This PDF file contains the front matter associated with SPIE Proceedings Volume 11282, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Two-dimensional (2D) atomic transition metal dichalcogenides (TMDCs) have distinct emission properties, which can be applied for ultrathin detectors and light-emitters in the future. In this study, strain and photoluminescence of MoS₂ monolayer on a 3D substrate through a two-step growth procedure was analyzed. The structural of materials and those optical properties of monolayer TMDCs fabricated on the planar and nonplanar substrate were examined. Monolayer MoS₂ grown on the nonplanar substrate exhibited uniform strain reduction and luminescence intensity. The fabrication of monolayer MoS₂ on a nonplanar substrate increased the light extraction efficiency. In the future, strain-reduced 2D TMDC materials grown on a nonplanar substrate can be employed as novel light-emitting devices for applications in light-emitters, communication, and displays for ultrathin optoelectronic integrated systems.

In this talk I will introduce hexagonal boron nitride (hBN) as a promising layered material that hosts ultra-bright quantum emitters. I will present several avenues to engineer these emitters in different forms of hBN hosts and then show unique tuning experiments and promising results for controlling the emission wavelength of these quantum emitters. At the second part of my talk, I will discuss promising avenues to integrate the emitters with plasmonic and photonic cavities to achieve improved collection efficiency and Purcell enhancement. Lastly, I will introduce the fabrication and characterization of monolithic hBN cavities.

We present our results on trapping a tunable number of interlayer excitons (IX) within a nanoscale confinement potential induced by placing a MoSe2-WSe2 hetero-bilayer onto an array of nanopillars. The mean occupation of the IX trap is controlled via the optical excitation level and discrete sharp-line emission from different configurations of interacting IXs is observed. The intensities of these features exhibit characteristic near linear, quadratic, cubic and quartic power dependencies, which allows us to identify them as different multiparticle configurations of 1-4 IX and we directly measure the hierarchy of dipolar and exchange interactions as the number of increases.

Transition metal dichalcogenide (TMDC) monolayers are promising materials for nanoscale light emitters. By coupling TMDCs to optical antennas, the spontaneous emission rate can be greatly enhanced, yielding faster device modulation. However, achieving high rate enhancement is difficult with electrically-injected light emitters compared to optical injection. We demonstrate two device designs to overcome these challenges: 1) a light-emitting capacitor (LEC) coupled to a slot antenna array, and 2) a light-emitting diode (LED) coupled to a nanosquare antenna array. The LEC shows high polarization ratios >30x, while the LED shows >10x enhanced electroluminescence for the antenna coupled device relative to control devices.

Monolayers of semiconducting transition metal dichalcogenides excel due to their strong exciton dominated light matter interaction. We focus on monolayer MoS2 field effect structures and demonstrate that the degree of valley polarization that typically vanishes at elevated temperature can be restored even at room temperature by increasing the electron density. The recovering of the valley polarization via doping is linked to the suppression of the Fröhlich exciton LO-phonon interaction that mediates a uniaxial long-range oscillating electric field braking the three-fold rotational symmetry. Our results provide a promising strategy to increase the degree of valley polarization towards room temperature valleytronic applications.

Microcavity exciton-polaritons based on transition metal dichalcogenide monolayers (TMDs) are a promising platform for coherent valleytronics, exhibiting valley-dependent phenomena at room-temperature. Using polarization-dependent transient reflectance, we demonstrate the valley-exclusive nature of the optical Stark effect in WS₂ exciton-polaritons. We observe a simultaneous shift of both polariton branches when pump and probe are co-polarized and no appreciable shift when they are cross-polarized, demonstrating a polarization-selective stark shift in exciton-polaritons. This work highlights how the unique features of TMD exciton-polaritons can give rise to new polaritonic phenomena.

In the flourishing field of valleytronics, demand for coherently manipulating valley information at elevated temperature continues to escalate. Monolayer transition metal dichalcogenide, due to its strongly bonded excitons and degenerated valleys, nominates itself as a promising candidate for room temperature operation of valley degree of freedom (DOF). Through the hybridization of valley-resolved exciton and helicity-resolved photon mode, the valley DOF will be inherited by half-light, half-matter polaritons. Here, we demonstrate non-vanishing valley coherence of exciton-polaritons at room temperature in a cavity-embedded monolayer Tungsten Diselenide. The extra decay path through the exciton-cavity coupling, which is free of decoherence, is the key for intervalley phase correlation. These observations pave the way for room temperaturevalleytronic devices.

