Similar ultrafast dynamics of several dissimilar Dirac and Weyl semimetals

Recent years have seen the rapid discovery of solids whose low-energy electrons have a massless, linear dispersion, such as Weyl, line-node, and Dirac semimetals. The remarkable optical properties predicted in these materials show their versatile potential for optoelectronic uses. However, little is known of their response in the picoseconds after absorbing a photon. Here we measure the ultrafast dynamics of four materials that share non-trivial band structure topology but that differ chemically, structurally, and in their low-energy band structures: ZrSiS, which hosts a Dirac line node and Dirac points; TaAs and NbP, which are Weyl semimetals; and Sr$_{1-y}$Mn$_{1-z}$Sb$_2$, in which Dirac fermions coexist with broken time-reversal symmetry. After photoexcitation by a short pulse, all four relax in two stages, first sub-picosecond, and then few-picosecond. Their rapid relaxation suggests that these and related materials may be suited for optical switches and fast infrared detectors. The complex change of refractive index shows that photoexcited carrier populations persist for a few picoseconds

Obtaining cross-sections of paint layers in cultural artifacts using femtosecond pulsed lasers

Recently, ultrafast lasers exhibiting high peak powers and extremely short pulse durations have created a new paradigm in materials processing. The precision and minimal thermal damage provided by ultrafast lasers in the machining of metals and dielectrics also suggests a novel application in obtaining precise cross-sections of fragile, combustible paint layers in artwork and cultural heritage property. Cross-sections of paint and other decorative layers on artwork provide critical information into its history and authenticity. However, the current methodology which uses a scalpel to obtain a cross-section can cause further damage, including crumbling, delamination, and paint compression. Here, we demonstrate the ability to make controlled cross-sections of paint layers with a femtosecond pulsed laser, with minimal damage to the surrounding artwork. The femtosecond laser cutting overcomes challenges such as fragile paint disintegrating under scalpel pressure, or oxidation by the continuous-wave (CW) laser. Variations in laser power and translational speed of the laser while cutting exhibit different benefits for cross-section sampling. The use of femtosecond lasers in studying artwork also presents new possibilities in analyzing, sampling, and cleaning of artwork with minimal destructive effects

Terahertz-frequency magnetoelectric effect in Ni-doped CaBaCo4O7

We present a study of the terahertz-frequency magnetoelectric effect in ferrimagnetic pyroelectric CaBaCo4O7 and its Ni-doped variants. The terahertz absorption spectrum of these materials consists of spin excitations and low-frequency infrared-active phonons. We studied the magnetic-field-induced changes in the terahertz refractive index and absorption in magnetic fields up to 17 T. We find that the magnetic field modulates the strength of infrared-active optical phonons near 1.2 and 1.6 THz. We use the Lorentz model of the dielectric function to analyze the measured magnetic-field dependence of the refractive index and absorption. We propose that most of the magnetoelectric effect is contributed by the optical phonons near 1.6 THz and higher frequency resonances. Our experimental results can be used to construct and validate more detailed theoretical descriptions of magnetoelectricity in CaBaCo4−xNixO7

Chalcogenide Glass-on-Graphene Photonics

Two-dimensional (2-D) materials are of tremendous interest to integrated photonics given their singular optical characteristics spanning light emission, modulation, saturable absorption, and nonlinear optics. To harness their optical properties, these atomically thin materials are usually attached onto prefabricated devices via a transfer process. In this paper, we present a new route for 2-D material integration with planar photonics. Central to this approach is the use of chalcogenide glass, a multifunctional material which can be directly deposited and patterned on a wide variety of 2-D materials and can simultaneously function as the light guiding medium, a gate dielectric, and a passivation layer for 2-D materials. Besides claiming improved fabrication yield and throughput compared to the traditional transfer process, our technique also enables unconventional multilayer device geometries optimally designed for enhancing light-matter interactions in the 2-D layers. Capitalizing on this facile integration method, we demonstrate a series of high-performance glass-on-graphene devices including ultra-broadband on-chip polarizers, energy-efficient thermo-optic switches, as well as graphene-based mid-infrared (mid-IR) waveguide-integrated photodetectors and modulators

