The mechanical stability and failures of two-dimensional (2D) materials on the compliant polymer substrates under biaxial strain are investigated. As the polymer substrate swells, the fracture of the system is initiated with the crack-formation in 2D materials (graphene and MoS2 flakes), which propagates deep into the substrates. According to the fracture theory, the generation of deep cracks in such a system (thin hard material on a compliant substrate) is expected to be a universal behavior due to a large mismatch in the elastic moduli between the film and the substrate. The properties of crack formation in systems with varying crystallinity and size of 2D materials are also investigated. The present results provide important insights into what to consider for mechanical stability in designing flexible devices based on 2D materials.

Molybdenum ditelluride (MoTe2) is a relatively unexplored layered transition metal dichalcogenide in nonlinear optics. Several recent studies have shown that MoTe2 has strong second-order optical nonlinearities originating from tellurium atoms. However, the third-order optical nonlinearities of MoTe2 have not been explored yet, except for the nonparametric saturable absorption process. Here we report the enhanced optical third-harmonic generation in phase-engineered MoTe2 thin films. MoTe2 films, including 2H and 1T' phases simultaneously, are synthesized by the flux-controlled phase-engineering method, and their nonlinear response is characterized. We observe that the 2H-MoTe2 film exhibits up to a 15-fold stronger nonlinear signal than that of the 1T'-phase film. The estimated third-order effective nonlinear susceptibility of 2H-MoTe2 is 9.3 x 10(-19) m(2) V-2 maximum, which is larger than highly nonlinear layered materials such as molybdenum disulfide. Our MoTe2 film synthesized with the desired phase over a large area will be a potential building block for ultrathin nonlinear photonics.

Twisted bilayer graphene (tBLG) has received substantial attention in various research fields due to its unconventional physical properties originating from Moiré superlattices. The electronic band structure in tBLG modified by interlayer interactions enables the emergence of low-energy van Hove singularities in the density of states, allowing the observation of intriguing features such as increased optical conductivity and photocurrent at visible or near-infrared wavelengths. Here, we show that the third-order optical nonlinearity can be considerably modified depending on the stacking angle in tBLG. The third-harmonic generation (THG) efficiency is found to significantly increase when the energy gap at the van Hove singularity matches the three-photon resonance of incident light. Further study on electrically tuneable optical nonlinearity reveals that the gate-controlled THG enhancement varies with the twist angle in tBLG, resulting in a THG enhanced up to 60 times compared to neutral monolayer graphene. Our results prove that the twist angle opens up a new way to control and increase the optical nonlinearity of tBLG, suggesting rotation-induced tuneable nonlinear optics in stacked two-dimensional material systems.

An efficient electro-optic transition control is reported in all-fiber graphene devices over a broad spectral range from visible to near-infrared. The ion liquid–based gating device fabricated onto a side-polished fiber with high numerical aperture significantly enhances the light-matter interaction with graphene, resulting in strong and nonresonant electro-optic absorption of up to 25.5 dB in the wavelength ranging from 532 to 1950 nm. A comprehensive analysis of the optical and electrical properties of the device fabricated with monolayer and bilayer graphene revealed that the number of graphene layers significantly impacts on the performance of the device, including modulation depth and driving voltage. Wavelength-dependent optical response is also measured, which clearly characterizes the electronic bandgap dispersion of graphene. The device exhibited more efficient electro-optic modulation in the longer wavelength region, where the maximum light modulation efficiency of 286.3%/V is achieved at a wavelength of 1950 nm.

We demonstrate an electrically tunable polarizer for terahertz (THz) frequency electromagnetic waves formed from a hybrid graphene-metal metasurface. Broadband (>3 THz) polarization-dependent modulation of THz transmission is demonstrated as a function of the graphene conductivity for various wire grid geometries, each tuned by gating using an overlaid ion gel. We show a strong enhancement of modulation (up to ∼17 times) compared to graphene wire grids in the frequency range of 0.2-2.5 THz upon introduction of the metallic elements. Theoretical calculations, considering both plasmonic coupling and Drude absorption, are in good agreement with our experimental findings.

Electrical characteristics of ion gels prepared by loading different amounts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) in Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) are investigated and compared with those of ion liquid, [EMIM][TFSI] for possible application as a gate stack for flexible electronic devices. Capacitance and impedance as a function of frequency are measured, which can be well accounted for by a simple circuit model identifying the local device components. The operation of a flexible field effect transistor based on graphene and the ion gel as a top gate stack is also demonstrated.

Electro-optic modulators which modulates the intensity or phase of the light through the electric signal control, have been extensively investigated for the diverse field applications including optical communication, bio-sensing, and security-monitoring based on lightwave. With recent technological advance of the fabrication of high quality graphene over large area, graphene have been intensively studied as a basic element to build novel photonic and electro-optic devices. However, low optical absorption in ultra-thin layered graphene often limits the performance of the device. Although there have been several attempts to increase graphene-light interaction, realization of efficient and broadband graphene-based electro-optic modulators is still challenging. In this work, we demonstrate an all-fiber graphene-based electro-optic modulator with a modulation depth of > 25 dB. In order to achieve non-resonant strong interaction with graphene, we employed a side-polished fiber (SPF) with high numerical aperture (NA) as a novel platform that evanescently interacts with graphene. The high NA fiber has about six times smaller mode-field area than that of the standard single-mode fiber, and we found that this can critically enhance the graphene-light interaction without significantly sacrificing the insertion loss. We experimentally fabricate the bi-layer graphene field-effect transistor onto the high-NA SPF, and covered index matched ion-liquid for further increase of the graphene-light interaction and effective gating. As a result, we observed that the fabricated device exhibits the modulation depth of 27.6 dB with low scattering loss at the applied voltage range within 2.5 V, which well agrees with our numerical expectation.

Graphene has emerged as a promising 2-dimensional (2D) material composed of a monolayer of carbon atoms, which is expected to be utilized for nano- and optoelectronic device applications. In order to fabricate high speed graphene transistors with low power consumption, the growth of insulating thin films with high dielectric constant (high-k) on graphene is essential. Atomic layer deposition (ALD) is one of the best deposition techniques to grow functional thin films, however, it is extremely challenging to grow high-k thin films on graphene by ALD because of the lack of surface functional groups (such as hydroxyl groups) on graphene. Here, we demonstrate that the graphene surface is fully covered by Al2O3 thin films (10-30 nm), with significantly reduced leakage current (decreased by a factor of similar to 10(7)), through simple surface treatment of the graphene in the ALD chamber prior to the deposition of the Al2O3 layer by ALD to provide surface nucleation sites on the graphene, without breaking vacuum and changing entire process temperature (100 degrees C). Physisorbed nuclei were created on the graphene as a form of Al2O3 with the surface treatment using trimethylaluminum (TMA) and H2O that are typical ALD precursors for Al2O3 growth. Negligible defects were generated during the graphene surface treatment, which provides promising opportunities in graphene electronics.