Deniz Turan/UCLA
Schematic of InAs lattice in contact with a nanoantenna array

Converting Light from One Wavelength to Another

Aug. 2, 2021
A built-in electric field created by surface states can enable wavelength conversion.

The ability to convert wavelengths of light is a crucial requirement in the improvement of many imaging, sensing and communication technologies. Endoscopy probes are just one example where this technology can make gains.

And now, electrical engineers have developed an efficient means of converting one wavelength to another by investigating semiconductor surface states.

In a paper published in Nature Communications, researchers noted that surface states are a natural phenomenon that occur when surface atoms have an insufficient number of other atoms to bind to. It causes a breakdown in atomic structure. Since these incomplete chemical bonds, known as “dangling bonds,” cause roadblocks for electric charges flowing through semiconductor devices, their performance is affected and the technique is not preferred.

Lead researcher, Mona Jarrahi, a professor of electrical and computer engineering at the UCLA Samueli School of Engineering, pointed out that efforts to suppress the effect of surface states have largely overlooked the fact that they have unique electrochemical properties that are not provided by bulk semiconductors and open the door to unprecedented device functionalities.

Since these incomplete bonds create a shallow but giant built-in electric field across the semiconductor surface, Jarrahi and her team recognized they could take advantage of surface states for improved wavelength conversion. 

Bending Light

According to the paper, incoming light can hit the electrons in the semiconductor lattice and move them to a higher energy state. This is when electrons are free to jump around within the lattice. 

The electric field created around the surface of the semiconductor further accelerates the photo-excited, high-energy electrons, explained the authors. In turn, they unload the excess energy gained by radiating it at different optical wavelengths, thereby converting the wavelengths.

Energy exchange happens only at the surface of a semiconductor. To enhance efficiency, the engineers used a nanoantenna array that bends incoming light so it is tightly confined around the shallow surface of the semiconductor. 

“Through this new framework, wavelength conversion happens easily and without any extra added source of energy as the incoming light crosses the field,” said Deniz Turan, an electrical engineer and a member of Jarrahi’s research team at the UCLA Terahertz Electronics Laboratory.

Applicability

The researchers converted a 1,550-nm wavelength light beam into the terahertz part of the spectrum, ranging from wavelengths of 100 μm up to 1 mm.

For their efforts, they were able to prove the wavelength-conversion efficiency by incorporating the new technology into an endoscopy probe that could be used for detailed in-vivo imaging and spectroscopy using terahertz waves.

Without this advancement in wavelength conversion, noted the authors, it would have required 100 times the optical power level to achieve the same terahertz waves, which the thin optical fibers used in the endoscopy probe cannot support.

The novel optical wavelength conversion method is applicable in other parts of the electromagnetic spectrum, they added, ranging from microwave to far-infrared wavelengths.

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