http://nanotechweb.org/cws/article/lab/54224

Terahertz emitters adapted for excitation at telecoms wavelengths (1.55 µm)

Terahertz (THz) time-domain spectroscopy (TDS) is a common technique for the investigation of many material systems. Typically, one of the most expensive parts of THz TDS systems is the laser providing ultrashort pulses, but new materials could allow cheaper lasers to be used and make systems more affordable.

Active region and transient profile of emitted THz pulse

In TDS systems, THz pulses are produced by generating free carriers in semiconductors with an ultrashort laser pulse. The free carriers are accelerated by the electric field of a bias voltage and thereby emit THz radiation. The most commonly used semiconductor material is GaAs that is excited by a Ti:sapphire laser. As these laser systems are quite expensive it is very interesting to develop TDS systems that can utilize cheaper laser systems such as fiber lasers emitting at the telecommunication wavelength of 1.55 µm. In this scenario, a low bandgap material like InGaAs is attractive, but there are issues to overcome. Typical InGaAs epitaxial layers are characterized by a resistivity, which is too low to apply a sufficient bias voltage.

To circumvent these restrictions, heterostructures have been designed to combine a low bandgap with a high resistivity. Trap states and Be acceptors for the carriers reduce the carrier density and the carrier lifetime. Up to now these materials have been used in antenna-based emitters. Due to the small active area in the center of the antenna, which is around 10 x 10 µm², the saturation power of these emitters is below 10 mW, while fibre laser systems usually exceed output powers of 100 mW. To avoid this low saturation power the team, which includes scientists from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the Heinrich Hertz Institute (HHI), have combined the heterostructured material with a large area interdigitated pattern with an area of up to 300 x 300 µm².

With these new emitters the generation of strong broadband THz pulses with a spectral range from 0.1 THz to 3 THz has been demonstrated. The saturation power is increased above 100 mW so that the full power of a fibre laser can be exploited. The spectral content of the emitted THz pulse is very stable against changes in the bias voltage or the pump power which is beneficial for spectroscopy measurements.

More information can be found in the journal Nanotechnology24 214007

Further reading

Nanoscale light source can change its colour (Aug 2009) 
THz laser source shines at room temperature (May 2008)

About the author

Martin Mittendorff is currently working towards a PhD degree at the Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany. His cresearch interests include THz pulse generation and ultrafast spectroscopy of graphene, especially in magnetic fields.

http://nanotechweb.org/cws/article/lab/54214

Metamaterials deliver broadband terahertz transparency window

Mimicking the quantum phenomena of electromagnetic-induced transparency (EIT) or plasmon-induced transparency (PIT) in metamaterials offers a unique route to exploit the coherent coupling effects in classical resonators. Within the transparency window, the high dispersion property of the meta-media may lead to remarkable slow-light phenomena and enhanced nonlinear effects. Such PIT metamaterials are promising candidates for designing slow light devices, highly sensitive sensors, and nonlinear elements operating over a broad frequency range.

Metamaterials induced broadband terahertz transparency

In a recent study, scientists from Tianjin University (TJU) in China, the King Abdullah University of Science and Technology in Saudi Arabia, Osaka University in Japan, and Oklahoma State University in the US have developed a unique and easily fabricated metamaterial and successfully achieved a flat PIT window over 0.4 THz broad bandwidth.

Dominant factors

It was found that the significantly strengthened coupling and the coexistence of electrical and magnetic power exchange are the dominant factors accounting for the broadband transparency feature. In addition, tuning the PIT bandwidth could also be accomplished through adjusting the geometrical parameters of the proposed unit resonators.

Further details can be found in the journal Nanotechnology 24214003.

Further reading

New material filters and polarizes terahertz radiation (Apr 2012)
Graphene could bridge terahertz gap (Aug 2007)

About the author

The TJU team and its collaborators are dedicated to exploring the frontiers of terahertz metamaterials, plasmonics, invisibility cloaking, terahertz radar, terahertz interactions with matters, and terahertz bio-science. The team is also interested in developing new terahertz technology for real-world applications. Miss Zhihua Zhu is a graduate student at TJU, under the supervision of Prof. Jiaguang Han and Prof. Weili Zhang. Her research focuses on the PIT phenomenon and chiral metamaterials.

