Controlling Laser Direction Paves the Way to the Discovery of Novel Functional Devices

A method to give photons, or light packets, their marching orders has been devised by a team of researchers at Washington University, St. Louis.

An artist's view showing the control of the emission direction of lasing at exceptional points in a whispering gallery mode microlaser. The tori and the spheres represent the microtoroid resonators and the scatterers, respectively. With two scatterers with appropriate sizes and locations in the field of the resonator, light is emitted in only one direction. The lasing is bidirectional when there is one or no scatterer. (Image: B. Peng, F. Monifi, S. K. Ozdemir and L. Yang)

The team was assisted by an energy state in the optical field to move the photons within the lasing system consistently, either in a clockwise or anticlockwise direction.

To achieve a reliable, strong photonic signal and light pulse for the various lasing systems and other applications, it is necessary to have consistent light propagation. Lasing has an enormous range of applications, from hair removal to communication to medicine, and is a several billion dollar industry. The consistent nature affects current and future optical sensing devices, right from aerosol detectors to cancer spotters.

The advantages of a physical phenomenon referred as an exceptional point to move photons either in the clockwise or anticlockwise direction instead of both the directions arbitrarily has been exploited by Lan Yang, the Edwin H. & Florence G. Skinner Professor of Electrical & Systems Engineering, and Şahin K. Özdemir, research associate professor, both in the School of Engineering & Applied Science, along with their colleagues Stefan Rotter at Vienna University of Technology, Austria and Jan Wiersig at Otto-von-Guericke University, Germany.

The discoveries were reported in an early edition of the June 6th issue of the Proceedings of the National Academy of Science.

In the physical fields when two complex eigenvalues and their eigenvectors combine or become the same, an exceptional point arises. These are considered to be mathematical tools that express a physical system. The exceptional point can be regarded as a complex bewitching atmosphere where counterintuitive and unpredictable phenomena often take place.

In a 2014 paper in Science, Yang and Özdemir have explained harnessing of the exceptional point to add the loss to a laser system to truly gain energy - gaining by losing. The exceptional point has given rise to numerous counterintuitive activities and findings in recent physics studies with much more to come.

By inserting two silica scatterers, or nanotips, which help to create the perfect storm that invokes an exceptional point, Yang and Özdemir stimulated the exceptional point existence in an optical field. Bo Peng, a doctoral student and one among the main authors of the 2014 Science paper, used nanopositioning systems to tune the distance between the scatterers and amplify their relative size in the optical field in order to disturb the microresonator and signal an exceptional point. The researchers succeeded in developing a controllable system.

One of the exciting things of this research is it presents an on-demand control of the laser. By changing the scatterer location, you can change the operating regime. This opens the possibilities for new functional devices based on lasers

Lan Yang, Professor of Electrical and Systems Engineering, Washington University

“This finding further enhances our capability to harness microlasers, not only by power boost with loss, as we achieved previously, but also by precisely controlling the lasing direction now,” said Peng.

The microresonator used was from a class of whispering gallery mode resonators (WGMRs) as they work in a similar fashion as the famous whispering gallery in London’s St. Paul’s Cathedral, where a message spoken by anyone to the wall on one side can be heard by a person standing on the other side of the dome. The WGMR device performs a similar action with light frequencies rather than with sound.

With the help of tapered fiber waveguides on both sides, light is introduced into and united out from the WGMR. The scatterers are regulated by nanopositioners, which explore the chirality of the WGM. When light is introduced into the WGMR in the absence of the scatterers, both in the clockwise or anticlockwise directions, a resonance peak emerges in the transmission and signals cannot be received in the reflection.

When the system is brought to an exceptional point by the help of the scatterers, this situation changes The reflection shows a pronounced resonance peak for only one of the injection directions. This asymmetric reflection is a distinguishing characteristic of chirality, an intrinsic property of a mode.

Şahin K. Özdemir, Research Associate Professor, Washington University

There could be many exceptional points within a physical system.

“Bringing the system to a different exceptional point results in the appearance of a reflection peak only for the other injection direction,” Özdemir explained.

Yang said, “Thus, we can control the chirality of a mode by going from one exceptional point of the system to another.”

The researchers then discovered the relation between the intrinsic chirality of lasing modes and asymmetric reflection at exceptional points. They utilized an erbium-doped WGM microresonator as erbium serves as a doping agent that promotes laser activity at a different wavelength than that used by the light to excite lasing.

The emitted light from the erbium could go both clockwise and anticlockwise simultaneously into the WGM resonator, when there is no exceptional point. This leads to the inefficiency in extracting laser light from WGM microlasers. The photons travel consistently either in the clockwise or counterclockwise direction at an exceptional point.

“We now have a control on the chirality of the whispering-gallery modes and hence the emission direction of lasing thanks to the exceptional points,” Yang explained. “Where we place the scatterers and how big we make them changes the operating regime, adding to the technique’s versatility.”

We can tune the whispering-gallery-modes to have a bidirectional (both clockwise and counterclockwise directions) microlaser or a unidirectional laser with emission in the clockwise or counterclockwise direction. We can completely reverse the emission direction by transiting from one exceptional point of the system to another exceptional point.

Şahin K. Özdemir, Research Associate Professor, Washington University

WGMR systems can gain new functions from these discoveries, which would be applicable in optomechanics, sensing, lasing, and quantum electrodynamics.

“The findings of the team prove yet again that if engineered properly, exceptional points, and thus engineering of loss and scattering, provide new tricks for optical sciences to solve problems, which have hindered progress and limited the usefulness of devices,” said Yang.

“This finding further enhances our capability to harness microlasers, not only by power boost with loss, as we achieved previously, but also by precise control of the lasing direction now,” stated Peng.

The scientists conclude that their discoveries would help in the development of unique technologies for regulating light flow, make room for chiral photonics on chip and could influence scientific fields apart from optics. The researchers have already planned to demonstrate and use other counterintuitive features of non-Hermitian photonics at the exceptional points.

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