Gas sensors have now become a part of our everyday routine. Particularly, the development of gas sensors for security, environmental and health applications has gained momentum in recent years. In most applications, gas sensors that are highly sensitive, highly selective, portable, robust, allow real-time data analysis and room temperature operation are desired. This dissertation explores the development of novel passive and active optical devices that are important in enhancing Quantum Cascade (QC) laser-based mid-infrared (mid-IR) gas spectroscopy. The thesis covers four major projects.
In the first part, a technique is developed for the fabrication and characterization of mesas with similar optical and electrical properties as QC lasers. The technique presented is quick and minimally invasive. More importantly, relevant intersubband QC laser information such as the peak emission wavelength and its full width at half maximum (FWHM), turn-on voltage and maximum operating current density, can be quickly verified. Characterization results of the mesas when compared with results from QC lasers made from the same wafer differ by less than 10%.
In the second part, the development and characterization of III-V semiconductor subwavelength resonant grating (SRG) structures are presented. Relative transmittance resonant depths up to 79% and a FWHM of 49 nm were achieved at 0°. In the reflection measurements, a relative reflectance of 76% and FWHM of 41.8 nm were reported at 20°. This is the first work on III-V semiconductor-based mid-IR SRGs. When coupled with a QC laser, single mode operation, a necessity in high resolution QC laser-based gas sensors for selectivity and specificity of wavelengths, can be achieved.
The third part is a presentation of the design, fabrication and characterization of a special kind of room temperature quantum well infrared photodetector (QWIP) which acts as both a passive optical element (beamsplitter) and a mid-IR detector. A 4.6 μm light incident on this QWIP is partially reflected, partially absorbed and the rest transmitted through it. Reflection, photocurrent and transmission signals simultaneously measured are presented. In a mid-IR gas sensor, these three signals are useful for different purposes: a main signal that interacts with the target gas; a reference signal; and a signal for laser source stabilization.
In the final project, the development and results of two successful deployments of a QC laser open path gas sensor (QCLOPS) to Ghana are presented. The main target gases are benzyl alcohol and 2-methylphenol (gases present in wood smoke used in fish smoking) and water vapor. Water vapor concentrations from QCLOPS when compared to results from a commercial humidity sensor show very close agreement. The wood smoke measurements reported are very spectrally complex. Possible techniques for resolving the combined spectrum into its individual component spectra and lessons learnt in using QCLOPS in resource constrained and harsh climatic conditions are discussed.