David Lowell

Bio: David Lowell is an academic researcher from University of North Texas. The author has contributed to research in topics: Photonics & Photonic crystal. The author has an hindex of 8, co-authored 23 publications receiving 158 citations.

Holographic fabrication of graded photonic super-crystals using an integrated spatial light modulator and reflective optical element laser projection system

Abstract: For the first time, to the best of our knowledge, we have combined a spatial light modulator with a single reflective optical element for the holographic fabrication of graded photonic super-crystals. The hybrid laser projection system takes advantage of the spatial light modulator for pixel-by-pixel phase control and the reflective optical element for large-area, small-feature fabrication. Graded photonic super-crystals with dual period, or with dual period and dual basis, have been fabricated with location dependence across the interference pattern. The fabricated samples have been explained by the simulation of eight-beam interference patterns.

Holographic fabrication of 3D photonic crystals through interference of multi-beams with 4 + 1, 5 + 1 and 6 + 1 configurations

Abstract: In this paper, we are able to fabricate 3D photonic crystals or quasi-crystals through single beam and single optical element based holographic lithography. The reflective optical elements are used to generate multiple side beams with s-polarization and one central beam with circular polarization which in turn are used for interference based holographic lithography without the need of any other bulk optics. These optical elements have been used to fabricate 3D photonic crystals with 4, 5 or 6-fold symmetry. A good agreement has been observed between fabricated holographic structures and simulated interference patterns.

Simultaneous direct holographic fabrication of photonic cavity and graded photonic lattice with dual periodicity, dual basis, and dual symmetry

Abstract: For the first time, to the authors' best knowledge, this paper demonstrates the digital, holographic fabrication of graded, super-basis photonic lattices with dual periodicity, dual basis, and dual symmetry. Pixel-by-pixel phase engineering of the laser beam generates the highest resolution in a programmable spatial light modulator (SLM) for the direct imaging of graded photonic super-lattices. This technique grants flexibility in designing 2-D lattices with size-graded features, differing periodicities, and differing symmetries, as well as lattices having simultaneously two periodicities and two symmetries in high resolutions. By tuning the diffraction efficiency ratio from the SLM, photonic cavities can also be generated in the graded super-lattice simultaneously through a one-exposure process. A high quality factor of over 1.56 × 106 for a cavity mode in the graded photonic lattice with a large super-cell is predicted by simulations.

Holographic fabrication of graded photonic super-quasi-crystals with multiple-level gradients.

Abstract: Photonic quasi-crystals and photonic crystals with certain degrees of disorder can have a broadband light–matter interaction. In this paper, we present the holographic fabrication of graded photonic super-quasi-crystals through pixel-by-pixel phase pattern engineering using a spatial light modulator. Using the same phase pattern arranged in a decagon, we have fabricated graded photonic super-quasi-crystals with five-fold symmetry and multiple levels of gradients and graded photonic super-crystals with rectangular unit super-cells, depending on the Fourier filter. Although a certain degree of disorder was incorporated in the quasi-crystals, we still observed the golden ratio in the diameters of the diffraction rings of the fabricated quasi-crystals, indicating five-fold symmetry. Using direct pixel-by-pixel phase engineering, the same laser projection system, consisting of an integrated spatial light modulator and a reflective optical element, can be used for the fabrication of graded photonic super-crystals with various symmetries. The multi-level gradient effects on the optical properties of an organic light-emitting diode were simulated. When the cathode of an organic light-emitting device is patterned in the graded photonic super-crystals, a light extraction efficiency up to 76% in the visible range can be achieved.

Electrically tunable diffraction efficiency from gratings in Al-doped ZnO

Abstract: Transparent conducting aluminum-doped zinc oxide (AZO) can be used as part of an active plasmonic device due to its electrically tunable permittivity, which is accomplished by changing the carrier concentration with electrical biasing. In this letter, we report a continuous electrical tuning of diffraction efficiency from AZO gratings in the visible range (specifically 532 nm) when the AZO is under bias voltages between −1 V and −3.5 V. The carrier concentration in AZO under negative bias has been measured and simulated. The diffraction efficiency changes have been explained by the carrier concentration variation and induced complex refractive index change at the Al2O3 and AZO interface. The reported results can lead toward the application of post-fabrication tuning of optoelectronic devices using AZO.

Photonic crystals

Abstract: The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

Gate-Variable Optical Transitions in Graphene

Abstract: Two-dimensional graphene monolayers and bilayers exhibit fascinating electrical transport behaviors. Using infrared spectroscopy, we find that they also have strong interband transitions and that their optical transitions can be substantially modified through electrical gating, much like electrical transport in field-effect transistors. This gate dependence of interband transitions adds a valuable dimension for optically probing graphene band structure. For a graphene monolayer, it yields directly the linear band dispersion of Dirac fermions, whereas in a bilayer, it reveals a dominating van Hove singularity arising from interlayer coupling. The strong and layer-dependent optical transitions of graphene and the tunability by simple electrical gating hold promise for new applications in infrared optics and optoelectronics.

3D printing of glass by additive manufacturing techniques: a review

Abstract: Additive manufacturing (AM), which is also known as three-dimensional (3D) printing, uses computer-aided design to build objects layer by layer. Here, we focus on the recent progress in the development of techniques for 3D printing of glass, an important optoelectronic material, including fused deposition modeling, selective laser sintering/melting, stereolithography (SLA) and direct ink writing. We compare these 3D printing methods and analyze their benefits and problems for the manufacturing of functional glass objects. In addition, we discuss the technological principles of 3D glass printing and applications of 3D printed glass objects. This review is finalized by a summary of the current achievements and perspectives for the future development of the 3D glass printing technique.

Active optical metasurfaces: comprehensive review on physics, mechanisms, and prospective applications

Abstract: Optical metasurfaces with subwavelength thickness hold considerable promise for future advances in fundamental optics and novel optical applications due to their unprecedented ability to control the phase, amplitude, and polarization of transmitted, reflected, and diffracted light. Introducing active functionalities to optical metasurfaces is an essential step to the development of next-generation flat optical components and devices. During the last few years, many attempts have been made to develop tunable optical metasurfaces with dynamic control of optical properties (e.g., amplitude, phase, polarization, spatial/spectral/temporal responses) and early-stage device functions (e.g., beam steering, tunable focusing, tunable color filters/absorber, dynamic hologram, etc) based on a variety of novel active materials and tunable mechanisms. These recently-developed active metasurfaces show significant promise for practical applications, but significant challenges still remain. In this review, a comprehensive overview of recently-reported tunable metasurfaces is provided which focuses on the ten major tunable metasurface mechanisms. For each type of mechanism, the performance metrics on the reported tunable metasurface are outlined, and the capabilities/limitations of each mechanism and its potential for various photonic applications are compared and summarized. This review concludes with discussion of several prospective applications, emerging technologies, and research directions based on the use of tunable optical metasurfaces. We anticipate significant new advances when the tunable mechanisms are further developed in the coming years.