Abstract -Attosecond synchronization of terahertz wave and high-harmonics

Zhihui Lü, Dongwen Zhang, Chao Meng, Xiyu Du, Zhaoyan Zhou, Yindong Huang, Zengxiu Zhao and Jianmin Yuan
Hide affiliations

zhao.zengxiu@gmail.com jmyuan@nudt.edu.cn
Department of Physics, National University of Defense Technology, Changsha 410073, People's Republic of China 

http://m.iopscience.iop.org/0953-4075/46/15/155602/

Argon atom ionizing in the ultrafast two-colour field has been used to investigate the physical mechanism of terahertz (THz) generation. We measured simultaneously pulse energies of THz and high-harmonics as the phase delay between the fundamental and its second harmonic was varied. The optimal phase delay of THz generation is determined according to the inherent attochirp of the emitted high-harmonics. The dynamic analysis of the tunnelling electron wave packet driven by the Coulomb–laser coupling shows that laser-assisted soft collisions of the electron wave packet with the atomic core play a key role. It is demonstrated that the rescattering process, being indispensable in high-harmonic generation processes, dominates THz wave generation as well in a more elaborate way.

Benefits and Applications of Terahertz (THz) Energy – An Interview with Dr. David Daughton

http://www.azom.com/article.aspx?ArticleID=9717

Interview conducted by Gary Thomas

David Daughton, Applications Scientist at Lake Shore Cryotronics, reveals some of the benefits of an often overlooked resource for the materials science community: terahertz (THz) energy. 
GT: Could you briefly introduce terahertz (THz) energy and its key properties?
DD: Terahertz refers to the portion of the electromagnetic spectrum which lies between the microwave and infrared frequency light.
The definitions are going to vary depending on who in the terahertz community that you talk to, but I generally like to think that terahertz frequency light is anything that lies between about 0.1 terahertz and 10 terahertz. This corresponds to a wavelength between 3 millimeters at the low end of that frequency domain and about 30 microns at the high end.

Diagram showing the position of terahertz (THz) energy in the electromagnetic spectrum. Image credit: Lake Shore Cryotronics.

