Mitigation of Si nanocrystal free carrier absorption loss at 1.5 microm in a concentric microdisk structure

We study the mitigation of Si nanocrystal (Si-nc) free carrier absorption (FCA) loss at telecom wavelengths in a concentric microdisk design. The concentric microdisk design relies on using the Si-nc emission as an optical pump for the surrounding Er-based lasing media without subjecting the lasing mode to the FCA loss present in the Si-ncs. We analyze the FCA loss as a function of overhang width in this design and show that for large enough overhang width the FCA loss is negligible. We also compute the FCA cross section from the FCA loss and number of excited Si-ncs, modeled by a four-level system, and show sigma(FCA)=1.08 +/- 2.3 x 10(-17) cm(2), which is in good agreement with reported cross sections for similar films.

Comparison of diurnal contrast changes for millimeter-wave and infrared imagery

Far-infrared outdoor imagery has a lower contrast in the morning/afternoon relative to the highest contrast, which is observed at 14:00. Millimeter-wave (mmW) imagery can also follow this pattern. However, in this paper, we show that the opposite can occur for mmW imagery, wherein a higher contrast can occur in the morning/afternoon and lower contrast at 14:00. To this end, we show that a wood and rubber sample are observed to have a difference in mmW radiometric temperature of 17 degrees C at 9:00 and a difference of only 7 degrees C at 14:00. Details of our observations are presented.

High yield fabrication of low threshold single-mode GaAs/AlGaAs semiconductor ring lasers using metallic etch masks

We demonstrate a novel high yield fabrication process for single-mode ridge-waveguide GaAs/AlGaAs ring lasers with significantly lower threshold currents than previously reported for similar devices. In this fabrication process, the ridge waveguide structure is patterned using a metallic etch mask, which survives ensuing fabrication steps to form a continuous metallic cover over the entire resonator structure. This metallic cover improves the uniformity of electrical contact between the resonator structure and the metallic biasing layer deposited at the conclusion of the fabrication process. This leads to optimum electrical pumping of the fabricated devices. This fabrication process also allows for the passivation of the ridge-waveguide device sidewalls and separation of the metallic biasing layer from the optical mode.

Pulsed pumping of silicon nanocrystal light emitting devices

Typical silicon nanocrystal light emitting devices (LEDs) operate under direct current (DC) biasing conditions that require high electric fields or high current densities. The electroluminescence (EL) under these conditions relies on impact excitation that can be damaging to the material. In this work, we present bipolar injection into silicon nanocrystal LEDs using a pulsed pumping scheme. We measured the frequency dependence of the integrated and time-resolved EL of the LEDs. The frequency dependent behavior of the time-resolved characteristics is used to explain the integrated EL measurements. In addition, the light output of the device was measured under pulsed excitation and was found to increase by a factor of 18 as compared to the case of DC excitation.

Comparison of raised-microdisk whispering-gallery-mode characterization techniques

We compare the two prevailing raised-microdisk whispering-gallery-mode (WGM) characterization techniques, one based on coupling emission to a tapered fiber and the other based on collecting emission in the far field. We applied both techniques to study WGMs in Si nanocrystal raised microdisks and observed dramatically different behavior. We explain this difference in terms of the radiative bending loss on which the far-field collection technique relies and discuss the regimes of operation in which each technique is appropriate.

Integration of silicon nanocrystals and erbium ring cavities for a silicon pumped Er:SiO2 laser

In this work we present a novel two-stage approach to achieve electrically pumped lasing on a CMOS compatible material platform in the telecom region. The proposed design consists of an electrically pumped silicon nanocrystal (Si-nc) light source acting as an optical pump for an Erbium doped silicate (Er:SiO2) lasing cavity. The integrated design, based on concentric disks of Si-nc and Er:SiO2, provides a means of coupling the Si-nc pump signal to the Er ions without requiring overlap of the Er based lasing mode with the Si-nc material. We present an electromagnetic analysis of the pump and lasing modes in the proposed configuration. We also present fabrication and characterization of Si-nc and Er:SiO2 microdisks as components of the integrated design.

