Grantee Research Project Results
1999 Progress Report: Development of a New Gas Sensing System Based on Terahertz Time-Domain Spectroscopy
EPA Grant Number: R827122Title: Development of a New Gas Sensing System Based on Terahertz Time-Domain Spectroscopy
Investigators: Mittleman, Daniel M. , Baraniuk, Richard G.
Institution: Rice University
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1998 through September 30, 2001
Project Period Covered by this Report: October 1, 1998 through September 30, 1999
Project Amount: $299,817
RFA: Exploratory Research - Environmental Engineering (1998) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Sustainable and Healthy Communities , Land and Waste Management
Objective:
Spectroscopic methods for the sensing and identification of gases have shown great promise, owing to their inherent non-invasive nature, relative simplicity, and high selectivity. The use of far-infrared or terahertz (1 THz = 1012 Hz, corresponding to a wavelength of ~300 µm) radiation for sensing purposes, though extremely valuable as a complement to well established mid-infrared technologies, has not achieved great success. This is largely due to the complexity of the instrumentation required for generation and detection of terahertz radiation, which renders existing spectrometers impractical for most real-world sensing applications. The objective of this research is to build a portable broadband spectrometer based on the recently developed technique of terahertz time-domain spectroscopy (THz-TDS).Progress Summary:
In the first year of the grant period, we reported a number of important advances towards the goals stated in the proposal. The first involved the development of a new algorithm for signal processing of Terahertz time-domain waveforms. A graduate student, Timothy Dorney, developed a model-based algorithm which can accurately extract both the absorption and refractive index of a planar slab of solid material, given the input THz waveform. This algorithm uses the time-domain Fabry-Perot effect to extract more information than one might obtain from the ballistic transmission. It employs a new error metric which exploits the fact that nearly all solids have spectra which vary smoothly with frequency in the THz range. This work was submitted for publication to the Journal of the Optical Society of America (JOSA) A. In Year 2, we have improved and elaborated on this model, in order to answer several important referee comments for this submission. The modified manuscript has been re-submitted to JOSA A, and is currently in review.
We have also developed a second method for extracting dielectric parameters of slabs, which avoids some of the issues with our first method. For example, the first method requires that the material have a sufficiently high refractive index to generate measurable multiple reflections. Low index materials do not generate a sufficient reflection at their interfaces, and thus could not be studied with this previous method. Our new method does not rely on multiple reflections to extract information. Rather, we measure a series of THz waveforms while rotating the sample, so that the effective path length of the beam inside the materials is varied. This method can be used on a material of any refractive index, including most common polymers. It also avoids the ambiguity inherent in the measurement of thick slabs, involving the unwrapping of the complex phase. We are currently preparing a manuscript on this new method.
These contributions have been valuable for reasons beyond the fact that they enable new and rapid methods for THz spectroscopy. They have also served as a valuable entry point for our continuing development of signal processing tools for the unique time-domain signals generated by the THz-TDS system. It has also helped Tim, whose background is much more related to signal processing than to optoelectronics, to become more familiar with the workings of the THz system, and to gain a deeper appreciation of the capabilities and limitations of the device.
A second important advance in this field has been the recent announcement of a commercial THz imaging system. Picometrix, a small optoelectronics firm based in Ann Arbor, MI, is now marketing these time-domain spectrometers through Coherent, the world's largest laser manufacturer. This is the culmination of a three year development effort, which began with the transfer of the THz imaging technology and intellectual property from Lucent Technologies to Picometrix. The PI, Prof. Mittleman, has been the technology transfer agent and a consultant to Picometrix since this process began. Beyond the extremely exciting prospects for the field of THz imaging, this development is significant to the project at hand because Picometrix intends to use our laboratory at Rice University as a beta-test site for their newly developed spectrometer. We anticipate that this system will be delivered to Rice by January 2001, although the delivery date has already been pushed back twice due to the long delivery time of various optical components.
