This PDF file contains the front matter associated with SPIE Proceedings Volume 10920, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Engineering solid state quantum systems is amongst grand challenges in engineering quantum information processing systems. While several 3D systems (such as diamond, silicon carbide, zinc oxide) have been thoroughly studied, solid state emitters in two dimensional (2D) materials have not been observed. 2D materials are becoming major players in modern nanophotonics technologies and engineering quantum emitters in these systems is a vital goal. In this talk I will first discuss the recently discovered single photon emitters in 2D hexagonal boron nitride (hBN). I will present several avenues to engineer these emitters in large exfoliated sheets using ion and electron beam techniques. I will also discuss potential atomistic structures of the defects supported by density functional theory. IN the 2nd part of my talk I will highlight promising avenues to integrate the emitters iwht plasmonic and photonic cavities to achieve improved collection efficiency. I will show preliminary results on nanofabrication of photonic crystal cavities from layered materials and pathway for an integrated quantum photonics with 2D materials. I will summarize by outlning challenges and promising directions in the field of quantum emitters and nanophotonics with 2D materials and other wide band gap materials.

The demonstration of stable quantum emitters in semiconductor transition metal dichalcogenides (TMDs) [1] and insulating hexagonal boron nitride (hBN) [2] has begun to impact the field of integrated quantum optics due to the promising properties of two-dimensional materials, such room-temperature operation, stretchability, heterogeneous device assembly and straightforward integration with photonic circuits. Nevertheless, major questions remain. Single photon emitters (SPEs) in hBN are associated with atom-like defects that confine electronic levels deep within the wide band gap. As recently reported, hBN can host several different families of emitters with emission energy that spans over a large spectral band [3]. On the other hand, quantum emission in TMDs is attributed to individual excitons bound to defects or impurities of the crystal structure. Recently, it has been shown that strain pockets can create potential traps able to confine single excitons producing SPEs with high spatial control [4,5]. Nevertheless, the relation between strain and emission energy is still not fully clear. The lack of a clear understanding of the nature of these defects [6] brings along the challenge of controlling the emission energy and of the deterministic creation of such emitters. These crucial aspects present central problems for developing identical single photon sources and for integration with photonic platforms. We developed a material processing based on ion irradiation and high temperature annealing of exfoliated hBN that sharply improves the single-photon purity with g(2)(0) = 0.08, and brightness with emission rate exceeding 10^7 counts/sec at room temperature. We also showed that the emitters persist material transfer process allowing to integrate them onto different platforms. To investigate the wide span of the emission energy we applied external compressive and tensile strain. Emitters in hBN show different tuning coefficients up to 6 meV per strain unit [7]. Furthermore, we performed photoluminescence excitation experiments at cryogenic temperature on different families of emitters. Our experimental results allow to identify the excited states of these atom-like systems and shine light on the characteristic level structures of the different families of emitters in hBN and can potentially help to reduce the spectral wandering with more efficient resonant excitation. Recently, new methods to develop quantum emitters in monolayer TMDs have been reported [4,5]. They rely on the strain induced by nanostructures in the substrate. This technique allows to create emitters with high spatial resolution. Unfortunately, it is still not possible to engineer the single photon energy with strain since the relation between strain and the emission energy of the localized excitons in TMDs is still not fully understood. To answer this question, we transferred CVD grown WSe2 on a pre-patterned Si3N4 substrate with pillars of 200 nm in diameter. We combined hyperspectral measurements, which allow to locate a single energy emission peak with a spatial resolution of around 10 nm, with strain measurements. Strain is measured optically by resolving in polarization the intensity of the second harmonic signal. This method allows to measure the photoelastic tensor of TMD monolayers and to retrieve the magnitude of biaxial strain with a spatial resolution of ~ 200 nm [8]. The combination of these techniques opens a new way to investigate the correspondence between the strain magnitude created by a nanopatterned substrate and the emission energy of the single photon emitted due to exciton localization in the nanostructures. [1] Perebeinos, V. Nat. Nano. 10, 485 (2015) [2] Tran, T. T., et al. Nat. Nano. 11, 37 (2016) [3] Tran, T. T. et al. ACS Nano 10, 7331 (2016) [4] Branny, A. et al. Nature Comm. 8, 15053 (2017) [5] Palacios-Berraquero, C. et al. Nature Comm. 8, 15093 (2017) [6] Tawfik, S.A. et al. Nanoscale 9, 13575 (2017) [7] Grosso, G., Moon, H, et al., Nature Comm. 8, 705 (2017) [8] L. Mennel, et al. Nature Comm. 9, 516 (2018)

