Similar frustrations motivated the development of ultrafast lasers within the scientific community. Instead of capturing a fluttering hummingbird, these lasers with pulse durations on the order of femtoseconds to picoseconds (10-15–10-12 seconds) are used to capture events like molecular vibrations,1 electronic motion,2 and even quantum phenomena.3  At timescales of one millionth of a billionth of a second, ultrafast lasers continue to expand our access to fundamental physical phenomena, as well as revolutionize industrial processes.

In the ultrafast community, we quantify this temporal distortion as group delay dispersion (GDD). GDD is a frequency-dependent value that, for a given material, scales linearly with thickness. Transmissive optical components like windows, lenses, and objectives normally apply positive GDD so your once-compressed pulse may emerge from the transmissive optical component with a longer duration than initially emitted by your laser.

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Beyond broad spectral bandwidths, incredibly high peak powers are another consequence of ultrashort pulse durations. For context, let’s examine the difference in the peak power output of a 10 W CW laser vs. that of a 10 W ultrafast laser with 150 fs pulses and a repetition rate of 80 MHz—properties common for many commercially available ultrafast laser sources.

A multi-layer AR coating contains multiple microscopic layers to improve performance and minimise reflection to less than 0.1% of incident light. Each thin layer is deposited onto the surface substrate to increase the destructive interference, maximising transmission. [3,5]

Antireflectivecoatingglasses vs blue light

Once light passes through the air and meets a medium, the Fresnel equations can determine the amount of light reflected and transmitted, depending on the refractive indices. [1,3] The following equation defines the fraction of reflected light:

3. Keshavarz Hedayati, M., & Elbahri, M. (2022). Antireflective coatings: Conventional stacking layers and ultrathin plasmonic metasurfaces, a mini-review.” Materials 9(6), 497. https://doi.org/10.3390/ma9060497

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The laser damage threshold (LDT) of optical components is another important distinction between ultrafast and other lasers that can pose a challenge for ultrafast laser users (see Fig. 3). When sourcing optical components for nanosecond lasers, it is common to see laser damage thresholds on the order of 5–10 J/cm2. For ultrafast optics, values this large are practically unheard of; you are more likely to see LDT values < 1 J/cm2, typically closer to 0.3 J/cm2. This dramatic difference in LDT values across different laser pulse durations reflects the different mechanisms for laser damage at play.

Studies to track electronic motion are already being conducted using these even shorter ultrafast pulses, and the field of attosecond science continues to improve our understanding of electronic structure and quantum mechanics.12 While integration of XUV attosecond pulses into industrial processes has not yet gained major traction, continued research and advancements in this field will almost certainly propel this technology out of the lab and onto the manufacturing floor, as has been the case with femtosecond and picosecond laser sources.

Next, we’ll summarise the different manufacturing processes for anti-reflection coatings and lenses. These processes fall under two primary categories: conventional techniques and non-conventional techniques. [5] Of course, cutting-edge equipment – such as the HEX Series deposition system we manufacture – is necessary for creating anti-reflection coatings. Conventional techniques include top-down and bottom-up technologies. [3,5]

An anti-glare coating works by splitting light waves into two reflections. The split creates destructive interference, causing the light waves to cancel each other partially or entirely. [4] How the light waves travel and behave through mediums and interfaces determines how the AR coating will work. [5]

Antireflectivecoatingmaterial

Sol-gel chemistry processing is one of the most commonplace techniques for creating anti-reflection coatings and lenses. It uses metal oxides and organic solvents to condense the compounds into an inorganic polymer bond. [5] Standard sol-gel techniques include meniscus coating, dip coating, and spin coating.

