Terahertz Modulation of Hexagonal-Shaped Metamaterials Employing ZnO Quantum Dots

The utilization of UV excitation to verify the terahertz (THz) wave modulation of hexagonal-shaped metamaterial (MM) arrays coated with synthesized photoluminescent, down-shifting ZnO quantum dots (QDs) of two different radius sizes, namely, 3.00 nm (pH 10) and 2.12 nm (pH 12), respectively is reported. In order to characterize the behavior of the MM before and after deployment of the ZnO QDs, THz time domain spectroscopy in transmission mode was employed. Upon exposure to UV excitation, the collected amplitude modulation values were 9.21% for the pH 12 and 4.55% for the pH 10 ZnO QDs, respectively. It is anticipated that the ability to actively tune the performance of otherwise passive structures will promote the proliferation of THz signal modulation devices in the near future.


Introduction
The terahertz (THz) portion of the electromagnetic spectrum is located between the microwave and infrared segments and has remained relatively underdeveloped because the power output of conventional electronic circuitry (i.e., IMPATT, MMIC, Gunn, TUNNET, multiplexers, photomixers, RTD, etc.) drops significantly as the transition is made from the microwaves to the THz region. Similarly, photonic structures (i.e., III-V lasers, QCL, etc.) also exhibit a similar and noticeable deterioration in the power output as the transition is made from infrared to THz frequencies. In view of the unrelenting saturation of the electromagnetic spectrum, there is an increased interest in developing passive and active devices with suitable performance in the THz portion of the spectrum, including tunable structures that could alleviate said saturation. Additionally, THz electromagnetic waves are nonionizing and, for the same reason, attractive for applications where microwave interactions with organic materials could be a source of concern [1]- [10]. One option to overcome the performance limitations of conventional electronic circuitry and/or photonic devices at THz frequencies is the utilization of metamaterials (MMs), which are subwavelength, unit-cell laboratory-made structures that are fabricated to exhibit specific electromagnetic properties not encountered in naturally available materials. The topology encountered in MMs customarily consists of a metallic or polymeric array, in which the passive electromagnetic properties are determined by the dimensions of the unit cell, the fill factor of said cells and the materials employed in their fabrication [11]. Incorporating functional materials to MM designs capable of responding in real time to an external stimulus would improve tunability and, more generally, enable the demonstration of THz modulation devices [12] by modifying signal intensity, phase, propagation direction, beam shape or resonant frequency. To this end, the utilization of UV excitation with a wavelength of 400 nm, to verify the modulation of THz waves of a metamaterial array coated with previously synthesized photoluminescent, down-shifting ZnO QDs synthesized in colloidal solutions with pH values of 10 and 12, respectively, is discussed in this work.

Experimental Details
The employed MM array (see Figure 1) which has been described elsewhere [13], was fabricated using a conventional lift-off process on high resistivity, <100> silicon wafers and had dimensions such that the array exhibited a resonance frequency of 1 THz. The metallization of the tested MM structures was performed by evaporating 200 nm of silver on a 20 nm layer of chromium that served as an adhesion layer.

ZnO QD Synthesis
Colloidal ZnO QDs were synthesized at room temperature by a sonochemical method [14] [15] in which acoustic cavitation precludes the formation of intermediate products, including Zn(OH) 2 , converts the reactants into ZnO QDs and calcination steps are not required. This method has proven useful for accelerating chemical reactions in liquid-solid systems [14]. Specifically, ZnO QD synthesis was accomplished by preparing an initial solution of 0.02 M zinc acetate (ZnAc) (99.99%, Sigma-Aldrich) in pure ethanol. A second and independent solution of 0.01 M of lithium hydroxide (LiOH) (≥98% Sigma-Aldrich) was prepared also in pure ethanol. Subsequently, the LiOH solution was incorporated dropwise to the ZnAc solution until a pH value of 10 or 12 was obtained. Once the targeted pH value was reached, the mixture was placed in a sonication bath for 3 hours [16] [17]. The chosen pH values were found to yield stable nanoparticles and relatively minimal agglomeration. Furthermore, the ultimate size of the obtained QDs was determined by the pH value of the final solution employed during synthesis and observed to be in agreement with the dependence of ZnO nanoparticle size on pH reported elsewhere [18]. A summary of the synthesis is shown in Figure 2. The ZnO QDs were then deposited onto the surface of the MM shown in Figure 1.
In order to characterize the synthesized ZnO QDs in the terahertz regime, the nanoparticles were dispersed in a polydimethylsiloxane (PDMS) matrix. To this end, the ZnO QDs were harvested by drying the final solution on a hotplate maintained at 70˚C, and prior to dispersing them in the aforementioned matrix, the QD samples were thoroughly pulverized using a mortar. Once the solvent had completely evaporated, the ZnO QD powder was mixed with a PDMS solution that had been previously prepared by mixing 5 wt% of a curing agent in a silicone elastomer [19] [20]. The mixture of PDMS and ZnO QDs was then cured at 90˚C for 3 hours (see Figure 3).

