All-Optical Reconfigurable Metamaterial Employing the Self-Assembly of CdTe Quantum Dots

Positively and negatively charged polyelectrolytes, namely, Poly(diallyldimethylammonium chloride) and Poly(styrene sulfonate), respectively, were employed to disperse and deploy negatively charged quantum dots on an otherwise passive metamaterial structure with a resonant frequency of 0.62 THz, by employing a layer-by-layer, self-assembly scheme. Upon exposure to a UV source with a wavelength of 365 nm the amplitude modulation was observed to increase with increases in the number of deposited bi-layers, until a modulation maximum of 2.68% was recorded enabling an all-optical, dynamically reconfigurable metamaterial geometry. Furthermore, amplitude modulation was subsequently observed to decrease with further increases in the number of layers employed due to quenching and shadowing effects. The experimental observations reported herein will enable the utilization of all-optical reconfigurable THz devices for communication and data transmission applications.


Introduction
The ability to control the amplitude, phase and/or polarization of electromagnetic waves remains of critical importance for all technological applications. This is also applicable to Terahertz (THz) radiation, ranging from 100 GHz to 5 THz that is of interest to scientists and technologists because electromagnetic waves at those frequencies can excite intra-and intermolecular resonances due to mo-lecular rotations or vibrations and Van de Waals forces [1] [2]. Additionally, THz electromagnetic waves exhibit a variety of useful characteristics, namely, THz photons are non-ionizing and most dielectrics are transparent to them, enabling a wide range of applications, from characterization of chemical substances and quality control [3] [4] [5] [6] [7] to contributions in communications, and conservation of cultural heritage [8] [9] [10] [11]. In terms of the size of the objects under consideration, it extends from the galactic scale to nanoscale systems [12] [13] [14] [15] [16]. Interestingly, THz devices are, in general, faster and more sensitive, compared to their counterparts in other regions of the electromagnetic spectrum [17]; however, the paucity of naturally occurring materials with a suitable performance in the THz regime has made the advance of terahertz devices and applications rather arduous. This shortcoming can be addressed by the judicious utilization of metamaterials, which are artificial media with subwavelength unit cells that can be designed to achieve arbitrary (negative or positive) permittivity and permeability values, and can be custom-tailored to exhibit resonances at a desired frequency even in the THz regime [18] [19] [20] [21] [22]. The resonance amplitude and frequency of operation of metamaterials are determined by the geometry and the materials utilized, and typically do not exhibit a reconfigurable behavior, which is a requirement in numerous applications. A variety of schemes have been suggested and explored to achieve reconfigurable metamaterial performance including the dynamic adjustment of the dimensions or the spacing of the structures which can be effected either by employing flexible surfaces [23] [24] or microactuating mechanisms frequently associated with MEMS technology [25] [26]. The use of phase-change materials, either thermal, electrical or electromagnetically controlled, can also attain such a goal [27] [28]. However, an all-optical reconfigurable device is highly desirable for THz communication and data transmission applications, and a dynamic reconfigurable performance could be achieved by modulating the dielectric characteristics of the media surrounding the metamaterial unit cell, for instance by the utilization of semiconducting quantum dots (QDs). Specifically, it has recently been reported that Cadmium Telluride (CdTe) QDs exhibit a variation in their dielectric characteristics when they are excited by an ultraviolet (UV) source [29]. However, their ability to modulate the response of a metamaterial array operating in the THz regime has not been demonstrated yet.

Quantum Dot Synthesis
The CdTe QDs employed as reconfigurable elements, were obtained by a hydrothermal synthesis method. In a typical synthesis, 0.0533 g (0.2 mmol) of Cadmium acetate dihydrate (Cd(CH 3 COO) 2 2H 2 O, 99.5%) were dissolved in 50 ml of deionized (DI) water in a 125-ml Erlenmeyer flask. Next, 18 µl of thioglycolic acid (TGA) were added and 1 M NaOH solution was incorporated dropwise to adjust the pH to a value between 10.5 -11. In a separate flask, 0.0101 g Open Journal of Inorganic Non-metallic Materials (0.04 mmol) of potassium tellurite (K 2 TeO 3 , 95%) were dissolved in 50 ml of DI water and added to the first solution. Subsequently, 0.08 g of sodium borohydride (NaBH 4 , 99.99%) was added to the mixture and the reaction was allowed to stir for 5 min. The solution was transferred to a three-neck round flask and attached to a Liebig condenser and refluxed under atmospheric conditions, at a temperature of 100˚C. QD size is determined by the reflux time. The solution was then washed with acetone and redispersed in deionized water to obtain a clean colloidal solution. A high-resolution TEM micrograph of the synthesized QDs is shown in the inset of Figure 1(b). Since the QDs are capped with TGA, their surfaces contain negatively-charged carboxylate groups [30] [31]. This makes CdTe QDs a suitable candidate for a layer-by-layer deposition employing positively and negatively charged polyelectrolytes, namely, Poly(diallyldimethylammonium chloride) and Poly(styrene sulfonate), respectively.

