Yanhan Zhu

Structural, electrical, and terahertz transmission properties of VO2 thin films grown on c-, r-, and m-plane sapphire substrates

The structure, metal-insulator transition (MIT), and related Terahertz (THz) transmission characteristics of VO2 thin films obtained by sputtering deposition on c-, r-, and m-plane sapphire substrates were investigated by different techniques. On c-sapphire, monoclinic VO2 films were characterized to be epitaxial films with triple domain structure caused by β-angle mismatch. Monoclinic VO2 β angle of 122.2° and the two angles of V4+–V4+ chain deviating from the am axis of 4.4° and 4.3° are determined. On r-sapphire, tetragonal VO2 was determined to be epitaxially deposited with VO2 (011)T perpendicular to the growth direction, while the structural phase transformation into lower symmetric monoclinic phase results in (2¯11) and (200) orientations forming a twinned structure. VO2 on m-sapphire has several growth orientations, related with the uneven substrate surface and possible inter-diffusion between film and substrate. Measurements of the electrical properties show that the sample on r-sapphire has MIT ...

VO2 multidomain heteroepitaxial growth and terahertz transmission modulation

We report the epitaxial relationship of VO2 thin-films on c-plane sapphire and their terahertz transmission modulation with temperature. The films exhibit a triple-domain structure caused by the lattice mismatch between monoclinic VO2 and sapphire hexagon. The epitaxial relationship is determined to be VO2[010]∥Al2O3[0001] and VO2(2¯02)∥Al2O3{112¯0}, with the in-plane lattice mismatch of 2.66% (tensile) along [2¯02] and the out-of-plane lattice mismatch of −2.19% (compressive). Terahertz measurements revealed that VO2 films have over fourfold change in transmission during the metal-insulator transition, indicating a strong potential for terahertz wave switching and modulation applications.

Tunable dual-band terahertz metamaterial bandpass filters

We report metamaterial terahertz (THz) bandpass filters with tunable dual-band selectivity. The shift in the center frequency of the device is achieved by actively modifying the effective length of the resonators. This was realized by introducing vanadium dioxide (VO2) bridges interconnecting specific regions of each resonator. Raising the temperature across the phase transition shifted the resonance frequency by ~32% due to changes in the electrical conductivity of the VO2. Measured THz transmission response of the proposed dual-band filter was in good correspondence with simulations.

Effect of substrate orientation on terahertz optical transmission through VO 2 thin films and application to functional antireflection coatings

We report studies of the terahertz (THz) transmission through vanadium dioxide (VO2) thin films grown on c-, m-, and r-plane sapphire substrates. Our results revealed THz amplitude modulation as large as 84% for VO2 films grown on r-plane sapphire substrates upon crossing the metal–insulator phase transition temperature. Complex optical conductivity and refractive indices were determined for all investigated samples in the metallic state. Results are consistent with electrical resistivity measurements and described based on the Drude model. The real and imaginary parts of the optical conductivity and refractive index are obtained, and associations with variations in the grain morphology and crystal quality are described. We show that VO2 thin films can be used as tunable broadband THz frequency antireflecting coatings.

Tuning the properties of VO2 thin films through growth temperature for infrared and terahertz modulation applications

Results are reported on tuning the electrical and optical properties of sputter-deposited vanadium dioxide (VO2) thin films through control of substrate growth temperature (Ts). As Ts increases from 550 to 700 °C, the morphology changes from granular to smooth film and finally to rough film. X-ray diffraction shows the presence of VO2 along with additional weak features related to the presence of non-stoichiometric phases. Electrical measurements show the phase transition to change from abrupt to gradual as both the below- and above-transition resistivities vary with Ts. The transition and hysteresis dependences observed in electrical resistivity are similarly observed in infrared transmission. Terahertz transmission measurements show that high conductivity above the phase transition is more important in achieving high modulation depth than obtaining high resistivity below the transition. We attribute changes in the electrical and optical properties to the formation of V and O vacancies, which result in diverse valence states from the ideal V4+ of VO2. Low Ts produces material with V5+ states resulting in higher resistivity in both the insulating and metallic phases. Alternatively, high Ts introduces material with V3+ states leading to lower resistivity in the insulating phase but slightly higher resistivity in the metallic phase.