ZnO QD Absorption and Emission Characterization
The UV-vis absorption characterization of the ZnO QDs was performed using  an Ocean Optics Flame-S-UV-VIS spectrometer and the collected absorption spectra corresponding to nanocrystals synthesized with the previously mentioned colloidal solution pH values of 10 and 12, are shown in Figure 4, along with their corresponding photoluminescent response. The emission wavelength is directly related to quantum dot size, therefore, the larger the nanoparticles the longer the wavelength of their characteristic photoluminescent peak [21] [22]. Thus, QDs synthesized with a pH value of 12 have an emission peak located at 510 nm, while QDs synthesized with a pH value of 10 have the peak located at 540 nm, indicating that lower pH values produce larger nanoparticles.
The observed QD absorbance measurements are related to QD bandgap by the following expression where α is the absorption coefficient, A is an optical constant, m represents the type of optical transition in the semiconducting material; specifically, m can only take the values of 1/2, 2, 3/2 or 3 corresponding to allowed direct, allowed indirect, forbidden direct and forbidden indirect, respectively [23], and g E is the band gap of the semiconducting quantum dots. Multiplying Equation (1) by hν , obtaining the natural logarithm and finally taking the derivative with respect to hν , the resultant equation is The bandgap can then be estimated by plotting Equation (2) with respect to energy [23], and this approach permits the identification of the inflection point (i.e., the bandgap value) where the slope changes sign (see Figure 5). In other words, the bandgap value will be determined as a discontinuity at   Since the wavelength emission is directly related to QD size, i.e., the larger the particle, the longer the wavelength of emission, the emission peak wavelength observations corroborate that higher pH values during synthesis indeed lead to smaller ZnO quantum dots. Once the bandgap value had been calculated, the size of the QDs was estimated employing Brus' quantum mechanical approximation [ where g E is the band gap energy of the semiconductor quantum dots, Bulk E is the band gap energy of the bulk semiconductor material,  is the reduced Planck constant, * e m and * h m represent the effective mass of electron and hole of the material under consideration,  is the bulk relative permittivity, and r is the QD radius. For the pH 10 ZnO QDs we estimated a radius of 3.00 nm and for the pH 12 ZnO QDs we obtained a radius of 2.12 nm. To corroborate the estimated sizes, transmission electron microscope (TEM) images for both samples were obtained and are shown in Figure 6. The measured values were approximately 3.2 nm and 1.85 nm radius for the ZnO pH 10 and ZnO pH 12 samples, respectively, in reasonable agreement with the sizes estimated via Equation (3).

Experimental Results and Discussion
The THz time domain spectroscopy (THz-TDS) characterization was performed by employing a Menlo Systems TeraSmart equipment [25], operated in transmission mode (see Figure 7). This system employs a photoconductive antenna based on an InGaAs/InAlAs multilayered mesa as the emitter and detector, which are activated by a femtosecond laser of wavelength 800 nm. The system  was aligned until the THz amplitude was at its maximum. Subsequently, the equipment was thermalized for ~1 hour, and the sample was positioned at the focal point of the THz beam path. Five measurements at room temperature were collected for each reference and samples, each of which corresponded to a thousand-point averaging.
Stand-alone PDMS membranes with and without QDs were tested, as well as MM arrays with QDs on the surface and exposed to UV laser excitation of 400 nm in wavelength to determine the prospective modulation behavior of the arrangement.
The obtained THz time domain waveform signal was processed by applying a Gaussian window to remove reflections after the main THz pulse [26] and, subsequently, a Fast Fourier transform (FFT) was employed to obtain the amplitude and phase information of both the reference as well as the sample under consideration. In both cases, i.e., a stand-alone PDMS membrane and MM array, samples without QDs were employed as a reference. The transmittance as a function of frequency of the samples was calculated using the following equation [26] ( )

