Bio


I study energy and climate policies in the United States. To be more specific, my focus is currently on natural gas infrastructure and the electricity sector. I use techno-economic analyzes, life-cycle assessments and statistical techniques to develop workable policy solutions to reduce greenhouse gas emissions. I have an eclectic background in electrical engineering, laser physics and environmental sensor systems. Outside of work, I'm an avid outdoor adventurer - I often go on or organize backpacking, hiking, mountaineering or canyoneering trips.

Professional Education


  • Master of Arts, Princeton University (2012)
  • Doctor of Philosophy, Princeton University (2015)

Stanford Advisors


Current Research and Scholarly Interests


I study energy and climate policies in the United States. To be more specific, my focus is currently on natural gas infrastructure and the electricity sector.

All Publications


  • Designing better methane mitigation policies: the challenge of distributed small sources in the natural gas sector ENVIRONMENTAL RESEARCH LETTERS Ravikumar, A. P., Brandt, A. R. 2017; 12 (4)
  • Are Optical Gas Imaging Technologies Effective For Methane Leak Detection? ENVIRONMENTAL SCIENCE & TECHNOLOGY Ravikumar, A. P., Wang, J., Brandt, A. R. 2017; 51 (1): 718-724

    Abstract

    Concerns over mitigating methane leakage from the natural gas system have become ever more prominent in recent years. Recently, the U.S. Environmental Protection Agency proposed regulations requiring use of optical gas imaging (OGI) technologies to identify and repair leaks. In this work, we develop an open-source predictive model to accurately simulate the most common OGI technology, passive infrared (IR) imaging. The model accurately reproduces IR images of controlled methane release field experiments as well as reported minimum detection limits. We show that imaging distance is the most important parameter affecting IR detection effectiveness. In a simulated well-site, over 80% of emissions can be detected from an imaging distance of 10 m. Also, the presence of "superemitters" greatly enhance the effectiveness of IR leak detection. The minimum detectable limits of this technology can be used to selectively target "superemitters", thereby providing a method for approximate leak-rate quantification. In addition, model results show that imaging backdrop controls IR imaging effectiveness: land-based detection against sky or low-emissivity backgrounds have higher detection efficiency compared to aerial measurements. Finally, we show that minimum IR detection thresholds can be significantly lower for gas compositions that include a significant fraction nonmethane hydrocarbons.

    View details for DOI 10.1021/acs.est.6b03906

    View details for Web of Science ID 000391346900079

    View details for PubMedID 27936621

  • Wavelength independent normal incident quantum cascade detectors OPTICS EXPRESS Ravikumar, A. P., Sivco, D., Gmachl, C. F. 2016; 24 (22): 25269-25276

    Abstract

    We demonstrate a novel technique for normal-incident absorption in intersubband infrared detectors by taking advantage of light scattering from the side-walls of a wet-etched mesa. We fabricate 'spiral' and 'hairpin' shaped quantum cascade detector at a peak wavelength of 6.6 μm, and compare their performance with a standard rectangular mesa. We achieve a peak responsivity of 6 mA/W for the spiral and 12 mA/W for the hairpin detectors at normal incidence, comparable to the 8.8 mA/W obtained for the mesa at 45 degree incidence. We obtain a background limited detectivity of about 3×1010 cmHz/W for the spiral and hairpin detectors at 80 K, compared to 3×108 cmHz/W for the standard mesa. This method to achieve normal incidence absorption is wavelength independent, and does not involve complicated fabrication procedures, paving the way for widespread use of intersubband detectors in sensor applications.

    View details for DOI 10.1364/OE.24.025269

    View details for Web of Science ID 000388413400057

    View details for PubMedID 27828465

  • II-VI quantum cascade emitters in the 6-8 mu m range PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS Garcia, T. A., De Jesus, J., Ravikumar, A. P., Gmachl, C. F., Tamargo, M. C. 2016; 253 (8): 1494-1497

    Abstract

    We present the growth and characterization of ZnCdSe/ZnCdMgSe quantum cascade (QC) heterostructures grown by molecular beam epitaxy (MBE) and designed to operate at 6-8μm. These structures utilize the better-understood ZnCdMgSe with InP lattice matched compositions yielding a bandgap of 2.80 eV as compared to previous work which used ZnCdMgSe compositions with bandgaps at 3.00 eV. Grown structures posses good structural and optical properties evidenced in X-ray diffraction and photoluminescence studies. Fabricated mesa devices show temperature dependent I-V measurements with differential resistance of 3.6 Ω, and a turn on voltage of 11V consistent with design specifications. Electroluminescence was observed in these devices up to room temperature with emission centered at 7.1 μm and line widths of ∼16%(ΔE/E) at 80K. The results show that these are well-behaved electroluminescent structures. Addition of waveguide layers and further improvements in well barrier interfaces are being pursued in efforts to demonstrate lasing.

    View details for DOI 10.1002/pssb.201600135

    View details for Web of Science ID 000383604300008

    View details for PubMedID 27990100

    View details for PubMedCentralID PMC5155511

  • Comparing Natural Gas Leakage Detection Technologies Using an Open-Source "Virtual Gas Field" Simulator ENVIRONMENTAL SCIENCE & TECHNOLOGY Kemp, C. E., Ravikumar, A. P., Brandt, A. R. 2016; 50 (8): 4546-4553

    Abstract

    We present a tool for modeling the performance of methane leak detection and repair programs that can be used to evaluate the effectiveness of detection technologies and proposed mitigation policies. The tool uses a two-state Markov model to simulate the evolution of methane leakage from an artificial natural gas field. Leaks are created stochastically, drawing from the current understanding of the frequency and size distributions at production facilities. Various leak detection and repair programs can be simulated to determine the rate at which each would identify and repair leaks. Integrating the methane leakage over time enables a meaningful comparison between technologies, using both economic and environmental metrics. We simulate four existing or proposed detection technologies: flame ionization detection, manual infrared camera, automated infrared drone, and distributed detectors. Comparing these four technologies, we found that over 80% of simulated leakage could be mitigated with a positive net present value, although the maximum benefit is realized by selectively targeting larger leaks. Our results show that low-cost leak detection programs can rely on high-cost technology, as long as it is applied in a way that allows for rapid detection of large leaks. Any strategy to reduce leakage should require a careful consideration of the differences between low-cost technologies and low-cost programs.

