Mid-infrared (MIR) radiation, in wavelengths from 2 to 10 micrometers is of particular interest for chemical and biological sensing because specific information based on the spectral absorption of MIR due to molecular rotational and vibrational transition is provided. To detect the absorption at a specific wavelength, two different wavelengths are used for reliable sensing where one wavelength is the same as the absorption wavelength and the other wavelength is used for reference. The sensing device requires a tunable MIR source and a spectrometer resulting in a bulky and expensive device.
For MIR radiation, a thermal source is widely used. The radiation spectrum of the thermal source is very broad as it follows Planck's radiation law and is therefore not easy to tune the spectrum without changing the temperature of the source. For detection of specific chemical information, a MIR source with narrow spectral distribution is advantageous to reduce power consumption and reduce background noise. There is a class of artificial micro-structures that can modify the thermal radiation spectrum by enhancing thermal radiation at a certain wavelength. Because the micro-structured thermal sources emit MIR radiation within a narrow range of wavelengths, adjustable by changing the structural parameters, it can be optimized as a MIR source for a specific absorption wavelength.
Thermal radiation can have two orthogonal polarizations and thermal radiation with one polarization that is independent of the thermal radiation of the other polarization. As a result, multiplexing is possible. Moreover, two thermal radiations with different polarizations reflect with different ratios when the incident angle is not normal to the reflecting surface, and this gives additional information for identifying chemical substances. Therefore, polarization engineering of MIR radiation brings additional advantages, in addition to the spectral narrowing of MIR radiation.
Generally, thermal radiation from a thermal source is considered unpolarized or weakly-polarized, which means the two polarizations of thermal radiation are equally distributed. The degree of polarization (DOP), defined by (P1−P2)/(P1+P2), is commonly used to show how much the thermal radiation is polarized, where P1 and P2 are the radiation powers of the two orthogonal polarizations, respectively. The DOP is 0% for unpolarized radiation and 100% for completely polarized radiation. Under special circumstances, DOP deviates far from 0%. For example, the thermal radiation emitted by flat metals starts with unpolarized radiation for normal emergence and increases in DOP with the angle of emergence, at first slowly, to about 90% percent at grazing emergence. Because the radiation power goes to zero at grazing emergence, it is not suitable for polarized MR radiation in spite of high DOP. The conventional way to produce polarized thermal radiation with reasonable power is to pass the unpolarized thermal radiation through a polarizer, which filters the thermal radiation having unwanted polarization resulting in the wasting of more than half of the power used.