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.
Figure 1 GEME Frequency Source Package Concepts
Table 1 shows the preliminary performance specifications for a GEME frequency source.
Table 1 Preliminary GEME Performance
Figure 2 Synchrotron Radiation Emission 
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.
Figure 3 Stanford Linear Accelerator Complex (SLAC) Undulator Assembly Photograph
Figure 4 Graphene Crystalline Structure 
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.
Figure 5 EM Radiation from a Graphene Layer
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 6 Electron Radiation Pattern Illustration
Figure 7 shows a simplified structure of the GEME. The geometry of the device forms a graphene Field Effect Transistor (FET)  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.
Figure 7 Graphene EM Emitter (GEME) Block Diagram
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.
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.
- 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.
Figure 8 THz Sources Power Vs. Frequency Plot (adapted from )
- 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)
Table 2 GEME Market Summary
Figure 9 Graphene EM Emitter Block Diagram
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 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?
Figure 10 Concept and Prototype GEME (Graphene EM Emitter)
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.
Figure 11 Source Frequency Vs. Power Research Focus
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.
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/
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Please send your letter to me at geme at p-brane.com . For more information please see the GEME Technical Description.
 “Undulator,” Wikipedia, the free encyclopedia. 27-Feb-2013.
 “Graphene,” Wikipedia, the free encyclopedia. 18-Jun-2013.
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 F. Schwierz, “Graphene transistors,” Nat. Nanotechnol., vol. 5, no. 7, pp. 487–496, 2010.
 “Terahertz radiation,” Wikipedia, the free encyclopedia. 20-Jun-2013.
 C. M. Armstrong, “The truth about terahertz,” IEEE Spectr., vol. 49, no. 9, pp. 36–41, 2012.
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