THz Quantum Dot Characterization
From the collected THz-TDS data, optical properties of interest such as refractive index, permittivity and conductivity can be extracted. The real part ( ) n ω and imaginary part ( ) κ ω of the refractive index can be calculated by using the following equations [26] respectively. Similarly, the permittivity and the index of refraction are related by where   is the complex permittivity, ′  is the real part of the permittivity, ′′  is the imaginary part of the permittivity. The extracted real effective refractive index of the PDMS + QDs membrane at 1 THz (see Figure 8), was of ~1.50 for the smaller QDs synthesized with a pH value of 12 (blue dotted line), and to ~1.85 (green solid line) for the larger QDs synthesized with a pH value of 10. Subsequently, the samples were exposed to UV excitation of 400 nm wavelength (see Figure 9 and Figure 10). The intensity of the UV source was measured using Open Journal of Inorganic Non-metallic Materials   an ILT2400 Light Meter and observed to be 2.5 × 10 −4 W/cm 2 . The observations indicate that larger quantum dots exhibit a relatively more pronounced response to UV excitation. This can be explained on the grounds that the synthesized semiconducting nanoparticles exhibit quantum confinement effects where exciton polarizability, reported to be proportional to the fourth power of the radius ( 4 R α ), can be defined as [27] ( ) were Δ s χ is the susceptibility of the material (related to the permittivity by is the permittivity of the quantum dots,  is the permittivity of the medium and s n is the sheet excitation density. This result is in agreement with the extracted real part of the effective refractive index of the media with the incorporated ZnO QDs shown in Figure 8. Ostensibly, studies of polarizability as a function of frequency and quantum dot dimensions can be performed employing THz-TDS.

THz Metamaterial Characterization
The experimental characterization of the metamaterial array previously described and shown in Figure 1, corroborated the anticipated resonant frequency at 1 THz (dotted lines Figure 11 and Figure 12). Subsequently, the synthesized ZnO QDs were deployed directly on the MM surface, and the resonance frequency was observed to shift to 0.95 THz and to 0.96 THz for the ZnO QDs synthesized with pH values of 10 and 12, respectively. The observed variation in the resonance frequency reflects the permittivity value trends presented in Figure 8, thus, larger permittivity values are associated with lower resonant frequencies. Figure 11. The modification of the effective permittivity of the medium by the deployment of the ZnO QDs synthesized with a pH value of 10, triggered the resonant frequency to decrease from 1 THz to 0.95 THz.  In order to measure the modulation of the metamaterial arrays with the deployed quantum dots, the samples were excited with a UV source of 400 nm while being tested with the THz-TDS (results are shown in Figure 13). The experimental observations yielded a modulation of 4.55% for the ZnO QDs synthesized with colloidal solutions with a pH value of 10, while the nanoparticles synthesized with a pH value of 12 exhibited a relatively large modulation of 9.21%. The observed results are thought to be due to a higher degree of confinement effects in smaller quantum dots [27]. These changes in modulation depth demonstrate the capability of hexagonal MM via the deployment of ZnO QDs to tune THz electromagnetic signals.

Conclusion
ZnO quantum dots were chemically synthesized employing colloidal solutions with pH values of 10 and 12, and the estimated bandgap and radii were calculated to be 3.37 eV (3.00 nm) and 3.50 eV (2.12 nm), respectively. Upon deploying the produced QDs on previously fabricated hexagonal metamaterial arrays and exposing the samples to UV excitation with a wavelength of 400 nm while performing THz spectroscopy in transmission mode, signal modulation values of 4.55% and 9.21% were observed. The results reported herein could promote the proliferation of THz regime modulation devices. Furthermore, since the photolithographic process for the MM fabrication is mass-production compatible [28], while the ZnO QDs are stable, biologically compatible, and low-cost [29], the MM-ZnO QD setup described herein is considered to be attractive for industrial applications.