Light-powered 3-D printer creates terahertz lens

http://www.nanowerk.com/news2/gadget/newsid=43278.php

(Nanowerk News) From visible light to radio waves, most people are familiar with the different sections of the electromagnetic spectrum. But one wavelength is often forgotten, little understood, and, until recently, rarely studied. It's called terahertz, and it has important applications in imaging and communications."Terahertz is somewhat of a gap between microwaves and infrared," said Northwestern University's Cheng Sun. "People are trying to fill in this gap because this spectrum carries a lot of information."Sun and his team have used metamaterials and 3-D printing to develop a novel lens that works with terahertz frequencies. Not only does it have better imaging capabilities than common lenses, but it opens the door for more advances in the mysterious realm of the terahertz.Supported by the National Science Foundation, the work was published online on April 22 in the journal Advanced Optical Materials ("Additive Manufacturing of a 3D Terahertz GradieRefractive Index Lens")GradieRefractive Index Lens").

                                            The design of Sun's lens with gradient refractive index.

"Typical lenses -- even fancy ones -- have many, many components to counter their intrinsic imperfections," said Sun, associate professor of mechanical engineering at Northwestern's McCormick School of Engineering. "Sometimes modern imaging systems stack several lenses to deliver optimal imaging performance, but this is very expensive and complex."The focal length of a lens is determined by its curvature and refractive index, which shapes the light as it enters. Without components to counter imperfections, resulting images can be fuzzy or blurred. Sun's lens, on the other hand, employs a gradient index, which is a refractive index that changes over space to create flawless images without requiring additional corrective components.There are two major factors that made this new lens possible. First, it is made from a novel metamaterial that exhibits properties not readily available in nature. "Such properties originate from its tiny structures that are much smaller than the terahertz wavelength," said Fan Zhou, the paper's first author and member of Sun's laboratory. "By assembling these tiny structures, we can create specific refractive index distribution."Second, the lens was manufactured with a 3-D printing technique called projection micro-stereo-lithography. The technique enables a scalable, rapid, and inexpensive way to produce the tiny features that are needed for the lens to operate at the terahertz frequency band. The printing technology allowed the researchers to fabricate the metamaterial to precisely fit their designs.

"For printing, we use a photo-polymer in liquid form," Sun said. "When we shine a light on the material, it converts it into a solid. The material forms to the shape of the light, allowing us to create a 3-D structure. You cannot accomplish a gradient index with traditional manufacturing processes."The lens could make terahertz imaging, which is particularly useful for security, cheaper, higher resolution, and more available. While X-rays can detect metal, they cannot detect plastic or chemicals. Terahertz scanners, however, can detect both of items to discover concealed weapons, biological weapons such as anthrax, and plastic explosives. And unlike X-rays, terahertz radiation is completely harmless to humans."This advance means we can unveil previously inaccessible information of some opaque materials in high resolution," said Wei Cao, Sun's collaborator at Oklahoma State University. "This opens up an entirely new technique for a massive range of potential uses from biomedical research to security."

Three-dimensional invisibility cloaks functioning at terahertz frequencies

Wei Cao, Fan Zhou, Cheng Sun, Jianqiang Gu, Dachuan Liang, Jiaguang Han, Shuang Zhang and 

Weili Zhang

http://spie.org/x108531.xml

Both the geometric and spectroscopic signatures of an object were completely concealed under 3D terahertz invisibility cloaks made of either homogeneous or inhomogeneous media.

3 June 2014, SPIE Newsroom. DOI: 10.1117/2.1201405.005440

Achieving invisibility cloaking in the terahertz regime has recently garnered a great deal of attention due to unique and promising emerging applications of terahertz technology. Particularly with recent advances in terahertz communications and radar, there has been an increasing demand for cloaking devices functioning at terahertz frequencies. The ultimate goal is to conceal a large object from being observed by terahertz radar in civilian or space communications.

So far, terahertz invisibility cloaks have been experimentally demonstrated in quasi-3D geometry based on both subwavelength building blocks and homogeneous uniaxial crystals.1, 2 These quasi-3D cloaks are also called ground plane or carpet cloaks, and they were proposed to overcome the difficulties of complete cloaks that require dielectric singularity, have high loss, and very narrow bandwidth.3, 4 One of the challenges that remains at terahertz frequencies is finding photo-curable dielectric materials with lower loss that allow for a broader response bandwidth. Indeed, the complete cloak that features 3D and broad bandwidth is the major challenge for the entire electromagnetic spectrum. Here we present experimental demonstrations of quasi-3D terahertz cloaks made from either 3D inhomogeneous metamaterials 1 or homogeneous 2 media.