GT: How much work has been previously undertaken using terahertz energy relative to the other regions of the electromagnetic spectrum?
DD: Unlike the infrared and microwave frequencies, there has been a historical lack of reliable terahertz sources and detectors, and that lack of reliable sources has limited a widespread adoption of Terahertz spectroscopies for materials characterization, compared with IR or microwave characterization techniques.
In the past 20 or 25 years, there’s been a number of terahertz sourcing technologies that have emerged and those technologies have matured to a point where terahertz is becoming a viable characterization tool.
There are tradeoffs between the various sources, but each particular application that you’re targeting with terahertz spectroscopy will really determine the kind of sourcing and detection that users are going to adopt.
GT: What are some of the unique measurement capabilities of THz spectroscopy compared with other optical characterisation technologies?
DD: Because terahertz is an electromagnetic wave, a lot of the conventional optical techniques that are used -- such as absorbance, reflectance, polarimetry, wave guiding, even near field microscopy -- can be extended into the terahertz regime.
I think what is unique is that terahertz interacts with materials in ways that are different than other electromagnetic radiation. For example, a lot of plastics, ceramics and even papers are transparent to terahertz, while, for ordinary light, those same materials are opaque.
Terahertz is also extraordinarily sensitive to water content, and so if the material has even a small amount of water content, it’s fairly absorptive to terahertz light, meaning there are ways of characterizing water content in materials using THz spectroscopy.
For Lake Shore, we’re interested in materials like semiconductors, and with terahertz light, the energy is sufficiently low so it can couple to the free carriers in many of the common semiconductors and characterize the mobility and carrier concentration..
Unlike visible light characterization, the THz energy is much smaller than the band gaps in most materials, and the analysis of THz absorption spectra need not include terms to describe photogenerated carriers.
GT: THz-TDS systems are commonly used in materials characterisation studies – could you explain how these systems work?
DD: Terahertz time-domain sources (THz-TDS) use optical pulses from mode-locked femtosecond lasers. The idea is that you convert that monochromatic, femtosecond pulse into a broadband, picosecond terahertz pulse which contains all frequencies, or a large number of frequencies, in the terahertz regime.
And so, for a transmission measurement, what you’ll do is generate that terahertz pulse and then propagate it through the sample before sending it into some sort of terahertz detector.
That detection can be done either with what’s called a photoconductive switch, a non-linear device or an electro-optic crystal. The idea is that you use a portion of the femtosecond energy to generate the THz and another portion to time gate the detector in order to measure both the amplitude as well as the time-delay of the terahertz pulse which passes through your sample.
From that, you’re able to derive the frequency dependent absorption and the phase delay from the Fourier Transform of your terahertz pulse. From those two bits of information, you can apply various models to understand how that terahertz pulse has interacted with the material.
I think the key thing with terahertz time domain spectroscopy is that, typically, you get somewhere between 3 and 10 gigahertz of spectral resolution and that actually comes out of the properties of the Fourier transform.
GT: How can “photomixing” aid these systems?
DD: Photomixing is related to some of the terahertz time domain sources in that we use the same semiconductor material, which has a fast carrier relaxation rate. With photo-conductive mixing, rather than using one of these costly femtosecond lasers, you use two low-cost continuous wave lasers and you mix those two together onto an antenna using these fast carrier recombination rate materials. 
By using similar terahertz sourcing technology used in some of these terahertz time domain sources but with lower cost laser systems, you’re able to develop a broad bandwidth, a continuously tunable terahertz source that offers better spectral resolution compared to those terahertz time domain systems -- often as low as about 100 megahertz.
One of the advantages is that because the photomixing generates a single frequency at a time, rather than all frequencies at the same time like a terahertz time domain source, you can focus your efforts on looking at a narrow spectral region. For example, if there’s a narrow absorption in a particular part of the spectral region, you can focus your acquisition time on that narrow spectral region instead of the entire spectral region.
GT: How can THz spectroscopy assist with the characterisation of dielectric materials?
DD: Most non-polar dielectrics are transparent to terahertz light, so from a spectroscopic standpoint, you can derive the absorption and the index at terahertz frequencies which can drive other terahertz applications such as making lenses or windows.
However, I think one of the key strengths of terahertz spectroscopy is using it as a non-destructive evaluation tool. For dielectric materials, for example, you could think about having a multilayer that’s glued together; because the terahertz light can pass through the multiple layers and picks up information from all of the layers, you could use terahertz spectroscopy to characterize the chemical composition of the adhesion layer over time in this composite structure.
Another possibility is polarization sensitive terahertz spectroscopy being used during manufacturing to look at the alignment of fibers in composite dielectrics. So there are lots of opportunities to look at dielectric materials, especially ones that have a little bit more structure to them, with terahertz spectroscopy and taking advantage of the transparency.
GT: How can THz spectroscopy aid the semiconductor industry?
On the semiconductor front, terahertz imaging has been a very hot topic of late and a number of manufacturers have been looking at terahertz imaging as a way of conducting fault detection in circuits. It’s a little bit away from the terahertz spectroscopy side of things, but it’s really an important application of terahertz energy.
I believe that the strength of terahertz spectroscopy for the semiconductor industry is really in developing new materials and new processes. On the roadmap of semiconductor development are things like high k dielectrics and new thin film deposition techniques, as well as a move away from silicon, so a new characterization tool that can be sensitive to some of the things that go on in those materials could be useful.
GT: How is Lake Shore Cryotronics utilising the unique capabilities of THz spectroscopy?
DD: Lake Shore is in the business of working with materials development scientists, both in academia and industry, to develop systems that they can use to characterize materials. We’ve been building and developing vibrating simple magnetometers and Hall systems for years now, and when terahertz started showing up on our radar, we immediately said “well, how can this help our core customer?”
Over the last few years, we’ve been developing a terahertz frequency characterization system product which bundles continuous-wave terahertz spectroscopy using photomixers with variable cryogenic temperatures and high magnetic field.
The design of the platform and the software is targeting users that are in the materials development community, not necessarily THz experts, who are looking for a platform that they can use to characterize the terahertz frequency properties of their pre-device stage materials.

