Mirrors for Electromagnetic Waves

COMPOUNDS

Mirrors for Electromagnetic Waves
Modified Materials. For communication in the future, dielectric mirrors made of custom-formulated polymer-based compounds could provide a key function. Their task would be to reflect THz waves for indoor data transmission.
STEFFEN WIETZKE CHRISTIAN JANSEN MARTIN KOCH

T

erahertz (THz) technology is suited not only for nondestructive testing of plastics [1]: a technology that may become commonplace in the future is communication by means of THz waves for the so-called last mile as data moves along its path to the end user. The demand for bandwidth for short-range wireless communication systems has grown continuously in the past 20 years. Today’s systems such as Bluetooth and wireless LAN work with carrier frequencies of a view gigahertz (GHz). The ultra-wideband system currently in development will use frequencies of up to 10.6 GHz and probably be able to transmit data at rates of 1.3 gigabits per second (Gbps). The first stationary pointto-point systems operating at 60 GHz are already currently available. Projecting the continuously growing demand for bandwidth into the future [2], it becomes obvious that the frequency ranges of existing and anticipated systems intended for local communication will no longer be adequate in 10 to 15 years. To provide the

Translated from Kunststoffe 12/2009, pp. 50–53 Article as PDF-File at www.kunststoffeinternational.com; Document Number: PE110274

projected data rates of several tens of Gbps, it will be necessary to move to even higher frequencies and unavoidably into the THz range. The Terahertz Communications Lab (www.tcl.tu-bs.de) is thus focusing its activities on the question of how to meet the future everyday demand for high-speed communication involving large amounts of data for our information society, e. g. for mobile electronic assistants used in the office, traffic, medicine, leisure and entertainment fields. Several concrete applications for these systems are currently pending. By way of example, the wireless expansion of EPONs (Ethernet Passive Optical Networks) should be mentioned here. The Japanese telecommunications giant NTT provides an even clearer vision of the future. For example, entire videos could be loaded wirelessly onto mobile devices such as the iPod in a few seconds at WiFi hotspots. In addition, high bit rate transmission systems will also play a role in the future for broadcasting sporting events from several mobile cameras for high-definition television (HDTV). During the 2008 Olympic Games in Beijing, compact, battery-operated 120-GHz transmitters were tested for transmission of six uncompressed HDTV channels. In addition to these already-identified applications (market

pull), this technology will pave the way in other fields of application (technology push). The infrared technology used today for remote control or networking of office equipment already employs even higher frequencies. However because of a variety of fundamental limitations (above all, the interfering thermal background of the surroundings), it is not suitable for communication systems that can provide such high data rates.

Mirrors as Key Components
If currently employed WLAN systems are compared to a light bulb that emits light in all directions, the new THz communication system is best visualized as the directional, narrow cone of light projected by a flashlight. A simulation of the new transmission scenarios quickly makes it >

i

Contact

Philipps-Universit?t Marburg Fachbereich Physik AG Experimentelle Halbleiterphysik Terahertz-Systemtechnik D-35032 Marburg / Germany TEL + 49 6421 28-22129 > www.tcl.tu-bs.de

Kunststoffe international 12/2009

37

COMPOUNDS

Fig. 1. Communication at THz frequencies requires an (indirect) visual connection between the transmitter and receiver (a). THz mirrors also permit shielded communication units (b)

clear that communication at THz frequencies requires visual contact between the transmitter and receiver. If individuals or objects interrupt this connection, the information packet must be bounced off the wall (Fig. 1a). A few reflecting hotspots on the walls (e. g. incorporated into the wall itself) are enough to fill the room satisfactorily with the signal. Moreover, such a reflectingly shielded unit ensures a high degree of data security (Fig. 1b).

Principle of a Dielectric Mirror
Because of the phase relationship between the reflected and transmitted waves, a layered structure with alternating indices of retraction (nH > nL) produces constructive and destructive interference (Fig. 2). For optimal functionality, a large difference between the indices of retraction in conjunction with a high degree of transparency of the materials used is desirable for the THz waves, since in contrast to

metal mirrors where the radiation is reflected only at the surface, it must traverse the entire structure in the case of a dielectric mirror – back and forth. Compared to metal mirrors, however, a dielectric mirror exhibits not only increased reflectivity, but also frequency selectivity: the layered structure can be designed to reflect defined frequency bands, allowing it to function simultaneously as a filter. Sensitive information is shielded and remains in the room, while wireless communication with the outside is possible on other channels.

