Requirement of the optimization of mixed phases for vanadium oxide systems in concern with sputtering parameters in order to produce high quality pure VO2 phase has been recognized. The resistivity drop at MIT is among the virtually significant characteristics in determination of film quality. It is pointed out that uniformity of the film, strain, grain size and boundaries are the dependants of the change in resistivity. In the measurement region 30°C and 90°C the resistivity of the bare VO2 films is presented in Fig. 3. At transition temperature, an evident decrease in the resistivity of bare film with greater than ~ 104 of magnitude has been observed. In the case of this bare thin film sample, MIT value is around 68°C, and the hysteresis loop width is around 4°C and for DC sputtered high quality VO2 thin films they are entirely acceptable values. Furthermore, the abrupt MIT in VO2 allows the amplitude variation of the resonant peaks of a metamaterial swiching device on a extremely short temperature region. In the VO2 transitional state, the film constitutes nanoscale configurations of metallic- and insulator-state VO2, and the developing medium operates as a tunable irregular metamaterial. A beneficial usage of this attitude could be a thermally triggered electrical switch which may operate by supplying minute however momentary heat pulses additionally cooling pulses of small amplitudes with the rank of a few Kelvins.
The measured transmission spectra of fourcross VO2 metamaterial filter (Fourcross (VO2)) at 299, 310, 330, 340, 350, 370 K is given in Fig. 5. The observed oscillations are by virtue of the etalon effect and turns to be more evident with increasing temperature. The ratio between the Fourier-transformed amplitude spectra of the sample and the heated stage with no sample mounted have specified the frequency dependent transmission. Accordingly, the measured transmission spectra incorporated the Fabry–Perot resonance effect taking place from the Al2O3 substrate (0.5 mm thick). In the case when the critical temperature of the VO2 sample is over 340 K the aforementioned effect becomes apparent to a greater extent. Furthermore, one could declare that the sample is an insulator providing that the temperature is under the critical temperature and on the other hand in the case when the temperature is over the critical temperature the sample is metallic where the THz radiation adding to the observed dips in the etalon is demonstrated. Eventhough the metamaterial configuration incorporating rectangular loops does not demonstrate any resonant properties the broadband THz transmission can be entirely turned off/on.
At room temperature the dielectric constant of VO2 is approximately 9 (insulating state) and when MIT occurs, the structural transformation occurs and VO2 turns into the rutile phase. The formation of the undesirable structural inhomogeneity provokes a monoclinic metal phase as an intermediate state that appears in a region between the MIT an the structural phase transition temperatures (SPT). This phenomena can cause a highly anisotropic THz dielectric function of VO2 films which can effect the resonance of the metamaterial device strongly.
Metamaterial device research recently has focused on development of advanced structures based on functional materials. Due to complexity of the overall metamaterial structure, e.g. chiral metamaterials, resultant device architecture may not manipulate the incident radiation effectively using optical and/or electrical means. Noting that the lower conductivity of VO2 compared to a pure metal is not sufficient to manipulate the incident radiation for patterned VO2 designs, a second design was implemented in order to better observe resonant THz interactions. Among the research areas of metamaterials, there exists a driving aspect namely; together with the optical resonance, the extensive utilization of nano-engineered metals has been documented to entitled complex functionalities, that furnishes the basis for accomplishing ‘self-sufficient’ metadevices. Nevertheless, an immediate investigation of the documented chiral metamaterials exposes that most patterns cannot endorse the extra functionalities due to the fact that they conventionally comprise of geometrically isolated units.
This second device comprising of one identical fourcross design with a structure equivalent to the one illustrated beforehand is produced that is supported by the metallic structure. Characterized by time-domain THz measurement systems this design showed more pronounce resonant effects. In Fig. 6 (a). it can be noticed that when bare VO2 thin film changes between the insulating and the metallic state, amplitude of the electric field of the transmitted wave varies significantly. Also, in Fig. 6 (b), the THz electric field peak to peak value is plotted with respect to temperature, showing this transition clearly.
Including semiconducting and metallic phases of VO2 film the frequency-dependent transmission spectra of our second design, Au/VO2 fourcross structure layer at 293, 303, 313, 318, 323, 328, 333, 343, 353, 363, and 373 K is shown in Fig. 7.
It is obviously seen from the terahertz measurement of the frequency selective surface composed of shaped gold layer that the resonance (0.7 THz) disappears around the VO2 transition temperature. In the Fig. 7. it is seen that; with a decrease in temperature, one observes increase in resonance strength and also a resonance shift to higher frequencies. With the increase of temperature (from 273 K up to MIT) resonance frequency is blue-shifted and it is observed that the resonance strength dissapeared as a broadening and reduction in amplitude of the transmission dip. In the present circumstance, as presented in Fig. 6 it is observed that low-lying second resonance frequency (around 0.370 THz) takes place close to the resonance frequency of the fourcross gold/VO2 structure. The aforementioned resonance peak is a consequence of the stimulation of the surface plasmons initiated by the periodically arranged ring arrays in the fourcross structure.
The resonance frequency that is obtained from experimental measurements of gold/VO2 design is similar to the simulations performed, where it is observed that the VO2 thin film base adversely affects the resonant effect in the metamaterial structure.
The associated THz transmission alongside altering the conductivity of VO2 through temperature, MIT nature was used to modulate their resonance frequencies. The agreement of the conclusion achieved from simulation estimates for these fourcross gold/VO2 metamaterials based on both semiconducting and metallic VO2 with those of the experimental measurements is outstanding.
THz transmission measurement and simulation results of gold fourcross patterned VO2 film at 293 K (room temperature), which is much lower than MIT, is shown in Fig. 8 on the same graph. The initial simulation results that performed before experimental studies give a resonance peak at around 0.65 THz (Sim. 1 fourcross Gold- VO2). It has been already known that the spectral response of metamaterial filters is sensitive to individual variation of each design parameters which can shift the peak frequency and bandwidth.
So, the simulation studies was repeated in compliance with measured unitcell parameters (Sim. 2 fourcross Gold- VO2). If we compare the room temperature transmission intensity of gold fourcross patterned VO2 film and fourcross patterned VO2 film, it is around only 4% for second design with bare film.
The modulation range is enhanced by 1 order magnitude in the fourcross patterned VO2 film metamaterial structure so that the THz transmission can be fully switched off.
The low transmission intensity of the bare VO2 layer can be attributed to the film thickness. The sharp alteration of refractive index in the course of the MIT process is the origin of the converse modulating tendency of transmittance with respect to temperature reported on thicker VO2 films. The converse modulating tendency of transmittance could be induced by the debate in between transmittance and absorption rate.
Hence, possessing latent applications in the fields of THz imaging, sensing and communications the device under consideration is a thermally governable intensity modulator in the THz regime. The thin film quality can adversely affect the device efficiency. The optical characteristics of VO2 films could be adjusted by numerous components some of which are structure, strain and defects. Well-controlled experimental condition is required in order to achieve the intented optical properties of VO2 film and this necessity restricts its utilizations in manipulating waves. For instance, oxygen vacancy or unsought V valence states might be stimulated by spatially discriminative defect engineering or other sequential methods to produce VO2 films. Accordingly, both the dielectric function and the effect of thickness of VO2 film on the optical characteristics has to be taken under consideration.