The recent advent of van der Waals (vdW) crystals are considered as a new class of material for optoelectronics or photonics applications, as they have a wide range of optical band gap energies and electrical transport properties. Furthermore, due to the nature of vdW interactions, these vdW materials can be transferred onto different substrates, making them a promising candidate for integrated photonics applications. Here, we will exploit these unique properties and describe how to realize ultrathin and integrable light sources, based on photonic crystal cavities integrated van der Waals light emitters. Our demonstrated device can be operated at room temperature with fast modulation speed and enhanced emission intensity. Additionally, we will show ultrathin (~0.14 λ) van der Waals metalenses, which not only can exhibit near diffraction-limited focusing and imaging, but also can be transferred onto flexible substrates to show strain- induced tunable focusing.

The fundamental study and realisation of practical devices for quantum nanophotonic systems stems from the development of hybridised devices, consisting of a single photon source and various other constituents, which aid in controlling light-matter interactions. Emitters hosted within hexagonal boron nitride (hBN) are such a source favoured for this role, owing to its high quantum efficiency, brightness, and robustness. In our work, we explore and demonstrate the integration of hBN emitters with plasmonics, in two distinct arrangements – gold nanospheres, and a gold plasmonic nanocavity array. The former involves the utilisation of an atomic force microscope (AFM) tip to precisely position gold nanospheres to within close proximity to the quantum emitters and observe the resulting emission enhancement and fluorescence lifetime reduction. A fluorescence enhancement of over 300% and a saturated count rate in excess of 5M counts/sec is achieved, emphasising the potential of this material for hybridisation. The latter arrangement involves the direct transfer of a gold plasmonic lattice on top of an emitter hosted within hBN, similarly, to achieve emission enhancement as well as a reduction in fluorescence lifetime and provides an approach for achieving scalable, integrated hybrid systems based on low-loss plasmonic nanoparticle arrays. Both these systems give promising solutions for future employment of quantum emitters in hBN for integrated nanophotonic devices and provide us insight into the complex photodynamics, which envelop the emitters hosted within the material.

Control over the interactions between light and matter underlies many classical and quantum applications. In recent years, 2D layered semiconductors have gained prominence for optoelectronics because of their strong excitonic effects and capacity for van der Waals assembly. One of the unique features of these monolayer materials, the valley pseudospin, can be manipulated by controlling the local properties of optical fields. Here, we discuss two manifestations of this optical control across different regimes of coupling. In a strongly coupled regime, we discuss the dynamics of valley-polarized hybrid light-matter states, or exciton-polaritons, in a monolayer MoS2 embedded in a microcavity. Different dynamics of valley-polarized exciton-polaritons can be accessed with microcavity engineering by tuning system parameters such as cavity decay rate and exciton-photon coupling strength. Comparison of predictions and measurements demonstrate the ability to intentionally modify exciton-polariton valley characteristics, illustrating the microcavity as a tool for manipulating and engineering valley dynamics in 2D materials. In the weak coupling regime optical selection rules give rise to the valley-selective optical Stark shift. We discuss recent advances in probing this effect with improved sensitivity. Both of these complementary approaches show how the valley structure of monolayer materials yield interesting light-matter phenomena that allow tuning of optical properties.

