Engineering Refractive Index Contrast in Thin Film Barium Titanate-on-Insulator.

Barium titanate-on-insulator has demonstrated excellent vertical optical confinement, low loss, and strong electro-optic properties. To fabricate a waveguide-based device, a region of higher refractive index must be created to confine a propagating mode, one way of which is through dry etching to form a ridge. However, despite recent progress achieved in etching barium titanate and similar materials, the sidewall and surface roughness resulting from the physical etching typically used limit the achievable ridge depth. This motivates the exploration of etch-free methods to achieve the required index contrast. Here, we introduce three etch-free methods to create a refractive index contrast in barium titanate-on-insulator, including a metal diffusion method, proton beam irradiation method, and crystallinity control method. Notably, molybdenum-diffused barium titanate leads to a large index change of up to 0.17. The methods provided in this work can be further developed to fabricate various on-chip barium titanate optical waveguide-based devices.

A Barium Titanate-on-Oxide Insulator Optoelectronics Platform.

Electro-optic modulators are among the most important building blocks in optical communication networks. Lithium niobate, for example, has traditionally been widely used to fabricate high-speed optical modulators due to its large Pockels effect. Another material, barium titanate, nominally has a 50 times stronger r-parameter and would ordinarily be a more attractive material choice for such modulators or other applications. In practice, barium titanate thin films for optical waveguide devices are usually grown on magnesium oxide due to its low refractive index, allowing vertical mode confinement. However, the crystal quality is normally degraded. Here, a group of scandate-based substrates with small lattice mismatch and low refractive index compared to that of barium titanate is identified, thus concurrently satisfying high crystal quality and vertical optical mode confinement. This work provides a platform for nonlinear on-chip optoelectronics and can be promising for waveguide-based optical devices such as Mach-Zehnder modulators, wavelength division multiplexing, and quantum optics-on-chip.

Design and fabrication of high-performance multimode interferometer in lithium niobate thin film.

We propose and demonstrate a type of high-performance transverse magnetic (TM) multimode interferometer (MMI) in Z-cut thin film lithium niobate (TFLN). Both 1 × 2 and 4 × 4 MMI designs are demonstrated. Simulation results show that the insertion losses (ILs) are nominally about 0.157 and 0.297 dB for the 1 × 2 and 4 × 4 MMI, respectively, with wide fabrication tolerances. Based on the designed structure, the MMIs are fabricated using an argon based induced coupled plasma (ICP) etching method in Z-cut TFLN. The measured ILs are 0.268 and 0.63 dB for these two kinds of devices. The presented TM mode MMI featuring compact size and low loss can be used for both multifunctional devices and on-chip integrated circuits on a Z-cut TFLN platform.

Analysis of perovskite oxide etching using argon inductively coupled plasmas for photonics applications.

We analyzed the dry etching of perovskite oxides using argon-based inductively coupled plasmas (ICP) for photonics applications. Various chamber conditions and their effects on etching rates have been demonstrated based on Z-cut lithium niobate (LN). The measured results are predictable and repeatable and can be applied to other perovskite oxides, such as X-cut LN and barium titanium oxide (BTO). The surface roughness is better for both etched LN and BTO compared with their as-deposited counterparts as confirmed by atomic force microscopy (AFM). Both the energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) methods have been used for surface chemical component comparisons, qualitative and quantitative, and no obvious surface state changes are observed according to the measured results. An optical waveguide fabricated with the optimized argon-based ICP etching was measured to have -3.7 dB/cm loss near 1550 nm wavelength for Z-cut LN, which validates this kind of method for perovskite oxides etching in photonics applications.

Solar Power Can Substantially Prolong Maximum Achievable Airtime of Quadcopter Drones.