Terahertz spectroscopy system from Lake Shore Cryotronics. Image credit Lake Shore Cryotronics.

GT: How has THz technology evolved over the last 20 years?
DD: I think the major change has been accessibility to terahertz technologies as a whole. 20 years ago you had to be a laser expert or have several degrees in microwave engineering in order to really access the technologies. These systems were built in labs for labs and they were large and not very cost effective.
Over the past 20 years, the number and variety of sources has grown rapidly. This has resulted in increased accessibility for what I’d call non-terahertz experts --- people who are looking at biological materials, semiconductors, or new composite materials who aren’t necessarily in the business of building a terahertz spectrometer but are interested in using its unique capabilities to look at their materials.
GT: What are some of the current limitations to THz spectroscopy and how can these be overcome?
DD: I think the biggest limitation to terahertz spectroscopy is in sourcing. As I previously mentioned, with the varieties of terahertz sources that are out there, there’s limitations to each of them, be it power, spectral bandwidth, or spectral resolution, and so what often happens is an application will pick the terahertz source.
I think over time and with the efforts of researchers in this field, what’s going to happen is an evolution towards a cost-effective, high-power, continuously tunable, high spectral resolution solution for terahertz spectroscopy. I think that the biggest limitation for users outside of the terahertz community that want to bring terahertz into the labs is that the systems are either severely limited or they’re really expensive. So I think, as technology evolves, the sources will become cheaper and there will be bundling of these sources into platforms that are similar to UV-VIS, Raman, or other light-based characterization systems, which are commonplace in biology, chemistry, and materials research settings.
This is why started where we have with our terahertz development system; it is really about coming up with the best sourcing technology we can that’s both cost effective and build it into a system that can be easily used by a technical customer-base.

About David Daughton

David Daughton is an Applications Scientist at Lake Shore Cryotronics in Westerville, OH. David joined Lake Shore in 2011 to develop new measurement platforms for terahertz frequency characterization of electronic and magnetic materials and support customer applications in Lake Shore’s cryogenic probe station products. 
He received B.S. and M.S. degrees from the University of Delaware, where he explored the effects of disorder on the superfluid and solid phases of helium, and a Ph.D. from the Ohio State University for scanning tunneling microscopy and optical studies of carbon-based electronic materials.

Abstract-Multiband Negative Permittivity Metamaterials and Absorbers

Yiran Tian, Guangjun Wen, and Yongjun Huang

http://www.hindawi.com/journals/aoe/2013/269170/abs/

Design and characteristics of multiband negative permittivity metamaterial and its absorber configuration are presented in this paper. The proposed multiband metamaterial is composed of a novel multibranch resonator which can possess four electric resonance frequencies. It is shown that, by controlling the length of the main branches of such resonator, the resonant frequencies and corresponding absorbing bands of metamaterial absorber can be shifted in a large frequency band.

Abstract-Self-mixing interferometry with terahertz quantum cascade lasers

Valavanis, Alex, Dean, Paul, Lim, Yah Leng, Alhathlool, Raed, Nikolic, Milan, Kliese, Russell, Khanna, Suraj P., Indjin, Dragan, Wilson, Stephen J., Rakic, Aleksandar D., Linfield, Edmund H. and Davies, Giles (2013) Self-mixing interferometry with terahertz quantum cascade lasers. IEEE Sensors Journal, 13 1: 37-43. 

http://espace.library.uq.edu.au/view/UQ:295646
Terahertz frequency quantum cascade lasers (THz QCLs) are compact sources of coherent THz radiation with potential applications that include astronomy, trace-gas sensing, and security imaging. However, the reliance on slow and incoherent thermal detectors has limited their practical use in THz systems. We demonstrate THz sensing using selfmixing (SM) interferometry, in which radiation is reflected from an object back into the QCL cavity, causing changes in the laser properties; the THz QCL thus acts simultaneously as both a source and detector. Well-established SM theory predicts a much weaker coupling in THz QCLs than in diode lasers, yielding a near-linear relationship between the phase of SM signals and the external cavity length. We demonstrate velocimetry of an oscillating reflector by monitoring SM-induced changes in the QCL drive voltage. We show that this yields data equivalent to that obtained by sensing the emitted THz power, thus allowing phase-sensitive THz-SM sensing without any external detector. We also demonstrate high-resolution SM-imaging at a round-trip distance of 21 m in air—the longestrange interferometric sensing with a THz QCL to date.

