Impact of interface roughness on the performance of broadband blackbody absorber based on dielectric-metal film multilayers.

We report on factors affecting the performance of a broadband, mid-IR absorber based on multiple, alternating dielectric / metal layers. In particular, we investigate the effect of interface roughness. Atomic layer deposition produces both a dramatic suppression of the interface roughness and a significant increase in the optical absorption as compared to devices fabricated using a conventional thermal evaporation source. Absorption characteristics greater than 80% across a 300 K black body spectrum are achieved. We demonstrate a further increase in this absorption via the inclusion of a patterned, porous anti-reflection layer.

Analysis of reflective Mach-Zehnder interferometry for electro-optic characterization of poled polymer films in multilayer structures.

We consider the Mach-Zehnder interferometer (MZI) method that specifically uses a poled organic thin film as one of the reflective mirrors in order to characterize the two independent electro-optic tensor elements r(13) and r(33). We discuss both a simple analysis based on a three-layer structure and a rigorous method including multiple reflection effects in a multilayer structure. In doing so, we find that the simple analysis of the reflective MZI method yields identical results to the reflection ellipsometric method (simple Teng-Man method), first introduced by Teng and Man as well as Shildkraut in 1990, when the ratio of r(13) to r(33) obtained from the MZI method is used in the analysis of the simple Teng-Man method. Error introduced by ignoring the multilayer nature of the sample structures in the MZI method is discussed and corrections are given for previous expressions in the literature for the simple analysis.

Broadband and mid-infrared absorber based on dielectric-thin metal film multilayers.

We propose a periodic multilayer structure of dielectric and metal interlayers to achieve a near-perfect broadband absorber of mid-infrared radiation. We examine the influence of four factors on its performance: (1) the interlayer metal conductance, (2) the number of dielectric layers, (3) a nanopatterned antireflective layer, and (4) a reflective metallic bottom layer for backreflection. Absorption characteristics greater than 99% of the 300 K and 500 K blackbody spectra are found for the optimized structures. Incident angle and polarization dependence of the absorption spectra are examined. We also investigate the possibility of fabricating a nanopatterned antireflective layer to maximize absorption.

Closed-form Maker fringe formulas for poled polymer thin films in multilayer structures.

We report new closed-form expressions for Maker fringes of anisotropic and absorbing poled polymer thin films in multilayer structures that include back reflections of both fundamental and second-harmonic waves. The expressions, based on boundary conditions at each interface, can be applied to multilayer structures containing a buffer and a transparent conducting oxide layer, which might enhance multiple reflections of fundamental and second-harmonic waves inside a nonlinear thin film layer. This formulation facilitates Maker fringe analysis for a sample containing additional multilayer structures on either side of a poled polymer thin film. Experimental data and numerical simulations are given to indicate the importance of inclusion of such a reflective layer in analyses for reliable characterization of second-harmonic tensor elements.

Effect of gold nanopillar arrays on the absorption spectrum of a bulk heterojunction organic solar cell.

We report on the effect of arrays of Au nanopillars of controlled size and spacing on the spectral response of a P3HT: PCBM bulk heterojunction solar cell. Prototype nanopillar-patterned devices have nearly the same overall power conversion efficiency as those without nanopillars. The patterned devices do show higher external quantum efficiency and calculated absorption in the wavelength range from approximately 640 nm to 720 nm, where the active layer is not very absorbing. The peak enhancement was approximately 60% at 675 nm. We find evidence that the corresponding resonance involves both localized particle plasmon excitation and multiple reflections/diffraction within the cavity formed by the electrodes. We explore the role of the attenuation coefficient of the active layer on the optical absorption of such an organic photovoltaic device.

Compact silicon microring-assisted directional couplers for optical signal processing applications.

We propose and demonstrate a compact microring-assisted directional coupler in the silicon-on-insulator material system. An analytical model based on the strongly coupled mode theory shows that the device can be designed to have a wide range of spectral characteristics, including sharp asymmetric Fano line shapes, which can be used for WDM, switching, and sensing applications. The measured transmission spectra of the fabricated device with a 5-mum-radius microring resonator exhibited an asymmetric spectral response characteristic of the Fano resonance as predicted by the analytical model.

Dispensed polymer waveguides and laser-fabricated couplers for optical interconnects on printed circuit boards.

Optical interconnects can provide chip-to-chip data communication with much needed bandwidth as processor speed and density keep growing. Optical waveguides and couplers are essential components for implementing optical interconnections. Techniques for directly dispensing polymer waveguides in laser-ablated trenches on printed circuit boards and for fabricating optical couplers are presented for quick prototype of optical interconnects. High-quality UV curable polymer waveguides were routinely fabricated. High-efficiency couplers, blazed grating couplers on sloped waveguides, sloped facet metal film couplers, and reflective-undercut facet couplers can be fabricated by using excimer laser ablation.

Measurement of complex Chi((3)) using degenerate four-wave mixing with an imaged 2-D phase grating.

We present a simple optical arrangement for phase sensitive detection of degenerate four-wave mixing (DFWM) to characterize the real and imaginary parts of Chi((3)) using an imaged 2-D phase grating. Phase sensitive coherent detection of DFWM signal is demonstrated. Phase stabilization of the interferometric arms is obtained passively with the 2-D grating. A processable polyacetylene sample is characterized at a wavelength of 1.5 microm using this technique. The observed nonlinearity is determined to be a fast (<250 fs) effect using a simple test.

Analysis of multiple reflection effects in reflective measurements of electro-optic coefficients of poled polymers in multilayer structures.

We present new closed-form expressions for analysis of Teng-Man measurements of the electro-optic coefficients of poled polymer thin films. These expressions account for multiple reflection effects using a rigorous analysis of the multilayered structure for varying angles of incidence. The analysis based on plane waves is applicable to both transparent and absorptive films and takes into account the properties of the transparent conducting electrode layer. Methods for fitting data are presented and the error introduced by ignoring the transparent conducting layer and multiple reflections is discussed.