I will present our recent works on confining visible frequency photons in heterostructures for plasmonic metals and excitonic transition metal dichalcogenides (TMDCs) of Mo and W. Evidence of strongly coupling between excitonic modes and Fabry-Perot like resonances will be presented in unpatterned case. When the TMDC layer is patterned, plasmonic and dielectric grating modes emerge which lead to further coupling with excitonic modes resulting in tunable strong coupling and light confiementment. Finally, I will extend this notion to monolayer TMDCs and show evidence of near-unity absorption in metamaterials of the same for applications ranging from optical modulators to photodetectors and photovoltaics.

Prototyping of van der Waals materials on dense nanophotonic devices requires high-precision monolayer discrimination to avoid bulk material contamination. We use the glass transition temperature of polycarbonate, used in the standard dry transfer process, to draw an in situ point for the precise pickup of two dimensional materials. We transfer transition metal dichalcogenide monolayers onto a large-area silicon nitride spiral waveguide and silicon nitride ring resonators to demonstrate the high-precision contamination-free nature of the modified dry transfer method.

2D materials have a number of intriguing value proposition that could be harnessed for compact, tunable, high-performance optoelectronic devices when heterogeneously integrated in photonic circuits. Here I review our latest work including; (1) tunable TMD-based microring resonator with engineered critical-coupling condition, (2) a broadband graphene plasmon-slot photodetector (R=0.7A/W), (3) a 200mV bandgap-shifted strain-engineered absorption-enhanced MoTe2 photodetector (R=0.5A/W, low-dark-current <10nA@-1V), (4) a record-high responsivity (R=1.36A/W) slot-plasmon exciton-modulated MoTe2 photodetector, (5) a MoS2 electro-absorption modulator all enabled by our recently developed method of cross-contamination-free yet deterministic dry transfer 2D material ‘printer’ mimicking a 3D printer for enabling rapid prototyping. These devices are based on co-integration of 2D materials into Silicon and SiN photonics, with the latter used for on-exciton modulation or exciton absorption.

In this paper, we will outline the architectures of photodetectors and light emitting diodes based on the van der Waals heterostructures. For the demonstrated photodetectors, we will show that they not only can exhibit the features of linear-dichroic, broadband and fast (> 200 MHz) photodetections at room temperature, but also can be useful for mid-infrared imaging applications. Regarding to the light emitters, we will show that the vdW-based light emitting diodes are applicable to the visible spectral region. In addition, the demonstrated vdW light emitting diodes can be further integrated with photonic crystal cavities, and the integration would significantly enhance the efficiency of light emission and lead to the high degree of linear polarized electroluminescence.

In this work, we demonstrate a photodetector (PD) based on heterogeneous integration of Few-layer MoTe2 integrated on planarized and non-planarized Si waveguide operating at 1550 nm. Under a strong local tensile strain (4%), the bandgap of few layers MoTe2 shifts from 1 eV to 0.8 eV, enabling higher responsivity as compared to unstrained one.

The propagation of both electrons and photons becomes chiral when their momentum and spin are correlated in forms such as spin-momentum locking. For the surface electrons in three-dimensional topological insulators (TIs), their spin is locked to the transport direction. For photons in optical fibers and photonic waveguides, they carry transverse spin angular momentum (SAM) which is also locked to the propagation direction. A direct connection between chiral electrons and chiral photons occurs in Tis with lifted spin degeneracy, which leads to spin-dependent selection rules of optical transition and results in phenomena such as circular photogalvanic effect (CPGE). Here, we demonstrate an optoelectronic device that integrates a TI with a chiral photonic waveguide. Interaction between the photons in the transverse-magnetic (TM) mode of the waveguide, which carries transverse SAM, and the surface electrons in a Bi2Se3 layer generates a directional, spin-polarized photocurrent. Because of

As a new group of advanced 2D layered materials, bismuth oxyhalides, i.e., BiOX (X = Cl, Br, I), have recently become of great interest. In this work, we characterize the third-order optical nonlinearities of BiOBr, an important member of the BiOX family. The nonlinear absorption and Kerr nonlinearity of BiOBr nanoflakes at both 800 nm and 1550 nm are characterized via the Z-Scan technique. Experimental results show that BiOBr nanoflakes exhibit a large nonlinear absorption coefficient β ~ 10^-7 m/W as well as a large Kerr coefficient n₂ ~ 10^-14 m²/W. We also note that the n₂ of BiOBr reverses sign from negative to positive as the wavelength is changed from 800 nm to 1550 nm. We further characterize the thickness-dependent nonlinear optical properties of BiOBr nanoflakes, finding that the magnitudes of β and n₂ increase with decreasing thickness of the BiOBr nanoflakes. Finally, we integrate BiOBr nanoflakes into silicon integrated waveguides and measure their insertion loss, with the extracted waveguide propagation loss showing good agreement with mode simulations based on ellipsometry measurements. These results confirm the strong potential of BiOBr as a promising nonlinear optical material for high-performance hybrid integrated photonic devices.