Quantum Dot Characterization
Quantum dot optical absorption characterization was performed employing an Ocean Optics Flame-S-UV-VIS spectrometer (Figure 1(a)). Bandgap energy and absorption are related by the following expression [32] ( ) where α is the experimentally collected absorption, h is Planck's constant, υ is the photon's frequency, E g is the quantum dot bandgap energy and A is a proportionality constant. The nominal value of r reflects the nature of the optical transition. For a direct allowed transition, r is set to be 2. The optical bandgap can then be estimated graphically employing a method described by Tauc [33] ( Figure 1(b)). The extracted bandgap value of CdTe QDs was estimated to be 3.11 eV. Alternatively, after a mathematical manipulation, the above equation can be rewritten as [34] ( ) According to the above equation, a plot of ( ) should exhibit a discontinuity when 0 g h E ν − → . However, due to impurities and QD size inhomogeneity, the discontinuity could be replaced by a maximum. Additionally, QD size can be estimated using Brus' quantum mechanical ap- where r is the radius, 10.2 =  is the relative permittivity of bulk CdTe, where D is the diameter of the QDs and λ is the wavelength of the first excitonic absorption peak. As shown in Figure 1( CdTe, and, therefore, larger than the size of the synthesized QDs, it evinces the strong confinement state of electrons and holes [37]. In order to conduct THz measurements employing colloidal quantum dots, water must be exchanged as the host solvent, due to its extremely high absorption coefficient [38]. A thiol exchange scheme was employed to make the QDs solvable on toluene, which has a much lower THZ absorption coefficient [39], [40]. To this end, 1 ml of 1-dodecanethiol was added to 1 ml of a clean solution of CdTe QDs. Subsequently, 2 ml of acetone were added to the mixture followed by 2 minutes of vortexing. The solution was then heated to 60˚C until the organic phase was reduced to 1 ml. After the QDs had been transferred to the organic phase, they were diluted with toluene and precipitated with methanol. The product was decanted and diluted once more with toluene.  ( ) where c is the volume fraction of colloidal QDs and h  is the permittivity of the host solvent, which, for CdTe QDs was toluene. Figure 4 shows the real and imaginary part of the permittivity of the QDs, after applying the aforementioned effective medium theory. Ostensibly, the real and imaginary parts of  increase when the nanoparticles are excited by an ultraviolet source. This change is thought to be related to the large exciton polarizability of the QDs, due to their atom-like response, which is present in the strong confinement state [44].   was a plane wave with the electric field aligned orthogonal to the capacitor plates in the center of the unit cell. An optical image of the microfabricated metamaterial structure is shown in Figure 5(b). Figure 6 shows the simulated and experimental transmission graphs featuring a resonance at 0.62 THz. The electric field distribution at the resonant frequency is illustrated in Figure 6(b). Ostensibly, the electric field is strongly confined within the capacitor area of the unit cell.

Metamaterial Fabrication
For Metamaterial fabrication a standard image reversal lift-off procedure was performed on a <100> high resistivity silicon substrate, with a nominal thickness of 400 μm. Silver was chosen as the material for the metallic array due to its relatively low-loss properties [47]. To this end, thermal evaporation was employed to deposit a silver layer with a thickness of 100 nm on top of a 20 nm layer of Open Journal of Inorganic Non-metallic Materials THz, the TM was observed to increase with all incident power density increases ( Figure 8(a)). This is thought to be due to the susceptibility of the photoexcited QDs dependence on the excitation density. That is, the number of incident photons per unit area has an impact on the polarizability of the excitons confined in the QDs [44].
In terms of the number of bilayers employed, the TM value increased as the number of bilayers increased from 6 to 10 and decreased for subsequent increases in the number of bilayers, i.e., 13 and 15 bilayers. Specifically, for 6 bilayers, the modulation increased from 0.2% to 1.3%, the modulation observed  with 10 bilayers increased from 1.84% to 2.68%, and with 13 bilayers, the modulation increased from 1.2% to 1.6%. Lastly, for 15 bilayers, the modulation increased from 0.13% to 0.48%, being the smallest modulation attained. Quenching and shadowing effects are thought to be responsible for the observed decrease in transmission modulation depth when employing 13 or more bilayers which is presented in Figure 8(b).

Conclusion
Colloidal CdTe QDs were chemically synthesized and utilized as a reconfigurable element in a previously fabricated metamaterial structure that exhibited a resonance frequency at 0.62 THz. Modulation due to an ultraviolet source with different power densities and different numbers of polyelectrolyte bilayers were probed. The experimental data revealed that the modulation is largest when 10 layers of QDs + PDDA are deployed and the maximum modulation achieved was 2.68%, with a power density reaching 20 mW/cm 2 . Furthermore, the proximity to a linear relation suggests that a larger modulation can be achieved at higher power densities. Additionally, the entire process is mass production compatible and can be integrated with other electronic circuitry microfabrication processes, making it attractive for a variety of technological applications