What is a GEME Frequency Source?

To date, sources and detectors operating in 0.1 to 10 THz spectrum have been inefficient or too difficult to fabricate because the frequencies are too high for electronics or too slow for optical devices. This section of the spectrum is known as the Terahertz (THz) gap. Providing solutions for this gap can result in many interesting and useful applications for communications, scientific instrumentation, and high frequency imaging.

The Graphene Electromagnetic Emitter (GEME) has been designed to fill this THz gap. It is a new type of frequency source designed to produce terahertz frequencies based on a synchrotron architecture and a graphene Field Effect Transistor (FET). This frequency source produces a single frequency output ranging from 10 GHz to 1000 GHz (0.01 to 1 THz) with 1 mW of output power. The preliminary power dissipation is expected to be less than 20 W for a device of this size for any frequency in this range.

Figure 1 shows the a GEME frequency source with a 1 mm coaxial connector for output frequencies to 110 GHz (left), and with a UG-387 style flange for WR-12 to WR-1 waveguides for frequencies ranging from 60 GHz to 1100 GHz (right). Two feedthroughs provide a filtered power input to the module and an output power level control.



Table 1 shows the preliminary performance specifications for a GEME frequency source.

What Are the GEME’s Applications?

The GEME is a component embedded in a customer’s instruments or equipment that is capable of producing millimeter-wave to Terahertz electromagnetic radiation (T-rays) that can be used for many applications in the following fields:

What is a Synchrotron?

A synchrotron is a machine that produces electromagnetic radiation by accelerating bunches of electrons at speeds approaching the speed of light [1]. The GEME uses this synchrotron architecture to produce terahertz radiation in a similar method. In a synchrotron, electrons circulate around a ring. At various points around the ring are magnetic assemblies that cause the electrons to follow an undulating motion as shown in Figure 2. This motion produces a force on the electron that makes them accelerate, and the acceleration causes the electrons to emit light. The wavelength of light depends on the period of the magnet assemblies, the strength of the magnetic field, and the velocity of the electrons. The light produced ranges from infrared to x-rays.



Each magnet assembly or undulator in the ring is designed to produce a particular wavelength of light for a specific purpose. These wavelengths are used for many applications that include spectroscopy, material science, x-ray microscopy, macromolecular crystallography, IR spectromicroscopy, chemical dynamics, tomography, diffraction imaging, molecular environmental science, small molecule crystallography, x-ray fluorescence, x-ray interferometry and photoemission, magnetic microscopy, and x-ray microdiffraction. An example undulator shown in Figure 3 and it is a very large and precise assembly.

Why Graphene?

The GEME is designed to integrate elements of the synchrotron into a solid state device. This integration provides significant advantages – it reduces the need for a vacuum line for the electron beam and miniaturizes many of the components. However, the electron velocity is much lower than the speed of light in solids. This is partially overcome when using graphene whose electron velocity is much higher than most materials since its electrons are nearly massless and has a velocity 300 times lower than the speed of light. Graphene has one of the highest room temperature electron velocities reported in the literature. Graphene [3] is a 2-D crystalline form of carbon with carbon atoms arranged in a hexagonal pattern as seen in Figure 4.



The hexagon arrangement of atoms results in two sublattices of carbon atoms designated A and B in Figure 4. Carbon atoms support four bonds. In graphene, each A atom bonds to three B atoms and each B atom bonds to three A atoms. The A and B atoms define the unit cell of the graphene lattice. The bond that forms between a A [B] and its three B [A] neighbors is a hybridizes sp2 bond. This leaves a p-electron that can move freely throughout the lattice and is the source of the conductivity in graphene. Further, the interaction of the electron wavefunctions with these two sublattices gives graphene some of its unique properties, such as massless electrons and high electron velocity.

How Does the GEME Work?

A GEME device contains a graphene layer that is biased to produce flowing electrons between two undulator magnet assemblies as shown in Figure 5. The magnet assemblies produce an alternating magnetic field that cause the electrons in the graphene to follow an undulating path and produce electromagnetic radiation at a single frequency that has some similarities to electromagnetic radiation produced from a synchrotron.



The frequency of the emitted radiation is determined by the period of the magnetic field and the velocity of the electrons in the graphene. Compared to a synchrotron, the radiation produced by the electrons in the graphene layer is a single frequency instead of broadband. Also, the radiation pattern produced by each electron looks like a donut shape oriented so that the donut hole aligns with the direction of acceleration, as shown in Figure 6, instead of a narrow cone as emitted by a synchrotron.



Figure 7 shows a simplified structure of the GEME. The geometry of the device forms a graphene Field Effect Transistor (FET) [5] structure with a graphene layer forming the channel between the drain and the source. A voltage, Vds, between the drain and the source produces a force that causes the electrons to flow from the source to the drain in the graphene layer. A voltage between the gate and the source, Vgs, controls the number of electrons present in the graphene layer. Magnet assemblies above and below the graphene channel cause the electrons to follow an undulation path and produce electromagnetic radiation.




