Abstract-Prospects of designing gold-nanoparticles-based soft terahertz radiation sources and terahertz-to-infrared converters for concealed object detection technology

K. A. Moldosanov, A. V. Postnikov, V. M. Lelevkin, N. J. Kairyev

https://arxiv.org/abs/1908.07991

The two-phonon scheme of generation of terahertz (THz) photons by gold nanobars (GNBs) is considered. It is shown that in GNBs, by choosing their sizes, it is possible to provide conditions for converting the energy of longitudinal phonons with THz frequencies into the energy of THz photons. The prospects of designing GNBs-based soft THz radiation sources (frequencies: 0.14; 0.24; 0.41 and 0.70 THz) with a large flow cross-section (diameter ~40 cm) intended for detection of hidden objects under clothing to ensure security in public places (airports, railway stations, stadiums, etc.) are assessed. The choice of the above frequencies is a compromise between the requirements of low absorption of THz radiation by water vapor in air, good penetration through the fabric of clothing, favoring a sufficient resolution of the imaging system, and an abundance of corresponding longitudinal phonons, capable of exciting Fermi electrons in GNBs. Estimates of the characteristics of the terahertz-to-infrared converter based on gold nanospheres (GNSs), which could work in tandem with these sources of THz radiation -- as a means of visualization of hidden objects -- are also given.

Abstract-Prospects for terahertz imaging the human skin cancer with the help of gold-nanoparticles-based terahertz-to-infrared converter

A. V. Postnikov, K. A. Moldosanov, N. J. Kairyev, V. M. Lelevkin

https://arxiv.org/abs/1901.02346

The design is suggested, and possible operation parameters are discussed, of an instrument to inspect a skin cancer tumour in the terahertz (THz) range, transferring the image into the infrared (IR) and making it visible with the help of standard IR camera. The central element of the device is the THz-to-IR converter, a Teflon or silicon film matrix with embedded 8.5 nm diameter gold nanoparticles. The use of external THz source for irradiating the biological tissue sample is presumed. The converter's temporal characteristics enable its performance in a real-time scale. The details of design suited for the operation in transmission mode (in vitro) or on the human skin in reflection mode {in vivo) are specified.

Abstract- On the plausible nature of the size effect in heterogeneous catalysis on gold nanoparticles

Kamil Moldosanov, Andrei Postnikovy

https://arxiv.org/abs/1808.10607

We suggest that the size effect in heterogeneous catalysis on gold nanoparticles (GNP) of ~1.3 - 9 nm size can be related to spontaneous terahertz (THz) emission of GNPs in the process of channeling the energy of longitudinal phonons therein into THz electromagnetic radiation. Since rotational and vibrational energy levels of molecules adsorbed on the GNP surface are in the THz range, the irradiation by self-induced THz photons would engage the molecules' swing and twisting, which in its turn would weaken or break the intramolecular bonds, facilitating their entering a chemical reaction on the GNP surface.

Abstract-Suggested design of gold-nanoobjects-based terahertz radiation source for biomedical research

Andrei Postnikov, Kamil Moldosanov, 

http://iopscience.iop.org/article/10.1088/1361-6528/aabf10/pdf

Gold nanoparticles (GNPs) may serve as "devices" to emit electromagnetic radiation in the terahertz (THz) range, whereby the energy is delivered by radio frequency or microwave photons which won't by themselves induce transitions between sparse confinement-shaped electron levels of a GNP, but may borrow the energy from longitudinal acoustic phonons to overcome the confinement gap. Upon excitation, the Fermi electron cannot relax otherwise than via emitting a THz photon, the other relaxation channels being blocked by force of shape and size considerations. Within this general scope that has been already outlined earlier, the present work specifically discusses two-phonon processes, namely (i) a combined absorption-emission of two phonons from the top of the longitudinal acoustic branch, and (ii) an absorption of two such phonons with nearly identical wavevectors. The case (i) may serve as a source of "soft" THz radiation (at 0.54 THz), the case (ii) the "hard" THz radia- tion at 8.7 THz. Numerical estimates are done for crystalline particles in the shape of rhombicuboctahedra, of 5 – 7 nm "diameter". A technical realisation of this idea is briefly discussed, assuming the deposition of GNPs onto / within the substrate of Teflon, the material sustaining high temperatures and transparent in the THz range.