In this talk we will discuss our recent work on electrical and optical control of strongly-coupled exciton-polaritons in two-dimensional Van der Waals materials. The possibility to optically address the valley degree of freedom of polariton states via optical excitation will be discussed [1]. Following this we will discuss the modulation of coupling strength between excitons and photons in a microcavity via electric field [2]. The possibility to enhance the nonlinear optical response of the polariton states by exploiting the higher order Rydberg states will also be presented. Finally, we will discuss the realization of room temperature quantum emitter array using strain engineered hexagonal boron nitride and coupling them to high quality factor resonators [3]. [1] Optical control of room temperature valley polaritons, Z. Sun, et al., Nature Photonics 11, 491 (2017). [2] Control of strong light-matter interaction in monolayer WS2 through electric field gating, B. Chakraborty et al., Nano. Lett. (2018) DOI: 10.1021/acs.nanolett.8b02932 [3] Near-deterministic activation of room temperature quantum emitters in hexagonal boron nitride, N. Proscia et al. Optica (In Press) [ArXiv: 1712.01352 ]

High sensitivity and fast response are the most important metrics for infrared sensing and imaging and together form the primary tradeoff space in bolometry. To simultaneously improve both characteristics requires a paradigm shift on the thermal properties of bolometric materials. Due to a vanishingly small density of states at the charge neutrality point, graphene has a record-low electronic heat capacity which can reach values approaching one Boltzmann constant Ce ~ kb. In addition, its small Fermi surface and the high energy of its phonons result in an extremely weak electron-phonon heat exchange. The combination will allow a strong thermal isolation of the electrons in graphene for higher sensitivity without sacrificing the detector response time. These unique thermal properties and its broadband photon absorption, make graphene a promising platform for ultrasensitive and ultra-fast hot electron bolometers, calorimeters and single photon detectors for low energy light. Here, we introduce a hot-electron bolometer based on a novel Johnson noise readout of the electron gas in graphene [1,2,3], which is critically coupled to incident radiation through a photonic nanocavity. This proof-of-concept operates in the telecom spectrum, achieves an enhanced bolometric response at charge neutrality with a noise equivalent power NEP < 5pW/√Hz, a thermal relaxation time of τ < 34ps, an improved light absorption by a factor ~3, and an operation temperature up to T=300K [3]. Altogether this shows that our proof-of-concept device can be a promising bolometer with efficient light absorption and a superior sensitivity-bandwidth product. Since the detector also has no limitations on its operation temperature, it provides engineering flexibility, which overall opens a new route for practical applications in the fields of thermal imaging, observational astronomy, quantum information and quantum sensing. In particular, since it is more than 5 times faster than the bandwidth of the intermediate frequency in the hot electron bolometer mixer, it can be employed as a cutting edge bolometric mixer material. [1] K. C. Fong, PRX, 2 (2012); [2] J. Crossno et.al., Science, 351 (2016); [3] D. K. Efetov et. al., Nature Nano. (2018));

Graphene has emerged as a promising material for photonics and optoelectronics due to its potential for ultrafast and broad-band photodetection. The photoresponse of graphene junctions is characterized by two competing photocurrent generation mechanisms: a built-in field driven photovoltaic effect and a more dominant hot- carrier-assisted photothermoelectric (PTE) effect. The hot-carrier PTE effect is understood to rely on abrupt variations in the Seebeck coefficient through the graphene doping profile. A second PTE effect can occur across a homogeneous graphene channel in the presence of an electronic temperature gradient. Here, we report on the latter effect facilitated by strongly localised plasmonic heating of graphene carriers in presence of nanostructured electrical contacts resulting in electronic temperatures of the order of 2000 K. At a certain gate bias, the plasmon-induced PTE photocurrent contribution can be isolated. In this regime, the device effectively operates as a sensitive electronic thermometer and as such represents an enabling technology for the development of hot carrier based plasmonic devices.