Chalcogenide Glass on Layered van der Waals Crystals for Integrated Photonic Devices

Layered van der Waals (vdW) materials have demonstrated huge potential for photonic devices with their varied and tunable optical properties. They can be integrated into planar photonic devices on virtually any substrate due to their van der Waals bonding, thereby introducing desirable material properties to existing integrated photonic platforms. Previously, their utilization has been limited to their transfer onto prefabricated photonic structures, limiting device design, and often introducing undesirable stress or fracture. Recently, the integration of vdW materials with chalcogenide glasses (ChG) has been developed for near and mid-infrared integrated photonic applications. This ChG-on-vdW platform enables new device architectures that can better utilize vdW material’s strong anisotropy and accelerates prototyping. In this work, we leverage the ChG-on-vdW material platform to demonstrate integrated photonic devices with enhanced performance, while also gaining further insight into the vdW material’s properties. First, we show that ChG processing does not damage vdW materials, while even serving as a passivation layer for unstable vdW materials such as black phosphorus. We then fabricate and characterize black phosphorus and tellurene based mid-infrared photodetectors, which not only achieve high sensitivity, but also give insight to the critical role of vdW material anisotropy in photodetection. Next, we utilize the strong second order nonlinearity in indium selenide and tellurene to investigate vdW semiconductor’s linear elecrooptic Pockels effect: an essential, yet elusive, effect to realize high-performance waveguide integrated optical modulators. Finally, we show how gallium sulfide’s use in hybrid waveguides can enhance the waveguide optical nonlinearity, which we use to demonstrate all-optical modulation. Cumulatively, this work demonstrates the power of the ChG-on-vdW platform and shows the promise of using vdW materials to engineer future generations of integrated photonic devices.Ph.D

2D-material-enabled multifunctional mid-IR optoelectronics

© 2020 SPIE. New narrow-gap two-dimensional (2-D) semiconductors exemplified by black phosphorus and tellurene are promising material candidates for mid-IR optoelectronic devices. In particular, tellurene, atomically thin crystals of elemental tellurium, is an emerging narrow-gap 2-D semiconductor amenable to scalable solution-based synthesis and large-area deposition. It uniquely combines tunable bandgap energies, high carrier mobility, exceptionally large electro-optic activity, and superior chemical stability, making it a promising and versatile material platform for mid-infrared photonics. Here we discuß the design and experimental realization of integrated photonic devices based on tellurene and other 2-D semiconductors specifically for the mid-IR spectral regime based on a chalcogenide glaß (ChG) photonic platform

Tellurene: A Multifunctional Material for Midinfrared Optoelectronics

The mid-infrared spectral band (2-20 μm) is of significant technological importance for thermal imaging, spectroscopic sensing, and free-space communications. Lack of optical materials compatible with common semiconductor substrates, however, presents a standing hurdle for integrated photonic device development in the mid-infrared domain. Tellurene, atomically thin crystals of elemental tellurium, is an emerging 2-D material amenable to scalable solution-based synthesis. It uniquely combines small and tunable bandgap energies, high carrier mobility, exceptionally large electro-optic activity, and superior chemical stability, making it a promising and versatile material platform for mid-infrared photonics. With these material properties in mind, we propose and design a waveguide-integrated tellurene photodetector and Pockels effect modulator. The photodetector boasts a record room temperature noise equivalent power of 0.03 fW/Hz1/2 at 3 μm wavelength, while the optimized modulator device claims a half-wave voltage-length product (V[subscript π]L) of 2.7 V·cm and a switching energy of 12.0 pJ/bit, both representing substantial improvements to current state-of-the-art devices.National Science Foundation (Awards 1453218, 1506605, 1122374