Abstract-Terahertz conductivity engineering in surface decorated carbon nanotube films by gold nanoparticles

Debanjan Polley, Animesh Patra, Anjan Barman, and Rajib Kumar Mitra

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-56-4-1107

We report the controllable conductivity of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes with their surface walls decorated by gold nanoparticles (Au NPs) with varying concentration in terahertz (THz) frequency range. Colloidal Au NPs of nominal diameter ∼15  nm$\sim 15\mathrm{nm}$ are synthesized by the reduction of gold chloride solution using tri-sodium citrate. A simple chemical route is followed to attach Au NPs on the surfaces of both types of carbon nanotubes (CNTs). The attachment of Au NPs on the sidewalls of CNTs is confirmed by UV-visible spectroscopy and scanning electron microscope images. THz spectroscopic measurements are carried out at room temperature in transmission geometry in the frequency range of 0.3–2.0 THz. It is found that the THz conductivity of the surface decorated SWNT composites can either be increased or decreased by ±15%$\pm 15\%$ than that of the as-prepared SWNT composites by carefully choosing the Au NP concentration. The conductivity variation is qualitatively explained in terms of carrier trapping potential for low Au NP density, and alternative carrier conduction pathways at higher Au NP density and analyzed with the help of a modified universal dielectric relaxation model.

© 2017 Optical Society of America

Full Article  |  PDF Article

Researchers develop technique for making light-bending 'raspberry-like metamolecules'

These raspberry-like metamolecules react to light’s magnetic field as a loop of wire does to an oscillating magnet.

 http://phys.org/news/2015-03-technique-light-bending-raspberry-like-metamolecules.html#jCp

by Evan Lerner

The field of metamaterials is all about making structures that have physical properties that aren't found in nature. Predicting what kinds of structures would have those traits is one challenge; physically fabricating them is quite another, as they often require precise arrangement of constituent materials on the smallest scales.

Researchers at the University of Pennsylvania have now devised a way of mass-producing metamaterials that exhibit magnetic resonancein optical frequencies. Called "raspberry-like metamolecules" due to their unique shape, these nanoscale structures could be used as building blocks for metamaterials that could scatter light as if they had magnetic properties, which could be relevant to applications in optical processing and signal handling. These raspberry-like metamolecules react to light's magnetic field as a loop of wire does to an oscillating magnet.

This ability stems from the precise arrangements of the raspberry-like metamolecule's "drupelets," which are composed ofgold nanoparticles. These drupelets need to be as close as possible without touching so as not to "short circuit" the optical electric fields around them. Through a carefully designed chemical process that coated each drupelet with an insulating surfactant, the Penn team was able to space these nanoparticles an average distance of just two nanometers apart.

And because the assembly of the nanoparticle drupelets and the surfactant coating can be done in a single step, vast quantities of these raspberry-like metamolecules can be fabricated at once, rather than being painstakingly assembled one at a time.

The research was conducted by lead author Zhaoxia Qian, who recently graduated with a doctorate in chemistry from Penn's School of Arts & Sciences; Nader Engheta, the H. Nedwill Ramsey Professor of Electrical and Systems Engineering in Penn's School of Engineering and Applied Science; Zahra Fakhraai, assistant professor of chemistry in Penn Arts & Sciences; and So-Jung Park, formerly associate professor of the Department of Chemistry, now professor of chemistry at South Korea's Ewha Womans University. Also contributing were Simon Hastings, who recently graduated with a doctorate in physics, and chemistry graduate student Chen Li, along with research specialist Brian Edwards and visiting undergraduate student Christine K. McGinn, both of Electrical and Systems Engineering

It was published in the journal ACS Nano.

If one takes a loop of wire and passes a magnet up and down through the center, the resulting oscillating magnetic field drives electrons around the wire, producing electrical current in the wire. That principle is in play in every generator, which has magnets that oscillate at around 50 hertz, or 50 times a second. But what if this principle could be extended into optical frequencies, on the order of 500 terahertz? Rather than generating electricity, the loop would be able to manipulate visible light.

