Nanotechnology Blog

In June’s blog, invisibility cloaking was covered.  While that is a type of optical metamaterial, the advantage of optical metamaterials is that they can provide the ability to extend the range of traditional optics.  (While the work has been ongoing for years, there is not the impact of the invisibility cloak.) There is a rule based on the wavelength of light called Rayleigh’s Limit that provides the limit of the smallest objects that can be defined.  Blue light is in the range of 450nm and green light is 550nm.  The Rayleigh Limit predicts that the smallest separation between two points that can be detected is 56nm using blue light and 69nm using green light.  This is the theoretical limit at which two points can be separately identified by perfect optics.  This is not the minimum that structure can be identified, which is much larger.

So what is the big deal?  Semiconductors devices have features in the low nanometer range and they can be inspected.  Yes, features smaller than 10 nm can be visualized employing various types of electron microscopy because the material being “viewed” is a solid surface.  The limitations of optical microscopy have the greatest impact on biological work. Many of the investigations in this filed work with objects that are small, transparent, and have little contrast difference in the object.  This includes viruses and DNA molecules.  Bright field microscopy limits the resolution to approximately 200nm. [Ref. 1] 

The challenges in manufacturing the optical metamaterial are a combination of both finding the proper materials to create a negative index of refraction and creating layers of the required thickness to become a metamaterial.  Work done and published in 2007 [Ref. 2] indicated that using positive and negative layers of refractive index material, they were able to achieve a resolution of 70nm.  Work presented in Reference 3 provides more information on the state of the effort in 2014.  “By using 15 nm TiO2 nanoparticles as building blocks, the fabricated 3D all-dielectric metamaterial-based solid immersion lens (mSIL) can produce a sharp image with a super-resolution of at least 45 nm under a white-light optical microscope, significantly exceeding the classical diffraction limit and previous near-field imaging techniques.”  Additional work in 2016 [Ref. 4] demonstrated 3D resolution of sub 50nm across the plane and 10nm in depth.  Current research efforts include the application of metamaterials and the inclusion of immersion techniques.

The focus of the metamaterial enhanced lenses is to provide a better understanding of the interaction of biostructures that are beyond the limit of optical microscopy.  The challenges moving forward are numerous.  The application of various layers to create the negative index is dependent on the material being employed and achieving the proper thickness of each layer.  Defects in the layers reduce the resolution of the image.  Fortunately, the production of precise layer thickness can be accomplished by available tools.  Atomic Layer Deposition (ALD) is available with existing semiconductor manufacturing tools.  Even the ability to create structures can be accomplished with existing tools.  The question that remains is how small a dimension will be able to be analyzed optically.  Progress is needed to advance biological/medical research.

References:

1. https://en.wikipedia.org/wiki/Superlens
2. https://ui.adsabs.harvard.edu/abs/2007Sci…315.1699S/abstract
3. https://research.bangor.ac.uk/portal/files/20635555/Bing_Yan_PhD_2018.pdf
4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4840372/

Metamaterials are usually identified by indicating it is a constructed material that has characteristics not observed in nature.  As the last blog demonstrated, metamaterials at the nanoscale can be designed to work well in the optical and near-optical portion of the electromagnetic spectrum.  There are also metamaterials that are on the millimeter scale.  This blog will cover modification of sound waves with metamaterials.

 A very generalized statement on metamaterials in the frequency domain is that it is possible to position material in such a manner as to inhibit the flow of the energy.  As with the optical examples in the last few blogs, it is possible to create structures that act as if they have negative properties of the normal material.  An interesting presentation [Ref. 1] provides a high level of various means of altering material properties in order to impact the acoustic (sound) waves. 

The function of the metamaterial to modify acoustic waves is created by developing geometries that create phase delays in portions of the wavefront that causes a cancellation of the wave front.  Figure 1a depicts a sinusoidal wave in red.  Figure 1b shows the addition of a second wavefront in blue that is precisely out of phase with the first wave.  The net result is the straight green line that is the sum of the two waves.  More details, including the mathematics involved, are presented in Reference 2. 

