EMAT Fundamentals – Discover
Science and Engineering are the key paradigms for organizing a study of natural phenomena. Science, the active study, seeks to understand and predict. Engineering, the pro-active study, seeks to co-opt and re-purpose. Hand in hand, the study of Science and Engineering, as applied to materials, can be mapped by four significant milestones. These milestones chart the progress of EMAT graduate students through their education and research as they struggle, learn, master and, finally, bequeath answers...
... that raise new questions.
Figure: Discovery of Si nanocrystal light emission (875-1050 nm spectral range) from sputtered SiON:Er and Si3N4:Er thin films confirms the co-existence of nanocrystals with Er.
It's about being able to ask reckless questions in a responsible way.
Discovery comes from pushing the envelope. From rigorous mastery of the laws of electromagnetism, thermodynamics and mechanics, and their assiduous application in an experimental laboratory setting. Then comes the moment of risk: daring to ask questions and devise measurements that seek to shatter the known laws-to rock the foundation of your lab space.
The process of discovery owes as much to a diligent survey of your peer's work, as it does to the lone researcher's creative inner reasoning. It is the artistic component of a science study-one that is honed by experience over time.
Past achievements in discovery for the EMAT group include:
- Discovery of room temperature Er electroluminescence in a forward-biased Si:Er LED, for an on-chip integrated LED light-source.
- Discovery of a low threading dislocation density deposition process for the direct epitaxy of Ge onto a Si substrate, thereby enabling the fabrication of silicon circuits with monolithically integrated Ge detectors with high responsivity.
- Discovery of a one-dimensional photonic crystal defect state, which was integrated into a Si waveguide to produce a wavelength-selective filter with ultra-high modal confinement.
EMAT Fundamentals – Model
Figure: Modeling distributed gain, pump and signal power profiles along
the length of a Waveguide Optical Amplifier.
You've found something new. But does it make sense based on what you already know?
Modeling is the step back-the humble acknowledgment that whatever you've discovered in the lab must ultimately connect by a train of physical laws to the prior knowledge that informs us all. There's something new afoot, but it only makes sense if we can connect it to the past.and in doing so, enable the researcher to predict novel, unprecedented phenomena.
The exercise of modeling completes the science study of a natural phenomenon. It is the attribution of a mathematical language with which to quantitatively describe and predict a phenomenon as a physical system of interacting internal parts. It isn't enough to discover something strange and mysterious-that is exploration. Modeling is what turns exploration into a scientific study and an unknown natural phenomenon into a known physical system.
Past achievements in modeling for the EMAT group include:
- Modeling the temperature-dependent energy transfer mechanism between Er and the Si conduction band, giving quantitative description to the quantum efficiency of a Si:Er LED.
- Modeling the absorption interaction cross-section of Si nanocrystals in Si-rich Oxide providing a quantitative measure of optical sensitization during Er co-doping.
- Modeling the bandgap shrinkage of Ge-on-Si expitaxial films under tensile compression to correlate the extended spectral responsivity of our photodetectors with spectral photo-reflectance measurements.
EMAT Fundamentals – Design
Figure: Design plot illustrating the trade-off between areal footprint (reduction in bending radius) versus scattering loss for,
for planar waveguides, as a function of index difference Delta-n.
You've cracked a secret of nature and learned to model its inner workings. Now, can you do better?
Design is where the researcher's ambition reasserts itself; where the engineer steps in, and the scientist takes a backseat. Modeling has helped develop a consistent description of a physical system whereby we can quantitatively predict the output of the system, subject to a controlled input. Now, can we alter the system and, in turn, alter the output in a predictable way?
Engineering is the study of modification and alteration; where a material's properties are manipulated by thermal, chemical, or physical processing. Can we heat a material, dope it or chemically bond it, etch it or shape it, in a way that alters the output of the material or its performance as a light-guiding device? Design involves assessing what we require from our material or device's properties and selectively engineering that enhancement or creating a desired effect.
Past achievements in design for the EMAT group include:
- Design of an optimized Er-doped Waveguide Optical Amplifier (2003) by enhancement of device gain efficiency and minimization of device footprint-as a function of waveguide index difference.
- Design of a tunable WDM filter (2002) that relies on micro-electromechanical tuning of an air defect layer, while maintaining a processing flow that allows for planar integration with a waveguide.
- Design of oxidation smoothing and waveguide engineering processes that enable the fabrication of Si waveguides with record-low propagation losses.
EMAT Fundamentals – Prototype
Figure: Prototype structure for a photonic crystal waveguide, fabricated using CVD deposition and UV lithographic processing tools.
Finally, the big picture. Discover, model, and design-to what do they amount?
Prototyping is the last step in our engineering study; where the researcher integrates the new knowledge set created through the design process by inventing a material or device with novel functionality. The prototype makes manifest future technology elements for Si Microphotonics; its successful demonstration in the form of novel materials, devices and integrated electronic-photonic circuits serves to advance the discipline. It is the culminating achievement of a research cycle and serves as the seed for new questions and the beginning of a new cycle of discover, model, design and prototype.
Past achievements in prototyping by the EMAT group include:
- Prototype of photonic crystal waveguides (2004) that demonstrate above light-line propagation within silicon oxide cores, for the planar integrated Si platform.
- Prototype of GHz-precision ring resonator filters, fabricated under 0.18 µm processing constraints. GHz-precision filter characteristics were achieved by trimming a polymerized hexamethyldisilane overcladding, using a controlled UV dose.
- Prototype of an integrated waveguide-detector that has decoupled the efficiency of charge collection, from the efficiency of signal absorption, for the planar integrated Si platform and enabled a high responsivity, high-speed performance.