"There are no known materials that have magnetic properties in optical frequencies," Fahkraai said. "If you could fabricate structures like this, they could be building blocks for metamaterials that could scatter light as if they had magnetic properties."

Engheta predicted that such a structure was possible in 2006, and in the intervening years other research groups have physically produced metamaterials that exhibit this trait. Such structures were mostly painstakingly constructed rings of metal nanoparticles, spaced on a flat surface such that electrons couldn't actually move between them.

"Because the metal doesn't touch," Engheta said, "the electrons can only oscillate within individual particles and can't move from one nanoparticle to its neighbor. This is known as a displacement current. It's like doing the wave in a stadium; no one fan is moving from their seat, but the wave moves around in a circle."

A raspberry-like configuration, where nanoparticles are spherically clustered around a core, rather than a ring, would be even better, as a cross-section of the raspberry acts like a ring of nanoparticles no matter which direction the magnetic field is applied. Other researchers have begun moving from mechanical assembly techniques toward the chemical self-assembly of such structures but have hit roadblocks.

The Penn team's approach solves the problems by adopting a synthetic approach.

"People have tried to make these kinds of structures in solution before, typically by assembling pre-synthesized nanoparticles," Qian said, "but it is hard to achieve high density of nanoparticles packing through that route."

"In our case," Park said, "we generate closely packed nanoparticle clusters by a synthetic approach where the nanoparticle growth and assembly occurs simultaneously. A challenge in such synthetic approach is that growing nanoparticles tend to form a fused shell. In our method, we use a special surfactant that forms a molecularly-thin, but tightly protecting, layer around the nanoparticles, which keeps them from touching each other."

The Penn team's synthetic method reduces some of the complexity that otherwise comes with making these raspberry-like metamolecules.

"It's like making a stew," Engheta said. "You throw everything into one pot."

The ingredients to the stew are polystyrene spheres decorated with small silver seed particles, silver nitrate, gold salts and reducing agents that break up those salts and allow the gold atoms to form nanoparticles. All of these ingredients are placed into a growth formula containing the insulating surfactant, which forms a thin layer on the exterior of the growing gold nanoparticles, cushioning them from each other.

Further research on the surfactant chemistry will enable the team to reduce the distance between the nanoparticles even more, to further strengthen the magnetic properties of the raspberry-like metamolecules. That trait is critical to the structures' abilities to manipulate light and thus be used in optical devices.

"If you want to make inductors at optical frequencies," Fahkraai said, "you need something that can respond at very high frequencies. The closer we can make the nanoparticles, the stronger we can make the scattering of light due to magnetic effects."

 Explore further: Researchers find easy way to deposit metal nanoparticles on a surface using tape

More information: ACS Nano, dx.doi.org/10.1021/nn5050678

Journal reference: ACS Nano  

Provided by University of Pennsylvania  

Terahertz Spectroscopy Promises Better Diagnosis, Safer Drugs

Marie Freebody, Contributing Editor, marie.freebody@photonics.com

With terahertz imaging systems getting smaller and cheaper – and performing better – applications are stacking up in cancer imaging as well as drug detection and development.

http://www.photonics.com/Article.aspx?AID=54396
Terahertz spectroscopy is a fast-growing area of research with some hugely promising applications, now that cumbersome, expensive and hard-to-use systems are in the past. Its low-photon-energy radiation makes it safe for tissue imaging, and its high absorption in water, which is often a key indicator of the presence of tumor cells, makes it a hopeful weapon in fighting cancer.

Revealing tumors in more detail

Cancer is the second most common cause of death in the US, second only to heart disease, and accounts for nearly 1 in every 4 deaths, according to the American Cancer Society. Early detection could mean the difference between life and death for many patients, but there is a shortfall in the current imaging techniques.

X-ray imaging and MRI provide images of living tissues at the macroscopic level, but with low resolution and specificity. Microscopic imaging can be used on biopsy samples to provide structural and functional information, but there is not yet a technique that can noninvasively image tissue at a high resolution – effectively combining both macroscopic and microscopic imaging.

