Mid-infrared sensors are gaining prominence in recent years with advancement in sensing technologies that operate in mid-wavelength infrared region of electromagnetic spectrum. Let us look into the details of mid-IR sensors and explore some important applications of this emerging technology.
What are Mid-IR Sensors?
The electromagnetic spectrum is broadly classified into different wavelength regions such as visible light, infrared, microwave etc. Mid-infrared wavelength region lies between 3-20 micrometer. Sensors that are sensitive to mid-infrared wavelengths are called mid-IR sensors. These sensors makes use of semiconductor materials such as Indium gallium arsenide (InGaAs), Mercury cadmium telluride (MCT), quantum-well infrared photodetectors (QWIPs) etc. that exhibit high absorption in mid-IR region.
Mid-IR sensors have advantages over other infrared sensors. For example, compared to thermal infrared sensors, Mid-IR Sensors provide spectroscopic signatures and does not sense only temperature variations. Compared to near-IR sensors, mid-IR sensors can detect weaker absorption features and are less affected by ambient light interference.
Principal of Operation
Mid-IR sensors detect infrared radiations emitted or reflected from an object based on the principle of photoconductivity or photovoltaic effect. The semiconductor material in the sensor absorbs infrared photons in the mid-wavelength range. This results in excitation of electrons from valence band to conduction band, increasing the concentration of free charge carriers. This change in conductivity or generation of voltage is measured as a signal corresponding to the intensity of incident infrared radiation. By analyzing the signal, properties of the object like thermal profile, chemical composition etc. can be determined.
Applications of Mid-IR Sensors
1. Thermal Imaging
One of the most common application of mid-IR sensors is thermal imaging. By detecting infrared radiations emitted by an object based on its temperature, mid-IR cameras can form thermal images and maps temperature distribution. They are used in applications like predictive maintenance of equipment, medical thermography, firefighting, law enforcement etc.
2. Gas Detection
Gases absorb infrared radiations at their characteristic wavelengths depending on their molecular structure. Mid-IR sensors coupled with gas cells and optical filters can be used to detect and identify trace gases. They find use in environmental monitoring, industrial process control, breathanalysis etc.
3. Biological and Medical Applications
Mid-IR spectroscopy coupled with imaging has potential in non-invasive medical diagnostics. Differences in molecular vibrations of healthy and diseased tissues can be detected. They are being explored for applications like cancer detection, study of bacterial infections, analysis of biopsy samples etc. Fingertip blood analysis is another emerging area.
4. Process Analytical Technologies
Mid-IR sensors enable real-time process monitoring and quality assurance in pharmaceutical/chemical manufacturing. By acquiring IR spectra directly from manufacturing lines, they aid in process understanding and product quality troubleshooting without interfering with production. Mid-IR fiber-optic probes make such in-situ analysis feasible even in hazardous conditions.
5. Homeland Security
Mid-IR spectroscopy proves useful in defense and security applications. For example, detection of hazardous chemicals and explosives at border checkpoints or detecting traces of drugs/explosives on people. Stand-off chemical identification without contact is an advantage over other techniques. Integrated mid-IR cameras and spectrometers are developed for such applications.
Technological Advancements
Significant progress is being seen in developing more sensitive, compact and cost-effective mid-IR sensor platforms:
- Nanopatterned quantum confined structures, 2D materials are being explored to enhance detection capabilities.
- Monolithic integration of detector arrays with optics and electronics on a single chip reduces size and cost.
- Focal plane arrays with hundreds or thousands of pixels allow imaging spectroscopy and hyper spectral imaging.
- Novel materials like quantum cascade lasers (QCLs) as illumination sources extend the tuneable wavelength range.
- Advances in signal and data processing enables real-time multivariate analysis and machine learning based applications.
- Optical components like micro-mirrors, filters are being miniaturized for on-chip spectrometers.
To summarize, mid-infrared sensors have tremendous potential that is still unfolding with ongoing research. Their non-contact, non-destructive sensing capabilities open up new possibilities across several domains. While transition from laboratory to real-world deployments take time, undoubtedly mid-IR technology will keep revolutionizing how we perceive and analyze the world around us.
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