Picometrix a subsidiary of Advanced Photonix supplied the Terahertz platform and tools used in INFRASTRUCTS

My Note: Mike Favale* posted a link to an article today which indicates that the recent news  (see: http://terahertztechnology.blogspot.com/2013/07/internal-thz-tagging-technique_23.html ) concerning the joint venture by Carnegie Melon and Microsoft to develop a new technology called Infrastructs which embeds information directly inside items which can then be read by a THz scanner and which can be used in 3D printing, used a Picometrix platform.

The researchers employed several off-the-shelf THz emitter-receivers, including Picometrix's T-Ray 4000 model. The team has also developed the software that analyzes the transmissions reflected from a scan of each item, and then links the results to a specific object. - See more at: http://www.rfidjournal.com/articles/view?10856/3#sthash.ylr1UnVB.dpuf

Gizmodo had this to say about Inftrastructs:

Say you have a 3D-printed item you're selling. Maybe you slap a barcode on it that identifies what it is, where it came from, and how much it costs. However, Microsoft has developed a new technology called InfraStructs that offers a better alternative. With it, you could embed information directly inside of an item.

InfraStructs is a passive, material-based tag that can be identified using Terahertz imaging. Terahertz imaging is sort of like X-ray vision, and it's often used in biomedical or aerospace fields for quality control. With InfraStructs, it can see through an item and immediately pick up all kinds of data. InfaStructs could be embedded in even very small items. It could be used for inventory control, interactive gaming, or pretty much anything you'd need to link an object to. You can see how it works in the demo above. And maybe it's our future without sticky labels and price tags or awkward QR codes. [Karl D. Willis, TG Daily]

http://gizmodo.com/microsoft-has-invented-a-way-to-embed-data-in-3d-printe-922028963

    *(Mike's blog is found here):

http://terahertzspectrum.blogspot.com/

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.

Dayton could be ‘Silicon Valley’ of sensors development

http://www.daytondailynews.com/news/business/dayton-could-be-silicon-valley-of-sensors-develo-1/nPSBd/

By Thomas Gnau

Staff Writer

DAYTON — The executive director of the Air Force Research Laboratory has no doubt that Dayton will be the international center of sensors technology development.

“In the next five to ten years, this will become the next Silicon Valley for sensors and sensors development,” Joe Sciabica, AFRL chief, told an audience Tuesday at the fourth annual Ohio Innovation Sensor Summit, held at the Tech Town business park.

Sensors are devices designed to help users detect everything from suspected terrorists to corrosion. Larrell Walters, director of the University of Dayton’s Institute for the Development and Commercialization of Sensors Technology, estimates that at least 2,000 people work in sensors development across Southwestern Ohio, not counting employees on Wright-Patterson Air Force Base, where AFRL is headquartered.

The base itself invests about $1 billion a year into sensors work, Walters estimated. At least $200 million to $300 million a year of sensor activity takes place outside the base, he estimated.

Sciabica said outside partnerships with contractors and small businesses “permeate the fenceline surrounding” the base. AFRL itself had a total budget of nearly $4.6 billion fiscal year 2011, and of that, just over $1.9 billion was spent on external contracts, he said. AFRL has a workforce of more than 10,000 across the Air Force.

“I think our future is extremely, extremely bright,” Sciabica added, calling the Dayton area, “a good area at a good time.”

Technology transfer between AFRL and academic and industry partners leads to increased production, jobs and sales, Sciabica said, and that was a theme for other summit speakers, as well.

Larry Hollingsworth, director of rapid response and irregular warfare at the Naval Air Station Command in Patuxent River, Md., praised the contractors with whom he works and urged listeners to tap into the expertise of small businesses. His projects have included helium-filled mini-blimps that can continuously monitor a region from 2,000 feet up to technologies that counter IEDs (improvised explosive devices) in Iraq and Afghanistan.

