Joel R. Wendt

InGaN/GaN quantum-well heterostructure light-emitting diodes employing photonic crystal structures

Electrical operation of InGaN/GaN quantum-well heterostructure photonic crystal light-emitting diodes (PXLEDs) is demonstrated. A triangular lattice photonic crystal is formed by dry etching into the top GaN layer. Light absorption from the metal contact is minimized because the top GaN layers are engineered to provide lateral current spreading, allowing carrier recombination proximal to the photonic crystal yet displaced from the metal contact. The chosen lattice spacing for the photonic crystal causes Bragg scattering of guided modes out of the LED, increasing the extraction efficiency. The far-field radiation patterns of the PXLEDs are heavily modified and display increased radiance, up to ∼1.5 times brighter compared to similar LEDs without the photonic crystal.

Realizing optical magnetism from dielectric metamaterials.

We demonstrate, for the first time, an all-dielectric metamaterial resonator in the mid-wave infrared based on high-index tellurium cubic inclusions. Dielectric resonators are desirable compared to conventional metallo-dielectric metamaterials at optical frequencies as they are largely angular invariant, free of ohmic loss, and easily integrated into three-dimensional volumes. With these low-loss, isotropic elements, disruptive optical metamaterial designs, such as wide-angle lenses and cloaks, can be more easily realized.

Optical magnetic mirrors without metals

The reflection of an optical wave from metal, arising from strong interactions between the optical electric field and the free carriers of the metal, is accompanied by a phase reversal of the reflected electric field. A far less common route to achieving high reflectivity exploits strong interactions between the material and the optical magnetic field to produce a “magnetic mirror” that does not reverse the phase of the reflected electric field. At optical frequencies, the magnetic properties required for strong interaction can be achieved only by using artificially tailored materials. Here, we experimentally demonstrate, for the first time to the best of our knowledge, the magnetic mirror behavior of a low-loss all-dielectric metasurface at infrared optical frequencies through direct measurements of the phase and amplitude of the reflected optical wave. The enhanced absorption and emission of transverse-electric dipoles placed close to magnetic mirrors can lead to exciting new advances in sensors, photodetectors, and light sources.

Optical properties of two‐dimensional photonic lattices fabricated as honeycomb nanostructures in compound semiconductors

We have experimentally studied two‐dimensional photonic lattices, honeycomb nanostructures, fabricated by electron beam lithography with (Al,Ga)As materials. Surface normal optical properties were investigated by measuring reflectance to determine the effective index of refraction and lattice stability against degradation. Also, continuous wave and time‐resolved luminescence spectroscopy was used to assess electron‐hole recombination. Finally, light scattering was employed to study photon coupling and propagation through the lattice. These measurements show that the structures are stable, that nonradiative surface recombination is present, and that resonant coupling of light into/out of the lattice occurs at selected wavelengths satisfying a Bragg condition.

Giant microwave photoresistance of two-dimensional electron gas

We measure microwave frequency (4–40 GHz) photoresistance at low magnetic field B, in high mobility two-dimensional electron gas samples, excited by signals applied to a transmission line fabricated on the sample surface. Oscillatory photoresistance vs B is observed. For excitation at the cyclotron resonance frequency, we find a giant relative photoresistance ΔR/R of up to 250%. The photoresistance is apparently proportional to the square root of applied power, and disappears as the temperature is increased.

Nanofabrication of photonic lattice structures in GaAs/AlGaAs

The nanofabrication of two‐dimensional photonic lattice structures in GaAs/AlGaAs is reported. The nanofabrication procedure combines direct‐write electron‐beam lithography and reactive‐ion‐beam etching to achieve etched features as small as 50 nm. The lattice comprises a hexagonal array of air cylinders etched into a semiconductor surface with a refractive index contrast of 3.54. A range of air volume fractions from 14% to 84% was investigated. The lithographic, masking, and etching processes necessary to fabricate the lattice are described along with practical limitations to achieving a lattice of arbitrary air volume fraction. Initial results from optical characterization of the lattice are also presented.