    View details for DOI 10.1021/acs.est.5b06068

    View details for Web of Science ID 000374707100045

    View details for PubMedID 27007771

  • High performance, room temperature, broadband II-VI quantum cascade detector APPLIED PHYSICS LETTERS Ravikumar, A. P., De Jesus, J., Tamargo, M. C., Gmachl, C. F. 2015; 107 (14)

    View details for DOI 10.1063/1.4932538

    View details for Web of Science ID 000363422100005

  • Phased-array sources based on nonlinear metamaterial nanocavities NATURE COMMUNICATIONS Wolf, O., Campione, S., Benz, A., Ravikumar, A. P., Liu, S., Luk, T. S., Kadlec, E. A., Shaner, E. A., Klem, J. F., Sinclair, M. B., Brener, I. 2015; 6

    Abstract

    Coherent superposition of light from subwavelength sources is an attractive prospect for the manipulation of the direction, shape and polarization of optical beams. This phenomenon constitutes the basis of phased arrays, commonly used at microwave and radio frequencies. Here we propose a new concept for phased-array sources at infrared frequencies based on metamaterial nanocavities coupled to a highly nonlinear semiconductor heterostructure. Optical pumping of the nanocavity induces a localized, phase-locked, nonlinear resonant polarization that acts as a source feed for a higher-order resonance of the nanocavity. Varying the nanocavity design enables the production of beams with arbitrary shape and polarization. As an example, we demonstrate two second harmonic phased-array sources that perform two optical functions at the second harmonic wavelength (∼5 μm): a beam splitter and a polarizing beam splitter. Proper design of the nanocavity and nonlinear heterostructure will enable such phased arrays to span most of the infrared spectrum.

    View details for DOI 10.1038/ncomms8667

    View details for Web of Science ID 000358858100011

    View details for PubMedID 26126879

  • High detectivity short-wavelength II-VI quantum cascade detector APPLIED PHYSICS LETTERS Ravikumar, A. P., Garcia, T. A., De Jesus, J., Tamargo, M. C., Gmachl, C. F. 2014; 105 (6)

    View details for DOI 10.1063/1.4893359

    View details for Web of Science ID 000341188700013

  • Improved electrical properties and crystalline quality of II-VI heterostructures for quantum cascade lasers JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B Garcia, T. A., Hong, S., Tamargo, M., De Jesus, J., Deligiannakis, V., Ravikumar, A., Gmachl, C., Shen, A. 2013; 31 (3)

    View details for DOI 10.1116/1.4803837

    View details for Web of Science ID 000320130500033

  • Characterization of the three-well active region of a quantum cascade laser using contactless electroreflectance JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B De Jesus, J., Garcia, T. A., Dhomkar, S., Ravikumar, A., Gmachl, C., Chen, G., Shen, A., Ferizovic, D., Munoz, M., Tamargo, M. C. 2013; 31 (3)

    View details for DOI 10.1116/1.4803838

    View details for Web of Science ID 000320130500034

  • MBE growth of ZnCdSe/ZnCdMgSe quantum-well infrared photodetectors JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B Shen, A., Ravikumar, A. P., Chen, G., Zhao, K., Alfaro-Martinez, A., Garcia, T., De Jesus, J., Tamargo, M. C., Gmachl, C. 2013; 31 (3)

    View details for DOI 10.1116/1.4794383

    View details for Web of Science ID 000320130500013

  • Room temperature and high responsivity short wavelength II-VI quantum well infrared photodetector APPLIED PHYSICS LETTERS Ravikumar, A. P., Chen, G., Zhao, K., Tian, Y., Prucnal, P., Tamargo, M. C., Gmachl, C. F., Shen, A. 2013; 102 (16)

    View details for DOI 10.1063/1.4802955

    View details for Web of Science ID 000318269300007

  • ZnCdSe/ZnCdMgSe quantum well infrared photodetector OPTICS EXPRESS Ravikumar, A. P., Alfaro-Martinez, A., Chen, G., Zhao, K., Tamargo, M. C., Gmachl, C. F., Shen, A. 2012; 20 (20): 22391-22397

    Abstract

    We report the design, fabrication and characterization of a II-VI Zn(0.51)Cd(0.49)Se / Zn0.45(Cd)0.42(Mg)(0.13)Se-based quantum well infrared photodetector (QWIP) with a bound to quasi-bound transition centered at 8.7 µm. The good growth quality of the epitaxial layers was verified by x-ray diffraction measurements. Absorption and photocurrent measurements yield results consistent with conventional III-V QWIPs. Photocurrent measurements reveal an exponential decrease with temperature. In addition, we also observe more than 4 orders of magnitude increase in photocurrent with applied bias. By compensating the drop in temperature performance with an increase in applied bias, we achieve an operating temperature of up to 140K and a responsivity of 1-10 µA/W.

    View details for DOI 10.1364/OE.20.022391

    View details for Web of Science ID 000309522400055

    View details for PubMedID 23037387