Sunlight energy is potentially excellent for small drones, which can often operate during daylight hours and fly high enough to avoid cloud blockade. However, the best solar cells provide limited power, compared to conventional power sources, making their use for aerial vehicles difficult to realize, especially in rotorcraft where significant lift ordinarily generated by a wing is already sacrificed for the ability to hover. In recent years, advances in materials (use of carbon-fiber components, improvement in specific solar cells and motors) have finally brought solar rotorcraft within reach. Here, the application is explored through a concise mathematical model of solar rotorcraft based on the limits of solar power generation and motor power consumption. Multiple solar quadcopters based on this model with majority solar power are described. One of them has achieved an outdoor airtime over 3 hours, 48 times longer than it can last on just battery alone with the solar cells carried as dead weight and representing a significant prolongation of drone operation. Solar-power fluctuations during long flight and their interaction with power requirements are experimentally characterized. The general conclusion is that solar cells have reached high enough efficiencies and can outperform batteries under the right conditions for quadcopters.

Planar Achiral Metasurfaces-Induced Anomalous Chiroptical Effect of Optical Spin Isolation.

Planar chiral structures respond differently for oppositely handed incident light, and thus can produce extraordinary chiroptical effects such as circular conversion dichroism (CCD) and asymmetric transmission (AT). Such chiroptical effects are powerful tools to realize the fundamental principle of optical spin isolation, which leads to a plethora of applications such as optical conversion diodes, chiral imaging, and sensing. Here, we demonstrate the chiroptical effects of simultaneous CCD and AT through meticulously designed single-layered achiral nanofins. Our metamolecule consists of four achiral hydrogenated amorphous silicon (a-Si:H) nanofins that are carefully oriented and optimized to exhibit considerable CCD and AT. The device demonstrates a circular conversion dichroism of 55% and an asymmetric transmission of 58% at a wavelength of 633 nm. Right-hand circularly polarized light (RHCP) is completely absorbed, while left-hand circularly polarized light (LHCP) is transmitted with a polarization conversion, making it a perfect circular polarization wave isolator with negligible backscattering (due to low reflectance). This unique design and its underlying working mechanism are described comprehensively with three different techniques. These methods validate the proposed design and its methodology. For practical applications such as imaging, the proposed design realizes the Pancharatnam-Berry (PB) phase, achieving a 0-2π phase coverage for transmitted circular polarization. For the proof of concept, a metahologram is designed and demonstrated by employing the achieved full-phase control. The measured response of the fabricated metadevice not only validates the CCD and AT but also exhibits a simulated polarization conversion efficiency of up to 71% and measured efficiency up to 52%, comparable to state-of-the-art metahologram demonstrations.

High pulsed power VCSEL arrays with polymer microlenses formed by photoacid diffusion.

We demonstrate millimeters-long VCSEL linear arrays with SU-8 epoxy-based microlenses that are directly patterned and cross-linked on the output apertures by a simple, photoacid-diffusion-aided photolithography technique. The linear arrays are capable of delivering >7 W of peak pulsed output power. By exploiting the photoacid diffusion effect, it is possible to produce a range of microlens structures with height and radius of curvature ranging from approximately ten to tens of microns. Simulation and experimental results show that the far-field beam divergence can be reduced by a factor of up to 7 in VCSELs integrated with optimal microlens dimensions.

Slippery and Wear-Resistant Surfaces Enabled by Interface Engineered Graphene.

Friction and wear remain the primary cause of mechanical energy dissipation and system failure. Recent studies reveal graphene as a powerful solid lubricant to combat friction and wear. Most of these studies have focused on nanoscale tribology and have been limited to a few specific surfaces. Here, we uncover many unknown aspects of graphene's contact-sliding at micro- and macroscopic tribo-scales over a broader range of surfaces. We discover that graphene's performance reduces for surfaces with increasing roughness. To overcome this, we introduce a new type of graphene/silicon nitride (SiNx, 3 nm) bilayer overcoats that exhibit superior performance compared to native graphene sheets (mono and bilayer), that is, display the lowest microscale friction and wear on a range of tribologically poor flat surfaces. More importantly, two-layer graphene/SiNx bilayer lubricant (<4 nm in total thickness) shows the highest macroscale wear durability on tape-head (topologically variant surface) that exceeds most previous thicker (∼7-100 nm) overcoats. Detailed nanoscale characterization and atomistic simulations explain the origin of the reduced friction and wear arising from these nanoscale coatings. Overall, this study demonstrates that engineered graphene-based coatings can outperform conventional coatings in a number of technologies.

