Abstract-Lateral heterogeneous integration of quantum cascade lasers

Yang Yang, Andrew Paulsen, David Burghoff, John L. Reno, and Qing Hu

https://pubsdc3.acs.org/doi/10.1021/acsphotonics.8b00507

Broadband terahertz radiation potentially has extensive applications, ranging from personal health care to industrial quality control and security screening. While traditional methods for broadband terahertz generation rely on bulky and expensive mode-locked lasers, frequency combs based on quantum cascade lasers (QCLs) can provide an alternative compact, high power, wideband terahertz source. QCL frequency combs incorporating a heterogeneous gain medium design can obtain even greater spectral range by having multiple lasing transitions at different frequencies. However, despite their greater spectral coverage, the comparatively low gain from such gain media lowers the maximum operating temperature and power. Lateral heterogeneous integration offers the ability to cover an extensive spectral range while maintaining the competitive performance offered from each homogeneous gain media. Here, we present the first lateral heterogeneous design for broadband terahertz generation: by combining two different homogeneous gain media, we have achieved a two-color frequency comb spaced by 1.5 THz.

Abstract-Hiding multi-level multi-color images in terahertz metasurfaces

Ashish Chanana, Andrew Paulsen, Sivaraman Guruswamy, and Ajay Nahata
https://www.osapublishing.org/optica/abstract.cfm?uri=optica-3-12-1466

Our work presents a novel technique to encode information onto terahertz metasurfaces comprised of geometrically identical unit cell arrays. Previous demonstrations on metasurfaces or frequency-selective surfaces have shown interesting concepts to engineer electromagnetic radiation, but such designs often require a spatial arrangement of geometrically varying unit cells, either by shape, size, orientation, etc. In some cases, the output response can be mapped by examining the arrangement of atoms. Here, we show that by fabricating an array of resonant structures that are nominally identical visually, but where individual structures can have different conductivities, we can hide image information that is revealed when imaged using the appropriate terahertz frequency and polarization. This is achieved because changes in the structure’s conductivity correspond to changes in the depth of the resonant absorption observed in transmission. Using the simplest unit cell consisting of a single dipole, we create images that have up to 9 different discernible gray levels when interrogated at a single frequency. When a slightly more complex cross structure is used in the unit cell, 36 discernible levels are encoded in the image using two different polarizations. Finally, when the unit cell consists of multiple dipoles designed for multiple frequencies, we observe 64 unique colors in an encoded image. We believe our results present a unique approach for hiding information that could be applied to security-related applications.

© 2016 Optical Society of America

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New filter could advance terahertz data transmission

University of Utah electrical and computer engineering professor Ajay Nahata, left, and U graduate student Andrew Paulsen hold up a terahertz frequency filter made through a process they developed with an inkjet printer. They have discovered a new approach for designing filters capable of separating different frequencies in the terahertz spectrum, the next generation of communications bandwidth that could allow cell-phone users and Internet surfers to download data a thousand times faster than today. 

http://unews.utah.edu/news_releases/terahertz-wireless-filter/

UNIVERSITY OF UTAH ENGINEERS CREATE FILTER FOR WIRELESS COMMUNICATIONS USING INKJET PRINTER

Feb. 27, 2015 – University of Utah engineers have discovered a new approach for designing filters capable of separating different frequencies in the terahertz spectrum, the next generation of communications bandwidth that could allow cellphone users and Internet surfers to download data a thousand times faster than today. Once the filter is designed, it can be fabricated using an off-the-shelf inkjet printer.

Filtering out different frequencies will be important in the development of the terahertz spectrum for communications. By filtering out unwanted frequencies users can download information from the Internet or talk on a cellphone, for example, with less noise or interfering signals.

The terahertz range refers to the band of frequencies between infrared light and radio waves. Engineers consider it the next frontier in communications because of the enormous potential for boosting data transmission rates. The technology also is being studied for next-generation medical imaging and airport scanners. Terahertz rays, or T-rays, can pass through many materials without using ionizing radiation, which makes them attractive for use in medical imaging and security screening devices.

This new methodology for creating filters was published in a paper Feb. 27 in The Optical Society’s online journal, Optica.

“Your cellphone operates at a frequency of 2.4 gigahertz. A terahertz is a thousand gigahertz,” said graduate student Andrew Paulsen, who co-authored the paper with U electrical and computer engineering professor, Ajay Nahata. “If we could effectively use the terahertz spectrum for communications, we could have a thousand times more bandwidth than we currently do.”

Paulsen and Nahata discovered that by creating certain computer-generated designs using engineering software called MATLAB and printing them on a plastic sheet via a regular inkjet printer, they could create a filter that allows certain terahertz frequencies to pass through while blocking others out. The printer uses silver-metal ink similar to what is used for the production of circuit boards and tiny antennas.

By using a terahertz generator, which shoots out an invisible beam of light, researchers can measure the frequencies as the beam passes through the filter. The dimensions and geometry of the printed designs, which can look like a wavy bull’s-eye for example, determine which frequencies get through and to what extent.

This method is an important step in utilizing the terahertz spectrum for commercial use, possibly as the basis for the next “5G” network for cellphones. If cellphones on a current “4G” network can download data at 10 to 15 megabits per second, terahertz technology can potentially send data back and forth at terabits per second (or millions of megabits per second). Using filters in such a network will be a vital component because it will be necessary to separate frequencies in order to create multiple communication channels. Many wireless devices use filters to single out frequencies, including Wi-Fi routers, televisions and cellphones.

It might be another 10 years before consumers are using Wi-Fi routers or cellphones with terahertz technology, but communications companies could use it for their network backones much sooner. A current limitation of terahertz frequencies is that they require line of sight and can transmit only over short distances. But some researchers have already achieved lightning download speeds with wireless terahertz chips, and others are interested in broadcasting super-high-definition 4K television signals over the air with cameras that use the terahertz spectrum.

“Terahertz technology is something there is a lot of interest in,” Nahata said. “I guarantee that people will come up with new ideas that can use all of that available bandwidth.”

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