The output power is determined by the number of electrons in the graphene channel, the area of the graphene channel, and can be controlled by adjusting the gate voltage. The output power can scale up with increasing graphene area. Large area devices are expected to produce more power output in the watts range for square meter size devices. This should be possible once graphene can be fabricated using roll-to-roll processes.

GEME’s Competitive Advantage

GEME’s competitive advantage: 1 mW continuous wave (CW) output power from 10 GHz to 1 THz, if the device can be fabricated as intended. The total power dissipation is estimated to be around 20 W based on a device that is built on a 100 mm wafer.

For comparison, products from Microtech Instruments offer sources from 0.5 mW at 600 GHz to 0.025 mW at 2 THz. These THz sources require a 6000 V power supply and a water cooling system. Products from Teraphysics look like they could be the strongest competitor as they make a 300 GHz micro-BWO on CVD diamond with 26 to 325 mW of output power. The output power and power dissipation of the GEME device are presently based on theory and estimated operating performance. These values may change as the development of the GEME becomes more mature.

Seventy-one companies have been identified to date that manufacture frequency sources between 10 GHz and 5000 GHz. Unlike companies that make frequency sources under 10 GHz, most companies in this the millimeter-wave and Terahertz market do not publish much information about their products, performance, or pricing so direct comparisons are difficult. Contacting sales is generally required to find out more. Determining companies that are competitors or customers has been challenging and currently being evaluated. For an interactive view of these companies see my blog THz Companies, Markets, and Applications.

GEME Product Comparison

Terahertz radiation is generally produced using a number of different technologies [6]. These include

• Vacuum: Gyrotrons, Backward Wave Oscillator (BWO), Klystrons, Grating Vacuum devices.
• Solid State: Multipliers, Monolithic Microwave Integrated Circuits (MMIC).
• Lasers: Optically pumped molecular lasers, Quantum Cascade Lasers (QCL), Free Electron Laser (FEL), synchrotron light sources.
• Photonics: Optical rectification, Photomixers, and Difference Frequency Generators.

The GEME device most closely approximates a synchrotron light source in function.

Figure 8 shows the various types of THz frequency sources by output power verses frequency. The plot shows where the GEME fits into the range of output power verses frequency. The GEME provides more power out than the photonic devices and some of the multiplier devices. In the chart, photonic devices are green, lasers are orange, vacuum devices are blue, and solid state devices are pink hues.

What are the Markets for GEME?

Table 2 shows opportunities for six different markets. The areas of the circles are scaled so that the relative areas show the relative sizes of each market. This information was extracted from published news reports and summaries of market forecasts and is spotty so making direct comparisons between markets are limited. The forecast dates range from 2016 to 2020 depending on the study and market. The graphene materials market for 2012 is shown for reference. The total addressable market and market share for each market is still being evaluated so a 0.1% market share is shown just for reference. The Markets in increasing frequency include:

• Microwave Com (0.3 GHz to 30 GHz)
• Satellite Com (18 GHz to 40 GHz)
• Millimeter wave (30 GHz to 300 GHz)
• THz Electronics (300 GHz to 3 THz)
• Pharmacy Automation (100 GHz to 10 THz)

What is the Status of GEME?

Over the past three years I’ve been studying graphene, semiconductor processing, designing component geometries, creating mathematical models to describe the GEME's operating range, and developing a business model for the product. In the summer of 2012, I went to two National Nanotechnology Infrastructure Network (NNIN) nano fabrication facilities to fabricate the graphene FET assembly. My attempt to fabricate the graphene FET assembly did not produce a graphene layer with acceptable quality. Since then, I have worked out a number of process issues and identified alternate graphene deposition approaches that should produce a graphene layer of reasonable quality over a 100 m wafer so now I have a number of possible ways to fabricate the graphene FET assemblies. The next fab run is expected to produce graphene FET assemblies that can be tested with the prototype magnet array assemblies as shown in Figure 9.



In addition, the following patents have been filed with the support of the Woodcock Washburn LLP intellectual property law firm: Graphene-based solid state devices capable of emitting electromagnetic radiation and improvements thereof, Jay P Morreale, U.S. Patent Application serial number 61/512,145 filed July 27, 2011, U.S. Patent Application serial number 61/642,038 filed May 3, 2012, and International application number PCT/US12/48453 filed July 27, 2012. The International Search Report (ISR) and the Written Opinion issued by the International Searching Authority indicates that provisions 1-44 of the invention are patentable.

What is the Focus of the GEME Prototype Effort?

