This PDF file contains the front matter associated with SPIE Proceedings Volume 10534, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Interest in transition metal dichalcogenides has been renewed by the discovery of emergent properties when reduced to single, two-dimensional (2D) layers. In the few-layer limit, the optical and electronic properties of TMDs are modified by a strongly reduced dielectric screening. As a consequence of the weak screening, these 2D materials are intrinsically susceptible to spatial disorder, which can arise due to defects from the growth or interactions with the substrate. Here, we use a set of complementary imaging techniques - Raman, photoluminescence, Kelvin probe, and photoelectron spectroscopy – to correlate locally the chemical state, electronic structure, and optical properties of 2D transition metal dichalcogenides. In particular, we employ spatially resolved angle resolved photoemission spectroscopy (nano-ARPES) to map the variations in band alignment, effective mass and chemical composition of CVD-grown monolayer WS2. By correlating the spectroscopic information from nano-ARPES with hyperspectral photoluminescence data, we reveal the interplay between local material properties, such as defect density or chemical composition, and the formation of charged trions, defect-bound excitons and neutral excitons. We compare these results to combined atomic force and scanning tunneling microscopy studies, where we can unambiguously identify point defects in the films at the atomic level.

Hot-electron (HE) transfer process in hybrid metal nanoparticle capped layered two-dimensional dichalcogenides (TMDs) can result in the modification of dielectric constant as well as doping of the semiconductor. Hot-carrier transfer process due to optical excitation is an attractive mechanism for enhancing the efficiency of photodetectors or photovoltaic devices. However, HE driven processes are usually non-radiative in nature and are not usually utilized in light emitting system. In this work, it has been demonstrated that by using the optical excitation resonant to plasmons in the hybrid structure, hot electrons are transferred from metal to semiconductor that causes the formation hybridized exciton-plasmon state that eventually gives rise to the enhanced photoluminescence (PL) emission. The localized surface plasmon (LSP) induced off-resonant to the emitted photons has also been utilized to enhance the PL emission. TMDs such as single monolayer Molybdenum Disulfide (MoS₂) material has high exciton binding energy, which can facilitate strong exciton-plasmon interaction. The light emitted from MoS₂ structure is weak due to intervalley scattering in indirect-gap multilayer structures or due to relatively low absorption of direct-bandgap monolayer structure. Hybrid Ag-MoS₂ plasmonic structures were realized by nucleating silver film of optimum thickness into islands by nucleation on chemical vapor deposition (CVD) grown monoloyer MoS2 structure. The LSP energy of the system is centered at 2.33 eV, with the two exciton absorption peaks at 1.85 and 2.0 eV. The carrier dynamics studied by ultrafast time-resolved spectroscopy reveals HE mediated hybridization of exciton within 700 fs. The emisson energy at 1.83 eV can be tuned by the input power due to HE transfer from Ag to MoS₂. A three-fold enhancement in the PL intensity has also been observed.

Atomically thin transition-metal dichalcogenide (TMD) semiconductors possess strong Coulomb interactions due to reduced dielectric screening, leading to the formation of excitons with exceptionally large binding energies. The enhanced stability of excitons in these materials provides a unique platform to investigate excitonic interactions at room temperature and to examine the role of plasma effects and excitonic interactions over a broad range of excitation densities. We report an excitation-density dependent crossover between two regimes: Using ultrafast absorption spectroscopy, we observe a pronounced red shift of the exciton resonance followed by an anomalous blue shift with increasing excitation density. Using both material-realistic computation and phenomenological modeling, we attribute this observation to long-range Coulomb interaction in the presence of plasma screening in an attraction-repulsion crossover with the short-ranged exciton-exciton interaction that mimics the Lennard-Jones potential between atoms, suggesting a strong analogy between excitons and atoms in respect of inter-particle interaction. Our findings underline the important role of many-particle renormalizations and screening due to excited carriers in the device-relevant regime of optically or electrically excited TMDs.