Dielectric multi-momentum meta-transformer in the visible.

Metasurfaces as artificially nanostructured interfaces hold significant potential for multi-functionality, which may play a pivotal role in the next-generation compact nano-devices. The majority of multi-tasked metasurfaces encode or encrypt multi-information either into the carefully tailored metasurfaces or in pre-set complex incident beam arrays. Here, we propose and demonstrate a multi-momentum transformation metasurface (i.e., meta-transformer), by fully synergizing intrinsic properties of light, e.g., orbital angular momentum (OAM) and linear momentum (LM), with a fixed phase profile imparted by a metasurface. The OAM meta-transformer reconstructs different topologically charged beams into on-axis distinct patterns in the same plane. The LM meta-transformer converts red, green and blue illuminations to the on-axis images of "R", "G" and "B" as well as vivid color holograms, respectively. Thanks to the infinite states of light-metasurface phase combinations, such ultra-compact meta-transformer has potential in information storage, nanophotonics, optical integration and optical encryption.

Boosting contact sliding and wear protection via atomic intermixing and tailoring of nanoscale interfaces.

Friction and wear cause energy wastage and system failure. Usually, thicker overcoats serve to combat such tribological concerns, but in many contact sliding systems, their large thickness hinders active components of the systems, degrades functionality, and constitutes a major barrier for technological developments. While sub-10-nm overcoats are of key interest, traditional overcoats suffer from rapid wear and degradation at this thickness regime. Using an enhanced atomic intermixing approach, we develop a ~7- to 8-nm-thick carbon/silicon nitride (C/SiN x ) multilayer overcoat demonstrating extremely high wear resistance and low friction at all tribological length scales, yielding ~2 to 10 times better macroscale wear durability than previously reported thicker (~20 to 100 nm) overcoats on tape drive heads. We report the discovery of many fundamental parameters that govern contact sliding and reveal how tuning atomic intermixing at interfaces and varying carbon and SiN x thicknesses strongly affect friction and wear, which are crucial for advancing numerous technologies.

Ultra-low loss ridge waveguides on lithium niobate via argon ion milling and gas clustered ion beam smoothening.

Lithium niobate's use in integrated optics is somewhat hampered by the lack of a capability to create low loss waveguides with strong lateral index confinement. Thin film single crystal lithium niobate is a promising platform for future applications in integrated optics due to the availability of a strong electro-optic effect in this material coupled with the possibility of strong vertical index confinement. However, sidewalls of etched waveguides are typically rough in most etching procedures, exacerbating propagation losses. In this paper, we propose a fabrication method that creates significantly smoother ridge waveguides. This involves argon ion milling and subsequent gas clustered ion beam smoothening. We have fabricated and characterized ultra-low loss waveguides with this technique, with propagation losses as low as 0.3 dB/cm at 1.55 µm.

Spiniform phase-encoded metagratings entangling arbitrary rational-order orbital angular momentum.

Quantum entanglements between integer-order and fractional-order orbital angular momentums (OAMs) have been previously discussed. However, the entangled nature of arbitrary rational-order OAM has long been considered a myth due to the absence of an effective strategy for generating arbitrary rational-order OAM beams. Therefore, we report a single metadevice comprising a bilaterally symmetric grating with an aperture, creating optical beams with dynamically controllable OAM values that are continuously varying over a rational range. Due to its encoded spiniform phase, this novel metagrating enables the production of an average OAM that can be increased without a theoretical limit by embracing distributed singularities, which differs significantly from the classic method of stacking phase singularities using fork gratings. This new method makes it possible to probe the unexplored niche of quantum entanglement between arbitrarily defined OAMs in light, which could lead to the complex manipulation of microparticles, high-dimensional quantum entanglement and optical communication. We show that quantum coincidence based on rational-order OAM-superposition states could give rise to low cross-talks between two different states that have no significant overlap in their spiral spectra. Additionally, future applications in quantum communication and optical micromanipulation may be found.