We report spatially resolved measurement of third-harmonic generation (THG) emission from a Tin diselenide (SnSe₂) multi-layer flake at a fundamental excitation wavelength of 1550 nm using a nonlinear optical microscopy system and study its thickness dependence. We also estimate the magnitude of the real part of the electronic nonlinearity susceptibility (χ⁽³⁾ coefficient) by analyzing the thickness-dependence and found to be approximately 1.6×10^-19 m²/V², which is around 1500 times higher than that of the glass when measured with the same settings. We find excellent agreement between the measured THG thickness dependence and the analytical model considering absorption of harmonic emission in SnSe₂ medium, phase mismatch and the multipath interference due to the underlying oxide/Si substrate. We also measure the second harmonic generation from same flake and find this to be maximum for thickness in the range of 10-12nm.

The far-field linear and nonlinear optical response of excitons in two-dimensional (2D) semiconducting transitional metal dichalcogenides (TMDs), such as MoS₂ and WSe₂, have been the subject of intense investigation over the past decade. Here, we report on our experimental results measuring the linear and nonlinear response of surface plasmon polaritons (SPPs) propagating on metallic waveguides interacting with excitons in a single WSe₂ monolayer. The WSe₂ monolayer was encapsulated in hexagonal boron nitride and transferred on top of a metallic waveguide. The measurements were carried out at low temperature (below 11 K). We measured the linear absorption of SPPs by excitons, resulting in a 73 % attenuation of the transmitted probe. To determine the nonlinear response, we performed both optical pump-SPP probe and SPP pump-SPP probe experiments. For the SPP pump case, a differential transmission response exceeding 4 % was achieved. Time-resolved pump-probe measurements reveal a fast component of the nonlinear response of 290 fs with a slower 13.7 ps component, consistent with previous optical measurements. These plasmonic structures could open up new opportunities to probe fundamental light-matter interactions in 2D material-plasmonic heterostructures.

Polarization selective devices, such as polarizers and polarization selective resonant cavities (e.g., gratings and ring resonators), are core components for polarization control in optical systems and find wide applications in polarizationdivision- multiplexing, coherent optical detection, photography, liquid crystal display, and optical sensing. In this paper, we demonstrate integrated waveguide polarizers and polarization-selective micro-ring resonators (MRRs) incorporated with graphene oxide (GO). We achieve highly precise control of the placement, thickness, and length of the GO films coated on integrated photonic devices by using a solution-based, transfer-free, and layer-by-layer GO coating method followed by photolithography and lift-off processes. The latter overcomes the layer transfer fabrication limitations of 2D materials and represent a significant advance towards manufacturing integrated photonic devices incorporated with 2D materials. We measure the performance of the waveguide polarizer for different GO film thicknesses and lengths versus polarization, wavelength, and power, achieving a very high polarization dependent loss (PDL) of ~ 53.8 dB. For GOcoated integrated MRRs, we achieve an 8.3-dB polarization extinction ratio between the TE and TM resonances, with the extracted propagation loss showing good agreement with the waveguide results. Furthermore, we present layer-by-layer characterization of the linear optical properties of 2D layered GO films, including detailed measurements that conclusively determine the material loss anisotropy of the GO films as well as the relative contribution of film loss anisotropy versus polarization-dependent mode overlap, to the device performance. These results offer interesting physical insights and trends of the layered GO films from monolayer to quasi bulk like behavior and confirm the high-performance of integrated polarization selective devices incorporated with GO films.

We present our computationally efficient approach to modeling graphene-based active metadevices followed by the design and optimization of a graphene-based tunable refractive index (RI) sensor with ultra-high sensitivity. The classical integral multi-variate surface conductivity is reformulated in the time and frequency domains with physically interpretable and fast-to-compute integration-free terms. The model reveals decomposition of graphene response into a universal constant term plus a damped oscillator (digamma functions in the frequency domain) plus non-oscillating correction terms for near-zero potentials. We showcase the advantage of our approach by optimizing an ultrasensitive, tunable RI sensor with graphene and hexagonal boron nitride nanoribbons.