In this talk we will discuss our recent work on strong light-matter coupling of excitons in two-dimensional Van der Waals materials [1, 2]. Formation of microcavity exciton-polaritons at room temperature in these materials and their valley polarization properties will be discussed. Prospects for electrical control and the nonlinear optical properties of the polariton states will also be addressed. Finally, we will also discuss the formation of room-temperature quantum emitters via strain engineering in these materials. [1] Strong light-matter coupling in two-dimensional atomic crystals, X. Liu, et al., Nature Photonics 9, 30 (2015) [2] Optical control of room temperature valley polaritons, Z. Sun, et al., Nature Photonics 11, 491 (2017).

We review¹ the fully-scalable fabrication of a large array of hybrid molybdenum disulfide (MoS₂) - silicon dioxide (SiO₂) one-dimensional (1D), freestanding photonic-crystal cavities (PCCs) capable of enhancement of the MoS₂ photoluminescence (PL) at the narrow cavity resonance. As demonstrated in our prior work [S. Hammer et al., Sci. Rep. 7, 7251 (2017)]¹, geometric mode tuning over the wide spectral range of MoS₂ PL can be achieved by changing the PC period. In this contribution, we provide a step-by-step description of the fabrication process and give additional detailed information on the degradation of MoS₂ by XeF₂ vapor. We avoid potential damage of the MoS₂ monolayer during the crucial XeF₂ etch by refraining from stripping the electron beam (e-beam) resist after dry etching of the photonic crystal pattern. The remaining resist on top of the samples encapsulates and protects the MoS₂ film during the entire fabrication process. Albeit the thickness of the remaining resists strongly depends on the fabrication process, the resulting encapsulation of the MoS₂ layer improves the confinement to the optical modes and gives rise to a potential enhancement of the light-matter interaction.

Coupling of an atom-like emitter to surface plasmons provides a path toward significant optical nonlinearity, which is essential in quantum information processing and quantum networks. A large coupling strength requires nanometer-scale positioning accuracy of the emitter near the surface of the plasmonic structure, which is challenging. We demonstrate the coupling of single localized defects in a tungsten diselenide (WSe₂) monolayer self-aligned to the surface plasmon mode of a silver nanowire. The silver nanowire induces a strain gradient on the monolayer at the overlapping area, leading to the formation of localized defect emission sites that are intrinsically close to the surface plasmon. We measured an average coupling efficiency with a lower bound of 26%±11% from the emitter into the plasmonic mode of the silver nanowire. This technique offers a way to achieve efficient coupling between plasmonic structures and localized defects of 2D semiconductors.

Recently, ultrafast optical modulators (OMs) based on atomic transition-metal dichalcogenides (TMDs) film have been intensively explored. Benefited from their remarkable nonlinear saturable absorption properties, TMDs based OMs could be employed as critical devices for pulsed lasers systems to transit continuous wave into pulse trains in laser cavity. Herein, the few-layer TMDs films were grown by chemical vapor deposition (CVD) method in possession of uniform thickness, large areas and high crystal quality. Then two types TMDs based OMs were fabricated by integrating single TMDs film or van der waals heterostructures (VdWHs) on the target substrates. As for VdWHs based OMs, different few-layer TMDs films were vertically stacked in turns on the target substrates to form heterointerfaces, which has been demonstrated with ultrafast carrier relaxation time between neighbor layers recently and is favor for ultrafast pulse generation. In our experiments, the nonlinear optical properties of two types TMDs based OMs were systematically investigated by measuring their nonlinear saturable absorption curves and further compared by embedded them into same fiber laser systems. The results indicate that the VdWHs based OMs owns more excellent nonlinear optical properties (such as larger modulation depth, smaller saturable intensity) and offers a feasible strategy to engineer desired ultrafast photonics devices by modifying the structure of VdWHs.