Highly efficient tunable and localized on-chip electrical plasmon source using protruded metal-insulator-metal structure.

A compact and highly efficient tunable and localized source of propagating surface plasmon-polaritons is proposed based on a protruded metal-insulator-metal (pMIM) structure. The protrusion along a segment of the pMIM forms a nanometer gap and allows a low voltage bias to generate a localized tunneling current. The tunneling current excited plasmons can be fully coupled to the metal-insulator-metal (MIM) waveguiding segment of the pMIM without leakage and propagate a long distance as the gap in the MIM waveguiding segment is much larger than the gap in the protruded segment of the pMIM. Eigenmode and numerical analyses show that by using MIM structures as a benchmark, the pMIM structure enhances the total amount of average power that is transferred from the tunneling current into the excitation of intrinsic eigenmodes of the MIM waveguiding segment. Depending on the magnitude of the applied voltage bias, the pMIM structure supports single, dual and multi modes for a typical Au-SiO2-Au design with a 500 nm-thick SiO2. Among all excited modes, the single mode operation allows highly efficient excitation of long travelling surface plasmon-polaritons (SPPs) of up to 30 µm. The electrical excitation of SPPs using pMIM structures opens up the possibility of integrating plasmon sources into nanoscale optoelectronic circuits to facilitate on-chip data communications.

Periodic Two-Dimensional GaAs and InGaAs Quantum Rings Grown on GaAs (001) by Droplet Epitaxy.

Growth of ordered GaAs and InGaAs quantum rings (QRs) in a patterned SiO2 nanohole template by molecular beam epitaxy (MBE) using droplet epitaxy (DE) process is demonstrated. DE is an MBE growth technique used to fabricate quantum nanostructures of high crystal quality by supplying group III and group V elements in separate phases. In this work, ordered QRs grown on an ordered nanohole template are compared to self-assembled QRs grown with the same DE technique without the nanohole template. This study allows us to understand and compare the surface kinetics of Ga and InGa droplets when a template is present. It is found that template-grown GaAs QRs form clustered rings which can be attributed to low mobility of Ga droplets resulting in multiple nucleation sites for QR formation when As is supplied. However, the case of template-grown InGaAs QRs only one ring is formed per nanohole; no clustering is observed. The outer QR diameter is a close match to the nanohole template diameter. This can be attributed to more mobile InGa droplets, which coalesce from an Ostwald ripening to form a single large droplet before As is supplied. Thus, well-patterned InGaAs QRs are demonstrated and the kinetics of their growth are better understood which could potentially lead to improvements in the future devices that require the unique properties of patterned QRs.

Visible-Frequency Metasurface for Structuring and Spatially Multiplexing Optical Vortices.

A multifocus optical vortex metalens, with enhanced signal-to-noise ratio, is presented, which focuses three longitudinal vortices with distinct topological charges at different focal planes. The design largely extends the flexibility of tuning the number of vortices and their focal positions for circularly polarized light in a compact device, which provides the convenience for the nanomanipulation of optical vortices.

On-chip discrimination of orbital angular momentum of light with plasmonic nanoslits.