So the question is “What can this type of device actually accomplish?”  Typically, large and heavy materials are employed to mitigate noises.  The typically “sound” barriers along the sides of expressways in the cities are a prime example of that approach.  But, for individuals who need shielding working in a noisy environment, heavy and bulky is not a solution.  Reference 3 presents work on a smaller, not large roadside structures, scale that was performed in Professor Xin Zhang’s Boston University’s lab. 

So, if one has the mathematics background and the computing power to calculate the desired structure shape, how do you prove it works.  The researchers decided to create a structure that would cancel the sound from a loudspeaker.  Figure 2 shown below is a picture of the experimental setup.  The video link to its operation is Reference 4.

The bottom portion of the Figure 2 shows the sound exiting the plastic pipe and the increase in sound, shown by the larger blue area, is obvious when the acoustic silencer is removed.  Their calculations indicate that they have reduced the noise level by 94%.

What does that acoustic “plug” look like.  The picture below, Figure 3, is the plug, which is a 3D printed metamaterial, that modifies acoustic waves in the video in Figure 2 [Ref. 4]. 

While this structure appears to be simple, the actual internal was not depicted in any of the articles on their work.  In order to accomplish the sound cancelling, the structure is complex.  There is a video from Duke University [Ref. 5] that provides an explanation of the modification of acoustic waves.  Figure 4 below is a picture from the video in Reference 5 with the researchers describing how the different shapes and lengths impact the sound in order to modify it. 

Obviously, the structure requires some serious design calculations.  The ability to change the properties of sound waves with metamaterials that are large in size is due to the fact that sound waves have wavelengths measured in inches.

It is possible that these types of devices could replace the heavy sound blocking partitions along highways with lighter and possibly less costly specifically designed to reduce traffic noises while permitting more traditional neighborhood sounds.

References:

1. https://www.esa.int/gsp/ACT/doc/EVENTS/acoustic_workshop/ACT-PRE-0914-valencia-review_acoustic_metamaterial.pdf
2. https://www.nature.com/articles/s41467-018-03839-z
3. https://phys.org/news/2019-03-acoustic-metamaterial-cancels.html
4. https://youtu.be/Fd1D42dVxS0
5. https://youtu.be/pNsnvRHNfho

Metamaterials are usually identified by indicating it is a constructed material that has characteristics not observed in nature.  While typically referring to nanoscale materials, there are metamaterials that are on the millimeter scale.  (That will be covered in a future blog.) 

Currently, in the public domain, there are two types of invisibility cloaks.  The one that exists in movies and a lot of videos is fake.  While the process is straightforward the effort takes some time.  The video in Reference 1 depicts how to make a video of a “invisibility” cloak.  The developer shows of background of the room he will be filming in.  He then walks into the view of the camera and picks up the “invisibility” cloak and moves it in front of him, which shows portions of the room behind him where ever the cloak is covering him (Figure 1).  The room is perfectly shown as it was in the beginning of the video. 

Figure 1

About 1:20 into the video, the developer then shows how this illusion is created.  He has a video (even a still picture would work as long as it has the same camera position) of the room.  When he walks into the viewing area, he picks up a green cloth and goes through whatever disappearing motions he will be showing.  The rest of the video shows how he can make the area with the green cloth transparent in that video.  Once that is completed, the modified video is overlaid on the background video/picture.  Where the green cloth had been on the film, the original background appears.  The more complex the background, the more the “invisibility” cloak appears to be real.  Based on examples of public domain information of actual modification of the light path, the clarity of the background indicates that things have been manipulated. 

There is a significant amount of information about the state of actual invisibility cloaks, most of it focuses on a Canadian company, HyperStealth Biotechnology Corp. [Ref. 2].  Their product is called Quantum Stealth.  The picture below is a screen capture from a video of the product [Ref. 3], which depicts a person emerging from behind the “cloak” (Figure 2). 

Figure 2

This is the latest version in a long series of inventions to improve the capabilities of the material.  So how does it work?  While the actual fabrication of the material is not available, there are certain fundamental principles that can be surmised.  In order to change the way light appears on an object, it is necessary to create something that will bend/transform light in a manner that does not occur in nature – a metamaterial. 

