Chunhui Ji

Optimization of metal-to-insulator phase transition properties in polycrystalline VO2 films for terahertz modulation applications by doping

Vanadium dioxide (VO2), due to its well-known metal–insulator phase transition (MIT), is a promising candidate to realize optical modulation devices operating at terahertz (THz) frequencies. Moreover, the application of VO2 on modulation devices requires a narrow hysteresis width associated with a significant change in its optical properties, which is quite challenging in Si-based polycrystalline VO2 films. In this paper, by doping high-valence metal ions (W6+ or Nb5+) into polycrystalline VO2 films, a narrowed hysteresis width and a decreased phase transition temperature are observed. Intriguingly, these doped VO2 films always maintain a high THz field modulation depth despite the low phase transition temperature resulting from the usage of either W or Nb dopants. To sum up, the optimized VO2 film with 6.5% Nb doping deposited on high-purity silicon substrates exhibits the best MIT characteristics with a giant field THz modulation depth of 62.5%, a small hysteresis width down to 4.8 °C and a low phase transition temperature of around 31.1 °C, which is very excellent for practical applications. Furthermore, we synthetically investigate the influences of W and Nb doping on the microstructures and MIT characteristics of the polycrystalline VO2 films. Doping Nb5+ and W6+ ions has similar effects through a similar mechanism. In addition, the excellent balance between the THz modulation ability and phase transition temperature is related to the high crystallinity degree of the doped films. The annealing process may play a key role in this peculiar case. These results show that the excellent MIT properties of the polycrystalline VO2 films can be effectively tailored by our distinctive preparation method, which provides a feasible solution to the design and fabrication of VO2 films with suitable MIT properties for THz devices.

Rebound effect of IMT properties by different doping form in Si-doped vanadium dioxide films

Vanadium dioxide is a promising material for THz modulations due to its remarkable insulator-to-metal transition (IMT) properties. Silicon-doped VO2 films, exhibiting excellent IMT properties with giant modulation amplitude and tunable phase transition temperature, greatly adapt in this area. In this paper, we report on a rebound effect of the IMT in Si-doped VO2 films. As the silicon dopants are increasingly introduced into VO2 films, the IMT is first tuned to lower temperature and then is anomalously shifted to higher temperature. This rebound effect is confirmed by crystal structure, valence concentration, and surface morphology. We attribute this rebound behavior to the interstitial and substitutive doping of Si atoms. Due to their distinct impactions on the crystallite, IMT properties of the VO2 films are depressed initially and recovered later.

Influence of infrared optical properties by transformation of the crystal structure in Al-doped vanadium dioxide films

Some infrared-active phonons in VO2 films suppress their modulation performance in the infrared region. Al-doped VO2 films, due to transforming VO2 crystal into the M2 phase, promptly eliminate absorption peaks in far-IR/THz bands and present widely modulating properties. Furthermore, we found high-frequency shifts of phonon vibration modes in Raman spectra by Al doping, indicating the stronger V-O bonds as the evidence of VO2 crystalline modification. However, although the high-frequency shifts and peak broadening were observed in V-O-V bending modes, mid-infrared spectra as the other phonon characterization show that its resonances are less involved, which is different from the remarkable variation of THz phonons. We attribute the difference to the distinct origins of phonon vibrations. As Al doped into films, the group-rotational peaks were rapidly erased with crystalline deformation whereas the high-frequency bending modes only slightly changed.

Broadband terahertz modulator based on graphene metamaterials

Tunable complementary split ring resonators (CSRRs) based on monolayer graphene are presented in terahertz regime. By applying different gate voltage, the Fermi level and optical conductivity of monolayer graphene pattern can be changed. Here, we employ a numerical simulation to study the interaction of light with graphene CSRRs. The results indicate that the extinction in transmission becomes stronger, and the resonance frequency presents blue shift with higher Fermi level of the graphene pattern. Three pronounced resonant peaks appear which can be modulated dynamically in the range of 1-2THz and 3-7THz, and realizing dynamic broadband terahertz modulation, the modulation depth exceeds 85% at all three resonant peaks, the highest modulation depth reaches 98.8% at 7.47THz.

Electrically tunable mid-infrared antennas based on VO2

Abstract A large change in optical constants of phase-change vanadium dioxide enables active control over the transmission and reflection properties of structures incorporating VO2. In this paper, we demonstrate electrically tunable mid-infrared strip array antennas based on metal–insulator transition of vanadium dioxide. The antennas consist of an interdigitated metal strip array separated from a metallic ground plane by a film of vanadium dioxide. The interdigitated metal strips serve as both antennas and electrodes. As the insulator-to-metal phase transition of vanadium dioxide is induced with applied voltages, resonance of the strip antenna array redshifts with reduced absorbance before it is eventually switched off. A tuning magnitude of 30% in reflectivity is measured at 25.5 THz. Our measurements of sample temperature reveal that tuning mechanism of the antennas is primarily thermal in nature. The demonstrated electrical tuning of mid-infrared antennas could be used for reconfigurable bolometric sensing, camouflaging and modulation of infrared radiations.

