Chaotic Cavity Gas Cell for Optical Trace Explosives Detection
R2-C.3

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Project Description

Overview and Significance

This project aims to develop compact cavities for mid-infrared (MIR) optical absorption trace gas sensing that support long optical path lengths (OPLs) and optical focusing. The research advances the overall ALERT research program by modeling, designing and characterizing, rotationally asymmetric cavities (RACs) and pairing these cavities with quantum cascade lasers (QCLs) for MIR trace explosives detection. The societal benefits include the development of technologies for MIR sensors that can be deployed in areas not currently accessible due to cost, space and personnel limitations, and the training of graduate and undergraduate students in technologies relevant to the homeland security enterprise (HSE).

The cavity plays a pivotal role in the performance, size and cost of a sensor incorporating a multipass cell. Cavities with long OPLs are favorable because the light interacts with the gas under test over a longer effective distance, increasing the amount of absorbed light. Sensors based on optical absorption benefit from the longer OPL because the change in the intensity of the optical source is more easily measured. The RACs investigated in this project support stable, long OPLs (>5m) by engineering rotational asymmetries in the cavity shape. Another favorable aspect of the RACs in this research is that beam can be engineered to focus as the light is reflected inside the cavity. This so-called “global focusing time” (GFT) is similar to the focusing by a lens or mirror and can be engineered via the cavity design. The ability to control the GFT is beneficial because it results in small beam spots on the surface of the cavity, enabling continuous wave operation of the optical source and a reduction in the size of the cavity.

This project also seeks to achieve gains in sensor performance by using MIR QCLs as the optical source. MIR light interacts strongly with many molecules because photons in this portion of the spectrum can excite the fundamental vibrational and rotational modes of many of these molecules. The resonant excitation of these modes results in large optical absorption coefficients. The change in the intensity of the light passing through the gas depends exponentially on the absorption coefficient and the OPL; increasing the interaction of light with the gas under test improves the sensor performance.

This research advances the goals of ALERT by developing technologies to detect airborne signatures of explosives including, for example, the vapor from triacetone triperoxide (TATP). The project leverages our expertise in MIR materials, detectors and sources to create optical sensors for this portion of the spectrum. Our work compliments other projects within ALERT that focus on optical detection using MIR QCLs (R2-C.2: Multiplexed Mid-infrared Imaging of Trace Explosives and R3-C: Standoff Detection of Explosives: Mid-infrared Spectroscopy and Chemical Sensing) and benefits from research within ALERT that focuses on other sensors for vapor phase detection (R2-B.2: Portable, Integrated Microscale Sensors (PIMS) for Explosives Detection).

The societal benefits of this project include the development of optical sensing technologies for homeland security that have dual use in fields such as law enforcement, medicine, industry and agriculture. A product of this research is the dissemination of results to the MIR and broader scientific communities via peer-reviewed publications. The project includes the training of undergraduate and graduate students in technologies relevant to the HSE. To this end, a relationship with Sandia National Laboratories (SNL) has been cultivated to support students from the PI’s research group in internships. Finally, we also engage local students through educational outreach events.

This research advances the goals of ALERT by developing technologies to detect airborne signatures of explosives including, for example, the vapor from triacetone triperoxide (TATP). The project leverages our expertise in MIR materials, detectors and sources to create optical sensors for this portion of the spectrum.
Phase 2 Year 2 Annual Report
Project Leader
  • Anthony J. Hoffman
    Assistant Professor
    University of Notre Dame
    Email

Students Currently Involved in Project
  • Galen Harden
    University of Notre Dame
  • Luis Enrique Cortes Herrera
    University of Notre Dame
  • Owen Dominguez
    University of Notre Dame