“Do not disregard small businesses for they are the engine of innovation,” Hollingsworth said. “I truly believe that.”

He acknowledged the reality of ever-tighter government budgets, but he said there should be an understanding that some sensor technologies will not pay dividends immediately. “Some technologies will have a return (on investment) in that fifth, sixth, seventh year,” he said.

Others praised Ohio government’s willingness to invest in new companies nurturing new technologies. Don Burdette, director of scientific research for Columbus-based Traycer Diagnostics, showed his audience a slide calling Ohio’s development programs “second to none.”

Ohio’s investment helped the company create the world’s first broadband camera for detecting terahertz radiation, a device that could be useful for looking past paint and silicon for defects, Burdette said.

“I don’t know anywhere else that is doing that,” Burdette said, referring to the Ohio Third Frontier and other programs meant to boost technology.

Contact this reporter at (937) 225-2390 or tgnau@DaytonDailyNews.com.

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IBM Demonstrates a Competitive Graphene Infrared Detector

Image: Tony Low

In the image, plasmon dispersion in graphene on silicon dioxide substrate, reveals coupling with long-lived substrate phonons. They can be excited by patterning graphene into nanoribbons.

By Dexter Johnson

Posted 24 Jul 2013 | 20:43 GMT

http://spectrum.ieee.org/nanoclast/semiconductors/nanotechnology/ibm-demonstrates-a-competitive-graphene-infrared-detector

Earlier this year, researchers at IBM’s Nanoscale Science and Technology group revealed some of the fundamental photoconductivity mechanisms of graphene.

The IBM researchers demonstrated that graphene can either be positive or negative depending on its gate bias. The positive is due to a photovoltaic effect and the negative is due to a bolometric effect.

The bolometric effect involves photo-generated carriers that, while propagating across graphene, emit quanta of lattice vibrations called phonons and thereby transfer their energy into the lattice. Heating up the lattice implies enhancing the electron-phonon scattering process and reducing the carrier’s mobility. The IBM researchers discovered this effect was dominant in the photo response of graphene and is what leads to the photocurrent flowing in the opposite direction of the source-drain current.

In new research, which was published both in Nature Communications(“Photocurrent in graphene harnessed by tunable intrinsic plasmons”) andNature Photonics (“Damping pathways of mid-infrared plasmons in graphene nanostructures”), the IBM team has begun to explore ways to amplify this bolometric effect in graphene.

The research team, which includes Hugen Yan, Tony Low, Wenjuan Zhu, YanqingWu, Marcus Freitag, Xuesong Li, Francisco Guinea, Phaedon Avouris, and Fengnian Xia, began by first studying the fundamental property of plasmons in graphene metamaterials by purely optical methods, revealing important information about its dispersion and damping mechanisms. This knowledge guided them in their design of graphene photodetectors, leading to the first demonstration of a graphene infrared detector driven by intrinsic plasmons.

Graphene’s high mobility and zero gap nature gives it fast optoelectronic response and detection in an extremely broad spectral range from the visible over the infrared and into the terahertz range.

In the visible and near-IR, semiconductors are more efficient in detecting light than graphene because they can have matched bandgaps to a particular spectral window, and because a single layer of graphene absorbs only a small fraction of the incoming light. So it is very unlikely that we will some day be able to buy a cell-phone or camera with a graphene photodetector in it.

However, at lower energies, for example in the mid-IR or terahertz regime, graphene could be much more competitive and provide a unique technology solution. Currently, superconducting transition-edge detectors and bolometers are state of the art in these regimes, and these detectors are very expensive. The absorption in a single layer of graphene can be as high as 40 percent in the terahertz, and the window of high absorption can be moved into the mid-IR by patterning the graphene and harvesting graphene plasmons.

The graphene-based photodetectors, which utilize their intrinsic plasmons, have been demonstrated to yield an order of magnitude improvement in the device’s photo-responsivity in comparison to its non-plasmonic counterpart.

The graphene used in the photodetectors were first grown by CVD on copper foil. Copper was then dissolved in etchant, and finally graphene was transferred to a silicon/silicon oxide chip. The researchers built the graphene photodetector itself by patterning graphene into superlattices of graphene nanoribbons using e-beam lithography. The ribbons widths range from 80 to 200 nm and lateral confinement in ribbons provides the necessary momentum to couple with the graphene plasmons. It is then illuminated with a chopped CO2 infrared laser beam.

