Post by Oliver Graydon

{credit}Image by Bethany Vukomanovic {/credit}

The electromagnetic spectrum spans a rich range of wavelengths – from short-wavelength, highly energetic x-rays at one end through to long-wavelength radiowaves at the other. While many regions of this spectrum have already been explored by mankind and put to good use there is one that is still largely underexploited – that of terahertz waves. Lying in the region between infrared light and microwaves, terahertz waves (photons with a frequency between ~300 GHz and 10 THz), fall into a gap between the worlds of photonics and electronics. However, in recent years scientists have been increasingly exploring how such terahertz waves can be exploited. Historically, difficulties in efficiently generating and manipulating terahertz waves served as a barrier for the area. However, several developments have changed the fortunes of the field.

The advent of the quantum cascade laser in the mid-1990s provided access to a solid-state laser technology that evolved to emit milliwatts of power of terahertz radiation from a convenient semiconductor device, albeit cooled. More recently, engineered structures called “metamaterials” have been designed that can switch and modulate terahertz waves.

As for applications, researchers have found that terahertz waves can be useful for security screening, in particular. For example, it’s known that terahertz waves can pass through thin layers of clothing and organic materials but are blocked by metals, such as a knife blade, and that certain explosives and pharmaceuticals have a clear absorption fingerprint. As a result, several companies have since commercialized the technology for security scanners for use at airports and elsewhere. Other potential applications include the use of terahertz waves instead of radiowaves as a high-frequency carrier wave for future mobile communications opening the door to ultra-high-bandwidth data connections. There are also efforts to use terahertz waves for biomedical applications such as skin cancer detection, however the strong water absorption of terahertz waves limits many biological applications.

One thing’s for sure: the region is evolving from being a once neglected gap in the electromagnetic spectrum to a highly active area of science that is rich in potential applications and opportunities.

Post by Nina Meizner

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.

Post by Lina Persechini

What?
As you move away from the visible region of the electromagnetic spectrum you encounter the Infrared region. Infrared light is divided into three spectral regions: near, mid- and far-infrared

Where?
The near-infrared (NIR) can be defined as the region between 750 nm and 2,500 nm, although the boundary between the NIR and the mid-Infrared (MIR) region can vary slightly.

When?
In the year 1800, astronomer and composer William Herschel had been using dark filters in a telescope to study the sun when he noticed that he could still feel the heat of the sun’s rays through the filters. It was through this ‘sensation of heat’ that he concluded that there must be invisible light beyond the visible part of the electromagnetic spectrum.

How….can we use it?
Herschel’s discovery led to the development of astronomical spectrophotometry as a means to understand our stars and galaxies. NIR spectroscopy can also be used to understand the chemical composition of different media we have right here on Earth. However, the most significant and relevant use of NIR radiation is in fibre-optic communication, where information is carried through NIR light rather than through electric cables. The telecom window between 800 – 900 nm was originally used for transmitting information, but it turned out the optical fibre losses over longer distances were too high in this region. Later, the range 1,260 – 1,360 nm (or more affectionately known as the original ‘O’-band) was used, as the optical fibres produced in the 1970s were of the lowest loss in this region. Nowadays the C-band (1,530 nm to 1,565 nm) is the most conventionally used band for long-haul and submarine optical transmission. In total we can boast around 6 bands (O, E, S, C, L, U) which have been defined and standardized to meet our information requirements.

Post by Mark Daly

The Earth’s Sun emits a tremendous amount of electromagnetic radiation in the Earth’s direction. Even though the entire spectrum of light is incident on this planet, why is it that we humans only see in a tiny band that we have — rather appropriately — named the visible spectrum?

At first, we could simply dismiss the problem as being purely evolutionary, but there must have been a driving factor — some evolutionary pressure — that led us down this path. With a little thought and some ‘light’ physics, we can use the process of elimination to lead us to a logical solution.

Let’s begin with the source of all our light, the aforementioned Sun. The Sun can be described as a ‘black body’. Without getting too bogged down in the somewhat confusing name for such a bright object, let’s just say it means that the Sun is capable of emitting light continuously over the entire frequency spectrum. (More interested readers can take a look at the Wikipedia entry for more information).  Luckily, our good friend Max Planck worked out the distribution of light across the different frequencies emitted by such a black body back in the 1900s. Using Planck’s law, we can input our Sun’s temperature and discover that the peak of its emission just so happens to be in the visible spectrum. You might be forgiven for thinking ‘Ah-ha! This is why we see in this range’, and you would be partially correct. Yet we can still dig a little deeper.

