Terahertz imaging spots microscopic twists in tissues

Spinning THz light: see the twists

University of Michigan project provides structural information without harmful X-rays.

http://optics.org/news/10/7/6

Researchers at the University of Michigan have detected microscopic twists in the internal structure of plant and animal tissue without using potentially harmful X-rays.

The approach involves rotating terahertz radiation in real time, and is said to be the first successful implementation of this approach, potentially opening up new applications in medical imaging, encrypted communications and cosmology. The work was reported in Nature Materials.

"Our bodies have a lot of twisted structures that are close enough to the surface for terahertz photons to penetrate: vessels, ligaments, muscle fibers, molecules and even some helical bacteria," said Nicholas Kotov of the University of Michigan.

Terahertz radiation can penetrate about six millimeters into the body, but unlike X-rays is non-ionizing, removing the risk of it creating damaging electrical charges in the body.

The project's breakthrough involves terahertz circular dichroism (TCD) spectroscopy, the differential absorption of left- and right-handed circular polarized light, which according to the project was impractical in the terahertz range until now. A plastic ribbon printed with a gold herringbone pattern and sliced with staggered rows of tiny cuts was designed, taking inspiration from the Japanese art of kirigami which uses arrangements of cuts to create 3D structures from paper.

Each cut measured 500 microns long and 5 microns in height, and the cuts were spaced with a horizontal period of 600 microns and a vertical period of 105 microns. When the ribbon is stretched, the cuts open up and the slices of ribbon twist. The gold lines then guide the radiation, twisting it in turn.

"The lack of dynamic polarization modulators comparable to polarization optics used for other parts of the electromagnetic spectrum is impeding the proliferation of TCD spectroscopy," commented the team in its published paper. "We show that tunable optical elements fabricated from patterned plasmonic sheets with periodic kirigami cuts make possible the polarization modulation of terahertz radiation under application of mechanical strain."

Encrypted communications

According to the paper, a herringbone pattern of microscale metal stripes enables a dynamic range of polarization rotation modulation exceeding 80 degrees over thousands of cycles. Following out-of-plane buckling, the plasmonic stripes function as reconfigurable semi-helices of variable pitch aligned along the terahertz propagation direction.

In trials on materials such as the forewing of a scarab beetle, distinct TCD fingerprints associated with the helical substructure in the biocomposite could be detected. Analogous kirigami modulators could also enable other applications in terahertz optics, such as polarization-based terahertz imaging, line-of-sight telecommunication, information encryption and space exploration, according to the project team.

The team proposes the same design could be scaled for other types of radiation as well, with larger patterns interacting with microwaves or radio waves, or shrinking the pattern down to manipulate infrared light.

In addition to imaging living tissues, terahertz circular dichroism spectroscopy could also aid the development of new medicines based on large biological molecules such as proteins and antibodies. The project team anticipates that an early application could be to encrypt and decrypt communications on the terahertz spectrum, along with use of such kirigami devices on satellites to measure the twist in the terahertz spectrum of the universe's background radiation, providing valuable information about the earliest stars.

Focus on: The Kotov Lab, University of Michigan

My Note: I just saw an interesting title to an abstract "Chiroptical Kirigami Modulators for Terahertz Circular Dichroism Spectroscopy of Biomaterials" which lead to me this page. So many interesting things are being done in areas I have never heard about before. 

http://www.umkotov.com/

Welcome to Biomimetic Nanostructures!

The “building blocks” of living organisms are nanoscale in dimension. Capable of spontaneous self-assembly, they form complex biological machinery with exceptional energy efficiencies of 89-95%. These nanoscale biological components also assemble into biocomposites with often-astounding combinations of properties – strength, density, transparency, ion conductivity and others. One can poses a fundamental question: can such machinery and materials can be reproduced using abiotic, inorganic building blocks that are also capable of self-assembly? The current body of knowledge accumulated for biomimetic nanostructures indicate that the answer is a definite “yes” for some and definite “no” for some others. We have dedicated our research efforts to understanding where the dividing line between these answers is.

Inorganic nanoparticles can produce multicomponent assemblies with sophisticated geometries, for instance left- and right-handed helices or spiky mesoscale hedgehog particles. These non-biological structures can have surprising similarities with biological nanostructures despite their vastly different ingredients (i.e. inorganic nanoparticles vs. biomacromolecules). These similarities arise because the forces that govern the solution dynamics of these two very different classes of nanoscale structures have a lot in common. While the accurate description of forces between nanoscale structures is an ongoing scientific challenge, the experimental and technological conditions needed to utilize the convergent technologies involving inorganic nanostructures is remarkably simple once the fundamental parallels in their interactions are recognized.

The rapid and controllable assembly of inorganic nanostructures can be realized because the attractive interactions between inorganic nanostructures are typically stronger than those between biological building blocks. The change in entropy and enthalpy for association of inorganic nanoparticles with each other are favorable. The complex interdependence of the forces between nanoscale structures, which determine their mutual organization and self-assembly patterns, can be simplified when nanoparticles with high anisotropy, and especially nanoplatelets, are employed. Materials designed in such a way form a large class of biomimetic layered nanocomposites, and examples of their technological implementations are abundant.

Replication of the brick-and-mortar structure of tough iridescent seashells has led to a family of biomimetic composites made from graphene, clay, cellulose, and other components. These composites have revealed previously unattainable combinations of useful properties, including mechanical robustness and rapid ion transport. High electrical conductivity and spectrally tunable optical absorption have became possible due to the translation of biological patterns to abiotic semiconductor and metallic nanocomponents. Such materials have in turn engendered the construction of a large family of energy storage and biomedical devices. They also serve as membrane, load-bearing, and tissue-mimicking components in electronic devices.

Our current studies are aimed at the further development and generalization of the toolbox of experimental, theoretical, and computational techniques for the engineering of biomimetic nanostructures. Current topics in this area include chiral inorganic nanomaterials, biosimilar inorganic organelles, and pollen-like hedgehog particles, among others. The motivations behind this research abound, including the ongoing quest for ultrastrong multifunctional composites, materials for energy storage, and biomedical implants. The new engineering fields emerging from these topics include the convergent nanosystems for catalysis of ‘hard’ reactions, safe and effective antimicrobial agents, and machine vision.

With an appreciation of the technical challenges inherent in these problems, we have forged collaborations with colleagues around the globe. We value creativity, integrity, and tenacity in every person with whom we work.

Nicholas A. Kotov