The inhomogeneous cloak made from dielectric metamaterials was lithographically fabricated using a scalable Projection Microstereolithography (PμSL) process (see Figure 1). The triangular cloaking structure has a total thickness of 4.4mm, comprised of 220 layers of 20μm thickness. The distribution of the varying hole geometry can be clearly identified in the scanning electron microscope (SEM) images. The space underneath the bump is designated as the cloaked region. The cloak operates at a broad frequency range between 0.3 and 0.6THz, and is placed over an α-lactose monohydrate absorber with rectangular shape. The α-lactose monohydrate exhibits a resonant attenuation signature at 0.53THz due to the presence of collective vibrational transition modes. We measured the reflected terahertz wave in four cases: (I) a flat reflective surface, (II) exposed lactose on a reflective surface, (III) a control structure with a reflective bump placed on top of the lactose, and (IV) the cloaking structure placed on top of the lactose. The measurements were carried out in an angular-resolved reflection terahertz time domain spectroscopy (THz-TDS) system. The terahertz cloak in case (IV) conceals both the geometric and spectroscopic features of the lactose, which then closely resembles case (I), demonstrating the successful design of the cloak.

 

Figure 1. Inhomogeneous quasi-3D terahertz cloak. (a) Schematic diagram illustrating the projection micro-stereolithography system fabricating a 3D metamaterial cloaking device. The grayscale of individual pixels within each 85.2×85.2μm unit cell can be adjusted so the holes can be fabricated with sub-pixel precision. (b) Optical and scanning electron microscope images of the fabricated cloaking device. The gradual change in hole size near the bump can be clearly observed.1

We also demonstrated a large-scale terahertz invisibility cloak made from birefringent crystalline sapphire (see Figure 2). This homogeneous cloaking device features a large concealed volume, low loss, and broad bandwidth. In particular, it is capable of hiding objects with a dimension nearly an order of magnitude larger than that of its lithographic counterpart, but without involving complex and time-consuming cleanroom processing. The cloak was made from two 20mm thick high-purity sapphire prisms. The area beneath the cloak is 1.75mm tall, nearly ten times taller than the inhomogeneous counterpart. In addition, the useful bandwidth increased from 0.3-0.6THz to 0.2-1.0THz due to different design approaches and material absorption properties. The volume of the cloaking region is approximately 5% of the whole sample.

 

Figure 2. Measured cloaking effect of a homogeneous terahertz cloak with respect to the relative positions (x-axis) and the frequency (y-axis). The color represents the relative spectral amplitude of (a) cloaking, (b) flat surface reflection, and (c) reference with the same cloak lens. A schematic of the cloak design is shown in (d).2

We characterized the homogeneous cloak using the same terahertz spectroscopy system, except that no test sample was placed in the cloak region. Instead, we used the uncloaked transverse electric polarized beam profile as the reference, which exhibits significant beam splitting: see Figure2(c). The THz-TDS results indicated that the terahertz invisibility cloak successfully concealed both the geometric and spectroscopic signatures of the object, making it undetectable to the observer. As shown in Figure 2, the reflected transverse magnetic beam from the cloak (a) shows nearly the same profile as that reflected by a flat mirror (b). On the other hand, for the transverse electric beam, the detector received two largely separated beam profiles at the left and right side of the cloaking profile, shown in (c).

This straightforward cloaking approach greatly reduces the complexity in design and fabrication of the metamaterial-based cloaks. More importantly, the initial work in the visible regime confirmed that this is a promising way toward practical macroscopic cloaking devices with frequency and incidence angle robustness.

In conclusion, we demonstrated that quasi-3D invisibility cloaks in the terahertz regime, comprised of either homogeneous or inhomogeneous media, can completely conceal both the geometrical and spectroscopic signatures of a rectangular absorber placed beneath them. Our next steps in this direction will explore innovative approaches toward a complete 3D cloak for terahertz wavelengths. The keys will be the transformation optics design, unique metamaterial building blocks, and novel fabrication processing.

The authors thank Y. Bao, C. T. Stuart, and Y. Yang for outstanding contributions in this work. We acknowledge financial support from the U.S. National Science Foundation and the China National Natural Science Foundation.

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Wei Cao

Oklahoma State University

Stillwater, OK

Fan Zhou, Cheng Sun

Northwestern University

Evanston, IL

Jianqiang Gu, Dachuan Liang, Jiaguang Han

Tianjin University

Tianjin, China

Shuang Zhang

University of Birmingham

Birmingham, United Kingdom

Weili Zhang
Oklahoma State University
Stillwater, OK
and
Tianjin University
Tianjin, China

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References:

1. F. Zhou, Y. J. Bao, W. Cao, C. T. Stuart, J. Gu, W. Zhang, C. Sun, Hiding a realistic object using a broadband terahertz invisibility cloak, Sci. Rep. 1, p. 78, 2011.

2. D. Liang, J. Gu, J. Han, Y. Yang, S. Zhang, W. Zhang, Robust large dimension terahertz cloaking, Adv. Mater. 24, p. 916, 2012.