New terahertz spectroscopic studies of cancer could potentially fill this niche. Since terahertz frequencies are readily absorbed by water and other polar liquids, the method lends itself well to imaging most organic tissue.

Although the high absorption loss limits the depth of imaging, it does promote extreme contrast between substances with lesser or higher degrees of water content, which helps to show distinctive contrast in

 medical imaging.

“High absorption of water and other polar liquids in terahertz frequency range limits the sensitivity of terahertz imaging in water-rich samples,” said Dr. Trung Quan Luong of the Applied Competence Cluster Terahertz at Ruhr University Bochum in Bochum, Germany. “The focus of current biomedical application is on skin imaging. Further improvements require high-power terahertz sources.”


(Left) Outside and inside a package of omeprazole. (Right) Terahertz image of the packaged omeprazole showing that the terahertz beam can penetrate the paper and plastic packages to observe the inner drug content. Courtesy of Applied Competence Cluster Terahertz, Ruhr University Bochum.

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In a bid to further improve image contrast, Korean scientists introduced gold nanoparticles to label cancer cells (Optics Express 2009;17:3469-75). In a paper reviewing various forays into terahertz imaging, published in March 2012, professor Emma MacPherson at the Chinese University of Hong Kong and colleagues explain the innovative approach in more detail.
Gold nanorods (GNR) are engineered to be readily absorbed by cancer cells and injected into the specimen. At first, terahertz imaging reveals nothing, but once an IR laser is used to heat the nanoparticles, a notable increase in reflected terahertz signal is detected. Interestingly, since the positioning of IR light illumination can be controlled with micron resolution, the image resolution of the terahertz-differential image measurement can also be on the order of microns – much higher than with conventional terahertz imaging.

What’s more, with high enough concentrations of GNRs and intense exposure of near-IR light, cells can be destroyed. This means that if the GNRs can target the cancer cells with high enough specificity, the technique also has potential for hyperthermia therapy for cancer.

But MacPherson is quick to point out that further research into the safety of the technique needs to be fully investigated first.

“In terms of developing terahertz imaging/spectroscopy into a clinical technique for cancer diagnosis, there is still a long way to go,” MacPherson said. “More studies and clinical trials are needed to determine whether terahertz imaging has significant advantages over existing techniques.”

At National Taiwan University, Dr. Chi-Kuang Sun and colleagues demonstrated in vivo breast cancer detection in a mouse model for the first time. The group used a terahertz-fiber-scanning transmission imaging system to continuously monitor the growth of human breast cancer in mice. Cancer cells were first implanted into the mice; then continuous screening distinguished cancer development from the surrounding fatty tissue.

“Early cancer development was detected on the twenty-third day, and the induced terahertz absorption coefficient change was 0.25 mm⁻¹, corresponding to a cancer volume of 1.3 mm³,” Sun said. “This change is mainly due to the change in the water content in these tissues.”

While this study indicates the potential for noninvasive early cancer detection with high sensitivity and without the need for labeling, it does not prove that this imaging system can be directly applied to humans in vivo.

The problem is enhancing the image contrast of cancerous cells buried within fibrous breast tissue rather than fatty breast tissue. In addition, gland tissue – which also has high water content – shows up with similar contrast to cancerous tissue under terahertz illumination. Since breast tissue can be fatty and originates from glandular tissue, the potential of this technology could be seriously limited for human screening.

One possible solution is to perform time-lapsed imaging to provide a base from which cancerous tissue can be distinguished from other gland tissues, but Sun admits that further work still needs to be done.

Although progress is being made in many areas, the competition from more developed imaging modalities is fierce. Ultrasound, OCT, near-IR and Raman spectroscopy, MRI, positron emission tomography, in situ
confocal microscopy, and x-ray techniques have all received much more attention and currently offer enhanced resolution, greater penetration, higher acquisition speeds and specifically targeted contrast mechanisms.

