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New resonator overcomes obstacle to ultimate nanolaser

Photonics Spectra
Apr 2012
Ashley N. Paddock, ashley.paddock@photonics.com

SAN DIEGO – Two very low power lasers that produce the smallest continuous-wave, room-temperature telecommunications frequency demonstrated to date may be a step closer to creating virus-size nanolasers. The new instruments – a 1.5-µm laser and a 1.38-µm “thresholdless” laser that funnels all of its protons into lasing without any waste – were developed by electrical engineers in the Ultrafast and Nanoscale Optics Group at the University of California, San Diego (UCSD), Jacobs School of Engineering.

The devices’ extremely low operating power is an important breakthrough because lasers usually require greater and greater pump power to begin lasing as they shrink to nano sizes. This quality, along with their small size, could make the lasers useful components for applications ranging from on-chip optical communication to ultrahigh-resolution and high-throughput imaging, sensing and spectroscopy.

“In this sense, the lasers are scalable in size down to the size of the smallest living organisms (20-nm thickness of the gain material),” said electrical engineer Mercedeh Khajavikhan of UCSD.

Khajavikhan and her colleagues in the group suggest that the thresholdless laser also may help researchers as they develop new metamaterials – artificially structured materials for applications ranging from superlenses for seeing individual DNA molecules or viruses, to devices that can bend light around an object and “cloak” it from view.

All lasers require a certain amount of pump power from an outside source to begin emitting a coherent beam of light, said Yeshaiahu “Shaya” Fainman, the principal investigator of the research and a professor at UCSD.

“As the laser becomes smaller in size, especially in microlasers and nanolasers, the volume of the active material becomes smaller and so does the amplification it can provide, but loss mechanisms in the cavity do not scale down with the same rate,” said Michael Kats, an electrical engineer at UCSD. “Because of this disproportionality between the loss and gain reduction with size, the pump required for lasing in nanolasers could be prohibitively large.”

To overcome this problem, the researchers designed a cavity that supports one mode, suppressed the coupling of emitters to the free-space radiation modes and provided a good confinement of the electromagnetic field because of its subwavelength metallic structure.


Upper left:
a schematic of the coaxial laser cavity. Lower left: a scanning electron micrograph image of the constituent ring of the coaxial structure containing the gain section and the cover silica layer. Right: The TEM-like mode of the coaxial structure, which is symmetric in the transverse plane, has no degeneracy and is extremely confined. Courtesy of Mercedeh Khajavikhan and Aleksandar Simic, UCSD.


The team built the thresholdless laser by modifying the geometry of this cavity. The design allowed them to build the smallest room-temperature, continuous-wave laser to date. The nanoscale coaxial laser is 10 times smaller in volume than the group’s previous record-smallest nanolaser (which did not operate in continuous-wave mode), described in Nature Photonics less than two years ago.

Fainman said that these highly efficient lasers would be useful in augmenting future computing chips with optical communications, where the lasers are used to establish communication links between distant points on the chip. Only a small amount of pump power would be required to reach lasing, reducing the number of photons needed to transmit information.

The scalability of the laser design is an extremely important feature that would make it possible to harvest laser light from even smaller nanoscale structures, the researchers said, adding that they expect to be able to build lasers with both radius and height at 20 nm.

“From an electromagnetic point of view, the lasers are infinitely scalable,” said electrical engineer Aleksandar Simic of UCSD. “This means that we can reduce the size to any small value, and it still supports a mode. However, the material properties start to change when we get to sizes below 20 nm; for example, the surface effects become significant, and the properties of the active medium change.”

This feature eventually could make them useful for creating and analyzing metamaterials with structures smaller than the wavelength of light currently emitted by lasers.

As for where their research is going to take them, “we think electrically pumped coaxial nanolasers [are] the most immediate extension of this work,” Khajavikhan said. “The electrically pumped nanolasers would have diverse applications in many areas of science and technology.”


GLOSSARY
laser cavity
A means of optical confinement intended to increase the gain length of radiation prior to emission from the device. The means of optical confinement used to increase gain path length vary depending upon the properties of the beam desired within the lasing medium. High light intensities occur within a laser cavity and dielectric mirrors coated for the lasing wavelength are used.The position and curvature of the optical cavity elements may be altered in order to optimize the laser performance as...
optical communications
The transmission and reception of information by optical devices and sensors.
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