The orbital angular momentum (OAM) of light can be taken as an independent and orthogonal degree of freedom for multiplexing in an optical communication system, potentially improving the system capacity to hundreds of Tbits per second. The high compactness and miniaturization of devices required for optical communications impose strict requirements on discriminating OAM modes of light at a small (micro- or even nano-meter) scale for demultiplexing; these requirements represent a challenge for traditional OAM sorting strategies. Here, we propose a semi-ring plasmonic nanoslit to directly and spatially sort various OAM modes of light into ∼120 nm-spaced mode intervals on the metallic surface. Making use of the constructive interference of a helical-phase modulated surface wave excited by a vortex beam, this on-chip interval can be stably demonstrated both theoretically and experimentally with a quasi-linear dependence on the plasmonic wavelength. Furthermore, its immunity to semi-ring geometry (i.e., the radius and number of rings) is verified by simulations. As a result, OAM discriminating is guaranteed by this stable sorting function. This technique shows a viable solution to discriminate the OAM of light at the nano-scale and might lead to broad benefits across the fields of optical communications, plasmonic physics and singular optics.

Physical mechanisms for tuning the nonlinear effects in photonic crystals.

By simultaneously taking field localization and slow light effects into account, in this paper we make use of a field averaging method to calculate the effective nonlinear refractive index coefficient (n2) of Kerr photonic crystals (PhCs) in the first band. Although the nonlinear PhC is beyond the traditional long-wavelength limit, interestingly, the theoretically calculated effective n2 agrees well with one numerically measured via the self-phase-modulation induced spectral broadening. Moreover, due to the cooperative influence of field localization and slow light effects, the effective n2 of the PhC decreases slowly at first and then goes up quickly with increasing frequency. This kind of dispersive nonlinearity is purely induced by the periodic nanostructures because the optical parameters of both components of the PhC we took are frequency-independent. Our results may pave the way for enhancing or limiting nonlinear effects and provide a method for producing the dispersive nonlinearity.

Characterization of C-apertures in a successful demonstration of heat-assisted magnetic recording.

An optical pump-probe setup was used to measure the coercivity change in a heat-assisted magnetic recording (HAMR) medium. The incident optical power required to attain the Curie temperature of the medium was determined by calculating its coercivity from BH loops under different illuminating laser powers through use of the Kerr signal in the pump-probe setup. The HAMR medium was then illuminated through an array of square and C-shaped nanoapertures so that the necessary laser power required for magnetic reversal could be compared to the bulk case. Magnetic force microscopy and Kerr microscopy revealed that C-apertures were able to permit heating of the magnetic medium and lower the coercivity to achieve magnetic reversal whereas the square apertures were not. The results show that aperture shape and design play a large role in HAMR head designs.

The Lissajous lens: a three-dimensional absolute optical instrument without spherical symmetry.

We propose a three dimensional optical instrument with an isotropic gradient index in which all ray trajectories form Lissajous curves. The lens represents the first absolute optical instrument discovered to exist without spherical symmetry (other than trivial cases such as the plane mirror or conformal maps of spherically-symmetric lenses). An important property of this lens is that a three-dimensional region of space can be imaged stigmatically with no aberrations, with a point and its image not necessarily lying on a straight line with the lens center as in all other absolute optical instruments. In addition, rays in the Lissajous lens are not confined to planes. The lens can optionally be designed such that no rays except those along coordinate axes form closed trajectories, and conformal maps of the Lissajous lens form a rich new class of optical instruments.

Exploiting design freedom in biaxial dielectrics to enable spatially overlapping optical instruments.

The optical behavior of gradient biaxial dielectrics has not been widely explored in the literature due to their complicated nature, but the extra degrees of freedom in the index tensor have the potential of yielding useful optical instruments which are otherwise unachievable. In this work, a design method is described in detail which allows one to combine the behavior of up to four totally independent isotropic optical instruments in an overlapping region of space. This is non-trivial because of the mixing of the index tensor elements in the Hamiltonians; previously known methods only handled uniaxial dielectrics (where only two independent isotropic optical functions could overlap). The biaxial method introduced also allows three-dimensional multi-faced Janus devices to be designed; these are worked out in an example of what is possible to design with the method.