Micro-replication is another type of non-conventional manufacturing process. It involves a roll-to-roll process replicating nanostructures on a thermoplastic film surface, such as PVC. The photo-aligning technique is another method that minimises transmission to 99.1%. [5]

Physical and chemical vapour deposition are two other common manufacturing methods and require using complex deposition systems like the HEX Series. Etching is another conventional technique, but it uses selective surface ablation to achieve the desired AR coating. [3,5]

“V” AR coatings are for highly specialised applications that single- and multi-layer coatings are unsuitable for, like high-frequency lasers. Other applications include high index lenses, anti-reflective glasses with UV protection and less glare, digital microscopy, fibre optics, engraving, and more. [5]

4. Nave, R. (n.d.). Anti-reflection coatings. HyperPhysics. Retrieved August 25, 2022, from http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/antiref.html

At Korvus Technology, we’re the UK’s premier source for thin film manufacturing, and over 25 organisations, universities, and brands trust our HEX Series deposition system. In this article, we’ll explain anti-reflection coatings, including different types, how they work, limitations, common uses, and more.

Most manufacturers switch between a low and high refractive index when depositing layers. Generally, anti-reflection coatings with multiple layers provide stronger broadband performance. However, the cost of manufacturing multi-layer anti-reflection coatings is prohibitive. [5] These coatings are more sophisticated than single-layer coatings and essential for optical applications, like lenses, astronomy, and aerospace telemetry. [1]

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Biological imaging. The very high peak powers of ultrafast lasers support nonlinear processes that can improve the resolution of biological imaging, such as in the case of multiphoton microscopy. In these studies, it is necessary to overlap two photons in both space and time to generate the nonlinear signal from the biological medium or fluorescent target. This nonlinear mechanism improves the imaging resolution by substantially reducing the signal background that plagues studies conducted with single photon processes.9 Figure 4 demonstrates how this reduced background can result in higher resolution.

Some manufacturers use non-conventional techniques when creating an anti-reflective coating. Lithography falls under this category and consists of patterning the substrate surface with microscopic features. [5]

As you can see, anti-reflective coatings offer modern-day technology a world of opportunities for improving products, efficiency, and our quality of life. At Korvus Technology, we’re proud to be the leading source for deposition systems in the UK. To learn more, check out our blog or contact us online.

Another common limitation occurs with quarter-wavelength anti-reflection coatings. To lower the refractive index, manufacturers must use a porous coating material, which occurs in a single processing step. However, the coating’s porous nature reduces its strength and could make it more vulnerable to contamination. [3,4,5]

Furthermore, chemical vapour deposition or sol-gel chemistry creates a durable, strong AR coating. However, the process is prohibitively expensive, particularly for multi-layer stacks. Additionally, multi-layer filters are highly sensitive to variations in the refractive index and coating thickness. [3,4,5]

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Nanostructured lenses with AR coatings that have a gradient to increase the refractive index have effective anti-reflection properties. However, the nanostructures in the topcoat are a double-edged sword as they decrease the mechanical strength of the coating. [3,4,5]

For the ultrafast laser, that 10 W is distributed across the 80 million pulses emitted each second, according to the repetition rate. At first glance, the sub-microjoule pulse energy of this laser may seem miniscule. But if we account for this energy being squeezed into only 150 fs of time, we arrive at a massive peak power of over 800,000 W for this laser, which is more than four orders of magnitude greater than the average power. While such enormously high peak powers and broad spectral bandwidths have made ultrafast lasers useful for a wide variety of application spaces, these features also give rise to some of their unique technical challenges.

However, the inherent differences and bonds between the coating’s thin layer and the front and back surfaces of the substrate impact durability, hardness, strength, refraction, and reflectability. [1,3] Therefore, most anti-glare coatings are vulnerable to abrasion, which can pull off the coating on the lens surface. Thermal cycling and solvents can also cause stress or damage to the bond. [5]

However, other applications like telephoto lens material, light-emitting diodes, and solar cell panels require AR coatings that maximise efficiency. [2] An anti-reflective lens coating that improves vision is also ideal for increasing available light transmission, enhancing contrast, eliminating ghost images, and sharpening visible focus.