Micrometer‐Scale Cubic Unit Cell 3D Metamaterial Layers

www.MaterialsViews.com C O M Micrometer-Scale Cubic Unit Cell 3D Metamaterial Layers M U N I By D. Bruce Burckel , * Joel R. Wendt , Gregory A. Ten Eyck , James C. Ginn , A. Robert Ellis , Igal Brener , and Michael B. Sinclair C A IO N The electromagnetic (EM) behavior of most bulk materials can be summarized by two frequency dependent tensors, the dielectric permittivity ( ε ) and the magnetic permeability ( μ ). The fi eld of metamaterials is predicated upon the possibility that man-made materials can exhibit absolute control over the magnitude and sign of both ε and μ within a specifi ed spectral band through the use of designed inclusions with specifi c engineered EM properties to create a new class of devices with enhanced functionality. For non-chiral, artifi cially structured materials, exotic EM behaviors such as perfect lensing [ 1 ] and negative refraction [ 2 ] are possible where ε and μ are simultaneously negative while cloaking requires the ability to spatially vary the permittivity and/or permeability over a wide range of values according to prescriptions from transformation optics. [ 3 ] In practice, achieving simultaneous negative ε and μ is diffi cult. The permittivity of metals below the bulk plasma frequency is inherently negative, and can be large; [ 4 ] however, naturally occurring negative permeability is quite rare, being presently limited to ferromagnetic materials at RF frequencies and lower. Furthermore, at high frequencies (optical and infrared) the response of materials to incident electromagnetic fi elds is dominated by the permittivity of the material, since the magnetic susceptibility is typically 4 orders of magnitude smaller in bulk materials than the electric susceptibility, [ 5 ] making μ r ≈ 1. In 1999 Pendry proposed an isotropic cubic unit cell with split ring resonators (SRR) located on its faces as a method for creating an artifi cially magnetic material in the microwave frequency range. [ 6 ] Shortly after Pendry proposed using SRRs to achieve magnetic permeability tuning of the resulting material, a negative index material was demonstrated with design wavelength λ = 30 mm (microwave) using a combination of SRRs and wires. [ 7 ] The individual SRRs measured 2.6 mm/side, were printed on circuit board material and then assembled into a macroscopic 2D array. One might expect that straightforward extension of the design, fabrication and characterization aspects of microwave metamaterials would lead to analog IR and visible metamaterials merely scaled to the wavelength of interest. In practice, however, translation of metamaterials

Polarization-sensitive subwavelength antireflection surfaces on a semiconductor for 975 nm.

We present the results of subwavelength antireflection surfaces etched into GaAs for use at 975 nm. These surfaces comprise linear gratings with periods less than the wavelength of light in GaAs. The structure appears as a homogeneous birefringent film. For one of the two polarizations, the film is directly analogous to the well-known quarter-wavelength antireflection coating. For the other polarization there is little effect on the surface reflectivity.

Fabrication of 3D Metamaterial Resonators Using Self-Aligned Membrane Projection Lithography

Fabrication of composite materials with designed constituent elements of sub-micrometer size typically requires cutting edge lithography techniques such as immersion lithography, [ 4 ] nanoimprint lithography, [ 5 ] or e-beam lithography. [ 6 ] While these techniques are capable of printing features with the requisite lateral dimensions, they are all planar patterning approaches, and hence offer limited options for creation of 3D structures, or structures with out-of-plane components. Other patterning techniques such as interferometric lithography are capable of creating 3D structures, [ 7 ] but are typically limited to periodic patterns, while direct write approaches are serial, [ 8 ] and hence do not scale well, severely limiting the design space. We introduce a fabrication technique called membrane projection lithography (MPL) which combines planar lithography with a sequence of processing steps to create micrometer-scale structures with out-of-plane components. The method is general, and can be repeated in a layer-by-layer fashion to create 3D volumetric materials with engineered inclusions. The basic premise behind MPL is to create a patterned membrane positioned over a cavity, and then use directional evaporation through the membrane to deposit instances of the membrane pattern on the interior face of the cavity. We fabricate micrometerscale metallic resonators using two separate MPL process fl ows: self-aligned MPL (SAMPL), and single-evaporation MPL (SEMPL). MPL is somewhat related to microstencil fabrication used in micro electromechanical systems (MEMS) fabrication, although the size scale, and linewidths of the patterns we present here are typically at least a factor of 10 smaller than those reported elsewhere. [ 9 ]

Experimental demonstration of a laterally deformable optical nanoelectromechanical system grating transducer

We experimentally demonstrate operation of a laterally deformable optical nanoelectromechanical system grating transducer. The device is fabricated in amorphous diamond with standard lithographic techniques. For small changes in the spacing of the subwavelength grating elements, lossy propagating resonant modes in the plane of the grating cause a large change in the optical reflection amplitude. An in-plane motion detection sensitivity of 160 fm/square root(Hz) was measured, exceeding that of any other optical microelectromechanical system transducer to our knowledge. Calculations predict that this sensitivity could be improved to better than 40 fm/square root(Hz) in future designs. In addition to having applications in the field of inertial sensors, this device could also be used as an optical modulator.