Coupling Si nanocrystal microdisk emission to whispering-gallery modes in a concentric SiO2 ring

We present a concentric microdisk design in which luminescence from an inner disk of Si nanocrystals (Si-ncs) contributes to resonant modes in an outer ring of SiO2. Photoluminescence from fabricated structures reveals the excitation of whispering-gallery modes (WGMs) with quality factors as high as 2850, limited by the spectral resolution of our spectrometer. Two-dimensional finite-difference time-domain simulations provide insight into the WGM properties and the role of disk and ring geometry. The presented concentric disk structure provides a means to use the efficient visible luminescence of Si-ncs as an optical pump for an extrinsic lasing material such as Er:SiO2.

Phase modulation using dual split ring resonators

In this paper, we studied phase modulation numerically using metamaterials such as stacked structures of dual split ring resonators (DSRRs). To demonstrate the modulation, a vertical and a planar design were considered, where the wave vectors were parallel and perpendicular to the proposed structures creating 70 degrees and 80 degrees of phase change, respectively. In both of the designs modulation was brought about by changing the effective index of the structure through switching between the open and short states of the DSRRs while maintaining high transmission. One of the attractive features of our design was the thin layers of DSRRs, where for the vertical and planar models the DSRRs layers were 5 mm and 2.28 mm respectively. The numerical results obtained by simulation matched well with the theoretical prediction.

Thin film solar cell design based on photonic crystal and diffractive grating structures

In this paper we present novel light trapping designs applied to multiple junction thin film solar cells. The new designs incorporate one dimensional photonic crystals as band pass filters that reflect short light wavelengths (400 - 867 nm) and transmit longer wavelengths(867 -1800 nm) at the interface between two adjacent cells. In addition, nano structured diffractive gratings that cut into the photonic crystal layers are incorporated to redirect incoming waves and hence increase the optical path length of light within the solar cells. Two designs based on the nano structured gratings that have been realized using the scattering matrix and particle swarm optimization methods are presented. We also show preliminary fabrication results of the proposed devices.

Electromagnetic modeling of active silicon nanocrystal waveguides

In this paper we propose an electromagnetic analysis of active silicon nano-crystal (Si-nc) waveguide devices. To account for the nonlinearity in the active medium we introduce a four level rate equation model whose parameters are based on experimentally reported material properties. The electromagnetic polarization serves to couple the quantum mechanical and electromagnetic behavior within the ADE-FDTD scheme. The developed modeling tool is used to simulate waveguide amplifiers, enhanced spontaneous emission microcavities, and the temporal lasing dynamics of active Si-nc based devices.

Lasing dynamics of a silicon photonic crystal microcavity

In this paper we propose a novel silicon microcavity design based on the dispersion engineered photonic crystals (PhCs). With the unique self-collimation property of PhCs, we optimize the passive cavity by tuning the design parameters, such as coupling gap size and array size, to achieve higher Q factor and drop efficiency. Highest cavity mode below the band edge is of particular interest. The strong mode confinement in the low index active material offers an opportunity to realize a lasing mechanism. To investigate the lasing dynamics we introduce the rate equations of atomic system into the electromagnetic polarization to fully describe the nonlinearity of active medium. With these auxiliary differential equations we solve the time evolutions of the electromagnetic waves and atomic populations by using the FDTD method.

Calculation of effective permittivity, permeability, and surface impedance of negative-refraction photonic crystals

We consider the eigen-fields of a two-dimensional negative-refraction photonic crystal and obtain negative effective permittivity and negative effective permeability. Effective permittivity, permeability, and surface impedance are calculated by averaging the eigen-fields. The value of the surface impedance is shown to be location-dependent and is validated by finite-difference time-domain simulations. The unique power propagation mechanism in the photonic crystal is demonstrated through time-evolution of eigen-fields.