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A single-layer AR coating may only become anti-reflective at a single wavelength, typically in the visible middle. [4] When depositing single-layer quarter-wavelength AR coatings, they can reduce surface reflectivity for incidence angle and limited wavelengths. [3]

The path length of the incident light will differ, reducing destructive interference. Many applications require single-layer anti-reflection coating, including photodiodes, lasers, and solar cells. However, the reflection dip in a single-layer anti-reflection coating makes it unfeasible for displays, lenses, and glasses. [3]

Generally, anti-reflection coating applications have two purposes (besides eliminating reflections): to improve an object’s aesthetic or efficiency. [2] Regarding aesthetics, applications include anti-glare glasses, picture glass, and electronic displays.

The equation calculates the index of refraction for an optimal AR coating that will reduce reflections off the surface. [1,5]

Broad spectral bandwidths, incredibly high peak powers, and the ultrashort pulse durations of ultrafast lasers must be considered and properly managed when applying an ultrafast laser to your project or process. Typically, the simplest of these is managing the broad spectral output of your laser. If you have mostly worked with CW or longer pulsed lasers in the past, your inventory of optical components may not reflect or transmit the entire bandwidth of your ultrafast laser pulse. The good news is many suppliers keep these needs in mind when designing ultrafast laser optics, so it is quite easy to source mirrors, lenses, and other optical components that sufficiently cover the bandwidth of ultrafast lasers.

A “V” anti-reflection coating follows the same transmission and light reflectance principles as a single-layer coating. However, it undergoes optimisation to improve performance within a small niche of wavelengths. [1] The name derives from its high refractive index, creating a “V” shape that curves over multiple wavelengths. The centre arcs around each design wavelength (DWL). [5]

Anti reflection coatingformula

Of course, the properties of an anti-reflection coating directly influence its useful lifespan. In particular, optoelectronic devices like camera lenses and touchscreens require the best anti-reflective coating possible. Ideally, the coating should have broadband, ultrathin thickness, and non-iridescent properties. [3]

Thanks to their ultrashort pulse durations, ultrafast lasers possess key features that distinguish them from longer pulse or continuous-wave (CW) lasers. Generating such a short pulse requires a broad spectral bandwidth. The minimum bandwidth to generate a pulse of a particular duration depends on the pulse shape and the central wavelength. In general, this relationship is described by the time-bandwidth product (TBP), which arises from the uncertainty principle. For a Gaussian pulse, the TBP is given by:

Laser materials processing. Ultrafast lasers have revolutionized the laser micromachining and materials processing worlds because of their ultrashort pulse durations. As mentioned in the context of LDT, the duration of the ultrafast laser pulse itself is faster than the time scale of thermal diffusion into a material’s lattice. As compared to nanosecond pulsed lasers, this means an ultrafast laser produces a much smaller heat-affected zone, resulting in lower kerf loss and more precise machining.10 This principle extends to the medical field as well, where the increased precision of ultrafast lasers cuts is routinely used to decrease damage to surrounding tissues and improve the patient experience.11

Imagine gazing out your window on a sunny spring afternoon. As you admire the green trees and blossoming flowers, you see a small blur dart past. You follow the motion until the blur finally pauses, and you realize it is a beautiful hummingbird—the first of the season! You pull out your phone to capture this beacon of warmer weather in a photo for your social media, only to startle the hummingbird with the sound of the capture button. A photo that once held the promise of many “likes” is now a giant blur across your screen.

For longer pulses, such as nanosecond and even picosecond pulse durations, GDD is not a major concern. In the case of femtosecond pulses, however, even 10 mm of N-BK7 in the path of your laser beam can broaden a 50 fs pulse centered at 800 nm by over 12%. This is roughly equivalent to having two windows or filters in the path of your beam. Because of this tendency toward temporal distortion, it is recommended to use specialized ultrafast optics that impart minimal to no additional GDD and decrease the chances of an elongated pulse duration.

The anti-reflective coating cost varies based on the manufacturing process, necessary equipment, intended use, surface substrate, etc. [2] However, we’re happy to answer questions regarding the cost of anti-reflection coatings and how they can add value to your business.

