Mid-infrared (mid-IR) radiation – typically defined as 2.5–10 µm wavelengths (although the exact values can vary) – is not something many of us come across in our daily lives. We can’t see it and we don’t use it for data transmission either. So, why do we care about it at all? Well, mid-infrared radiation can help us identify many materials by their characteristic spectra.
To understand this better, let’s take a little detour into the world of molecules. Even if they don’t move around widely, molecules can be in different excited states: they can stretch along their bonds, vibrate around their centre of mass and rotate around one of their axes. Like a stretching spring or the balance of a watch, these excitations store energy and, of course, energy can be expressed in wavelengths and the energy of many of these rotational and vibrational (ro-vibrational) excitations correspond to mid-IR wavelengths. And because the energy stored in each of these excitations is characteristic to a specific molecule, we can use these spectral fingerprints to identify materials using spectroscopy. That’s why airport security use spectrometers to check for dangerous substances when they swipe your laptop.
While we can use mid-IR radiation to identify materials, materials are also one of the biggest challenges for mid-IR applications because all the materials typically used at shorter wavelengths don’t work the same way at longer wavelengths. For example, the silica glass used for microscope slides, lenses and optical fibres in the visible and near-infrared, is no longer transparent above about 4 µm, forbidding such applications in the mid-IR. Two common silica replacements for the mid-IR are calcium fluoride or chalcogenide glasses, but the search continues, especially for nonlinear applications.
But how do we make mid-IR light in the first place? Well, that has long been a bottle neck in mid-IR technology, but the last few years have brought some improvement and mid-IR sources can these days even be bought off the shelf. These are usually frequency combs – trains of sharp spectral lines with an equidistant frequency spacing – often derived from quantum cascade lasers, but many of them do not cover the full mid-IR range. Compared to the adjacent near-infrared, however, the choice of sources is limited, and more advanced frequency converters like optical parametric oscillators (OPO), which are standard lab-equipment in the near-IR, are still few and far between in the longer mid-IR.
Luckily, these challenges are opportunities rather than obstacles, and although we’ll never be able to see it in a literal sense, we will for sure see a lot more (figuratively) of the mid-IR in the near future.