MoS2 has attracted substantial attention due to its atomic thickness and outstanding electronic and mechanical properties. As one of the thinnest semiconductors in the world, MoS2 is promising to build flexible electronics that can be integrated with objects with arbitrary shapes and inspires a vision of distributed ubiquitous electronics. Despite recent advances in two-dimensional materials-based electronics (e.g. 2D materials-based transistors, memory devices and sensors), an efficient and flexible energy harvesting solution is necessary, but still missing, to enable a self-powered system. At the same time, the electromagnetic (EM) radiation in the Wi-Fi band (2.4 GHz and 5.9 GHz) is becoming increasingly ubiquitous and it would be beneficial to be able to wirelessly harvest it to power future distributed electronics. However, the rectennas (i.e. RF energy harvesters) based on flexible semiconductors have not been fast enough to cover the Wi-Fi band due to their limited transport properties. Here we present a unique MoS2 semiconducting-metallic phase heterojunction, which enables a flexible and high-speed Schottky diode with a cutoff frequency of 10 GHz. Due to a novel lateral architecture and self-aligned phase engineering, our MoS2 Schottky diode exhibits significantly reduced parasitic capacitance and series resistance. By integrating the MoS2 rectifier with a flexible Wi-Fi band antenna, we successfully fabricate a fully flexible rectenna that demonstrates direct energy harvesting of EM radiation in the Wi-Fi band with zero external bias (battery-free). Moreover, taking advantage of the nonlinearity of the MoS2 Schottky diode, a frequency mixing in the gigahertz range is also successfully demonstrated on flexible substrates.
Birefringence is a fundamental property of materials that enables optical components such as wave plates and polarizers, and is quantified by the difference between extraordinary and ordinary refractive indices. Solid homogeneous crystals like calcite and rutile are some of the most birefringent materials at visible and near-infrared wavelengths. However, at longer wavelengths (i.e., mid to far infrared) these materials become highly lossy. In the mid infrared, the most birefringent materials that are transparent are significantly less birefringent than their visible counterparts. While structured materials with strong optical anisotropy exist at these wavelengths (i.e., with form birefringence), their utility is limited by fabrication constraints.
In the talk, we will report on a rationally designed and synthesized material, barium titanium sulfide (BaTiS3), which has broadband and giant birefringence surpassing that of any known transparent anisotropic crystal throughout the infrared. We will detail our extensive optical characterization to extract the anisotropic complex refractive index spanning the ultraviolet to the mid infrared by combining generalized spectroscopic ellipsometery and polarized reflection and transmission measurements. We report a difference between the ordinary and extraordinary refractive index of up to 0.76 in a mid-infrared region of transparency, more than twice that of rutile in the visible, and show that the unprecedented optical anisotropy extends to the limit of our detection capabilities (16.7 μm). This material also features highly anisotropic Raman scattering, and we are currently working on measuring polarized infrared photoluminescence measurements to provide further insight into the anisotropy of this unique material.
The emerging class of low symmetry 2D materials, which include black phosphorus, its isoelectronic materials such as the monochalcogenides of Group IV elements and other layered materials with reduced in-plane symmetry, exhibit strong in-plane anisotropy in their optical and phonon properties that allow for the realization of conceptually new electronic and photonic devices. High mobility, narrow gap BP thin film (0.3 eV in bulk), for example, fill the energy space between zero-gap graphene and large-gap transition metal dichalcogenides, making it a promising material for mid-infrared wavelength infrared optoelectronics. Here, we will first present our work in understanding the fundamental electronic and optical properties of low-symmetry 2D materials such as black phosphorus and rhenium diselenide using a newly developed scanning ultrafast electron microscopy (SUEM) technique and photoluminescence spectroscopy. A few novel photonic device concepts will then be discussed that utilize these new materials, particularly for applications in the infrared wavelength range. We will also discuss about promising future research directions of low-symmetry optoelectronic devices based on anisotropic 2D materials and how their novel properties is expected to benefit the next-generation photonics technologies.