The market for printed and flexible electronics, key attributes for internet of things, is estimated to reach $45 billion by 2016 and paper-based electronics shows great potential to meet this increasing demand due to its popularity, flexibility, low cost, mass productivity, disposability, and ease of processing. Solar-blind deep ultraviolet (DUV) photodetectors (PDs) can be widely applied in wearable applications such as military sensing, automatization, short-range communications security and environmental detection. In this work, we present flexible DUV paper PDs consisting of 2D boron nitride (h-BN) with good detectivity, fast recovery time (down to 0.393 s), great thermal stability (146 W/m K, 3-order-of-magnitude larger than conventional flexible substrates), high working temperature (up to 200 oC), excellent flexibility and bending durability (showing repeatable ON/OFF switching during 200-time bending cycles), which opens avenues to the flexible electronics.

Black phosphorus stands out from the family of two-dimensional materials as a semiconductor with a direct, layer-dependent bandgap in energy corresponding to the spectral range from the visible to the mid-infrared (mid-IR), as well as many other attractive optoelectronic attributes. It is, therefore, a very promising material for various optoelectronic applications, particularly in the important mid-IR range. While mid-IR technology has been advancing rapidly, both photodetection and electro-optic modulation in the mid-IR rely on narrow-band compound semiconductors, which are difficult and expensive to integrate with the ubiquitous silicon photonics. For mid-IR photodetection, black phosphorus has been proven to be a viable alternative. Here, we demonstrate electro-optic modulation of mid-IR absorption in few-layer black phosphorus under field applied by an electrostatic gate. Our experimental and theoretical results find that, within the doping range obtainable in our samples, the quantum confined Franz-Keldysh effect is the dominant mechanism of electro-optic modulation. Spectroscopic study on samples with varying thickness reveals strong layer-dependence in the inter-band transition between different sub-bands. Our results show black phosphorus is a very promising material to realizing efficient mid-IR modulators.

Transition metal dichalcogenides (TMDs) have attracted a great deal of attention for potential applications in a variety of areas including integrated optoelectronic devices. Most of TMDs transform from indirect to direct band-gap semiconductors when their thickness is reduced to a monolayer. Therefore, monolayer TMDs could serve as efficient optical gain materials, especially for making nanolasers with the smallest possible volume of active medium. Such lasers with small gain media could be important as light sources for integrated photonics for future on-chip interconnects, where extreme low energy consumption is required. Furthermore, the large exciton binding energy in monolayer TMDs exceeding 0.5 eV makes them important as potential candidate for exciton lasers at room temperature. So far, lasers based on monolayer TMDs have been reported at cryogenic temperatures, employing a monolayer WSe2 coupled onto a GaP photonic-crystal cavity, or a monolayer WS2 with a Si3N4 micro-disk. One of the possible reasons for the low temperature operation is the low Q-cavity as a result of the choices of cavity materials and structures and the associated fabrication precision. Here, we demonstrate the first room-temperature operation of a nanolaser using a monolayer molybdenum ditelluride nanolaser combined with a silicon nanobeam cavity. Lasing operation at 1132 nm is supported by the exciton emission with a linewidth of 0.202 nm and a Q factor of 5605. The room temperature operation shows the feasibility of TMDs nanolaser towards practical application. In addition, the silicon nanobeam cavity would make such 2D-based lasers more attractive for silicon photonics integration.

The locking of the electron spin to the valley degree of freedom in transition metal dichalcogenide (TMD) monolayers has seen them emerge as a promising platform in valleytronics. When embedded in optical microcavities the large oscillator strength of excitonic transitions in TMDs allows the formation of hybrid quasiparticles which are a superposition of the exciton and cavity photon states called polaritons. Here, we report the valley addressability of MoSe2 cavity polaritons under non-resonant helical excitation with a polarisation degree observed in photoluminescence that can be controlled by the exciton-cavity frequency detuning. Moreover, we observe both strong trion-cavity and exciton-cavity coupling demonstrating the formation of valley-polarised polaritons which are a linear superposition of excitons, trions and photons. In contrast to the very low circular polarisation degrees seen in MoSe2 exciton and trion resonances, we observe a significant enhancement of up to 20% when in the polaritonic regime. We further extend this work to the control of valley coherence in magnetic field in the polariton regime in WSe2 observed as rotation of linear polarisation imprinted by the pump laser. An unexpected feature here is the very different rotation angles of the linear polarisation in the lower and upper polariton branches, that is in addition controlled by the exciton-cavity-mode detuning. The effect originates from the modified exciton dynamics of the high-momentum excitons in the presence of the low-momentum polariton states.