Once installed, this system will enable a host of new measurements, since the emitter and detector modules are fully fiber-pigtailed and integrated. These new capabilities will include the possibility of longer time-base measurements for high-resolution spectroscopy, as well as measurements in which either the transmitter or receiver are repositioned during the experiment. We have already exploited this latter capability, in two sets of measurements performed in the Picometrix laboratories in Ann Arbor, MI. The first set of measurements involved a study of the cross-polarized component of the radiation from THz dipole emitters. We studied the angular dependence of the polarization component orthogonal to the orientation of the emitter dipole, and found that it exhibits a quadrupole radiation pattern. This was explained using a simple model for the origin of the quadrupole term, plus a Fresnel-Kirchoff diffraction calculation to describe the angular dependence. This work was published in Optics Letters. The second set of measurements was a preliminary attempt to apply Kirchoff migration imaging algorithms to THz reflection imaging. By moving the detector in a reflection imaging geometry, one can in principle obtain sufficient information for a three-dimensional reconstruction, similar to a holographic measurement. We have demonstrated the effectiveness of this technique using migration algorithms borrowed from the field of geophysical prospecting. A manuscript describing this work is in preparation.
A final project involving THz imaging has been performed at Rice University. This involves reflection imaging with few-cycle pulses. Because the technique permits measurement of the THz electric field (not merely the intensity), one can directly measure the phase of the radiation. This can be exploited in an interferometric imaging measurement, to provide a dramatic enhancement of the sensitivity to small or subtle features in a sample. The idea of this technique is as follows. An object to be imaged is placed at the focus of a lens in one arm of a Michaelson interferometer. This introduces a phase shift of approximately p between the two arms of the interferometer, via the Gouy phase shift. The resulting destructive interference provides a nearly background-free measurement, and a dramatic enhancement in depth resolution. We demonstrated that it is possible to resolve features thinner than 2 percent of the coherence length of the radiation. This technique could have important applications in low-coherence optical tomographic measurements. A paper describing these measurements has been submitted to Applied Physics Letters. We are also in the process of preparing a patent application for this novel method. In the coming year, we anticipate that we will investigate the application of this interferometric cancellation to the problem of THz gas sensing.
The funds provided during Years 1 and 2 of the grant period have been used as originally envisioned in the initial proposal, with one minor modification. Just after the grant was awarded, the PI was promoted from Faculty Fellow (non-tenure-track) to Assistant Professor (tenure-track) in the ECE Department. As a result, the salary support requested for Mittleman was used instead for his summer salary. This promotion also complicated the laboratory development somewhat, as the PI was provided with new laboratory space, which required extensive (and time-consuming) renovation. However, these are now completed, and the equipment portion of the budget has been used to purchase a femtosecond laser for this laboratory which is operational as of February 2000. The student salary portion of the budget has been used to pay Tim Dorney, and will support him in the coming year as well.
Journal Articles on this Report : 4 Displayed | Download in RIS Format
Other project views: | All 10 publications | 10 publications in selected types | All 10 journal articles |
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Dorney TD, Baraniuk RG, Mittleman DM. Material parameter estimation using terahertz time-domain spectroscopy. Journal of the Optical Society of America A-Optics Image Science and Vision 2001;18(7):1562-1571. |
R827122 (1999) R827122 (Final) |
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Dorney T, Johnson J, Mittleman D. THz reflection image formation using Kirchoff migration (manuscript in preparation). |
R827122 (1999) |
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Johnson JL, Dorney TD, Mittleman DM. Enhanced depth resolution in terahertz imaging using phase-shift interferometry. Applied Physics Letters 2001;78(6):835-837. |
R827122 (1999) R827122 (Final) |
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Van Rudd J, Johnson JL, Mittleman DM. Quadrupole radiation from terahertz dipole antennas. Optics Letters 2000;25(20):1556-1558. |
R827122 (1999) R827122 (Final) |
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Supplemental Keywords:
innovative technology, engineering, monitoring., RFA, Scientific Discipline, Air, Toxics, Environmental Chemistry, climate change, HAPS, Engineering, Chemistry, & Physics, Electron Microscopy, 33/50, environmentally conscious manufacturing, portable spectrometers, Terahertz time-domain, PCBs, green house gas concentrations, infrared spectroscopy sensor, optical sensor, radiation balance, benzene, Trichloroethylene, gas sensing system, electro-optic sensing, fiber laser, Acetonitrile, industrial solvents, Benzene (including benzene from gasoline)Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.