Negative refraction imaging in a hybrid photonic crystal device at near-infrared frequencies

We present the experimental demonstration of imaging of a point source by negative refraction at near-infrared frequencies using a hybrid photonic crystal device. The photonic crystal device, fabricated by patterning holes in 260nm silicon-on-insulator, integrates a triangular-lattice photonic crystal with a large photonic bandgap and square-lattice photonic crystal with negative refraction. Experimental results show that the output of a line-defect photonic bandgap waveguide provides a nearly ideal point source and then is imaged through the photonic crystal by negative refraction.

Multiscale free-space optical interconnects for intrachip global communication: motivation, analysis, and experimental validation

The use of optical interconnects for communication between points on a microchip is motivated by system-level interconnect modeling showing the saturation of metal wire capacity at the global layer. Free-space optical solutions are analyzed for intrachip communication at the global layer. A multiscale solution comprising microlenses, etched compound slope microprisms, and a curved mirror is shown to outperform a single-scale alternative. Microprisms are designed and fabricated and inserted into an optical setup apparatus to experimentally validate the concept. The multiscale free-space system is shown to have the potential to provide the bandwidth density and configuration flexibility required for global communication in future generations of microchips.

Two bit optical analog-to-digital converter based on photonic crystals

In this paper, we demonstrate a 2-bit optical analog-to-digital (A/D) converter. This converter consists of three cascaded splitters constructed in a self-guiding photonic crystal through the perturbation of the uniform lattice. The A/D conversion is achieved by adjusting splitting ratios of the splitters through changing the degree of perturbation. In this way, output ports reach a state of '1' at different input power levels to generate unique states desired for an A/D converter. To validate this design concept, we first experimentally characterize the relation between the splitting ratio and the degree of lattice perturbation. Based on this understanding, we then fabricate the 2-bit A/D converter and successfully observe four unique states corresponding to different power levels of input analog signal.

Optically tunable silicon photonic crystal microcavities

We demonstrate the use of silicon photonic crystal based microcavity structures to perform light modulation at potentially giga-Hertz speeds through the use of optically induced plasma dispersion. The cavity configurations considered have the potential to operate at low pump power when the Q of the cavity involved is maximized.

Experimental demonstration of self-collimation inside a three-dimensional photonic crystal

We present our experimental demonstration of self-collimation inside a three-dimensional (3D) simple cubic photonic crystal at microwave frequencies. The photonic crystal was designed with unique dispersion property and fabricated by a high precision computer-controlled machine. The self-collimation modes were excited by a grounded waveguide feeding and detected by a scanning monopole. Self-collimation of electromagnetic waves in the 3D photonic crystal was demonstrated by measuring the 3D field distribution, which was shown as a narrow collimated beam inside the 3D photonic crystal but a diverged beam in the absence of the photonic crystal.

Electromagnetic simulation of quantum well structures

We present an auxiliary differential equation Finite-difference Time-domain (ADE-FDTD) approach to numerically model the wave propagation within a gain or absorbing medium such as quantum well structures. Start from traditional quantum electronics theory, the macroscopic susceptibility of the semiconductor is derived and expressed by a multiple-Lorentz-like model based on Prony's method. With the auxiliary differential equation method each Lorentz-like model can be simulated in the time domain and the induced polarization is then determined by summing all the models. By incorporating the induced polarization into the time-domain Maxwell's equations, electromagnetic wave propagation in the quantum well medium can be accurately modeled using the FDTD method.

Perfect lens makes a perfect trap

In this work, we present for the first time a new and realistic application of the "perfect lens", namely, electromagnetic traps (or tweezers). We combined two recently developed techniques, 3D negative refraction flat lenses (3DNRFLs) and optical tweezers, and experimentally demonstrated the very unique advantages of using 3DNRFLs for electromagnetic traps. Super-resolution and short focal distance of the flat lens result in a highly focused and strongly convergent beam, which is a key requirement for a stable and accurate electromagnetic trap. The translation symmetry of 3DNRFL provides translation-invariance for imaging, which allows an electromagnetic trap to be translated without moving the lens, and permits a trap array by using multiple sources with a single lens. Electromagnetic trapping was demonstrated using polystyrene particles in suspension, and subsequent to being trapped to a single point, they were then accurately manipulated over a large distance by simple movement of a 3DNRFL-imaged microwave monopole source.