Reference 4 has a good explanation of how light bends going through a material (Figure 3 – time 2:10 in video), which is a lenticular lens.  Further on in the video (Figure 4 – time 3:45). The author demonstrates a “invisibility shield” created from two opposing lenticular lenses filled with water between the two lenses. 

Figure 3

Figure 4

What is noticeable is that the vertical window line has disappeared at the shield.  The property of the device is such that is takes the horizontal structure and in-effect creates the continuous horizontal structures across the “shield”. 

There is another issue that is impacted by Snell’s Law.  Different wavelengths (colors) are bent (refracted) differently based on the wavelength.  Work has been done to address both visible and light outside the visible portion to be part of the reimaging of the background.  Obviously, there have been significant improvements based on the Quantum Stealth material’s latest version.    

For those who would like a more detailed, scientific explanation, Reference 5 provides a good description with the required mathematical details.  Quoting from the beginning of Metamaterials explanation and explaining that this was first applied to the cloak:

“The immense potential of refractive index engineering in metamaterials can be further exemplified by recent progress in the field of transformation optics that enabled novel opportunities in the design of graded-index structures … The basic idea of the transformation method is that, to guide waves along a certain trajectory, either the space should be deformed, assuming that material properties remain the same, or the material properties should be properly modified.”

Remember: Metamaterials are a novel class of functional materials that are designed around unique micro-and nanoscale patterns of structures, which cause them to interact with light and other forms of energy in ways not found in nature.  The properties of these materials are directly due to the combination of various material characteristic involving both their shape and thickness. 

References:

1. https://youtu.be/4z1yGdEPo0Q
2. https://www.hyperstealth.com
3. https://youtu.be/pZMyWEWHCTM
4. https://youtu.be/TJvGOI263po 
5. https://www.sciencedirect.com/topics/materials-science/metamaterials

An excellent example of how metamaterials work comes from Stanford University [Ref. 1].  The concept can be described as creating nanoscale antenna to affect the performance of electromagnetic spectrum waves.  Over a half century ago, TVs had external metal antenna.  The design was for the frequencies of the various television stations.  If your reception was inadequate, changing the position of the antenna provided the opportunity to improve the signal being received.  Working with visible and infrared light, the wavelengths are roughly 1,000nm or less.  So, the work at modifying the light beam requires structures that are much smaller than the wavelength of the light.  Fortunately, there are semiconductor processes that are able to manufacture devices at a fraction of the wavelength. 

One example that is employed to explain metamaterials involves light.  When the light rays enter a material, the light is distorted by an amount that is related to the index of refraction, which is related to the amount of speed reduction of the speed of light in that material.  Metamaterials can actually have a negative index of refraction.  That means that the light will bend in the opposite direction that is observed in nature.  AND, the apparent speed of light in the material will appear to be greater than the speed of light in a vacuum!

The picture below is an example of a simple demonstration that the index of refraction of the water, which is different from that of air, creates the appearance of a displacement of the image of the pencil.  If one could create a liquid with a negative index, the pencil would be displaced in the opposite direction.

While the ability to create negative indices of refraction in liquids has not been reported, work is being done on solid materials that create the impact of a negative index.  The key to this “cloaking” device is a lenticular lens, which will be described in next month’s blog. 

What the lenticular lens does is “bends” the light outward instead of either focusing or reflecting the light.  While this appears to be promising, there are still issues.  As with an existing, all the colors (frequencies) of light do not bend at the same amount.  This is compensated in optical lenses by using different shapes and materials to compensate for the different amount of bending of the light.  One example of the difference in various colors (frequencies) of light being bent differently is a rainbow.  The color separation is due to different amounts of bending the water particles have on sunlight. 

There has been some work done on using pairs of either cell phones of iPads to create “holes” in objects/people since 2006 [Ref. 2].  The real issue is that while small “see-through” objects can be created to appear to see through an object, covering an entire, large object is not practical with this technique. 

Science moves forward finding novel means to apply new technology as it is developed.  The application of lenticular lenses permits some very interesting situations.  The picture below shows only the head and shoulders of a person [Ref. 3]. 

