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Potentially, THz waves may accelerate telecom technologies and break new ground in understanding the fundamental properties of photonics. Challenges related to efficiently generating and detecting THz waves has primarily limited their use.


Traditional methods seek to either compress oscillating waves from the electronic range or stretch waves from the optical range. But when compressing waves, the THz frequency becomes too high to be generated and detected by conventional electronic devices.


So, this approach normally requires either a large-scale electron accelerator facility or highly electrically-biased photoconductive antennas that produce only a narrow range of waves.


To stretch optical waves, most techniques include mixing two laser frequencies inside an inorganic or organic crystal. However, the natural properties of these crystals result in low efficiency.


So, to address these challenges, the Ames Laboratory team looked outside natural materials for a possible solution. They used man-made materials called metamaterials, which exhibit optical and magnetic properties not found in nature.


Costas Soukoulis, an Ames Laboratory physicist and expert in designing metamaterials, along with collaborators at Karlsruhe Institute of Technology in Germany, created a metamaterial made up of a special type of meta-atom called split-ring resonators. Split-ring resonators, because of their u-shaped design, display a strong magnetic response to any desired frequency waves in the THz to infrared spectrum.


significant enhancement at magnetic-dipole resonance of the metamaterials emitter (shown in inset image). This approach has potential to generate gapless spectrum covering the entire THz band, which is key to developing practical THz technologies and to exploring fundamental understanding of optics


Ames Laboratory physicist Jigang Wang, who specializes in ultra-fast laser spectroscopy, designed the femto- second laser experiment to demonstrate THz emission from the metamaterial of a single nanometre thickness.


“The combination of ultra-short laser pulses with the unique and unusual properties of the metamaterial generates efficient and broadband THz waves from emitters of significantly reduced thickness,” says Wang, who is also an associate professor of Physics and Astronomy at Iowa State University.


The team demonstrated its technique using the wavelength used by telecommunications (1.5 µm), but Wang says that the THz generation can be tailored simply by tuning the size of the meta-atoms in the metamaterial.


“In principle, we can expand this technique to cover the entire THz range,” says Soukoulis, from Iowa State University.


What’s more, the team’s metamaterial THz emitter measured only 40 nm thick and performed as well as traditional emitters that are thousands of times thicker.


“Our approach provides a potential solution to bridge the ‘THz technology gap’ by solving the four key challenges in the THz emitter technology: efficiency; broadband spectrum; compact size; and tunability,” adds Wang.


This work is partially supported by Ames Laboratory’s Laboratory Directed Research and Development (LDRD) funding.


This research has been published in the paper, “Broadband terahertz generation from metamaterials,” by Liang Luo et al in Nature Communications,5, (3055). DOI:10.1038/ncomms4055


A team led by Ames Laboratory physicists demonstrated broadband, gapless terahertz emission (red line) from split-ring resonator metamaterials (background) in the telecomm wavelength. The THz emission spectra exhibit


January / February 2014 www.compoundsemiconductor.net 159


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