Atomically-thin two-dimensional (2D) materials including graphene and transition metal dichalcogenide (TMD) atomic layers (e.g. Molybdenum disulfide, MoS2) are attractive materials for optoelectronic and plasmonic applications and devices due to their exceptional flexural strength led by atomic thickness, broadband optical absorption, and high carrier mobility. Here, we show that crumple nanostructuring of 2D materials allows the enhancement of the outstanding material properties and furthermore enables new, multi-functionalities in mechanical, optoelectronic and plasmonic properties of atomically-thin 2D materials. Crumple nanostructuring of atomically thin materials, graphene and MoS2 atomic layers are used to achieve flexible/stretchable, strain-tunable photodetector devices and plasmonic metamaterials with mechanical reconfigurability. Crumpling of graphene enhances optical absorption by more than an order of magnitude (~12.5 times), enabling enhancement of photoresponsivity by 370% to flat graphene photodetectors and ultrahigh stretchability up to 200%. Furthermore, we present a novel approach to achieve mechanically reconfigurable, strong plasmonic resonances based on crumple-nanostructured graphene. Mechanical reconfiguration of crumple nanostructured graphene allows wide-range tunability of plasmonic resonances from mid- to near-infrared wavelengths. The mechanical reconfigurability can be combined with conventional electrostatic tuning. Our approach of crumple nanostructuring has potential to be applicable for other various 2D materials to achieve strain engineering and mechanical tunability of materials properties. The new functionalities in mechanical, optoelectronic, plasmonic properties created by crumple nanostructuring have potential for emerging flexible electronics and optoelectronics as well as for biosensing technologies and applications.

The fast carrier relaxation time, high carrier mobility and electrostatic tunability make graphene a prospective ideal material for electronics and optoelectronics. However, its low optical absorption is a big obstacle. Moreover, for using graphene in the large area optoelectronic devices, any scheme for enhancing the light-matter interaction in graphene should be polarization and incident angle-independent. Here, we demonstrate a novel design of an optical cavity-coupled hexagonal nanohole and nanodisk array to excite Dirac plasmon. We compare the Dirac plasmon lifetimes of the graphene nanohole and nanodisk arrays and their role in the enhanced light-matter interaction. Coupling the patterned graphene to an optical cavity creates a temporal and spatial overlap between the graphene plasmon and cavity modes. This complex geometry gives rise to an unprecedented polarization independent light absorption of 60% on nanohole and 90% on nanodisk arrays in low carrier mobility CVD-grown monolayer graphene in the 8-12 um atmospheric transparent infrared imaging band. Electrostatically doping of the patterned graphene tune the surface plasmon resonance wavelength up to 2.5 um by applying a small gate voltage (4V). We show theoretically, and also for the first time the experimental results of the enhanced light absorption for the non-normal incidence. While the light absorption up to 40° (incident angle) is almost constant, the trend of the angular optical response for s- and p-polarized light are different which is validated by our analytical coupled-dipole approximation modeling. This electronically tunable wide angle extraordinary light absorption paves the path towards new generation of graphene-based optoelectronics devices.

2D materials enable quantum well-like performance, while enjoying substrate independence. Together with their unique band-engineering potential, they pose an opportunity for exploring next generation devices. The rationale for heterogeneous integration is material function separation; that is to perform electrooptic switching in light-matter-enhanced or polaritonic material-mode combinations, while reserving the bosonic and weak-interacting character for photonics, ideally Si or SiN platforms for cost and loss competitiveness, respectively. Here we report on the first 2D material (TMD) integration into microring resonators (MRR), and demonstrated tunability to critical coupling regime. This system allows determining the TMD index via a semi-empirical approach, which is challenged by traditional ellipsometry due to the atom-thin TMD. We further discuss MRR-TMD electrooptic modulation contrasting spectrally ON versus OFF exciton tuning. We conclude by discussing optical nanocavity-TMD systems with applications in QED or LED emission, such as radiation and emission-channel engineering.