If one observes the picture closely, there is a slight change in the coloring and texture, which is due to the “cloaking” device.  There has been considerable efforts in creating the “invisibility” or “stealth” material that works through a reasonable portion of both the visible and infrared spectrum.  Remember the rainbow example above.  If that were to happen to the light being “bent” around the object, it would be easy to determine the attempt at cloaking. 

There is a material advertised as “Quantum Stealth” that has demonstrated a material that can “hide” large objects through light bending technology [Ref. 4].  There are videos of the performance of the material on their site.  While there have been examples of cloaking large objects since at least 2011 [Ref. 5], the ability to cover a wider spectrum of light has improved steadily.  Next month, how this metamaterial works and can be mass produced.

References:

1. https://engineering.stanford.edu/magazine/article/what-are-metamaterials-and-why-do-we-need-them 2016
2. https://www.flickr.com/photos/evanbooth/291214973
3. https://www.freethink.com/technology/invisibility-cloak 
4. https://www.hyperstealth.net/
5. https://www.wired.com/2011/09/invisibility-cloak-tanks-cows/

In many cases semiconductor industry leads the application of technology shrinkage.  Each “generation” of new devices employs smaller and smaller dimensional structures in the circuitry.  There is work being done on qualifying the processes for the 3nm and 2nm generation.  Since future shrinkage will be fractions of a nanometer, Intel Corporation has indicated it  is switching terminology from the 2nm to 20 Angstroms.  (The term Angstrom has been employed in optics for centuries.)  The next metric dimension identified is “pico” for 10^-12 meters.  That scale is well into the atomic structure. 

Research has demonstrated that materials have different properties at the nanoscale from their bulk properties, which is commonly associated with the materials.  One example is that silver has anti-bacteria properties in the low double-digit nanometers.  Another example is that aluminum becomes highly reactive in the low double-digit nanoscale.

There are two phrases “floating” around in various publications:  metamaterials and mesoscale materials.  The former is human constructs of materials that are not normally found in nature and the term prominently employed in publications.  Mesoscale, as defined by the Los Alamos National Labs [Ref.1] “is the spatial scale beyond atomic, molecular, and nanoscale where a material’s structure strongly influences its macroscopic behaviors and properties.”  Stated differently, mesoscale can include the behaviour of a particular section of a large weather event, i.e., how the tornado interacts with the entire storm system, to the behaviour of a submicron section of material within the bulk material.  It is the properties of a small section of a larger system that may have significantly different characteristics.

Metamaterials is usually identified by indicating this material has characteristics that are not seen in nature.  “Metamaterials are a novel class of functional materials that are designed around unique micro-and nanoscale patterns of structures, which cause them to interact with light and other forms of energy in ways not found in nature. [Ref. 2]  The properties of these materials are directly due to the combination of various material characteristic involving both their shape and thickness. 

The ability to produce two-dimensional materials has provided the means of being able to investigate novel material properties.  The vast majority of the work in metamaterials is focused on the modifying or influencing the behaviour of electromagnetic waves.  These waves include the RF (radio spectrum), microwaves, and even infrared and visible light.  Some details on these applications and what they can accomplish will be the subject of future blogs.

The point of this blog is to focus on the application of nanomaterials to create structures that do not exist in nature.  As mentioned in previous blogs, during the Middle Ages, the artisans knew that adding certain size gold nanoparticles to the manufacture of glass would create the red color in stained glass windows.  They could not measure the particles, but probably had a process or recipe that they followed to create the desired size particle.  There are industries where ball milling is employed to provide a means of obtaining a sufficiently uniform particle size to enable the manufacture of the final product. 

Next month, the blog will start covering details of the structure of metamaterials.  One example that is employed to explain metamaterials involves light.  When the light rays enter a material, the light is distorted by an amount that is related to the index of refraction, which is related to the amount of speed reduction of the speed of light in that material.  Metamaterials can actually have a negative index of refraction.  That means that the light will been in the opposite direction that is observed in nature. 

There are many applications that are being considered.  This could be a very important field to develop novel products that can produce effects that are currently impossible.  More next time.

References:

1. https://lanl.gov/science-innovation/pillars/materials-science/mesoscale/index.php
2.  https://www.nanowerk.com/what-are-metamaterials.php