n0

nH
d= λ0 4nH

nL
λ0 4nL

nH
λ0 4nH

n0

Compounds as the Optimal Functional Materials
A large step in the index of refraction between layers means not only clearly delimited frequency windows in which the mirror reflects, but also that fewer layers are needed to achieve high reflectivity. Dielectric mirrors based on purely polymer layers for the THz frequency range were first proposed in a publication dating from 2002, since nonpolar polymers are almost transparent in most of the THz frequency range. However, because of the minimal difference between the indices of retraction of the two components, their performance (reflectivity, frequency selectivity) was extremely limited [3].While use of ceramic-based materials would increase the difference between the indices of refraction of the layers, it will also increase the manufacturing cost and the rigidity of the structure. A very large difference between the indices of refraction was achieved with a combination of silicon and polypropylene layers [4], but this layered system provides high, frequencyselective reflectivity only in conjunction
Kunststoffe international 12/2009

+3π
Partial reflections

+3π
+

+3π
+


+



π 2 π 2 π 2

+

+

π 2 π 2

+

π 2

+

Incident wave

π 2

+

π 2

+

π 2

Transmitted wave
? Kunststoffe





Fig. 2. Principle of operation of a dielectric mirror in the form of a three-layer structure by way of example: the optical thickness (index of refraction n times the geometric thickness d) corresponds to one-quarter of the wavelength for which the mirror was designed. The indices 0, H, L stand for air, the high-refracting and the low-refracting materials, respectively. At the interface where the index of refraction changes from low to high the reflected partial wave undergoes an additional phase shift. This structure produces constructive interference in the direction of reflection; the transmitted waves cancel each other

38

? Carl Hanser Verlag, Munich

COMPOUNDS

with the drawback of a brittle, mechanically sensitive component. The solutions proposed cannot combine the needed dielectric performance of the component with the material properties desired from a manufacturing standpoint namely, rollto-roll processes for mass production. In this regard, an alternating layered structure of a thermoplastic base polymer and a compound means new degrees of freedom for design. The incorporated additives increase the index of refraction of every second layer. Figure 3 compares the benefits of polymer-compound multi-layer systems with present solutions.

Pure polymer structure Reflectivity Stability of the structure Mechanical flexibility Manufacturing costs

Ceramicbased structure

Polymer-Si structure

Polymercompound structure

Compound

Base polymer

? Kunststoffe

Fig. 3. The most beneficial material combination for a dielectric mirror is an alternating sequence of polymer and compound layers. The photo at the right illustrates the flexibility of this layered structure

Custom Compound Modification
The major benefit of the compound layer is that its index of refraction can be adjusted by means of the amount of additive. This allows the dielectric properties of the material to be predicted exactly on the basis of an effective-medium theory (EMT) – this reduces the number of development steps. After a number of simulations and trials with various fillers, rutile titanium dioxide (TiO2) was found to be a suitable additive. It possesses a significantly higher index of retraction than polymers and can be incorporated into the polymeric matrix easily with the aid of a compatibilizer (maleic acid anhydride-grafted PP). Figure 4 shows the real part of the permittivity of a filled polypropylene (PP) as a function of the TiO2 content. The size of the additive particles (see SEM image in the graphic) is considerably below that of the wavelengths used, so that scattering effects can be neglected and the compound can be considered an effective medium. An excessively high filler level would, on the one hand, have a negative impact on processability, while also causing too

much absorption of the THz radiation, on the other. To date, the extruded PP and PP-TiO2 films have been pressed together only on the laboratory scale or combined to form stacks of layers through use of an epoxy adhesive. The next challenge is to optimize processing in order to achieve highly filled, thin compound layers and produce dielectric mirrors by means of coextrusion on a technical scale, which would permit installation in private households and conference rooms in the future. In spite of limited access to production equipment, the initial results are promising. Figure 5a shows the frequency-selective reflectivity of a five-layer dielectric mirror for various angles of incidence of the THz waves. Every second PP layer contains 50 vol.-% TiO2. The stop and pass bands are sharp and confirm omnidirectional reflection capability. The measured data validate the simulation results. The slight discrepancies can be explained on the basis of variations in layer thickness as the result of fluctuations during production. A polymer-based mirror is not only flexible; it also does not

lose its ability to function as a reflector when curved. In fact, it allows the waves to be focused (Fig. 5b).

Prize-winning Patent Application
This material innovation for dielectric mirrors, which may be the key component in communication technology of the future, is based on the selection and development of suitable layer materials. Using a custom-formulated polymer-based compound for every second layer of the multi-layer structure permits not only maximum design freedom in terms of functional layout, but also the manufacturing prerequisites for a mass market. The patent-pending material combination [5] from the inventor group headed by Prof. Martin Koch was honored at the beginning of the year with the Patent Award, the Prize of the Future, by the IP Bewertungs AG (IPB). The inventors look forward to working with partners from the plastics industry who are interested in marketing the invention.
ACKNOWLEDGMENT We thank Dr. Jens Helbig, Frank Neubauer and Christoph Reinhard, formerly with Neue Materialien Würzburg GmbH, Germany, for producing the compound and the layered structures. REFERENCES 1 S. Wietzke, F. Rutz, M. Koch: The Terahertz View. Kunststoffe international 97 (2007) 5, pp. 31–34 2 S. Cherry: Edholm’s law of bandwidth. IEEE Spectrum (Juli 2004), pp. 58–60 3 D. Turchinovich et al.: Flexible all-plastic mirrors for the THz range. Applied Physics A: Materials Science & Processing 74 (2002), pp. 291–293 4 N. Krumbholz et al.: Omnidirectional terahertz mirrors: A key element for future terahertz communication systems, Applied Physics Letters 88 (2006). pp. 202905-1–202905-3