For nanosecond lasers or even longer pulses, the predominant damage mechanism is thermal in nature. In these cases, the material is absorbing incident photons and heating up, which can result in deformation of the lattice. Effects like thermal expansion, lattice strain, cracking, and melting are common outcomes for thermal pathways of laser damage.5 In the case of ultrafast lasers, the duration of the pulse itself is actually faster than the timescale of heat transfer into the surrounding material lattice. Instead, the high peak powers of ultrafast lasers shift the damage mechanism toward more nonlinear pathways, such as multiphoton absorption and ionization.6 For these same reasons, one cannot scale the LDT rating for nanosecond pulses down for ultrafast pulses. As a result, the most appropriate optic for your particular application is one with a sufficiently high LDT rating obtained under the same conditions (laser wavelength, pulse duration, repetition rate, etc.) it will experience in your application.

The ultrashort pulse durations and high peak powers of ultrafast lasers provide benefits to a wide variety of applications, including:

5. Raut, H. K., Ganesh, V. A., Nair, A. S., & Ramakrishna, S. (2011). Anti-reflective coatings: A critical, in-depth review. Energy & Environmental Science, 4(10), 3779–3804. https://doi.org/10.1039/c1ee01297e

2. Burghoorn, M., et al. (2013). Single layer broadband anti-reflective coatings for plastic substrates produced by full wafer and roll-to-roll step-and-flash nano-imprint lithography. Materials, 6(9), 3710–3726. Retrieved August 25, 2022, from www.ncbi.nlm.nih.gov/pmc/articles/PMC5452668/, 10.3390/ma6093710.

The mechanical and chemical properties of anti-reflective lens coatings make them invaluable for modern-day applications, including anti-glare glasses, lasers, display screens, optic lenses, and solar panels.

1. Bauer, G. (n.d.). Anti-reflection coatings. PVEducation. Retrieved August 25, 2022, from https://www.pveducation.org/pvcdrom/design-of-silicon-cells/anti-reflection-coatings

As research into the applications of ultrafast lasers continues, so too does development of new and improved ultrafast laser sources. To gain insight into even faster physical processes, many researchers are turning their attention to the generation of attosecond pulses—pulses on the order of 10-18 seconds in the extreme ultraviolet (XUV) spectral region.

Anti-reflective coating and anti-glare lenses have dozens of practical uses for modern-day technology thanks to their unique properties. However, that doesn’t mean manufacturing AR coatings is easily accessible or affordable for the masses. As with any delicate and complex manufacturing process, there are certain limitations to consider.

Through thin film and vacuum deposition technology, you can apply an AR coating to an object’s surface (like that of a standard lens), reducing light reflections and eye strain. [3] Anti-reflection coatings also depend on their refractive index to minimise light loss on lens surfaces. [1,4,5]

If you’ve ever squinted reflexively after a bright sunbeam reflected off your windshield, you probably wished for a pair of sunglasses with an anti-reflective coating on the lenses to cut the glare. While light reflection is necessary for objects like mirrors, it causes absorption in glasses, telescopes, and lenses. However, depositing a special coating on the object’s surface (as in anti-reflective lenses) reduces reflections and glare, improving visual acuity. [1]

One of the most difficult technical challenges associated with ultrafast lasers is maintaining the ultrashort pulse duration provided by your laser. Ultrashort pulses are highly susceptible to temporal distortion, which worsens as the pulse duration gets shorter. Though your laser may emit a 50 fs pulse, relaying this pulse to a target position using mirrors and lenses, or even just passing it through air, has the potential to temporally broaden your ultrafast pulse.

The manufacturing process for anti-reflection coatings presents significant limitations. Most techniques cannot accommodate the deposition of AR coating on large-scale surfaces.

Spectroscopy. Since the inception of the ultrafast laser, its application in spectroscopy has been ubiquitous. By decreasing the pulse duration down to femtosecond timescales, dynamic processes in physics, chemistry, and biology were suddenly observable.7 The advent of ultrafast lasers has provided access to atomic motions—improving our understanding of fundamental processes ranging from molecular vibrations and dissociation all the way to energy transfer in photosynthetic proteins.8