To help reduce the development risk, I’m working on a NSF proposal to build a proof-of-concept prototype of the graphene FET assembly. Figure 10 shows a conceptual model of the proof-of-concept prototype of the GEME built with the graphene FET structure on a wafer sandwiched between two external magnet assemblies (top left). Figure 10 also shows the physical implementation of the external magnet assemblies using off-the-self magnets and a partially processed wafer in a plastic container to keep it clean (bottom right). In operation, the magnet assemblies will be slid together as close as possible to provide the strongest periodic magnetic fields near the graphene layer. This proof-of-concept prototype is estimated to emit electromagnetic radiation in the range of up to 700 MHz. The magnet assemblies were built separately to simplify and lower the wafer processing complexity and cost. The lower frequency is a result of the physical size of the magnets chosen for the prototype. The purpose of this prototype is to prove that the assembly can produce synchrotron-like light from electrons propagating in a graphene layer. The prototype should help to answer or provide the means to answer the following technical questions:

• What are the key process issues associated with a quality graphene layer deposition and integration into a standard silicon process flow that covers an entire 100 mm wafer?
• What are the process issues associated with fabricating graphene FET(s) across a wafer without degrading the properties of the graphene layer?
• What are the IV curve characteristics of the graphene FET(s)?
• What are the issues associated with integrating the graphene FET assembly with the magnet array assemblies?
• What are the output characteristics of the EM radiation from the assembly?

The last question is a very important. If the electrons radiate independently in the graphene then the output power will be in the 1 mW range as described above, but if the electrons radiate in unison then the output power could be in the 100 W range. This is a big jump and has to do with the number of electrons squared contributing to the EM radiation (n vs. n²). The prototyping effort will focus on what process and design parameters are needed to achieve this performance. Even with a modest improvement in output power to 1 W, a GEME would out perform most all other devices on the market at 1 THz and below except for low end klystrons.


Higher frequencies are possible in theory. The size and strength of the magnetic domains producing the alternating magnetic field set the limit on the highest frequency possible and will be the focus of the next development effort pending favorable results as described above.

Who am I? What is p-brane LLC?

I’m fascinated and enthralled with the concepts, ideas, interactions and possibilities that nanotechnology can produce. I want to build nanotechnology devices to help people explore and understand the world around them. When I learned that electrons in graphene can travel at very high velocities, I began developing the GEME to take advantage of the nanoscale nature of graphene. With the GEME, I can build tools to image tumors under the skin, measure materials and identify their composition, explore astronomical objects, and communicate with satellites.

I built undersea telecommunications and optical networking hardware for 16 years for AT&T Bell Labs and Lucent Technologies. In 2002, I co-founded Red Sky Systems, which made regional undersea transmission systems with partners like Siemens and Corning. We raised almost $20M in venture capital before it was acquired by Global Marine Systems in 2006. In 2006, I started my consulting business, p-brane LLC, by designing and building proof-of-concept prototypes for high frequency and power electronics working in extreme environments. Some of my projects included an undersea power and communication junction box as part of an undersea observatory, a microwave delay line, and an undersea transponder. I earned a BS and MS in Electrical Engineering from the University of Arizona, and a certificate in nanoscale materials science from the Stanford Center of Professional Development in 2008.

p-brane LLC is small business incorporated in New Jersey in 2006. Follow my blogs on nanotechnology and entrepreneurship on p-brane.com or on twitter @0brane. Invite me to join your LinkedIn network at www.linkedin.com/in/pbrane/

Why do I need your help?

I am working on a proposal for grant money to build the graphene FET assembly for the agencies like the National Science Foundation (NSF) and DARPA. It's important to show a commercial interest for this technology by providing letters of support from customers, partners, or investors. If you are interested in supporting this technology, please just write a letter to me on your company stationary containing:

• Your company, contact information, title
• Description of support to be provided

The description of support should show an interest in commercializing the product. This might include for example:

• Possibly purchasing this technology when it comes to market assuming it still applies to your business and market conditions support such a purchase.
• Partnering or a joint development
• Licensing the technology
• Joining my technical advisory board
• Contributing temporary lab facilities
• Loaning equipment
• Donating old or unused test equipment
• Providing business or technical expertise

I need to show the the agencies that I have enough resources and expertise to complete the proposal, and have a good chance to commercialize this technology. That being said any support, even informal support, will make a significant contribution. Any one of these examples of support would be greatly appreciated.

• Passing this onto others who might provide support
• Making introductions to potential supporters
• Advocating for this technology
• Identifying potential customers
• Providing a recommendation
• Giving feedback on this proposal
• Providing your expertise or guidance

Please send your letter to me at geme at p-brane.com . For more information please see the GEME Technical Description.

References

[1] A. Hofmann, The Physics of Synchrotron Radiation, 1st ed. Cambridge University Press, 2007.
[2] “Undulator,” Wikipedia, the free encyclopedia. 27-Feb-2013.
[3] “Graphene,” Wikipedia, the free encyclopedia. 18-Jun-2013.
[4] “Advanced Quantum Mechanics II PHYS 40202.” Accessed: 21-Jun-2013.
[5] F. Schwierz, “Graphene transistors,” Nat. Nanotechnol., vol. 5, no. 7, pp. 487–496, 2010.
[6] “Terahertz radiation,” Wikipedia, the free encyclopedia. 20-Jun-2013.
[7] C. M. Armstrong, “The truth about terahertz,” IEEE Spectr., vol. 49, no. 9, pp. 36–41, 2012.



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