Introduction

   Ensuring that specific chemicals are used, stored, and disposed of properly can be a dangerous and hazardous occupation. Some toxic gases like carbon monoxide ([math]CO[/math]) and carbon dioxide ([math]CO_2[/math]) are odorless and colorless, making them particularly dangerous. Other gases like ammonia, chlorine, or hydrogen sulfide can inflame and destroy the integrity of the respiratory tract, so breathing in any trace amount can quickly become life threatening. So how are deadly gases detected?

Modern gas detectors

   One technique utilized by portable gas detectors is through electrochemical processes. The gas detector contains an exposed porous membrane that allow gas molecules to pass through and interact with an electrode. Upon reaching the electrode, the gas is chemically reduced or oxidized, producing a an electrical current indicating the presence of a gas. Alternative methods include catalytic beading, photoionization, or other optical techniques to determine the presence of a gas and characterize its density.

An optical alternative

   An alternative solution is to utilize micro-resonant structures based on optical gratings. Best known for their ability to separate and disperse colors, optical gratings often comprise a periodic geometry the parameters of which are designed to interact with a known wavelength of incident light. A typical optical grating is illustrated below (left). Light of a known wavelength [math] \lambda_{inc} [/math] impinges on a grating with effective refractive index [math] n_{grat} [/math] and period [math] \Lambda [/math]. The grating rests on a monolithic substrate of refractive index [math] n_{sub} [/math]. Since the refractive index of the environment is less than [math] n_{grat} [/math] and [math] n_{grat} < n_{sub} [/math], the incident light passes through the grating and diffracts into the substrate. Depending on the relationship between [math] \lambda_{inc} [/math] and [math] \Lambda [/math], light striking the grating will either reflect, diffract, or both.

   But this isn’t always the case. As [math] \lambda_{inc} [/math] approaches [math] \Lambda [/math], there are fewer and fewer diffractive orders; eventually, [math] \lambda_{inc} > \Lambda [/math] and the grating is said to be “subwavelength”. Within the subwavelength regime, very interesting things begin to happen. For instance, placing a thin layer of material on top of the grating (characterized by refractive index [math] n_{wg} [/math]) causes the [math] 0^{th} [/math] diffractive order to reflect along the positive [math] \eta [/math] axis and become trapped within the newly added layer. This causes a spike in the spectral reflectivity (shown below) of the device at the (narrowband) resonant wavelength. 

   If the grating has a linear geometry, reflected diffraction only occurs for one polarization state as shown below; the other polarization state will not meet the resonance conditions. Since the resonance occurs because of a mode being guided within a higher-index waveguide, the phenomenon has come to be known as guided-mode resonance, and the devices that enable them are referred to as guided-mode resonance filters, or GMRFs.

   However, the resonance conditions can be very sensitive to changes in the geometry’s parameters. Increasing [math] \Lambda [/math], for instance, would spectrally shift the resonance, while decreasing the thickness of the top layer would shift the resonance to shorter wavelengths.

   By the same token, it would be relatively easy to design the GMRF such that the top waveguide layer is destroyed or damaged in the presence of a harmful or corrosive chemical. With the resonance destroyed, the reflectance profile optically displayed by the GMRF would also change in essence creating a passive chemically-activated optical sensor that could be checked remotely. This type of sensor would ultimately appear to be much more safe than walking into a room with conventional gas detectors — instead, a single sensor (or multiple sensors placed around a room) could be monitored optically via cameras or video streaming devices.

Conclusion

   Although there are many ways to detect gas leaks and similar environmental contaminations, GMRFs provide yet another means to safely monitor an environment without the need to get close to the environment. But GMRFs have similarly shown promise in other applications as well, such as laser wavelength stabilization, underwater communication, and optical signaling.