Timothy Creazzo

Thin film silicon 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.

Integrated tunable CMOS laser

An integrated tunable CMOS laser for silicon photonics, operating at the C-band, and fabricated in a commercial CMOS foundry is presented. The III-V gain medium section is embedded in the silicon chip, and is hermetically sealed. The gain section is metal bonded to the silicon substrate creating low thermal resistance into the substrate and avoiding lattice mismatch problems. Optical characterization shows high performance in terms of side mode suppression ratio, relative intensity noise, and linewidth that is narrow enough for coherent communications.

Design and fabrication of transparent silicon solar cells for high efficiency

Transparent silicon solar cells can lead to increased energy conversion efficiency by enabling the collection and conversion of photons near and below the silicon band gap. This can be accomplished through the use of a low band gap solar cell in multiple junction systems. This analysis shows that the potential energy conversion efficiency gain for a one sun system is 7%. The design and fabrication of a silicon cell transparent to low energy, infrared (IR), light is presented.

Thin-film coating of vibro-fluidized microparticles via R. F. Magnetron sputtering

Recent improvements in microparticle synthesis and handling have prompted new research into the engineering and fabrication of single and multilayered microspheres through traditional physical and chemical vapor depositions. At the University of Delaware, we have developed a custom batch coating process utilizing a vibro-fluidized mixing vessel to deposit thin-films onto the surface of microparticle substrates through R.F. magnetron sputtering. This process opens up a number of design possibilities for single and multilayered microsphere technologies that can be used to improve the optical performance of several optical filtering applications. Through the use of custom design and simulation software, we have optimized a number of filter designs and validated these findings through commercial software. Specifically, we have aimed to improve upon the mass extinction performance seen by traditional materials in the long wave infrared spectrum (LWIR, λ=8-12μm). In order to do this, we have run a series of experiments aimed at creating ultra-lightweight metallic hollow-spheres. Aluminum thin-films have been successfully deposited onto a number of substrates including hollow glass microspheres, high density polyethylene microspheres, and polystyrene foam spheres. By depositing the thin-films onto polymer substrates we have been able to remove the solid core after deposition through a thermal decomposition or chemical dissolution process, in an effort to reduce particle mass and improve mass extinction performance of the filter. A quantum cascade laser measurement system has been used to characterize the optical response of these fabricated aluminum hollow-spheres and have largely agreed with the expected simulated results.

Frequency selective infrared optical filters for micro-bolometers

Current micro-bolometers are broadband detectors and tend to absorb a broad window of the IR spectrum for thermal imaging. Such systems are limited due to their lack of sensitivity to blackbody radiation, as well as the inability to spectrally discern multiple wavelengths in the field of view for hyperspectral imaging (HSI). As a result, many important applications such as low concentration chemical detection cannot be performed. One solution to this problem is to employ a system with thermoelectrically cooled or liquid nitrogen cooled sensors, which can lead to higher sensitivity in detection. However, one major drawback of these systems is the size, weight and power (SWaP) issue as they tend to be rather bulky and cumbersome, which largely challenges their use in unmanned aerial vehicles. Further, spectral filtering is commonly performed with large hardware and moving gratings, greatly increasing the SWaP of the system. To this point, Lumilant’s effort is to develop wavelength selective uncooled IR filters that can be integrated onto a microbolometer, to exceed the sensitivity imposed by the blackbody radiation limit. We have demonstrated narrowband absorbers and electrically tunable filters addressing the need for low-SWaP platforms.

Engineered micro-spheres for optical filtering

As infrared (IR) imaging technologies improve for the commercial market, optical filters complementing this technology are critical to aid in the insertion and benefit of thermal imaging across markets of industry and manufacturing. Thermal imaging, specific to shortwave infrared (SWIR) through longwave infrared (LWIR) provides the means for an observer to collect thermal information from a scene, whether being temperature gradients or spectral signatures of materials. This is beneficial to applications such as chem/bio sensing, where the identification of a chemical species being present or emitted could compromise personnel or the environment. Due to the abundant amount of information within an environment, the difficulty lies within the observer’s ability to extract the information. The use of optical filters paired with thermal imaging provides the means to interrogate a scene by looking at unique infrared signatures. The more efficient the optical filter can either transmit the wavelengths of interest, or suppress other wavelengths increases the finesse of the imaging system. Such optical filters can be fabricated in the form of micro-spheres, which can be dispersed into a scene, where the optical filter’s intimate interaction with the scene can supply information to the observer, specific to material properties and temperature. To this extent, Lumilant has made great progress in the design and fabrication of such micro-sphere optical filters. By engineering the optical filter’s structure, different optical responses can be tuned to their individual application.

Composite-CMOS Integrated Photonics for High Bandwidth WDM Optical Interconnects

Bandwidth requirements continue to drive the need for low-power, high speed interconnects. Harnessing the mature CMOS technology for high volume manufacturing, Silicon Photonics is a top candidate for providing a viable solution for high bandwidth, low cost, low power, and high packing density, optical interconnects. The major drawback of silicon, however, is that it is an indirect bandgap material, and thus cannot produce coherent light. Consequently, different integration schemes of III/V materials on silicon are being explored. An integrated CMOS tunable laser is demonstrated as part of a composite-CMOS integration platform that enables high bandwidth optical interconnects. The integration platform embeds III-V into silicon chips using a metal bonding technique that provides low thermal resistance and avoids lattice mismatch problems. The performance of the laser including side mode suppression ratio, relative intensity noise, and linewidth is summarized.

Design, fabrication, and testing of enhanced EO materials for mmW modulators

We seek to incorporate enhanced electro-optic (EO) materials into mmW imaging systems. EO based mmW detection systems have demonstrated sub-picowatt noise equivalent power while overcoming many of the drawbacks inherent in other systems including size, cooling, and cost. Current EO imaging systems rely on LiNbO3 modulators because of its strong EO effect. The linear EO effect, which only exists in non-centro-symmetric materials, has been shown to increase by orders of magnitude in quantum confined materials. III-V quantum dot materials offer the potential for a stronger EO effect than LiNbO3 while providing the advantages of III-V semiconductor integration. We focus on MBE grown InAs quantum dots in a GaAs matrix and offer XRD and AFM characterization These quantum dot materials are incorporated into an external Mach-Zehnder Interferometer setup where the Vπ is measured experimentally allowing us to extract an EO coefficient for the InAs/GaAs quantum dot layer of 39.4pm/V, an order of magnitude improvement relative to the bulk coefficients.