the 78-MHz oscillator and 120-kHz regenerative amplifier as femtosecond light sources will be employed at the IPST.

In order to obtain the transmission coefficients of circularly polarized THz waves, we will measure the pump-induced change of the co- and cross-polarization transmission coefficients for orthogonal linear polarizations. By varying the geometry of meta-atoms and/or their mutual arrangement on the substrate along with conductivity/carrier mobility of graphene we will maximize the performance of the chiral metamaterial in the circular polarization control. To achieve control of the circular polarization by electric field, we will employ chiral metamaterial with solid electrolyte layer. By changing the gate voltage we will change the electronic properties of the 2D sheet and hence will arrive at the modulation of the circular birefringence and dichroism of the chiral metamaterial.

One of the key advantages of the proposed approach is flexibility. By varying the meat-atoms structure and their mutual arrangement in the array CHARTIST will develop and fabricate the prototype of THz component for generation and control of the circularly polarized THz radiation. R&D KPIs are collected through the theoretical estimations, THz preliminary experiments and benchmarking. The next step, that is integration of the THz polarizer component with other photonics components will be done beyond CHARTIST at later stage.