Van der Waals Heteroepitaxial VO₂/Mica Films with Extremely Low Optical Trigger Threshold and Large THz Field Modulation Depth

Abstract

More interestingly, the phase transition of VO 2 can be triggered by thermal, [7,8] electrical, [9,10] and optical stimuli [11][12][13][14][15][16] as well as strain, [17,18] making it very appealing to practical applications. Across the MIT, VO 2 changes from a monoclinic phase into a rutile phase. The insulating monoclinic phase has high THz transparency due to its low-density free carrier absorption, while the metallic rutile phase is opaque in the THz regime due to its high free carrier concentration (>10 21 cm −3 ). [19] The huge change of free carrier concentration across MIT gives rise to VO 2 's large M d in the THz regime; this characteristic is significant since there are very few natural materials with suitable optical properties for THz wave manipulation.For VO 2 -based THz switches and modulators, a very high laser intensity is normally required to trigger MIT. Émond et al. [12] reported a pump laser fluence of F th = 3.8 mJ cm −2 in order to trigger MIT of VO 2 /Al 2 O (11 3 02) films, while Cocker et al. [13] reported F th = 2 mJ cm −2 for VO 2 /Al 2 O (112 3 0) films. Hilton et al. [14] reported F th = 7 mJ cm −2 for a VO 2 /MgO(100) film, while Xiao et al. [15] reported the same value for VO 2 /SiO 2 films. The high F th values of VO 2 films are attributed to the facts that photoinduced heat accumulation or high concentration of photoinduced carriers are needed to trigger the MIT.The high optical trigger threshold F th of VO 2 films significantly increases the cost of VO 2 -based THz devices, so it is highly desirable to explore means to reduce F th of VO 2 films for applications to THz (opto)electronics. One way is to heat VO 2 films close to the phase transition temperature, T c . Cocker et al. [13] obtained a temperature-dependent phase diagram of ultrafast photoinduced MIT of VO 2 , and found that F th of VO 2 films decreases from 8 to 2 mJ cm −2 when the sample is heated from 41.2 to 295 K. However, relatively high F th is still needed to drive the MIT even at high temperatures. Pashkin et al. [16] reported a reduction in F th from 4.6 to 3.5 mJ cm −2 when a VO 2 /diamond film was heated from 295 to 320 K. Heating a VO 2 film to T c does reduce F th , but the effect is insignificant.Another approach to reduce F th is doping, which tunes T c closer to room temperature but also increases photoinduced carriers at a certain pump fluence. W is the most used element for VO 2 doping, highly effective in reducing T c and increasing Materials with low optical trigger threshold and large THz field modulation depth are highly desirable for THz switches and modulators. VO 2 , a classical strongly correlated oxide undergoing a sharp metal-insulator transition (MIT), is advantageous for its large THz field modulation depth by virtue of MIT. However, for optically triggered VO 2 -based THz switches and modulators, high pump laser intensity is normally required to trigger the MIT, adding considerable costs to THz systems. Reducing optical trigger threshold of VO 2 is of substantial interest in the THz scien...

Dynamic Manipulation of THz Waves Enabled by Phase-Transition VO2 Thin Film

“…Another approach involves introducing epitaxial strain to influence the microstructures of VO 2 film. This approach highly relies on epitaxial growth techniques and avoids the degradation of modulation phenomenon [ 129 ]. Other optimizations, such as the broadening of the phase transition temperature window [ 91 ] and anisotropic modulation [ 130 ], can also be realized by controlling the synthesis process of VO 2 film.…”

“…Except for ion doping, another effective approach to optimize the IMT properties is to alter the film structure through substrate influence. Recent research reported by Liang et al took a novel approach to reduce the excitation energy of ultrafast IMT [ 129 ]. They deposited Van der Waals (vdW) heteroepitaxial VO 2 film on ultrathin (~13 um) mica substrate.…”

“…Additionally, the poor heat conduction in the vdW epitaxial interface prevents heat from transferring to substrate, improving the energy efficiency of the pump laser. Both factors are considered responsible for the significant fluence threshold reduction [ 129 , 134 ].…”

“…( g ) Schematics of laser-induced lattice changes of heteroepitaxial VO 2 films on substrates with strong chemical bonding (covalent or ionic) and vdW bonding. Reproduced from [ 129 ], with permission from WILEY-VCH.…”

Recent Advances in Fabrication of Flexible, Thermochromic Vanadium Dioxide Films for Smart Windows

“…This weak vdW bonding between the VO 2 film and the mica sheet does not induce any significant lattice strain in the VO 2 layer. Therefore, the VO 2 film behaves as a nearly freestanding film on the mica sheet, which enables MIT with exceedingly low energy stimuli [90]. Moreover, owing to the weak vdW bonding between adjacent mica sheets, the thin mica sheet can be peeled off from the substrate, creating transparent and flexible VO 2 (M)/mica sheets.…”