Daryl I. Vulis

A simple two-dimensional model system to study electrostatic-self-assembly

This paper surveys the variables controlling the lattice structure and charge in macroscopic Coulombic crystals made from electrically charged, millimeter-sized polymer objects (spheres, cubes, and cylinders). Mechanical agitation of these objects inside planar, bounded containers caused them to charge electrically through contact electrification, and to self-assemble. The processes of electrification and self-assembly, and the characteristics of the assemblies, depended on the type of motion used for agitation, on the type of materials used for the objects and the dish, on the size and shape of the objects and the dish, and on the number of objects. Each of the three different materials in the system (of the dish and of the two types of spheres) influenced the electrification. Three classes of structures formed by self-assembly, depending on the experimental conditions: two-dimensional lattices, one-dimensional chains, and zero-dimensional ‘rosettes’. The lattices were characterized by their structure (disordered, square, rhombic, or hexagonal) and by the electrical charges of individual objects; the whole lattices were approximately electrically neutral. The lattices observed in this study were qualitatively different from ionic crystals; the charge of objects had practically continuous values which changed during agitation and self-assembly, and depended on experimental conditions which included the lattice structure itself. The relationship between charge and structure led to the coexistence of regions with different lattice structures within the same assembly, and to transformations between different lattice structures during agitation.

On-chip all-dielectric fabrication-tolerant zero-index metamaterials

Zero-index metamaterials (ZIMs) offer unprecedented ways to manipulate the flow of light, and are of interest for wide range of applications including optical cloaking, super-coupling, and unconventional phase-matching properties in nonlinear optics. Impedance-matched ZIMs can be obtained through a photonic Dirac-cone (PDC) dispersion induced by an accidental degeneracy of an electric monopole and a transverse magnetic dipole mode at the center of the Brillouin zone. Therefore, PDC is very sensitive to fabrication imperfections. In this work, we propose and demonstrate fabrication-tolerant all-dielectric ZIM in telecom regime that supports near PDC dispersion over much wider parameter space than conventional designs. The prism device integrated with Si photonics is fabricated and measured for the verification.

Intracellular Delivery Using Nanosecond-Laser Excitation of Large-Area Plasmonic Substrates

Efficiently delivering functional cargo to millions of cells on the time scale of minutes will revolutionize gene therapy, drug discovery, and high-throughput screening. Recent studies of intracellular delivery with thermoplasmonic structured surfaces show promising results but in most cases require time- or cost-intensive fabrication or lead to unreproducible surfaces. We designed and fabricated large-area (14 × 14 mm), photolithography-based, template-stripped plasmonic substrates that are nanosecond laser-activated to form transient pores in cells for cargo entry. We optimized fabrication to produce plasmonic structures that are ultrasmooth and precisely patterned over large areas. We used flow cytometry to characterize the delivery efficiency of cargos ranging in size from 0.6 to 2000 kDa to cells (up to 95% for the smallest molecule) and viability of cells (up to 98%). This technique offers a throughput of 50000 cells/min, which can be scaled up as necessary. This technique is also cost-effective as each large-area photolithography substrate can be used to deliver cargo to millions of cells, and switching to a nanosecond laser makes the setup cheaper and easier to use. The approach we present offers additional desirable features: spatial selectivity, reproducibility, minimal residual fragments, and cost-effective fabrication. This research supports the development of safer genetic and viral disease therapies as well as research tools for fundamental biological research that rely on effectively delivering molecules to millions of living cells.

Monolithic CMOS-compatible zero-index metamaterials

Zero-index materials exhibit exotic optical properties that can be utilized for integrated-optics applications. However, practical implementation requires compatibility with complementary metallic-oxide-semiconductor (CMOS) technologies. We demonstrate a CMOS-compatible zero-index metamaterial consisting of a square array of air holes in a 220-nm-thick silicon-on-insulator (SOI) wafer. This design supports zero-index modes with Dirac-cone dispersion. The metamaterial is entirely composed of silicon and offers compatibility through low-aspect-ratio structures that can be simply fabricated in a standard device layer. This platform enables mass adoption and exploration of zero-index-based photonic devices at low cost and high fidelity.

Integrated Super-Couplers Based on Zero-Index Metamaterials

There has been strong interest in the confinement of electromagnetic energy in sub-diffraction limit waveguide configurations. Such an achievement would offer applications in in telecommunications, subwavelength imaging, optical memory storage, and on-chip photonic processes. Materials with a refractive index of zero have been considered as strong contenders for such “super-coupling” applications (Silveirinha MG, Engheta N, Phys Rev B 76:245109, 2007; Engheta N, Science 340:286–287, 2013). Though e-near-zero and μ-near-zero metamaterials, where zero index is obtained by tuning the effective electric permittivity or permeability to zero, have been proposed as a possible candidate, their infinite or zero impedance causes large reflections which pose a challenge for coupling applications.

Lossless integrated dirac-cone metamaterials

Zero-index metamaterials exhibit inherent losses due to poor confinement. We present a 2D photonic crystal that achieves isotropic impedance-matched zero-index propagation, but suffers no material or radiation loss due to a bound-state in the continuum.