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.

Monolayer transition metal dichalcogenides (TMD) with confined 2D Wannier-Mott excitons are intriguing for the fundamental study of strong light-matter interactions and the applications of exciton-polaritons based devices at high temperatures. However, the research of 2D exciton-polaritons has been hindered, because the polaritons in these atomically thin semiconductors discovered so far can hardly support strong nonlinear interactions and quantum coherence due to uncontrollable polariton dynamics and weakened coherent coupling. In this work, we demonstrate, for the first time, precisely controlled hybrid composition with angular dependence and dispersion-correlated polariton emission by tuning the polariton dispersion in TMD over a broad temperature range of 110-230 K in a single cavity. This tamed polariton emission is achieved by the realization of robust coherent exciton-photon coupling in a monolayer tungsten disulphide (WS2) with large splitting-to-linewidth ratios (SLR, >3.3). The unprecedented ability to manipulate the dispersion and correlated properties of TMD exciton-polariton at will offers new possibilities to explore important quantum phenomena such as Bose–Einstein condensation (BEC) and superfluidity, but also holds great promise to applications for the inversionless lasers and valleytronic devices.

Semiconducting 2D transition metal dichalcogenides (TMDs) show outstanding electrical and optical properties for novel optoelectronic applications. In this talk, I will first discuss optical measurements of the local strain matrix in 2D semiconductors. Our method is based on second harmonic generation. I will then review our activities on devices for electrically driven light emission from TMDs.

Two-dimensional transition metal dichalcogenides (2D TMDCs) are promising candidates for the formation of planar optoelectronic devices on a large range of substrates (including flexible substrates). In this talk, different techniques for the synthesis of TMDC alloys with high degree of control over the in-plane spatial profile of the alloying process for precise tuning of the optoelectronic properties of the resulting material will be demonstrated. The main challenges in using this process for forming high-quality lateral heterostructures (e.g., for the formation of quantum wells and quantum dots in a single-layer 2D material), especially the effect of alloying-induced strain and its impact on the structural and optoelectronic properties of TMDC alloys, will be discussed in depth; and solutions to address this challenge will be presented.

Understanding the band alignment at metal/2D semiconductor (SC) contacts is essential for electrical characterizations of 2D SC materials and for fabrication of high performance 2D SC devices. Many researchers have attempted to understand the electrical properties of metal/2D SC contacts and have revealed that they have unique features distinct from those of 3D SC counterparts. In this work, we investigated the surface potential (Vsurf) of exfoliated MoS2 flakes on bare and Au-coated SiO2/Si substrates using Kelvin probe force microscopy. The Vsurf of MoS2 single layers was larger on the Au-coated substrates than on the bare substrates; our theoretical calculations indicate that this may be caused by the formation of a larger electric dipole at the MoS2/Au interface leading to a modified band alignment. Vsurf decreased as the thickness of the flakes increased until reaching the bulk value at a thickness of ~20 nm on the bare and ~80 nm on the Au-coated substrates, respectively. This thickness-dependence of Vsurf was attributed to electrostatic screening in the MoS2 layers. Thus, a difference in the thickness at which the bulk Vsurf appeared suggests that the underlying substrate has an effect on the electric-field screening length of the MoS2 flakes. This work provides important insights to understand the band alignment and the charge transport at the metal/2D SC interfaces.