Three-dimensional subwavelength imaging by a photonic-crystal flat lens using negative refraction at microwave frequencies

We experimentally demonstrate subwavelength resolution imaging at microwave frequencies by a three-dimensional (3D) photonic-crystal flat lens using full 3D negative refraction. The photonic crystal was fabricated in a layer-by-layer process. A subwavelength pinhole source and a dipole detector were employed for the measurement. By point-by-point scanning, we obtained the image of the pinhole source shown in both amplitude and phase, which demonstrated the imaging mechanism and subwavelength feature size in all three dimensions. An image of two pinhole sources with subwavelength spacing showed two resolved spots, which further verified subwavelength resolution.

All-optical switching in silicon photonic crystal waveguides by use of the plasma dispersion effect

Silicon photonic crystals offer new ways of controlling the propagation of light as well as new tools for the realization of high-density optical integration on monolithic substrates. However, silicon does not possess the strong nonlinearities that are commonly used in the dynamic control of optical devices. Such dynamic control is nevertheless essential if silicon is to provide the higher levels of functionality that are required for optical integration. We demonstrate that the combination of the refractive index change caused by the presence of photoexcited carriers, or so-called plasma dispersion, and photonic crystal properties such as photonic bandgaps, constitutes a powerful tool for active control of light in silicon integrated devices. We show close to 100% modulation depth near the photonic crystal band edge.

Efficient terahertz coupling lens based on planar photonic crystals on silicon on insulator

We present a promising coupling device, namely, a terahertz (THz) planar photonic crystal (PhC) lens based on the effective refractive-index contrast between the PhC and the surrounding unpatterned area. Three-dimensional finite-difference time-domain calculations show a 90% power transfer from a 100-microm silicon waveguide to a 10-microm waveguide, and 45% coupling efficiency is confirmed experimentally. These results demonstrate the utility of the PhC lens as an effective approach to coupling into PhC THz circuits.

Design of two-dimensional polarization-selective diffractive optical elements with form-birefringent microstructures

We describe a design methodology for synthesizing polarization-sensitive diffractive optical elements based on two-dimensional form-birefringent microstructures. Our technique yields a single binary element capable of producing independent phase transformations for horizontally and vertically polarized illumination. We designed two elements for operation at 10.6 microm and fabricated them in silicon. Qualitative experimental results agree with design predictions.

Plane-wave expansion method for calculating band structure of photonic crystal slabs with perfectly matched layers

We present a new algorithm for calculation of the band structure of photonic crystal slabs. This algorithm combines the plane-wave expansion method with perfectly matched layers for the termination of the computational region in the direction out of the plane. In addition, the effective-medium tensor is applied to improve convergence. A general complex eigenvalue problem is then obtained. Two criteria are presented to distinguish the guided modes from the PML modes. As such, this scheme can accurately determine the band structure both above and below the light cone. The convergence of the algorithm presented has been studied. The results obtained by using this algorithm have been compared with those obtained by the finite-difference time-domain method and found to agree very well.

Total internal reflection-evanescent coupler for fiber-to-waveguide integration of planar optoelectric devices

We present a method for parallel coupling from a single-mode fiber, or fiber ribbon, into a silicon-on-insulator waveguide for integration with silicon optoelectronic circuits. The coupler incorporates the advantages of the vertically tapered waveguides and prism couplers, yet offers the flexibility of planar integration. The coupler can be fabricated by use of either wafer polishing technology or gray-scale photolithography. When optimal coupling is achieved in our experimental setup, the coupler can be packaged by epoxy bonding to form a fiber-waveguide parallel coupler or connector. Two-dimensional electromagnetic calculation predicts a coupling efficiency of 77% (- 1.14-dB insertion loss) for a silicon-to-silicon coupler with a uniform tunnel layer. The coupling efficiency is experimentally achieved to be 46% (-3.4-dB insertion loss), excluding the loss in silicon and the reflections from the input surface and the output facet.