The design of an integrated Al-doped BP p-n homojunction with 2D Photonic crystal coupled with strip waveguide in SOI to enhance Photodetection for 3.7 μm is presented. Peak-Q of 1600 has been achieved by systematically optimizing the radius of the circularly shaped unit cell and period by performing 3D FDTD simulations. The thickness of the BP layer is calculated to be 17 nm for 3.7 μm absorption. The energy band structure and density-of-states for the monolayers are calculated using quantum expresso. The available 400 nm thick SOI lithography, in a standard multiproject wafer run, can be employed to ac hive the monolithic integration of our designed photodetector for mid-IR wavelengths. We believe this platform could help in realization of chip scale system for sensing application, optical wireless applications, interconnects (on-chip optical) and phased array for LIDAR applications.

Thermal poling, a technique to create permanently effective second-order susceptibility in silica optical fibers, has recently been improved by the discovery of an “induction poling” technique¹ and the adoption of liquid electrodes², allowing for poling fibers of any length and geometry. Nevertheless, the nonlinearity created via thermal poling is always limited by the 𝜒⁽³⁾of the optical fiber material and by the maximum electric field that can be frozen inside the glass. For these reasons research is ongoing to determine routes for further improving the nonlinear effects due to the thermal poling process. In this work, we propose to enhance the effects of the thermal poling by exploiting the intrinsic nonlinear properties of some 2D materials³, which are deposited inside the cladding holes of a twin-hole silica fiber. The materials we focused on are 2D Transition Metal Chalcogenide (2D TMDC) MoS₂ and WS₂ and the technique adopted to realize the deposition inside the cladding channels of a twin-hole step index silica fiber consists of a thermal decomposition process4 of the precursor ammonium tetrathiomolybdate (NH4)2MoS⁴ in 6% H₂/Ar flow. The technique has allowed us to uniformly coat the two cladding channels for a length of ≈25 cm with a film nominally consisting in a bi-layer of the 2D materials. A Raman based analysis has been used to test the morphology of the coating. The fiber deposited with 2D materials was later thermally poled and periodically erased via exposure to UV light to reach the QPM condition at a wavelength of ≈1550 nm. The effective 𝜒⁽²⁾ of the fiber was measured via SHG for both the deposited and the pristine fiber, showing an enhancement of the nonlinearity in favor of the deposited one. The phenomenon can be explained by the exploitation of a higher 𝜒⁽³⁾ seen by the pump wave due to the presence of the 2D layer deposited inside the cladding holes and opens the possibility of exploiting the higher intrinsic material 𝜒⁽²⁾, in case of a periodic patterning/synthesis of the TMDC.

Layered MoS₂ is a promising transition metal dichalcogenides (TMDC) material due to its outstanding physical and chemical properties. Chemical vapor deposition (CVD) is the most effective method to bring this layered TMDC material into mass production. During CVD synthesis of MoS₂, sulfurization of MoO₃ reactants by sulfur powers or an H₂S/H₂ mixture is an essential reaction step. However, the reaction processes associated with the sulfurization of MoO₃ by the H2_S/H₂ mixture are not fully understood. In addition, effects of H₂S/H₂ mixture on the sulfurization of MoO₃ still remain unclear. This is because the atomic scale resolutions of the reaction pathways for the reactions of MoO₃ and the H₂/H₂S mixture have yet to be obtained. Here, quantum molecular dynamics (QMD) simulations were performed to investigate the sulfurization of the MoO₃ slab using two different environments, i.e., A pure H₂S system and an H₂S/H₂ mixture. The QMD results reveal that the H₂S/H₂ mixture indeed reduce and sulfurize the MoO₃ slab effectively, when compared with pure H₂S precursors. This is primarily due to additional reactions of MoO₃ and H₂ molecules, leading to additional molybdenum oxyhydride intermediates during CVD processes. As such, the identification of these reaction pathways and the Mo-O-H reaction intermediates from QMD simulations may help experimental synthesis of higher quality MoS₂ layers.

Ultra-fast graphene optoelectronic devices require a stable voltage-free doped graphene that is achieved through meticulous molecular surface adsorption in this work. The graphene film resistance is significantly reduced from 40 to less than 10.