Multiplexed Mid-Infrared Imaging of Trace Explosives
R2-C.2

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Project Description

Overview and Significance

The overall goal of this project is to develop technology to enhance the ALERT research programs by providing lower cost, wide-bandwidth, mid-infrared (MIR, λ=4-12 μm) laser sources and developing MIR laser-based imaging technology for the detection of trace explosives. ALERT projects are already exploring the MIR region of the spectrum and identifying features unique to explosives, and ALERT industrial partners are producing commercial MIR laser arrays. However, these efforts are restricted by technological questions:

1) How do you perform sensitive spectroscopic imaging in a wavelength range that lacks robust and widely available focal plane arrays and optical components.

2) How do you combine several high-cost laser chips into a single module for sensitive imaging and detection?

This project seeks to answer these questions through novel semiconductor fabrication techniques and laser-scanning/coded aperture imaging technology. This will enhance ongoing ALERT efforts be enabling higher resolution imaging technology to detect signatures characterized by other ALERT thrusts, provide a technology to enhance the commercial offerings of ALERT industrial partners and provide new spectroscopic tools that could be useful to the security enterprise as well as other fields where MIR technology is employed, such as medical diagnostics, industrial process control, environmental monitoring and pharmaceutical process analytical technology.

The MIR region of the electromagnetic spectrum contains unique absorption “fingerprints” corresponding to vibrational modes of molecules. ALERT projects led by Hernández (R3-C), Hoffman (R2-C.3), Rinaldi (R2-B.3) and ALERT industrial collaborator, EOS Photonics, are using mid-infrared light to identify molecular fingerprints corresponding to trace amounts of explosives. For example, two MIR lasers tuned to relevant on-and off resonant wavelengths (e.g., 1348 cm-1 on and 1370 cm-1 off resonance for TNT) can be used to measure the differential absorption of light between those two frequencies attributed to the presence of an explosive. The ability to detect explosives with MIR light is limited by the availability of high-performance MIR laser sources and imaging platforms. MIR lasers, especially lasers that can identify many unique explosives simultaneously, are very expensive. Additionally, imaging platforms are difficult to produce since few MIR “cameras” exist (or are very expensive).

This project develops technology that will make it cheaper to produce laser systems that can identify multiple explosives at the same time, as well as a complementary imaging platform. Thus, this project serves as a translation layer between fundamental spectroscopic characterization and deployable detection platforms. This is done through two complimentary efforts:

1) The development of an MIR spectroscopic imaging platform that overcomes the technical limitations in MIR imaging and a lack of cost-sensitive MIR imaging devices; and

2) The integration of low-cost, and highly robust devices geometries with new MIR imaging platforms to form a single integrated wide-bandwidth single-output-port high-power-spectral-density sources.

Sensitive MIR spectroscopic detection platforms have been limited due to a lack of strong signal-to-background detection. This is due to the common use of larger spectroscopy systems (i.e., Fourier transform infrared spectrometers, FTIR), which have low power-spectral-density at a given wavelength, as well as a lack of MIR imaging systems that require large-area sampling. Thus, systems attempt to detect a narrow trace explosive spectral bandwidth and small sample size amongst an excessively large spectrometer bandwidth and beam spot size. We address these challenges by employing laser spectroscopic and spatial frequency multiplexed imaging to obtain high power spectral density and high resolution imaging, respectively.

To obtain high power spectral density and high resolution imaging, a high spectral power density light source, such as a laser, must be employed. Quantum cascade lasers (QCLs) are ideal devices for MIR imaging; however, widely tunable QCLs require external cavity tuning and are, thus, susceptible to mode-hopping instabilities and mechanical failure, which severely limits performance. EOS Photonics has overcome these issues by developing QCL arrays where each device in the array has a unique output wavelength. To maintain high signal-to-background contrast with small trace samples on substrates, high resolution imaging is required. In these imaging systems, however, each of the lasers in the array must follow the same beam path, which is not possible from laser waveguide arrays. This project also develops integrated wide-bandwidth single-output-port high-power-spectral-density sources by developing new micro-fabrication techniques to build a system which combines the light out of several expensive MIR lasers together into an inexpensive silicon based chip. The light can then be used for imaging or possibly for detection of explosives in small quantities.

Both of these technologies form an integrated MIR explosives imaging platform unlike any that is currently available. Additionally, this platform is modular and can be extended for rapid development of further advanced screening and detection systems.

This project develops technology that will make it cheaper to produce laser systems that can identify multiple explosives at the same time, as well as a complementary imaging platform. Thus, this project serves as a translation layer between fundamental spectroscopic characterization and deployable detection platforms.
Phase 2 Year 2 Annual Report
Project Leader
  • Scott Howard
    Assistant Professor
    University of Notre Dame
    Email

Students Currently Involved in Project
  • Tahsin Ahmed
    University of Notre Dame
  • David Benirschke
    University of Notre Dame
  • Bruna Liborio
    Duke University
  • Zechariah Pfaffenberger
    Indiana Wesleyan University