Atmospheric absorption of light from the sun. Image credit: NASA, SVG by Mysid

Not all of the light that is incident on Earth makes it down to the ground in any meaningful abundance. In fact, there are huge absorption bands in our atmosphere. The only bands of light that could be candidates for vision are ultraviolet, visible, near-infrared, and longer radio wavelengths.

Technically speaking, all of these types of light would be viable candidates for providing some information about the world around us. However, the scale of the fine details on Earth limits it further. Radio wavelengths are very long, ranging from a few millimetres to kilometres! Because of their long wavelengths they turn out to be very useful, because they aren’t absorbed much by thin obstructions. However, because of their large dimensions, if we relied on radio wavelengths to see we wouldn’t be able to make out any fine details in our surroundings, so we can rule them out.

What about the infrared spectrum? Some animals do see in the infrared, but these animals are typically cold-blooded. Why? Well, hot objects emit quite a bit of infrared radiation, so we would be all but blinded by the heat from our own bodies if we could see in this regime. Ultraviolet light contains a lot of energy – the shorter the wavelength, the higher the energy of a single photon. Although some animals do see in the UV, we humans cannot. We know that even on a seemingly cloudy day, UV radiation can damage our skin, so it’s best not to focus this powerful light onto our delicate retinas.

Ultimately, after exhausting all other options, we are left with one contender: the good old reliable visible spectrum.

Post by Gaia Donati

Ultraviolet (UV) radiation identifies the region of the electromagnetic spectrum where wavelengths are longer than those of X-rays. The extreme UV (XUV) range identifies light with a wavelength around 10 nm and up to 100 nm; far-UV and middle-UV regions are characterised by wavelengths between about 100 nm and 200 nm and between 200 nm and 300 nm, respectively. Near-UV radiation extends to around 400 nm, which is commonly taken as the lower value for visible wavelengths. You may wonder – why should one bother to label these intervals so diligently? While the precise boundaries of these ranges are not set in stone, the UV region peculiarly spans two different orders of magnitude in wavelength (or, equivalently, in frequency): the fastidious labels for each sub-region are there to remind us that the features and applications of light in the longer-wavelength near-UV region are distinct from those characterising short-wavelength XUV radiation, for example.

If you check the label on your sunscreen, this will probably read “UVB protection” or “UVB and UVA protection” – and it just so happens that UVB and UVA identify wavelengths ranges which almost coincide with the middle- and near-UV regions, respectively. Of all kinds of UV light, UVB and UVA rays are the ones that penetrate the deepest into human skin – with its longer wavelength, UVA light reaches all the way into the epidermis and the dermis. Therefore, it is important to protect our skin from these rays: overexposure to UVA and UVB radiation – including tanning beds – is now known to increase the risk of developing various forms of skin cancer. On a more positive health-related note, light in the UVB region is instrumental to the activation of vitamin D in the organism, and is thus crucial for calcium absorption.

Let’s put aside the interaction of light with our human bodies and look instead at what happens when light interacts with media such as a crystal slab or a gas. Under specific conditions, it is possible to observe a variety of so-called nonlinear interactions between these states of matter and electromagnetic radiation: nonlinear frequency conversion, for instance, describes processes whereby shining light at a given wavelength through a crystal produces an output radiation at a different wavelength. An ‘extreme’ form of nonlinear frequency conversion involves the interaction between an intense, focussed laser pulse (which has a fixed temporal duration, as opposed to the continuous light beam produced by a laser pointer) and a gas, resulting in the generation of light at high multiples of the optical frequency of the original pulse: this process, known as high harmonic generation, has proven incredibly successful for obtaining short laser pulses in the XUV range. These short pulses, which have durations around and below 100 attoseconds (where an attosecond equals one quintillionth of a second), can be turned into probes for the study of physical phenomena taking place on the same temporal scale – think of a very fast camera capturing the dynamics of electrons in matter, for example. Ultraviolet? Ultrafast, too.