High thermochromic performance of Fe/Mg co-doped VO2 thin films for smart window applications

In the field of energy-efficient smart windows, vanadium dioxide (VO2) is a promising material due to its reversible metal–insulator transition. However, the development of VO2-based smart windows has been restricted by several drawbacks in performance, including high phase transition temperature (TC), low luminous transmittance (Tlum), insufficient solar energy modulation ability (ΔTsol) and unpopular color. In this paper, Fe/Mg co-doping is proposed for the first time to improve the thermochromic performance of VO2 films. Interestingly, the Fe/Mg co-doped films can exhibit outstanding thermochromic performance with a balanced solar regulation efficiency of 12.8%, suitable luminous transmittance up to 42.1%, and low phase transition temperature around 38.2 °C, which is very promising for practical applications. In addition, the color of films is modified to increase their brightness and lighten the brown color. Different from previous work on co-doped VO2 focusing on W6+ and low-valence (<4+) elements, Fe3+/Mg2+ co-doping is found to have the synergistic combination of the advantages of single doping with Fe and Mg by getting rid of the neutralization between charge carriers e− and h+. Such a feature is especially desired for optimizing the thermochromic performance with co-doping methods. This study provides a new solution for improving the optical properties of VO2 films for smart window applications.

Broadband planar multilayered absorbers tuned by VO2 phase transition

The metal-insulator transition makes vanadium dioxide an attractive material for developing reconfigurable optoelectronic components. Here we report on dynamically tunable broadband absorbers consisting of planar multilayered thin films. By thermally triggering the phase transition of vanadium dioxide, the effective impedance of multilayered structures is tuned in or out of the condition of impedance matching to free-space, leading to switchable broadband absorptions. Two types of absorbers are designed and demonstrated by using either the insulating or metallic state of vanadium dioxide at the impedance matched condition. The planar multilayered absorbers exhibit tunable absorption bands over the wavelength ranges of 5–9.3 μm and 3.9–8.2 μm, respectively. A large modulation depth up to 88% is measured. The demonstrated broadband absorbance tunability is of potential interest for reconfigurable bolometric sensing, camouflaging, and modulation of mid-infrared lights.The metal-insulator transition makes vanadium dioxide an attractive material for developing reconfigurable optoelectronic components. Here we report on dynamically tunable broadband absorbers consisting of planar multilayered thin films. By thermally triggering the phase transition of vanadium dioxide, the effective impedance of multilayered structures is tuned in or out of the condition of impedance matching to free-space, leading to switchable broadband absorptions. Two types of absorbers are designed and demonstrated by using either the insulating or metallic state of vanadium dioxide at the impedance matched condition. The planar multilayered absorbers exhibit tunable absorption bands over the wavelength ranges of 5–9.3 μm and 3.9–8.2 μm, respectively. A large modulation depth up to 88% is measured. The demonstrated broadband absorbance tunability is of potential interest for reconfigurable bolometric sensing, camouflaging, and modulation of mid-infrared lights.

Effects of annealing time on the application of vanadium dioxide films in smart windows

Vanadium dioxide (VO2) films have great potential applications in photoelectric switching, storage devices, terahertz modulators and smart windows, due to the abruptly insulator-metal phase transition (IMT) near room temperature. In this research, vanadium oxide films were deposited by DC reactive magnetron sputtering in different annealing time of 450°C on glass substrates. As for electrical properties, the increasing of annealing time turns out sheet resistance increases at first, and then decreases in insulating phase, vice versa in metallic phase. In optical properties, the visible transmittance of VO2 films initially drops with annealing time prolonging, afterwards the transmittance slightly recovers. Differences between the electrical and optical are due to the grain size. Moreover, VO2 film annealing 15 min presents excellent visible transmittance, highly near-IR modulation efficiency (about 92% at a wavelength of 1100nm) and the lowest phase transition temperature (55.7°C). This result indicates that an appropriate annealing ambient can facilitate the application of VO2 film in smart windows.

THz transmittance and electrical properties of silicon doped vanadium dioxide films tuning by annealing temperature

Silicon doped vanadium dioxide (VO2) films were successfully prepared on high purity Si(111) substrate. Confirmed by X-ray diffraction, all samples showed a preference orientation of (011) direction. Introducing silicon led grain sizes decreasing comparing to undoped VO2 film, and this result induced a narrow hysteresis width in MIT performance. Furthermore, silicon doped VO2 films annealing in different temperature presented different phase transition properties. In the electrical, a higher annealing temperature resulted in a decrease of sheet resistance and lowering the transition temperature. In terahertz optical transmittance, silicon doped VO2 films keep an excellent modulation ratio, indicating a great potential in the application of terahertz modulator devices.