Manipulating the flow of light using Dirac-cone zero-index metamaterials

Metamaterials with a refractive index of zero exhibit properties that are important for integrated optics. Possessing an infinite effective wavelength and zero spatial phase change, zero-index metamaterials may be especially useful for routing on-chip photonic processes and reducing the footprint of nonlinear interactions. Zero-index has only been achieved recently in an integrated platform through a Dirac-cone dispersion, enabling some of these more exciting applications in an integrated platform. This paper presents an overview of Dirac-cone zero-index metamaterials, including the fundamental physics, history and demonstration in the optical regime, as well as current challenges and future directions.

Monolithic CMOS-compatible zero-index metamaterials (Conference Presentation)

Zero-index metamaterials exhibit exotic optical properties such as uniform spatial phase and infinite wavelength. These extreme properties can be utilized for integrated-optics applications. However, practical implementation of zero-index-based photonic devices requires compatibility with complementary metallic-oxide-semiconductor (CMOS) technologies. Zero-index metamaterials have been previously demonstrated in both out-of-plane and integrated configurations by taking advantage of a photonic Dirac-cone dispersion at the center of the Brillouin zone. Such metamaterials feature a square matrix of high aspect-ratio pillars and offer matched impedance through simultaneously zero effective permittivity and permeability. However, these configurations are inherently incompatible with integrated devices due to out-of-plane excitation, metallic inclusions, or high aspect-ratio structures. This work demonstrates a CMOS-compatible zero-index metamaterial consisting of a square array of air-holes in a 220-nm-thick silicon-on-insulator wafer. To experimentally verify the refractive index, we measure the angle of refraction of light through a triangular prism consisting of the metamaterial. The index is extracted using Snells Law to verify a refractive index of zero at a wavelength of 1625 nm. Through the air-hole in silicon configuration, the proportion of silicon is increased as compared to designs based on high aspect-ratio silicon pillars. This enables a platform with low-aspect-ratio features, improved confinement of transverse electric polarized light, as well as the original benefit of matched impedance. Featuring a trivial monolithic fabrication and capacity for integration with the expansive library of existing silicon photonic devices, this metamaterial enables implementation of proposed zero-index devices and offers a powerful platform for exploring the future applications of zero-index materials.

Reusable titanium nitride plasmonic microstructures for intracellular delivery (Conference Presentation)

Efficient drug and biomolecular delivery into cells is an important area of biomedical research. Intracellular delivery relies on porating cell membranes to allow exterior molecules to enter the cell efficiently and viably. Various methods, including optoporation, electroporation, and viral techniques, can deliver molecules to cells, but come with significant drawbacks such as low efficiency, low throughput, and low viability. We present a new laser-based delivery method that uses laser pulses to excite plasmonic, Titanium Nitride (TiN) microstructures for cell poration and offers high efficiency, throughput, and viability. TiN is a promising plasmonic material for laser-based delivery methods due to its high levels of hardness and thermal stability. We fabricate these microstructures by sputtering thin films of TiN on patterned sapphire substrates. We then optimize plasmonic enhancement and stability by investigating different fabrication conditions. We deliver dye molecules, siRNA, and microspheres to cells to quantify poration efficiency and viability by using flow cytometry and by imaging the target cells at defined time intervals post laser irradiation. Additionally, we study temperature effects via simulations and experiments, as well as oxidation of the TiN films over time. We also use scanning electron microscopy (SEM) techniques to study microstructure damage and cell adhesion. Overall, TiN presents a promising opportunity for use as a reusable material in future biomedical devices for intracellular biomolecular delivery and regenerative medicine.

Pulsed Laser-Activated Plasmonic Pyramids for Intracellular Delivery

We use pulsed laser-activated plasmonic micropyramids to deliver molecules to living cells with high efficiency, viability, and throughput. Cellular therapy holds great promise for applications in gene therapy and fundamental biomedical research, and it is essential to develop a universal delivery platform that can safely deliver biomolecules to different cell types effectively. Such a platform would be an important stepping stone towards treatment of hematologic diseases such as leukemia and primary immunodeficiency disorder treatments. An idea molecular delivery platform would exhibit advantages such as high delivery efficiency, low toxicity, minimal immune reaction, and reusability. None of the currently available commercial methods, such as viral-based or electroporation, offer all desirable characteristics at once. We present a new optical method for molecular delivery that uses laser-activated microstructures. Our micropyramids produce a strong plasmonic effect under laser illumination by focusing energy in a small volume at the tip of each pyramid. This leads to the formation of microbubbles which temporarily porate the cell membrane and allow dye molecules and siRNA to diffuse into the cytoplasm. We fabricate large-area micropyramid arrays using photolithograpy, anisotropic etching of silicon, metal deposition, and template stripping. The silicon pyramid templates can be used repeatedly to fabricate gold pyramids. We optimize our laser parameters for high efficiency delivery of small dye molecules like calcein (>80 %) at high cell viability (>90 %). Alongside small dyes, we also deliver different-sized fluorescently labeled dextrans (70 kDa–2000 kDa) and fluorescent microspheres. Our method delivers molecules with high efficiency and high cell viability in different cell types, and our substrates can be reused for repeated high efficiency poration. Our scalable technique offers an innovative approach to delivering molecules to living cells for important applications in regenerative medicine.