Terahertz bandpass filters using double-stacked metamaterial layers

Bandpass filters are reported based on double-stacked metamaterial layers separated by an air gap for operation at terahertz frequencies. Several stacking configurations were investigated designed for a ~0.5 THz center frequency. The filters exhibited improved spectral transmission properties when compared with conventional ones based on single metamaterial layers. 3 dB bandwidth of ~78 GHz and sidelobe suppression ratio >16 dB were determined when symmetric or asymmetric double layers were stacked. We demonstrate that superior frequency selectivity can be achieved when metamaterial layers with different unit cells are used. Good agreement was found between measured and simulated transmission response.

Terahertz Two-Layer Frequency Selective Surfaces With Improved Transmission Characteristics

In this paper, a cascaded configuration for two-layer frequency selective surfaces (FSSs) at terahertz (THz) frequencies with improved filtering characteristics is realized for electrically thick substrates. At THz frequencies, the thicknesses of commercially available substrates are comparable to the free-space wavelength. As a result, the substrate plays a critical role in determining the transmission characteristics of THz multilayer FSS structures. Proper coupling method between FSS structures should be chosen to avoid unwanted substrate resonances or Fabry-Pérot resonances, which otherwise degrade the transmission characteristics of the cascaded FSS structure. In this paper, a cascaded structure to avoid multiple reflections within the substrate is presented and the same is used to realize two double-layered FSS structures to improve the transmission response. The transmission response is improved by introducing an extra transmission zero at a frequency location lower than the resonant frequency, thereby achieving high roll-off rate for the lower side of the stop band, and to suppress unwanted resonances, thereby increasing the rejection bandwidth of the filter. The proposed cascaded FSS structures were fabricated and tested using THz time-domain spectroscopy. Good agreement between simulations and experiments were obtained.

Reconfigurable terahertz frequency selective structures using vanadium dioxide

Terahertz frequency selective surfaces with tunable characteristics are presented. Vanadium dioxide patches and metallic resonators were used. Vanadium dioxide behaves as an insulator at low temperatures and as a metal at high temperatures with transition temperature of ~ 68°C. Single to dual polarized bandpass FSS structure at 0.5 THz and tunable bandpass FSS structure were realized. We measured extinction ratio of 25 dB for the polarizer and 31% shift in the resonance frequency for the FSS.

Terahertz Imaging With a Time-Reversed Finite Difference Time-Domain Method

A novel approach to three-dimensional terahertz (THz) imaging based on a time-reversed finite difference time-domain (FDTD) method is presented. Time-reversed convolutional perfectly matching layers (CPML) were implemented as boundaries to truncate the simulation domain. The proposed computationally efficient technique is capable of producing detailed images using a much smaller set of measured data compared to conventional terahertz imaging methods. A coarser grid of transmitter/receiver locations is needed, saving significant data acquisition time. Transmission and reflection imaging configurations in far-field were investigated. Simulations as well as experimental results, including comparisons with conventional methods, are presented. Our approach was validated experimentally using a time-domain terahertz measurement system.

Terahertz frequency selective surface with reconfigurable polarization characteristics using vanadium dioxide

In this paper, we present a design technique to realize reconfigurable terahertz (THz) frequency selective surface (FSS) polarizer. Our approach relies on combining vanadium dioxide (VO2) patches with metallic resonators. Vanadium dioxide behaves as an insulator at room temperatures and as a metal at high temperatures with a characteristic insulator–metal transition temperature of ~68 °C. We used this attribute to realize a reconfigurable single- to dual-polarized bandpass FSS structure at 0.5 THz. Along with the simulation results, FSS structures fabricated on sapphire substrates were measured using THz time-domain spectroscopy. Measured extinction ratio of ~25 dB was achieved for the THz polarizer with just a single FSS layer. Good agreement between simulation and experiments were obtained.