But terahertz imaging is catching up fast and offers some elements that existing techniques do not. For example, no technique can readily distinguish benign from malignant lesions macroscopically at the surface or subdermally. Furthermore, the sensitivity of terahertz signals to skin moisture (which is often a key indicator) is very high, and competing techniques such as high-resolution MRI are less convenient and more costly.

Drug detection, development

Today, more terahertz systems are commercially available than ever before – and with more competition come many improvements.

“Advances in recent years have been on the source and detector side,” said Dr. David Armstrong, marketing director at M Squared Lasers in Glasgow, Scotland. “At M Squared, we have developed a turnkey tunable terahertz source, the Firefly-THz, using licensed technology developed at [the University of] St. Andrews [in Scotland] – namely, a pulse OPO [optical parametric oscillator] approach that yields high peak power and narrow linewidth, suitable for standoff imaging and resolving spectral components.”

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M Squared Lasers’ Firefly-THz is a hands-free, pulsed nanosecond terahertz laser source that provides a tunable narrow-linewidth output, thanks to its novel optical parametric oscillator design. Courtesy of M Squared Lasers.

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Building on 15 years of experience in developing parametric generators for the UV-VIS-IR spectral ranges, researchers at St. Andrews have developed a compact, efficient and transportable parametric generator of terahertz radiation.

The system has the potential to identify

The system has the potential to identify and characterize illegal drugs that are concealed about a person, sent
through the mail or on a contaminated surface. Unlike x-ray scanners, canine detection or external swiping,
terahertz waves can noninvasively penetrate a wide range of materials such as paper, wood, plastic, fabrics, ceramics, bone and tissue.
Not only can terahertz imaging detect a concealed drug, but it can also provide a spectral fingerprint, making drug identification possible.

Advances in 3-D terahertz imaging are opening up new avenues in pharmaceutical applications, where it can improve the quality and uniformity of pharmaceutical products. Many pharmaceutical materials can exist in multiple solid forms, known as polymorphic forms: They have the same chemical composition but different crystalline structures and as such behave differently in terms of solubility and stability.

Terahertz spectroscopy can pinpoint at which step in the manufacturing process the polymorphic transitions are emerging, giving pharmaceutical makers a better understanding of the causes of changes in structure.

But it’s not just what’s on the inside that matters; the tablet coating is also a crucial component that impacts how the drug interacts with our body. For oral drugs, the coating protects the active ingredients from being released before the tablet reaches the small intestine, where they are absorbed.

Coatings can also prolong a tablet’s shelf life by protecting the components from moisture and oxygen degradation. Therefore, characterizing the tablet coating and ensuring uniformity within a single tablet or an entire batch is important.

Here, terahertz pulse imaging measures the thickness of coatings by a change in refractive index of reflected light. Although the technique provides results similar to ultrasound measurements, there remains one common practice in the pharmaceutical industry in which terahertz triumphs.

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The interaction of a terahertz pulse with a protein and its hydration shell. Courtesy of Trung Quan Luong, Konrad Meister and Martina Havenith, Ruhr University Bochum.

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Terahertz data compiled from measuring the coating thickness has been found to strongly correlate with the dissolution profile of the tablet. Determining how a drug dissolves is currently performed using a dissolution apparatus, often requiring lengthy time analysis. Terahertz, on the other hand, offers a promising way to reduce the cost and time involved.

In terms of pharmaceutical applications, extending the spectral range of current instruments from 100 to 300 cm⁻¹ would allow coverage of both the intermolecular and some of the intramolecular vibration modes of pharmaceuticals. This would not only provide better spectral specificity for chemical discrimination and mapping, but would also help to characterize thinner coatings with higher spatial resolution.
References

1. C. Yu et al (March 2012). The potential of terahertz imaging for cancer diagnosis: A review of investigations to date. Quant Imaging Med Surg, Vol. 2, pp. 33-45.

2. American Cancer Society Cancer Facts & Figures 2011. American Cancer Society, Atlanta. Available online at http://www.cancer.org/Research/CancerFactsFigures/CancerFactsFigures/cancer-facts-figures-2011.

3. L.M. Ingle et al (2013). Terahertz spectroscopy for pharmaceutical applications. Int J Pharm Pharm Sci Res, Vol. 3, pp. 48-52.