Solid state molecular sensor for detection of chemical and biological agents

The U.S. government is greatly concerned with the growing threat of terrorism and armed conflict with the use of chemical or biological (CB) weapons. The urgency in developing CB sensors in order to help mitigate this threat is critical. Presently, several optical detection concepts are being explored using lasers, optical fibers, and linear waveguided structures. A review of the various techniques has been provided by Boisdé et al.¹ Each has a niche market for a specific application. Most, however, lack the potential for further development due to limitations in either sensitivity, integration, handling, or discrimination capabilities.

The essential criteria that must be considered in the development of compact, integrated, CB detection systems are summarized in Table 1 and focus on localized detection

i.) high sensitivity /low detection limits

ii.) simultaneous detection and discrimination of CB agents

iii.) self-calibration and adjustable warning and alarm level functions

iv.) robust maintenance and easy exchangeable modules

v.) operational at normal environmental conditions

vi.) low power consumption

vii.) functional reliability/long life

viii.) portability

The numerical specifics for each criterion vary with target and the operational environment, and each has different application requirements that mandate probing over large distances or volumes. In this case, integration is less important, and the key technological task is the development of high-intensity lasers operating at wavelengths in the mid- (3 – 5μm) and far-IR (8 – 12μm). ^2–6 The limitations imposed by the transparency windows of the earth's atmosphere in such high power applications do not apply to the highly-localized SSMS systems, which generally provides for a wider range of detection.

The solid-state molecular sensors (SSMS) concept^7–11 is based on birefringent, confined nonlinear, chalcopyrite (CP) heterostructures. This differs significantly from conventional passive waveguide sensors where the evanescent wave (phase- and/or amplitude) is analyzed either interferometrically (phase shift) or by integral means (absorption loss). The SSMS concept uses a nonlinear, birefringent medium to translate phase and amplitude changes encountered in the evanescent wave—in an optical parametric oscillator (OPO) process—into a frequency shift, that can be detected with high accuracy.

Figure 1 illustrates the physical principles of detection and discrimination employed by the SSMS. Assuming that the phase matching condition is established for a given pump laser wavelength, an evanescence wave probing an impurity free collector surface in an OPO process will generate an output wave with base frequency, f_o, and corresponding intensity peak. The presence of a target CB agent produces a phase shift resulting in a series of frequency-shifted and intensity-reduced side bands. These may be viewed as a unique, target-specific, frequency spectrum whose peak intensities are a function of the collector surface concentration. A general approach to the signal-processing/discrimination of single and multiple target agents has been developed. This approach is based on Bayesian inference.¹²

In summary, the SSMS is a nonlinear, birefringent, chalcopyrite, waveguided technology. It detects, discriminates, and measures concentrations of target molecules in an ambient background in real time. It does so by employing resonant phase- and/or amplitude sensitive detection over a large, tunable spectral range. The SSMS can be made to be sensitive to one specific group of molecules by setting up appropriate phase matching conditions. The SSMS is a miniaturized technology that can be easily interfaced with existing Si and III-V compound electronic components. Applications include the remote screening of air pollutants, recognition of CB hazards in the environment, monitoring of surface corrosion/etching processes, and bio-medical testing.