The researchers believe that graphene plasmonics could potentially provide a natural platform for a range of technologies in the infrared regime such as light detection and modulation, optical communications, photovoltaics, and spectroscopy.

With this basic understanding of how graphene plasmon disperses, damps, and generates photocurrent, the IBM team is now more confident about this line of research. The merging of graphene plasmonics with optoelectronics is a field that has essentially just began so there remain fundamental and technological issues to resolve.

Abstract-Metamaterial-based gradient index beam steerers for terahertz radiation

Jens Neu¹, René Beigang², and Marco Rahm¹

¹Department of Electrical and Computer Engineering and Research Center OPTIMAS, University of Kaiserslautern, 67663 Kaiserslautern, Germany
²Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, 67663 Kaiserslautern, Germany 

http://apl.aip.org/resource/1/applab/v103/i4/p041109_s1?bypassSSO=1

We designed, fabricated, and optically characterized single and double layer metamaterial-based gradient index beam steerers for terahertz radiation. We measured a maximal deflection angle of 6°. The operation bandwidth of the beam steerers was 300 GHz around a center frequency of 1.3 THz. Within this bandwidth, the amplitude transmission was higher than 50%. Due to a thickness of only 100 μm or below, the implemented beam steerers are ideally suited for integration in compact terahertz measurement systems.

© 2013 AIP Publishing LLC

Millimeter Waves May Be the Future of 5G Phones

Samsung’s millimeter-wave transceiver technology could enable ultrafast mobile broadband by 2020

By Ariel Bleicher / July 2013 

Beyond 4G: Samsung engineers [from left] Wongsuk Choi, Daeryong Lee, and Byunghwan Lee test next-generation cellular equipment at a lab in Suwon, South Korea.

http://m.spectrum.ieee.org/telecom/wireless/millimeter-waves-may-be-the-future-of-5g-phones

Clothes, cars, trains, tractors, body sensors, and tracking tags. By the end of this decade, analysts say, 50 billion things such as these will connect to mobile networks. They’ll consume 1000 times as much data as today’s mobile gadgets, at rates 10 to 100 times as fast as existing networks can support. So as carriers rush to roll out 4G equipment, engineers are already beginning to define a fifth generation of wireless standards.
What will these “5G” technologies look like? It’s too early to know for sure, but engineers at Samsung and at New York University say they’re onto a promising solution. The South Korea–based electronics giant generated some buzz when it announced a new 5G beam-forming antenna that could send and receive mobile data faster than 1 gigabit per second over distances as great as 2 kilometers. Although the 5G label is premature, the technology could help pave the road to more-advanced mobile applications and faster data transfers.
Samsung’s technology is appealing because it’s designed to operate at or near “millimeter-wave” frequencies (3 to 300 gigahertz). Cellular networks have always occupied bands lower on the spectrum, where carrier waves tens of centimeters long (hundreds of megahertz) pass easily around obstacles and through the air. But this coveted spectrum is heavily used, making it difficult for operators to acquire more of it. Meanwhile, 4G networks have just about reached the theoretical limit on how many bits they can squeeze into a given amount of spectrum.
So some engineers have begun looking toward higher frequencies, where radio use is lighter. Engineers at Samsung estimate that government regulators could free as much as 100 GHz of millimeter-wave spectrum for mobile communications—about 200 times what mobile networks use today. This glut of spectrum would allow for larger bandwidth channels and greater data speeds.
Wireless products that use millimeter waves already exist for fixed, line-of-sight transmissions. And a new indoor wireless standard known as WiGig will soon allow multigigabit data transfers between devices in the same room. But there are reasons engineers have long avoided millimeter waves for broader mobile coverage.

Illustration: Erik Vrielink

5g Beam Scheme: Steerable millimeter-wave beams could enable multigigabit mobile connections. Phones at the edge of a 4G cell [blue] could use the beams to route signals around obstacles. Because the beams wouldn’t overlap, phones could use the same frequencies [pink] without interference. Phones near the 4G tower could connect directly to it [green].

