(Source: Wright State University)
The use of metamaterials on the laser has made its beams less divergent

Teams of scientists from Harvard University and the University of Leeds in the United Kingdom have created a terahertz (THz) semiconductor laser that is much different than traditional THz lasers – it releases less divergent beams, which will be helpful to the areas of astronomy, security screening and chemical sensing. 

The study was led by postdoctoral fellow Nanfang Yu and Federico Capasso along with Robert L. Wallace, professor of Applied Physics at Harvard's School of Engineering and Applied Sciences (SEAS) and Vinton Hayes, senior research fellow in Electrical Engineering at SEAS. This team of Harvard researchers collaborated with a team from the University of Leeds, which was led by Edmund Linfield from the School of Electronic and Electrical Engineering. 

Terahertz rays, or T-rays, can detect items such as weapons inside of materials such as clothing, plastic and paper. They are also used to find tiny concentrations of interstellar chemicals, cracks within certain materials and to image tumors without side effects. Though, the problem with these rays is that their beams are "widely divergent," making it difficult for conventional types of terahertz lasers to perform the aforementioned tasks. 

According to Capasso, the teams of scientists were able to create beams with a smaller divergence by developing an artificial optical structure, which consists of the sculpting of sub-wavelength-wide grooves (metamaterial), right on the facet of the laser. This causes the device to emit beams at a frequency of 3 THz into the far-infrared, which is also known as the invisible part of the spectrum. 

Metamaterials, which are artificial materials used to provide properties "not readily available in Nature," were a very important part of the study, since metamaterials being used in semiconductor devices has been limited until now. They were only mainly intended for negative refraction, high resolution imaging and cloaking. Metamaterials have now successfully confined the THz light in the laser, ultimately tightening the direction of the beams. These artificial materials have also eliminated the need for traditional, bulky lenses through the high concentration and efficient collection of power it creates in the facet of the laser.

"Our team was able to reduce the divergence angle of the beam emerging from these semiconductor lasers dramatically, whilst maintaining the high output optical power of identical unpatterned devices," said Linfield. "This type of laser could be used by customs officials to detect illicit substances and by pharmaceutical manufacturers to check the quality of drugs being produced and stored."

This research was published in Nature Materials in August 2010. 

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