G. Kenanakis

Flexible chiral metamaterials in the terahertz regime: a comparative study of various designs

Five different chiral metamaterials in the terahertz (THz) regime, fabricated on fully flexible polyimide substrates, are comparatively studied via numerical calculations and experimental measurements. The chiral properties of these metamaterials, which are discussed based on their optical activity, circular dichroism, and the retrieved effective parameters, show pronounced pure optical activity (larger than 300°/wavelength), as well as important circular polarization generation and filtering capabilities. Negative refractive index is also obtained for all the considered designs.

Optically controllable THz chiral metamaterials

Switchable and tunable chiral metamaterial response is numerically demonstrated here in different uniaxial chiral metamaterial structures operating in the THz regime. The structures are based on the bi-layer conductor design and the tunable/switchable response is achieved by replacing parts of the metallic components of the structures by photoconducting Si, which can be transformed from an insulating to an almost conducting state through photoexcitation, achievable under external optical pumping. All the structures proposed and discussed here exhibit frequency regions with giant tunable circular dichroism, as well as regions with giant tunable optical activity, showing unique potential in the achievement of active THz polarization components, like tunable polarizers and polarization filters.

Self-organization approach for THz polaritonic metamaterials

In this paper we discuss the fabrication and the electromagnetic (EM) characterization of anisotropic eutectic metamaterials, consisting of cylindrical polaritonic LiF rods embedded in either KCl or NaCl polaritonic host. The fabrication was performed using the eutectics directional solidification self-organization approach. For the EM characterization the specular reflectance at far infrared, between 3 THz and 11 THz, was measured and also calculated by numerically solving Maxwell equations, obtaining good agreement between experimental and calculated spectra. Applying an effective medium approach to describe the response of our samples, we predicted a range of frequencies in which most of our systems behave as homogeneous anisotropic media with a hyperbolic dispersion relation, opening thus possibilities for using them in negative refractive index and imaging applications at THz range.

Parametric investigation and analysis of fishnet metamaterials in the microwave regime

We study through experiments and associated simulations the electromagnetic response of microwave fishnet metamaterials and its dependence on the systems geometrical parameters. Our study verifies the validity of an earlier proposed inductor-capacitor (LC) circuit description of the fishnet design and reveals a left-handed response with high transmittance for a wide variety of geometrical structure parameters. This study paves the way to achieve optimized left-handed fishnet metamaterial designs.

Temperature induced modification of the mid-infrared response of single-walled carbon nanotubes

The temperature dependences of the absorbance spectra of thin free-standing single-walled carbon nanotube (SWCNT) films were studied in the infrared range (700–6200 cm−1) while heating the air from 300 to 575 K. The observed temperature variation in the infrared absorbance spectra has been explained by two different physical factors. The first one is the strong temperature dependence of the conductivity of p-type doped semiconducting SWCNTs. The second one is the temperature dependence of electron relaxation time of intraband electron transitions in metallic SWCNTs. The possibility of the separation of contributions from the interband and intraband transitions to the infrared spectra of SWCNT films has been demonstrated.

Distinction between breast cancer cell subtypes using third harmonic generation microscopy

Third Harmonic Generation (THG) microscopy as a non-invasive, label free imaging methodology, allows linkage of lipid profiles with various breast cancer cells. The collected THG signal arise mostly from the lipid droplets and the membrane lipid bilayer. Quantification of THG signal can accurately distinguish HER2-positive cells. Further analysis using Fourier transform infrared (FTIR) spectra reveals cancer-specific profiles, correlating lipid raft-corresponding spectra to THG signal, associating thus THG to chemical information. THG imaging of a cancer cell.

Electromagnetic response of anisotropic eutectic metamaterials in THz range

We study the electromagnetic (EM) response of anisotropic eutectic metamaterials, consisting in cylindrical polaritonic LiF rods embedded in a KCl host. The specular reflectance of the samples was measured at far infrared (3–12 THz). The sample reflection was simulated by modeling the eutectic structure and solving numerically Maxwell equations for the EM fields. The reflectance was also calculated from simple effective response functions models. A good agreement was obtained between experimental and calculated spectra. From the effective response functions calculations, we obtained a range of frequencies in which the system behaves as a homogeneous effective anisotropic media, with a hyperbolic dispersion relation, opening possibilities for negative refraction and focusing applications.

Detection of the T cell activation state using non-linear optical microscopy

The ability to monitor the activation state of T-cells during immunotherapy is of great importance. Although specific activation markers do exist, their abundance and complicated regulation cannot definitely define the activation state of the cells. Previous studies have shown that Third Harmonic Generation (THG) imaging could distinguish between activated versus resting microglia and healthy versus cancerous cells, mainly based on their lipid-body profiles. In the present study, mitogen or antigen-stimulated T-cells were subjected to THG imaging microscopy. Qualitative and quantitative analysis showed statistically significant increase of THG mean area and intensity in activated versus resting T-cells. The connection of THG imaging to chemical information was achieved using Raman spectroscopy, which showed significant differences between the activation processes and controls, correlating of THG signal area with cholesterol and lipid compounds, but not with triglycerides. The obtained results suggested a potential employment of nonlinear microscopy in evaluating of T-cell activation, which is expected to be largely appreciated in the clinical practice.

THz polarization control with chiral and bianisotropic metamaterials and metasurfaces

Chiral metamaterials, where magnetoelectric coupling is orders of magnitude larger than that of natural chiral media, offer unique possibilities for the control and manipulation of the electromagnetic wave polarization. Such a control is particularly useful in the THz range, where natural materials do not show strong response, and therefore the optical components available are limited, while the possibilities in terms of applications are huge.

Chiral metamaterials: A tool for THz polarization control

Metamaterials, i.e. engineered composite materials structured in sub-wavelength building blocks, attract nowadays continuously increasing attention. This is mainly due to the unique properties of those materials, which entail new possibilities for the manipulation of electromagnetic (EM) waves. With metamaterials one is able to achieve permittivity and permeability values ranging from very large to negative, combined negative permittivity and permeability leading to negative index of refraction, unusual anisotropic response, giant chirality values in chiral metamaterial structures, etc. The giant chirality values achievable in chiral metamaterials, orders of magnitude larger than the chirality of natural chiral materials, empowers chiral metamaterials with unique possibilities for the control of electromagnetic wave polarization. Such a control is extremely useful in the THz regime, where there is a serious lack of polarization control components. In this talk we present a variety of chiral metamaterials operating in the THz regime and we demonstrate, both theoretically and experimentally, the THz-polarization control capabilities of those metamaterials. The main structures discussed are shown in Fig. 1 [1]. These structures are planar designs based on the bi-layer conductor configuration, i.e. the chiral response derives from the electromagnetic coupling of a pair of conducting elements (along z-direction on Fig. 1), mutually twisted as to break the mirror symmetry of the structures along propagation direction. The structures are of micrometer length scale, they have been fabricated by UV lithography in a flexible polyimide substrate, they have been characterized by FTIR spectroscopy and analyzed theoretically and computationally by the finite element method. The results show in all cases extremely large optical activity, large circular dichroism and negative refractive index response for both left- and right-handed circularly polarized waves [1]. Incorporating properly in the structures a photoconducting material (e.g. Si), which can be transformed from an insulating to a conducting state using photo-excitation, we show that one can achieve dynamically tunable or switchable chiral response, demonstrated as switchable optical activity or switchable transmitted wave ellipticity [2]. Thus the structures can offer a great tool for the realization of dynamic polarization control components, such switchable polarization filters, modulators, wave plates and others, extremely valuable in the THz regime.