For one thing, these waves don’t penetrate solid materials very well. They also tend to lose more energy than do lower frequencies over long distances, because they are readily absorbed or scattered by gases, rain, and foliage. And because a single millimeter-wave antenna has a small aperture, it needs more power to send and receive data than is practical for cellular systems.
Samsung’s engineers say their technology can overcome these challenges by using an array of multiple antennas to concentrate radio energy in a narrow, directional beam, thereby increasing gain without upping transmission power. Such beam-forming arrays, long used for radar and space communications, are now being used in more diverse ways. The Intellectual Ventures spin-off Kymeta, for instance, is developing metamaterials-based arrays in an effort to bring high-speed satellite broadband to remote or mobile locations such as airplanes.
Samsung’s current prototype is a matchbook-size array of 64 antenna elements connected to custom-built signal-processing components. By dynamically varying the signal phase at each antenna, this transceiver generates a beam just 10 degrees wide that it can switch rapidly in any direction, as if it were a hyperactive searchlight. To connect with one another, a base station and mobile radio would continually sweep their beams to search for the strongest connection, getting around obstructions by taking advantage of reflections.
“The transmitter and receiver work together to find the best beam path,” says Farooq Khan, who heads Samsung’s R&D center in Dallas. Khan and his colleagues Zhouyue Pi and Jianzhong Zhang filed the first patent describing a millimeter-wave mobile broadband system in 2010. Although the prototype revealed this year is designed to work at 28 GHz, the Samsung engineers say their approach could be applied to most frequencies between about 3 and 300 GHz. “Our technology is not limited to 28 GHz,” Pi says. “In the end, where it can be deployed depends on spectrum availability.”
In outdoor experiments near Samsung’s Advanced Communications Lab, in Suwon, South Korea, a prototype transmitter was able to send data at more than 1 Gb/s to two receivers moving up to 8 kilometers per hour—about the speed of a fast jog. Using transmission power “no higher than currently used in 4G base stations,” the devices were able to connect up to 2 km away when in sight of one another, says Wonil Roh, who heads the Suwon lab. For non-line-of-sight connections, the range shrank to about 200 to 300 meters.
Theodore Rappaport, a wireless expert at the Polytechnic Institute of NYU, has achieved similar results for crowded urban spaces in New York City and Austin, Texas. His NYU Wireless lab, which has received funding from Samsung, is working to characterize the physical properties of millimeterwave channels. In recent experiments, he and his students simulated beam-forming arrays using megaphone-like “horn” antennas to steer signals. After measuring path losses between two horn transceivers placed in various configurations, they concluded that a base station operating at 28 or 38 GHz could provide consistent signal coverage up to about 200 meters.
Millimeter-wave transceivers may not make useful replacements for current cellular base stations, which cover up to about a kilometer. But in the future, many base stations will likely be much smaller than today’s, Rappaport points out. Already carriers are deploying compact base stations, known as small cells, in congested urban areas to expand data capacity. Not only could millimeter-wave technology add to that capacity, he says, it could also provide a simple, inexpensive alternative to backhaul cables, which link mobile base stations to operators’ core networks.
“The beauty of millimeter waves is there’s so much spectrum, we can now contemplate systems that use spectrum not only to connect base stations to mobile devices but also to link base stations to other base stations or back to the switch,” Rappaport says. “We can imagine a whole new cellular architecture.”
Other wireless experts remain skeptical that millimeter waves can be widely used for mobile broadband. “This is still theoretical; it has to be proven,” says Afif Osseiran, a master researcher at Ericsson and project coordinator for the Mobile and wireless communication Enablers for the Twenty-twenty Information Society (METIS). The newly formed consortium of European companies and universities is working to identify the most promising 5G solutions by early 2015.
Osseiran says METIS is considering a variety of technologies, including new data coding and modulation techniques, better interference management, densely layered small cells, multihop networks, and advanced receiver designs. He emphasizes that a key characteristic of 5G networks will be the use of many diverse systems that must work together. “Millimeter-wave technology is only one part of a bigger pie,” he says.
This article originally appeared in print as “The 5G Phone Future.”