Fully microscopic many-body models based on the Dirac-Bloch equations and quantum-Boltzmann type scattering equations are used to study the carrier dynamics in monolayer transition metal dichalcogenides (TMDCs) under conditions as typical for applications as lasers, diodes or saturable absorbers. The carrier-carrier scattering is shown to be happening on an ultra-fast few-femtosecond timescale for excitations high above the bandgap. Once the carriers have relaxed into quasi-equilibrium distributions near the bandgap, the scattering is slowed dramatically by phase-space filling and screening of the Coulomb interaction. Here, the scatterings and resulting dephasing of the optical polarizations happen on a 100fs timescale and lead to similar broadenings as found in conventional III-V semiconductor materials. Also like the case in III-V materials, the carrier phonon scattering times are found to be in the picosecond range. The scatterings are shown to allow for gain spectra as needed for good lasing operation. It is shown that the weak interaction between the two bands associated with the two different sub-lattices can potentially allow for simultaneous lasing at two different frequencies. Strong absorption and ultrafast carrier relaxation could allow for TMDCs to be used in saturable absorption applications.

Graphene has emerged as an attractive nonlinear-optical material due to the high coefficient of two-photon absorption and four-wave mixing. Four-wave mixing in graphene has been previously studied in silicon-photonic platform. Enhancement of the four-wave mixing using optical cavities such as silicon micro-ring resonator (MRR) has been demonstrated. Recently, similar experiments have been extended to silicon-nitride (SiN) waveguides and micro-ring resonators. Electrostatic tuning of the four-wave mixing, and generation of frequency combs have been demonstrated using SiN MRRs having a Q-factor of 10⁶ at input pump powers ≥ 1 W. On-chip pump powers of the order of 10 mW to 100 mW are desirable to obtain high conversion efficiency of the four- wave mixing. However, such high on-chip powers are challenging to handle in integrated-optic platforms. We report preliminary experimental result of four-wave mixing in graphene-on-SiN MRRs with CW pump power of 120 μW, which is coupled to the MRR. The MRR used has a modest Q-factor of the order of 10³ after transferring graphene. We observe four-wave mixing even with a 50 % coverage of monolayer graphene on the MRR. Such low power level allows low-power on-chip nonlinear process. Furthermore, low photon count could be used for quantum photonic process and fundamental research where high conversion efficiency may not be necessary.

Owing to the ease of preparation as well as the tunability of its material properties, graphene oxide (GO) has become a rising star of the graphene family. In our previous work, we found that GO has an ultra-high Kerr nonlinear optical response - several orders of magnitude higher than that of silica and even silicon. Moreover, as compared with graphene, GO has much lower linear loss as well as nonlinear loss (two photon absorption (TPA)), arising from its large bandgap (2.4~3.1 eV) being more than double the photon energy in the telecommunications band. Here, we experimentally demonstrate enhanced four-wave mixing (FWM) in hybrid integrated waveguides coated with GO films. Owing to strong mode overlap between the integrated waveguides and the high Kerr nonlinearity GO films as well as low linear and nonlinear loss, we demonstrate significant enhancement in the FWM efficiency. We achieve up to ~9.5-dB enhancement in the conversion efficiency for a 1.5-cm-long waveguide with 2 layers of GO. We perform FWM measurements at different pump powers, wavelength detuning, GO film lengths and numbers of layers. The experimental results verify the effectiveness of introducing GO films into integrated photonic devices in order to enhance the performance of nonlinear optical processes.