3. J. S. Li, J. B. Pendry, Hiding under the carpet: a new strategy for cloaking, Phys. Rev. Lett. 101, p. 203901, 2008. doi:10.1103/PhysRevLett.101.203901

4. D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, J. B. Starr, D. R. Smith, Metamaterial electromagnetic cloak at microwave frequencies, Science 314, p. 977, 2006.

Hiding objects with a terahertz invisibility cloak

MY NOTE: THIS STORY IS OLD NEWS, ON THIS BLOG, BUT IS A NEW "TAKE" ON THE INTERESTING STORY.

http://www.physorg.com/news/2011-09-terahertz-invisibility-cloak.html

Though this design can't translate into an invisibility cloak for the visible spectrum, it could have implications in diagnostics, security, and communication.

The cloak, designed by Cheng Sun, assistant professor of mechanical engineering at Northwestern's McCormick School of Engineering and Applied Science, uses microfabricated gradient-index materials to manipulate the reflection and refraction of light.

Sun's research was published Sept. 1 in Scientific Reports, a new online, open-source journal that provides rapid publication and high visibility of research for all areas of science.

Humans generally recognize objects through two features: their shape and color. To render an object invisible, one must be able to manipulate light so that it will neither scatter at an object's surface nor be absorbed or reflected by it (the process which gives objects color).

In order to manipulate light in the terahertz frequency, which lies between infrared and microwaves, Sun and his group developed metamaterials: materials that are designed at the atomic level. Sun's tiny, prism-shaped cloaking structure, less than 10 millimeters long, was created using a technique called electronic transfer microstereolithography, where researchers use a data projector to project an image on a liquid polymer, then use light to transform the liquid layer into a thin solid layer. Each of the prism's 220 layers has tiny holes that are much smaller than terahertz wavelengths, which means they can vary the refraction index of the light and render invisible anything located beneath a bump on the prism's bottom surface; the light then appears to be reflected by a flat surface.

Sun says the purpose of the cloak is not to hide items but to get a better understanding of how to design materials that can manipulate light propagation.

"This demonstrates that we have the freedom to design materials that can change the refraction index," Sun said. "By doing this we can manipulate light propagationmuch more effectively."

The terahertz range has been historically ignored because the frequency is too high for electronics. But many organic compounds have a resonant frequency at the terahertz level, which means they could potentially be identified using a terahertz scanner. Sun's research into terahertz optics could have implications in biomedical research (safer detection of certain kinds of cancers) and security (using terahertz scanners at airports).

Next Sun hopes to use what he's learned through the cloak to create its opposite: a terahertz lens. He has no immediate plans to extend his invisibility cloak to visible frequencies.

"That is still far away," he said. "We're focusing on one frequency range, and such a cloak would have to work across the entire spectrum."

Provided by Northwestern University (news : web)

Latest news from CLEO 2011 on terahertz “invisibility cloak"

Perspective view of the terahertz cloaking structure made by researchers from Northwestern University and Oklahoma State University.

Patricia Daukantas

http://www.osa-opn.org/OpenContent/NewsRoom/Broadband-Terahertz-Cloaking-at-CLEO2011.aspx
Scientists from two U.S. universities have created a three-dimensional terahertz-spectrum “invisibility cloak” that hides a bump from a wide range of radiation frequencies.
The six-member group, led by Cheng Sun of Northwestern University, reported its results at the CLEO:2011 conference in Baltimore, Md. (U.S.A.). Three of the researchers are from Oklahoma State University.
The terahertz cloak is reminiscent of the “carpet bump” cloak devised in the near-infrared region by a German team (see OPN, June 2010, p. 7). From a distance, the device looks almost like a polymer resin near-cube cut in half diagonally.
Sun and his colleagues used a technique called “projection microstereolithography” to fabricate a varying array of holes through the resin block. The size of the holes changes around the negative “bump” on the bottom side of the block, so that the refractive index of the cloak increases in the vicinity of the bump.
The researchers coated the side containing the bump with a 200-nm-thick layer of gold to provide a reflecting surface. Then they shone broadband terahertz radiation into the block at an incident angle of 45 degrees with respect to the reflecting surface. Underneath the bump, they placed a block of pressed lactose powder, which has a strong absorption feature at 0.53 THz. They performed similar tests with a flat reflecting surface, a chunk of lactose covered by a non-cloaking reflecting surface and a bare chunk of lactose.
Imaging of the reflected terahertz light showed that the cloaked block of lactose showed the same spectroscopic signature as the flat reflecting surface—no scattering or absorption of the rays. According to the researchers, the result demonstrates that the cloak has hidden both the geometrical and spectroscopic signatures of the concealed material.
Previous cloaking studies have focused on the microwave and optical regions of the spectrum, leaving a gap at terahertz frequencies—a spectroscopically interesting band that researchers have been studying intensely for potential security and industrial applications.