Real fraction of the dielectric constant ε’

25 Measurement EMT model 20

Fig. 4. The dielectric properties of the compound layer can be modeled and adjusted: with increasing TiO2 content, the material’s index of refraction increases. The SEM image shows the compound with 50 vol-% TiO2

15 1?m 10

5

0

0

10

20

30

40

50

%

60

Vol. of TiO2

? Kunststoffe

>

Kunststoffe international 12/2009

39

COMPOUNDS

a) a)
1.0 0.5 0 0.1 1.0 0.5 0 0.1 1.0 0.5 0 0.1 1.0 0.5 0 0.1

Measurement

Simulation 30°

b)

Coefficient of reflection (intensity) for s-polarization

0.2

0.3 THz 0.4 40°

0.2

0.3 THz 0.4 50°

0.2

0.3 THz 0.4 60°

0.2

0.3 THz 0.4
? Kunststoffe

Frequency f

Fig. 5. Measurement and simulation results for a five-layer compound mirror that reflects not only at various angles of incidence (a), but also when curved as well – and even focuses (b) 5 DE 10 2007 021 954.9-41: ?Vorrichtung zum Reflektieren elektromagnetischer Strahlung“, Patent pending THE AUTHORS PROF. DR. MARTIN KOCH, born in 1963, was the head of the “Terahertz systems group” at the Institute for High-Frequency Technology” (Institut für Hochfrequenztechnik / IHF) at the Technische Universit?t Braunschweig, Germany, until 2008 and is now a professor in the Experimental Semiconductor Physics working group at Philipps Univeristy, Marburg. He established the Terahertz Communications Lab (TCL). DIPL.-ING. STEFFEN WIETZKE, born in 1980, is a research associate at the IHF and works in the field of THz spectroscopy of polymers and plastics. He is a member of the TCL; steffen.wietzke@ihf.tu-bs.de DIPL.-ING. CHRISTIAN JANSEN, born in 1983, is a research associate at the IHF and works on industrial process control in the field of terahertz system technology. He is a member of the TCL. The “Terahertz System Technology” working group is relocating to Marburg with Prof. Martin Koch, who accepted a position at the Philipps University at the beginning of the year.

Fewer Heating Zones in Long Nozzles
Hot-runner Technology. At Fakuma 2009, Incoe International Europe, R?dermark, Germany, exhibited the AXR option for long hot runner nozzles. This permits operation with two instead of the usual three heating zones. This has been made possible by the new ultralong AXR heaters. Over the greater length of these extended heaters, the temperature profile can now be adjusted so as to render a third heating zone unnecessary. According to the manufacturer, omitting the third heating zone is appropriate for processing less technically demanding plastics such as PP, PE or PS. This reduction in complexity benefits customers, because – assuming the desired processing quality can be maintained – it allows one heatTranslated from Kunststoffe 12/2009, p. 70

ing zone to be saved per nozzle, for which customers would otherwise have to provide the required controller capacity. The AXR option also offers some other useful design features. An additional centering collar on the nozzle body protects the nozzle and cable from damage during installation and dismantling. Since the wires running along the nozzle body are countersunk or snug-fitting, it is possible to dispense with the cable groove in the mold. A circular clearance in the mold is sufficient. According to Incoe, the AXR option is of particular interest to the automobile market, in which large systems with long nozzles are used, e. g. for the production of bumpers, wheel-arch liners and interior door panels.
> www.incoe.com

Simplifies production without sacrificing quality: the AXR nozzle is doubly protected and reduces the number of heating zones (photo: Incoe)

40

? Carl Hanser Verlag, Munich

Kunststoffe international 12/2009


相关文档

Do mirrors for gravitational waves exist
8 Plane Electromagnetic Waves
Propagation of electromagnetic waves
Chapter 34 - Electromagnetic Waves
Multiple Rayleigh scattering of electromagnetic waves
Electromagnetic Acoustic Transducer for Generation and Detection of Guided Waves
guide13Maxwell’s Equations and Electromagnetic Waves
Invariant Imbedding Equations for Electromagnetic Waves in Stratified Magnetic Media Applic
Resonant Atom Traps for Electromagnetic Waves
Mesoscopic Correlation with Polarization of Electromagnetic Waves
电脑版