Van der Waals semiconductors provide a platform for creating two-dimensional crystals layer-by-layer and engineering excitonic states therein with exceptional properties. We discuss here a few interesting opportunities enabled by interlayer excitons in bilayer transitional metal dichalcogenides (TMDs), including high valley polarizations, lasing in 2D cavities, and tunable interlayer excitons in homo-bilayers. Monolayer TMDs feature spin-valley locking, enabling valleytronic phenomena and applications. However, strong inter-valley scattering due to electron hole exchange interactions leads to rapid valley depolarization in picoseconds, making it difficult to achieve a high degree of valley polarization. The electron-hole exchange interaction becomes suppressed for interlayer excitons in heterobilayers with type II band alignment. We show highly polarized interlayer excitons with a long valley lifetimein in both spin singlet and brightened triplet states in hetero-bilayers. TMDs have also garnered intense interest as an active medium, for they feature very strong exciton-photon interactions in a monolayer. However, the rapid radiative decay makes it challenging to establish population inversion. Interlayer excitons decay much more slowly, comparable to excitons in quantum wells of conventional semiconductors. When the interlayer exciton are integrated on a cavity, lasing was established at the cavity resonance accompanied by increased temporal and spatial coherence. Lastly, due to the strong intra-layer localization of the carriers in 2D materials, interlayer excitons co-exist with intra-layer excitons as meta-stable states even in homo-bilayers without artificial interfaces. These interlayer excitons also feature oscillator strengths between intra-layer excitons and inter-layer ones in hetero-structures, offering a potentially highly tunable system for 2D optoelectronics.

Heterogeneous integration of ultra-thin atomic layer is expected to dominate future photonic and electronic devices due to their rich unique physical properties. Hence, an understanding of the role played by excitons of transition metal dichalcogenides (TMDs) is a prerequisite for achieving compact, cost-effective, efficient, fast and broadband optical modulator for advancement of optical communication. Here, we demonstrate a novel method of transferring 2D materials resembling the functionality known from printing; utilizing a combination of a sharp micro-stamper and viscoelastic polymer, we show precise placement of individual 2D materials resulting in vanishing cross-contamination to the substrate. Our 2D printer-method results in an aerial cross-contamination improvement of two to three orders of magnitude relative to state-of-the-art transfer methods from a source of average area for single flake (~50 μm2). Testing this 2D material printer on taped-out integrated Silicon photonic chips, we further demonstrate passive tunable coupling i.e. from overcoupled to undercoupled regime via critical coupling condition by integrating few layers of MoTe2 on a micro-ring resonator (MRR) for the first time which is important for active device functionality such as energy efficient modulator. Using this TMD–ring heterostructure, we further demonstrate a semi-empirical method to determine the index of the unknown TMD material near for telecommunication-relevant wavelengths. This novel approach bears further development potential to determine the index of the monolayers with high accuracy as compared to conventional methods. Such accurate and substrate-benign transfer method for 2D materials could be industrialized for rapid device prototyping due to its high time-reduction, accuracy, and contamination-free process.

We present that a thin layer of phthalocyanine (Pc) molecules can efficiently improve the optical quality of defective MoSe₂ grown by CVD method. Since MoSe₂ flakes are coupled with Pc molecules through van der Waals force, the structural change can be minimized, but the pristine properties can be significantly recovered. This defect healing effect can be achieved via ultrathin coverage of Pc molecules using a simple solution-mediated process. We also observed that Pc molecules with nanometer-scale lateral dimensions is useful in suppression of point defects commonly observed in mechanically exfoliated 2D TMDCs when it is improperly annealed at sub-decomposition temperatures.

Wavelength-selective integrated photonic devices in silicon-photonic platform require tuning to match the operating wavelength of multiple devices. The operating wavelengths are generally in the near-IR band. The conventional method of choice is to thermally tune the refractive index of silicon using metal micro-heaters. However, metals absorb near-IR wavelengths and must be placed away from the waveguides to avoid optical losses. This significantly reduces the power-efficiency of the heaters. Graphene-based local heaters on top of waveguides have been recently explored. Although the absorption in graphene is less than that of metals, it is still large enough to necessitate the placement of a thin spacer between the waveguide and the heater. We observe that metallic carbon-nanotubes (CNTs) are comparatively more transparent in the C-band. We implement heaters made of solution-processed metallic CNTs directly on top of a silicon-on-insulator micro-ring resonator. We demonstrate thermo-optic tuning of 60 pm/mW on a micro-ring resonator having a free-spectral range (FSR) of 1.75 nm. The estimated power efficiency is 29 mW/FSR, which is at par with previously implemented graphene-based heaters, that has higher absorption and better than conventional metal heaters. The proposed configuration offers compact and efficient thermal-tuner integration.