Ray Chen, Ph.D. - US grants

This proposal was submitted in response to the solicitation "Nanoscale Science and Engineering" (NSF 00-119). The goal of this project is to develop laser and optoelectronic device technologies that achieve photon and electron confinement to generate 0-dimensional states, based on advances in nanolithography and dry etching to fabricate nanocrystals containing self-organized quantum dots. A decrease of a semiconductor laser's volume to its minimum size, while maintaining high Q, along with a decrease in the electronic confinement potential, may result in revolutionary advances in device operation. These include high-speed operation below and at threshold, and high efficiency in the spontaneous regime below threshold. In the ultimate limits of small active volume and sufficiently high Q the system can enter the quantum reversible regime necessary to create quantum-entangled states. Both these quantum limits of the photons and electron-hole pairs are possible using III-V nanostructured active material and nanostructured photonic crystals. The materials to be employed in these studies will be GaAs/AlGaAs/InGaAs strained layer heterostructures grown by molecular beam epitaxy, which will be fabricated into photonic crystal lasers and microcavities. The III-V heterostructures will be grown at the University of Texas/Austin Microelectronics Research Center, and photonic crystal fabrication will take place at the California Institute of Technology(CIT) and at UT-Austin. The III-V nanostructures will be optimized for high-speed operation based on studies to be carried out at CIT. Manufacturable processes for the nanolithography will be developed at UT-Austin. Graduate research assistants working towards Ph.D. degrees represent a major component of this research. The expected impact of the research is the development of a new technology for low power, high speed optoelectronic interconnects suitable for wavelength division multiplexing and low power transceivers for optical interconnects, and new devices useful for exchange of quantum information.
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The project addresses basic materials science and engineering research issues in a topical area of materials science with high technological relevance. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area. The project will develop strong technical, communication, and organizational/management skills in students through unique educational experiences made possible by a collaborative forefront research environment. The project is co-supported by the DMR/EM, ECS/PFET, and EEC Divisions.
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This project is to develop a materials platform for mid-infrared integrated photonics and create mid-infrared photonic crystal sensors with parts per billion sensitivity. Such a platform and sensors do not currently exist. High sensitivity will be achieved by targeting strong fundamental vibrational absorption lines of chemical components in the mid-infrared spectral range, also known as the molecular fingerprint region, and employing the principle of slow light in slotted photonic crystal waveguides. The photonic crystal waveguides will be monolithically integrated with mid-infrared quantum cascade lasers and detectors using epitaxial transfer techniques to produce highly compact integrated-photonic chemical sensors operating in the mid-infrared spectral region for a wide range of applications, such as chemical and biomedical sensing, environmental monitoring, and security applications.

The scientific objective of this project to investigate a mid-infrared photonic waveguideing platform spanning molecular fingerprint region (3-14 micrometer) and develop integrated-photonic sensors in the molecular fingerprint region. Silicon-on-insulator devices cannot be operated beyond 3.5 micrometer wavelength range owing to mid-infrared absorption in SiO2, silicon-on-sapphire platform is limited to wavelengths below 5 micrometer owing to sapphire absorption, and silicon itself is only transparent down to 7-8 micrometers. The lack of a suitable photonic waveguiding platform currently limits the realization of a portable absorption spectrometer surrogate of benchtop infrared spectroscopic systems that can effectively probe the molecular fingerprint region below 1500 cm-1. Epitaxially-grown GaAs/AlGaAs waveguides offer a well-established lattice-matched, very low defect density and high index contrast platform for low loss optical circuits in the entire 3-14 micrometer wavelength range. This project aims at developing a GaAs/AlGaAs integrated photonics platform for on-chip mid-infrared photonics spanning the entire mid-infrared spectral range, including the molecular fingerprint region, and using this platform to demonstrate high sensitivity low parts per billion on-chip absorption sensing in the molecular fingerprint region. High sensitivity will be achieved using slotted photonic crystal waveguide devices. The proposed platform is expected to have a detection limit of 0.3 parts per billion for CO2 and 1 part per billion for toluene at their respective peak absorbance wavelengths at 4.23 micrometer and at 13.8 micrometer, respectively.

The mid-infrared spectrum represents a fingerprint of a material with absorption peaks corresponding to the frequencies of vibrations between the bonds of the atoms making up the material, and thus can identify and quantify all kinds of materials. Mid-infrared spectroscopy has a vast range of applications including pharmacy, biotechnology, industrial chemistry, food safety, and environmental monitoring. The preferred infrared spectroscopy method is Fourier transform infrared (FTIR) spectroscopy. However, conventional FTIR systems with moving components are bulky, heavy, and sensitive to environment fluctuations (vibration, etc). These disadvantages make it mainly a laboratory-only tool requiring extensive human involvement and therefore unsuitable for field applications. In this project, the team at the University of Texas at Austin proposes to use integrated photonics technology to build the FTIR on a chip. The weight of the FTIR can be amazingly reduced to a few grams and the size to less than 1 cm2. Moving parts are no longer needed. With these revolutionary improvements, FTIR can be used in many unprecedented areas such as toxic gas detection in battle fields, greenhouse gas monitoring on airborne platform, and standalone environment monitoring.

Integrated photonics has been experiencing explosive growth in the past few years. While many components and systems have been demonstrated with impressive performance, the development of on-chip spectroscopy is slow due to the limited absorption length and strength on a chip, and the lack of high resolution, wide wavelength range spectrometers. To address these issues, the proposed on-chip FTIR involves two major innovations. First, subwavelength grating metamaterial waveguide is used as an absorption enhancement medium for the first time. It solves the dilemma between guiding (which requires the optical field to be constrained inside the high index dielectric region) and absorption (which prefers that the optical field propagates outside of the waveguide). The absorption of light by the analyte can be enhanced over 400 times compared to a conventional strip waveguide. Second, on-chip FTIRs formed by an array of asymmetric MZIs with increasing path differences between the two arms suffers from the extremely limited wavelength bandwidth. In this project, a thermo-optic phase shifter is added to each MZI. The combination of thermal phase shifters and incremental waveguide length differences makes it possible to achieve high resolution and large spectral range simultaneously, which has never been demonstrated before. As a proof-of-concept demonstration, this project will design, fabricate and experimentally demonstrate a subwavelength grating metamaterial waveguide enhanced, on-chip FTIR centered at 3.4 microns on the silicon-on-sapphire platform for Methane detection. The concept can also be readily extended to cover other wavelength ranges.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.